explanation blue bibcodes open ADS page with paths to full text
Author name code: spruit
ADS astronomy entries on 2022-09-14
author:"Spruit, Hendrik C." OR author:"Spruit, Henk C."
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Title: dopmap: Fast Doppler mapping program
Authors: Spruit, Hendrik C.
2021ascl.soft06002S Altcode:
dopmap constructs Doppler maps from the orbital variation of line
profiles of (mass transferring) binaries. It uses an algorithm related
to Richardson-Lucy iteration and includes an IDL-based set of routines
for manipulating and plotting the input and output data.
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Title: The formation of (very) slowly rotating stars
Authors: Spruit, H. C.
2018arXiv181006106S Altcode:
The slow rotation of some young stars and the extreme rotation
periods of some Ap stars have so far defied explanation. The absence
of sufficiently efficient braking mechanisms for newly formed stars
points to the star formation process itself as the origin. I find that
a mode of star formation exists by which a protostar can form without
accreting angular momentum. It depends on the survival of a magnetic
connection between the accreting matter and the birth cloud. The
conditions for this process to operate are analyzed, and illustrated
with a generic (scale-free) model. Depending on the initial rotation
rate of the accreting matter, either a Keplerian disk forms, or the
gas ends up rotating with the rotation period of the cloud, even if
it is at a large distance. The boundary in parameter space between
these two outcomes is sharp.
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Title: Disruption of a Planet Spiraling into its Host Star
Authors: Jia, Shi; Spruit, H. C.
2018ApJ...864..169J Altcode: 2018arXiv180800467J
The processes leading to the deformation and destruction of
planets spiraling into the convective envelope of their host
stars are described. The planet is compressed by ram pressure and
deformed into a flattened shape, for which a quantitative model is
developed. Compression increases the planet’s density contrast with
the envelope and its gravitational binding energy. This increases
the survivability, especially of gas planets. An estimate is given
for the depth of disruption by ram pressure, and for the subsequent
fragmentation of the remnants. We show how the debris of rocky or
iron planets, instead of mixing through the convection zone (CZ),
sinks below the base of the CZ. The timescale of the entire sequence
of events is of the order of a few orbital times of the planet. If
spiral-in of (partly) icy, rocky or iron planets has happened to the
pre-main sequence Sun, then this could account for the higher opacity
below the base of the CZ, as inferred from helioseismology.
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Title: Electromagnetic deformable mirror for space applications
Authors: Kuiper, S.; Doelman, N.; Overtoom, T.; Nieuwkoop, E.;
Russchenberg, T.; van Riel, M.; Wildschut, J.; Baeten, M.; Spruit,
H.; Brinkers, S.; Human, J.
2017SPIE10562E..30K Altcode:
To increase the collecting power and to improve the angular imaging
resolution, space telescopes are evolving towards larger primary
mirrors. The aerial density of the telescope mirrors needs to be
kept low, however, to be compatible with the launch requirements. A
light-weight (primary) mirror will introduce additional optical
aberrations to the system. These may be caused by for instance
manufacturing errors, gravity release and thermo-elastic effects. Active
Optics (AO) is a key candidate technology to correct for the resultant
wave front aberrations [1].
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Title: Flux canceling in three-dimensional radiative
magnetohydrodynamic simulations
Authors: Thaler, Irina; Spruit, H. C.
2017A&A...601A..88T Altcode:
We aim to study the processes involved in the disappearance of
magnetic flux between regions of opposite polarity on the solar
surface using realistic three-dimensional (3D) magnetohydrodynamic
(MHD) simulations. "Retraction" below the surface driven by magnetic
forces is found to be a very effective mechanism of flux canceling
of opposite polarities. The speed at which flux disappears increases
strongly with initial mean flux density. In agreement with existing
inferences from observations we suggest that this is a key process
of flux disappearance within active complexes. Intrinsic kG strength
concentrations connect the surface to deeper layers by magnetic forces,
and therefore the influence of deeper layers on the flux canceling
process is studied. We do this by comparing simulations extending to
different depths. For average flux densities of 50 G, and on length
scales on the order of 3 Mm in the horizontal and 10 Mm in depth,
deeper layers appear to have only a mild influence on the effective
rate of diffusion.
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Title: Magnetic fields in non-convective regions of stars
Authors: Braithwaite, Jonathan; Spruit, Henk C.
2017RSOS....460271B Altcode: 2015arXiv151003198B
We review the current state of knowledge of magnetic fields inside
stars, concentrating on recent developments concerning magnetic fields
in stably stratified (zones of) stars, leaving out convective dynamo
theories and observations of convective envelopes. We include the
observational properties of A, B and O-type main-sequence stars,
which have radiative envelopes, and the fossil field model which
is normally invoked to explain the strong fields sometimes seen in
these stars. Observations seem to show that Ap-type stable fields
are excluded in stars with convective envelopes. Most stars contain
both radiative and convective zones, and there are potentially
important effects arising from the interaction of magnetic fields at
the boundaries between them; the solar cycle being one of the better
known examples. Related to this, we discuss whether the Sun could
harbour a magnetic field in its core. Recent developments regarding
the various convective and radiative layers near the surfaces of
early-type stars and their observational effects are examined. We look
at possible dynamo mechanisms that run on differential rotation rather
than convection. Finally, we turn to neutron stars with a discussion
of the possible origins for their magnetic fields.
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Title: Instability of mass transfer in a planet-star system
Authors: Jia, Shi; Spruit, H. C.
2017MNRAS.465..149J Altcode: 2016arXiv160703919J
We show that the angular momentum exchange mechanism governing the
evolution of mass-transferring binary stars does not apply to Roche
lobe filling planets, because most of the angular momentum of the
mass-transferring stream is absorbed by the host star. Apart from a
correction for the difference in specific angular momentum of the stream
and the centre of mass of the planet, the orbit does not expand much on
Roche lobe overflow. We explore the conditions for dynamically unstable
Roche lobe overflow as a function of planetary mass and mass and radius
(age) of host star and equation of state of planet. For a Sun-like host,
gas giant planets in a range of mass and entropy can undergo dynamical
mass transfer. Examples of the evolution of the mass transfer process
are given. Dynamic mass transfer of rocky planets depends somewhat
sensitively on equation of state used. Silicate planets in the range
1 < M<SUB>p</SUB> < 10 M<SUB>⊕</SUB> typically go through a
phase of dynamical mass transfer before settling to slow overflow when
their mass drops to less than 1 M<SUB>⊕</SUB>.
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Title: The growth of helium-burning cores
Authors: Spruit, H. C.
2015A&A...582L...2S Altcode: 2015arXiv150900659S
Helium burning in the convective cores of horizontal branch and red
clump stars appears to involve a process of "ingestion" of unburnt
helium into the core, the physics of which has not been clearly
identified yet. I show here that a limiting factor controlling the
growth is the buoyancy of helium entering the denser C+O core. It yields
a growth rate that scales directly with the convective luminosity
of the core and agrees with constraints on core size from current
asteroseismology.
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Title: Convective settling in main sequence stars: Li and Be depletion
Authors: Andrássy, R.; Spruit, H. C.
2015A&A...579A.122A Altcode: 2015arXiv150604635A
The process of convective settling is based on the assumption that a
small fraction of the low-entropy downflows sink from the photosphere
down to the bottom of the star's envelope convection zone retaining
a substantial entropy contrast. We have previously shown that this
process could explain the slow Li depletion observed in the Sun. We
construct a parametric model of convective settling to investigate
the dependence of Li and Be depletion on stellar mass and age. Our
model is generally in good agreement with the Li abundances measured
in open clusters and solar twins, although it seems to underestimate
the Li depletion in the first ~1 Gyr. The model is also compatible
with the Be abundances measured in a sample of field stars.
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Title: Small-scale dynamos on the solar surface: dependence on
magnetic Prandtl number
Authors: Thaler, I.; Spruit, H. C.
2015A&A...578A..54T Altcode: 2015arXiv150504575T
The question of possible small-scale dynamo action in the surface layers
of the Sun is revisited with realistic 3D MHD simulations. As in other
MHD problems, dynamo action is found to be a sensitive function of the
magnetic Prandtl number P<SUB>m</SUB> = ν/η; it disappears below
a critical value P<SUB>c</SUB> which is a function of the numerical
resolution. At a grid spacing of 3.5 km, P<SUB>c</SUB> based on the
hyperdiffusivities implemented in the code (STAGGER) is ≈1, increasing
with increasing grid spacing. As in other settings, it remains uncertain
whether small scale dynamo action is present in the astrophysical limit
where P<SUB>m</SUB> ≪ 1 and magnetic Reynolds number R<SUB>m</SUB>
≫ 1. The question is discussed in the context of the strong effect
that external stray fields are observed to have in generating and
maintaining dynamo action in other numerical and laboratory systems,
and in connection with the type-II hypertransient behavior of dynamo
action observed in the absence of such external fields.
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Title: Overshooting by differential heating
Authors: Andrássy, R.; Spruit, H. C.
2015A&A...578A.106A Altcode: 2015arXiv150205628A
On the long nuclear time scale of stellar main-sequence evolution,
even weak mixing processes can become relevant for redistributing
chemical species in a star. We investigate a process of "differential
heating", which occurs when a temperature fluctuation propagates by
radiative diffusion from the boundary of a convection zone into the
adjacent radiative zone. The resulting perturbation of the hydrostatic
equilibrium causes a flow that extends some distance from the convection
zone. We study a simplified differential-heating problem with a static
temperature fluctuation imposed on a solid boundary. The astrophysically
relevant limit of a high Reynolds number and a low Péclet number (high
thermal diffusivity) turns out to be interestingly non-intuitive. We
derive a set of scaling relations for the stationary differential
heating flow. A numerical method adapted to a high dynamic range in flow
amplitude needed to detect weak flows is presented. Our two-dimensional
simulations show that the flow reaches a stationary state and confirm
the analytic scaling relations. These imply that the flow speed drops
abruptly to a negligible value at a finite height above the source
of heating. We approximate the mixing rate due to the differential
heating flow in a star by a height-dependent diffusion coefficient
and show that this mixing extends about 4% of the pressure scale
height above the convective core of a 10 M<SUB>⊙</SUB> zero-age main
sequence star. <P />Appendix A is available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201425125/olm">http://www.aanda.org</A>
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Title: The B Fields in OB Stars (BOB) Survey
Authors: Kholtygin, A. F.; Castro, N.; Fossati, L.; Hubrig, S.;
Langer, N.; Morel, T.; Przybilla, N.; Schöller, M.; Carroll, T.;
Ilyin, I.; Irrgang, A.; Oskinova, L.; Schneider, F. R. N.; Díaz,
S. S.; Briquet, M.; González, J. F.; Kharchenko, N.; Nieva, M. -F.;
Scholz, R. -D.; de Koter, A.; Hamann, W. -R.; Herrero, A.; Maíz
Apellániz, J.; Sana, H.; Arlt, R.; Barbá, R.; Dufton, P.; Mathys,
G.; Piskunov, A.; Reisenegger, A.; Spruit, H.; Yoon, S. -C.
2015ASPC..494...79K Altcode:
The B fields in OB stars (BOB) survey is an ESO large program collecting
spectropolarimetric observations for a large number of early-type stars
in order to study the occurrence rate, properties, and ultimately
the origin of magnetic fields in massive stars. As of July 2014,
a total of 98 objects were observed over 20 nights with the FORS2
and HARPSpol. Our preliminary results indicate that the fraction of
magnetic OB stars with an organized, detectable field is small. This
conclusion, now independently reached by two different surveys, has
profound implications for any theoretical model attempting to explain
the field formation in these objects. We discuss in this contribution
some important issues addressed by our observations (e.g., the lower
boundary of the field strength) and the discovery of some remarkable
objects.
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Title: Instability of magnetic equilibria in barotropic stars
Authors: Mitchell, J. P.; Braithwaite, J.; Reisenegger, A.; Spruit,
H.; Valdivia, J. A.; Langer, N.
2015MNRAS.447.1213M Altcode: 2014arXiv1411.7252M
In stably stratified stars, numerical magnetohydrodynamics simulations
have shown that arbitrary initial magnetic fields evolve into stable
equilibrium configurations, usually containing nearly axisymmetric,
linked poloidal and toroidal fields that stabilize each other. In
this work, we test the hypothesis that stable stratification is a
requirement for the existence of such stable equilibria. For this
purpose, we follow numerically the evolution of magnetic fields in
barotropic (and thus neutrally stable) stars, starting from two
different types of initial conditions, namely random disordered
magnetic fields, as well as linked poloidal-toroidal configurations
resembling the previously found equilibria. With many trials, we
always find a decay of the magnetic field over a few Alfvén times,
never a stable equilibrium. This strongly suggests that there are
no stable equilibria in barotropic stars, thus clearly invalidating
the assumption of barotropic equations of state often imposed on the
search of magnetic equilibria. It also supports the hypothesis that,
as dissipative processes erode the stable stratification, they might
destabilize previously stable magnetic field configurations, leading
to their decay.
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Title: The B Fields in OB Stars (BOB) Survey
Authors: Morel, T.; Castro, N.; Fossati, L.; Hubrig, S.; Langer, N.;
Przybilla, N.; Schöller, M.; Carroll, T.; Ilyin, I.; Irrgang, A.;
Oskinova, L.; Schneider, F. R. N.; Díaz, S. Simon; Briquet, M.;
González, J. F.; Kharchenko, N.; Nieva, M. -F.; Scholz, R. -D.;
de Koter, A.; Hamann, W. -R.; Herrero, A.; Maíz Apellániz, J.;
Sana, H.; Arlt, R.; Barbá, R.; Dufton, P.; Kholtygin, A.; Mathys,
G.; Piskunov, A.; Reisenegger, A.; Spruit, H.; Yoon, S. -C.
2015IAUS..307..342M Altcode: 2014arXiv1408.2100M
The B fields in OB stars (BOB) survey is an ESO large programme
collecting spectropolarimetric observations for a large number of
early-type stars in order to study the occurrence rate, properties, and
ultimately the origin of magnetic fields in massive stars. As of July
2014, a total of 98 objects were observed over 20 nights with FORS2 and
HARPSpol. Our preliminary results indicate that the fraction of magnetic
OB stars with an organised, detectable field is low. This conclusion,
now independently reached by two different surveys, has profound
implications for any theoretical model attempting to explain the
field formation in these objects. We discuss in this contribution some
important issues addressed by our observations (e.g., the lower bound
of the field strength) and the discovery of some remarkable objects.
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Title: The B Fields in OB Stars (BOB) Survey
Authors: Morel, T.; Castro, N.; Fossati, L.; Hubrig, S.; Langer,
N.; Przybilla, N.; Schöller, M.; Carroll, T.; Ilyin, I.; Irrgang,
A.; Oskinova, L.; Schneider, F. R. N.; Díaz, S. S.; Briquet, M.;
González, J. F.; Kharchenko, N.; Nieva, M. -F.; Scholz, R. -D.;
de Koter, A.; Hamann, W. -R.; Herrero, A.; Maíz Apellániz, J.;
Sana, H.; Arlt, R.; Barbá, R.; Dufton, P.; Kholtygin, A.; Mathys,
G.; Piskunov, A.; Reisenegger, A.; Spruit, H.; Yoon, S. -C.
2014Msngr.157...27M Altcode:
The B fields in OB stars (BOB) survey is an ESO Large Programme
collecting spectropolarimetric observations for a large number of
early-type stars in order to study the occurrence rate, properties,
and ultimately the origin of magnetic fields in massive stars. A total
of 98 objects was observed over 20 nights with FORS2 and HARPSpol
to July 2014. Preliminary results indicate that the fraction of
magnetic OB stars with an organised, detectable field is low. This
conclusion, now independently reached by two different surveys, has
profound implications for any theoretical model attempting to explain
the field formation in these stars. We also discuss some important
issues addressed by our observations (e.g., the lower bound of the
field strength) and the discovery of some remarkable objects.
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Title: Magnetic Fields throughout Stellar Evolution (IAU S302)
Authors: Petit, Pascal; Jardine, Moira; Spruit, Hendrik C.
2014IAUS..302.....P Altcode:
Foreword; Introduction; 1. Magnetized stellar formation; 2. Magnetic
activity in the Sun and main-sequence stars with convective outer
layers; 3. Origin and impact of magnetic fields in higher-mass stars
with radiative outer layers; 4. Magnetic fields in the ultimate stages
of stellar evolution; Author index.
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Title: Search for Stable Magnetohydrodynamic Equilibria in Barotropic
Stars.
Authors: Mitchell, J. P.; Braithwaite, J.; Langer, N.; Reisenegger,
A.; Spruit, H.
2014IAUS..302..441M Altcode: 2013arXiv1310.2595M
It is now believed that magnetohydrodynamic equilibria can exist in
stably stratified stars due to the seminal works of Braithwaite &
Spruit (2004) and Braithwaite & Nordlund (2006). What is still
not known is whether magnetohydrodynamic equilibria can exist in a
barotropic star, in which stable stratification is not present. It
has been conjectured by Reisenegger (2009) that there will likely not
exist any magnetohydrodynamical equilibria in barotropic stars. We
aim to test this claim by presenting preliminary MHD simulations of
barotropic stars using the three dimensional stagger code of Nordlund
& Galsgaard (1995).
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Title: Brightness of the Sun's small scale magnetic field: proximity
effects
Authors: Thaler, I.; Spruit, H. C.
2014A&A...566A..11T Altcode: 2014arXiv1404.2871T
The net effect of the small scale magnetic field on the Sun's
(bolometric) brightness is studied with realistic 3D MHD
simulations. The direct effect of brightening within the magnetic
field itself is consistent with measurements in high-resolution
observations. The high "photometric accuracy" of the simulations,
however, reveal compensating brightness effects that are hard to
detect observationally. The influence of magnetic concentrations on
the surrounding nonmagnetic convective flows (a "proximity effect")
reduces the brightness by an amount exceeding the brightening by the
magnetic concentrations themselves. The net photospheric effect of
the small scale field (≈-0.34% at a mean flux density of 50 G)
is thus negative. We conclude that the main contribution to the
observed positive correlation between the magnetic field and total
solar irradiance must be magnetic dissipation in layers around the
temperature minimum and above (not included in the simulations). This
agrees with existing inferences from observations.
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Title: Overshooting by convective settling
Authors: Andrássy, R.; Spruit, H. C.
2013A&A...559A.122A Altcode: 2013arXiv1310.8117A
We study a process of slow mixing in stars with convective envelopes,
which is driven by the settling of cool downward plumes below the base
of the convection zone. If a small fraction (of order 10<SUP>-7</SUP>)
of the material cooled at the surface retains a significant entropy
deficit while descending in plumes, it can reach the depth where
lithium burning takes place. The model calculates the thermal response
and mixing below the convection zone due to the settling process,
assuming that the plumes arrive at the base of the convection zone with
a broad range of entropy contrasts. We obtain a good fit to the observed
lithium depletion in the Sun by assuming that the settling mass flux is
distributed with respect to the entropy contrast as a power law with
a slope around -2. We find convective settling to have a negligible
influence on the stratification below the convection zone, although
mixing induced by it could modify the gradient of helium concentration.
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Title: Semiconvection: numerical simulations
Authors: Zaussinger, F.; Spruit, H. C.
2013A&A...554A.119Z Altcode: 2013arXiv1303.4522Z
A grid of numerical simulations of double-diffusive convection is
presented for the astrophysical case where viscosity (Prandtl number
Pr) and solute diffusivity (Lewis number Le) are much lower than
the thermal diffusivity. As in laboratory and geophysical cases,
convection takes place in a layered form. The proper translation
of subsonic flows in a stellar interior and an incompressible
(Boussinesq) fluid is given, and the validity of the Boussinesq
approximation for the semiconvection problem is checked by
comparison with fully compressible simulations. The predictions
of a simplified theory of mixing in semiconvection given in a
companion paper are tested against the numerical results, and used
to extrapolate these to astrophysical conditions. The predicted
effective He-diffusion coefficient is nearly independent of the
double-diffusive layering thickness d. For a fiducial main sequence
model (15 M<SUB>⊙</SUB>) the inferred mixing time scale is of the
order of 10<SUP>10</SUP> yr. An estimate for the secular increase in
d during the semiconvective phase is given. It can potentially reach
a significant fraction of the pressure scale height. <P />Movies
associated to Figs. 5 and 7 are available in electronic form at <A
href="http://www.aanda.org">http://www.aanda.org</A>
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Title: Semiconvection: theory
Authors: Spruit, H. C.
2013A&A...552A..76S Altcode: 2013arXiv1302.4005S
A model is developed for the transport of heat and solute in a system
of double-diffusive layers under astrophysical conditions (where
viscosity and solute diffusivity are low compared with the thermal
diffusivity). The process of formation of the layers is not part of
the model but, as observed in geophysical and laboratory settings, is
assumed to be faster than the life time of the semiconvective zone. The
thickness of the layers is a free parameter of the model. When the
energy flux of the star is specified, the effective semiconvective
diffusivities are only weakly dependent on this parameter. An
estimate is given of the evolution of layer thickness with time in a
semiconvective zone. The model predicts that the density ratio has a
maximum for which a stationary layered state can exist, R<SUB>ρ</SUB>
≲ Le<SUP>-1/2</SUP>. Comparison of the model predictions with a grid
of numerical simulations is presented in a companion paper.
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Title: The Large-scale Magnetic Fields of Thin Accretion Disks
Authors: Cao, Xinwu; Spruit, Hendrik C.
2013ApJ...765..149C Altcode: 2013arXiv1301.4543C
Large-scale magnetic field threading an accretion disk is a key
ingredient in the jet formation model. The most attractive scenario
for the origin of such a large-scale field is the advection of the
field by the gas in the accretion disk from the interstellar medium
or a companion star. However, it is realized that outward diffusion of
the accreted field is fast compared with the inward accretion velocity
in a geometrically thin accretion disk if the value of the Prandtl
number P <SUB>m</SUB> is around unity. In this work, we revisit this
problem considering the angular momentum of the disk to be removed
predominantly by the magnetically driven outflows. The radial velocity
of the disk is significantly increased due to the presence of the
outflows. Using a simplified model for the vertical disk structure,
we find that even moderately weak fields can cause sufficient angular
momentum loss via a magnetic wind to balance outward diffusion. There
are two equilibrium points, one at low field strengths corresponding
to a plasma-beta at the midplane of order several hundred, and one for
strong accreted fields, β ~ 1. We surmise that the first is relevant
for the accretion of weak, possibly external, fields through the outer
parts of the disk, while the latter one could explain the tendency,
observed in full three-dimensional numerical simulations, of strong
flux bundles at the centers of disk to stay confined in spite of strong
magnetororational instability turbulence surrounding them.
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Title: Essential Magnetohydrodynamics for Astrophysics
Authors: Spruit, H. C.
2013arXiv1301.5572S Altcode:
This text is intended as an introduction to magnetohydrodynamics in
astrophysics, emphasizing a fast path to the elements essential for
physical understanding. It assumes experience with concepts from fluid
mechanics: the fluid equation of motion and the Lagrangian and Eulerian
descriptions of fluid flow. In addition, the basics of vector calculus
and elementary special relativity are needed. Not much knowledge of
electromagnetic theory is required. In fact, since MHD is much closer in
spirit to fluid mechanics than to electromagnetism, an important part
of the learning curve is to overcome intuitions based on the vacuum
electrodynamics of one's high school days. The first chapter (only 39
pp) is meant as a practical introduction including exercises. This is
the `essential' part. The exercises are important as illustrations
of the points made in the text (especially the less intuitive
ones). Almost all are mathematically unchallenging. The supplement in
chapter 2 contains further explanations, more specialized topics and
connections to the occasional topic somewhat outside MHD. The emphasis
is on physical understanding by the visualization of MHD processes,
as opposed to more formal approaches.
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Title: Accretion discs trapped near corotation
Authors: D'Angelo, Caroline R.; Spruit, Hendrik C.
2012MNRAS.420..416D Altcode: 2011arXiv1108.3833D; 2011MNRAS.tmp.1988D
We show that discs accreting on to the magnetosphere of a rotating star
can end up in a trapped state, in which the inner edge of the disc
stays near the corotation radius, even at low and varying accretion
rates. The accretion in these trapped states can be steady or cyclic;
we explore these states over a wide range of parameter space. We find
two distinct regions of instability: one related to the buildup and
release of mass in the disc outside corotation, and the other to mass
storage within the transition region near corotation. With a set of
calculations over long time-scales, we show how trapped states evolve
from both non-accreting and fully accreting initial conditions, and
also calculate the effects of cyclic accretion on the spin evolution
of the star. Observations of cycles such as found here would provide
important clues on the physics of magnetospheric accretion. Recent
observations of cyclic and other unusual variability in T Tauri stars
(EXors) and X-ray binaries are discussed in this context.
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Title: The relative significance of the H-index
Authors: Spruit, H. C.
2012arXiv1201.5476S Altcode:
Use of the Hirsch-index ($h$) as measure of an author's visibility
in the scientific literature has become popular as an alternative to
a gross measure like total citations (c). I show that, at least in
astrophysics, $h$ correlates tightly with overall citations. The mean
relation is $h=0.5(\sqrt c+1)$. Outliers are few and not too far from
the mean, especially if `normalized' ADS citations are used for $c$
and $h$. Whatever the theoretical reasoning behind it, the Hirsch index
in practice does not appear to measure something significantly new.
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Title: Theories of the Solar Cycle and Its Effect on Climate
Authors: Spruit, H.
2012PThPS.195..185S Altcode:
In the first part, some views of the solar magnetic cycle are discussed
and confronted with observations, with focus on two aspects at the
core of most models: the role of convective turbulence, and the role
of the `tachocline' at the base of the convection zone. The shorter
second part discusses the possibility that the magnetic field of the
Sun might influence its brightness enough to have an effect on the
Earth's climate. The standard view, which treats the solar cycle as
a result of the interaction between turbulent convection and magnetic
fields is shown to be misplaced. The main ingredient of the solar cycle,
apart from differential rotation, is instead buoyant instability of the
magnetic field itself. The source of the magnetic field of the solar
cycle is usually assumed to be located in the `tachocline': the shear
zone at the base of the convection zone. The cycle cannot be powered
by the radial shear of the tachocline as assumed in these models,
however, since the radiative interior does not support significant shear
stresses. Instead, it must be powered by the latitudinal gradient of
the rotation rate in the convection zone, as in early models of the
solar cycle. The Sun's brightness is known to vary in sync with the
sunspot cycle, but our understanding of the mechanisms involved make it
unlikely that it has a significant effect on climate, whether on short
(decades) or longer time scales.
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Title: A binary merger origin for inflated hot Jupiter planets
Authors: Martin, E. L.; Spruit, H. C.; Tata, R.
2011A&A...535A..50M Altcode: 2011arXiv1102.3336M
We hypothesize that hot Jupiters with inflated sizes represent
a separate planet formation channel, the merging of two low-mass
stars. We show that the abundance and properties of W UMa stars
and low mass detached binaries are consistent with their being
possible progenitors. The degree of inflation of the transiting hot
Jupiters correlates with their expected spiral-in life time by tidal
dissipation, and this could indicate youth if the stellar dissipation
parameter Q<SUB>∗</SUB><SUP>'</SUP> is sufficiently low. Several
Jupiter-mass planets can form in the massive compact disk formed in
a merger event. Gravitational scattering between them can explain the
high incidence of excentric, inclined, and retrograde orbits. If the
population of inflated planets is indeed formed by a merger process,
their frequency should be much higher around blue stragglers than
around T Tauri stars.
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Title: Magnetically powered jets
Authors: Spruit, H. C.
2011AIPC.1381..227S Altcode:
This is an update on a previous, more extended conference review
text [61], with emphasis on outstanding problem areas. The role of
dissipation of magnetic energy in accelerating the flow is discussed,
and its importance for explaining high Lorentz factors. The transition
between disk and outflow is one of the least understood parts of the
magnetic theory; its role in setting the mass flux in the wind, in
possible modulations of the mass flux, and the problems in treating it
realistically are discussed. Current views on most of these problems
are still strongly influenced by the restriction to 2 dimensions
(axisymmetry) in previous analytical and numerical work; 3-D effects
likely to be important are suggested. An interesting problem area is
the nature and origin of the strong, preferably highly ordered magnetic
fields known to work best for jet production. The presence or absence
of such fields may well be the `second parameter' governing not only
the presence of jets but also the X-ray spectra and timing behavior
of X-ray binaries.
---------------------------------------------------------
Title: Long-term evolution of discs around magnetic stars
Authors: D'Angelo, Caroline R.; Spruit, Hendrik C.
2011MNRAS.416..893D Altcode: 2011arXiv1102.3697D; 2011MNRAS.tmp.1254D
We investigate the evolution of a thin viscous disc surrounding a
magnetic star, including the spin-down of the star by the magnetic
torques it exerts on the disc. The transition from an accreting to
a non-accreting state, and the change of the magnetic torque across
the corotation radius r<SUB>c</SUB> are included in a generic way,
the widths of the transition taken in the range suggested by numerical
simulations. In addition to the standard accreting state, in which the
star gradually moves into spin equilibrium with the disc, two more
states are found. An accreting state can develop into a 'dead' disc
state, with inner edge r<SUB>in</SUB> well outside corotation. More
often, a 'trapped' state develops, in which r<SUB>in</SUB> stays close
to corotation even at very low accretion rates. The long-term evolution
of these two states is different. In the dead state the star spins
down incompletely, retaining much of its initial spin. In the trapped
state the star can asymptotically spin-down to arbitrarily low rates,
with its angular momentum transferred to the disc. We identify these
outcomes with respectively the rapidly rotating and the very slowly
rotating classes of Ap stars and magnetic white dwarfs.
---------------------------------------------------------
Title: Extremely Inflated Hot Jupiters Could Be Extremely Young
Authors: Martin, Eduardo L.; Spruit, Henk; Tata, Ramarao
2011ESS.....2.4301M Altcode:
Extremely inflated hot Jupiters could have formed recently from the
merger of two low-mass stars. <P />The frequency of W UMa stars is not
enough <P />to account for many inflated hot Jupiters, <P />so low mass
detached binaries could also contribute to the progenitor population. <P
/>We find that the degree of inflation of the transiting hot Jupiters
correlates with their expected spiral-in life time by tidal dissipation,
and this <P />could be an indication of youth if the Q dissipation
parameter is sufficiently low, as suggested by the <P />studies of the
Jupiter-Io system. <P />There is also a correlation between radius
anomaly and host star rotational velocity as expected in the merger
scenario. <P />The distribution of rotational velocities among the host
stars is statistically similar to that of blue <P />stragglers in the
globular cluster 47 Tuc. A significant challenge to the binary merger
hypothesis <P />is the efficient angular momentum loss required to
explain the slow rotation of some inflated hot Jupiter host stars. <P
/>As observational tests we point out that if hot Jupiters are mainly
formed as a result of binary mergers, the frequency of this kind of
planets should be higher around blue stragglers than around T Tauri
stars. <P />The presenter is supported by funding <P />from the Spanish
Ministry of Science <P />and Technology and the ROPACS european <P
/>training network.
---------------------------------------------------------
Title: The brightness of magnetic field concentrations in the
quiet Sun
Authors: Schnerr, R. S.; Spruit, H. C.
2011A&A...532A.136S Altcode: 2010arXiv1010.4792S
In addition to the "facular" brightening of active regions, the
quiet Sun also contains a small scale magnetic field with associated
brightenings in continuum radiation. We measure this contribution of
quiet regions to the Sun's brightness from high spatial resolution
(0.16 arcsec-0.32 arcsec) observations of the Swedish 1-m Solar
Telescope (SST) and Hinode satellite. The line-of-sight magnetic field
and continuum intensity near Fe i 6302.5 Å are used to quantify the
correlation between field strength and brightness. The data show that
magnetic flux density contains a significant amount of intrinsically
weak fields that contribute little to brightness. We show that with
data of high spatial resolution a calibration of magnetic flux density
as a proxy for brightness excess is possible. In the SST data, the
magnetic brightening of a quiet region with an average (unsigned)
flux density of 10 G is about 0.15%. In the Hinode data, and in SST
data reduced to Hinode resolution, the measured brightening is some
40% lower. With appropriate correction for resolution, magnetic flux
density can be used as a reliable proxy in regions of small scale
mixed polarity. The measured brightness effect is larger than the
variation of irradiance over a solar cycle. It is not clear, however,
if this quiet Sun contribution actually varies significantly.
---------------------------------------------------------
Title: Invited Speaker: Hale Prize - How the Cycle Does and Does
Not Work
Authors: Spruit, Hendrik
2011SPD....42.0501S Altcode: 2011BAAS..43S.0501S
The talk will present a personal view of our current understanding of
the solar cycle. Central points are the role of magnetic buoyancy as
the main driving force (together with differential rotation), and the
absence of an essential role for the radial shear in the tachocline. The
resulting view is essentially that of Babcock and Leighton of half
a century ago, but now supported by much more observational and
theoretical evidence. I will show how the traditional interpretation
of the cycle as a dynamo driven by convective turbulence is (and has
always been) incompatible with the observations, as well as with the
numerical results that have accumulated over the past decade. Instead,
I will argue that to make progress it will pay off i) to directly
confront a couple of currently somewhat neglected theoretical problems,
and ii) to address some well-known observational puzzles that have not
yet found convincing interpretations in theories of the cycle but may
well hold critical clues.
---------------------------------------------------------
Title: The Sun's Magnetic Surface
Authors: Spruit, Hendrik
2011AAS...21822301S Altcode: 2011BAAS..43G22301S
An overview is given of the history and present status of observations
and theory of magnetic fields as observed at the solar surface:
their structure, their evolution, and their effect on the Sun's
brightness. By a fortunate coincidence, the photosphere is the
region most accessible to direct numerical MHD simulations. The
spectacular level of qualitative and quantitative realism now
possible is illustrated by comparisons with the equally remarkable
advances high-resolution observations achieved in recents years. These
comparisons are now yielding confident physical interpretations of
many of the observed properties of the Sun's surface magnetic fields,
including, for example the bewildering detail of sunspot structure. The
controversial question of a possible connection between climate and
brightness variations over the Sun's magnetic cycle will be discussed
in the light of recent observational and numerical results.
---------------------------------------------------------
Title: Inflated Hot Jupiters may not Require Inflated Physics
Authors: Martin, Eduardo L.; Spruit, H.
2011AAS...21840608M Altcode: 2011BAAS..43G40608M
Due to the Darwin instability, hot Jupiters are expected to spiral in
and merge with their host stars. The time scale for this spiral-in
can be readily calculated for transiting hot Jupiters, but it is
subject to uncertainty in the tidal dissipation parameter Q. Using
data available for a sample of over one hundred transiting planets, we
calculate the time it takes for hot Jupiters to spiral in from their
current distance to their host stars. It is found that the spiral in
times are strongly correlated with the excess of the planet's radius
relative to its equilibrium radius in the sense that larger radius
anomalies correspond to shorter spiral in times. An energy source
has to be invoked to keep planets inflated longer than their natural
cooling time. Irradiation by the host star has been considered but a
plausible mechanism to transport the irradiating flux to the planet
interior where it is needed for significant inflation has not yet
been identified. A 1 Jupiter mass planet needs an thermal energy
excess of the order of its gravitational binding energy in order to
inflate it by as much as 50 percent. This rules out a source like
dissipation of tides in the planet due to nonsynchronous rotation,
since the maximum rotational energy of a planet is only a fraction of
its binding energy. We propose that the cause of inflation is that the
hot Jupiters are young, typically a few hundred Myr. The reason for
this youth is hot Jupiter formation in the merger of a binary. The
likely binary populations include W UMa stars (contact binaries)
and low mass detached binaries. This scenario also explains other
puzzling properties of hot Jupiters, such as their high abundance in
orbits close to the host stars and enhanced lithium depletion.
---------------------------------------------------------
Title: The Total Solar Irradiance and Small Scale Magnetic Fields
Authors: Schnerr, R. S.; Spruit, H. C.
2011ASPC..437..167S Altcode:
“What is the contribution of the weak small scale fields in the quiet
Sun to the Total Solar Irradiance (TSI)?.” This is the question we
try to answer by investigating the relation between magnetic fields and
continuum intensity in SST and Hinode quiet Sun observations. We find
that the increase in brightness of the quiet Sun due to weak (B <
30 G) fields could be as large as 0.3%, which would make it a major
contributor to TSI variations. We also find that most of the magnetic
flux appears to be resolved at the 0.16" resolution of the SST data.
---------------------------------------------------------
Title: High time resolution optical/X-ray cross-correlations for
X-ray binaries: anticorrelations and rapid variability
Authors: Durant, Martin; Shahbaz, Tariq; Gandhi, Poshak; Cornelisse,
Remon; Muñoz-Darias, Teodoro; Casares, Jorge; Dhillon, Vik; Marsh,
Tom; Spruit, Hendrik; O'Brien, Kieran; Steeghs, Danny; Hynes, Rob
2011MNRAS.410.2329D Altcode: 2010arXiv1008.4522D; 2010MNRAS.tmp.1581D
Using simultaneous observations in X-rays and optical, we have
performed a homogeneous analysis of the cross-correlation behaviours
of four X-ray binaries: SWIFT J1753.5-0127, GX 339-4, Sco X-1 and Cyg
X-2. With high-time-resolution observations using ULTRACAM and RXTE,
we concentrate on the short time-scale, δt < 20 s, variability
in these sources. Here we present our data base of observations,
with three simultaneous energy bands in both the optical and the
X-ray, and multiple epochs of observation for each source, all with
∼second or better time resolution. For the first time, we include a
dynamical cross-correlation analysis, i.e. an investigation of how the
cross-correlation function changes within an observation. We describe a
number of trends which emerge. We include the full data set of results,
and pick a few striking relationships from among them for further
discussion. <P />We find, that the surprising form of X-ray/optical
cross-correlation functions, a positive correlation signal preceded by
an anticorrelation signal, is seen in all the sources at least some of
the time. Such behaviour suggests a mechanism other than reprocessing as
being the dominant driver of the short-term variability in the optical
emission. This behaviour appears more pronounced when the X-ray spectrum
is hard. Furthermore, we find that the cross-correlation relationships
themselves are not stable in time, but vary significantly in strength
and form. This all hints at dynamic interactions between the emitting
components which could be modelled through non-linear or differential
relationships.
---------------------------------------------------------
Title: Theories of the Solar Cycle: A Critical View
Authors: Spruit, Hendrik C.
2011sswh.book...39S Altcode: 2010arXiv1004.4545S
Some established views of the solar magnetic cycle are discussed
critically, with focus on two aspects at the core of most models:
the role of convective turbulence, and the role of the `tachocline'
at the base of the convection zone. The standard view which treats the
solar cycle as a manifestation of the interaction between convection
and magnetic fields is shown to be misplaced. The main ingredient
of the solar cycle, apart from differential rotation, is instead
buoyant instability of the magnetic field itself. This view of the
physics of the solar cycle was already established in the 1950s, but
has been eclipsed mathematically by mean field turbulence formalisms
which make poor contact with observations and have serious theoretical
problems. The history of this development in the literature is discussed
critically. The source of the magnetic field of the solar cycle is
currently assumed to be located in the `tachocline': the shear zone
at the base of the convection zone. While the azimuthal field of the
cycle is indeed most likely located at the base of the convection
zone, it cannot be powered by the radial shear of the tachocline
as assumed in these models, since the radiative interior does not
support significant shear stresses. Instead, it must be the powered
by the latitudinal gradient in rotation rate in the convection zone,
as in early models of the solar cycle. Possible future directions for
research are briefly discussed.
---------------------------------------------------------
Title: Can a "propelling" disc stay trapped near corotation?
Authors: D'Angelo, C.; Spruit, H. C.
2011fxts.confE..39D Altcode: 2011PoS...122E..39D
No abstract at ADS
---------------------------------------------------------
Title: Semiconvection
Authors: Zaussinger, F.; Spruit, H. C.
2010arXiv1012.5851Z Altcode:
A grid of numerical simulations of double-diffusive convection is
presented for astrophysical conditions. As in laboratory and geophysical
cases convection takes place in a layered form. A translation between
the astrophysical fluid mechanics and incompressible (Boussinesq)
approximation is given, valid for thin layers. Its validity is
checked by comparison of the results of fully compressible and
Boussinesq simulations of semiconvection. A fitting formula is given
for the superadiabatic gradient as a function of this parameter. The
superadiabaticity depends on the thickness $d$ of the double diffusive
layers, for which no good theory is available, but the effective
He-diffusion coefficient is nearly independent of $d$. For a fiducial
main sequence model (15 $M_\odot$) the inferred mixing time scale is
of the order $10^{10}$ yr.
---------------------------------------------------------
Title: Modeling the Subsurface Structure of Sunspots
Authors: Moradi, H.; Baldner, C.; Birch, A. C.; Braun, D. C.; Cameron,
R. H.; Duvall, T. L.; Gizon, L.; Haber, D.; Hanasoge, S. M.; Hindman,
B. W.; Jackiewicz, J.; Khomenko, E.; Komm, R.; Rajaguru, P.; Rempel,
M.; Roth, M.; Schlichenmaier, R.; Schunker, H.; Spruit, H. C.;
Strassmeier, K. G.; Thompson, M. J.; Zharkov, S.
2010SoPh..267....1M Altcode: 2009arXiv0912.4982M; 2010SoPh..tmp..171M
While sunspots are easily observed at the solar surface, determining
their subsurface structure is not trivial. There are two main
hypotheses for the subsurface structure of sunspots: the monolithic
model and the cluster model. Local helioseismology is the only means
by which we can investigate subphotospheric structure. However, as
current linear inversion techniques do not yet allow helioseismology to
probe the internal structure with sufficient confidence to distinguish
between the monolith and cluster models, the development of physically
realistic sunspot models are a priority for helioseismologists. This
is because they are not only important indicators of the variety of
physical effects that may influence helioseismic inferences in active
regions, but they also enable detailed assessments of the validity of
helioseismic interpretations through numerical forward modeling. In
this article, we provide a critical review of the existing sunspot
models and an overview of numerical methods employed to model wave
propagation through model sunspots. We then carry out a helioseismic
analysis of the sunspot in Active Region 9787 and address the serious
inconsistencies uncovered by Gizon et al. (2009a, 2009b). We find that
this sunspot is most probably associated with a shallow, positive
wave-speed perturbation (unlike the traditional two-layer model)
and that travel-time measurements are consistent with a horizontal
outflow in the surrounding moat.
---------------------------------------------------------
Title: Striation and convection in penumbral filaments
Authors: Spruit, H. C.; Scharmer, G. B.; Löfdahl, M. G.
2010A&A...521A..72S Altcode: 2010arXiv1008.0932S
Observations with the 1-m Swedish Solar Telescope of the flows seen in
penumbral filaments are presented. Time sequences of bright filaments
show overturning motions strikingly similar to those seen along
the walls of small isolated structures in the active regions. The
filaments show outward propagating striations with inclination
angles suggesting that they are aligned with the local magnetic
field. We interpret it as the equivalent of the striations seen in
the walls of small isolated magnetic structures. Their origin is then
a corrugation of the boundary between an overturning convective flow
inside the filament and the magnetic field wrapping around it. The
outward propagation is a combination of a pattern motion due to
the downflow observed along the sides of bright filaments, and the
Evershed flow. The observed short wavelength of the striation argues
against the existence of a dynamically significant horizontal field
inside the bright filaments. Its intensity contrast is explained by
the same physical effect that causes the dark cores of filaments,
light bridges and “canals”. In this way striation represents an
important clue to the physics of penumbral structure and its relation
with other magnetic structures on the solar surface. We put this in
perspective with results from the recent 3-D radiative hydrodynamic
simulations. <P />4 movies are only available in electronic form at
<A href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: Rapid optical and X-ray timing observations of GX339-4:
multicomponent optical variability in the low/hard state
Authors: Gandhi, P.; Dhillon, V. S.; Durant, M.; Fabian, A. C.; Kubota,
A.; Makishima, K.; Malzac, J.; Marsh, T. R.; Miller, J. M.; Shahbaz,
T.; Spruit, H. C.; Casella, P.
2010MNRAS.407.2166G Altcode: 2010arXiv1005.4685G; 2010MNRAS.tmp.1137G
A rapid timing analysis of Very Large Telescope (VLT)/ULTRACAM
(optical) and RXTE (X-ray) observations of the Galactic black hole
binary GX339-4 in the low/hard, post-outburst state of 2007 June is
presented. The optical light curves in the r',g' and u' filters show
slow (~20s) quasi-periodic variability. Upon this is superposed fast
flaring activity on times approaching the best time resolution probed
(~50ms in r' and g') and with maximum strengths of more than twice
the local mean. Power spectral analysis over ~0.004-10Hz is presented,
and shows that although the average optical variability amplitude is
lower than that in X-rays, the peak variability power emerges at a
higher Fourier frequency in the optical. Energetically, we measure
a large optical versus X-ray flux ratio, higher than that seen on
previous occasions when the source was fully jet dominated. Such a
large ratio cannot be easily explained with a disc alone. Studying the
optical-X-ray cross-spectrum in Fourier space shows a markedly different
behaviour above and below ~0.2Hz. The peak of the coherence function
above this threshold is associated with a short optical time lag with
respect to X-rays, also seen as the dominant feature in the time-domain
cross-correlation at ~150ms. The rms energy spectrum of these fast
variations is best described by distinct physical components over the
optical and X-ray regimes, and also suggests a maximal irradiated disc
fraction of 20 per cent around 5000Å. If the constant time delay is
due to propagation of fluctuations to (or within) the jet, this is the
clearest optical evidence to date of the location of this component. The
low-frequency quasi-periodic oscillation is seen in the optical but
not in X-rays, and is associated with a low coherence. Evidence of
reprocessing emerges at the lowest Fourier frequencies, with optical
lags at ~10s and strong coherence in the blue u' filter. Consistent
with this, simultaneous optical spectroscopy also shows the Bowen
fluorescence blend, though its emission location is unclear. However,
canonical disc reprocessing cannot dominate the optical power easily,
nor explain the fast variability.
---------------------------------------------------------
Title: Local Helioseismology: Three-Dimensional Imaging of the
Solar Interior
Authors: Gizon, Laurent; Birch, Aaron C.; Spruit, Henk C.
2010ARA&A..48..289G Altcode: 2010arXiv1001.0930G
The Sun supports a rich spectrum of internal waves that are continuously
excited by turbulent convection. The Global Oscillation Network
Group (GONG) network and the SOHO/MDI (Solar and Heliospheric
Observatory/Michelson Doppler Imager) space instrument provide
an exceptional database of spatially resolved observations of
solar oscillations, covering more than an entire sunspot cycle (11
years). Local helioseismology is a set of tools for probing the solar
interior in three dimensions using measurements of wave travel times
and local mode frequencies. Local helioseismology has discovered
(a) near-surface vector flows associated with convection, (b) 250 m
s<SUP>-1</SUP> subsurface horizontal outflows around sunspots, (c)
∼50 m s<SUP>-1</SUP> extended horizontal flows around active regions
(converging near the surface and diverging below), (d) the effect of
the Coriolis force on convective flows and active region flows, (e)
the subsurface signature of the 15 m s<SUP>-1</SUP> poleward meridional
flow, (f) a ±5 m s<SUP>-1</SUP> time-varying depth-dependent component
of the meridional circulation around the mean latitude of activity,
and (g) magnetic activity on the farside of the Sun.
---------------------------------------------------------
Title: Episodic accretion on to strongly magnetic stars
Authors: D'Angelo, Caroline R.; Spruit, Hendrik C.
2010MNRAS.406.1208D Altcode: 2010arXiv1001.1742D; 2010MNRAS.tmp..908D
Some accreting neutron stars and young stars show unexplained episodic
flares in the form of quasi-periodic oscillations or recurrent
outbursts. In a series of two papers, we present new work on an
instability that can lead to episodic outbursts when the accretion disc
is truncated by the star's strong magnetic field close to the corotation
radius (where the Keplerian frequency matches the star's rotational
frequency). In this paper we outline the physics of the instability and
use a simple parametrization of the disc-field interaction to explore
the instability numerically, which we show can lead to repeated bursts
of accretion as well as steady-state solutions, as first suggested by
Sunyaev and Shakura. The cycle time of these bursts increases with
a decreasing accretion rate. These solutions show that the usually
assumed `propeller' state, in which mass is ejected from the system,
need not occur even at very low accretion rates.
---------------------------------------------------------
Title: Rapid timing studies of black hole binaries in Optical and
X-rays: correlated and non-linear variability
Authors: Gandhi, P.; Dhillon, V. S.; Durant, M.; Fabian, A. C.;
Makishima, K.; Marsh, T. R.; Miller, J. M.; Shahbaz, T.; Spruit, H. C.
2010AIPC.1248..119G Altcode:
In a fast multi-wavelength timing study of black hole X-ray
binaries (BHBs), we have discovered correlated optical and X-ray
variability in the low/hard state of two sources: GX 339-4 and SWIFT
J1753.5-0127. After XTE J1118+480, these are the only BHBs currently
known to show rapid (sub-second) aperiodic optical flickering. Our
simultaneous VLT/Ultracam and RXTE data reveal intriguing patterns
with characteristic peaks, dips and lags down to very short
timescales. Simple linear reprocessing models can be ruled out as
the origin of the rapid, aperiodic optical power in both sources. A
magnetic energy release model with fast interactions between the disk,
jet and corona can explain the complex correlation patterns. We also
show that in both the optical and X-ray light curves, the absolute
source variability r.m.s. amplitude linearly increases with flux,
and that the flares have a log-normal distribution. The implication is
that variability at both wavelengths is not due to local fluctuations
alone, but rather arises as a result of coupling of perturbations
over a wide range of radii and timescales. These `optical and X-ray
rms-flux relations' thus provide new constraints to connect the outer
and inner parts of the accretion flow, and the jet.
---------------------------------------------------------
Title: Accretion disks
Authors: Spruit, H. C.
2010arXiv1005.5279S Altcode:
In this lecture the basic theory of accretion disks is reviewed,
with emphasis on aspects relevant for X-ray binaries and Cataclysmic
Variables. The text gives a general introduction as well as a selective
discussion of a number of more recent topics.
---------------------------------------------------------
Title: Theory of Magnetically Powered Jets
Authors: Spruit, H. C.
2010LNP...794..233S Altcode: 2008arXiv0804.3096S
The magnetic theory for the production of jets by accreting objects is
reviewed with emphasis on outstanding problem areas. An effort is made
to show the connections behind the occasionally diverging nomenclature
in the literature, to contrast the different points of view about basic
mechanisms and to highlight concepts for interpreting the results of
numerical simulations. The role of dissipation of magnetic energy in
accelerating the flow is discussed and its importance for explaining
high Lorentz factors. The collimation of jets to the observed narrow
angles is discussed, including a critical discussion of the role
of “hoop stress.” The transition between disk and outflow is
one of the least understood parts of the magnetic theory; its role
in setting the mass flux in the wind, in possible modulations of
the mass flux, and the uncertainties in treating it realistically
are discussed. Current views on most of these problems are still
strongly influenced by the restriction to two dimensions (axisymmetry)
in previous analytical and numerical work; 3-D effects likely to be
important are suggested. An interesting problem area is the nature and
origin of the strong, preferably highly ordered magnetic fields known
to work best for jet production. The observational evidence for such
fields and their behavior in numerical simulations is discussed. I
argue that the presence or absence of such fields may well be the
“second parameter” governing not only the presence of jets but
also the X-ray spectra and timing behavior of X-ray binaries.
---------------------------------------------------------
Title: Dissecting the Accretion Environments of X-ray Binaries with
High Speed Coordinated Optical and X-ray Timing Observations
Authors: Gandhi, Poshak; Durant, M.; Fabian, A. C.; Malzac, J.;
Miller, J. M.; Shahbaz, T.; Dhillon, V. S.; Marsh, T. R.; Spruit,
H. C.; Makishima, K.
2010HEAD...11.4314G Altcode: 2010BAAS...42..733G
We are uncovering significant optical variability in low/hard state
observations of several X-ray binaries on the fastest time-scales of
just tens of milliseconds typically probed with modern rapid imaging
cameras. The optical light curves are remarkable in that they display
properties very characteristic of X-ray variations: 1) power spectra
with band-limited, red noise over broad time ranges of 10 ms - 1000
s, and in some cases, a low-frequency quasi-periodic oscillation;
2) an instantaneous variability amplitude linearly scaling with
source flux; and, 3) log-normal distributions of fluxes. Aperiodic
optical variability components can dominate over simple linear X-ray
reprocessing expectations, and are much faster than viscous time-scales
of the outer accretion disk or flow. Cross-correlated optical vs. X-ray
time delays not only constrain emission mechanisms, but can also be used
to probe characteristic size scales of the physical components (jet,
corona), and to understand how they are coupled. Rapid, multiwavelength
timing studies are thus opening a new window on the hearts of accreting
sources, though the broad-band spectral plus timing properties remain
to be unified consistently. I will briefly review recent results on
rapid optical variability, including our new data on black hole and
neutron star binary systems. The fact that the sources were all in
typical low/hard states (with relatively-bright optical counterparts)
suggests that correlated optical/X-ray activity may be a general
feature, waiting to be uncovered in more systems. The continuance of
RXTE is vital for such work.
---------------------------------------------------------
Title: Solar Surface Magnetism and Irradiance on Time Scales from
Days to the 11-Year Cycle
Authors: Domingo, V.; Ermolli, I.; Fox, P.; Fröhlich, C.; Haberreiter,
M.; Krivova, N.; Kopp, G.; Schmutz, W.; Solanki, S. K.; Spruit, H. C.;
Unruh, Y.; Vögler, A.
2009SSRv..145..337D Altcode:
The uninterrupted measurement of the total solar irradiance during the
last three solar cycles and an increasing amount of solar spectral
irradiance measurements as well as solar imaging observations
(magnetograms and photometric data) have stimulated the development
of models attributing irradiance variations to solar surface
magnetism. Here we review the current status of solar irradiance
measurements and modelling efforts based on solar photospheric
magnetic fields. Thereby we restrict ourselves to the study of solar
variations from days to the solar cycle. Phenomenological models
of the solar atmosphere in combination with imaging observations of
solar electromagnetic radiation and measurements of the photospheric
magnetic field have reached high enough quality to show that a large
fraction (at least, about 80%) of the solar irradiance variability
can be explained by the radiative effects of the magnetic activity
present in the photosphere. Also, significant progress has been made
with magnetohydrodynamic simulations of convection that allow us to
relate the radiance of the photospheric magnetic structures to the
observations.
---------------------------------------------------------
Title: The source of magnetic fields in (neutron-) stars
Authors: Spruit, Hendrik C.
2009IAUS..259...61S Altcode:
Some arguments, none entirely conclusive, are reviewed about the origin
of magnetic fields in neutron stars, with emphasis of processes during
and following core collapse in supernovae. Possible origins of the
magnetic fields of neutron stars include inheritance from the main
sequence progenitor and dynamo action at some stage of evolution of
progenitor. Inheritance is not sufficient to explain the fields of
magnetars. Energetic considerations point to differential rotation in
the final stages of core collapse process as the most likely source
of field generation, at least for magnetars. A runaway phase of
exponential growth is needed to achieve sufficient field amplification
during relevant phase of core collapse; it can probably be provided
by a some form of magnetorotational instability. Once formed in
core collapse, the field is in danger of decaying again by magnetic
instabilities. The evolution of a magnetic field in a newly formed
neutron star is discussed, with emphasis on the existence of stable
equilibrium configurations as end products of this evolution, and the
role of magnetic helicity in their existence. A particularly puzzling
problem is the large range of field strengths observed in neutron stars
(as well as in A stars and white dwarfs). It implies that a single,
deterministic process is insufficient to explain the origin of the
magnetic fields in these stars.
---------------------------------------------------------
Title: Kink instabilities in jets from rotating magnetic fields
Authors: Moll, R.; Spruit, H. C.; Obergaulinger, M.
2008A&A...492..621M Altcode: 2008arXiv0809.3165M
We have performed 2.5D and 3D simulations of conical jets driven by
the rotation of an ordered, large-scale magnetic field in a stratified
atmosphere. The simulations cover about three orders of magnitude
in distance to capture the centrifugal acceleration as well as the
evolution past the Alfvénsurface. We find that the jets develop kink
instabilities, the characteristics of which depend on the velocity
profile imposed at the base of the flow. The instabilities are
especially pronounced with a rigid rotation profile, which induces a
shearless magnetic field. The jet's expansion appears to be limiting
the growth of Alfvénmode instabilities.
---------------------------------------------------------
Title: Rapid optical and X-ray timing observations of GX 339-4:
flux correlations at the onset of a low/hard state
Authors: Gandhi, P.; Makishima, K.; Durant, M.; Fabian, A. C.; Dhillon,
V. S.; Marsh, T. R.; Miller, J. M.; Shahbaz, T.; Spruit, H. C.
2008MNRAS.390L..29G Altcode: 2008arXiv0807.1529G; 2008MNRAS.tmpL..87G
We present the discovery of optical/X-ray flux correlations on
rapid time-scales in the low/hard state of the Galactic black hole
GX339-4. The source had recently emerged from outburst and was
associated with a relatively faint counterpart with mag V ~ 17. The
optical [Very Large Telescope (VLT)/ULTRACAM] and X-ray (Rossi X-ray
Timing Explorer) data show a clear positive cross-correlation function
(CCF) signal, with the optical peak lagging X-rays by ~150 ms, preceded
by a shallow rise and followed by a steep decline along with broad
anticorrelation dips. An examination of the light curves shows that
the main CCF features are reproduced in superpositions of flares and
dips. The CCF peak is narrow and the X-ray autocorrelation function
(ACF) is broader than the optical ACF, arguing against reprocessing as
the origin for the rapid optical emission. X-ray flaring is associated
with spectral hardening, but no corresponding changes are detected
around optical peaks and dips. The variability may be explained in the
context of synchrotron emission with interaction between a jet and a
corona. The complex CCF structure in GX 339-4 has similarities to that
of another remarkable X-ray binary XTE J1118+480, in spite of showing
a weaker maximum strength. Such simultaneous multiwavelength, rapid
timing studies provide key constraints for modelling the inner regions
of accreting stellar sources. <P />Based on observations carried out
in ESO programmes 079.D-0535 and 279.D-5021, and Rossi X-ray Timing
Explorer proposal number 93119. <P />E-mail: pg@crab.riken.jp
---------------------------------------------------------
Title: Soft X-ray components in the hard state of accreting black
holes
Authors: D'Angelo, C.; Giannios, D.; Dullemond, C.; Spruit, H.
2008A&A...488..441D Altcode: 2008arXiv0807.1274D
Recent observations of two black hole candidates (GX 339-4 and
J1753.5-0127) in the low-hard state (L<SUB>{X</SUB>}/L<SUB>{Edd</SUB>}
≃ 0.003-0.05) suggest the presence of a cool accretion disk very
close to the innermost stable orbit of the black hole. This runs
counter to models of the low-hard state in which the cool disk is
truncated at a much larger radius. We study the interaction between a
moderately truncated disk and a hot inner flow. Ion-bombardment heats
the surface of the disk in the overlap region between a two-temperature
advection-dominated accretion flow and a standard accretion disk,
producing a hot (kT<SUB>{e</SUB>} ≃ 70 keV) layer on the surface of
the cool disk. The hard X-ray flux from this layer heats the inner parts
of the underlying cool disk, producing a soft X-ray excess. Together
with interstellar absorption these effects mimic the thermal spectrum
from a disk extending to the last stable orbit. The results show that
soft excesses in the low-hard state are a natural feature of truncated
disk models.
---------------------------------------------------------
Title: Rapid Optical/X-ray flux correlations in the low/hard state
of GX 339-4
Authors: Gandhi, Poshak; Makishima, K.; Kubota, A.; Fabian, A. C.;
Marsh, T.; Dhillon, V.; Durant, M.; Shahbaz, T.; Miller, J.; Spruit, H.
2008xru..confE..33G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Simultaneous Optical And X-ray Flickering Observations Of GX
339-4: Correlations On Sub-second Timescales
Authors: Gandhi, Poshak; Makishima, K.; Fabian, A.; Durant, M.;
Shahbaz, T.; Dhillon, V.; Marsh, T.; Miller, J.; Spruit, H.
2008HEAD...10.0106G Altcode:
We present a rapid timing analysis of VLT (optical) and RXTE data of
the Galactic black hole GX 339-4 in the low/hard state in June 2007,
soon after emerging from outburst. Both optical and X-ray power density
spectra are qualitatively similar with QPO-like feature or a break
around 0.05-0.1 Hz. A cross-correlation function (CCF) analysis finds
no positive correlation on timescales on 1 s or longer, similar to
previous observations in the bright state. Searching for signal on the
fastest (50 ms) timescales available, on the other hand, a significant
cross-correlation peak is detected, with the optical lagging the X-rays
by 150 ms. The CCF is narrow with a shallow rise and sharp decline
to an anti-correlation dip. The X-ray auto-correlation function (ACF)
is broader than the optical ACF, arguing against re-processing as the
emission mechanism for the optical power. Similarities and differences
with respect to the complex behaviour of XTE J1118+480 are discussed and
the need for further rapid variability observations of this fascinating
source are emphasized.
---------------------------------------------------------
Title: Origin of neutron star magnetic fields
Authors: Spruit, H. C.
2008AIPC..983..391S Altcode: 2007arXiv0711.3650S
Possible origins of the magnetic fields of neutron stars include
inheritance from the main sequence progenitor and dynamo action at
some stage of evolution of progenitor. Inheritance is not sufficient
to explain the fields of magnetars. Energetic considerations point
to differential rotation in the final stages of core collapse
process as the most likely source of field generation, at least for
magnetars. A runaway phase of exponential growth is needed to achieve
sufficient field amplification during relevant phase of core collapse;
it can probably be provided by a some form of magnetorotational
instability. Once formed in core collapse, the field is in danger of
decaying again by magnetic instabilities. The evolution of a magnetic
field in a newly formed neutron star is discussed, with emphasis on
the existence of stable equilibrium configurations as end products of
this evolution, and the role of magnetic helicity in their existence.
---------------------------------------------------------
Title: Soft X-ray components in the hard state of accreting black
holes
Authors: D'Angelo, C.; Giannios, D.; Spruit, H.; Dullemond, C.
2008mqw..confE..10D Altcode: 2008PoS....62E..10D
No abstract at ADS
---------------------------------------------------------
Title: OPTIMA: A High Time Resolution Optical Photo-Polarimeter
Authors: Kanbach, G.; Stefanescu, A.; Duscha, S.; Mühlegger, M.;
Schrey, F.; Steinle, H.; Slowikowska, A.; Spruit, H.
2008ASSL..351..153K Altcode:
A high-speed photo-polarimeter, "OPTIMA" short for Optical Pulsar
Timing Analyzer, has been designed and developed in the group for
gamma-ray astronomy of the Max-Planck-Institut für Extraterrestrische
Physik. This sensitive, portable detector is used to observe optical
emissions of sources that radiate mainly at X- and gamma-ray energies,
like pulsars and other highly variable compact sources. The single
photon counting instrument is based on fiber fed avalanche photodiodes
(APDs), a GPS timing receiver,a CCD camera for target acquisition and a
stand-alone data acquisition and control system. Several configurations
are available: for photometry a hexagonal bundle with seven channels
and one fiber offset for sky background monitoring; for polarimetry
a rotating polarization filter in front of the photometer or a newly
developed 4-channel double Wollaston system; and for coarse spectroscopy
a 4-colour prism spectrograph.
---------------------------------------------------------
Title: MHD Simulations of Penumbra Fine Structure
Authors: Heinemann, T.; Nordlund, Å.; Scharmer, G. B.; Spruit, H. C.
2007ApJ...669.1390H Altcode: 2006astro.ph.12648H
We present the results of numerical 3D magnetohydrodynamic (MHD)
simulations with radiative energy transfer of fine structure in a small
sunspot of about 4 Mm width. The simulations show the development
of filamentary structures and flow patterns that are, except for
the lengths of the filaments, very similar to those observed. The
filamentary structures consist of gaps with reduced field strength
relative to their surroundings. Calculated synthetic images show dark
cores like those seen in the observations; the dark cores are the result
of a locally elevated τ=1 surface. The magnetic field in these cores
is weaker and more horizontal than for adjacent brighter structures,
and the cores support a systematic outflow. Accompanying animations
show the migration of the dark-cored structures toward the umbra,
and fragments of magnetic flux that are carried away from the spot by
a large-scale “moat flow.” We conclude that the simulations are in
qualitative agreement with observed penumbra filamentary structures,
Evershed flows, and moving magnetic features.
---------------------------------------------------------
Title: Spectral and timing properties of a dissipative γ-ray burst
photosphere
Authors: Giannios, D.; Spruit, H. C.
2007A&A...469....1G Altcode: 2006astro.ph.11385G
We explore the observational appearance of the photosphere of
an ultrarelativistic flow with internal dissipation of energy as
predicted by the magnetic reconnection model. Previous study of the
radiative transfer in the photospheric region has shown that gradual
dissipation of energy results in a hot photosphere. There, inverse
Compton scattering of the thermal radiation advected with the flow leads
to powerful photospheric emission with spectral properties close to
those of the observed prompt GRB emission. Here, we build on that study
by calculating the spectra for a large range of the characteristics
of the flow. An accurate fitting formula is given that provides the
photospheric spectral energy distribution in the 10 keV to 10 MeV
energy range (in the central engine frame) as a function of the basic
physical parameters of the flow. It facilitates the direct comparison
of the model predictions with observations, including the variability
properties of the lightcurves. We verify that the model naturally
accounts for the observed clustering in peak energies of the E \cdot
f(E) spectrum. In this model, the Amati relation indicates a tendency
for the most luminous bursts to have more energy per baryon. If this
tendency also holds for individual GRB pulses, the model predicts the
observed narrowing of the width of pulses with increasing photon energy.
---------------------------------------------------------
Title: Gamma-ray Bursts from X-ray Binaries
Authors: Spruit, H. C.
2007ralc.conf..407S Altcode:
In this contribution I review the mechanism proposed earlier for
producing a gamma-ray burst from the rapidly spinning neutron star in
an X-ray binary (Spruit 1999), with a discussion of some more recent
developments and outstanding issues.
---------------------------------------------------------
Title: Magnetostatic penumbra models with field-free gaps
Authors: Scharmer, G. B.; Spruit, H. C.
2006A&A...460..605S Altcode: 2006astro.ph..9130S
We present numerical 2D magnetostatic models for sunspot penumbrae
consisting of radially aligned field-free gaps in a potential magnetic
field, as proposed by Spruit & Scharmer (2006, A&A, 447,
343). The shape of the gaps and the field configurations around them
are computed consistently from the condition of magnetostatic pressure
balance between the gap and the magnetic field. The results show that
field-free gaps in the inner penumbra are cusp-shaped and bounded
by a magnetic field inclined by about 70° from the vertical. Here,
the magnetic component has a Wilson depression on the order 200-300
km relative to the top of the field-free gap; the gaps should thus
appear as noticeably elevated features. This structure explains the
large variations in field strength in the inner penumbra inferred from
magnetograms and two-component inversions, and the varying appearance
of the inner penumbra with viewing angle. In the outer penumbra, on the
other hand, the gaps are flat-topped with a horizontal magnetic field
above the middle of the gap. The magnetic field has large inclination
variations horizontally, but only small fluctuations in field strength,
in agreement with observations.
---------------------------------------------------------
Title: Variations in solar luminosity and their effect on the
Earth's climate
Authors: Foukal, P.; Fröhlich, C.; Spruit, H.; Wigley, T. M. L.
2006Natur.443..161F Altcode:
Variations in the Sun's total energy output (luminosity) are caused
by changing dark (sunspot) and bright structures on the solar
disk during the 11-year sunspot cycle. The variations measured from
spacecraft since 1978 are too small to have contributed appreciably to
accelerated global warming over the past 30 years. In this Review,
we show that detailed analysis of these small output variations
has greatly advanced our understanding of solar luminosity change,
and this new understanding indicates that brightening of the Sun is
unlikely to have had a significant influence on global warming since
the seventeenth century. Additional climate forcing by changes in the
Sun's output of ultraviolet light, and of magnetized plasmas, cannot
be ruled out. The suggested mechanisms are, however, too complex to
evaluate meaningfully at present.
---------------------------------------------------------
Title: The role of kink instability in Poynting-flux dominated jets
Authors: Giannios, Dimitrios; Spruit, Henk
2006AIPC..848..530G Altcode:
The role of kink instability in magnetically driven jets is
explored through numerical one-dimensional steady relativistic MHD
calculations. The instability is shown to have enough time to grow
and influence the dynamics of Poynting-flux dominated jets. In
the case of AGN jets, the flow becomes kinetic flux dominated at
distances >~1000rg because of the rapid dissipation of Poynting
flux. When applied to GRB outflows, the model predicts more gradual
Poynting dissipation and moderately magnetized flow at distances of ~
10<SUP>16</SUP> cm where the deceleration of the ejecta due to its
interaction with the external medium is expected. The energy released
by the instability can power the compact “blazar zone” emission and
the prompt emission of GRB outflows with high radiative efficiencies.
---------------------------------------------------------
Title: Magnetic instability in a differentially rotating star
Authors: Spruit, H. C.
2006astro.ph..7164S Altcode:
Instabilities in magnetic fields wound up by differential rotation
as reviewed in Spruit (1999) are discussed with some detail and new
developments added. In stellar models which include magnetic torques,
the differential rotation tends to accumulate in the gradients in
composition. In view of this, instability in a $\mu$-gradient is
studied in more detail here, resulting in the detection of a second
instability. Its relevance for angular momentum transport is uncertain,
however, since it requires high horizontal field gradients and would
not operate near the pole. Finally, the possibility is discussed that
magnetic instability in a $\mu$-gradient will lead to {\em layer
formation}: the gradients breaking up into small steps of uniform
composition and rotation rate. This would enhance the angular momentum
transport across inhomogeneous zones, and decrease the rotation rates
of the end products of stellar evolution. A recent {\tt astro-ph}
submission by Dennisenkov and Pinsonneault proposing a modification
of the instability conditions is shown to contain a mathematical error.
---------------------------------------------------------
Title: Evolution of the magnetic field in magnetars
Authors: Braithwaite, J.; Spruit, H. C.
2006A&A...450.1097B Altcode: 2005astro.ph.10287B
We use numerical MHD to look at the stability of a possible poloidal
field in neutron stars (Flowers & Ruderman 1977, ApJ, 215, 302),
and follow its unstable evolution, which leads to the complete decay
of the field. We then model a neutron star after the formation of
a solid crust of high conductivity. As the initial magnetic field
we use the stable "twisted torus" field which was the result of our
earlier work (Braithwaite & Nordlund 2006, A&A, 450, 1077),
since this field is likely to exist in the interior of the star at the
time of crust formation. We follow the evolution of the field under
the influence of diffusion, and find that large stresses build up in
the crust, which will presumably lead to cracking. We put this forward
as a model for outbursts in soft gamma repeaters.
---------------------------------------------------------
Title: The role of kink instability in Poynting-flux dominated jets
Authors: Giannios, D.; Spruit, H. C.
2006A&A...450..887G Altcode: 2006astro.ph..1172G
The role of kink instability in magnetically driven jets is
explored through numerical one-dimensional steady relativistic MHD
calculations. The instability is shown to have enough time to grow and
influence the dynamics of Poynting-flux dominated jets. In the case
of AGN jets, the flow becomes kinetic flux dominated at distances
⪆1000~r<SUB>g</SUB> because of the rapid dissipation of Poynting
flux. When applied to GRB outflows, the model predicts more gradual
Poynting dissipation and moderately magnetized flow at distances
of ~10<SUP>16</SUP> cm where the deceleration of the ejecta due to
interaction with the external medium is expected. The energy released
by the instability can power the compact "blazar zone" emission and
the prompt emission of GRB outflows with high radiative efficiencies.
---------------------------------------------------------
Title: Fine structure, magnetic field and heating of sunspot penumbrae
Authors: Spruit, H. C.; Scharmer, G. B.
2006A&A...447..343S Altcode: 2005astro.ph..8504S
We interpret penumbral filaments as due to convection in field-free,
radially aligned gaps just below the visible surface of the penumbra,
intruding into a nearly potential field above. This solves the
classical discrepancy between the large heat flux and the low vertical
velocities observed in the penumbra. The presence of the gaps causes
strong small-scale fluctuations in inclination, azimuth angle and
field strength. The field is nearly horizontal in a region around
the cusp-shaped top of the gap, thereby providing an environment for
Evershed flows. We identify this region with the recently discovered
dark penumbral cores. Its darkness has the same cause as the dark lanes
in umbral light-bridges, reproduced in numerical simulations by Nordlund
& Stein (2005, in preparation). We predict that the large vertical
and horizontal gradients of the magnetic field inclination and azimuth
in the potential field model will produce the net circular polarization
seen in observations. The model also explains the significant elevation
of bright filaments above their surroundings. It predicts that dark
areas in the penumbra are of two different kinds: dark filament cores
containing the most inclined (horizontal) fields, and regions between
bright filaments, containing the least inclined field lines.
---------------------------------------------------------
Title: Relativistic jet production from accreting compact objects
Authors: Spruit, Hendrik
2006smqw.confE..44S Altcode: 2006PoS....33E..44S
The magnetic theory for the production of jets by accreting objects
is summarized with attention to some outstanding problem areas. An
effort is made to show the connections behind the somewhat diverging
nomenclature in the literature, and to contrast the different points
of view about basic mechanisms. A key problem is the acceleration of
the jet flow to high Lorentz factors. In this context, the role of
dissipation of magnetic energy in accelerating the flow is discussed,
the importance of which has been realized only recently. A second theme
is the ‘collimation problem’, discussion of which is hampered by
incorrect ideas about the role of ‘hoop stress’. Current views
on these problems are still strongly biased by the restriction to
2 dimensions (axisymmetry) in previous analytical and numerical
work. Estimates are given how jets will behave differently in the
3-dimensional world. An interesting problem area is the nature and
origin of the strong, preferably highly ordered magnetic fields known
to work best for jet production, and observational evidence for such
fields. I argue that the presence or absence of such fields may well
be the ‘second parameter’ governing the X-ray spectra and timing
behavior in X-ray binaries.
---------------------------------------------------------
Title: Magnetic Field Dissipation in GRBs
Authors: Topinka, Martin; Spruit, Henk; Karlický, Marián
2005AIPC..801..141T Altcode:
We report on the first steps towards the three-dimensional simulation
of magnetic field dissipation in gamma-ray burst prompt emission. The
model is based on magnetically driven Poynting flux outflow.
---------------------------------------------------------
Title: Mid-infrared emission from cataclysmic variables
Authors: Dubus, G.; Campbell, R.; Kern, B.; Taam, R. E.; Spruit, H. C.
2005ASPC..330...55D Altcode:
CVs could be surrounded by cold material that has condensed out of
the ejecta from the compact binary. The strong braking resulting from
tidal interactions between such a circumbinary (CB) disc and the red
dwarf might explain several puzzles in our current understanding of
CV evolution. The thermal emission from CB discs is expected to peak
in the mid-IR. We report on a survey of the mid-IR emission from 8
CVs. AE Aqr and SS Cyg are detected above 5 μm but the observed rapid
variability makes CB material unlikely in both cases. We discuss the
upper limits on CB discs resulting from this study and the prospects
for their detection.
---------------------------------------------------------
Title: Magnetic Flux Captured by an Accretion Disk
Authors: Spruit, H. C.; Uzdensky, Dmitri A.
2005ApJ...629..960S Altcode: 2005astro.ph..4429U
We suggest a possible mechanism of efficient transport of the
large-scale external magnetic flux inward through a turbulent
accretion disk. The outward drift by turbulent diffusion that limits
the effectiveness of this process is reduced if the external flux
passes through the disk in concentrated patches. Angular momentum loss
by a magnetocentrifugal wind from these patches of strong field adds
to the inward drift. We propose that magnetic flux accumulating in
this way at the center of the disk provides the “second parameter”
determining X-ray spectral states of black hole binaries and the
presence of relativistic outflows.
---------------------------------------------------------
Title: Presupernova Evolution of Differentially Rotating Massive
Stars Including Magnetic Fields
Authors: Heger, A.; Woosley, S. E.; Spruit, H. C.
2005ApJ...626..350H Altcode: 2004astro.ph..9422H
As a massive star evolves through multiple stages of nuclear burning
on its way to becoming a supernova, a complex, differentially rotating
structure is set up. Angular momentum is transported by a variety
of classic instabilities and also by magnetic torques from fields
generated by the differential rotation. We present the first stellar
evolution calculations to follow the evolution of rotating massive
stars including, at least approximately, all these effects, magnetic
and nonmagnetic, from the zero-age main sequence until the onset of
iron-core collapse. The evolution and action of the magnetic fields is
as described by Spruit in 2002, and a range of uncertain parameters is
explored. In general, we find that magnetic torques decrease the final
rotation rate of the collapsing iron core by about a factor of 30-50
when compared with the nonmagnetic counterparts. Angular momentum
in that part of the presupernova star destined to become a neutron
star is an increasing function of main-sequence mass. That is, pulsars
derived from more massive stars rotate faster and rotation plays a more
important role in the star's explosion. The final angular momentum of
the core has been determined-to within a factor of 2-by the time the
star ignites carbon burning. For the lighter stars studied, around 15
M<SUB>solar</SUB>, we predict pulsar periods at birth near 15 ms, though
a factor of 2 range is easily tolerated by the uncertainties. Several
mechanisms for additional braking in a young neutron star, especially
by fallback, are explored.
---------------------------------------------------------
Title: Structure and Evolution of Compact Binary Systems: Infrared
Spectroscopy of SS Cygni
Authors: Froning, Cynthia; Ciardi, David; Dubus, Guillaume; Hoard,
Donald; Howell, Steve; Long, Knox; Spruit, Hendrik; Szkody, Paula;
Taam, Ron; Wachter, Stefanie; van Belle, Gerard
2005sptz.prop20074F Altcode:
Cataclysmic variables (CVs) are interacting binary systems in which
a white dwarf accretes mass from a late-type donor star. In dwarf
novae (DN), accretion occurs via a disk around the white dwarf that
undergoes semi-regular outbursts caused by thermal-viscous disk
instabilities. Because of their proximity and lack of obscuration,
CVs are excellent systems in which to study the physics of disk
accretion and binary star evolution. Ground-based mid-IR (4 -- 18
micron) photometry of the quiescent DN SS Cygni revealed a flux well in
excess of that expected from the accretion disk and the donor star. The
IR emission also appeared to vary on 15~min timescales, although the
limited sensitivity of the ground-based observations precluded detailed
variability study. The source of this emission is not known. Possible
IR emission regions in DN include circumbinary material, a remnant
of the common envelope evolutionary phase of the binary; a previously
unobserved component of the accretion disk; or an outflow from the disk
or the donor star. We propose Spitzer IRS staring mode observations
to identify and characterize the source of the quiescent IR emission
in SS Cyg. We will use the observations to study both the high S/N,
time-averaged spectrum of the IR source and to construct time-resolved
lightcurves in multiple wavebands. The spectra and lightcurves will
allow us to distinguish between circumbinary emission, likely to be
cold and dusty, and a disk or outflow source, which will be warmer and
possibly show molecular and/or shocked gas emission features. Spitzer is
the only instrument capable of obtaining these data, which will enable
us to open a new window on the accretion physics and evolution of CVs.
---------------------------------------------------------
Title: Evaporation of ion-irradiated disks
Authors: Dullemond, C. P.; Spruit, H. C.
2005A&A...434..415D Altcode: 2005astro.ph..1463D
We calculate the evaporation of a cool accretion disk around a black
hole due to the ion-bombardment by an ion supported accretion flow
(here ISAF, or optically thin ADAF). As first suggested by Spruit
& Deufel (2002), this evaporation takes place in two stages: ion
bombardment of the cool disk (Shakura-Sunyaev disk: SSD) produces an
intermediate-temperature layer on top of the disk (“warm layer”)
which constitutes an independent accretion flow on both sides of the
SSD. As this warm material accretes inward of the inner radius of the
SSD, it becomes thermally unstable by lack of cooling of photons,
and evaporates into the ISAF, thereby feeding the latter. Angular
momentum conservation forces a certain fraction of the ISAF material
to move outward, where it can bombard the SSD with its hot ions. The
flow geometry is derived by computing stationary solutions of the
continuity- and angular momentum equations for the three components
(ISAF, warm flow and SSD). The overall radiative output is dominated
by hard X-rays. They are produced mostly from the warm component,
rather than the ISAF. The expected time dependence and stability of
the flow, not computed here, is discussed briefly.
---------------------------------------------------------
Title: Magnetohydrodynamics at the Interface Between a Disk and a Jet
Authors: Spruit, Henk
2005paoa.progE..26S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Physics of Astrophysical Outflows and Accretion Disks
Authors: Blaes, O.; Gammie, C.; Spruit, H.
2005paoa.progE....B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Introduction of the Jets and Disks Program
Authors: Spruit, Henk
2005kbls.confE..29S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Accretion Disks
Authors: Spruit, H. C.
2005ASIB..210..203S Altcode: 2005esns.conf..203S
No abstract at ADS
---------------------------------------------------------
Title: Spectra of Poynting-flux powered GRB outflows
Authors: Giannios, D.; Spruit, H. C.
2005A&A...430....1G Altcode: 2004astro.ph..1109G
We investigate the production of the gamma-ray spectrum of a
Poynting-flux dominated GRB outflow. The very high magnetic field
strengths (super-equipartition) in such a flow lead to very efficient
synchrotron emission. In contrast with internal shocks, dissipation of
magnetic energy by reconnection is gradual and does not produce the
spectrum of cooling electrons associated with shock acceleration. We
show that a spectrum with a break in the BATSE energy range is produced,
instead, if the magnetic dissipation heats a small (∼10<SUP>-4</SUP>)
population of electrons.
---------------------------------------------------------
Title: Comment on “Variations of Total Solar Irradiance Produced
by Structural Changes in the Solar Interior”
Authors: Foukal, P.; Spruit, H.
2004EOSTr..85..524F Altcode:
In a recent Eos article, Sofia [2004] argues for the influence on
irradiance variation of global changes in the Sun's structure associated
with its magnetic dynamo. These changes would act in addition to the
relatively well understood modulation by dark sunspots and bright
faculae at the surface. His assessment of the present observational
evidence for such a global change agrees with our earlier conclusion
that it is not widely convincing at the present time [Foukal, 2003]. But
Sofia's article also claims (1) that the numerical results obtained by
him and his collaborators at Yale disagree with and correct earlier
work, and (2) that a hydrostatic approximation is not adequate for
variations on the 11-year solar cycle timescale. These surprising
claims are based on the results of recently published hydrostatic models
[e.g., Sofia and Li, 2004] using the same mixing length approximation
for convective heat transport used in earlier work [Spruit, 1982,
1991; Gilliland, 1988].
---------------------------------------------------------
Title: Excitation of low-frequency QPOs in black hole accretion
Authors: Giannios, D.; Spruit, H. C.
2004A&A...427..251G Altcode: 2004astro.ph..7474G
We study possible mechanisms of excitation of quasiperiodic oscillations
in the accretion flow of black hole accreters in their hard spectral
states, in the context of the “truncated disk” model. Quasi-spherical
oscillations of the inner ion-supported accretion flow (ISAF) can be
excited by the interaction of this hot flow with the cool disk extending
outward from it. The fundamental mode of (p-mode) oscillation is most
easily excited, and has a frequency near the Kepler frequency at the
inner edge of the cool disk. The strongest excitation mechanism is a
feedback loop involving cooling of the ISAF by soft photons from the
cool disk and heating of the cool disk by the ISAF, while synchrotron
emission can be a relatively strong damping effect. Growth times are
computed by detailed Comptonization calculations of the interaction of
an idealized ISAF with a cool disk. Typical growth times as short as a
few dynamical times are found, while amplitudes can be as large as 10%.
---------------------------------------------------------
Title: A fossil origin for the magnetic field in A stars and white
dwarfs
Authors: Braithwaite, Jonathan; Spruit, Hendrik C.
2004Natur.431..819B Altcode: 2005astro.ph..2043B
Some main-sequence stars of spectral type A are observed to have a
strong (0.03-3tesla), static, large-scale magnetic field, of a chiefly
dipolar shape: they are known as `Ap stars', such as Alioth, the fifth
star in the Big Dipper. Following the discovery of these fields, it
was proposed that they are remnants of the star's formation, a `fossil'
field. An alternative suggestion is that they could be generated by a
dynamo process in the star's convective core. The dynamo hypothesis,
however, has difficulty explaining high field strengths and the observed
lack of a correlation with rotation. The weakness of the fossil-field
theory has been the absence of field configurations stable enough
to survive in a star over its lifetime. Here we report numerical
simulations that show that stable magnetic field configurations, with
properties agreeing with those observed, can develop through evolution
from arbitrary, unstable initial fields. The results are applicable
equally to Ap stars, magnetic white dwarfs and some highly magnetized
neutron stars known as magnetars. This establishes fossil fields as
the natural, unifying explanation for the magnetism of all these stars.
---------------------------------------------------------
Title: The optical and X-ray flickering of XTE J1118+480
Authors: Malzac, J.; Belloni, T.; Spruit, H. C.; Kanbach, G.
2004NuPhS.132..400M Altcode:
We study the complex correlations between optical and X-ray variability
in the black hole X-ray nova XTE J1118+480. We find that the optical
and X-ray fluctuations of very different shapes, amplitudes and
time-scales are correlated in a similar mode where the optical light
curve is seemingly related to the time derivative of the X-rays.
---------------------------------------------------------
Title: Angular Momentum Transport and Mixing by Magnetic Fields
(Invited Review)
Authors: Spruit, H. C.
2004IAUS..215..356S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Presupernova Evolution of Rotating Massive Stars and the
Rotation Rate of Pulsars (Invited Review)
Authors: Heger, A.; Woosley, S. E.; Langer, N.; Spruit, H. C.
2004IAUS..215..591H Altcode: 2003astro.ph..1374H
Rotation in massive stars has been studied on the main sequence and
during helium burning for decades, but only recently have realistic
numerical simulations followed the transport of angular momentum
that occurs during more advanced stages of evolution. The results
affect such interesting issues as whether rotation is important to
the explosion mechanism, whether supernovae are strong sources of
gravitational radiation, the star's nucleosynthesis, and the initial
rotation rate of neutron stars and black holes. We find that when only
hydrodynamic instabilities (shear, Eddington-Sweet, etc.) are included
in the calculation, one obtains neutron stars spinning at close to
critical rotation at their surface -- or even formally in excess of
critical. When recent estimates of magnetic torques (Spruit 2002) are
added, however, the evolved cores spin about an order of magnitude
slower. This is still more angular momentum than observed in young
pulsars, but too slow for the collapsar model for gamma-ray bursts.
---------------------------------------------------------
Title: Magnetically powered prompt radiation and flow acceleration
in GRB
Authors: Spruit, H. C.; Drenkhahn, G. D.
2004ASPC..312..357S Altcode: 2003astro.ph..2468S
The physics of GRB powered by a magnetic energy flux is
reviewed. Magnetic fields are natural for transmitting the energy
from the central compact object to the small amount of baryons
required for a GRB. When dissipation of the flux of magnetic energy
by reconnection inside the flow is taken into account, the magnetic
model assumes several more convincing properties. For baryon loading
typical of observed GRB, most of the dissipation takes place just
outside photosphere, so that prompt emission is produced efficiently,
and the magnetic field strength in this region is high, resulting
in efficient synchrotron emission. Remarkably, the dissipation also
causes very efficient acceleration of the bulk flow. This effect is
illustrated with a classical hydrodynamic equivalent. In this context,
the distinction between the flux of magnetic energy cB^2/8π and
the Poynting flux cB^2/4π is important, and an interpretation of the
Poynting flux as a `magnetic enthalpy flux' illuminating. Numerical and
analytical results for flow acceleration and the relative contribution
of photospheric (thermal) and nonthermal emission as functions of the
asymptotic bulk Lorentz factor are given. The transition between X-ray
flashes and true GRB is predicted at Γ≈ 100.
---------------------------------------------------------
Title: Magnetically powered prompt radiation and flow acceleration
in GRB
Authors: Drenkhahn, G.; Spruit, H. C.
2004ASPC..312..357D Altcode:
Magnetic fields are natural for transmitting the energy from the
central compact object to the small amount of baryons required for
a GRB. If dissipation of the flux of magnetic energy by reconnection
inside the flow is taken into account, the magnetic model has several
more convincing properties. For baryon loading typical of observed
GRB, most of the dissipation takes place just outside photosphere, so
that prompt emission is produced efficiently, and the magnetic field
strength in this region is high, resulting in efficient synchrotron
emission. The dissipation also causes remarkably efficient acceleration
of the bulk flow. In this context, an interpretation of the Poynting
flux as a `magnetic enthalpy flux' is illuminating. The transition
between X-ray flashes and true GRB is predicted at Γ≈ 100.
---------------------------------------------------------
Title: Local Axisymmetric Diffusive Stability of Weakly Magnetized,
Differentially Rotating, Stratified Fluids
Authors: Menou, Kristen; Balbus, Steven A.; Spruit, Henk C.
2004ApJ...607..564M Altcode: 2004astro.ph..2150M
We study the local stability of stratified, differentially rotating
fluids to axisymmetric perturbations in the presence of a weak
magnetic field and of finite resistivity, viscosity, and heat
conductivity. This is a generalization of the Goldreich-Schubert-Fricke
(GSF) double-diffusive analysis to the magnetized and resistive,
triple-diffusive case. Our fifth-order dispersion relation admits a
novel branch that describes a magnetized version of multidiffusive
modes. We derive necessary conditions for axisymmetric stability in
the inviscid and perfect-conductor (double-diffusive) limits. In each
case, rotation must be constant on cylinders and angular velocity
must not decrease with distance from the rotation axis for stability,
irrespective of the relative strength of viscous, resistive, and heat
diffusion. Therefore, in both double-diffusive limits, solid-body
rotation marginally satisfies our stability criteria. The role of
weak magnetic fields is essential to reach these conclusions. The
triple-diffusive situation is more complex, and its stability criteria
are not easily stated. Numerical analysis of our general dispersion
relation confirms our analytic double-diffusive criteria but also
shows that an unstable double-diffusive situation can be significantly
stabilized by the addition of a third, ostensibly weaker, diffusion
process. We describe a numerical application to the Sun's upper
radiative zone and establish that it would be subject to unstable
multidiffusive modes if moderate or strong radial gradients of angular
velocity were present.
---------------------------------------------------------
Title: Accretion onto Fast X-Ray Pulsars
Authors: Rappaport, S. A.; Fregeau, J. M.; Spruit, H.
2004ApJ...606..436R Altcode: 2003astro.ph.10224R
The recent emergence of a new class of accretion-powered, transient,
millisecond X-ray pulsars presents some difficulties for the
conventional picture of accretion onto rapidly rotating magnetized
neutron stars and their spin behavior during outbursts. In particular,
it is not clear that the standard paradigm can accommodate the wide
range in M (i.e., >~ a factor of 50) over which these systems
manage to accrete and the high rate of spin-down that the neutron stars
exhibit in at least a number of cases. When the accretion rate drops
sufficiently, the X-ray pulsar is said to become a “fast rotator,”
and in the conventional view, this is accompanied by a transition
from accretion to “propellering,” in which accretion ceases and
the matter is ejected from the system. On the theoretical side,
we note that this scenario for the onset of propellering cannot be
entirely correct because it is not energetically self-consistent. We
show that, instead, the transition is likely to take place through
disks that combine accretion with spin-down and terminate at the
corotation radius. We demonstrate the existence of such disk solutions
by modifying the Shakura-Sunyaev equations with a simple magnetic
torque prescription. The solutions are completely analytic and have the
same dependence on M and α (the viscosity parameter) as the original
Shakura-Sunyaev solutions, but the radial profiles can be considerably
modified, depending on the degree of fastness. We apply these results
to compute the torques expected during the outbursts of the transient
millisecond pulsars and find that we can explain the large spin-down
rates that are observed for quite plausible surface magnetic fields
of ~10<SUP>9</SUP> G.
---------------------------------------------------------
Title: Excess mid-infrared emission in cataclysmic variables
Authors: Dubus, G.; Campbell, R.; Kern, B.; Taam, R. E.; Spruit, H. C.
2004MNRAS.349..869D Altcode: 2003astro.ph.12408D
We present a search for excess mid-infrared emission due to circumbinary
(CB) material in the orbital plane of cataclysmic variables (CVs). Our
motivation stems from the fact that the strong braking exerted by
a CB disc on the binary system could explain several puzzles in our
current understanding of CV evolution. Since theoretical estimates
predict that the emission from a CB disc can dominate the spectral
energy distribution (SED) of the system at λ > 5 μm, we obtained
simultaneous visible to mid-infrared (mid-IR) SEDs for eight systems. We
report detections of SS Cyg at 11.7 μm and AE Aqr at 17.6 μm, both
in excess of the contribution from the secondary star. In AE Aqr, the
IR likely originates from synchrotron-emitting clouds propelled by the
white dwarf. In SS Cyg, we argue that the observed mid-IR variability is
difficult to reconcile with simple models of CB discs and we consider
free-free emission from a wind. In the other systems, our mid-IR upper
limits place strong constraints on the maximum temperature of a putative
CB disc. The results show that if any sizeable CB discs are present
in these systems, they must be self-shadowed or perhaps dust-free,
with the peak thermal emission shifted to far-IR wavelengths.
---------------------------------------------------------
Title: Search for fast correlated X-ray and optical variability in
Cir X-1 and XTE J1746-321
Authors: Spruit, H. C.; Steinle, H.; Kanbach, G.
2004astro.ph..4524S Altcode:
Coordinated observations X-ray+optical observations of two southern
X-ray binaries, the black hole candidate XTE J1746-321 and the neutron
star accreter Cir X-1 (a `microquasar') are reported. With a photon
counting optical photometer on the 1.9m telescope at Sutherland,
South Africa and the PCA detector on RXTE, 4h each of simultaneous
data were obtained on XTE J1746 and Cir X-1. Cir X-1 showed no X-ray
variability at the 2% level, XTE J1746 was variable at 5-7% with a 5Hz
QPO. Cross-correlation yielded no correlated signals on either source,
to a level of 1%. A problem with a recently published orbital ephemeris
of Cir X-1 is pointed out.
---------------------------------------------------------
Title: Gamma-Ray Burst Central Engines
Authors: Spruit, H. C.
2004BaltA..13..266S Altcode: 2004OAst...13..266S
A brief overview of currently popular central engine models of GRBs
is given, with particular emphasis on magnetic models for powering
the outflow and the dissipation of energy into radiation.
---------------------------------------------------------
Title: Coordinated Millisecond X-RAY+OPTICAL too for Black Hole
Transients
Authors: Spruit, Hendrik
2004rxte.prop90019S Altcode:
This is a target of opportunity proposal for simultaneous X-ray +optical
observations of new soft X-ray transients or new outbursts from known
transient sources. Strong X-ray/optical correlations at 0.03--5 s time
scales were observed in XTE J1118+480. These turn out to be difficult
to fit into a disk reprocessing model. Cyclo-synchrotron emission in
the inner accretion regions and/or an outflow may be involved. The high
quality of the data obtained on XTE J1118, combined with the puzzling
properties of the correlation suggest that observations of this kind
provide unique new diagnostics of the accretion proicess in black hole
transients. This is a resubmission of a cycle 8 TOO.
---------------------------------------------------------
Title: Coordinated Millisecond RXTE+OPTICAL Observations of
Persistent LMXB
Authors: Spruit, Hendrik
2004rxte.prop90026S Altcode:
In RXTE+optical observatipons of XTE J1118+480 we found the visible
light to be strongly correlated with the X-rays on time scales of 0.03
- 5s. The short time scales, and a `premonition dip' in the visible
preceding the X-rays, turn out to be difficult to understand in existing
models. Simultaneous fast X-ray+optical obervations thus may give new
clues on the poorly understood inner regions of the accretion flow in
XRB, and their relation to outflows. We propose to observe for this
purpose the brightest persistent low-mass black hole accreters, and
two well studied neutron star accreters. (Cyg X-1, GX 339-4, Cyg X-
2 and the neutron star jet source Cir X-1).
---------------------------------------------------------
Title: The optical and X-ray flickering of XTE J1118+480
Authors: Malzac, J.; Belloni, T.; Spruit, H. C.; Kanbach, G.
2003A&A...407..335M Altcode: 2003astro.ph..6256M
We use both time-domain and Fourier techniques to study the
correlated optical/X-rays variability in the black hole X-ray nova
XTE J1118+480. Its X-ray timing properties such as the shape of the
X-ray power spectrum and cross-correlation functions (CCFs) between
X-ray energy bands, the slight decrease of rms variability from 30%
in the 2-5.9 keV to 19% in the 15.5-44.4 keV band, as well as the
X-ray hardness/flux anti-correlation are very similar to what is found
in other black hole binaries in the hard state. The optical/X-ray
CCF is virtually independent of the X-ray energies. The optical flux
appears to be correlated not only with the X-ray flux but also with the
X-ray spectral variability. Both the coherence function and the lags
between optical and the X-rays are Fourier frequency dependent. The
optical/X-ray coherence function reaches its maximum ( ~ 0.3) in the
0.1-1 Hz range and the time-lags decrease with frequency approximatively
like f<SUP>-0.8</SUP>. The correlation between X-ray and optical
light curves appears to have time-scale-invariant properties. The
X-ray/optical CCF maintains a similar but rescaled shape on time-scales
ranging at least from 0.1 s to few 10 s. Using the event superposition
method we show that the correlation is not triggered by a single type
of event (dip or flare) in the light curves. Instead, optical and X-ray
fluctuations of very different shapes, amplitudes and time-scales are
correlated in a similar mode where the optical light curve is seemingly
related to the time derivative of the X-rays.
---------------------------------------------------------
Title: X-ray/Optical Correlations in the Transient Black Hole System
KV UMa (XTEJ1118+48)
Authors: Kanbach, G.; Steinle, H.; Spruit, H. C.; Straubmeier, C.;
Belloni, T.
2003ANS...324Q..23K Altcode: 2003ANS...324..E02K
No abstract at ADS
---------------------------------------------------------
Title: Interpretations of Torsional Oscillations
Authors: Spruit, H.
2003SPD....34.2608S Altcode: 2003BAAS...35..855S
The processes proposed for the torsional oscillation pattern of solar
rotation are reviewed. Helioseismology shows that its amplitude declines
with depth from the surface, though significant amplitudes survive
over most of the convection zone. Its relation with the solar cycle,
with largest positive (negative) amplitudes at the leading (trailing)
latitudes of the main activity belt, suggests a magnetic cause. The
early start of the oscillation, a few years before the start of the
new activity cycle is difficult to interpret in magnetic models and
has led to suggestions that the primary cause is not magnetic but a
hydrodynamic oscillation somehow synchronized with the cycle. `Direct'
magnetic models using the Lorentz force of the cycle's magnetic field
(likely located at the base of the convection zone) do not fit the
depth dependence easily. The latitude dependence, interpreted as
a flow around the active latitudes, suggests a strong influence of
rotation. <P />An `indirect' magnetic model is proposed, in which
the flow is driven by the thermal effects of the surface magnetic
fields. The additional radiative loss in the small scale magnetic
field (which exceeds the reduced radiation from spots) causes a small
temperature decrease below active regions. The associated pressure
effect causes a geostrophic flow with the correct amplitude, latitude
dependence and depth dependence. The early start of the oscillation
may indicate that the cycle starts with small scale mixed fields
that do not show up on synoptic magnetograms. <P />It is pointed out
that any long-lived temperature fluctuations near the surface must
be associated with systematic flows. These flows are much easier to
detect with current technology than the surface brightness variations
due to the temperature effects themselves.
---------------------------------------------------------
Title: Origin of the torsional oscillation pattern of solar rotation
Authors: Spruit, H. C.
2003SoPh..213....1S Altcode: 2002astro.ph..9146S
A model is presented that explains the `torsional oscillation' pattern
of deviations in the solar rotation rate as a geostrophic flow. The
flow is driven by temperature variations near the surface due to the
enhanced emission of radiation by the small-scale magnetic field. The
model explains the sign of the flow, its amplitude and the fact that
the maxima occur near the boundaries of the main activity belts. The
amplitude of the flow decreases with depth from its maximum at the
surface but penetrates over much of the depth of the convection zone,
in agreement with the data from helioseismology. It predicts that
the flow is axisymmetric only on average, and in reality consists of
a superposition of circulations around areas of enhanced magnetic
activity. It must be accompanied by a meridional flow component,
which declines more rapidly with depth.
---------------------------------------------------------
Title: Design and results of the fast timing photo-polarimeter OPTIMA
Authors: Kanbach, Gottfried; Kellner, Stefan; Schrey, Fritz Z.;
Steinle, Helmut; Straubmeier, Christian; Spruit, Henk C.
2003SPIE.4841...82K Altcode:
A high-speed photometer, "OPTIMA" short for Optical Pulsar Timing
Analyzer, has been designed as a sensitive, portable detector to observe
optical pulsars and other highly variable sources. The detector contains
eight fiber fed avalanche photodiode single photon counters, a GPS
timing receiver, a CCD camera for target acquisition and a computerized
control unit. The central fibers are configured as a hexagonal bundle
around the target fiber, while one fiber is located at a distance of
~1' as a monitor for the night sky background. Recently a rotating
polarization filter and a 4-color prism spectrograph have been added
to the system as optional equipment. Since January 1999 OPTIMA has
been used on different telescopes to measure detailed lightcurves and
polarization of the Crab Pulsar, in a search for optical emission from
the Geminga pulsar, and for the timing of cataclysmic variables and
X-ray transients.
---------------------------------------------------------
Title: Disks Surrounding Cataclysmic Binaries
Authors: Spruit, Henk C.; Taam, Ronald E.
2003ASPC..308..323S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Coordinated Millisecond X-RAY+OPTICAL too for Black Hole
Transients
Authors: Spruit, Hendrik
2003rxte.prop80144S Altcode:
This is a target of opportunity proposal for simultaneous X-ray
+optical observations of new soft X-ray transients. Strong X-
ray/optical correlations at 0.03--5 s time scales were observed in
XTE J1118+480. These turn out to be difficult to fit into a disk
reprocessing model. Cyclo-synchrotron emission in the inner accretion
regions and/or an outflow may be involved. The high quality of the data
obtained on XTE J1118, combined with the puzzling properties of the
correlation suggest that observations of this kind provide unique new
diagnostics of the accretion proicess in black hole transients. This
is a resubmission of a cycle 7 TOO (not triggered).
---------------------------------------------------------
Title: A search for excess mid-IR emission from cataclysmic variables
Authors: Dubus, G.; Campbell, R.; Kern, B.; Taam, R.; Spruit, H.
2003sf2a.conf..589D Altcode: 2003sf2a.confE.244D
Cataclysmic variables (CVs) regularly eject matter through winds and
novae outbursts. A small fraction of this material may stay bound and
accumulate around the binary in a disk. The strong braking exerted on
the red dwarf by such a circumbinary (CB) disk would explain several
puzzles in our current understanding of CV evolution. Thermal emission
from CB disks could easily have gone unnoticed since the mid-IR emission
of CVs remains largely unexplored. We present the results of a near-
and mid-IR survey of 8 CVs obtained at the Keck I in Sep. 2002.
---------------------------------------------------------
Title: Orbital Bulk-flow Comptonization in Accretion Disks around
Black Holes
Authors: Kylafis, Nikolaos D.; Reig, Pablo; Spruit, Henk C.
2003ASPC..308..153K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Coordinated Millisecond RXTE+OPTICAL Observations of
Persistent LMXB
Authors: Spruit, Hendrik
2003rxte.prop80114S Altcode:
We propose to search for correlated X-ray/optical emission from black
hole candidates and neutron star accreters. The unexpectedly fast
and complex correlations between visible light and X-rays seen in
our previous observations of XTE J1118+480 indicate that the visible
light may be be an important source of information on the nature of
the accretion flow in the inner regions of the accretion flow, and its
relation with outfl;ows. Targets to be observed are the persistent
black hole candidates GX 339-4 and Cyg X-1, and the neutron star
accreters Cyg X-2 and the jet-source Cir X-1.
---------------------------------------------------------
Title: Efficient acceleration and radiation in Poynting flux powered
GRB outflows
Authors: Drenkhahn, G.; Spruit, H. C.
2002A&A...391.1141D Altcode: 2002astro.ph..2387D
We investigate the effects of magnetic energy release by local magnetic
dissipation processes in Poynting flux-powered GRBs. For typical GRB
parameters (energy and baryon loading) the dissipation takes place
mainly outside the photosphere, producing non-thermal radiation. This
process converts the total burst energy into prompt radiation at an
efficiency of 10-50%. At the same time the dissipation has the effect
of accelerating the flow to a large Lorentz factor. For higher baryon
loading, the dissipation takes place mostly inside the photosphere, the
efficiency of conversion of magnetic energy into radiation is lower, and
an X-ray flash results instead of a GRB. We demonstrate these effects
with numerical one-dimensional steady relativistic MHD calculations.
---------------------------------------------------------
Title: Correlated X-ray and optical variability in KV UMa
Authors: Spruit, H. C.; Kanbach, G.
2002A&A...391..225S Altcode: 2002astro.ph..5431S
We present coordinated X-ray and optical observations of the X-ray
transient KV UMa (=XTE J1118+480) at high time resolution. The
optical variation associated with spikes in the X-rays consist of
a dip preceding the spike by 0-5 s, and a peak following it. The
cross-correlation function between X-rays and optical has the same
shape. It rises sharply, within 30 ms of the peak of the X-rays. The
shape of the cross correlation shows significant intrinsic variability
on time scales as short as 30 s. Analyzing this variability with
principal component analysis (calibrated with simulated data), we
find two statistically independent components. The first is similar
to the average cross-correlation. The second also consists of a dip
followed by a peak, but on a 3x shorter time scale. These properties
of the optical light, together with the high optical brightness of
the transient, are not easily explained by reprocessing of the X-rays.
---------------------------------------------------------
Title: Hydrodynamical simulations of the stream-core interaction in
the slow merger of massive stars
Authors: Ivanova, N.; Podsiadlowski, Ph.; Spruit, H.
2002MNRAS.334..819I Altcode: 2001astro.ph..9524I
We present detailed simulations of the interaction of a stream emanating
from a mass-losing secondary with the core of a massive supergiant in
the slow merger of two stars inside a common envelope. The dynamics
of the stream can be divided into a ballistic phase, starting at the
L<SUB>1</SUB> point, and a hydrodynamical phase, where the stream
interacts strongly with the core. Considering the merger of a 1-
and 5-M<SUB>solar</SUB> star with a 20-M<SUB>solar</SUB> evolved
supergiant, we present two-dimensional hydrodynamical simulations
using the PROMETHEUS code to demonstrate how the penetration depth
and post-impact conditions depend on the initial properties of the
stream material (e.g. entropy, angular momentum, stream width) and the
properties of the core (e.g. density structure and rotation rate). Using
these results, we present a fitting formula for the entropy generated
in the stream-core interaction and a recipe for the determination of
the penetration depth based on a modified Bernoulli integral.
---------------------------------------------------------
Title: A New Class of High-Mass X-Ray Binaries: Implications for
Core Collapse and Neutron Star Recoil
Authors: Pfahl, Eric; Rappaport, Saul; Podsiadlowski, Philipp;
Spruit, Hendrik
2002ApJ...574..364P Altcode: 2001astro.ph..9521P
We investigate an interesting new class of high-mass X-ray binaries
(HMXBs) with long orbital periods (P<SUB>orb</SUB>>30 days)
and low eccentricities (e<~0.2). The orbital parameters suggest
that the neutron stars in these systems did not receive a large
impulse, or “kick,” at the time of formation. After considering
the statistical significance of these new binaries, we develop a
self-consistent phenomenological picture wherein the neutron stars
born in the observed wide HMXBs receive only a small kick (<~50
km s<SUP>-1</SUP>), while neutron stars born in isolation, in the
majority of low-mass X-ray binaries, and in many of the well-known
HMXBs with P<SUB>orb</SUB><~30 days receive the conventional large
kicks, with a mean speed of ~300 km s<SUP>-1</SUP>. Assuming that this
basic scenario is correct, we discuss a physical process that lends
support to our hypothesis, whereby the magnitude of the natal kick to
a neutron star born in a binary system depends on the rotation rate
of its immediate progenitor following mass transfer-the core of the
initially more massive star in the binary. Specifically, the model
predicts that rapidly rotating precollapse cores produce neutron stars
(NSs) with relatively small kicks, and vice versa for slowly rotating
cores. If the envelope of the NS progenitor is removed before it has
become deeply convective, then the exposed core is likely to be a rapid
rotator. However, if the progenitor becomes highly evolved prior to
mass transfer, then a strong magnetic torque, generated by differential
rotation between the core and the convective envelope, may cause the
core to spin down to the very slow rotation rate of the envelope. Our
model has important implications for the dynamics of stellar core
collapse, the retention of neutron stars in globular clusters, and
the formation of double neutron star systems in the Galaxy.
---------------------------------------------------------
Title: X-ray spectra from accretion disks illuminated by protons
Authors: Deufel, B.; Dullemond, C. P.; Spruit, H. C.
2002A&A...387..907D Altcode: 2001astro.ph..8496D
The X-ray spectrum from a cool accretion disk heated by virialized
protons is computed. The cool disk is either embedded in a magnetically
heated accretion disk corona or partly extends into an ion supported
torus (or ADAF). We calculate the stationary equilibrium between
proton heating, electron thermal conduction and the radiative losses
by bremsstrahlung and Compton scattering. A heated surface layer on
top of the accretion disk is produced with temperatures between 60-90
keV above a cool layer with temperatures of 0.01 keV (AGN) and 1 keV
(galactic black hole candidates). The spectra produced by the surface
layer are reminiscent of hard state spectra, but a bit too steep,
especially for AGN's. Near the inner edge of the disk, where the optical
depth of the disk tau <~ 1, we find that the cool component of the
disk disappears. Instead, the hot protons from the corona/ADAF heat the
disk, on a dynamical time-scale, to temperatures of several 100 keV,
limited by pair production. This region, here called a “warm disk”,
could contribute significantly to the hard X-ray spectra and could be
important for feeding material into an ADAF.
---------------------------------------------------------
Title: The transition from a cool disk to an ion supported flow
Authors: Spruit, H. C.; Deufel, B.
2002A&A...387..918S Altcode: 2001astro.ph..8497S
We show that the inner regions of a cool accretion disk in an X-ray
binary can transform into an advective, ion supported accretion flow (an
optically thin ADAF, here called ISAF), through events involving only
the known properties of the Coulomb interaction in a two-temperature
plasma, standard radiation processes, and viscous heating. The optically
thin inner edge of the disk is heated to a few 100 keV by the strong
flux of hot ions from the surrounding hot ISAF. We show that he
resident ions in this “warm” disk are thermally unstable due to
internal viscous heating, and heat up to their virial temperature. The
innermost disk regions thus evaporate and feed the ISAF. These processes
are demonstrated with time dependent calculations of a two-temperature
plasma in vertical hydrostatic equilibrium, including heating by
external ions, internal proton-electron energy exchange, and viscous
heating. The process complements the “coronal” evaporation mechanism
which operates at larger distances from the central object.
---------------------------------------------------------
Title: Investigation of the Sources of Irradiance Variation on the
Sun (ISIS)
Authors: LaBonte, B. J.; Bernasconi, P. N.; Rust, D.; Foukal, P.;
Hudson, H.; Spruit, H.
2002AAS...200.5608L Altcode: 2002BAAS...34..736L
There is a persistent correlation of the longterm climate change and
solar irradiance. ISIS is designed to understand the physical basis of
this correlation. ISIS combines an innovative bolometric imager and a
multiband CCD imager. The bolometric imager has uniform response from
200 nm to 3000 nm, spatial resolution < 5 arcseconds, and precision
of < 0.1% in a one minute integration. The multiband imager records
ultraviolet irradiance variation in the band from 200 to 350 nm,
measures photospheric temperature structure, and provides chromospheric
structure in Ca II K and H-alpha, with spatial resolution <1.0
arcsecond. Designed for flight on the Solar Dynamics Observatory,
ISIS will provide the comprehensive photometric measurements needed
to characterize the irradiance variation from identifiable structures
and challenge theoretical models of convection and the solar dynamo.
---------------------------------------------------------
Title: The Structure and Evolution of Circumbinary Disks in
Cataclysmic Variable Systems
Authors: Dubus, Guillaume; Taam, Ronald E.; Spruit, H. C.
2002ApJ...569..395D Altcode: 2001astro.ph.12389D
We investigate the structure and evolution of a geometrically thin,
viscous, Keplerian circumbinary (CB) disk, using detailed models of
their radiative/convective vertical structure. We use a simplified
description for the evolution of the cataclysmic binary and focus
on cases where the circumbinary disk causes accelerated mass transfer
(>~10<SUP>-8</SUP> M<SUB>solar</SUB> yr<SUP>-1</SUP>). The inner edge
of the disk is assumed to be determined by the tidal truncation radius,
and the mass input rate into the disk is assumed to be a small fraction
(10<SUP>-5</SUP> to 0.01) of the mass transfer rate. Under the action
of the viscous stresses in the disk the matter drifts outward with the
optically thick region extending to several AU. The inner part of the
disk is cool with maximum effective temperatures <~3000 K, while
the outermost parts of the disk are <~30 K and optically thin. We
calculate the effects of thermal instability on a sufficiently massive
CB disk. It leads to outbursts reminiscent of those in thermally
unstable accretion disks, with the instability remaining confined to
the inner regions of the CB disk. However, for most of the evolutionary
sequences, the surface densities required to trigger instability are
not reached. The spectral energy distributions from circumbinary disks
are calculated, and the prospects for the detection of such disks in
the infrared and submillimeter wavelength regions are discussed.
---------------------------------------------------------
Title: Instability of an accretion disk with a magnetically driven
wind
Authors: Cao, X.; Spruit, H. C.
2002A&A...385..289C Altcode: 2001astro.ph..8484C
We present a linear analysis of the stability of accretion disks in
which angular momentum is removed by the magnetic torque exerted by a
centrifugally driven wind. The effects of the dependence of the wind
torque on field strength and inclination, the sub-Keplerian rotation
due to magnetic forces, and the compression of the disk by the field
are included. A WKB dispersion relation is derived for the stability
problem. We find that the disk is always unstable if the wind torque
is strong. The growth time scale of the instability can be as short
as the orbital time scale. The instability is mainly the result of
the sensitivity of the mass flux to changes in the inclination of the
field at the disk surface. Magnetic diffusion in the disk stabilizes
if the wind torque is small.
---------------------------------------------------------
Title: ORGANIZED DISCUSSION: What can the Sun teach us about black
hole accretion disks?
Authors: Spruit, Hendrik
2002bhty.confE..38S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Do Sunspots Influence the Earth's Climate?
Authors: Spruit, Hendrik
2002kbls.confE..28S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Evaporation of the Inner Disk in Black Hole Candidates
Authors: Spruit, Henk C.; Deufel, Bernhard
2002luml.conf..479S Altcode:
One of the promising models for the inner accretion flow in black hole
accreters is a combination of an ion supported flow (ADAF) inside,
and red by, a cool standard disk extending to larger distances. This
model is suggested by the observed correlations between the reflection
components in the X-ray spectrum (Fe line and Compton bump) with the
slope of the power law component in the same spectrum and with the
frequency of quasiperiodic oscillations in the X-rays (Churazov et
al. 2001, Zdziarski et al. 1999).
---------------------------------------------------------
Title: Coordinated Millisecond Rxte/optical too for Black Hole
Transients
Authors: Spruit, Hendrik
2002rxte.prop70129S Altcode:
This a target of opportunity proposal for simultaneous X-ray+optical
observations of new soft X-ray transients. Strong X-ray/optical
correlations at 0.02--3 sec time scales were observed in last year's
transient XTE J1118+480 (RXTE+OPTIMA July 2000). These turn out to be
difficult to fit into a disk reprocessing model. Cyclo-synchrotron
emission in the inner accretion regions and/or an outflow may be
involved. The high quality of the data data obtained on XTE J1118,
combined with the puzzling properties of the correlation suggest
that observations of this kind provide unique new diagnostics of the
accretion process in black hole transients.
---------------------------------------------------------
Title: X-ray/optical correlations in the transient black hole system
KV UMa (XTE J1118+48).
Authors: Kanbach, G.; Steinle, H.; Spruit, H. C.; Straubmeier, C.;
Belloni, T.
2002AGAb...19Q..34K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Coordinated Millisecond Rxte/optical Observations of XRB
Authors: Spruit, Hendrik
2002rxte.prop70104S Altcode:
We propose simultaneous X-ray+optical observations of Cyg X-1 and
Cyg X-2 to follow up on the puzzling X-ray/optical correlations at
0.02--3 sec time scales observed in XTE J1118+480 (RXTE+OPTIMA July
2000). These turn out to be difficult to fit into a disk reprocessing
model, thus reviving an older suggestion (made for GX 339-4) that
black holes in their hard spectral states produce significant amounts
of optical/UV radiation by cyclo-synchrotron emission in the inner
accretion regions. If optical emission like that in XTE J1118 exists
in Cyg X-1, it will be easily detectable through its correlation with
the X-rays, in spite of the bright O-star secondary. The second target
Cyg X-2 is proposed to see if/how optical emission from neutron star
accreters differs.
---------------------------------------------------------
Title: Dynamo action by differential rotation in a stably stratified
stellar interior
Authors: Spruit, H. C.
2002A&A...381..923S Altcode: 2001astro.ph..8207S
Magnetic fields can be created in stably stratified (non-convective)
layers in a differentially rotating star. A magnetic instability in
the toroidal field (wound up by differential rotation) replaces the
role of convection in closing the field amplification loop. Tayler
instability is likely to be the most relevant magnetic instability. A
dynamo model is developed from these ingredients, and applied to
the problem of angular momentum transport in stellar interiors. It
produces a predominantly horizontal field. This dynamo process is found
to be more effective in transporting angular momentum than the known
hydrodynamic mechanisms. It might account for the observed pattern of
rotation in the solar core.
---------------------------------------------------------
Title: A New Class of High-Mass X-ray Binaries: Implications for
Core Collapse and Neutron-Star Recoil
Authors: Pfahl, E.; Rappaport, S.; Podsiadlowski, Ph.; Spruit, H.
2001AAS...199.9508P Altcode: 2001BAAS...33.1446P
We investigate the statistical significance and implications of a new
class of high-mass X-ray binaries (HMXBs), with P<SUB>orb</SUB> >
30 days and e < 0.2. The orbital parameters suggest that the neutron
stars in these systems did not receive a very large kick at the time
of formation. We develop a scenario wherein the neutron stars born
in the new class of HMXBs receive kick speeds of only < 50 km/s,
while neutron stars born in isolation and in low-mass X-ray binaries
receive the conventional large kicks of 200-300 km/s. This apparent
dichotomy may be explained if the kick speed depends on the rotation
rate of the pre-collapse core, such that rapidly rotating core yield
small kicks, and vice versa for slowly rotating cores.
---------------------------------------------------------
Title: Disks surrounding Cataclysmic Binaries
Authors: Spruit, H. C.; Taam, R. E.
2001astro.ph.11420S Altcode:
Observations of Cataclysmic Variables show a number of phenomena that
do not fit easily into the standard magnetic braking scenario. These
include the large spread in mass transfer rates, the low surface
temperatures of many of the companion stars, and evidence for material
at low velocities. We propose that these anomalies have a common cause:
the presence of a circumbinary (CB) disk. This may be a significant
component of mass transferring binaries in general. Direct detection
of such CB disks may be possible but difficult, because of their low
luminosity Disks surrounding Cataclysmic Binaries and spectral energy
distribution peaking in the mid-IR.
---------------------------------------------------------
Title: Correlated fast X-ray and optical variability in the black-hole
candidate XTE J1118+480
Authors: Kanbach, G.; Straubmeier, C.; Spruit, H. C.; Belloni, T.
2001Natur.414..180K Altcode:
Black holes become visible when they accrete gas, a common source of
which is a close stellar companion. The standard theory for this process
(invoking a `thin accretion disk') does not explain some spectacular
phenomena associated with these systems, such as their X-ray variability
and relativistic outflows, indicating some lack of understanding of
the actual physical conditions. Simultaneous observations at multiple
wavelengths can provide strong constraints on these conditions. Here we
report simultaneous high-time-resolution X-ray and optical observations
of the transient source XTE J1118+480, which show a strong but puzzling
correlation between the emissions. The optical emission rises suddenly
following an increase in the X-ray output, but with a dip 2-5 seconds
in advance of the X-rays. This result is not easy to understand within
the simplest model of the optical emission, where the light comes from
reprocessed X-rays. It is probably more consistent with an earlier
suggestion that the optical light is cyclosynchrotron emission that
originates in a region about 20,000km from the black hole. We propose
that the time dependence is evidence for a relatively slow (<0.1c),
magnetically controlled outflow.
---------------------------------------------------------
Title: The Evolution of Cataclysmic Variable Binary Systems with
Circumbinary Disks
Authors: Taam, Ronald E.; Spruit, H. C.
2001ApJ...561..329T Altcode:
The effect of a circumbinary (CB) disk on the evolution of a binary
consisting of a low-mass secondary star with a white dwarf primary
is investigated, taking into account the viscous spreading of matter
within the circumbinary disk and the response of the secondary to mass
loss. The CB disk is assumed to be fed a constant fraction δ of the
mass transfer from the secondary to the primary through a wind such
as those observed in nova-like systems. The CB disk is effective in
draining orbital angular momentum from the system provided that δ
exceeds about 0.01. In this case, the mass transfer rates are elevated
and the evolution accelerated in comparison with an evolution with
the standard angular momentum loss processes of magnetic braking and
gravitational radiation. The mass transfer rates for a given system
can vary by more than an order of magnitude during its evolution. With
a CB disk, a binary can thus evolve between the various subclasses
of cataclysmic variables. A large spread in transfer rates at a given
orbital period results, reflecting a range of ages of the systems, the
possible presence of a CB disk already at the beginning of the evolution
of the binary, and the mass and evolutionary state of the donor. For
high mass input rates into the circumbinary disk (δ>~0.015),
the secondary can be completely dissolved in less than a Hubble time.
---------------------------------------------------------
Title: X-ray spectra from protons illuminating a neutron star
Authors: Deufel, B.; Dullemond, C. P.; Spruit, H. C.
2001A&A...377..955D Altcode: 2001astro.ph..8438D
We consider the interaction of a slowly rotating unmagnetized neutron
star with a hot (ion supported, ADAF) accretion flow. The virialized
protons of the ADAF penetrate into the neutron star atmosphere,
heating a surface layer. Detailed calculations are presented of
the equilibrium between heating by the protons, electron thermal
conduction, bremsstrahlung and multiple Compton scattering in this
layer. Its temperature is of the order 40-70 keV. Its optical depth
increases with the incident proton energy flux, and is of the order
unity for accretion at 10<SUP>-2</SUP>-10<SUP>-1</SUP> of the Eddington
rate. At these rates, the X-ray spectrum produced by the layer has a
hard tail extending to 100 keV, and is similar to the observed spectra
of accreting neutron stars in their hard states. The steep gradient
at the base of the heated layer gives rise to an excess of photons
at the soft end of the spectrum (compared to a blackbody) through an
“inverse photosphere effect”. The differences with respect to previous
studies of similar problems are discussed, they are due mostly to
a more accurate treatment of the proton penetration process and the
vertical structure of the heated layer.
---------------------------------------------------------
Title: Orbital Comptonization in accretion disks around black holes
Authors: Reig, P.; Kylafis, N. D.; Spruit, H.
2001hell.confE.114R Altcode:
We have performed Monte Carlo simulations of Compton upscattering
of low-energy photons in an accretion disk around a Schwarzschild
black hole. The photons gain energy from the rotational motion of the
electrons in the disk. The upscattering occurs near the black hole
horizon, where the flow velocity of the electrons approaches the speed
of light. We show that this type of bulk-flow Comptonization can produce
power-law X-ray spectra similar to the ones observed in black-hole X-ray
transients in the high/soft state, i.e., a soft bump dominating the
spectrum below ∼ 10 keV and a power-law tail with photon index in the
range 2--3. In order to reproduce the observed hard to soft flux ratio
the disk has to have vertical optical depth above 3 at the last stable
orbit.We conclude that the power-law component of the high/soft state of
black-hole transients may be due to an intrinsically cool disk extending
all the way to the hole, without a separate hot plasma component.
---------------------------------------------------------
Title: Comptonization in rotating flows
Authors: Reig, Pablo; Kylafis, N. D.; Spruit, H. C.
2001ESASP.459..255R Altcode: 2001egru.conf..255R
Although the thermal Comptonization theory has been quite successful in
describing the high energy spectra of black hole candidates there are
still a number of unresolved issues. Due to the turbulent nature of the
accretion disk and high radiation pressure in the vicinity of the black
hole, it seems "natural" for the disk to possess a corona. However,
neither the geometry nor the heating mechanism of such a corona are
known. In bulk Comptonization models a purely radial flow of plasma has
been considered, making the assumption that the radiation force on the
accreting matter is negligible. On the other hand, a general agreement
on the presence of an accretion disk exists. In this work we show
that Comptonization in a rotating flow can also give rise to power-law
spectra. Soft photons get upscattered by collisions with fast-moving
electrons, which follow Keplerian trajectories in the accretion disk.
---------------------------------------------------------
Title: Orbital Comptonization in accretion disks around black holes
Authors: Reig, P.; Kylafis, N. D.; Spruit, H. C.
2001A&A...375..155R Altcode: 2001astro.ph..4389R
We have performed Monte Carlo simulations of Compton upscattering
of low-energy photons in an accretion disk around a Schwarzschild
black hole. The photons gain energy from the rotational motion of the
electrons in the disk. The upscattering occurs near the black hole
horizon, where the flow velocity of the electrons approaches the
speed of light. We show that this type of bulk-flow Comptonization
can produce power-law X-ray spectra similar to the ones observed in
black-hole X-ray transients in the high/soft state, i.e., a soft bump
dominating the spectrum below ~ 10 keV and a power-law tail with photon
index in the range 2-3. In order to reproduce the observed hard to
soft flux ratio the disk has to have vertical optical depth above ~
3 at the last stable orbit. We conclude that the power-law component
of the high/soft state of black-hole transients may be due to an
intrinsically cool disk extending all the way to the hole, without a
separate hot plasma component.
---------------------------------------------------------
Title: Correlated msec X-ray and optical emission from XTE J1118+480
Authors: Kanbach, G.; Straubmeier, C.; Spruit, H. C.; Belloni, T.
2001astro.ph..8199K Altcode:
We report on fast (msec) simultaneous observations in X-rays and
optical light of the black hole candidate XTE J1118+480, showing
correlation on time scales from 20msec to several seconds. The shape
of the correlation is not easily explained with existing models. We
suggest that is is caused by optical cyclosynchrotron emission in a
dense, relatively slow outflow from the inner regions of the accretion.
---------------------------------------------------------
Title: Stability of accretion discs threaded by a strong magnetic
field
Authors: Stehle, R.; Spruit, H. C.
2001MNRAS.323..587S Altcode: 2001astro.ph..3409S
We study the stability of poloidal magnetic fields anchored in a
thin accretion disc. The two-dimensional hydrodynamics in the disc
plane is followed by a grid-based numerical simulation including the
vertically integrated magnetic forces. The three-dimensional magnetic
field outside the disc is calculated in a potential field approximation
from the magnetic flux density distribution in the disc. For uniformly
rotating discs we confirm numerically the existence of the interchange
instability as predicted by Spruit, Stehle & Papaloizou. In
agreement with predictions from the shearing sheet model, discs with
Keplerian rotation are found to be stabilized by the shear, as long
as the contribution of magnetic forces to support against gravity
is small. When this support becomes significant, we find a global
instability which transports angular momentum outwardly and allows
mass to accrete inwardly. The instability takes the form of a m=1
rotating `crescent', reminiscent of the purely hydrodynamic non-linear
instability previously found in pressure-supported discs. A model where
the initial surface mass density Σ(r) and B<SUB>z</SUB>(r) decrease
with radius as power laws shows transient mass accretion during about
six orbital periods, and settles into a state with surface density and
field strength decreasing approximately exponentially with radius. We
argue that this instability is likely to be the main angular momentum
transport mechanism in discs with a poloidal magnetic field sufficiently
strong to suppress magnetic turbulence. It may be especially relevant
in jet-producing discs.
---------------------------------------------------------
Title: Structure and Evolution of Circumbinary Disks in Cataclysmic
Variable Systems
Authors: Taam, R. E.; Dubus, G.; Spruit, H. C.
2001AAS...198.1101T Altcode: 2001BAAS...33..802T
Circumbinary disks have been proposed as an additional means by which
angular momentum can be lost from a cataclysmic variable binary
system. Provided that the mass input rate is sufficiently high,
the evolution of the system can be significantly accelerated. The
structure and evolution of such disks is investigated based on
radiative/convective vertical structure models combined with time
dependent radial diffusion models. The results of the numerical
calculations will be presented and the possible observational
consequences of such disks will be discussed. This research is supported
by NSF under grant AST-9727875, by NASA under grants NAG5-7007 and
NAG5-70034, and by the European Commission under grant TMR grant
ERBFMRX-CT98-0195.
---------------------------------------------------------
Title: Overview of Solar Luminosity Variation Mechanisms
Authors: Spruit, H. C.
2001AGUSM..SP31B01S Altcode:
The theory of stellar structure can be used to put constraints on
proposed mechanisms for the irradiance variations associated with
the solar cycle. Changes in the emissivity of the solar surface
turn out to be particularly effective. Such changes are produced by
sunspots and the small scale magnetic field, either directly (spots
being dark, for example) or indirectly through their effect on the
convective flow in their immediate vicinity. They can account for
most of the observed variation. Sources of variation deeper below the
solar surface either have much smaller effects, or require ad-hoc new
physics. Helioseismology may soon be able to distinguish between surface
emissivity changes and interior sources, since their predicted effects
on p-mode frequencies and travel times tend to have opposite sign.
---------------------------------------------------------
Title: Large scale magnetic fields and their dissipation in GRB
fireballs}
Authors: Spruit, H. C.; Daigne, F.; Drenkhahn, G.
2001A&A...369..694S Altcode: 2000astro.ph..4274S
We consider possible geometries of magnetic fields in GRB outflows,
and their evolution with distance from the source. For magnetically
driven outflows, with an assumed ratio of magnetic to kinetic energy
density of order unity, the field strengths are sufficient for
efficient production of gamma -rays by synchrotron emission in the
standard internal shock scenario, without the need for local generation
of small scale fields. In these conditions, the MHD approximation is
valid to large distances (ga 10<SUP>19</SUP> cm). In outflows driven by
nonaxisymmetric magnetic fields, changes of direction of the field cause
dissipation of magnetic energy by reconnection. Much of this dissipation
takes place outside the photosphere of the outflow, and can convert
a significant fraction of the magnetic energy flux into radiation.
---------------------------------------------------------
Title: Circumbinary Disks and Cataclysmic Variable Evolution
Authors: Spruit, H. C.; Taam, Ronald E.
2001ApJ...548..900S Altcode: 2000astro.ph.10194S
The influence of a circumbinary (CB) disk on the evolution of
cataclysmic variable (CV) binary systems is investigated. We show that
CB mass surface densities sufficient to influence the evolution rate
are plausibly provided by the outflows observed in CVs, if the net
effect of these winds is to deliver 10<SUP>-4</SUP> to 10<SUP>-3</SUP>
of the mass transfer rate to the CB disk. The torque exerted by the
CB disk provides a positive feedback between mass transfer rate and
CB disk mass that can lead to mass transfer rates of ~10<SUP>-8</SUP>
to 10<SUP>-7</SUP> M<SUB>solar</SUB> yr<SUP>-1</SUP>. This mechanism
may be responsible for causing the range of variation of mass transfer
rates in CVs. In particular, it may explain rates inferred for the
nova-like variables and the supersoft X-ray binary systems observed near
the upper edge of the period gap (P~3-4 hr), as well as the spread in
mass transfer rates above and below the period gap. Consequences and
the possible observability of such disks are discussed.
---------------------------------------------------------
Title: Common-Envelope Evolution: the Nucleosynthesis in Mergers of
Massive Stars
Authors: Ivanova, Natalia; Podsiadlowski, Philipp; Spruit, Henk
2001ASPC..229..261I Altcode: 2001ebms.conf..261I; 2001astro.ph..2141I
We study the merging of massive stars inside a common envelope for
binary systems consisting of a red supergiant with a mass of 15-20
Msun and a main-sequence companion of 1-5 Msun. We are particularly
interested in the stage when the secondary, having overfilled its
Roche lobe inside the common envelope, starts to transfer mass
to the core of the primary at a very high mass-transfer rate and
the subsequent nucleo-synthesis in the core-impact region. Using a
parametrized model for the structure of the envelope at this stage,
we perform 2-dimensional hydrodynamical calculations with the Munich
Prometheus code to calculate the dynamics of the stream emanating from
the secondary and its impact on the core of the primary. We find that,
for the lower end of the estimated mass-transfer rate, low-entropy,
hydrogen-rich material can penetrate deep into the primary core where
nucleosynthesis through the hot CNO cycle can take place and that the
associated neutron exposure may be sufficiently high for significant
s-processing. For mass-transfer rates at the high end of our estimated
range and higher densities in the stream, the stream impact can lead
to the dredge-up of helium, but the neutron production is too low for
significant s-processing.
---------------------------------------------------------
Title: Accretion Discs
Authors: Spruit, H. C.
2001nsbh.conf..111S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Origin of the Rotation Rates of Single White Dwarfs and
Neutron Stars
Authors: Spruit, Henk C.
2001ASPC..229...43S Altcode: 2001ebms.conf...43S
No abstract at ADS
---------------------------------------------------------
Title: Gamma-ray Bursts from Neutron Stars Spun up in X-ray Binaries
Authors: Spruit, H. C.
2001nsbh.conf..467S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Radiatively Inefficient Accretion Discs
Authors: Spruit, H. C.
2001nsbh.conf..141S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Circumbinary Disks and Cataclysmic Variable Evolution
Authors: Taam, R. E.; Spruit, H. C.
2000AAS...19712601T Altcode: 2000BAAS...32.1612T
The influence of a circumbinary (CB) disk on the evolution of
cataclysmic variable (CV) binary systems is investigated. The torque
exerted by the CB disk can provide a positive feedback on the mass
transfer rate between the stellar components of the system and the CB
disk. Such a mechanism may be responsible for producing a variation of
mass transfer rates in CV's. The results of evolutionary calculations
will be presented and the consequences of such disks on CV evolution
will be discussed. This research is supported by NSF under grant
AST-9727875 and by the European Commission under grant TMR grant
ERBFMRX-CT98-0195.
---------------------------------------------------------
Title: Theory of solar irradiance variations
Authors: Spruit, H.
2000SSRv...94..113S Altcode:
The theory of stellar structure can be used to identify the most
plausible mechanisms for the irradiance variations associated with the
solar cycle. Changes in surface emissivity, i.e. the reduced cooling
in spots and enhanced emission by small scale magnetic fields, are
the most effective mechanisms and account for most of the observed
variation. New views of stellar surface convection developed from
realistic numerical simulations changes the physical description of
thermal perturbations of the solar envelope. Helioseismology will soon
be able to test the consequences of changes in surface emissivity,
and distinguish them from other scenarios for irradiance variability.
---------------------------------------------------------
Title: Accretion Disks
Authors: Spruit, H.
2000eaa..bookE2124S Altcode: 2000astro.ph..3144S
By accretion one understands in astrophysics the accumulation of matter
onto a heavy object, under the influence of its pull of gravity. The
object can be, for example, a star, a black hole or a neutron star. Most
of the objects of these types are quite isolated, without much gas
around that could be pulled in by their gravity. Accordingly, the vast
majority of stars do not show any evidence of...
---------------------------------------------------------
Title: Solar Photosphere
Authors: Spruit, H.
2000eaa..bookE2015S Altcode:
The photosphere (Gr φως, light) of the Sun are those layers in
its atmosphere that one observes in white light (broad band visible
light). An image in such light (figure 1) shows a sharp edge, as if
the light were emitted from a single layer, the `visible surface'
of the Sun. Since stars are gaseous, they do not have sharp surfaces,
however. Moreover, the radiation at different wavelengths is gen...
---------------------------------------------------------
Title: Comptonization in an accretion disk illuminated by protons
Authors: Deufel, B.; Spruit, H. C.
2000A&A...362....1D Altcode:
We compute the X-ray spectrum from a cool, optically thick,
geometrically thin accretion disk embedded in a hot ion supported
torus (or ADAF). Most of the gravitational power is assumed to be
dissipated in the torus, where the protons are near their local virial
temperature. The protons are slowed down in the accretion disk via
Coulomb interactions, producing a hot surface layer with an optical
thickness tau_<SUB>T</SUB> ~1-2. The heating of this layer is balanced
by cooling through Comptonization of soft photons. The soft photons
are produced by the reprocessing (treated in an approximate way)
of hard photons penetrating into the cool disk. Solving the energy
balance problem of the heated layer together with the Comptonization
by a Monte-Carlo simulation, we find X-ray spectra reminiscent of
observed spectra of AGN and black hole candidates in their hard states.
---------------------------------------------------------
Title: Disks around Mass Transferring Binaries
Authors: Spruit, Henk
2000smyn.progE..44S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Gamma Ray Bursts from Rapidly Rotating Neutron Stars
Authors: Spruit, Hendrik
2000smyn.progE..43S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The sign of temperature inhomogeneities deduced from
time-distance helioseismology
Authors: Brüggen, M.; Spruit, H. C.
2000SoPh..196...29B Altcode: 2000astro.ph..4140B
Inhomogeneities in wave propagation conditions near and below
the solar surface have been detected by means of time-distance
helioseismology. Here we calculate the effect of temperature
inhomogeneities on the travel times of sound waves. A temperature
increase, e.g., in active regions, not only increases the sound speed
but also lengthens the path along which the wave travels because the
expansion of the heated layers shifts the upper turning of the waves
upward. Using a ray-tracing approximation we find that in many cases the
net effect of a temperature enhancement is an increase of the travel
times. We argue that the reduced travel times that are observed are
caused by a combination of magnetic fields in the active region and
reduced subsurface temperatures. Such a reduction may be related to
the increased radiative energy loss from small magnetic flux tubes.
---------------------------------------------------------
Title: Gamma-ray bursts from rapidly spinning neutron stars
Authors: Spruit, H. C.
2000AIPC..526..589S Altcode: 2000hgrb.symp..589S
A neutron star in an X-ray binary (XRB) is spun up by mass
transfer from its companion to rotation periods of the order of a
millisecond. Evidence for this are the existence of ms pulsars, believed
to descend from X-ray binaries. Direct evidence of such rotation rates
in XRB is the of 2.5 ms rotation in SAX 1808. The rotation energy in
a neutron star spinning at 1 ms is 10<SUP>52</SUP> erg, in the range
of that required for GRB. The environment also satisfies the baryon
loading constraint, since the accretion disk contains only of the order
10<SUP>-9</SUP> M<SUB>solar</SUB>, and the companion star blocks only
of the order 1% of the sky as seen from the neutron star. .
---------------------------------------------------------
Title: The magnetosphere of an oscillating neutron star. Non-vacuum
treatment
Authors: Timokhin, A. N.; Bisnovatyi-Kogan, G. S.; Spruit, H. C.
2000MNRAS.316..734T Altcode: 1999astro.ph..9354T
We generalize a formula for the Goldreich-Julian charge density
(ρ<SUB>GJ</SUB>), originally derived for a rotating neutron star,
for arbitrary oscillations of a neutron star with an arbitrary
magnetic field configuration under the assumption of low current
density in the inner parts of the magnetosphere. As an application,
we consider the toroidal oscillation of a neutron star with a dipole
magnetic field and calculate the energy losses. For some oscillation
modes, the longitudinal electric field cannot be cancelled by putting
charged particles in the magnetosphere without the presence of a strong
electric current (j~=ρ<SUB>GJ</SUB>c×cωr). It is shown that the
energy losses are strongly affected by plasma in the magnetosphere,
and cannot be described by vacuum formulae.
---------------------------------------------------------
Title: The X-ray spectrum of a disc illuminated by ions
Authors: Spruit, H. C.; Haardt, F.
2000MNRAS.315..751S Altcode: 2000astro.ph..1458S
The X-ray spectrum from a cool disc embedded in an ion-supported
torus is computed. The interaction of the hot ions with the disc
increases the hard X-ray luminosity of the system. A surface layer
of the disc is heated by the protons from the torus. The Comptonized
spectrum produced by this layer has a shape that depends only weakly
on the incident energy flux and the distance from the accreting compact
object. It consists of a `blue bump' of un-Comptonized soft photons and
a flat high-energy tail, reminiscent of the observed spectra. The hard
tail becomes flatter as the thermalization depth in the cool disc is
increased. Further evidence for ion illumination is the Li abundance
in the secondaries of low-mass X-ray binaries and the 450-keV lines
sometimes seen in black hole transient spectra.
---------------------------------------------------------
Title: Jets from Compact Objects
Authors: Spruit, H. C.
2000IAUS..195..113S Altcode: 2000astro.ph..3043S
Some topics in the theory of jets are reviewed. These include jet
precession, unconfined jets, the origin of knots, the internal shock
model as a unifying theme from protostellar jets to gamma-ray bursts,
relations between the Blandford-Znajek and MHD disk-wind models,
and jet collimation in magnetic acceleration models.
---------------------------------------------------------
Title: Spirals and the size of the disk in EX Dra
Authors: Joergens, V.; Spruit, H. C.; Rutten, R. G. M.
2000A&A...356L..33J Altcode: 2000astro.ph..2302J
Observations at high spectral and temporal resolution are presented
of the dwarf nova EX Dra in outburst. The disk seen in the He I line
reconstructed by Doppler tomography shows a clear two-armed spiral
pattern pointing to spiral shocks in the disk. The Balmer and He
Ii maps also give evidence for the presence of spirals. The eclipse
as seen in the red continuum indicates a disk radius of 0.31 times
the orbital separation, which might be large enough to explain the
observed spiral shocks through excitation by the tidal field of the
secondary. The eclipse in the Balmer line profiles, well resolved in
our observations, indicates a somewhat smaller disk size (0.25). We
discuss the possibility that this is related to an optical depth effect
in the lines.
---------------------------------------------------------
Title: Internal shock model for microquasars
Authors: Kaiser, C. R.; Sunyaev, R.; Spruit, H. C.
2000A&A...356..975K Altcode: 2000astro.ph..1501K
We present a model for the radio outbursts of microquasars based on
the assumption of quasi-continuous jet ejection. The jets are `lit
up' by shock fronts traveling along the jets during outbursts. The
shocks accelerate relativistic particles which emit the observed
synchrotron radiation. The observed comparatively flat decay light
curves combined with gradually steepening spectral slopes are explained
by a superposition of the radiation of the aging relativistic particle
population left behind by the shocks. This scenario is the low energy,
time-resolved equivalent to the internal shock model for GRBs. We
show that this model predicts energy contents of the radiating plasma
similar to the plasmon model. At the same time, the jet model relaxes
the severe requirements on the central source in terms of the rate
at which this energy must be supplied to the jet. Observations of
`mini-bursts' with flat spectral slopes and of infrared emission far
from the source centre suggest two different states of jet ejections:
(i) A `mini-burst' mode with relatively stable jet production and
weak radio emission with flat spectra and (ii) an outburst mode with
strong variations in the jet bulk velocities coupled with strong radio
emission with steeper spectra. We also show that the continuous jets
in microquasars should terminate in strong shocks and possibly inflate
radio lobes similar to extragalactic jet sources. We investigate the
possibility of testing the predictions of this model with resolved
radio observations. Finally, we suggest that Doppler-shifted X-ray
iron lines, and possibly H-alpha lines, may be emitted by the jet flow
of microquasars if thermal instabilities analogous to those in SS433
exist in their jets.
---------------------------------------------------------
Title: Radiatively inefficient accretion disks
Authors: Spruit, H. C.
2000astro.ph..3143S Altcode:
Radiatively inefficient (or advection dominated) disks are discussed
at an introductory level. Ion supported and radiation supported flows
are discussed, the different consequences of advection dominated flows
onto black holes vs. solid surfaces (neutron stars, white dwarfs),
hydrodynamics, the role of the ratio of specific heats, and the possible
connection between ADAFs and outflows.
---------------------------------------------------------
Title: Theory of solar luminosity variations
Authors: Spruit, H. C.
2000ASIC..558..289S Altcode: 2000astro.ph..3044S; 2000asre.conf..289S
The theory of stellar structure can be used to identify the most
plausible mechanisms for the irradiance variations associated with the
solar cycle. Changes in surface emissivity, i.e. the reduced cooling
in spots and enhanced emission by small scale magnetic fields, are
the most effective mechanisms and account for most of the observed
variation. Helioseismology will soon be able to test the consequences
of changes in surface emissivity, and distinguish them from other
scenarios for irradiance variability.
---------------------------------------------------------
Title: Max-Planck-Institut für Astrophysik. Annual report 1999.
Authors: Spruit, H.; Hillebrandt, W.; White, S. D. M.
2000mfaa.book.....S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Magnetosphere of oscillating neutron star
Authors: Timokhin, A. N.; Bisnovatyi-Kogan, G. S.; Spruit, H. C.
2000NuPhS..80C1117T Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Theory of Solar Irradiance Variations
Authors: Spruit, H.
2000svc..book..113S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Differential rotation and magnetic fields in stellar interiors
Authors: Spruit, H. C.
1999A&A...349..189S Altcode: 1999astro.ph..7138S
The processes contributing to the evolution of an initially weak
magnetic field in a differentially rotating star are reviewed. These
include rotational smoothing (akin to convective expulsion) and a list
of about 5 instabilities, among them magnetorotational instability,
buoyancy instability, and pinch-type instabilities. The important
effects of thermal and magnetic diffusion on these instabilities are
analyzed in some detail. The first instability to set in is a pinch-type
instability. It becomes important in modifying the field configuration
before magnetic buoyancy-driven instabilities set in. The evolution
of an initially strong field remains a more open question, including
the old problem whether dynamically stable magnetic equilibria exist
in stars.
---------------------------------------------------------
Title: Hydrodynamics of accretion discs of variable thickness
Authors: Stehle, R.; Spruit, H. C.
1999MNRAS.304..674S Altcode:
We present a new model for the hydrodynamics of thin accretion discs. In
the (r, phi ) plane full hydrodynamics is taken into account, while
the time evolution of the disc thickness H is followed by assuming
uniform expansion and contraction in the vertical direction. This
amounts to a `one-zone' approximation for the vertical equation of
motion. We show that the model correctly incorporates the fundamental
mode of vertical oscillation of a disc for waves long compared with
the disc thickness. A numerical scheme to solve these equations on an
Eulerian grid is introduced and tested for validity. As an application
we compute the excitation of disc thickness oscillations by the tidal
field of a companion. Strong excitation occurs at the 2{:}1 resonance,
at r ~= 0.32 a, where a is the binary separation.
---------------------------------------------------------
Title: Gamma-ray bursts from X-ray binaries
Authors: Spruit, H. C.
1999A&A...341L...1S Altcode: 1998astro.ph.11007S
A weakly magnetized ( ~ 10(7) G) neutron star, slowly spun up by
accretion in an X-ray binary, crosses the instability boundary for
r-mode instability at P=1-2 msec. The amplitude of the oscillation,
which initially increases only at the spinup time scale, is secularly
unstable due to the negative temperature dependence of the viscosity in
neutron star matter, and diverges after a few hundred years. Angular
momentum loss by the gravitational wave causes strong differential
rotation, in which the magnetic field is wound up to 10(17) G on a time
scale of a few months. When this field becomes unstable to buoyancy
instability, a surface field strength of a few 10(16) G is produced
on a time scale of seconds, which then powers a GRB with energies of
10(51) -10(52) and duration of 1-100 sec.
---------------------------------------------------------
Title: The stream impact region in the disc of WZ SGE
Authors: Spruit, H. C.; Rutten, R. G. M.
1998MNRAS.299..768S Altcode:
We report the observation of new features in the spectrum of the
cataclysmic variable WZ Sge. The disc eclipse is seen as a well-defined
structure in the Hα line. From phases 0.25 to 0.5 an absorption
feature of the same shape as the emission S wave is seen in this line,
but redshifted by ~200 kms^-1. Two possible interpretations of this
feature are given, both of which imply that it originates at the
impact point of the stream on the disc edge. In addition, evidence
is found for substructure in the velocity map of the spot. Emission
from line-emitting post-shock material, extending to about 60 deg
downstream from the continuum hotspot, is seen in the Doppler map in
the form of a tail extending from the hotspot. A theoretical estimate
shows that such a tail is to be expected as a consequence of the
post-impact hydrodynamics of the stream. A new determination of the
system parameters is made. They agree with those of Gilliland et al.;
in particular, the data support a high primary mass. The variation
of Hα surface brightness with distance from the primary is flatter,
in the inner regions, than the r^-1.5 dependence found by Horne for
other cataclysmic variables in quiescence.
---------------------------------------------------------
Title: Fast maximum entropy Doppler mapping
Authors: Spruit, H. C.
1998astro.ph..6141S Altcode:
A numerical code is described for constructing Doppler maps from the
orbital variation of line profiles of (mass transfering) binaries. It
uses an algorithm related to Richardson-Lucy iteration, and is much
faster than the standard algorithm used for ME problems. The method has
been tested on data of cataclysmic variables (including WZ Sge and SS
Cyg), producing maps of up to 300X300 points. It includes an IDL-based
set of routines for manipulating and plotting the input and output data,
and can be downloaded from http://www.mpa-garching.mpg.de/~henk
---------------------------------------------------------
Title: Bolometric Light Curves of Supernovae and Postexplosion
Magnetic Fields
Authors: Ruiz-Lapuente, P.; Spruit, H. C.
1998ApJ...500..360R Altcode: 1997astro.ph.11248R
The various effects leading to diversity in the bolometric light curves
of supernovae are examined; nucleosynthesis, kinematic differences,
ejected mass, degree of mixing, and configuration and intensity of
the magnetic field are discussed. In Type Ia supernovae, a departure
in the bolometric light curve from the full-trapping decline of
<SUP>56</SUP>Co can occur within the 2.5 years after the explosion,
depending on the evolutionary path followed by the white dwarf (WD)
during the accretion phase. If convection has developed in the WD
core during the presupernova evolution, starting several thousand
years before the explosion, a tangled magnetic field close to the
equipartition value should have grown in the WD. Such an intense
magnetic field would confine positrons where they originate from
the <SUP>56</SUP>Co decays and preclude a strong departure from the
full-trapping decline as the supernova expands. This situation is
expected to occur in C + O Chandrasekhar WDs as opposed to edge-lit
detonated sub-Chandrasekhar WDs. If the preexplosion magnetic field of
the WD is less intense than 10<SUP>5</SUP>-10<SUP>8</SUP> G, a lack
of confinement of the positrons emitted in the <SUP>56</SUP>Co decay
and a departure from full trapping of their energy would occur. The
time at which the departure takes place can provide estimates of
the original magnetic field of the WD, its configuration, and also
of the mass of the supernova ejecta. In SN 1991BG, the bolometric
light curve suggests an absence of a significant tangled magnetic
field: its intensity is estimated to be lower than 10<SUP>3</SUP>
G. Chandrasekhar-mass models do not reproduce the bolometric light curve
of this supernova. For SN 1972E, on the contrary, there is evidence
for a tangled configuration of the magnetic field and its light curve
is well reproduced by a Chandrasekhar WD explosion. A comparison is
made for the diagram of absolute magnitude and rate of decline in Type
Ia supernovae coming from different explosion mechanisms. The effects
of mixing and ejected mass in the bolometric light curve of Type Ib,
Ic supernovae are also discussed.
---------------------------------------------------------
Title: Birth kicks as the origin of pulsar rotation
Authors: Spruit, H.; Phinney, E. S.
1998Natur.393..139S Altcode: 1998astro.ph..3201S
Radio pulsars are thought to born with spin periods of 0.02-0.5s and
space velocities of 100-1,000kms<SUP>-1</SUP>, and they are inferred to
have initial dipole magnetic fields of 10<SUP>11</SUP>-10<SUP>13</SUP>G
(refs 1-5). The average space velocity of their progenitor stars
is less than 15kms<SUP>-1</SUP>, which means that pulsars must
receive a substantial `kick' at birth. Here we propose that the birth
characteristics of pulsars have a simple physical connection with each
other. Magnetic fields maintained by differential rotation between
the core and envelope of the progenitor would keep the whole star in
a state of approximately uniform rotation until10 years before the
explosion. Such a slowly rotating core has 1,000 times less angular
momentum than required to explain the rotation of pulsars. The specific
physical process that `kicks' the neutron star at birth has not been
identified, but unless its force is exerted exactly head-on it will also
cause the neutron star to rotate. We identify this process as the origin
of the spin of pulsars. Such kicks may cause a correlation between the
velocity and spin vectors of pulsars. We predict that many neutron stars
are born with periods longer than 2s, and never become radio pulsars.
---------------------------------------------------------
Title: Origin of the rotation rates of single white dwarfs
Authors: Spruit, H. C.
1998A&A...333..603S Altcode: 1998astro.ph..2141S
I argue that the rotation of white dwarfs is not a remnant of the
angular momentum of their main sequence progenitors but a result of
the mass loss process on the AGB. Weak magnetic fields, if present in
stellar interiors, are likely to maintain approximately uniform rotation
in stars, both on the main sequence and on the giant branches. The
nearly uniform rotation of the core of the Sun is evidence for the
existence of such fields. Exactly axisymmetric mass loss on the AGB
from uniformly rotating stars would lead lead to white dwarfs with very
long rotation periods (> 10 yr). Small random non-axisymmetries ( ~
10(-3) ) in the mass loss process, on the other hand, add sufficient
angular momentum to explain the observed rotation periods around one
day. The process illustrated with a computation of the probability
distribution of the rotation periods under the combined influence of
random forcing by weak nonaxisymmetries and angular momentum loss in
the AGB superwind. Such asymmetries can in principle be observed by
proper motion studies of the clumps in interferometric images of SiO
maser emission.
---------------------------------------------------------
Title: Origin of the rotation rates of single white dwarfs and
neutron stars
Authors: Spruit, H. C.
1998ASPC..138..335S Altcode: 1998stas.conf..335S
No abstract at ADS
---------------------------------------------------------
Title: Solar Irradiance Variations: Theory
Authors: Spruit, H. C.
1998IAUS..185..103S Altcode:
The theory of stellar structure can be used to make definite predictions
about the effect disturbances in the solar interior or atmosphere on the
observed irradiance. For stars like the sun, the extended convective
envelope plays an important role as a heat storage reservoir. The
irradiance is easily modulated on observable time scales by changes
in heat transport efficiency close to the surface, such as those due
to spots and the small scale magnetic field. The nonmagetic part
of the surface is predicted to be unaffected, to high accuracy,
by such perturbations. The enhanced overall irradiance at solar
maximum is interpreted as due to the high emissivity of the small
scale magnetic field, which dominates over the lower emissivity of
spots. The meachnisms causing these emissivity changes are known
and have been recovered in numerical simulations. Quantititative
predictions, however, are still hampered limits in our knowledge of
the sub-arcsecond structure of the surface magnetic fields.
---------------------------------------------------------
Title: Is stellar granulation turbulence?
Authors: Nordlund, A.; Spruit, H. C.; Ludwig, H. -G.; Trampedach, R.
1997A&A...328..229N Altcode:
We show that power spectra of granulation images or velocity fields
cannot be compared meaningfully with spectra from theoretical models
based on turbulent cascades. The small scale power in these images
is due almost entirely to the sharp edges between granules and
intergranular lanes, not to turbulence in the usual sense. This is
demonstrated with a number of experiments with result from numerical
simulations, and with simpler synthetic data with power spectra similar
to that of granulation. The reason for the seemingly laminar behavior of
the granulation flow, in spite of the high Reynolds numbers involved,
is the influence of stratification on the local ratio of turbulence
to bulk flow. The rapid expansion of upflows, their deep origin and
near-adiabatic stratification lead to low levels of turbulence in
the overturning fluid at the surface. Higher levels of turbulence
are expected in the converging flows near downdrafts, but mostly at
scales that are below current observational resolution limits, and
contributing relatively little to the total convective flux and to
spectral line broadening.
---------------------------------------------------------
Title: Collimation of magnetically driven jets from accretion discs
Authors: Spruit, H. C.; Foglizzo, T.; Stehle, R.
1997MNRAS.288..333S Altcode:
We argue that the toroidal fields developing in a magnetically
accelerated jet are sufficiently unstable that they cannot contribute
much to collimation. We show how an initially collimated jet is
decollimated by decay of the toroidal field and by the build-up of
internal pressure due to kink instability. We propose that most of the
collimation of observed jets is due to the poloidal field anchored in
the disc. We show how the collimation by this mechanism depends on the
distribution of poloidal field strength in the disc. We find that the
maximum achievable collimation increases with the ratio of outer to
inner disc radius, and can be of the order of 1 deg. This dependence
is found to be consistent with the available data, in particular the
absence of collimated outflows from cataclysmic variables. Because of
the decay of the toroidal field a new characteristic distance plays a
role: the collimation distance, z_c, of the order of the disc radius
or less. Beyond z_c the jet is entirely ballistic and only weakly
magnetic. No external medium is needed beyond this distance to explain
observed narrow opening angles of jets, and no interaction with an
external medium is necessary to explain the parallel field orientation
observed in fast jets.
---------------------------------------------------------
Title: X-ray spectrum of a disk illuminated by ions
Authors: Spruit, H. C.
1997LNP...487...67S Altcode: 1997adna.conf...67S
The X-ray spectrum from a cool disk inside an ion supported torus
is computed. A surface layer of the disk is heated by the protons
from the torus. It produces a comptonized spectrum with a shape that
is only very weakly dependent on the incident energy flux and the
distance from the accreting compact oject, and is similar to observed
spectra. Further evidence for 'ion illuminated' disks are the Li
abundance in the secondaries of low mass X-ray binaries and the 450
keV lines sometimes seen in black-hole transient spectra.
---------------------------------------------------------
Title: Convection in stellar envelopes: a changing paradigm.
Authors: Spruit, H. C.
1997MmSAI..68..397S Altcode: 1996astro.ph..5020S
Progress in the theory of stellar convection over the past decade
is reviewed. The similarities and differences between convection in
stellar envelopes and laboratory convection at high Rayleigh numbers
are discussed. Direct numerical simulation of the solar surface layers,
with no other input than atomic physics, the equations of hydrodynamics
and radiative transfer is now capable of reproducing the observed heat
flux, convection velocities, granulation patterns and line profiles
with remarkably accuracy. These results show that convection in stellar
envelopes is an essentially non-local process, being driven by cooling
at the surface. This differs distinctly from the traditional view
of stellar convection in terms of local concepts such as cascades of
eddies in a mean superadiabatic gradient. The consequences this has
for our physical picture of processes in the convective envelope
are illustrated with the problems of sunspot heat flux blocking,
the eruption of magnetic flux from the base of the convection zone,
and the Lithium depletion problem.
---------------------------------------------------------
Title: Accretion Disks - New Aspects
Authors: Meyer-Hofmeister, Emmi; Spruit, Henk
1997LNP...487.....M Altcode: 1997adna.conf.....M
The most luminous compact objects are powered by accretion of
mass. Accretion disks are the one common and fundamental element of
these sources on widely different scales, ranging from close stellar
binaries, galactic black holes and X-ray pulsars to active galactic
nuclei (AGN). Key new developments in theory and observations, reviewed
by experts in the field, are presented in this book. The contributions
to the workshop cover the puzzles presented by the X-UV spectra of AGN
and their variability, the recent numerical simulations of magnetic
fields in disks, the remarkable behavior of the superluminal source
1915+105 and the "bursting pulsar" 1744-28, to mention a few of
the topics.
---------------------------------------------------------
Title: Mechanisms of p-mode absorption by active regions
Authors: Spruit, H. C.
1996BASI...24..211S Altcode:
Proposed mechanisms for explaining the observed absorption of p-mode
waves by active regions are briefly reviewed. Absorption by conversion
into downward propagating slow mode waves is identified as particularly
promising. The observed dependence of absorption and scattering
amplitudes on wavenumber allows in principle the determination of the
field strength in active region and sunspot fields down to a depth of
about 20000 km.
---------------------------------------------------------
Title: A `curve of growth' of astronomers on the Citation Index
Authors: Spruit, H. C.
1996QJRAS..37....1S Altcode:
The average citation career of a sample of 72 astronomers who received
their PhD between 1977 and 1988, and who were employed in astronomy
in 1994, is analysed using data from the Science Citation Index. The
citation rates n_i(t) are fairly well represented by the assumption
n_i(t)=alpha_i f(t) where f(t) is a `universal' function and alpha_i an
individual `amplitude factor'. The shape of f shows that the citation
rate 2 years after PhD is, on average, already one half of what it will
be 12 years after PhD. Probability intervals are given for the later
citation rate, for given rates early in the career. Early citation
rates are found to have a modest but measurable predictive value.
---------------------------------------------------------
Title: Differential rotation, circulation and turbulence in radiative
zones of stars.
Authors: Urpin, V. A.; Shalybkov, D. A.; Spruit, H. C.
1996A&A...306..455U Altcode:
We examine the steady state circulation and differential rotation in
the radiative zones of early-type stars, following Zahn's assumption
of latitude-independent rotation. The formulation includes in a
consistent way the turbulence produced by shear, and the rotational
and baroclinic forces driving the circulation. The angular momentum
transfer by the combined action of meridional circulation and turbulence
results in a differential rotation in the radiative zone. In its turn,
the differential rotation causes the circulation (due to deviations
from the spherical symmetry) and turbulence (due to instabilities
associated with the shear stresses). Numerical results are given
for the steady state in a 20M<SUB>sun</SUB>_ main sequence star. The
circulation flow is very weak near the surface, and mixing between
the internal and surface layers is practically absent even at high
rotation rates. However, during the initial stage of evolution before
the steady state is reached, a fraction of matter from the deep layers
can be transported to the surface.
---------------------------------------------------------
Title: Magnetohydrodynamic winds and jets from accretion disks
Authors: Spruit, H. C.
1996astro.ph..2022S Altcode:
The theory of magnetically accelerated outflows and jets from
accretion disks is reviewed at an introductory level, with special
attention to problem areas like the launching conditions of the flow
at the disk surface, stability of the magnetic field, and collimation
mechanisms. This text will appear in R.A.M.J. Wijers, M.B. Davies and
C.A. Tout, eds., Physical processes in Binary Stars, Kluwer Dordrecht,
1996 (NATO ASI series).
---------------------------------------------------------
Title: Magnetohydrodynamic jets and winds from accretion disks
Authors: Spruit, H. C.
1996ASIC..477..249S Altcode: 1996epbs.conf..249S
The theory of magnetically accelerated outflows and jets from accretion
disks is reviewed at an introductory level, with special attention to
problem areas like the launching conditions of the flow at the disk
surface, stability of the magnetic field, and collimation mechanisms.
---------------------------------------------------------
Title: Eclipse Mapping of the Cataclysmic Variable GS Pav
Authors: Barziv, O.; Augusteijn, T.; Spruit, H.; Kuulkers, E.; Berger,
M.; van Paradijs, J.; Seiradakis, J. H.
1996hell.conf..246B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Strategies for detecting Thorne-Zytkow objects.
Authors: van Paradijs, J.; Spruit, H. C.; van Langevelde, H. J.;
Waters, L. B. F. M.
1995A&A...303L..25V Altcode:
Thorne-Zytkow objects (TZO) are likely to have high mass loss
rates, such that their optical photospheres are obscured by a dusty
envelope. Indentification of massive TZO through abundance anomalies
associated with the irp process in the core is more likely to be
possible by observing molecular transitions in the 4-8 μm region and
at mm wavelengths than in the optical. A promising molecule is SiO,
with anomalously high abundances predicted for ^29^SiO and ^30^SiO. An
upper limit for ^30^SiO is reported for the unusually bright galactic
center SiO maser OH359.762+0.120.
---------------------------------------------------------
Title: Switch-off conditions for radio pulsars
Authors: Tsygan, A. I.; Spruit, H.
1995AstL...21..790T Altcode: 1995PAZh...21..877T
No abstract at ADS
---------------------------------------------------------
Title: Interchange instability in and accretion disc with a poloidal
magnetic field
Authors: Spruit, H. C.; Stehle, R.; Papaloizou, J. C. B.
1995MNRAS.275.1223S Altcode: 1995astro.ph..4043S
We investigate the stability to nonaxisymmetric perturbations of
an accretion disc in which a poloidal magnetic field provides part
of the radial support against gravity. Interchange instability due
to radial gradients in the magnetic field are strongly stabilized
by the shear flow in the disc. For smooth field distributions this
instability is restricted to discs in which the magnetic energy is
comparable to the gravitational energy. An incompressible model for
the instability akin to the Boussinesq approximation for convection is
given which predicts the behaviour of the instability accurately. Global
axisymmetric disturbances are also considered and found to be stable
for a certain class of models. The results indicate that accretion
discs may be able to support poloidal fields which are strong enough to
suppress other forms of magnetic instability. These strong and stable
field distributions are likely to be well suited for the magnetic
acceleration of jets and winds.
---------------------------------------------------------
Title: Magnetic Interchange Instability in Accretion Disks
Authors: Lubow, Stephen H.; Spruit, Hendrik C.
1995ApJ...445..337L Altcode:
We investigate the stability of a disk to magnetic interchange in
the disk plane, when a poloidal magentic field provides some radial
support of the disk. The disk is assumed to be geometrically thin and
may possess rotation and shear. We assume the unperturbed magnetic
field vertically threads the disk and has a comparable radial component
at the disk surface. We formulate the linear stability problem as an
initial value problem in shearing coordinates and ignore any effects
of winds. Shear stabilizes the interchange instability strongly
compared to the uniformly rotating case studied previously and
makes the growth algebraic rather than exponential. A second form of
instability with long wavelengths is identified, whose growth appears
to be transient. If the field strength is measured by the travel
time tau<SUB>A</SUB> of an Alfven wave across the disk thickness,
significant amplification for both forms of instability requires
(tau<SUB>A</SUB> Omega)<SUP>-2</SUP> greater than or approximately
equal to L/H, where L is the radial length scale of the field gradient
and H is the disk thickness. Field strengths such that 1 less than or
approximately equal (tau<SUB>A</SUB> Omega)<SUP>-2</SUP> less than or
approximately equal L/H are stable to these instabilities as well as
the instability recently investigated by Balbus & Hawley (1991). The
results suggest that in dark environments in which the magnetic energy
density is greater than the thermal energy density, disks are stable
over a substantial range of parameter space, with radial advection of
magnetic flux limited by the interchange instability possibly near the
disk center. Such environments may be relevant for the production of
magnetic winds or jets in young stars or active galactic nuclei.
---------------------------------------------------------
Title: Cyclic Accretion from a Disk onto a Neutron Star Magnetosphere
Authors: Spruit, H. C.
1995ASIC..450..377S Altcode: 1995lns..conf..377S; 1996lns..proc..377S
No abstract at ADS
---------------------------------------------------------
Title: Accretion Disks
Authors: Spruit, H. C.
1995ASIC..450..355S Altcode: 1995astro.ph..2098S; 1995lns..conf..355S
In this lecture the basic concepts used in accretion disk theory are
reviewed, with emphasis on aspects relevant for X-Ray Binaries and
Cataclysmic Variables.
---------------------------------------------------------
Title: Energy implications of Li production in X-ray transients.
Authors: Martin, E. L.; Spruit, H. C.; van Paradijs, J.
1994A&A...291L..43M Altcode:
The Li-abundances recently observed in the low mass secondaries of
several soft X-ray transients have been proposed to come from nuclear
reactions in the vicinity of the compact objects during the outbursts
of these systems. This scenario implies a large flux of energetic
particles in X-ray transients. We show that Li newly produced in
an outburst is mixed into the secondary very quickly, so that the
observed abundance must represent the average production over many
outbursts. With known limits on the Li-depletion rates in low mass
stars, we derive minimal energy fluxes in energetic protons and/or
α-particles, assuming that either the secondary or the accretion
disk is the main target of this flux. In both cases, at least a few
per cent of the accretion energy must be converted to fast particles
with energies exceeding 10MeV/nucleon.
---------------------------------------------------------
Title: Fast eclipse mapping.
Authors: Spruit, H. C.
1994A&A...289..441S Altcode:
A recent algorithm by Lucy is used for the problem of mapping the
intensity distribution of an object elipsed by a companion star, and
is found to be an efficient way of obtaining solutions of the maximum
entropy type. More general default maps than the standard axisymmetric
one are explored; they allow some reduction of the artefacts in eclipse
maps. It is shown that reconstruction of the intensity distribution on
a surface parallel to the orbital plane is a special case in which the
forward and backward projection steps can be done much more efficiently.
---------------------------------------------------------
Title: Magnetically driven wind from an accretion disk with
low-inclination field lines.
Authors: Cao, Xinwu; Spruit, H. C.
1994A&A...287...80C Altcode:
The inclination of the field lines at the surface of an accretion disk
has a strong influence on the nature of the resulting magnetically
driven wind. If the field lines are steep, a hot corona must be present
to feed the wind, at low inclinations the wind is fed directly from
the surface. With a simple model for the field distribution in a disk
we show how the transition between the two regimes takes place. At
high inclinations an ordinary wind of the stellar type results. At low
inclinations, a high- ˙(m) wind results with very low terminal speed
and highly wound-up field lines. We argue that the latter type of wind
will be highly unstable to nonaxisymmetric (Parker type) instabilities,
and that it gives rise to a magnetically driven circulation along the
disk surface rather than a high density wind.
---------------------------------------------------------
Title: Theoretical interpretation of solar and stellar irradiance
variations
Authors: Spruit, H. C.
1994svsp.coll..270S Altcode: 1994IAUCo.143P.270S
The main cause of variability of solar type stars are their varying
magnetic fields. To compute irradiance variations one has to compute
the magnetic field (the dynamo problem), and from this the irradiance
effects. The second problem is considered here. The theoretical work of
the past decade has shown that the dominant effect of magnetic fields is
a surface effect: a change of effective emissivity of the magnetic parts
of the surface while the nonmagnetic part of the surface contributes
very little to the irradiance variation on almost all time scales. No
other processes have yet been found that would cause variations
exceeding (at the current level of magnetic activity) the observed 0.1%
irradiance fluctuation of the Sun. This implies that a knowledge of the
surface magnetic fields [separated into its bright small scale (faculae,
network) and dark large scale (spots) components] is sufficient for
pre- or post-dicting the solar irradiance. It is hypothesized that the
discrepancy remaining between the measured irradiance variations and
values reconstructed from proxies is due to the difficulty of finding
a proxy that accurately correlates with the continuum contrast of
a dispersed small scale magnetic field. Stellar structure theory
predicts that the variations in the solar radius associated with
magnetic activity are quite small. For stars, color and brightness
variations should primarily be interpreted in terms of variations in
the fraction of the surface covered by magnetic patches. Their (long
term) displacement from the main sequence is not very large.
---------------------------------------------------------
Title: Theories of radius and luminosity variations
Authors: Spruit, H. C.
1994seit.conf..107S Altcode:
The main cause of variability of the Sun is its magnetic field. The
theoretical work of the past decade has shown that the dominant effect
of magnetic fields is a surface effect: a change of effective emissivity
of the magnetic parts of the surface while the nonmagnetic part of the
surface contributes very little to the irradiance variation on almost
all time scales. No other processes have yet been found that would
cause variations exceeding (at the current level of magnetic activity)
the observed 0.1% irradiance fluctuation of the Sun. This implies that
a knowledge of the surface magnetic fields [separated into its bright
small scale (faculae, network) and dark large scale (spots) components]
is sufficient for pre- or post-dicting the solar irradiance. It is
hypothesized that the discrepancy remaining between the measured
irradiance variations and values reconstructed from proxies is due to
the difficulty of finding a proxy that accurately correlates with the
continuum contrast of a dispersed small scale magnetic field. Stellar
structure theory predicts that the variations in the solar radius
associated with magnetic activity are quite small, of the order of a
few times 10^{-7} Observations of radius variations much above this
value would constitute a serious challenge for stellar structure theory.
---------------------------------------------------------
Title: Magnetic winds from stars and disks
Authors: Spruit, H. C.
1994ASIC..422...33S Altcode: 1994coma.conf...33S
Some current problems in magnetic wind theory are discussed: i) the
uncertainties in quantitative theories of magnetic braking of stars
and how these influence the theory of the evolution of Cataclysmic
Variables, ii) the instability of the toroidal field component in
winds and jets from accretion disks, iii) the importance of poloidal vs
toroidal collimation of jets, and iv) what determines the distribution
of field strength in the disk from which the wind originates. It is
argued, in particular, that the geometrical properties of the poloidal
field of a disk may well be the main contributor to the observed
collimation of jet.
---------------------------------------------------------
Title: Numerical Simulations of Shock-driven Accretion
Authors: Rozyczka, M.; Spruit, H. C.
1993ApJ...417..677R Altcode:
We calculate how accretion in a mass transferring binary system
takes place if shock waves are the only means of angular momentum
transport and energy dissipation. Cooling by radiation from the disk
is included. In the absence of a mass transferring stream, with
shocks excited by the tidal force only, the disk quickly settles
into a quasi-stationary shock pattern. The presence of a stream
impacting on the disk has a profound effect by keeping the flow very
nonsteady. From simulations covering several hundred binary orbits,
we find the following sequence of events. After an initial transient
(which lasts on the order of 20 orbits) most of the mass transferred
accumulates in a ring while a lower level accretion takes place
from the ring onto the central object. For disk temperatures of a few
percent of the local virial temperature, the effective alpha-viscosity,
as measured by the accretion rate, during this phase is of the order
10<SUP>-3</SUP>. The size of the disk and the shape of the brightness
distribution across it agree well with observations of quiescent CV
disks. The rotation profile in the ring approaches a constant angular
momentum distribution and then becomes violently unstable by a process
observed earlier by Blaes and Hawley. During the instability, the
accretion rate onto the central object is enhanced. Storage of mass
in a ring alternating with accreting phases due to instability of
the torus is expected to take place in general at low disk viscosity,
whatever the process responsible for the viscosity. This provides a
new mechanism for soft X-ray transients and the superoutburst cycle
in cataclysmic variables.
---------------------------------------------------------
Title: An Instability Associated with a Magnetosphere-Disk Interaction
Authors: Spruit, H. C.; Taam, Ronald E.
1993ApJ...402..593S Altcode:
The evolution of a thin accretion disk surrounding a rapidly rotating
magnetosphere is considered. By taking account of the variations of the
magnetospheric boundary in response to the conditions at the inner edge
of the disk, we find from linear analysis and numerical computation
that the accretion disk can become unstable. Mass can be accreted
by the central object in a cyclic fashion, with the cycle involving
the storage and release of mass in the inner parts of the disk. The
physical origin of the instability is associated with the variations
of the magnetospheric boundary about corotation. The recurrence time
scale of the cycle can vary by several orders of magnitude depending
on the details of the conditions at the magnotosphere. The possible
applicability of this instability process to the 'rapid burster'
MXB 1730-335 is briefly discussed.
---------------------------------------------------------
Title: Physical Processes and Conditions Associated with the Formation
of Protoplanetary Disks
Authors: Morfill, G.; Spruit, H.; Levy, E. H.
1993prpl.conf..939M Altcode:
Stars and planetary systems are thought to develop more or less
contemporaneously from extended disk-shaped nebulae. Because the
dynamical states of such disks are far from their final equilibrium
configurations, the nebulae can dissipate large amounts of energy,
as matter accumulates in the center and angular momentum moves to
the peripheries. Experience with cosmical systems that are far from
equilibrium indicates that such rapid dissipative evolution frequently
occurs through a variety of collective behaviors, sometimes producing
phenomena of surprising violence. This chapter reviews the range of
types of collective processes that may occur in protostellar disks
and play significant roles in speeding the evolution of the disks,
as well as affecting the physical state of the disk and altering the
state of protoplanetary matter. The processes considered here include
collective angular momentum transport processes, electrostatic lightning
and magnetic flares.
---------------------------------------------------------
Title: Unstable accretion from a disk onto a neutron star
magnetosphere
Authors: Spruit, H.
1993heac.conf...60S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Conversion of p-Modes to Slow Modes and the Absorption
of Acoustic Waves by Sunspots
Authors: Spruit, H. C.; Bogdan, T. J.
1992ApJ...391L.109S Altcode:
The study considers the possibility that the acoustic absorption by
sunspots and the surrounding plage, reported by Braun et al. (1988,
1990), is a consequence of the conversion of p-modes to slow modes
(s-modes) by the principally vertical magnetic fields within these
structures. It is found that for the f-mode, the absorption coefficient
increases monotonically from small to large horizontal wavenumbers, and,
along the nth p-mode ridge, this same general trend is modulated by
the presence of n localized absorption minima. These characteristic
signatures of acoustic absorption by p-mode/s-mode conversion
distinguish this mechanism from other competing processes and afford
the diagnostic possibility of determining the sunspot magnetic field
strength from the location in wavenumber of the predicted absorption
minima.
---------------------------------------------------------
Title: The Fate of the Heat Flux Blocked by Sunspots
Authors: Spruit, H. C.
1992ASIC..375..163S Altcode: 1992sto..work..163S
A sunspot blocks a part of the heat flux from the solar surface, causing
a thermal adjustment in the convection zone. Two very different time
scales are involved, and the adjustments are different on the two time
scales. On time scales longer than the thermal time scale tau(t) of
the convection zone, a significant fraction of the heat flux blocked
by spots can reappear elsewhere at the surface. The average spot
coverage over this time scale leads to a small change in position of
the sun in the HRD. On time scales short compared with tau(t), almost
all changes in the heat flux are absorbed by the convection zone. The
surface temperature outside spots and the solar radius do not change on
such time scales. Thus for all time scales of observational interest,
the irradiance variations directly reflect the instantaneous surface
coverage by spots (and faculae if these are present).
---------------------------------------------------------
Title: Influence of Starspots on Internal Stellar Structure (Invited)
Authors: Spruit, H. C.
1992LNP...397...78S Altcode: 1992sils.conf...78S
No abstract at ADS
---------------------------------------------------------
Title: The rate of mixing in semiconvective zones.
Authors: Spruit, H. C.
1992A&A...253..131S Altcode:
The present theory for mixing in semiconvective zones, under the
assumption that the layered convection observed in similar laboratory
circumstances occurs, takes into account the development of boundary
layers with steep gradients in entropy and mean weight at the interface
of adjacent layers. The most important property of the derived mixing
rate is that it is proportional to the square root of the microscopic
diffusion coefficient, reflecting the essential role played by
both diffusion and advection of overturning cells in the transport
mechanism. The diffusion coefficient depends only on the energy flux,
the thermodynamic variables, and the chemical composition.
---------------------------------------------------------
Title: On the mechanism of angular momentum transport in accretion
disks.
Authors: Livio, M.; Spruit, H. C.
1991A&A...252..189L Altcode:
Recently, a new mechanism for angular momentum transport in accretion
disks has been proposed. The mechanism is based on tidally induced
perturbations which develop into coherent spiral shocks. Angular
momentum transfer by the spiral shocks was shown to be quite effective
for hot, two-dimensional disks. Observational data on the mean
recurrence time-scale of dwarf nova eruptions are used in conjunction
with the disk instability model, to obtain a relation between the
viscosity parameter alpha and the mass ratio of the system. This
relation can be compared to the effective 'viscosity' obtained from
the spiral-shocks scheme. The data show a weaker dependence on the mass
ratio than predicted. Possible causes for the discrepancy are discussed.
---------------------------------------------------------
Title: Li Depletion in F Stars by Internal Gravity Waves
Authors: Garcia Lopez, Ramon J.; Spruit, Hendrik C.
1991ApJ...377..268G Altcode:
It is argued that internal gravity waves generated by the convective
envelope of a star may be effective in producing a weak mixing in
its radiative interior. The spectrum of wavelengths and frequencies
generated is estimated. Only the largest horizontal wavelengths and the
lowest frequencies contribute significantly to the wave energy flux and
the mixing at the Li-burning depth. At this layer, the combination of
the dependence on spectral type, both for the wave energy flux and the
shear rate induced by the waves, would produce a distribution of Li
abundances similar to that of the Li gap found among F-type stars of
several open clusters, and the field. Quantitative agreement with the
observed location and time scale of the gap is obtained, but only by
increasing the intensity of the waves generated by a factor 15 above
straightforward mixing-length estimates. The ratio of mixing length
to scale height needed to get the gap in the right spectral range is
1.6. The blue edge of the gap is predicted to be sharp.
---------------------------------------------------------
Title: Filigree and flux tube physics.
Authors: Spruit, H. C.; Schuessler, M.; Solanki, S. K.
1991sia..book..890S Altcode:
This review covers the properties of the small-scale (outside
sunspots) magnetic field from observational and theoretical points
of view. Special emphasis is put on the physics of small isolated
magnetic concentrations (flux tubes). Topics discussed include the
basic observational properties, the origin and disappearance of the
small-scale field, the properties of individual magnetic elements,
their influence on the solar irradiance and their interaction with
solar oscillations.
---------------------------------------------------------
Title: Absorption of p-Mode Waves by Magnetic Fields
Authors: Spruit, H. C.
1991ctsm.conf..121S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Absorption of p-Mode Waves by Magnetic Fields
Authors: Spruit, H. C.
1991LNP...388..121S Altcode:
Vertical magnetic flux tubes can leak energy out of the p-mode
wave guide in the form of transversal and longitudinal tube waves. A
procedure is developed for calculating this energy loss for tubes whose
diameter is much smaller than the wavelength of the exciting p-mode.
---------------------------------------------------------
Title: Theory of Luminosity and Radius Variations
Authors: Spruit, H. C.
1991suti.conf..118S Altcode:
Theoretical calculations of luminosity and radius variations are
reviewed. As sources for these variations, two kinds of effects are
considered: (1) temporary conversion of thermal energy inside the
convection zone into a different energy form (such as magnetic fields);
and (2) the effect of surface magnetic fields on the energy emissivity
of the solar surface. Because strength, extent, and location of the
subsurface fields are uncertain, emphasis is on upper limits, derived
from general thermodynamic considerations, and from the buoyancy of
strong fields. If the toroidal fields from which active regions erupt
are located at a depth greater than 10,000 km, their effect on the solar
radius is likely to be below 3 x 10 exp -6 during a solar cycle and
their efect on the luminosity below 10 exp -4. The vertical fields seen
at the surface itself have effects on the luminosity that compare well
with observations. The dominant effect in this case is a modulation of
the surface emissivity (reduction in spots, enhancement in small-scale
fields). The radius changes associated with this effect are negligible.
---------------------------------------------------------
Title: Shock Waves in Accretion Disks.
Authors: Spruit, H. C.
1991RvMA....4..197S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The influence of internal gravity waves on the light elements
depletion among F-type stars.
Authors: Garcia Lopez, Ramon J.; Spruit, H. C.
1991MmSAI..62..183G Altcode:
It is proposed that the Li abundances in F-type stars can be affected by
the weak mixing generated by internal gravity waves since the absence of
Li is so strong among this spectral type. The mixing mechanism caused
by the waves is describe theoretically and is applied to the F-stars
because of the inherent similarity of the Li gap. The wave-energy flux
is found to grow with age, the flux values of the Li and Be burning
layers are similar, and the resulting predictions for young stellar
systems are compatible with observational data.
---------------------------------------------------------
Title: Shock Waves in Accretion Disks
Authors: Spruit, H. C.
1991heac.conf..333S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Mass transfer by tidally induced spiral shocks in an accretion
disk
Authors: Matsuda, T.; Sekino, N.; Shima, E.; Sawada, K.; Spruit, H.
1990A&A...235..211M Altcode:
Numerical simulations are presented of the evolution of disks in binary
systems under the influence of accretion through spiral shock waves. The
calculations are followed for at least one radial drift time scale,
thereby improving upon previous results. The accretion time scale
(radial drift time from the outer edge of the disk) decreases from
about 1000 orbital periods at a mass ratio of 1000 to about 15 orbits
at q = 0.1. Due to the neglect of radiative losses the disks studied
are still significantly hotter than realistic disks, however.
---------------------------------------------------------
Title: Mass transport in a neutron star magnetosphere
Authors: Spruit, H. C.; Taam, R. E.
1990A&A...229..475S Altcode:
The interaction between a thin Keplerian accretion disk and a
magnetosphere surrounding a central object is investigated within
the framework of an analytical description for the magnetic field
configuration. The commonly held assumption that all accreting plasma
flows from the magnetospheric boundary to the stellar surface is shown
to be overly restrictive. If the magnetospheric boundary is defined
as the distance where the rotation starts deviating significantly from
the Kepler rate, it is found that there is an extensive region inside
this boundary where gas, nearly corotating with the star, drifts inward
across the field by an interchange instability. The linear analysis
of this instability is presented. It is also found that gas tied
to field lines can be in equilibrium at positions off the midplane,
and that gas can plausibly flow from the midplane to these positions,
in certain circumstances. The observational consequences of such a
picture are briefly discussed.
---------------------------------------------------------
Title: Angular momentum transport and magnetic fields in the solar
interior
Authors: Spruit, H. C.
1990ASSL..159..415S Altcode: 1990insu.conf..415S; 1990IAUCo.121..415S
The possible mechanisms of angular momentum transport in convectively
stable regions of a star are reviewed, with emphasis on transport
by magnetic torques. The strength and configuration of the fields
in such layers is quite uncertain, because it is not known if the
field strength is obtained by assuming that the field can reach a
dynamically stable configuration. A lower limit to the field strength
is obtained by assuming that the field is always dynamically unstable,
and decaying at the (rotation modified) dynamical time scale. The
present field in the sun would then be of the order 1 G, with poloidal
and toroidal components of similar strength. The differential rotation
in the core, if due only to the solar wind torque, would be very small
for this field strength, and instead would more likely be governed
by magnetic coupling to the differential rotation of the convection
zone. If smnall scale hydrodynamic transport mechanisms are present,
their properties would also be influenced by a field of this strength.
---------------------------------------------------------
Title: Will a Magnetic Field Inhibit Turbulent Transport?
Authors: Spruit, H. C.
1990LNP...366..151S Altcode: 1990rmsi.conf..151S
No abstract at ADS
---------------------------------------------------------
Title: Solar convection.
Authors: Spruit, H. C.; Nordlund, A.; Title, A. M.
1990ARA&A..28..263S Altcode:
The current understanding of solar convection is examined in connection
with optical observations of the surface, helioseismological
observations of the interior, and theories and simulations of
compressible convection. Recent progress in these fields has been
documented in workshops on solar granulation, the solar photosphere,
and helioseismology.
---------------------------------------------------------
Title: The Disrupted Magnetic Braking Hypothesis and the Period Gap
of Cataclysmic Variables
Authors: Taam, Ronald E.; Spruit, H. C.
1989ApJ...345..972T Altcode:
The consequences of a rearrangement of the surface magnetic field
of a late-type star on the rate of angular momentum loss associated
with a stellar wind are explored. The rearrangement of the field
from a low-order to a higher order multipole may plausibly reflect
the changing character of a stellar dynamo as a rapidly rotating
mass-losing star in a short-period binary system enters into a fully
convective state. Upon generalizing the results of Mestel and Spruit
(1987) to higher order magnetic field configurations, it is found that
the fraction of open field lines decreases by a factor ranging from 2
to 200 (depending on which multipole order is assumed) for rotational
periods near 3 hr. The angular momentum loss rate corresponding to this
reduction can decrease by a factor ranging from 3 to 2000. This loss in
effectiveness of the stellar wind can be accomplished without a sudden
decline in magnetic activity. It is suggested that the operation of a
mechanism of this type may be responsible for the origin of the period
gap of cataclysmic variable binary systems.
---------------------------------------------------------
Title: Stability of Sunspots to Convective Motions. I. Adiabatic
Instability
Authors: Moreno-Insertis, F.; Spruit, H. C.
1989ApJ...342.1158M Altcode:
For determining the adiabatic stability of a uniform vertical field
in an arbitrary stratification it is sufficient to consider the
limit of infinitesimal horizontal wavelength. It is shown how the
behavior of the instability can be estimated qualitatively from the
dependence of the equipartition field strength on depth. Modes are
calculated numerically for analytic stratification models and for a
detailed sunspot stratification, including the effects of partial
ionization. It is concluded that for the observed field strengths
of umbrae the stratification is indeed unstable, with a growth time
of about 18 minutes. The unstable eigenfunctions have a maximum at
about 2300 km below the surface of the umbra and are about 3900 km
deep. Deeper layers may also be unstable depending on unknown details
of the stratification. A connection between fluting instability and
convective instability is noted.
---------------------------------------------------------
Title: Accretion and particle acceleration by spiral shock waves
Authors: Spruit, H. C.
1989ASSL..156...59S Altcode: 1989admf.proc...59S
The physics of accretion through a system of stationary spiral shaped
shock waves is reviewed. The angular momentum transport by such waves
can be described in terms of the negative angular momentum carried by
trailing waves. Excitation processes are discussed. In `large' disks
(inner radius r_i &lt&lt&lt outer radius r_o) there is
a range of distances r_i &lt&lt r &lt&lt r_o where
the shock-accretion solution is insensitive to conditions at both r_i
and r_o. For a simple equation of state and opacity this part of the
solution has a selfsimilar (scale invariant) form. These properties make
spiral shocked accretion especially attractive in protostellar, neutron
star and AGN disks, where the traditional `angular momentum problem'
is most severe. It is shown that spiral shocks are also good locations
for producing energetic protons, and I propose that this is the way
in which the energetic particles inferred in many AGN's are produced.
---------------------------------------------------------
Title: Mass transfer by tidally induced spiral shocks in an accretion
disc
Authors: Matsuda, T.; Sekino, N.; Shima, T.; Sawada, K.; Spruit, H. C.
1989ASIC..290..355M Altcode: 1989tad..conf..355M
Numerical simulations of two-dimensional adiabatic inviscid flow in
an accretion disc in a binary system or a proto-planetary system are
performed using a second-order accurate implicit Roe upwind scheme. A
purpose of the work is to estimate quantitatively the amount of
mass accretion rate due to spiral shocks produced by a less massive
component. The mass ratio of the binary system, q, is varied to see the
tidal effect of the less-massive component on the formation of spiral
shocks. Five cases, namely q=0.001, 0.01, 0.1 and 1 are examined. In
the case q &gt=0.01, steady spiral shocks persist until the end
of the calculations, i.e. about 15-20 rotation periods. If q is a
small as 0.001, i.e. the case of the Jupiter, the amplitude of the
spiral waves is not very strong. On the basis of calculated mass
accretion rate, we can determine an effective alpha parameter in the
standard accretion disc model. It depends on q, and we found that 4e-4
&ltalpha&lt0.04 for 1e-3 &ltq&lt1.
---------------------------------------------------------
Title: Spiral shocks in accretion disks: a preliminary numerical study
Authors: Rozyczka, M.; Spruit, H. C.
1989ASIC..290..341R Altcode: 1989tad..conf..341R
The response of accretion disks to tidal forces is studied numerically
with the help of a 2-D, second order hydrodynamical code. The code
is tested on analytical self-similar models of spiral shocks and
proven capable of maintaining them stationary in the grid. The disks
are assumed to reside in potential wells of primary components of
binary systems. The models are not structured perpendicularly to the
orbital plane, and only the flow in the orbital plane is studied. A
uniform inflow condition applied to the outer boundary of the disk
invariably leads to the formation of two-armed spiral waves resembling
the analytical solutions. Provided the disk approaches the Roche lobe,
fairly strong shocks are observed even for mass ratios (secondary to
primary) as small as 0.01. Stopping the inflow at the outer boundary
modifies the spiral pattern and causes the disk to shrink rapidly.
---------------------------------------------------------
Title: Physics of accretion by spiral shock waves
Authors: Spruit, H. C.
1989ASIC..290..325S Altcode: 1989tad..conf..325S
The properties of disks accreting by global spiral shocks are
reviewed. The discussion includes the formation of these waves, the way
they transport angular momentum, a selfsimilar model for spiral shocked
accretion and recent numerical simulations. These simulations suggest
that at least in disks produce by mass overflow in close binaries
the efficiency of accretion (in terms of an effective alpha-value)
can be substantial.
---------------------------------------------------------
Title: Polar observatories
Authors: Spruit, H. C.; Title, A. M.; Peterson, R. C.
1988Natur.334..466S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Particle acceleration in a flow accreting through shock waves
Authors: Spruit, H. C.
1988A&A...194..319S Altcode:
It was shown recently (Donner, 1979; Sawada et al., 1986; Spruit,
1987) that accretion with angular momentum onto a compact object is
possible by a process in which the dissipation and angular momentum
transport are due to a system of stationary shock waves. The accreting
mass spirals many times through the shocks before reaching the central
object. In this kind of shocked flow diffusive particle acceleration
works well even for low shock strengths, in contrast to the case of an
isolated shock. The process is illustrated with analytical examples for
diffusion of relativistic particles in a periodically shocked flow, and
with a numerical solution for diffusion in a detailed two-dimensional
flow model. Flat particle spectra (spectral index around 2) are found
under rather mild assumptions on the diffusion coefficient and the
probability for escape of particles from the accreting flow. It is
proposed that this process is involved in the production of energetic
particles in active galactic nuclei.
---------------------------------------------------------
Title: Influence of magnetic activity on the solar luminosity
and radius
Authors: Spruit, H. C.
1988srov.proc..254S Altcode:
Theoretical calculations of the effect of surface magnetic fields and
fields inside the convection zone on the solar radius and luminosity
are reviewed. Since the strength, extent and location of the subsurface
fields is uncertain, emphasis is on upper limits, derived from i)
general thermodynamic considerations, and ii) the buoyancy of strong
fields. In either case fields located at the base of the convection
zone do not produce luminosity or radius changes of observational
interest. Fields closer to the surface, but still embedded entiurelyt
inside the convection zone can in principle have measurable effects
through their influence on the efficiency of convective transport. The
vertical fields seen at the surface itself have effects on the
luminosity that compare well with observations. The dominating effect
in this case is a modulation of the surface emissivity (reduction in
spots, enhancements in small scale fields).
---------------------------------------------------------
Title: Spiral shocks and accretion in discs.
Authors: Spruit, H. C.; Matsuda, T.; Inoue, M.; Sawada, K.
1987MNRAS.229..517S Altcode:
Recent numerical and analytical results on disc-like accretion with
shock waves as the only dissipation mechanism are compared. The
global properties of the process are similar to those of the viscous
(α) disc model, but precise values of the effective α value as a
function of the accretion rate can be calculated. At low values of the
ratio of specific heats (γ < 1.45) accretion is possible without
radiative losses. Such adiabatic accretion can occur in practice at
high accretion rates on to low mass objects and may be important in
the formation of planets. Following Donner, and Lynden-Bell, it is
pointed out that non-axisymmetric perturbations in the outer parts of
a disc increase in amplitude as they propagate in and cause spiral
shocks more easily in a disc than perturbations originating in the
inner parts. It is suggested for this reason that the cause of spiral
structure in normal spiral galaxies lies in moderate non-axisymmetries
in their gaseous outer discs.
---------------------------------------------------------
Title: Stationary shocks in accretion disks
Authors: Spruit, H. C.
1987A&A...184..173S Altcode:
Special solutions of the equations of motion and continuity near the
midplane of a thin accretion disk are obtained by analytic and numerical
means. They have the form of stationary, self-similar flows containing
two or more spiral shaped shock waves. They represent shocks initiated
by a disturbance at the outer edge of the disk. Self-similar shocks
excited at the center of the disk do not seem to exist. No solutions
were found containing only one shock. Adiabatically accreting solutions
exist in which there is a unique relation between the ratio of specific
heats γ and the opening angle of the shock. When radiative losses
from the disk are included, solutions exist for γ above a certain
minimum value. The strength of the shock decreases with decreasing disk
thickness. An effective "α parameter" can be defined, it varies with
disk thickness like (h/r)<SUP>1.5</SUP>. An analogy between the present
problem and the case of spherically symmetric accretion is discussed.
---------------------------------------------------------
Title: What Determines the Temperature of a Sunspot?
Authors: Spruit, H. C.; Simon, G. W.
1987BAAS...19..943S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On magnetic braking of late-type stars
Authors: Mestel, L.; Spruit, H. C.
1987MNRAS.226...57M Altcode:
The paper reports on a preliminary study of the variation with angular
velocity of the rate of braking of a late-type star. The strength
of the dynamo-built field B will normally increase with rotation, so
that the surface where the wind speed becomes Alfvénic moves further
out from the star and the angular momentum transported per unit mass
is correspondingly higher. The stronger field tends to trap more gas
within a 'dead zone' which does not contribute to the braking, but
the extent of the dead zone is limited by the pressure required to
balance the higher centrifugal force. These effects are illustrated
by a simply parametrized field model. Braking rates are estimated
for the two cases with the coronal base density independent of B and
proportional to B, respectively.
---------------------------------------------------------
Title: Is there a weak mixed polarity background field? Theoretical
arguments
Authors: Spruit, H. C.; Title, A. M.; van Ballegooijen, A. A.
1987SoPh..110..115S Altcode:
A number of processes associated with the formation of active regions
produce `U-loops': fluxtubes having two ends at the photosphere but
otherwise still embedded in the convection zone. The mass trapped on the
field lines of such loops makes them behave in a qualitative different
way from the `omega-loops' that form active regions. It is shown that
U-loops will disperse though the convection zone and form a weak (down
to a few gauss) field that covers a significant fraction of the solar
surface. This field is tentatively identified with the inner-network
fields observed at Kitt Peak and Big Bear. The process by which these
fields escape through the surface is described; a remarkable property is
that it can make active regions fields apparently disappear in situ. The
mixed polarity moving magnetic features near sunspots are interpreted
as a locally intense form of this disappearance by escape of U-loops.
---------------------------------------------------------
Title: Is there a weak mixed polarity background field? Theoretical
arguments.
Authors: Spruit, H. C.; Title, A. M.; van Ballegooijen, A. A.
1987MPARp.271.....S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: How is the Penumbra formed?
Authors: Spruit, H. C.
1987rfsm.conf..199S Altcode:
Simple theoretical arguments are reviewed that restrict the directions
in which the answer should be sought. They make plausible that the field
of the penumbra is in quasistatic equilibrium except with respect to
motions along the field, that penumbral filaments are unlikely to be
elevated with respect to the photosphere or their bright neighbors,
that the boundary between the field and the underlying convection
zone is at a shallow depth of the order of a pressure scale height
and that convective processes in the penumbra take the form of flows
nearly parallel to the field rather than Danielson's (1961) rolls. Why
there should be a penumbra at all, i.e. why a spot does not look like
a giant pore, seems to be a more difficult question.
---------------------------------------------------------
Title: Angular momentum transport in the radiative interior of
the sun.
Authors: Spruit, H. C.
1987MPARp.273.....S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: How is the penumbra formed?
Authors: Spruit, H. C.
1987MPARp.274.....S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Angular Momentum Transport in the Radiative Interior of the Sun
Authors: Spruit, H. C.
1987ASSL..137..185S Altcode: 1987isav.symp..185S
The known mechanisms for transporting angular momentum in the core of
the Sun are reviewed. All hydrodynamic mechanisms are insufficient to
account for the present low rotation rate with the possible exception
of internal gravity waves. Several mechanisms are likely to be far too
effective in depleting lithium, however. It is argued that weak magnetic
fields can easily solve both problems. Their stability properties
are reviewed and a scenario is presented for the evolution of such
fields. It predicts that the present field in the Sun has poloidal
and toroidal components of similar strengths, on the order of a few
Gauss. The differential rotation in the core, if due to such a field
alone, would be extremely small.
---------------------------------------------------------
Title: Mixing and transport of angular momentum in the solar interior.
Authors: Spruit, H. C.
1987ppcs.work...78S Altcode:
The observational indicators of mixing in the solar radiative interior,
and the known mechanisms for mixing are reviewed critically. It is
concluded that the three most important indicators, Li-depletion, the
neutrino discrepancy and the internal rotation, probably each have a
different, rather than a common origin. The low internal rotation is
interpreted as a strong evidence for the presence of a weak magnetic
field in the core of the sun.
---------------------------------------------------------
Title: Spiral shocks and accretion in disks.
Authors: Spruit, H. C.; Matsuda, T.; Inoue, M.; Sawada, K.
1987MPARp.301.....S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Particle acceleration in a flow accreting through shock waves.
Authors: Spruit, H. C.
1987MPARp.304.....S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Colors and luminosities of stars with spots.
Authors: Spruit, H. C.; Weiss, A.
1986A&A...166..167S Altcode:
Theoretical zero-age main sequence models from 0.2 to 1.9 solar masses
have been made for stars with large sunspot-like spots, and their black
body UBVRIJK colors calculated. The effect of spots is different on
long and short time scales (compared with the thermal time scale of
the convection zone) and differs between mainly convective and mainly
radiative stars. During a short term increase in spot area the stars
always become redder in all color indices, but the long term change
can be either to the blue or the red, depending on stellar mass
and the color index used. The anomalies in color diagrams predicted
for Hyades stars are much smaller than those discussed by Campbell
(1984), unless a spot coverage factor close to unity is assumed. It
is suggested that these color anomalies may reflect the brightening
in short wavelength bands due to chromospheric emission rather than
the effect of spots themselves.
---------------------------------------------------------
Title: Baroclinic waves in a vertically stratified thin accretion disk
Authors: Knobloch, E.; Spruit, H. C.
1986A&A...166..359K Altcode:
The conditions for steady equilibrium and energy transport in general
require a thin accretion disk to be baroclinic, i.e. the rotation speed
to vary with height in the disk. A necessary condition for baroclinic
instability is derived which is analogous to that encountered in
geophysics. The instabilty can occur only if the radial scale length is
locally comparable to the vertical scale height H, or if the vertical
stratification is close to adiabatic. The maximum level of turbulence
expected is limited by the dominant effect of the shear in the Kepler
flow. The role of the thermal wind and of meridional circulations in
establishing the mean state is elucidated. Attention is drawn to the
role of buoyancy resonances which may play an important role in many
wave phenomena in disks.
---------------------------------------------------------
Title: Energy transport in sunspot penumbrae.
Authors: Schmidt, H. U.; Spruit, H. C.; Weiss, N. O.
1986A&A...158..351S Altcode:
It is proposed that the magnetic field in the outer penumbra of a
sunspot is almost horizontal and that the penumbra itself is very
shallow, with a sharp lower boundary. A simple model of energy transport
in the outer penumbra predicts that there is a Wilson depression
of about 100 km, below which the penumbra extends for only 80 km. A
two-component model with bright and dark filaments suggests that the
associated differences in observed field strength and corrugations
of the visible surface of the penumbra will be small. The authors
argue that flows along the field are needed to explain the existence
of bright and dark filaments while the Evershed flow is driven by
pressure differences along flux tubes.
---------------------------------------------------------
Title: Stationary shocks in accretion disks.
Authors: Spruit, H. C.
1986MPARp.232.....S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Colors and luminosities of stars with spots.
Authors: Spruit, H. C.; Weiss, A.
1986MPARp.225.....S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Baroclinic waves in a vertically stratified thin accretion
disk.
Authors: Knobloch, E.; Spruit, H. C.
1986MPARp.217.....K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: No Title Provided
Authors: Spruit, H. C.
1986rfsm.proc..199S Altcode:
Instead of answering the question posed by the title, simple theoretical
arguments are revuewd that restrict the directions in which the answer
should be sought. They make plausible that the field of the penumbra
is in quasistatic equilibrium except to motions along the field, that
the penumbral filaments are unlikely to be elevated with respect to
the photosphere or their bright neighbors, that the boundary between
the field and the underlying convection zone is at a shallow depth
of the order of a scale height and that convective processes in the
penumbra take the form of flows nearly parallel to the field rather
than Danielson's (1961) rolls. Why there should be a penumbra at all,
i.e. why a spot does not look like a giant pore, seems to be a more
difficult question.
---------------------------------------------------------
Title: Observational Signature of Tube Waves
Authors: Spruit, H. C.
1985tphr.conf..158S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Propagation of nonlinear, radiatively damped longitudinal
waves along magnetic flux tubes in the solar atmosphere
Authors: Herbold, G.; Ulmschneider, P.; Spruit, H. C.; Rosner, R.
1985A&A...145..157H Altcode:
For solar magnetic flux tubes the authors compare three types of
waves: longitudinal MHD tube waves, acoustic tube waves propagating
in the same tube geometry but with rigid walls and ordinary acoustic
waves in plane geometry. They find that the effect of distensibility
of the tube is small and that longitudinal waves are essentially
acoustic tube waves. Due to the tube geometry there is considerable
difference between longitudinal waves or acoustic tube waves and
ordinary acoustic waves. Longitudinal waves as well as acoustic tube
waves show a smaller amplitude growth, larger shock formation heights,
smaller mean chromospheric temperature but a steeper dependence of
the temperature gradient on wave period.
---------------------------------------------------------
Title: Baroclinic instability in the presence of a strong horizontal
shear
Authors: Knobloch, E.; Spruit, H. C.
1985GApFD..32..197K Altcode:
The theory of baroclinic instability is extended to include strong
horizontal shears. The f-plane and anelastic approximations are used,
but the assumption that the Rossby number is small is relaxed. Although
this problem is nongeostrophic, much of the standard theory, including
necessary conditions for instability and a semicircle theorem,
generalize readily for disturbances of low zonal wavenumber.
---------------------------------------------------------
Title: Energy transport in sunspot penumbrae.
Authors: Schmidt, H. U.; Spruit, H. C.; Weiss, N. O.
1985MPARp.182.....S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Theorie des solaren Magnetfeldes.
Authors: Spruit, H. C.
1985Sonne...9....6S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Observational signature of tube waves.
Authors: Spruit, H. C.
1985MPARp.212..158S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Mixing in the solar interior.
Authors: Spruit, H. C.
1984ESASP.220...21S Altcode: 1984ESPM....4...21S
It seems appropriate to study in detail which mixing processes can
occur in a star, and why the effective diffusivity resulting from
these processes should be a weakly varying multiple of the kinematic
viscosity. The author reviews the extent to which these questions can
be answered at present.
---------------------------------------------------------
Title: Baroclinic instability in stars
Authors: Spruit, H. C.; Knobloch, E.
1984A&A...132...89S Altcode:
The conditions for the occurrence of adiabatic baroclinic instability in
differentially rotating stars are studied using the quasi-geostrophic
approximation on a β-plane. In rapidly rotating stars (near breakup)
the instability can occur throughout the star. In slowly rotating stars,
it can only occur in very narrow regions located near (1) the center
of the star, (2) the boundary of a convection zone, (3) a jump in the
molecular weight or in the buoyancy frequency, or (4) a jump in the
gradient of the rotation rate. It is concluded that the instability
cannot be responsible for the mixing needed to explain the Li abundance
anomalies in main sequence stars or the low neutrino flux of the Sun. It
could however, be important for mixing near the cores of red giants.
---------------------------------------------------------
Title: Evolution of the Sun with Mixing by Hydrodynamic Instabilities
Authors: Law, W. Y.; Knobloch, E.; Spruit, H. C.
1984IAUS..105..523L Altcode:
Following Schatzman and Maeder (1981) the authors compute the evolution
of the sun with partial mixing by hydrodynamic instabilities. Instead of
simply assuming a turbulent diffusion coefficient which is a constant
multiple of the viscosity, they incorporate some of the properties of
hydrodynamic instabilities. This puts limits on the amount of diffusion
that can be obtained, and makes it dependent on time and position in
the star.
---------------------------------------------------------
Title: Interaction of Fluxtubes With Convection (Keynote)
Authors: Spruit, H. C.
1984ssdp.conf..249S Altcode:
The author discusses the interaction of flux tubes with convection from
two points of view: (1) the formation of flux tubes by interaction
of a weak field with convection and (2) the interaction of already
established flux tubes with the convective flow. The first part
describes the current understanding of the processes that produce
flux tubes of the observed high field strength. The second gives a
classification of the types of motion that a (thin) flux tube can
execute, the ways in which these can be excited by granulation,
and describes a model for the propagation of these motions into the
higher atmosphere.
---------------------------------------------------------
Title: No Title Provided
Authors: Spruit, H. C.
1984ssdp.proc..249S Altcode:
In this contribution I discuss the interaction of flux tubes with
convection from two points of view: i) the formation of flux tubes by
interaction of a weak field with convection and ii) the interaction of
already established flux tubes with the convective flow. The first part
describes current understanding of the processes that produce flux tubes
of the observed high field strength. The second gives a classification
to the types of motion that a (thin) flux tube can execute, the way
in which these can be excited by granulation, and describes a model
for the propagation of these motions into the higher atmosphere.
---------------------------------------------------------
Title: Baroclinic instability in the presence of a strong horizontal
shear. I. Instability conditions.
Authors: Knobloch, E.; Spruit, H. C.
1984MPARp.170.....K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Mixing in the solar interior.
Authors: Spruit, H. C.
1984MPARp.161.....S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Stellar activity and the period gap in cataclysmic variables
Authors: Spruit, H. C.; Ritter, H.
1983A&A...124..267S Altcode:
The evolution of a cataclysmic binary is computed under the assumption
of angular momentum loss due primarily to a magnetic wind from the
secondary star, in order to investigate the possible relation between
the period gap in cataclysmic variables and the sudden decrease in
magnetic activity observed in main sequence stars at B-V values of about
1.65. A decrease in the angular momentum loss rate at the point where
the secondary becomes fully convective can temporarily switch off mass
transfer. A period gap of the correct width and location is obtained if
the decrease is rapid and large enough, and in addition if the initial
relative angular momentum loss rate lies between about 4 and 7 x 10 to
the -9th/year. The calculation is based on homologous stellar models
calibrated with the Grossman et al. (1974) main sequence. Attention
is given to the influence of star spots, by way of their effect on
the radius of the secondary star.
---------------------------------------------------------
Title: Magnetic flux tubes on the Sun
Authors: Spruit, H. C.; Roberts, B.
1983Natur.304..401S Altcode:
Magnetic fields are the cause of almost all forms of solar
activity. Near the solar surface, and possibly in the entire convection
zone, these fields occur in the form of isolated flux tubes. In
recent years, new views have been developed (and older ones revived)
in which this property plays a central role. Here we review these
ideas, dealing with the nature of the solar cycle, sunspot structure,
the origin of spicules and the source of mechanical heating in the
solar atmosphere. The ideas are illustrated with the aid of a simple
mathematical model for the behaviour of thin magnetic flux tubes. The
properties of inhomogeneities in the corona (coronal loops) are also
discussed.
---------------------------------------------------------
Title: The molecular weight barrier and angular momentum transport
in radiative stellar interiors.
Authors: Knobloch, E.; Spruit, H. C.
1983A&A...125...59K Altcode:
The stability of a differentially rotating fluid with respect
to axisymmetric instabilities is investigated in the presence of
finite viscosity (nu), thermal diffusion (kappa-T) and a gradient
of chemical composition, of diffusivity kappa-S. The results confirm
that the Goldreich-Schubert-Fricke instability is suppressed already
at very small stabilizing composition gradients of the order N(S)
approximately Omega, where N(S) is the buoyancy frequency due to
the chemical gradient and Omega the rotation rate. This excludes the
GSF instability as a mixing agent in chemically inhomogeneous stellar
interiors. However, two other modes of instability exist, one of which
is stabilized only by much stronger composition gradients of the order
N(S) approximately N(T) where N(T) is the thermal buoyancy frequency. It
is an unstable inertial oscillation, operating on the baroclinicity of
the stratification. It is argued that this instability might produce
significant mixing in radiative stellar interiors. The other instability
is triply diffusive in nature and is probably not very important since
like the GSF instability it is limited to composition gradients with
N(S) approximately equal to or less than Omega.
---------------------------------------------------------
Title: Internal rotation of the Sun
Authors: Spruit, H. C.; Knobloch, E.; Roxburgh, I. W.
1983Natur.304..520S Altcode:
The low surface rotation rate of the Sun and other main-sequence
stars is believed to be the result of angular momentum loss due to
a stellar wind<SUP>1</SUP>. This loss also leads to a differential
rotation, the interior spinning more rapidly than the surface. The
rate of increase with depth of the rotation speed is limited by
hydrodynamic instabilities, which cause an outward diffusion of
angular momentum<SUP>2</SUP>. The conditions for the occurrence of
hydrodynamic instabilities in a radiative stellar interior are reviewed
here assuming that the rotation is constant on spheres. The instability
with the lowest threshold is a double diffusive one, the axisymmetric
baroclinic diffusive (ABCD) instability. A minimum rotation curve for
the present Sun is calculated using the assumption that the efficiency
of this instability is sufficiently high that the rotation of the
Sun is close to marginal stability. This lower limit to the internal
rotation rate is roughly consistent with present observations of the
rotational splitting of solar oscillations.
---------------------------------------------------------
Title: The Sun's Magnetic Field. (Book Reviews: Solar
Magnetohydrodynamics)
Authors: Spruit, H. C.
1983Sci...220..191S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Sun's Magnetic Field. (Book Reviews: Solar
Magnetohydrodynamics)
Authors: Spruit, H. C.
1983Sci...220..191P Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Theory of photospheric magnetic fields
Authors: Spruit, H. C.
1983IAUS..102...41S Altcode:
The magnetic fields at the solar (stellar) surface are discussed from
a theoretical point of view. A description of the general structure of
the magnetic field near the photosphere is presented, and the formation
of flux tubes is discussed. The characteristics of flux tubes in a
stratified atmosphere are examined, taking into account the stability
of a tube and convective collapse, aspects of buoyancy, and the energy
balance of tubes. Attention is given to the wave modes of a cylindrical
flux tube of finite diameter, nonlinear tube waves, the energy transport
from the photosphere to higher layers, the 'missing flux' problem, the
eruption of new flux, and the distribution of fields over the surface.
---------------------------------------------------------
Title: Magnetic Activity of Stars: Theoretical Aspects
Authors: Spruit, H. C.
1983MitAG..60...83S Altcode:
Theories of the magnetic activity of stars derived from studies of solar
activity are reviewed. The structure of the magnetic field of a star,
the formation of flux tubes and the strength of the small scale field,
magnetic heating of stellar atmospheres, dynamo action, and star spots
are discussed. Extensive references are provided.
---------------------------------------------------------
Title: Erratum - Stability of Toroidal Flux Tubes in Stars
Authors: Spruit, H. C.; van Vallegooijen, A. A.
1982A&A...113..350S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Stability of differential rotation in stars
Authors: Knobloch, E.; Spruit, H. C.
1982A&A...113..261K Altcode:
Consideration of the conditions for the occurrence of hydrodynamic
instabilities in differentially rotating, stably stratified stars leads
to the suggestion that under normal stellar conditions, buoyancy is
strong enough to suppress baroclinic instabilities. Shear instability
is important for angular momentum transport across constant pressure
surfaces in rapidly rotating stars, as well as the thin shear layers
of slow rotators. The evolution of the angular momentum distribution
through the Golreich-Schubert-Fricke instability is sketched for the
case of a star which is slowly spun down by a stellar wind torque. The
evolution is toward a state in which angular momentum is nearly constant
on equipotentials.
---------------------------------------------------------
Title: Effect of spots on a star's radius and luminosity
Authors: Spruit, H. C.
1982A&A...108..348S Altcode:
The effects of the sudden appearance of spots on a star's radius and
luminosity are calculated. Variations in luminosity associated with
the birth and disappearance of spots and with the activity cycle
are studied on time scales much less than the Kelvin-Helmholtz time
scale, and luminosity changes are calculated for a simple model of the
distribution of spots on the stellar surface. An analytic model for
the stratification of the convection envelope is used to show how the
diffusive time scale and the Kelvin-Helmholtz time scale follow from an
energy equation, and how they relate to each other. Changes in surface
luminosity and radius due to changes in efficiency of convection in
the envelope are also calculated, and radius changes, in particular,
are very small.
---------------------------------------------------------
Title: The flow of heat near a starspot
Authors: Spruit, H. C.
1982A&A...108..356S Altcode:
The thermal disturbance due to a single starspot of radius R and depth
d in a polytropic convective envelope is calculated using a turbulent
diffusion approximation. The effect of systematic fluid flows is
neglected. Times large compared with the turbulent diffusion time scale
and short compared with the Kelvin-Helmholtz time scale are considered
so that the heat flow is quasi-static. Of the heat flux blocked by the
spot, a fraction is stored in the deeper layers of the envelope and the
remaining fraction reappears at the surface. It is concluded that most
of the blocked heat flux is stored in the convection zone, in agreement
with recent observations of the solar luminosity variation. The results
imply that the light variations (in the visual band) in spotted stars
can be interpreted directly in terms of the fraction of the projected
surface area covered by spots.
---------------------------------------------------------
Title: Stability of toroidal flux tubes in stars
Authors: Spruit, H. C.; van Ballegooijen, A. A.
1982A&A...106...58S Altcode:
A thin tube approximation is used to study the stability of a magnetic
flux tube in the equatorial plane of a star, where (1) only adiabatic
disturbances are considered, (2) the tubes are unstable to poleward
motion and to (3) poleward motions within the equatorial plane, if
the superadiabaticity of the stratification is large enough. While the
curvature of the tube in a spherical geometry has a stabilizing effect,
it is not strong enough to stabilize flux tubes in the convective
envelopes of main sequence stars. The longer wavelengths are favored
by the instability and, for the case of the sun, modes m equals 0-4
are unstable. It is suggested in view of this instability that toroidal
fields, in a stellar dynamo, occur at the interface between convection
zone and radiative interior rather than within the convection zone.
---------------------------------------------------------
Title: Propagation Speeds and Acoustic Damping of Waves in Magnetic
Flux Tubes
Authors: Spruit, H. C.
1982SoPh...75....3S Altcode:
Propagation speeds are derived for the wave modes of a thin magnetic
tube in an otherwise homogeneous magnetized or unmagnetized fluid. These
results generalize results obtained by previous authors. There are
three types of wave, a (torsional) Alfvén wave and two waves which
are specific for the thin tube. These are named the longitudinal and
transversal tube waves, according to their polarization properties. They
can be camped by radiating an MHD or acoustic wave into the surroundings
of the tube. Conditions for occurrence of this acoustic damping, and
the damping rates, are derived. The behavior of the waves in the solar
convection zone and corona is discussed.
---------------------------------------------------------
Title: Magnetohydrodynamics of sunspots
Authors: Spruit, H. C.
1981SSRv...28..435S Altcode:
Current theories are reviewed concerning the pressure equilibrium and
thermal balance of sunspots, their hydromagnetic stability, energy
transport mechanisms, and propagation of waves.
---------------------------------------------------------
Title: Equations for thin flux tubes in ideal MHD
Authors: Spruit, H. C.
1981A&A...102..129S Altcode:
A Hamilton-Lagrange formalism is used to derive the equation of motion
for an isolated magnetic flux tube embedded in a static gravitating
fluid. An equation of motion is derived from the Lagrangian density, and
a procedure analogous to that in Berstein et al. (1958) is used to show
that a system of non-interacting flux tubes has a self-adjoint force
operator so that the system is either stable or monotonically unstable.
---------------------------------------------------------
Title: Magnetic flux tubes.
Authors: Spruit, H. C.
1981NASSP.450..385S Altcode: 1981suas.nasa..385S
The magnetohydrodynamics of flux tubes are considered. The sections on
equilibrium of flux tubes, and stability and waves deal with sunspots,
the largest members of the general class of photospheric flux tubes.
---------------------------------------------------------
Title: Motion of magnetic flux tubes in the solar convection zone
and chromosphere.
Authors: Spruit, H. C.
1981A&A....98..155S Altcode:
General equations of motion are derived for a thin untwisted magnetic
flux tube embedded in a non-magnetic compressible fluid, in the presence
of gravity. Special equations governing small perturbations in purely
horizontal or vertical flux tubes are derived from these. There is in
general a longitudinal and a transversal wave mode. The transversal
wave in a vertical tube is studied in detail. This wave propagates
under the combined influences of buoyancy and magnetic tension along
the tube. In an exponential atmosphere it has a cutoff frequency which
is at least twice as low as the acoustic cutoff frequency. It can
transport a significant amount of energy from the convection zone to
the chromosphere. The transversal motions seen in the magnetic field
of the chromosphere are interpreted as transversal tube waves generated
in the convection zone.
---------------------------------------------------------
Title: The Size Dependence of Contrasts and Numbers of Small Magnetic
Flux Tubes in an Active Region
Authors: Spruit, H. C.; Zwaan, C.
1981SoPh...70..207S Altcode:
Intensity contrasts and number densities of bright points, knots and
pores ranging in size between 0″.15 and 4″ are studied using high
resolution pictures in Mg b<SUB>1</SUB> of a young active region. On the
average, the contrast in the wing of the line increases very strongly
with decreasing size, while the continuum contrast increases more
slowly. The ratio of contrast in the line to contrast in the continuum
increases rapidly with decreasing size. The possibility is explored of
using this contrast ratio as an indicator of size. The distribution
of the contrast ratio in a part of the active region is used in this
way to derive a size distribution of facular points. The resulting
distribution has a limited accuracy, but is free from systematic
distortion due to selection effects. Validity checks on the method are
presented. We measure the size distribution of the pores in the same
area, and combine the result with that for the facular points. The
combined distribution shows that the surface area covered by magnetic
elements with diameter δ has a maximum near δ = 0″.8. It increases
roughly proportional to δ for δ < 0″.3 and falls off as δ for
δ > 1″.5. It is inferred that elements with 0″.5 < δ <
1″.6, which show no conspicuous contrast in the line wing or in the
continuum, occupy as much area as the pores, and twice as much as the
facular points. We suggest that the changing appearance of a facular
area with increasing height of formation reflects at least as much
the increasing weight of the small elements in the contrast as a real
change in intrinsic properties (such as the diameter) of individual
elements. A spatial resolution better than 0″.1 may be needed to
resolve the individual elements in plages and the chromospheric network.
---------------------------------------------------------
Title: Magnetohydrodynamics of thin flux tubes.
Authors: Spruit, H. C.
1981ASIC...68..289S Altcode: 1981spss.conf..289S
In this contribution a formalism is presented for studying the
magnetohydrodynamics of a magnetic field consisting of isolated flux
tubes. Applications are given for the following problems: (a) the value
of the field strength in magnetic elements at the stellar surface, (b)
the transfer of energy from the convection zone to the chromosphere
via the magnetic field, and (c) the instability of toroidal magnetic
fields in a convection zone and its effect on the dynamo process.
---------------------------------------------------------
Title: A cluster model for sunspots
Authors: Spruit, H. C.
1981phss.conf...98S Altcode:
A flux tube model for sunspots is presented, including a critical review
of its deficiencies and an analysis of the points of accuracy. The
umbral field is assumed to be divided into thin flux tubes with
diameters around 500 km, beginning some distance below the umbral
photosphere. The tube model was chosen from evidence that sunspots are
an agglomeration of smaller elements. The stability of the flux tubes is
examined qualitatively, suggesting that they are rooted in a field of
10,000 gauss, thereby being kept stable by the field lines. The tubes
are modeled as having a fixed field strength at the sunspot cluster,
and buoyancy keeps them clustered. Their repellant force is quantified
as that between two half-monopoles, and conditions are defined for
buoyancy overcoming the repulsion in order to confine the tube tops as
a spot. Suggestions are offered for further development of the model
to accurately describe the field strength variability across the spot,
the merging depth, and the right field strength.
---------------------------------------------------------
Title: Small scale phenomena in umbras and penumbras - The role of
convective processes
Authors: Spruit, H. C.
1981phss.conf..359S Altcode:
Small scale phenomena in sunspots that are related to convection are
examined. Topics discussed include umbral dots and umbral granulations,
umbral flashes, small scale motions, mechanisms of energy transport in
the umbra, the structure and dynamics of penumbral filaments, and the
Evershed flow. For all of these topics, the available observational data
are compared with the various theoretical ideas that have been advanced.
---------------------------------------------------------
Title: Effect of spots on a star's radius and luminosity
Authors: Spruit, H. C.
1981phss.conf..480S Altcode:
The effects of sunspot size variations on the temperature fields
of the unspotted surface fraction and the solar luminosity are
examined. Thermal relaxation of a convective envelope is discussed
in a polytropic model, showing that a small temperature fluctuation
is a sufficient force to drive a large-scale flux disturbance. The
Kelvin-Helmholtz time scale is demonstrated to be relevant only in
the convection zone and not the entire sun. The heat flux disturbances
caused by many small sunspots or a single sunspot are quantified, taking
into account the increase in thermal conductivity with depth below the
surface. The single spot is assumed to have a zero heat conductivity,
and changes in the heat flux are calculated in terms of changes in
the spot's size. It is shown that changes in the size of sunspots
do not affect the stellar radius, and that luminosity variations are
proportional to the size of the surface which is blocked by sunspots.
---------------------------------------------------------
Title: A Formalism for Differential Rotation
Authors: Durney, B. R.; Spruit, H. C.
1980HiA.....5..121D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On the Dynamics of the Solar Convection Zone
Authors: Durney, B. R.; Spruit, H. C.
1980LNP...114...15D Altcode: 1980IAUCo..51...15D; 1980sttu.coll...15D
No abstract at ADS
---------------------------------------------------------
Title: On the dynamics of stellar convection zones - The effect of
rotation on the turbulent viscosity and conductivity
Authors: Durney, B. R.; Spruit, H. C.
1979ApJ...234.1067D Altcode:
We derive expressions for the turbulent viscosity and turbulent
conductivity applicable to convection zones of rotating stars. We assume
that the relative dimensions of the dominant convective cell are known
and derive a simple distribution function for the turbulent convective
velocities under the influence of rotation. From this distribution
function (which includes, in particular, the stabilizing effect of
rotation on convection) we calculate in the mixing-length approximation:
(i) the turbulent Reynolds stress tensor and (ii) the expression for the
heat flux in terms of the superadiabatic gradient. The contributions
of the turbulent convective motions to the mean momentum and energy
equation (which determine the large-scale motions in stellar convection
zones) are treated consistently, and assumptions about the turbulent
viscosity and heat transport are replaced by assumptions about the
turbulent flow itself. The free parameters in our formalism are the
relative cell dimensions and their dependence on depth and latitude.
---------------------------------------------------------
Title: Convective instability of thin flux tubes.
Authors: Spruit, H. C.; Zweibel, E. G.
1979SoPh...62...15S Altcode:
The stability of magnetic flux tubes embedded vertically in a
convection zone is investigated. For thin tubes, the dominant
instability is of the convective type, i.e. it is driven by buoyancy
forces associated with displacements along the tube. The stability
is determined by β = 8πP/B<SUP>2</SUP>; if β ≤ β<SUB>c</SUB>
the tube is convectively stable, otherwise it is unstable, where the
critical value β<SUB>c</SUB> depends on the stratification of the
convection zone. For a solar convection zone model, β<SUB>c</SUB>
= 1.83, corresponding to a magnetic field strength of 1350 G at the
surface of the Sun. It is concluded that the flux tubes making up the
small scale field of the Sun are probably hydrodynamically stable.
---------------------------------------------------------
Title: Convective collapse of flux tubes.
Authors: Spruit, H. C.
1979SoPh...61..363S Altcode:
Flux tubes of constant β extending vertically through the solar
convection zone are unstable to a convective instability if the surface
field strength is less than 1270 G. By downward displacement of matter
along the tube an unstable tube can transform into a new equilibrium
state with lower energy which has a higher field strength. Numerical
calculations of these `collapsed' states are presented. If the collapse
starts in a field with a strength corresponding to equipartition
with kinetic energy in the convection zone, it yields a surface field
strength of about 1650 G. It is proposed that the small scale magnetic
field in active regions consists of such tubes. The collapsed state
is not in thermal equilibrium. In the deeper layers the heat exchange
following the collapse is very slow but the surface layers return
rapidly to temperature equilibrium. It is argued that during the gradual
thermal evolution of the collapsed state its surface layers may start
an overstable oscillation. A brightness-velocity correlation in this
oscillation could account for the observed downdraft in the tubes.
---------------------------------------------------------
Title: Convection instability of flux tubes in the solar photosphere.
Authors: Spruit, H. C.
1978BAAS...10Q.729S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Heat flow near obstacles in the solar convection zone.
Authors: Spruit, H. C.
1977SoPh...55....3S Altcode:
Disturbances in the heat flow in the solar convection zone are
calculated with a turbulent thermal diffusion coefficient based on
a mixing length approximation. As a consequence of the radiative
boundary condition at the surface and the strong increase of the
diffusion coefficient with depth, the convection zone resembles a
thermally superconducting shell enclosed between a thin surface layer
and an interior core of low thermal conductivity. Thermal disturbances
originating in the convection zone do not penetrate into the interior,
and penetrate only weakly through the solar surface. A thermally
isolating obstacle buried entirely in the convection zone casts a
`shadow' of reduced temperature at the solar surface; the brightening
surrounding this shadow is undetectable. The shadow is weak unless the
object is located close to the surface (less than 2000 km). Assuming
a sunspot to be an area of reduced thermal conductivity which extends
a finite depth into the convection zone, the heat flow around this
obstacle is calculated. The heat flux blocked below the spot (`missing
flux') spreads over a very extended area surrounding the spot. The
brightening corresponding to this `missing flux' is undetectable if
the reduction of the thermal conductivity extends to a depth greater
than 1000 km. It is concluded that no effect other than a decrease
of the convective efficiency is needed to explain the temperature
change observed at the solar surface in and around a sunspot. The
energy balance is calculated between magnetic flux tubes, oriented
vertically in the solar surface, (magnetic elements in active regions
and the quiet network) and their surroundings. Near the visible surface
radiation enters the tube laterally from the surrounding convection
zone. The heating effect of this influx is important for small tubes
(less than a few arcseconds). Due to this influx tubes less than about
1″ in diameter can appear as bright structures irrespective of the
amount of heat conveyed along the tube itself. Through the lateral
influx, small tubes such as are found in the quiet network act as little
`leaks' in the solar surface through which an excess heat flux escapes
from the convection zone.
---------------------------------------------------------
Title: Magnetic flux tubes and transport of heat in the convection
zone of the Sun.
Authors: Spruit, H. C.
1977PhDT.......237S Altcode:
This thesis consists of five papers dealing with the transport of heat
in the solar convection zone on te one hand, and with the structure
of magnetic flux tubes in the top of the convection zone on the other
hand. Included is a table of the solar convection zone (improved and
more detailed version of the model published in Solar physics 34 p.270).
---------------------------------------------------------
Title: Appearance at the solar surface of disturbances in the heat
flow associated with differential rotation.
Authors: Spruit, H. C.
1977A&A....55..151S Altcode:
Surnrnary It is shown that the variation of the solar surface flux
due to large disturbances in the deeper layers is usually very small
(depending on the horizontal scale and on the depth of origin). This
is due to the high efficiency of turbulent diffusion of heat in the
deeper layers of the convection zone, and to the behaviour of the
solar surface, which acts as an isolating layer with respect to flux
disturbances. It is suggested that the observed homogeneity of the
surface flux is a much less severe constraint on theories of solar
rotation than has been assumed. The large disturbances in the heat flux
in the deeper layers of the convection zone found in these theories are
compatible with observed fluxes, if their horizontal length scale is
less than a solar radius. Stars with efficient convective envelopes
will show a negligible variation of the heat flux at their surface
unless the variation is due to a disturbance just below the surface,
or is due to a strong magnetic field. Key words: Differential rotation,
turbulent diffusion, convection zones.
---------------------------------------------------------
Title: Small magnetostatic flux tubes.
Authors: Spruit, H. C.
1977IAUS...62..265S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Magnetic flux tubes and transport of heat in the convection
zone of the sun.
Authors: Spruit, H. C.
1977mftt.book.....S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Pressure equilibrium and energy balance of small photospheric
fluxtubes.
Authors: Spruit, H. C.
1976SoPh...50..269S Altcode:
Field configurations and temperature distributions of axially symmetric
fluxtubes are determined on the basis of pressure equilibrium and energy
balance of the tubes. The description concentrates on layers below
≈ 600 km above the photosphere; a magnetostatic field, and energy
transport by a diffusion process are assumed. It is assumed also that
the magnetic field of the tubes prevents convective flow across the
field lines, so that only radiative energy exchange between the tube
and the convection zone is present. A set of model tubes is presented
ranging in size from facular points (150 km) to small pores (1000 km),
for different values of the field amplitude and the asymptotic energy
flux F<SUB>0</SUB> flowing along the tube from the deeper layers. Radial
influx of heat into the tube at the photospheric level influences the
temperature in the tube strongly for all these models. For a pore-like
tube f<SUB>0</SUB> = 0.25 (similar to the flux from a spot umbra) seems
appropriate (F<SUB>0</SUB> in units of the normal photospheric flux). If
in the smallest fluxtubes F<SUB>0</SUB> is also 0.25, a comparison of
the intensity contrast with observations of facular points indicates
that the radius of tubes corresponding to facular points is 50-100
km. In the continuum the structure looks like a depression in the
photosphere (similar to the Wilson depression of spots). The magnitude
of this depression is ≈ 200 km for pores of 1000 km diameter and
≈ 100 km for facular points. The walls of the hole created by the
depression contribute considerably to the contrast of structures
observed near the solar limb. It is shown how this contribution may
explain the centre to limb behaviour of facular contrast as seen in
the continuum, and why the continuum CLV differs so strongly from that
in line cores. Over the first 400 km above the photosphere the tube
expands by a factor of ≈ 2 for all the tubes calculated.
---------------------------------------------------------
Title: A model of the solar convection zone
Authors: Spruit, H. C.
1974SoPh...34..277S Altcode:
A model of the convection zone is presented which matches an empirical
model atmosphere (HSRA) and an interior model. A mixing length
formalism containing four adjustable parameters is used. Thermodynamical
considerations provide limits on two of these parameters. The average
temperature-pressure relation depends on two or three combinations of
the four parameters. Observational information on the structure of the
outermost layers of the convection zone, and the value of the solar
radius limit the range of possible parameter combinations. It is shown
that in spite of the remaining freedom of choice of the parameters,
the mean temperature-pressure relation is fixed well by these data.
---------------------------------------------------------
Title: Magnetische velden en convectie in de zon.
Authors: Spruit, H. C.; Zwaan, C.
1974NTNA...40.....S Altcode:
No abstract at ADS
