explanation blue bibcodes open ADS page with paths to full text
Author name code: shu
ADS astronomy entries on 2022-09-14
author:Shu, Frank H.
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Title: The impact of new estimates of models of stellar motion from
VLBI on the alignment of the optically bright Gaia frame to ICRF3
Authors: Lunz, S.; Anderson, J.; Xu, M.; Heinkelmann, R.; Titov, O.;
Lestrade, J. F.; Johnson, M. C.; Shu, F.; Chen, W.; Melnikov, A.;
McCallum, J.; Lopez, Y.; Mikhailov, A.; Abad, P. de Vicente; Schuh, H.
2022evlb.confE..32L Altcode: 2022PoS...399E..32L
No abstract at ADS
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Title: A superconductor filter installed in the broadband feed of
Ishioka VLBI station
Authors: Takagi, Y.; Ueshiba, H.; Nakakuki, T.; Matsumoto, S.; Hayashi,
K.; Yutsudo, T.; Mori, K.; Kobayashi, T.; Sekido, M.; McCallum, J.;
Shu, F.
2021evga.conf...24T Altcode:
No abstract at ADS
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Title: Improving the S/X Celestial Reference Frame in the South:
A Status Update
Authors: de Witt, A.; Basu, S.; Charlot, P.; Gordon, D.; Jacobs,
C.; Johnson, M.; Krásná, H.; Le Bail, K.; Shu, F.; Titov, O.;
Schartner, M.
2021evga.conf...85D Altcode:
No abstract at ADS
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Title: The K Band Geodesy with the East Asian VLBI Network
Authors: Xu, S.; Jike, T.; Jung, T.; Shu, F.; Cui, L.; Melnikov,
A.; McCallum, J.; Yi, S.; Zhang, B.; Sakai, N.; He, X.; Imai, H.;
Kawaguchi, N.; Sakai, D.; Oh, C.; Jiang, P.; Xu, M.; Wang, G.
2021evga.conf...71X Altcode:
No abstract at ADS
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Title: Observations of radio stars in geodetic VLBI experiments
Authors: Titov, O.; Shu, F.; Chen, W.
2020jsrs.conf..173T Altcode:
Proper motion and parallaxes of radio stars will enable us to align
the latest International Celestial Reference Frame (ICRF3) based on
VLBI observations with the optical astrometric catalogue obtained by
the Gaia mission. <P />The Gaia mission has observed approximately
500,000 extragalactic objects in optical with a competitive precision,
and produced an independent astrometric catalogue of 2820 objects
common to the ICRF2 (Gaia Collaboration et al. 2018) <P />and contains
astrometric solutions of parallaxes and proper motions for significant
amount of stars. This provides a rare opportunity for independent
verification of the VLBI astrometric catalogues. However, due to an
average parallax <P />zero-point of -29 µas of the Gaia catalogue
(Lindegren et al. 2018), independent assessment of the Gaia parallaxes
is required. Unfortunately, very few optically bright radio stars were
observed with VLBI, <P />therefore it is important to identify more
Galactic stars that could be observed by both VLBI and optical mission
for the parallax verification. <P />Here we show the astrometric results
for five radio stars (HR1099, UX Ari, HR132742, HR5907 and LSI+61 303)
in several VLBI experiments between 2015 and 2019.
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Title: Observations of Radio Sources Near the Sun
Authors: Titov, O.; Lambert, S.; Soja, B.; Shu, F.; Melnikov, A.;
McCallum, J.; McCallum, L.; Schartner, M.; de Witt, A.; Ivanov, D.;
Mikhailov, A.; Yi, S. O.; Chen, W.; Xia, B.; Ishigaki, M.; Gulyaev,
S.; Natusch, T.; Weston, S.
2019evga.conf..138T Altcode: 2019arXiv190800973T
Geodetic Very Long Baseline Interferometry (VLBI) data are capable of
measuring the light deflection caused by the gravitational field of
the Sun and large planets with high accuracy. The parameter $\gamma$
of the parametrized Post-Newtonian (PPN) formalism estimated using
observations of reference radio sources near the Sun should be
equal to unity in the general relativity. We have run several VLBI
experiments tracking reference radio sources from 1 to 3 degrees from
the Sun. The best formal accuracy of the parameter $\gamma$ achieved
in the single-session mode is less than 0.01 percent, or better than
the formal accuracy obtained with a global solution included all
available observations at arbitrary elongation from the Sun. We are
planning more experiments starting from 2020 using better observing
conditions near the minimum of the Solar activity cycle.
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Title: Mixed-mode VLBI Experiment with Chinese Stations in APSG40
Authors: He, X.; Shu, F.; Jiang, W.; Ma, L.; Yang, X.; Chen, L.
2019evga.conf..148H Altcode:
No abstract at ADS
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Title: Efforts and Attempts to Develop VGOS-like Stations in China
Authors: Li, J.; Liu, C.; Zhang, H.; Zhao, D.; Fan, Q.; Shu, F.; Zhao,
R.; Wang, J.; Zhang, X.; Wang, G.; Yang, X.; Yu, L.; Jiang, Y.; Guo,
S.; Wang, L.
2019evga.conf...12L Altcode:
No abstract at ADS
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Title: Comparison of Results Between CVN and K5 Software Correlators
Authors: Kondo, T.; Zheng, W.; Liu, L.; Zhang, J.; Shu, F.; Tong,
L.; Tong, F.
2019evga.conf...62K Altcode:
No abstract at ADS
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Title: Preliminary Work on Promoting Radar Astronomical Study
Authors: Sun, J.; Ping, J.; Han, S.; Shu, F.; Ma, L.; Chen, W.; Cui, L.
2019evga.conf..134S Altcode:
No abstract at ADS
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Title: Research and Application of VLBI Differential Phase Delays
in Lunar Exploration
Authors: Zheng, X.; Liu, Q.; Shu, F.; Huang, Y.; Li, P.; Xu, Z.;
Chu, Z.
2019evga.conf...57Z Altcode:
No abstract at ADS
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Title: VLBI monitoring of two distant quasars as a showcase for
'EVN Lite'
Authors: Frey, S.; Titov, O.; Melnikov, A.; de Vicente, P.; Shu, F.
2018evn..confE..97F Altcode: 2018PoS...344E..97F
No abstract at ADS
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Title: High-resolution radio imaging of two luminous quasars beyond
redshift 4.5
Authors: Frey, S.; Titov, O.; Melnikov, A. E.; de Vicente, P.; Shu, F.
2018A&A...618A..68F Altcode: 2018arXiv180706837F
Context. Radio-loud active galactic nuclei in the early Universe are
rare. The quasars J0906+6930 at redshift z = 5.47 and J2102+6015 at
z = 4.57 stand out from the known sample with their compact emission
on milliarcsecond (mas) angular scale with high (0.1 Jy level) flux
densities measured at GHz radio frequencies. This makes them ideal
targets for very long baseline interferometry (VLBI) observations. <BR
/> Aims: By means of VLBI imaging we can reveal the inner radio
structure of quasars and model their brightness distribution to
better understand the geometry of the jet and the physics of the
sources. <BR /> Methods: We present sensitive high-resolution VLBI
images of J0906+6930 and J2102+6015 at two observing frequencies, 2.3
and 8.6 GHz. The data were taken in an astrometric observing programme
involving a global five-element radio telescope array. We combined the
data from five different epochs from 2017 February to August. <BR />
Results: For one of the highest redshift blazars known, J0906+6930,
we present the first-ever VLBI image obtained at a frequency below 8
GHz. Based on our images at 2.3 and 8.6 GHz, we confirm that this source
has a sharply bent helical inner jet structure within ∼3 mas from the
core. The quasar J2102+6015 shows an elongated radio structure in the
east-west direction within the innermost ∼2 mas that can be described
with a symmetric three-component brightness distribution model at 8.6
GHz. Because of their non-pointlike mas-scale structure, these sources
are not ideal as astrometric reference objects. Our results demonstrate
that VLBI observing programmes conducted primarily with astrometric
or geodetic goals can be utilized for astrophysical purposes as well.
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Title: Testing general relativity with geodetic VLBI. What a single,
specially designed experiment can teach us
Authors: Titov, O.; Girdiuk, A.; Lambert, S. B.; Lovell, J.; McCallum,
J.; Shabala, S.; McCallum, L.; Mayer, D.; Schartner, M.; de Witt,
A.; Shu, F.; Melnikov, A.; Ivanov, D.; Mikhailov, A.; Yi, S.; Soja,
B.; Xia, B.; Jiang, T.
2018A&A...618A...8T Altcode: 2018arXiv180611299T
Context. We highlight the capabilities of geodetic VLBI technique
to test general relativity in the classical astrometric style,
i.e. measuring the deflection of light in the vicinity of the Sun. <BR
/> Aims: In previous studies, the parameter γ was estimated by
global analyses of thousands of geodetic VLBI sessions. Here we
estimate γ from a single session where the Sun has approached two
strong reference radio sources, 0229+131 and 0235+164, at an elongation
angle of 1-3°. <BR /> Methods: The AUA020 VLBI session of 1 May 2017
was designed to obtain more than 1000 group delays from the two radio
sources. The solar corona effect was effectively calibrated with the
dual-frequency observations even at small elongation. <BR /> Results:
We obtained γ with a greater precision (0.9 × 10<SUP>-4</SUP>) than
has been obtained through global analyses of thousands of standard
geodetic sessions over decades. Current results demonstrate that
the modern VLBI technology is capable of establishing new limits on
observational tests of general relativity.
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Title: The Progress of VLBI Terminal and Software Correlator in SHAO
Authors: Zheng, W.; Renjie, Z.; Zhang, J.; Tong, L.; Liu, L.; Li,
J.; Guo, S.; Tong, F.; Shu, F.
2017evga.conf...56Z Altcode:
No abstract at ADS
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Title: VizieR Online Data Catalog: VLBI Ecliptic Plane Survey: VEPS-1
(Shu+, 2017)
Authors: Shu, F.; Petrov, L.; Jiang, W.; Xia, B.; Jiang, T.; Cui, Y.;
Takefuji, K.; McCallum, J.; Lovell, J.; Yi, S. -O.; Hao, L.; Yang,
W.; Zhang, H.; Chen, Z.; Li, J.
2017yCat..22300013S Altcode:
We began observations in the search mode in 2015 February. The
participating stations included the three core stations of the Chinese
VLBI Network (CVN): seshan25, kunming, and urumqi. Depending on the
participating stations, the longest baseline length in each session can
be varied from 3200km to 9800km. Our observations were performed at a
2048Mbps data rate, with 16 Intermediate Frequency (IF) channels and
2-bit sampling. The first eight IFs of 32MHz bandwidth were distributed
in the range of [8.188, 8.444]GHz, and the remaining eight IFs of
32MHz bandwidth were in the range of [8.700, 8.956]GHz. <P />Table
1: Summary of the VLBI Ecliptic Plane Survey (VEPS) observations in
search mode: ---------------------------------------------------
Date Dur. Code Stations Number of (Y/M/D) (h) Targets
--------------------------------------------------- 2015 Feb 13 24
VEPS01 ShKmUr 293 2015 Feb 14 24 VEPS02 ShKmUr 338 2015 Apr 23 24
VEPS03 UrKv 300 2015 Apr 24 24 VEPS04 ShKmUrKv 400 2015 Aug 10 25
VEPS05 ShKmKvHo 252 2015 Aug 19 25 VEPS06 ShKmKvHo 277 2016 Mar 02
24 VEPS07 ShKmUrKb 333 2016 Mar 11 24 VEPS08 ShKmUrKb 477 2016 May
13 24 VEPS09 ShUrHo 291 2016 May 14 22 VEPS10 ShUrKv 322 2016 Jul 06
24 VEPS11 ShUrKb 307 2016 Sep 02 23 VEPS12 ShUr 424 2016 Sep 03 23
VEPS13 ShKmUr 344 ---------------------------------------------------
Sh=Seshan25; Km=Kunming; Ur=Urumqi; Kv=Sejong; Kb=Kashim34;
Ho=Hobart26. ---------------------------------------------------
<P />We ran two absolute astrometry dual-band VLBA programs that
targeted ecliptic plane compact radio sources: the dedicated survey
of weak ecliptic plane calibrators with the VLBA BS250 program in
2016 March-May, and the VLBA Calibrator Survey 9 (VCS-9) in 2015
August-2016 September. <P />The International VLBI Service for Geodesy
and Astrometry (IVS) runs a number of VLBI observing programs. We made
an attempt to improve the coordinates of some VEPS sources detected
in the search mode and provide additional measurements of telescope
position with the same experiments in two such 24hr sessions, AOV010
in July and AUA012 in 2016 August. <P />(2 data files).
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Title: Results from the VLBI Analysis Software Comparison Campaign
2015
Authors: Klopotek, G.; Artz, T.; Bellanger, A.; Bourda, G.; Gerstl,
M.; Gordon, D.; Haas, R.; Halsig, S.; Hjelle, G. A.; Hobiger, T.;
Hugentobler, U.; Iddink, A.; Kirkvik, A. S.; Lambert, S.; Plank, L.;
Schmid, R.; Shu, F.; Titov, O.; Tong, F.; Wang, G.; Xu, M.; Zheng, W.
2016ivs..conf..203K Altcode:
The aim of the VLBI Analysis Software Comparison Campaign 2015
(VASCC2015) was to compare different VLBI analysis software packages on
the basis of computed theoretical delays. Eleven research groups and
institutes participated in this project, which allowed us to compare
software packages that are used in operational VLBI analyses or that
are still under development. We present the first results, and we show
how well the individual software packages agree at this stage.
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Title: Six Decades of Spiral Density Wave Theory
Authors: Shu, Frank H.
2016ARA&A..54..667S Altcode:
The theory of spiral density waves had its origin approximately
six decades ago in an attempt to reconcile the winding dilemma of
material spiral arms in flattened disk galaxies. We begin with the
earliest calculations of linear and nonlinear spiral density waves
in disk galaxies, in which the hypothesis of quasi-stationary spiral
structure (QSSS) plays a central role. The earliest success was the
prediction of the nonlinear compression of the interstellar medium
and its embedded magnetic field; the earliest failure, seemingly,
was not detecting color gradients associated with the migration of
OB stars whose formation is triggered downstream from the spiral
shock front. We give the reasons for this apparent failure with an
update on the current status of the problem of OB star formation,
including its relationship to the feathering substructure of galactic
spiral arms. Infrared images can show two-armed, grand design spirals,
even when the optical and UV images show flocculent structures. We
suggest how the nonlinear response of the interstellar gas, coupled
with overlapping subharmonic resonances, might introduce chaotic
behavior in the dynamics of the interstellar medium and Population I
objects, even though the underlying forces to which they are subject
are regular. We then move to a discussion of resonantly forced spiral
density waves in a planetary ring and their relationship to the ideas
of disk truncation, and the shepherding of narrow rings by satellites
orbiting nearby. The back reaction of the rings on the satellites led
to the prediction of planet migration in protoplanetary disks, which
has had widespread application in the exploding data sets concerning
hot Jupiters and extrasolar planetary systems. We then return to the
issue of global normal modes in the stellar disk of spiral galaxies and
its relationship to the QSSS hypothesis, where the central theoretical
concepts involve waves with negative and positive surface densities
of energy and angular momentum in the regions interior and exterior,
respectively, to the corotation circle; the consequent transmission
and overreflection of propagating spiral density waves incident on the
corotation circle; and the role of feedback from the central regions.
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Title: The CVN Geodetic Observation and its Result
Authors: Wang, G.; Xu, M.; Zhang, Z.; Xu, S.; Li, L.; Shu, F.
2015evga.conf..228W Altcode:
No abstract at ADS
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Title: The Asia-Oceania VLBI Group for Geodesy and Astrometry
Authors: Lovell, J.; Kawabata, R.; Kurihara, S.; Shu, F.; Cho, J.
2015evga.conf..117L Altcode:
No abstract at ADS
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Title: On the Role of Tianma Radio Telescope for Improving Celestial
Reference Frames
Authors: Shu, F.; Wang, J.; Jiang, W.; Wang, G.; Shen, Z.
2015evga.conf...71S Altcode:
No abstract at ADS
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Title: VLBI Phase-referencing Experiments for Deep Space Probes
Authors: Zheng, W.; Tong, F.; Zhang, J.; Shu, F.; Liu, L.
2015evga.conf..268Z Altcode:
No abstract at ADS
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Title: Erratum: "Gas Dynamics of Semidetached Binaries" <A
href="/abs/1975ApJ...198..383L">(1975, ApJ, 198, 383)</A>
Authors: Lubow, Stephen H.; Shu, Frank H.
2014ApJ...788...95L Altcode:
No abstract at ADS
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Title: Erratum: "Gas Dynamics of Semidetached
Binaries. II - The Vertical Structure of the Stream" <A
href="/abs/1976ApJ...207L..53L">(1976, ApJ, 207, L53)</A>
Authors: Lubow, Stephen H.; Shu, Frank H.
2014ApJ...787L..38L Altcode:
No abstract at ADS
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Title: First 3-Way Lunar Radio Phase Ranging and Doppler Experiment
in Chang'E-3 Lander Mission
Authors: PING, J.; Meng, Q.; Tang, G.; Jian, N.; Wang, Z.; Li, W.;
Chen, C.; Wang, M.; Wang, M.; Lu, Y.; Yu, Q.; Mao, Y.; Miao, C.; Lei,
Y.; Shu, F.; Cao, J.
2014EPSC....9..226P Altcode:
Radio science experiments have been involved in all of the Chinese
lunar missions with different research objectives. In Chang'E-3 landing
mission, a 3-way open loop lunar radio phase ranging and Doppler
technique was suggested and tested. This technique is modified and
updated from early multi-channel oneway Doppler deep space tracking
technique developed for Chinese Mars mission Yinghuo-1. In the 1st
preliminary experiments, we obtained 1sps continuous phase ranging data
before and after the successful landing period, with a resolution of
0.5 millimeter or better. This method, called Lunar Radio Phase Ranging
(LRPR) can be a new space geodetic technique to measure the station
position, earth tide and rotation, lunar orbit, tide and liberation,
by means of independent observation, or to work together with Lunar
Laser Ranging. Also, it can be used in future Mars mission.
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Title: Spiral Arm Substructure and Massive Star Formation
Authors: Lee, Wing-Kit; Shu, F. H.
2013AAS...22122502L Altcode:
A theoretical framework is developed to understand the substructures
of spiral arms in galaxies and its relation to the massive star
formation. In particular, quasi-periodic gas density enhancements
along the spiral arms, which are called feathers, can be formed by the
instability of spiral arms. Such feathering instability may trigger
the formation of Giant Molecular Clouds (GMCs) by the collapse of high
density region. The analytic framework has advantages for the large
parameter space in the problem. Parameter study of the problem using
the framework are done to understand which factor is most important
among self-gravity, magnetic field or local shear in the inter-arm
region. Observational applications (on nearby spiral galaxies) are also
being developed to understand the physics of large-scale star formation
quantitatively. Ultimately, we hope to apply what we know about star
formation in the nearby galaxies to the high-redshift universe.
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Title: Feathering Instability of Spiral Arms. I. Formulation of
the Problem
Authors: Lee, Wing-Kit; Shu, Frank H.
2012ApJ...756...45L Altcode: 2012arXiv1207.0875L
In this paper, we study the feathering substructures along spiral
arms by considering the perturbational gas response to a spiral
shock. Feathers are density fluctuations that jut out from the
spiral arm to the interarm region at pitch angles given by the
quantum numbers of the doubly periodic structure. In a localized
asymptotic approximation, related to the shearing sheet except that
the inhomogeneities occur in space rather than in time, we derive
the linearized perturbation equations for a razor-thin disk with
turbulent interstellar gas, frozen-in magnetic field, and gaseous
self-gravity. Apart from the modal quantum numbers, the individual
normal modes of the system depend on seven dimensionless quantities that
characterize the underlying time-independent axisymmetric state plus its
steady, nonlinear, two-armed spiral-shock response to a hypothesized
background density wave supported by the disk stars of the galaxy. We
show that some of these normal modes have positive growth rates. Their
overdensity contours in the post-shock region are very reminiscent
of observed feathering substructures in full magnetohydrodynamic
simulations. The feathering substructures are parasitic instabilities
intrinsic to the system; thus, their study not only provides potential
diagnostics for important parameters that characterize the interstellar
medium of external galaxies, but also yields a deeper understanding of
the basic mechanism that drives the formation of the giant molecular
clouds and the OB stars that outline observed grand-design spirals.
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Title: ERRATUM: "Gravitational Collapse of Magnetized
Clouds. I. Ideal Magnetohydrodynamic Accretion Flow" <A
href="/abs/2006ApJ...647..374G">(2006, ApJ, 647, 374)</A>
Authors: Galli, Daniele; Lizano, Susana; Shu, Frank H.; Allen, Anthony
2012ApJ...754...78G Altcode:
No abstract at ADS
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Title: Feathering Instability of Spiral Arms and OB Star Formation
Authors: Lee, Wing-Kit; Shu, F. H.
2012AAS...21944109L Altcode:
Quasi-regular substructures of the spiral arm are commonly found in
spiral galaxies. These substructures are known as feathers or spurs,
and they jut out perpendicularly into the inter-arm region. They also
associate with the Giant Molecular Clouds where massive star formation
occurs. The formation of these density fluctuation can be studied from
the perspective of perturbation of galactic spiral shock. We investigate
the gas response under the influcence of the shock perturbation, and
formulate the MHD equations in a local two-dimensional quasi-rectangular
region between tightly-winding spiral arms. Our theoretical model
includes the effect of magnetic field and self-gravity of the gas,
we are able to reproduce feather-like structures in the post-shock
region. In this semi-analytical framework, the periodic density
fluctuations depends on the various background parameters such as
pattern speed, strength of spiral arm, surface density of the gas and
strength of magnetic field. Potentially this study can help understand
the inter-arm environment that will be observed in the nearby galaxies
using submillimeter telescope such ALMA in the coming years.
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Title: Magnetic Interactions in Pre-main-sequence Binaries
Authors: Adams, Fred C.; Cai, Michael J.; Galli, Daniele; Lizano,
Susana; Shu, Frank H.
2011ApJ...743..175A Altcode: 2011arXiv1110.4562A
Young stars typically have strong magnetic fields, so that the
magnetospheres of newly formed close binaries can interact, dissipate
energy, and produce synchrotron radiation. The V773 Tau A binary
system, a pair of T Tauri stars with a 51 day orbit, displays such
a signature, with peak emission taking place near periastron. This
paper proposes that the observed emission arises from the change in
energy stored in the composite magnetic field of the system. We model
the fields using the leading order (dipole) components and show that
this picture is consistent with current observations. In this model,
the observed radiation accounts for a fraction of the available energy
of interaction between the magnetic fields from the two stars. Assuming
antisymmetry, we compute the interaction energy E <SUB>int</SUB> as a
function of the stellar radii, the stellar magnetic field strengths,
the binary semimajor axis, and orbital eccentricity, all of which can be
measured independently of the synchrotron radiation. The variability in
time and energetics of the synchrotron radiation depend on the details
of the annihilation of magnetic fields through reconnection events,
which generate electric fields that accelerate charged particles, and
how those charged particles, especially fast electrons, are removed
from the interaction region. However, the major qualitative features
are well described by the background changes in the global magnetic
configuration driven by the orbital motion. The theory can be tested
by observing a collection of pre-main-sequence binary systems.
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Title: From Magnetized Cores to Protoplanetary Disks
Authors: Lizano, Susana; Shu, Frank H.
2010HiA....15..440L Altcode:
We highlight several recent theoretical results that show how magnetic
fields, with the magnitudes currently observed in molecular clouds,
affect the structure and evolution of dense cores and protoplanetary
disks to form stars and planets.
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Title: The Role of Magnetic Fields in the Protostellar Accretion Phase
Authors: Galli, Daniele; Shu, Frank H.; Lizano, Susana; Cai, Michael J.
2010AIPC.1242..231G Altcode:
We summarize recent work addressing the role of magnetic fields in
the process of star formation and disk accretion. After a short review
of the basic observational results, we concentrate on the efficiency
of magnetic braking during cloud collapse and its consequences on the
formation of centrifugally supported disks around young stars. Then,
we relate this issue to the well-known magnetic flux problem of star
formation, and we show that the introduction of non-ideal MHD effects is
a necessary step toward the development of self-consistent models for
the collapse of molecular clouds and the formation of disks. Finally,
we discuss the structure and evolution of magnetized accretion disks
around young stars that have dragged their magnetic field in the phase
of gravitational collapse and we focus on two main diffusive processes:
viscous stresses that redistribute mass and angular momentum, and the
resistive diffusion of mass across magnetic field lines.
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Title: The SMA and Galactic Star Formation
Authors: Shu, Frank H.
2010AAS...21523202S Altcode: 2010BAAS...42R.595S
We review observations of the SMA telescope that have had a large
impact in the field of Galactic star formation. We divide the subject
into the origin of low-mass and high-mass stars, and we focus on the
subjects of protostellar collapse and outflow. We conclude that the
SMA indicates that (a) magnetic fields have a stronger influence in
controlling star formation than interstellar turbulence, (b) until
the appearance of H II regions, high-mass star formation is just
a scaled-up version of low-mass star formation, and (c) the basic
drivers for bipolar outflows originate near the inner edges of the
accretion disks that surround young stellar objects.
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Title: Current status of Chinese VLBI network software correlator
Authors: Zheng, W.; Shu, F.; Luo, W.; Yu, Y.; Wang, W.
2009evga.conf...84Z Altcode:
No abstract at ADS
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Title: Shanghai correlation system upgrade for geodetic application
Authors: Shu, F.; Zheng, W.; Zhang, X.; Xu, Z.; Wang, W.; Chen, Z.
2009evga.conf...87S Altcode:
No abstract at ADS
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Title: Magnetized Disks around Young Stars
Authors: Lizano, S.; Shu, F. H.; Galli, D.; Glassgold, A.
2009RMxAC..36..149L Altcode:
We discuss the structure and evolution of a magnetized accretion
disks around young stars that have dragged their magnetic field in
the process of gravitational collapse. The disk evolves due to two
diffusive processes: viscous stresses that redistribute mass and
angular momentum, and the resistive diffusion of mass across magnetic
field lines due to imperfect conduction. In steady-state there is an
analytic model of the structure of these magnetized disks. We discuss
the application of this model to disks around low and high mass young
stars and recent results of time dependent models.
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Title: Magnetic Braking and Field Dissipation in the Protostellar
Accretion Phase
Authors: Galli, D.; Cai, M.; Lizano, S.; Shu, F. H.
2009RMxAC..36..143G Altcode:
We summarize recent theoretical work addressing the role of magnetic
fields in the process of star formation. First, we concentrate on
the efficiency of magnetic braking during cloud collapse and its
consequences on the formation of centrifugally supported disks around
young stars. Then, we relate this issue to the well-known magnetic
flux problem of star formation, and we show that the introduction of
non-ideal MHD effects is a necessary step toward the development of
self-consistent models for the collapse of molecular clouds and the
formation and evolution of accretion disks around young stars.
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Title: Generalized Multipole X-Wind Model
Authors: Mohanty, Subhanjoy; Shu, Frank H.
2009ASSP...13...51M Altcode: 2009pjc..book...51M
The X-wind model for magnetospheric accretion and outflow in classical
T Tauri stars (CTTS) has gained credence in recent years for a variety
of theoretical and observational reasons. However, both this model as
well as other theoretical scenarios for explaining magnetospheric disk
accretion assume that the stellar field, were it not perturbed by an
electrically conducting accretion disk, would have a dipolar geometry
(e.g., [5]; OS95 hereafter). Observations of accretion hot spot sizes
and net field polarization on the surface of CTTS, however, clearly
indicate that the stellar field has a complex multipolar structure. To
overcome this discrepancy between theory and data, we reformulate X-wind
theory without the dipole constraint. This contribution represents
a brief summary of the paper by Mohanty and Shu [6]. In Sect. 1 we
present the fundamental physical ideas of the generalized theory,
and the associated equations; in Sect. 2 we compare the resulting
theoretical prediction to recent observations, and provide some
illustrative numerical simulations with multipole stellar fields.
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Title: Magnetocentrifugally Driven Flows from Young Stars and
Disks. VI. Accretion with a Multipole Stellar Field
Authors: Mohanty, Subhanjoy; Shu, Frank H.
2008ApJ...687.1323M Altcode: 2008arXiv0806.4769M
Previous analyses of magnetospheric accretion and outflow in classical
T Tauri stars (CTTSs), within the context of both the X-wind model
and other theoretical scenarios, have assumed a dipolar geometry for
the stellar magnetic field if it were not perturbed by the presence
of an accreting, electrically conducting disk. However, CTTS surveys
reveal that accretion hot spots cover a small fraction of the stellar
surface and that the net field polarization on the stellar surface
is small. Both facts imply that the magnetic field generated by the
star has a complex nondipolar structure. To address this discrepancy
between theory and observations, we reexamine X-wind theory without
the dipole constraint. Using simple physical arguments based on
the concept of trapped flux, we show that a dipole configuration
is in fact not essential. Independent of the precise geometry of
the stellar magnetosphere, the requirement for a certain level of
trapped flux predicts a definite relationship among various CTTS
observables. Moreover, superposition of multipole stellar fields
naturally yield small observed hot spot covering fractions and small
net surface polarizations. The generalized X-wind picture remains viable
under these conditions, with the outflow from a small annulus near the
inner disk edge little affected by the modified geometry, but with
inflow highly dependent on the details of how the emergent stellar
flux is linked and trapped by the inner disk regions. Our model is
consistent with data, including recent spectropolarimetric measurements
of the hot spot sizes and field strengths in V2129 Oph and BP Tau.
---------------------------------------------------------
Title: The Challenge of Sub-Keplerian Rotation for Disk Winds
Authors: Shu, Frank H.; Lizano, Susana; Galli, Daniele; Cai, Mike J.;
Mohanty, Subhanjoy
2008ApJ...682L.121S Altcode: 2008arXiv0806.2137S
Strong magnetization makes the disks surrounding young stellar objects
rotate at rates that are too sub-Keplerian to enable the thermal
launching of disk winds from their surfaces unless the rate of gas
diffusion across field lines is dynamically fast. This underappreciated
implication of disk magnetization poses a considerable challenge for
disk-wind theory.
---------------------------------------------------------
Title: Astrophysics of X-ray Irradiated Protoplanetary Disks
Authors: Shu, Frank H.
2008HEAD...10.1703S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: X-Winds in Action
Authors: Cai, Mike J.; Shang, Hsien; Lin, Hsiao-Hsuan; Shu, Frank H.
2008ApJ...672..489C Altcode: 2007arXiv0709.3087C
The interaction of accretion disks with the magnetospheres of young
stars can produce X-winds and funnel flows. With the assumption of
axial symmetry and steady state flow, the problem can be formulated
in terms of quantities that are conserved along streamlines, such as
the Bernoulli integral (BI), plus a partial differential equation
(PDE), called the Grad-Shafranov equation (GSE), that governs the
distribution of streamlines in the meridional plane. The GSE plus BI
yields a PDE of mixed type, elliptic before critical surfaces where the
flow speed equals certain characteristic wave speeds are crossed and
hyperbolic afterward. The computational difficulties are exacerbated by
the locations of the critical surfaces not being known in advance. To
overcome these obstacles, we consider a variational principle by which
the GSE can be attacked by extremizing an action integral, with all
other conserved quantities of the problem explicitly included as part of
the overall formulation. To simplify actual applications we adopt the
cold limit of a negligibly small ratio of the sound speed to the speed
of Keplerian rotation in the disk where the X-wind is launched. We also
ignore the obstructing effects of any magnetic fields that might thread
a disk approximated to be infinitesimally thin. We then introduce trial
functions with adjustable coefficients to minimize the variations that
give the GSE. We tabulate the resulting coefficients so that other
workers can have analytic forms to reconstruct X-wind solutions for
various astronomical, cosmochemical, and meteoritical applications.
---------------------------------------------------------
Title: Ambipolar Diffusion In Molecular Cloud Cores and the
Gravomagneto Catastrophe
Authors: Adams, Fred C.; Shu, Frank H.
2007ApJ...671..497A Altcode: 2007arXiv0708.4238A
This paper reexamines the problem of ambipolar diffusion as a
mechanism for the production and runaway evolution of centrally
condensed molecular cloud cores, a process that has been termed the
gravomagneto catastrophe. Our calculation applies in the geometric limit
of a highly flattened core and allows for a semianalytic treatment of
the full problem, although physical fixes are required to resolve a
poor representation of the central region. A noteworthy feature of the
overall formulation is that the solutions for the ambipolar diffusion
portion of the evolution for negative times (t<0) match smoothly
onto the collapse solutions for positive times (t>0). The treatment
shows that the resulting cores display nonzero, but submagnetosonic,
inward velocities at the end of the diffusion epoch, in agreement with
current observations. Another important result is the derivation of
an analytic relationship between the dimensionless mass-to-flux ratio
λ<SUB>0</SUB>≡f<SUP>-1</SUP><SUB>0</SUB> of the central regions
produced by runaway core condensation and the dimensionless measure
of the rate of ambipolar diffusion ɛ. In conjunction with previous
work showing that ambipolar diffusion takes place more quickly in the
presence of turbulent fluctuations, i.e., that the effective value
of ɛ can be enhanced by turbulence, the resultant theory provides a
viable working hypothesis for the formation of isolated molecular cloud
cores and their subsequent collapse to form stars and planetary systems.
---------------------------------------------------------
Title: Mean Field Magnetohydrodynamics of Accretion Disks
Authors: Shu, Frank H.; Galli, Daniele; Lizano, Susana; Glassgold,
Alfred E.; Diamond, Patrick H.
2007ApJ...665..535S Altcode: 2007arXiv0705.0421S
We consider the accretion process in a disk with magnetic fields
that are dragged in from the interstellar medium by gravitational
collapse. Two diffusive processes are at work in the system: (1)
“viscous” torques exerted by turbulent and magnetic stresses, and
(2) “resistive” redistribution of mass with respect to the magnetic
flux arising from the imperfect conduction of current. In steady
state, self-consistency between the two rates of drift requires
that a relationship exists between the coefficients of turbulent
viscosity and turbulent resistivity. Ignoring any interactions with
a stellar magnetosphere, we solve the steady-state equations for a
magnetized disk under the gravitational attraction of a mass point and
threaded by an amount of magnetic flux consistent with calculations of
magnetized gravitational collapse in star formation. Our model mean
field equations have an exact analytical solution that corresponds
to magnetically diluted Keplerian rotation about the central mass
point. The solution yields the strength of the magnetic field and
the surface density as functions of radial position in the disk and
their connection with the departure from pure Keplerian rotation in
representative cases. We compare the predictions of the theory with the
available observations concerning T Tauri stars, FU Orionis stars, and
low- and high-mass protostars. Finally, we speculate on the physical
causes for high and low states of the accretion disks that surround
young stellar objects. One of the more important results of this study
is the physical derivation of analytic expressions for the turbulent
viscosity and turbulent resistivity.
---------------------------------------------------------
Title: Review of Theory
Authors: Shu, Frank
2007sftn.confE..32S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Magnetohydrodynamics of Star Formation
Authors: Shu, Frank H.
2007DDA....38.0101S Altcode:
We present self-consistent models of disk accretion driven by the
magneto-rotational instability associated with magnetic fields dragged
in by a process of gravitational collapse from rotating, magnetized,
molecular cloud cores. We compare such star-formation models with
constraints from astronomical observations, meteoritic investigations,
and comet sample returns. We show that previous theoretical studies have
missed two crucial effects: (1) the fact that diffusion is occurring
not only via a viscous redistribution of angular momentum but also
by a non-ideal drift of inwardly moving matter across magnetic field
lines that thread vertically through the disk, and (2) that realistic
circumstances may result in magnetically pinched disks which rotate
at substantially sub-Keplerian speeds. We also argue that the complete
data set cannot be understood for sunlike stars without incorporating
the interaction of the inner edge of the accretion disk with the
magnetosphere of the central star that results in X-winds and funnel
flows, with important, incompletely examined, consequences for the
processes of planet formation.
---------------------------------------------------------
Title: Formation of OB Associations in Galaxies
Authors: Shu, Frank H.; Allen, Ronald J.; Lizano, Susana; Galli,
Daniele
2007ApJ...662L..75S Altcode: 2007arXiv0705.0634S
We consider the formation of OB associations from two perspectives: (1)
the fractional gas consumption in star formation, ɛ, per dynamical
timescale t<SUB>dyn</SUB> in a galaxy, and (2) the origin of the
so-called Kennicutt-Schmidt law, that the rate of star formation per
unit area is proportional to a power, α, of the surface density in
H I and H<SUB>2</SUB> gas when certain thresholds are crossed. The
empirical findings that ɛ~10<SUP>-2</SUP> and α~1.4 or 1.5 have
simple explanations if the rate of star formation is magnetically
regulated. An empirical test of the ideas resides in an analysis of
why giant OB associations are “strung out like pearls along the arms”
of spiral galaxies.
---------------------------------------------------------
Title: Magnetization, accretion, and outflows in young stellar objects
Authors: Shu, Frank H.; Galli, Daniele; Lizano, Susana; Cai, Mike J.
2007IAUS..243..249S Altcode:
We review the theory of the formation and gravitational collapse of
magnetized molecular cloud cores, leading to the birth of T Tauri stars
surrounded by quasi-Keplerian disks whose accretion is driven by the
magnetorotational instability (MRI). Some loss of magnetic flux during
the collapse results typically in a dimensionless mass-to-flux ratio
for the star plus disk of λ<SUB>0</SUB> ≈ 4. Most of the mass ends
up in the star, while almost all of the flux and the angular momentum
ends up in the disk; therefore, a known mass for the central star
implies a computable flux in the surrounding disk. A self-contained
theory of the MRI that drives the viscous/resistive spreading in
such circumstances then yields the disk radius needed to contain
the flux trapped in the disk as a function of the age t. This theory
yields analytic predictions of the distributions with distance from
the central star of the surface density Σ(), the vertical magnetic
field B<SUB>z</SUB>(), and the (sub-Keplerian) angular rotation rate
Ω (). We discuss the implications of this picture for disk-winds,
X-winds, and funnel flows, and we summarize the global situation by
giving the energy and angular-momentum budget for the overall problem.
---------------------------------------------------------
Title: Star formation and radio astronomy
Authors: Shu, F.
2007mru..confE..60S Altcode: 2007PoS....52E..60S
No abstract at ADS
---------------------------------------------------------
Title: Gravitational Collapse of Magnetized Clouds. II. The Role of
Ohmic Dissipation
Authors: Shu, Frank H.; Galli, Daniele; Lizano, Susana; Cai, Mike
2006ApJ...647..382S Altcode: 2006astro.ph..4574S
We formulate the problem of magnetic field dissipation during the
accretion phase of low-mass star formation, and we carry out the first
step of an iterative solution procedure by assuming that the gas is
in free fall along radial field lines. This so-called “kinematic
approximation” ignores the back reaction of the Lorentz force on the
accretion flow. In quasi-steady state and assuming the resistivity
coefficient to be spatially uniform, the problem is analytically
soluble in terms of Legendre's polynomials and hypergeometric confluent
functions. The dissipation of the magnetic field occurs inside a region
of radius inversely proportional to the mass of the central star (the
“Ohm radius”), where the magnetic field becomes asymptotically
straight and uniform. In our solution the magnetic flux problem
of star formation is avoided because the magnetic flux dragged in
the accreting protostar is always zero. Our results imply that the
effective resistivity of the infalling gas must be higher by at least
1 order of magnitude than the microscopic electric resistivity, to
avoid conflict with measurements of paleomagnetism in meteorites and
with the observed luminosity of regions of low-mass star formation.
---------------------------------------------------------
Title: Gravitational Collapse of Magnetized Clouds. I. Ideal
Magnetohydrodynamic Accretion Flow
Authors: Galli, Daniele; Lizano, Susana; Shu, Frank H.; Allen, Anthony
2006ApJ...647..374G Altcode: 2006astro.ph..4573G
We study the self-similar collapse of an isothermal magnetized rotating
cloud in the ideal magnetohydrodynamic (MHD) regime. In the limit
of small distance from the accreting protostar, we find an analytic
solution that corresponds to free fall onto a central mass point. The
density distribution is not spherically symmetric but depends on the
mass loading of magnetic field lines, which can be obtained by matching
our inner solution to an outer collapse solution previously computed
by Allen et al. The concentration of magnetic field trapped by the
central mass point under field freezing, independent on the details of
the starting state, creates a split-monopole configuration in which
the magnetic field strength increases as the inverse square of the
distance from the center. Under such conditions, the inflow eventually
becomes sub-Alfvènic and the outward transfer of angular momentum by
magnetic braking very efficient, thus preventing the formation of a
centrifugally supported disk. Instead, the azimuthal velocity of the
infalling gas decreases to zero at the center, and the gas spirals
into the star. Therefore, the dissipation of dynamically important
levels of magnetic field is a fundamental requisite for the formation
of protoplanetary disks around young stars.
---------------------------------------------------------
Title: The Irradiation Origin of Beryllium Radioisotopes and Other
Short-lived Radionuclides
Authors: Gounelle, Matthieu; Shu, Frank H.; Shang, Hsien; Glassgold,
A. E.; Rehm, K. E.; Lee, Typhoon
2006ApJ...640.1163G Altcode: 2005astro.ph.12517G
Two explanations exist for the short-lived radionuclides
(T<SUB>1/2</SUB><=5 Myr) present in the solar system when the
calcium-aluminum-rich inclusions (CAIs) first formed. They originated
either from the ejecta of a supernova or by the in situ irradiation of
nebular dust by energetic particles. With a half-life of only 53 days,
<SUP>7</SUP>Be is then the key discriminant, since it can be made only
by irradiation. Using the same irradiation model developed earlier
by our group, we calculate the yield of <SUP>7</SUP>Be. Within model
uncertainties associated mainly with nuclear cross sections, we obtain
agreement with the experimental value. Moreover, if <SUP>7</SUP>Be
and <SUP>10</SUP>Be have the same origin, the irradiation time must be
short (a few to tens of years), and the proton flux must be of order
F~2×10<SUP>10</SUP> cm<SUP>-2</SUP> s<SUP>-1</SUP>. The X-wind model
provides a natural astrophysical setting that gives the requisite
conditions. In the same irradiation environment, <SUP> 26</SUP>Al,
<SUP>36</SUP>Cl, and <SUP>53</SUP>Mn are also generated at the measured
levels within model uncertainties, provided that irradiation occurs
under conditions reminiscent of solar impulsive events (steep energy
spectra and high <SUP>3</SUP>He abundance). The decoupling of the <SUP>
26</SUP>Al and <SUP>10</SUP>Be observed in some rare CAIs receives
a quantitative explanation when rare gradual events (shallow energy
spectra and low <SUP> 3</SUP>He abundance) are considered. The yields
of <SUP>41</SUP>Ca are compatible with an initial solar system value
inferred from the measured initial <SUP> 41</SUP>Ca/<SUP>40</SUP>Ca
ratio and an estimate of the thermal metamorphism time (from Young
et al.), alleviating the need for two-layer proto-CAIs. Finally, we
show that the presence of supernova-produced <SUP>60</SUP>Fe in the
solar accretion disk does not necessarily mean that other short-lived
radionuclides have a stellar origin.
---------------------------------------------------------
Title: Indeterminate-origin nozzles to control jet structure and
evolution
Authors: Shu, F.; Plesniak, M. W.; Sojka, P. E.
2005JTurb...6...26S Altcode: 2005JTurb...6N..26S
An indeterminate-origin (IO) nozzle consisting of a four-point tapered
crown geometry is used to control jet structure and evolution. The
near-field structure and flow field of round water jets were studied
with particle image velocimetry (PIV) and laser-induced fluorescence
(LIF) techniques. Typical jet Reynolds numbers (based on the nozzle
diameter) are 5 000 10 000. The jet structures for IO nozzles are
compared with those produced by conventional round nozzles. The IO
nozzles introduce strong streamwise vortex pairs, which influence the
near-nozzle structure and spreading by deforming the Kelvin Helmholtz
vortex rings. A pair of counter-rotating vortices forms at each valley
plane, within the interior of the jet. Adjacent vortex pairs reorganize
themselves and form another set of counter-rotating vortex pairs
that propagate radially outwards from the jet. The evolution of these
effects with streamwise distance and their implications are discussed.
---------------------------------------------------------
Title: A Theory of the IMF
Authors: Shu, Frank H.; Li, Zhi-Yun; Allen, Anthony
2005ASSL..327..401S Altcode: 2005imf..conf..401S
No abstract at ADS
---------------------------------------------------------
Title: Collapse of Singular Isothermal Spheres to Black Holes
Authors: Cai, Mike J.; Shu, Frank H.
2005ApJ...618..438C Altcode:
We study the gravitational collapse of a relativistic singular
isothermal sphere that is initially in unstable equilibrium. In the
subsequent collapse, the dynamic spacetime is self-similar. The infall
proceeds in an inside-out fashion, mimicking its Newtonian counterpart
in star formation. A spherical expansion wave propagates outward at the
speed of sound, initiating an inward collapse relative to local static
observers. Outside of the expansion wave front, matter remains in local
equilibrium. Inside, fluid elements are accelerated from rest toward
the expanding black hole event horizon. When the singular isothermal
sphere is initially threaded by a uniform but weak magnetic field,
the frozen-in field lines accumulate above the horizon according to
a distant observer, while assuming a split-monopole configuration on
a larger scale. When the magnetized system also possesses rotation,
such a configuration may naturally develop a vigorous outflow in
the simultaneous presence of an accretion inflow. We speculate that
such a process underlies the well-known relationship between mass and
bulge velocity dispersion of supermassive black holes in the nuclei
of galaxies.
---------------------------------------------------------
Title: Making Radio Jets from the X-wind Model
Authors: Shang, H.; Lizano, S.; Glassgold, A.; Shu, F.
2004ASPC..323..299S Altcode:
We calculate free-free emission at centimeter wavelengths from jets
from Young Stellar Objects (YSOs). We use the x-wind model which is
heated and ionized by several physical mechanisms, being of primary
importance ionization by x-rays and heating by dissipation of mechanical
energy (Shang et al. 2002). With stellar parameters, mass-loss rates,
and x-ray luminosities characteristic of Class 0 or Class I YSOs, we
recover realistic radio spectral indices and radio maps. In particular,
the 3.6 cm flux in models with mass loss rates ∼ 10<SUP>-6</SUP>
M<SUB>⊙</SUB>/yr are comparable to existing observations of radio jets
at 0”.1 resolution. Lower mass-loss rates characteristic of Class II
YSOs with revealed optical jets generally produce radio jets that are
too weak to be detected at current sensitivity levels. We discuss the
implications of these comparisons for theoretical models of the YSO
outflow phenomenon.
---------------------------------------------------------
Title: The Stellar Initial Mass Function
Authors: Shu, F. H.; Li, Z. -Y.; Allen, A.
2004ASPC..323...37S Altcode:
If magnetic fields are frozen in gravitational collapse, the resulting
magnetic tension can prevent the outer part of the subcritical
envelope of a molecular cloud from falling in with the supercritical
core. However, the implied surface magnetic fields much exceed measured
values for young stars. Moreover, it is virtually impossible for
Keplerian disks to form in these circumstances. Magnetic reconnection
can eliminate the long lever arms of the split monopole formed by
the gravitational collapse that contibutes to catastrophic magnetic
braking. The natural appearance then of a Keplerian disk adjoining a
rotating star with an outer convective envelope will lead to an X-wind
driven magnetocentrifugally from the inner edge of the disk. This wind
can cut off the continued infall from the envelope and build-up of the
central stellar mass. We use these ideas and results to calculate the
initial mass function and star formation efficiency for the distributed
and clustered modes of star formation.
---------------------------------------------------------
Title: The Origin of Short-lived Radionuclides and Early Solar
System Irradiation
Authors: Gounelle, M.; Shu, F. H.; Shang, H.; Glassgold, A. E.; Rehm,
K. E.; Lee, T.
2004cpd..work.9051G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Singular isothermal sphere and black hole formation
Authors: Cai, Mike J.; Shu, Frank H.
2004IAUS..222..551C Altcode:
We present here the self-similar collapse of a relativistic
singular isothermal sphere (SIS) that leads to monolithic black
hole formation. The dynamic evolution is triggered by the central
portion of the SIS collapsing to for an infinitesimal black hole. This
perturbation removes the pressure support for the layer immediately
above, which causes it to collapse, and so on. The influence of
gravitational collapse propagates out as an expansion wave moving at
the speed of sound. Outside of the expansion wave, matter remains in
local equilibrium.
---------------------------------------------------------
Title: Jet Flows: Formation and Thermal Processing of Solids in
Protoplanetary Disks
Authors: Shu, F. H.; Shang, H.; Lee, T.
2004cpd..work.9008S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Free-free Radio Emission from Young Stellar Objects
Authors: Shang, Hsien; Lizano, Susana; Glassgold, Al; Shu, Frank
2004ApJ...612L..69S Altcode:
We calculate the centimeter wavelength free-free emission of the jets
of young stellar objects (YSOs) with the X-wind model enhanced by a
variety of physical processes. Using parameters characteristic of a
Class I YSO with a mass-loss rate of ~10<SUP>-6</SUP> M<SUB>solar</SUB>
yr<SUP>-1</SUP>, we obtain a 3.6 cm map and a spectral index that
compare well with high spatial resolution observations of L1551 IRS
5. Models with lower mass-loss rates, appropriate for Class II YSOs
with revealed optical jets, produce radio jets that are too weak to be
detected at current sensitivity levels. In addition to demonstrating
the consistency of the density distribution of the X-wind model with
observations, we are able to obtain information on the processes that
heat and ionize the inner jet, i.e., X-ray ionization and shock heating
and ionization.
---------------------------------------------------------
Title: Nonaxisymmetric Neutral Modes in Relativistic Disks
Authors: Cai, Mike J.; Shu, Frank H.
2004ApJ...611.1054C Altcode: 2004astro.ph..5247C
We perform a linear stability analysis of the axisymmetric,
relativistic, self-similar, isothermal disk against nonaxisymmetric
perturbations. Two sets of neutral modes are discovered. The first
set corresponds to marginally unstable perturbations driven by
gravitational radiation, and the other signals the onset of bifurcation
to nonaxisymmetric equilibrium solutions to the Einstein equations.
---------------------------------------------------------
Title: The Origin of Short-lived Radionuclides and Early Solar
System Irradiation
Authors: Gounelle, M.; Shu, F. H.; Shang, H.; Glassgold, A. E.; Rehm,
K. E.; Lee, T.
2004LPI....35.1829G Altcode:
Using the irradiation model developed by Gounelle et al. (2001),
we can reproduce the abundance of ^7Be measured by Chaussidon,et
al. (2004, this conference). We also provide a tentative explanation
for the hibonite grains that show a decoupling betwen ^26Al and ^10Be
(Marhas et al. 2002).
---------------------------------------------------------
Title: Does Magnetic Levitation or Suspension Define the Masses of
Forming Stars?
Authors: Shu, Frank H.; Li, Zhi-Yun; Allen, Anthony
2004ApJ...601..930S Altcode: 2003astro.ph.11426S
We investigate whether magnetic tension can define the masses of forming
stars by holding up the subcritical envelope of a molecular cloud that
suffers gravitational collapse of its supercritical core. We perform an
equilibrium analysis of the initial and final states assuming perfect
field freezing, no rotation, isothermality, and a completely flattened
configuration. The sheet geometry allows us to separate the magnetic
tension into a levitation associated with the split monopole formed by
the trapped flux of the central star and a suspension associated with
curved field lines that thread the static pseudodisk and envelope of
material external to the star. We find solutions where the eigenvalue
for the stellar mass is a fixed multiple of the initial core mass of
the cloud. We verify the analytically derived result by an explicit
numerical simulation of a closely related three-dimensional axisymmetric
system. However, with field freezing, the implied surface magnetic
fields much exceed measured values for young stars. If the pinch by the
central split monopole were to be eliminated by magnetic reconnection,
then magnetic suspension alone cannot keep the subcritical envelope
(i.e., the entire model cloud) from falling onto the star. We argue
that this answer has general validity, even if the initial state lacked
any kind of symmetry, possessed rotation, and had a substantial level
of turbulence. These findings strongly support a picture for the halt
of infall that invokes dynamic levitation by YSO winds and jets, but
the breakdown of ideal magnetohydrodynamics is required to allow the
appearance in the problem of a rapidly rotating, centrifugally supported
disk. We use these results to calculate the initial mass function and
star formation efficiency for the distributed and clustered modes of
star formation.
---------------------------------------------------------
Title: Planetesimal Formation by Gravitational Instability - The
Goldreich-Ward Hypothesis Revisited
Authors: Youdin, Andrew N.; Shu, Frank H.
2004IAUS..202..250Y Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Chaos in Spiral Galaxies
Authors: Shu, F. H.; Chakrabarti, S.; Laughlin, G.
2004ASSL..319..581S Altcode: 2004pbmc.conf..581S
No abstract at ADS
---------------------------------------------------------
Title: Chaos in Spiral Galaxies
Authors: Shu, Frank H.; Chakrabarti, Sukanya; Laughlin, Gregory
2004csg..book.....S Altcode:
We review spiral density wave theory and its relationship to the
disordered appearance of population I objects in some disk galaxies. We
discuss mechanisms proposed for the formation of (a) feathers by
gravitational instability behind galactic shocks, (b) branches by
the action of ultraharmonic resonances, (c) spurs by reflection of
leading waves off sharp features induced by nonlinear dredgin, and
(d) flocculence by the chaos produced from overlapping resonances. We
conclude that disorder arises is spiral galaxies not so much from
disorderly causes as from too sensitive a response of the interstellar
medium to an orderly but nonlinear spiral gravitational field.
---------------------------------------------------------
Title: Non-Axisymmetric Modes in Relativistic Singular Isothermal
Disks
Authors: Cai, M. J.; Shu, F. H.
2004PThPS.155..317C Altcode:
The relativistic singular isothermal disk equilibriumsolutions are
self-similar and form a two-parameter family described by the sound
speed and the linear rotational velocity. Using the equilibrium model
as the base state, we study its stability against self-similar but
non-axisymmetric perturbations. We discovered that instability can
be manifested through radiation driven neutral modes and bifurcation
points to non-axisymmetric equilibria.
---------------------------------------------------------
Title: Collapse of Magnetized Singular Isothermal
Toroids. II. Rotation and Magnetic Braking
Authors: Allen, Anthony; Li, Zhi-Yun; Shu, Frank H.
2003ApJ...599..363A Altcode: 2003astro.ph.11377A
We study numerically the collapse of rotating magnetized molecular
cloud cores, focusing on rotation and magnetic braking during
the main accretion phase of isolated star formation. Motivated by
previous numerical work and analytic considerations, we idealize the
precollapse core as a magnetized singular isothermal toroid, with a
constant rotational speed everywhere. The collapse starts from the
center and propagates outward in an inside-out fashion, satisfying
exact self-similarity in space and time. For rotation rates and field
strengths typical of dense low-mass cores, the main feature remains the
flattening of the mass distribution along field lines-the formation of
a pseudodisk, as in the nonrotating cases. The density distribution
of the pseudodisk is little affected by rotation. On the other hand,
the rotation rate is strongly modified by pseudodisk formation. Most of
the centrally accreted material reaches the vicinity of the protostar
through the pseudodisk. The specific angular momentum can be greatly
reduced on the way, by an order of magnitude or more, even when
the precollapse field strength is substantially below the critical
value for dominant cloud support. The efficient magnetic braking is
due to the pinched geometry of the magnetic field in the pseudodisk,
which strengthens the magnetic field and lengthens the level arm for
braking. Both effects enhance the magnetic transport of angular momentum
from inside to outside. The excess angular momentum is carried away
in a low-speed outflow that has, despite claims made by other workers,
little in common with observed bipolar molecular outflows. We discuss
the implications of our calculations for the formation of true disks
that are supported against gravity by rotation.
---------------------------------------------------------
Title: Collapse of Magnetized Singular Isothermal Toroids. I. The
Nonrotating Case
Authors: Allen, Anthony; Shu, Frank H.; Li, Zhi-Yun
2003ApJ...599..351A Altcode: 2003astro.ph.11376A
We study numerically the collapse of nonrotating self-gravitating
magnetized singular isothermal toroids characterized by sound speed, a,
and level of magnetic to thermal support, H<SUB>0</SUB>. In qualitative
agreement with treatments by Galli & Shu and other workers, we find
that the infalling material is deflected by the field lines toward
the equatorial plane, creating a high-density flattened structure,
a pseudodisk. The pseudodisk contracts dynamically in the radial
direction, dragging the field lines and threading them into a highly
pinched configuration that resembles a split monopole. The oppositely
directed field lines across the midplane and the large implied stresses
may play a role in how magnetic flux is lost in the actual situation
in the presence of finite resistivity or ambipolar diffusion. The
infall rate into the central regions is given to 5% uncertainty by the
formula M=(1+H<SUB>0</SUB>)a<SUP>3</SUP>/G, where G is the universal
gravitational constant, anticipated by semianalytical studies of the
self-similar gravitational collapses of the singular isothermal sphere
and isopedically magnetized disks. The introduction of finite initial
rotation results in a complex interplay between pseudodisk and true
(Keplerian) disk formation that is examined in a companion paper.
---------------------------------------------------------
Title: Radio Free-Free Emission from Young Stellar Objects
Authors: Shang, H.; Lizano, S.; Glassgold, A.; Shu, F.
2003AAS...203.3305S Altcode: 2003BAAS...35R1257S
We use x-winds heated and ionized by the physical mechanisms
discussed in earlier communications by our group to model the
free-free emission at centimeter wavelengths of the jets from young
stellar objects (YSOs). With stellar parameters, mass-loss rates,
and x-ray luminosities characteristic of Class 0 or Class I YSOs,
we recover realistic spectral indices and radio maps. In particular,
the 3.6 cm flux in models with mass loss rates ∼ 10<SUP>-6</SUP>
M<SUB>⊙</SUB>/yr or higher are comparable to existing observations of
radio jets at 0".1 resolution. Lower mass-loss rates characteristic of
Class II YSOs with revealed optical jets generally produce radio jets
that are too weak to be detected at current sensitivity levels. We
discuss the implications of these comparisons for theoretical models
of the YSO outflow phenomenon.
---------------------------------------------------------
Title: Branch, Spur, and Feather Formation in Spiral Galaxies
Authors: Chakrabarti, S.; Laughlin, G.; Shu, F. H.
2003ApJ...596..220C Altcode: 2003astro.ph..6472C
We use hydrodynamic simulations to investigate the response of
geometrically thin, self-gravitating, singular isothermal disks of
gas to imposed rigidly rotating spiral potentials. By minimizing
reflection-induced feedback from boundaries, and by restricting
our attention to models where the swing parameter X~10, we minimize
the swing amplification of global normal modes even in models where
Toomre's Q<SUB>g</SUB>~1-2 in the gas disk. We perform two classes of
simulations: short-term ones over a few galactic revolutions where
the background spiral forcing is large, and long-term ones over
many galactic revolutions where the spiral forcing is considerably
smaller. In both classes of simulations, the initial response of the
gas disk is smooth and mimics the driving spiral field. At late times,
many of the models evince substructure akin to the so-called branches,
spurs, and feathers observed in real spiral galaxies. We comment on the
parts played respectively by ultraharmonic resonances, reflection off
internal features produced by nonlinear dredging, and local, transient,
gravitational instabilities within spiral arms in the generation of such
features. Our simulations reinforce the idea that spiral structure in
the gaseous component becomes increasingly flocculent and disordered
with the passage of time, even when the background population of old
disk stars is a grand-design spiral. We speculate that truly chaotic
behavior arises when many overlapping ultraharmonic resonances develop
in reaction to an imposed spiral forcing that has itself a nonlinear,
yet smooth, wave profile.
---------------------------------------------------------
Title: Chondritic meteorites and X-wind
Authors: Shu, Frank H.
2003GeCAS..67R.435S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Epoch of Planetesimal Formation
Authors: Youdin, A. N.; Shu, F. H.
2003AAS...202.2405Y Altcode: 2003BAAS...35..729Y
The Epoch of Planetesimal Formation <P />Our research addresses the
hypothesis that kilometer sized planetesimals form directly by the
gravitational fragmentation of a layer of small (⪉ cm) midplane
solids. The advantage of this mechanism is that one need not rely
on uncertain particle sticking efficiencies. It is particularly
advantageous to bypass the difficulties of pairwise collisional growth
in the meter size range. However small solids are tightly coupled to the
gas motions. Any midplane turbulence must therefore be characterized by
very low α values to allow efficient settling to the midplane. Thus,
for the gravitational instability mechanism, planetesimals should
form relatively late in the lifetime of a YSO, e.g. near the boundary
between Class II (classical) and Class III (weak-lined) T-Tauri
stars. Additionally our model argues for a gradual buildup of the
solid/gas ratio of protoplanetary disks which greatly aids particle
settling to the midplane. Several mechanisms (bipolar outflows, radial
migration, photoionization, and layered accretion) can contribute
to this enrichment on relevant 10<SUP>5</SUP> to 10<SUP>6</SUP> year
timescales. <P />The late formation of planetesimals has implications
for the mass of planetary systems and for the metallicity enrichment
of gas giant planets and their host stars. The talk will also present
recent results on the non-linear development of the two phase (gas and
solid) gravitational instability. The focus will be on the timescale
of this process and the implications for subsequent phases of planet
formation.
---------------------------------------------------------
Title: Early Solar System Irradiation and Beryllium-7 Synthesis
Authors: Gounelle, M.; Shang, S.; Glassgold, A. E.; Shu, F. H.; Rehm,
E. K.; Lee, T.
2003LPI....34.1833G Altcode:
In the framework of the x-wind model, we have calculated the yields
of the extinct short-lived 7Be (T=53 days) produced via irradiation
by protoSolar Energetic Particles. 7Be -and other radionuclides-
yields are compatible with initial abundances inferred from meteorites.
---------------------------------------------------------
Title: Relativistic Singular Isothermal Toroids
Authors: Cai, Mike J.; Shu, Frank H.
2003ApJ...583..391C Altcode: 2003astro.ph..2474C
We construct self-similar, axisymmetric, time-independent solutions
to Einstein's field equations for an isothermal gas with a
flat rotation curve in the equatorial plane. The metric scales
as ds<SUP>2</SUP>-->α<SUP>2</SUP>ds<SUP>2</SUP> under the
transformation r-->αr and t-->α<SUP>1-n</SUP>t, where n is a
dimensionless measure of the strength of the gravitational field. The
solution space forms a two-parameter family characterized by the ratios
of the isothermal sound speed and the equatorial rotation speed to the
speed of light. The isodensity surfaces are toroids, empty of matter
along the rotation axis. Unlike the Newtonian case, the velocity field
is not constant on a cylindrical radius because of frame dragging. As
the configuration rotates faster, an ergoregion develops in the form of
the exterior of a cone centered about the rotation axis. The sequence
of solutions terminates when frame dragging becomes infinite and the
ergocone closes onto the axis. The fluid velocity of the last solution
has a modest value in the midplane but reaches the speed of light on
the axis.
---------------------------------------------------------
Title: Collapse of Magnetized, Rotating, Toroidal Clouds
Authors: Allen, Anthony; Shu, Frank; Li, Zhi-Yun; Choi, Minho;
Chuang, Hui-Wen
2003IAUS..221P..85A Altcode:
Models of molecular cloud cores have been proposed by Frank Shu
and Zhi-Yun Li. These models have been used as initial states for
magnetohydrodynamic collapse calculations to study the dynamics of
the early stages of star formation. Implications about accretion rates
outflows and angular momentum transport will be summarized. Synthetic
maps and spectra generated from simulation data will be shown.
---------------------------------------------------------
Title: Champagne Flows and Winds in H II Regions
Authors: Lizano, S.; Galli, D.; Shu, F.; Cantó, J.
2003RMxAC..15..166L Altcode:
We discuss the expansion of an initially self-gravitating, static,
singular cloud core characterized by a power-law density distribution,
rho ~ r^-n, with 3/2 < n < 3. This core is heated out of
mechanical balance by the formation of a massive star at its
center. If the initial ionization and heating is approximated to
occur instantaneously at t = 0, the subsequent flow (for r >>
100 AU) caused by the resulting imbalance between self-gravity and
thermal pressure is self-similar. Because of the steep density profile,
pressure gradients produce a shock front that travels into the cloud,
accelerating the gas to supersonic velocities in what has been called
the “champagne phase”. The expansion of the inner region at t >
0 is connected to the outer envelope of the now ionized cloud core
through this shock whose strength is an increasing function of the
exponent n. We also discuss the evolution of the strong stellar winds
of massive stars inside these champagne flows.
---------------------------------------------------------
Title: CVN Correlator and Its Future
Authors: Zhang, X.; Zheng, W.; Shu, F.; Han, Z.; Xiang, Y.; Chen,
Z.; Zhu, R.
2003ASPC..306..287Z Altcode: 2003ntvl.conf..287Z
No abstract at ADS
---------------------------------------------------------
Title: Self-similar Champagne Flows in H II Regions
Authors: Shu, Frank H.; Lizano, Susana; Galli, Daniele; Cantó, Jorge;
Laughlin, Gregory
2002ApJ...580..969S Altcode: 2002astro.ph..9036S
We consider the idealized expansion of an initially self-gravitating,
static, singular, isothermal cloud core. For t>=0, the gas is
ionized and heated to a higher uniform temperature by the formation of
a luminous but massless star in its center. The approximation that the
mass and gravity of the central star are negligible for the subsequent
motion of the H II region holds for distances r much greater than
~100 AU and for the massive cloud cores that give rise to high-mass
stars. If the initial ionization and heating are approximated to occur
instantaneously at t=0, then the subsequent flow (for r>>100 AU)
caused by the resulting imbalance between self-gravity and thermal
pressure is self-similar. Because of the steep density profile
(ρ~r<SUP>-2</SUP>), pressure gradients produce a shock front that
travels into the cloud, accelerating the gas to supersonic velocities in
what has been called the “champagne phase.” The expansion of the inner
region at t>0 is connected to the outer envelope of the now ionized
cloud core through this shock, whose strength depends on the temperature
of the H II gas. In particular, we find a modified Larson-Penston
(L-P) type of solution as part of the linear sequence of self-similar
champagne outflows. The modification involves the proper insertion of
a shock and produces the right behavior at infinity (v-->0) for an
outflow of finite duration, reconciling the long-standing conflict on
the correct (inflow or outflow) interpretation for the original L-P
solution. For realistic heating due to a massive young central star
that ionizes and heats the gas to ~10<SUP>4</SUP> K, we show that even
the self-gravity of the ionized gas of the massive molecular cloud
core can be neglected. We then study the self-similar solutions of the
expansion of H II regions embedded in molecular clouds characterized
by more general power-law density distributions: ρ~r<SUP>-n</SUP>
with 3/2<n<3. In these cases, the shock velocity is an increasing
function of the exponent n and diverges as n-->3. We show that this
happens because the model includes an origin where the pressure driving
the shock diverges because the enclosed heated mass is infinite. Our
results imply that the continued photoevaporation of massive reservoirs
of neutral gas (e.g., surrounding disks and/or globules) near the
embedded ionizing source is required in order to maintain over a
significant timescale the emission measure observed in champagne flows.
---------------------------------------------------------
Title: Planetesimal Formation by Gravitational Instability
Authors: Youdin, Andrew N.; Shu, Frank H.
2002ApJ...580..494Y Altcode: 2002astro.ph..7536Y
We investigate the formation of planetesimals via the gravitational
instability of solids that have settled to the midplane of a
circumstellar disk. Vertical shear between the gas and a subdisk
of solids induces turbulent mixing that inhibits gravitational
instability. Working in the limit of small, well-coupled particles,
we find that the mixing becomes ineffective when the surface density
ratio of solids to gas exceeds a critical value. Solids in excess of
this precipitation limit can undergo midplane gravitational instability
and form planetesimals. However, this saturation effect typically
requires increasing the local ratio of solid to gaseous surface
density by factors of 2-10 times cosmic abundances, depending on the
exact properties of the gas disk. We discuss existing astrophysical
mechanisms for augmenting the ratio of solids to gas in protoplanetary
disks by such factors and investigate a particular process that
depends on the radial variations of orbital drift speeds induced by
gas drag. This mechanism can concentrate millimeter-sized chondrules
to the supercritical surface density in <=few×10<SUP>6</SUP> yr,
a suggestive timescale for the disappearance of dusty disks around T
Tauri stars. We discuss the relevance of our results to some outstanding
puzzles in planet formation theory-the size of the observed solar
system and the rapid type I migration of Earth-mass bodies.
---------------------------------------------------------
Title: Activity in Very Cool Stars: Magnetic Dissipation in Late M
and L Dwarf Atmospheres
Authors: Mohanty, Subhanjoy; Basri, Gibor; Shu, Frank; Allard, France;
Chabrier, Gilles
2002ApJ...571..469M Altcode: 2002astro.ph..1518M
Recent observations show that chromospheric Hα activity in late M and L
dwarfs is much lower than in the earlier M types. This is particularly
surprising given that the late M and L dwarfs are comparatively very
rapid rotators: in the early M dwarfs, rapid rotation is associated
with high activity levels. One possibility is that the drop-off
in activity in the late M and L dwarfs is a result of very high
electrical resistivities in their dense, cool, and predominantly
neutral atmospheres.We calculate the magnetic field diffusivity in the
atmospheres of objects with T<SUB>eff</SUB> in the range 3000-1500 K
(mid M to late L) using the atmospheric structure models of Allard
and Hauschildt. We find that the combination of very low ionization
fraction and high density in these atmospheres results in very large
resistivities and thus efficient field diffusion. While both ambipolar
diffusion and Ohmic decay of currents due to ion-electron collisions
occur, the primary diffusion effects are due to current decay through
collisions of charged particles with neutrals. Moreover, the latter
resistivity is a strong function of both effective temperature and
optical depth, increasing rapidly as either T<SUB>eff</SUB> or optical
depth decreases. This has two implications: (1) Any magnetic field
present is increasingly decoupled from atmospheric fluid motions as one
moves from mid M to L. In the late M and L dwarfs, atmospheric motions
cannot lead to equilibrium field configurations very different from
potential ones. That is, the magnitude of magnetic stresses generated
by atmospheric motions is very small in these objects. We quantify
this effect by a simple Reynolds number calculation. (2) Even if
magnetic stresses are easily produced by fluid motions in the hot
interior (where the coupling between field and matter is good), their
propagation up through the atmosphere will be increasingly hampered
by the growing atmospheric resistivity as one moves from mid M to
late L. Thus both the generation and propagation of magnetic stresses
are increasingly damped with decreasing T<SUB>eff</SUB> in these cool
dwarfs. As a result, the magnetic free energy available for the support
of a chromosphere, and activity becomes smaller and smaller with later
type. This can account for the observed drop in Hα activity from mid M
to L, assuming that activity in these dwarfs is magnetically driven. To
check the latter assumption, we estimate the emergent acoustic fluxes
in these objects through a Lighthill-Proudman calculation. While
the acoustic fluxes also decrease with decreasing T<SUB>eff</SUB>,
they appear inadequate to explain the observed Hα fluxes in mid M to
L dwarfs. In the absence of acoustic heating, magnetic heating indeed
seems the most viable way of generating activity. We emphasize that our
calculations are equilibrium ones and do not address time-dependent
phenomena. We also do not examine the highest atmospheric layers,
where correction is not expected, but which we show are rarefied enough
to permit charged particles to remain coupled to the field. Finally,
while our calculations do not address flares in late M and L dwarfs,
we speculate that the latter could be created by buoyant flux tubes that
are generated in the interior and rise rapidly through the atmosphere,
dissipating their associated currents in the upper atmospheric layers.
---------------------------------------------------------
Title: Star-Disk Coupling Mechanisms
Authors: Shu, F. H.
2002AAS...200.5406S Altcode: 2002BAAS...34..731S
We attempt to clarify the confusion concerning angular-momentum coupling
mechanisms when closed and open magnetic fields originating from a young
star thread through a surrounding disk. We argue that the traditional
Ghosh & Lamb description represents only a transient behavior that
does not account for important longer-term effects that arise because
of accretion and if the disk is highly, but imperfectly, electrically
conducting. In the latter case, we argue that the steady-state response
of the system is to form a funnel-flow/x-wind geometry. We describe
approximate, self-consistent, calculations of the gas flow for the case
when the unperturbed magnetic-field configuration of the star would
have been a pure dipole in the absence of the disk. We show that the
disk-star interactions considerably modifies the actual magnetospheric
structure of the system. We also show calculations where we drop the
assumption that the unperturbed magnetosphere is a pure dipole. As long
as the radius of the inner edge of the disk is a few or more times the
radius of the star, we find that the properties of the x-wind are little
changed by the relaxation of the dipole assumption. However, the size
and geometry of the hot spots where the funnel flow impacts the star can
be greatly affected by the exact mixture of multipoles chosen to model
the magnetic fields on the stellar surface. The crucial invariant in our
theory is the amount of trapped flux required to truncate a disk of a
certain accretion rate before the flow reaches the equator of a star of
given mass. We present empirical evidence that trapped flux is indeed
the relevant concept for the explanation of the hot-spot properties of
T Tauri stars. We close with a qualitative discussion of the limits of
the validity of the concept of disk locking. This research is supported
in part by grants from NASA and the National Science Foundation.
---------------------------------------------------------
Title: Relativistic Self-similar Equilibria and Non-axisymmetric
Neutral Modes
Authors: Cai, Mike J.; Shu, F. H.
2002AAS...200.8004C Altcode: 2002BAAS...34..781C
We have constructed semi-analytic axisymmetric scale free solutions
to Einstein field equations with perfect fluid matter source. These
spacetimes are self-similar under the simultaneous transformation r'=
ar and t'=a<SUP>1-n</SUP>t. We explored the two dimensional solution
space parameterized by the rescaling index n and the isothermal
sound speed γ <SUP>1/2</SUP>. The isopycnic surfaces are in general
toroids. As the equilibrium configuration rotates faster, an ergo
region develops in the form of the exterior of a cone centered
about the symmetry axis. The sequence of solution terminates when
frame dragging becomes infinite and the ergo cone closes onto the
axis. In the extreme flattening limit, we have also searched for
non-axisymmetric neutral modes in a self-similar disk. Two separate
sets of tracks are discovered in the solution space. One corresponds
to the bifurcation points to non-axisymmetric equilibria, which is
confined in the non-ergo solutions. The other track signals the onset
of instability driven by gravitational radiation. These solutions are
formally infinite in extent, and thus can not represent realistic
astrophysical systems. However, if these properties do not alter
qualitatively when the self-similar configurations are truncated,
then these solutions may serve as initial data for dynamic collapse
in super massive black hole formation.
---------------------------------------------------------
Title: Relativistic Self-similar Disks
Authors: Cai, Mike J.; Shu, Frank H.
2002ApJ...567..477C Altcode: 2001astro.ph.11344C
We formulate and solve by semianalytic means the axisymmetric
equilibria of relativistic self-similar disks of infinitesimal
vertical thickness. These disks are supported in the horizontal
directions against their self-gravity by a combination of isothermal
(two-dimensional) pressure and a flat rotation curve. The dragging of
inertial frames restricts possible solutions to rotation speeds that
are always less than 0.438 times the speed of light, a result first
obtained by Lynden-Bell & Pineault in 1978 for a cold disk. We show
that prograde circular orbits of massive test particles exist and are
stable for all of our model disks but that retrograde circular orbits
cannot be maintained with particle velocities less than the speed of
light once the disk develops an ergoregion. We also compute photon
trajectories, planar and nonplanar, in the resulting spacetime for
disks with and without ergoregions. We find that all photon orbits,
except for a set of measure zero, tend to be focused by the gravity
of the flattened mass-energy distribution toward the plane of the
disk. This result suggests that strongly relativistic, rapidly rotating,
compact objects may have difficulty ejecting collimated beams of matter
or light along the rotation axes until the flows get well beyond the
flattened parts of the relativistic mass distribution (which cannot
happen in the self-similar models considered in this paper).
---------------------------------------------------------
Title: Heating and Ionization of X-Winds
Authors: Shang, Hsien; Glassgold, Alfred E.; Shu, Frank H.; Lizano,
Susana
2002ApJ...564..853S Altcode: 2001astro.ph.10539S
In order to compare the X-wind with observations, one needs to be able
to calculate its thermal and ionization properties. We formulate the
physical basis for the streamline-by-streamline integration of the
ionization and heat equations of the steady X-wind. In addition to the
well-known processes associated with the interaction of stellar and
accretion funnel hot spot radiation with the wind, we include X-ray
heating and ionization, mechanical heating, and a revised calculation
of ambipolar diffusion heating. The mechanical heating arises from
fluctuations produced by star-disk interactions of the time-dependent
X-wind that are carried by the wind to large distances where they
are dissipated in shocks, MHD waves, and turbulent cascades. We model
the time-averaged heating by the scale-free volumetric heating rate,
Γ<SUB>mech</SUB>=αρv<SUP>3</SUP>s<SUP>-1</SUP>, where ρ and v
are the local mass density and wind speed, respectively, s is the
distance from the origin, and α is a phenomenological constant. When
we consider a partially revealed but active young stellar object, we
find that choosing α~10<SUP>-3</SUP> in our numerical calculations
produces temperatures and electron fractions that are high enough for
the X-wind jet to radiate in the optical forbidden lines at the level
and on the spatial scales that are observed. We also discuss a variety
of applications of our thermal-chemical calculations that can lead to
further observational checks of X-wind theory.
---------------------------------------------------------
Title: Collapse of Magnetised, Singular Isothermal Toroids
Authors: Allen, Anthony; Shu, Frank; Li, Zhi-Yun
2001JKAS...34..325A Altcode:
This poster summarizes numerical collapse calculations of non-rotating
and rotating singular, isothermal toroids that employed the
zeus2d (Norman and Stone 1992) magnetohydrodynamics package. In the
non-rotating collapse calculations, it is seen that infall proceeds at
a constant rate and magnetically supported, high density pseudo-disks
form in the equatorial plane. With rotating clouds, however, toroidal
magnetic fields grow as infall proceeds, teaming with angular momentum
to slow the inflow to the center and generate outflow.
---------------------------------------------------------
Title: Heating and Ionization of X-Winds
Authors: Shang, H.; Glassgold, A. E.; Shu, F. H.; Lizano, S.
2001AAS...198.2801S Altcode: 2001BAAS...33..824S
We formulate the physical basis for a streamline-by-streamline
integration of the ionization and heat equations of the steady
x-wind. In addition to th well-known processes associated with the
interaction of stellar and accretion-funnel hot-spot radiation with
the wind, we include X-ray heating and ionization, mechanical heating,
and a new calculation of ambipolar diffusion heating. The mechanical
heating arises from the fluctuations produced by the time dependent
x-wind star-disk interaction, which generates magnetic fluctuations at
the source that are carried by the wind to large distances where they
are dissipated in shocks, MHD waves, and turbulent cascades. We model
the time-averaged heating by the scale-free volumetric heating rate Γ
<SUB></SUB> mech = α ρ v<SUP>3</SUP> s<SUP>-1</SUP>, where ρ and
v are the local mass density and wind speed, respectively, s is the
distance from the origin, and α is a phenomenological constant. When
we consider a revealed but active young stellar object, we find that
choosing α ~ 10<SUP>-3</SUP> in our numerical calculations produces
temperatures and electron fractions that are high enough for the
x-wind jet to radiate in the optical forbidden lines at the level and
on the spatial and kinematic scales that are observed. We also discuss
a variety of applications of our thermal-chemical calculations that
lead to further observational checks of the theory. This research has
been supported in part by the NSF.
---------------------------------------------------------
Title: The X-Wind Theory for the Origin of Chondritic Meteorites
Authors: Shu, F. H.
2001eag..conf.3700S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Ambipolar Diffusion and X-ray Heating of the X-wind Jet
Authors: Glassgold, A. E.; Shang, H.; Shu, F. H.; Lizano, S.
2001AAS...198.2802G Altcode: 2001BAAS...33..824G
Ambipolar diffusion and X-ray interactions have the potential to
heat the inner part (or jet) of the X-wind. We have recalculated the
ambipolar diffusion coupling coefficient for atomic regions (where
H<SUP>+</SUP> is the dominant ion) using recently calculated cross
sections by Krstíc and Schultz and by others. The coefficient for
hot regions (T ≈ 10<SUP>4</SUP> K) is an order of magnitude larger
than the values used in the past because the short-range part of the
H-H<SUP>+</SUP> interaction (including exchange effects) is larger
than previously assumed. Consequently ambipolar diffusion plays only
a minor role in heating the inner part of the X-wind and very likely
in YSO jets in general. We also find that X-ray heating is enhanced
in atomic regions through the significant probability for populating
excited levels of the H atom in collisions with X-ray induced secondary
electrons (e.g., Dalgarno, Yan, and Liu 1999). The standard Lyman-alpha
cooling must then be reduced by an amount that can be greater than
direct collisional X-ray heating. The excess population in the n = 2
level leads to heating by collisional de-excitation in the presence of
trapped Lyman-alpha line radiation. However, even with this enhancement,
X-ray heating is usually less important than the main heating mechanism
for the inner X-wind, which involves the dissipation of shock and
turbulent energy. This research has been supported in part by the NSF.
---------------------------------------------------------
Title: Self-similar Relativisitic Disks revisted
Authors: Cai, M. J.; Shu, F. H.
2001AAS...198.3706C Altcode: 2001BAAS...33..836C
We revisit the rotating self-similar disk first studied by Lynden-Bell
and Pineault and extend it to include pressure. A two-parameter family
of solutions is constructed numerically. These disks are parameterized
by the constant linear rotation velocity v, and the isothermal
sound speed γ <SUP>1/2</SUP>. For sufficiently high velocities,
an ergo region develops in the form of the exterior of a cone. For
each value of γ , there is a maximum velocity v<SUB>c</SUB> above
which there is no equilibrium solutions. For this solution the frame
dragging is infinite and the ergo cone closes on the rotation axis. The
null geodesic equations are also integrated numerically. Due to the
infinite extend and mass of the system, all photon trajectories are
focused towards the disk. The behavior of equatorial photons orbits
is qualitatively the same as that of cold disks.
---------------------------------------------------------
Title: Singular Isothermal Disks. II. Nonaxisymmetric Bifurcations
and Equilibria
Authors: Galli, Daniele; Shu, Frank H.; Laughlin, Gregory; Lizano,
Susana
2001ApJ...551..367G Altcode: 2000astro.ph.12242G
We review the difficulties of the classical fission and fragmentation
hypotheses for the formation of binary and multiple stars. A crucial
missing ingredient in previous theoretical studies is the inclusion of
dynamically important levels of magnetic fields. As a minimal model
for a candidate precursor to the formation of binary and multiple
stars, we therefore formulate and solve the problem of the equilibria
of isopedically magnetized, singular isothermal disks, without the
assumption of axial symmetry. Considerable analytical progress can be
made if we restrict our attention to models that are scale-free, i.e.,
that have surface densities that vary inversely with distance ϖ from
the rotation axis of the system. In agreement with earlier analysis by
Syer & Tremaine, we find that lopsided (M=1) configurations exist at
any dimensionless rotation rate, including zero. Multiple-lobed (M=2,
3, 4,...) configurations bifurcate from an underlying axisymmetric
sequence at progressively higher dimensionless rates of rotation, but
such nonaxisymmetric sequences always terminate in shock waves before
they have a chance to fission into M=2, 3, 4,... separate bodies. On
the basis of our experience in this paper and the preceding Paper I,
we advance the hypothesis that binary and multiple star formation
from smooth (i.e., not highly turbulent) starting states that are
supercritical but in unstable mechanical balance requires the rapid
(i.e., dynamical) loss of magnetic flux at some stage of the ensuing
gravitational collapse.
---------------------------------------------------------
Title: Refractory Phases of Micrometeorites and the “Primitivity”
of Cometary Nuclei
Authors: Gounelle, M.; Maurette, M.; Engrand, C.; Kurat, G.; Shu, F.
2001LPI....32.1626G Altcode:
We used common characteristics of refractory phases in modern and early
micrometeorites to both further constrain the scenarios proposed for
the origin of the solar system and confirm that cometary nuclei are
not as primitive as previously thought.
---------------------------------------------------------
Title: The Origin of Chondrules and Refractory Inclusions in
Chondritic Meteorites
Authors: Shu, Frank H.; Shang, Hsien; Gounelle, Matthieu; Glassgold,
Alfred E.; Lee, Typhoon
2001ApJ...548.1029S Altcode:
Examples of calcium-aluminum-rich inclusions (CAIs) surrounded by
thick chondrule mantles have been found in chondritic meteorites
and cast doubt on the conventional belief that CAIs and chondrules
possessed different spacetime origins in the primitive solar nebula. We
study specific processes by which such objects, and the more common
ordinary CAIs and chondrules, might have formed by flare heating of
primitive rocks interior to the inner edge of a gaseous accretion
disk that has been truncated by magnetized funnel flow onto the
central proto-Sun. Motivated by the appearance of the chains of
Herbig-Haro knots that define collimated optical jets from many
young stellar objects (YSOs), we adopt the model of a fluctuating
X-wind, where the inner edge of the solar nebula undergoes periodic
radial excursions on a timescale of ~30 yr, perhaps in response
to protosolar magnetic cycles. Flares induced by the stressing of
magnetic fields threading both the star and the inner edge of the
fluctuating disk melt or partially melt solids in the transition zone
between the base of the funnel flow and the reconnection ring, and in
the reconnection ring itself. The rock melts stick when they collide
at low velocities. Surface tension pulls the melt aggregate into a
quasi-spherical core/mantle structure, where the core consists mainly
of refractories and the mantle mainly of moderate volatiles. Orbital
drift of rocks past the inner edge of the disk or infall of large
objects from the funnel flow replaces the steady loss of material by
the plasma drag of the coronal gas that corotates with the stellar
magnetosphere. In quasi-steady state, agglomeration of molten or
heat-softened rocks leads to a differential size-distribution in radius
R proportional to R<SUP>-3</SUP>e<SUP>-Lt/t<SUB>L</SUB>R</SUP>, where
t<SUB>L</SUB>~20 yr is the drift time of an object of fiducial radius
L≡1 cm and t is the time since the last inward excursion of the
base of the funnel flow and X-wind. Thus, during the ~30 yr interval
between successive flushing of the reconnection ring, flash-heated and
irradiated rocks have a chance to grow to millimeter and centimeter
sizes. The evaporation of the moderately volatile mantles above large
refractory cores, or the dissolving of small refractory cores inside
thick ferromagnesian mantles before launch, plus extended heating in
the X-wind produce the CAIs or chondrules that end up at planetary
distances in the parent bodies of chondritic meteorites.
---------------------------------------------------------
Title: Extinct Radioactivities and Protosolar Cosmic Rays:
Self-Shielding and Light Elements
Authors: Gounelle, Matthieu; Shu, Frank H.; Shang, Hsien; Glassgold,
A. E.; Rehm, K. E.; Lee, Typhoon
2001ApJ...548.1051G Altcode:
We study the effects of self-shielding in the X-wind model of
protosolar cosmic-ray irradiation of early solar-system rocks. We
adopt a two-component picture of protoCAIs consisting of cores with the
elemental abundances of type B1 CAIs (calcium-aluminum-rich inclusions)
and mantles of less refractory material. The cores have a power-law
distribution of sizes between R<SUB>min</SUB> and R<SUB>max</SUB>. The
mantles have a uniform thickness, whose value is chosen to bring
the total inventory of elements at least as refractory as sulfur to
cosmic abundances for the entire population of protoCAIs. Each object is
irradiated with a fluence consistent with the product of their residence
time in the reconnection ring and the flux of solar cosmic rays obtained
by a scaling of impulsive flares from the hard X-rays observed from
low-mass protostars. For R<SUB>min</SUB> in the 50 μm regime and
R<SUB>max</SUB> in the few centimeter regime, which corresponds to the
range of sizes of observed CAIs in micrometeorites and chondrites,
we recover approximately the canonical values quoted for the ratios
<SUP>26</SUP>Al/<SUP>27</SUP>Al, <SUP>53</SUP>Mn/<SUP>55</SUP>Mn,
and <SUP>41</SUP>Ca/<SUP>40</SUP>Ca in CV3 meteorites. Moreover, the
excess <SUP>138</SUP>La (denoted as <SUP>138</SUP>La<SUP>*</SUP>)
produced by proton bombardment of <SUP>138</SUP>Ba lies within the
CAI range obtained in the experiments of Shen et al. When we include
fragmentation reactions that produce <SUP>10</SUP>Be from the impact of
protons, alphas, and <SUP>3</SUP>He on the <SUP>16</SUP>O that is bound
up in rocks, we further obtain a level of <SUP>10</SUP>Be/<SUP>9</SUP>Be
that agrees approximately with the report of McKeegan et al. for a
CAI from the Allende meteorite. Similar calculations for the expected
anomalies in the stable isotopes of lithium show rough consistency
with the measured values and further support our interpretation. The
value for <SUP>10</SUP>Be/<SUP>9</SUP>Be is particularly difficult to
produce by any other astrophysical mechanism. Thus, the <SUP>10</SUP>Be
discovery greatly strengthens the case for an origin in early
solar-system irradiation, rather than external stellar seeding, for
the shortest-lived radionuclides inferred from CAIs in chondritic
meteorites.
---------------------------------------------------------
Title: Singular Isothermal Disks and the Formation of Multiple Stars
Authors: Galli, D.; Shu, F. H.; Laughlin, G.; Lizano, S.
2001ASPC..243...93G Altcode: 2000astro.ph.12019G; 2001fdtl.conf...93G
A crucial missing ingredient in previous theoretical studies of
fragmentation is the inclusion of dynamically important levels of
magnetic fields. As a minimal model for a candidate presursor to the
formation of binary and multiple stars, we therefore consider the
equilibrium configuration of isopedically magnetized, scale-free,
singular isothermal disks, without the assumption of axial
symmetry. We find that lopsided (M=1) configurations exist at any
dimensionless rotation rate, including zero. Multiple-lobed (M = 2,
3, 4, ...) configurations bifurcate from an underlying axisymmetric
sequence at progressively higher dimensionless rates of rotation, but
such nonaxisymmetric sequences always terminate in shockwaves before
they have a chance to fission into separate bodies. We advance the
hypothesis that binary and multiple star-formation from smooth (i.e.,
not highly turbulent) starting states that are supercritical but in
unstable mechanical balance requires the rapid (i.e., dynamical) loss
of magnetic flux at some stage of the ensuing gravitational collapse.
---------------------------------------------------------
Title: Time Scale of Disk and Planet Formation
Authors: Shu, Frank H.
2001ASPC..245...89S Altcode: 2001aats.conf...89S
No abstract at ADS
---------------------------------------------------------
Title: Excitation Conditions and Line Ratios from X-wind Jets
Authors: Shang, H.; Glassgold, A. E.; Shu, F. H.; Lizano, S.
2000AAS...19713209S Altcode: 2001BAAS...33R.718S
Thermal structures of the X-wind proves to be a necessary yet very
important foundation for realistic modeling of the jet phenomena. X-rays
and magnetic disturbances emanating from the central source, are
identified as the self-sustaining energy generators in the x-wind
star-disk interacting system. Following the demonstration of density
structure and kinematic information in X-winds in Shang et al. (1998)
with the assumption of uniform ionization conditions to calculate
the emissions in the [S II]λ 6716/6731 and [O I]λ 6300/6364 lines,
we investigate the self-consistent excitation conditions powered by the
energy sources in the X-wind system. We perform streamline-by-streamline
integrations of the rate and heat equations on the steady-state
x-wind. Results show that X-rays are capable of providing typical
ionization fractions inferred from the optical jets. We model the
external heating mechanism as he averaging of mechanical energies
deposited by shocks and/or MHD turbulence cascades. Shockwaves or
turbulence cascades of magnetic origins are needed to provide the
sufficiently high temperatures in enough emitting areas of the jets
observed in the forbidden lines at thousands of AU from the stars to
maintain the excitation. A good demonstration of such diagnostics would
be line ratios from the optical forbidden lines. We will report in this
presentation that the electron densities and temperatures achieved in
the overall self-consistent model yield line ratios similar to those
observed in a large set of YSO jet sources.
---------------------------------------------------------
Title: What Do Meteorites Tell Us about the Formation of the Solar
System?
Authors: Shu, F. H.
2000AAS...197.0201S Altcode: 2000AAS...197..201S; 2000BAAS...32.1385S
The existence of chondritic meteorites has posed deep puzzles
for our understanding of the origin of the solar system for two
hundred years. Chondrites are believed to be primitive material that
recorded the physical conditions under which solids in the solar
nebula aggregated to form the interiors of the terrestrial planets
and the cores of the giant planets. Chondritic meteorites contain
a grainy matrix that has obviously never been heated to more than a
few hundred K, intimately mixed with mm- to cm-sized balls of rock
that have been brought briefly (hours for chondrules to days for
refractory inclusions) to the melting temperatures of iron-magnesium
silicates or calcium-aluminum silicates and oxides. Moreover, the
calcium-aluminum-rich inclusions (CAIs) contain evidence for short-lived
radioactivities that should have been long extinct at the birth of
the sun and its planets, and yet were clearly alive at the origin
of the meteoritical parent bodies. According to modern astronomical
observations and theory, the solar nebula was not nearly hot enough to
melt rocks at the distance of the asteroid belt where the parent bodies
of meteorites reside, nor are time scales as short as hours to days
natural for such regions. I will review a new theory developed by our
group which suggests that chondrules and CAIs originated much closer
to the protosun, and that they were irradiated by protosolar flares
and cosmic-ray particles in the fierce interaction region between a
strongly magnetized protosun and its surrounding protoplanetary disk,
before being thrown to the distances of their modern host bodies by a
powerful bipolar jet that is another byproduct of this interaction. I
will present evidence unearthed since the initial development of the
theory that supports its assumptions and conclusions, and I will
conclude with some brief remarks about the changes brought to the
question of our astronomical origins by the new perspective. This work
was funded by NASA's program on Origins of Solar Systems.
---------------------------------------------------------
Title: Making Calcium-Aluminum-rich Inclusions and Chondrules near
the Young Sun by Flares
Authors: Shu, F. H.; Shang, H.; Gounelle, M.; Glassgold, A. E.; Lee, T.
2000M&PSA..35Q.145S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Extinct Radioactivities in Meteorites and Protosolar Cosmic
Rays: The Effect of Self-Shielding
Authors: Gounelle, M.; Shu, F. H.; Shang, S.; Glassgold, A. E.; Rehm,
K. E.; Lee, T.
2000M&PSA..35R..62G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Toy Model of Giant Molecular Clouds
Authors: Allen, Anthony; Shu, Frank H.
2000ApJ...536..368A Altcode:
We idealize giant molecular clouds as flattened sheets, incorporating
the concepts of strong magnetization, star formation from dense cores,
and efficient bipolar outflows. This toy model reproduces the observed
tendency of molecular clouds to form filaments without the need to
invoke large-scale overall gravitational collapse that would yield a
rate of star formation far in excess of empirical Galactic values. It
yields line width-size relationships that are in rough accord with
observations, although better simulations are needed that remove
the imposition of periodic boundary conditions and allow for a more
systematic treatment of subgrid turbulence. The model lends credence
to earlier ideas concerning the self-regulation of star formation by
turbulence and photoionization.
---------------------------------------------------------
Title: X-Winds Theory and Observations
Authors: Shu, F. H.; Najita, J. R.; Shang, H.; Li, Z. -Y.
2000prpl.conf..789S Altcode:
We review the theory of x-winds in young stellar objects (YSOs),
and we compare its predictions with a variety of astronomical
observations. Such flows arise magnetocentrifugally from accretion
disks when their inner edges interact with strongly magnetized central
stars. X-winds collimate logarithmically slowly into jets, and their
interactions with the surrounding molecular cloud cores of YSOs yield
bipolar molecular outflows.
---------------------------------------------------------
Title: Singular Isothermal Disks. I. Linear Stability Analysis
Authors: Shu, Frank H.; Laughlin, Gregory; Lizano, Susana; Galli,
Daniele
2000ApJ...535..190S Altcode:
As part of a larger effort to understand how binary and single stars
form from the collapse of magnetized molecular cloud cores, we perform
a global stability analysis of isopedically magnetized, singular
isothermal disks (SIDs). The work described here has precedents in
earlier studies of disturbances in power-law disks by Zang in 1976,
Toomre in 1977, Lynden-Bell & Lemos in 1993, Syer & Tremaine in
1996, and Goodman & Evans in 1999. We find the analytic criteria
for the bifurcation of axisymmetric disks into nonaxisymmetric forms
with azimuthal periodicities m=1 (eccentric displacements), 2 (oval
distortions), 3 (triangular distortions), etc. These bifurcations,
which occur at zero frequency, are the compressible and differentially
rotating analogs of how the classical sequence of incompressible and
uniformly rotating Maclaurin spheroids bifurcate (secularly, under
dissipative forces) to become Dedekind ellipsoids with figure axes that
remain fixed in space. Like Syer & Tremaine and Lynden-Bell &
Lemos, we also find that zero-frequency logarithmic spirals are possible
scale-free disturbances, but our interpretation of the existence of
such steadily propagating wavetrains is different. We give a dynamical
instability interpretation based on the onset of swing amplification
by overreflection at the corotation circle of prograde spiral
density waves the pattern speeds of which have nonzero and positive
values. Our analysis yields identical instability criteria as the global
normal-modes treatment of Goodman & Evans, and we tentatively also
identify dynamical barred-spiral instabilities as the “breathing mode”
limit of two-armed ordinary-spiral instabilities. We prove a general
“reciprocity theorem,” which states that the overreflection factors
are identical for spiral density waves launched from cavities interior
or exterior to Q-barriers that straddle the corotation circle. This
globally valid result supports a unifying interpretation, advocated
for many years by C. C. Lin and his colleagues (see, e.g., work by
Bertin & Lin): the coexistence of spiral structure in galaxies
arising from the instability of internal normal modes in the combined
star/gas disk or from driving by external tidal influences associated
with the chance passages of companion bodies.
---------------------------------------------------------
Title: Protostellar Winds and Chondritic Meteorites
Authors: Shang, Hsien; Shu, Frank H.; Lee, Typhoon; Glassgold,
Alfred E.
2000SSRv...92..153S Altcode:
We discuss the interaction between the magnetosphere of a young star
and its surrounding accretion disk. We consider how an X-wind can
be driven magnetocentrifugally from the inner edge of the disk where
accreting gas is diverted onto stellar field lines either to flow onto
the Sun or to be flung outwards with the wind. The X-wind satisfies
many observational tests concerning optical jets, Herbig-Haro objects,
and molecular outflows. Connections may exist between primitive solar
system materials and X-winds. Chondrules and calcium-aluminum-rich
inclusions (CAIs) experienced short melting events uncharacteristic
of the asteroid belt where meteorites originate. The inner edge of
the solar nebula has the shortest orbital timescale available to the
system, a few days. Protosolar flares introduce another timescale,
tens of minutes to hours. CAIs may form when solids are lifted from
shaded portions of the disk close to the Sun and are exposed to its
intense light for a day or so before they are flung by the X-wind to
much larger distances. Chondrules were melted, perhaps many times,
by flares at larger distances from the Sun before being launched
and annealed, but not remelted, in the X-wind. Aerodynamic sorting
explains the narrow range of sizes with which CAIs and chondrules
are found in chondritic meteorites. Flare-generated cosmic-rays may
induce spallation reactions that produce some of the short-lived
radioactivities associated with primitive solar system rocks.
---------------------------------------------------------
Title: Protostellar Winds and Chondritic Meteorites
Authors: Shang, H.; Shu, F. H.; Lee, T.; Glassgold, A. E.
2000fdtp.conf..153S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Singular Isothermal Disks
Authors: Galli, Danielle; Shu, Frank H.; Laughlin, Gregory; Lizano,
Susana
2000STIN...0107251G Altcode:
We review the difficulties of the classical fission and fragmentation
hypotheses for the formation of binary and multiple stars. A crucial
missing ingredient in previous theoretical studies is the inclusion of
dynamically important levels of magnetic fields. As a minimal model
for a candidate presursor to the formation of binary and multiple
stars, we therefore formulate and solve the problem of the equilibria
of isopedically magnetized, singular isothermal disks, without the
assumption of axial symmetry. Considerable analytical progress can be
made if we restrict our attention to models that are scale-free, i.e.,
that have surface densities that vary inversely with distance omega from
the rotation axis of the system. In agreement with earlier analysis by
Syer and Tremaine, we find that lopsided (M = 1) configurations exist
at any dimensionless rotation rate, including zero. Multiple-lobed (M =
2, 3, 4, ...) configurations bifurcate from an underlying axisymmetric
sequence at progressively higher dimensionless rates of rotation,
but such nonaxisymmetric sequences always terminate in shockwaves
before they have a chance to fission into M = 2, 3, 4, ... separate
bodies. On the basis of our experience in this paper, we advance the
hypothesis that binary and multiple star-formation from smooth (i.e.,
not highly turbulent) starting states that are supercritical but in
unstable mechanical balance requires the rapid (i.e., dynamical) loss
of magnetic flux at some stage of the ensuing gravitational collapse.
---------------------------------------------------------
Title: Chandrasekhar's Study of Dynamical Friction
Authors: Shu, Frank H.
1999ApJ...525C.347S Altcode: 1999ApJC..525..347S
No abstract at ADS
---------------------------------------------------------
Title: Scale-free Equilibria of Isopedic Polytropic Clouds
Authors: Galli, D.; Lizano, S.; Li, Z. Y.; Adams, F. C.; Shu, F. H.
1999ApJ...521..630G Altcode: 1999astro.ph..4028G
We investigate the equilibrium properties of self-gravitating magnetized
clouds with polytropic equations of state with negative index n. In
particular, we consider scale-free isopedic configurations that have
constant dimensionless spherical mass-to-flux ratio λ<SUB>r</SUB>
and that may constitute “pivotal” states for subsequent dynamical
collapse to form groups or clusters of stars. For given Γ=1+1/n,
equilibria with smaller values of λ<SUB>r</SUB> are more flattened,
ranging from spherical configurations with λ<SUB>r</SUB>=∞
to completely flattened states for λ<SUB>r</SUB>=1. For a given
amount of support provided by the magnetic field as measured by the
dimensionless parameter H<SUB>0</SUB>, equilibria with smaller values
of Γ are more flattened. However, logatropic (defined by Γ=0) disks
do not exist. The only possible scale-free isopedic equilibria with
logatropic equation of state are spherical uniformly magnetized clouds.
---------------------------------------------------------
Title: Annual review of astronomy and astrophysics. Vol. 37.
Authors: Burbidge, G.; Sandage, A.; Shu, F. H.
1999ARA&A..37.....B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Our Changing Society
Authors: Shu, Frank H.
1999aasf.book..307S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Low-Mass Star Formation: Theory
Authors: Shu, Frank H.; Allen, Anthony; Shang, Hsien; Ostriker,
Eve C.; Li, Zhi-Yun
1999ASIC..540..193S Altcode: 1999osps.conf..193S
Magnetic Support of Clouds Fragmentation Empirical Measurements of the
Mass-to-Flux Ration Turbulent Support of Molecular Clouds Filamentary
Structure in Molecular Clouds Sustenance of Cloud Turbulence Through
YSO Outflows A Toy Model of GMC Structure and Evolution Formation
and Evolution of Molecular Cloud Cores The Pivotal State Self-Similar
Collapse of Singular Isothermal Toroids Mass Loss from Stars and Disks
Generalized X-wind Model Summary
---------------------------------------------------------
Title: Protostellar Cosmic Rays and Extinct Radioactivities in
Meteorites
Authors: Lee, Typhoon; Shu, Frank H.; Shang, Hsien; Glassgold, Alfred
E.; Rehm, K. E.
1998ApJ...506..898L Altcode:
Calcium-aluminum-rich inclusions (CAIs) and chondrules of chondritic
meteorites may originate with the melting of dustballs launched by
a magnetically driven bipolar outflow from the inner edge of the
primitive solar nebula. Bombardment by protostellar cosmic rays
may make the rock precursors of CAIs and chondrules radioactive,
producing radionuclides found in meteorites that are difficult to
obtain with other mechanisms. Reasonable scalings from the observed
hard X-rays for the cosmic-ray protons released by flares in young
stellar objects yield the correct amounts of <SUP>41</SUP>Ca,
<SUP>53</SUP>Mn, and <SUP>138</SUP>La inferred for meteorites, but
proton- and α-induced transformations underproduce <SUP>26</SUP>Al by
a factor of about 20. The missing <SUP>26</SUP>Al may be synthesized
by <SUP>3</SUP>He nuclei accelerated in impulsive flares reacting
primarily with <SUP>24</SUP>Mg, an abundant isotope in the target
precursor rocks. The mechanism allows a simple explanation for the very
different ratios of <SUP>26</SUP>Al/<SUP>27</SUP>Al inferred for normal
CAIs, CAIs with fractionated and unidentified nuclear (FUN) anomalies,
and chondrules. The overproduction of <SUP>41</SUP>Ca by analogous
<SUP>3</SUP>He reactions and the case of <SUP>60</SUP>Fe inferred for
eucritic meteorites require special interpretations in this picture.
---------------------------------------------------------
Title: Star Formation, Near and Far
Authors: Shu, Frank H.
1998AAS...192.3107S Altcode: 1998BAAS...30..855S
We review what has been learned during the past two decades from
observations of nearby star-forming regions, and the theory developed
to describe the simplest case of the birth of low-mass stars from
isolated molecular cloud cores. We then outline the principal
difficulties in understanding the formation of high-mass stars,
especially when they occur under crowded and turbulent conditions,
as they usually do. Finally, we speculate on possible differences
made by much lower metallicities and magnetic field strengths in the
case of the formation of the first generation of stars. Throughout our
discussion, we emphasize the importance of having measurements at the
highest spatial and spectral resolutions if we are to make breakthroughs
on the most obstinate problems in the field. Large-aperture telescopes
and interferometric arrays at infrared, submillimeter, and millimeter
wavelengths, operating from high mountaintops, or borne on airplanes
or into space, are the crucial components for the observational attack
on these very challenging problems.
---------------------------------------------------------
Title: Synthetic Images and Long-Slit Spectra of Protostellar Jets
Authors: Shang, Hsien; Shu, Frank H.; Glassgold, Alfred E.
1998ApJ...493L..91S Altcode:
We construct synthetic images and long-slit spectra of protostellar jets
modeled as steady state X-winds. Assuming uniform ionization fractions
and electron temperatures, we calculate non-LTE level populations for
five-level atoms. Synthetic images in the [S II] λ6716/λ6731 and [O I]
λ6300/λ6364 lines have roughly the same surface brightness as observed
jets if we ignore their knotty structure. Long-slit spectra taken
with the slit placed along the central axis of the jet, or slightly
displaced laterally from it, provide strong evidence in support of the
X-wind theory. In both the models and the actual objects, wide line
profiles, containing both large positive and negative velocities,
are often seen at the base of the flow, indicative of a wide-angle
wind. As one progresses up the length of the slit, the line profiles
narrow to straddle the projected velocity of a highly collimated jet.
---------------------------------------------------------
Title: Annual review of astronomy and astrophysics. Vol. 36.
Authors: Burbidge, G.; Sandage, A.; Shu, F. H.
1998ARA&A..36.....B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Synthetic Images and Long-Slit Spectra of Protostellar Jets
Authors: Shu, F. H.; Shang, Hsien; Glassgold, A. E.
1997AAS...19112107S Altcode: 1997BAAS...29.1404S
We construct synthetic images and long-slit spectra of protostellar jets
modeled as steady-state x-winds. Assuming uniform ionization fractions
and electron temperatures, we calculate non-LTE level populations
for five-level atoms. Synthetic images in the [S II]lambda 6716/6731
and [O I]lambda 6300/6364 lines have approximately the same surface
brightness distribution as observed jets. Thus, observed jets may be
optical illusions at their base, since x-winds become highly focused
only at large distances from the source. Long-slit spectra taken with
the slit placed along the central axis of the jet, or slightly diplaced
from it, provide strong evidence in support of the x-wind theory. In
both the models and the actual objects, wide line-profiles, containing
both large positive and negative velocities, are often seen at the
base of the flow, indicative of a wide-angle wind. As one progresses
uo the length of the slit, the line profiles narrow to straddle the
projected velocity of a well-collimated jet.
---------------------------------------------------------
Title: The Origin of Chondrites and Extinct Radioactivities in the
Solar System
Authors: Shu, F. H.; Shang, H.; Lee, T.; Glassgold, A. E.
1997AAS...190.4905S Altcode: 1997BAAS...29..845S
We will review current ideas concerning the formation of sunlike
stars and planetary systems. In particular, we discuss the idea that
the chondrules and CAIs found in chondritic meteorites might have
been formed as solids entrained and melted in the bipolar wind that
results from the interaction of the accreting protosolar nebula and
the magnetosphere of the young protosun. Aerodynamic sorting and a
mechanical selection for molten droplets that rain back onto the disk
at planetary distances explain the size distributions and patterns
of element segregation that we observe in carbonaceous and ordinary
chondrties. Cosmic ray ions generated in the flares that accompany
the general magnetic activity of the inner region may irradiate the
precursor rocks before they are launched in the bipolar wind. Under
reasonable scaling assumptions for the efficiency of the process in
protostars, cosmic-ray bombardment suffices to generate the short-lived
radionuclides (26) Al, (41) Ca, and (53) Mn at their inferred meteoritic
levels.
---------------------------------------------------------
Title: Magnetic Forces in an Isopedic Disk
Authors: Shu, Frank H.; Li, Zhi-Yun
1997ApJ...475..251S Altcode:
We consider the magnetic forces in electrically conducting thin disks
threaded by magnetic fields originating in the external (interstellar)
medium. We focus on disks that have dimensionless ratios λ of the
mass to flux that are spatially constant, a condition that we term
isopedic. For arbitrary distributions of the surface density Σ
(which can be nonaxisymmetric and time dependent), we show that the
magnetic tension exerts a force in the plane of the disk equal to
-1/λ<SUP>2</SUP> times the self-gravitational force. In addition,
if the disk maintains magnetostatic equilibrium in the vertical
direction, the magnetic pressure, integrated over the z-height of the
disk, may be approximated as (1 + η<SUP>2</SUP>)/(λ<SUP>2</SUP> +
η<SUP>2</SUP>) times the gas pressure integrated over z, where η
≡ f<SUB>||</SUB>/2πGΣ and f<SUB>||</SUB> is the component of the
local gravitational field parallel to the plane of the disk. We apply
these results to the problem of the stability of magnetized isothermal
disks to gravitational fragmentation into subcondensations of a
size comparable to the vertical scale height of the disk. Contrary
to common belief, such dynamical fragmentation probably does not
occur. In particular, the case of the magnetized singular isothermal
disk undergoes not dynamical fragmentation into many subcondensations,
but inside-out collapse into a single compact object, a self similar
problem that is studied in a companion paper (Li & Shu 1997).
---------------------------------------------------------
Title: Self-Similar Collapse of an Isopedic Isothermal Disk
Authors: Li, Zhi-Yun; Shu, Frank H.
1997ApJ...475..237L Altcode:
We study the gravitational collapse of an isothermal disk which is
isopedically magnetized (i.e., with a mass-to-flux ratio that is
spatially constant). The two theorems concerning magnetic forces in
such a disk proven in a companion paper (Shu & Li 1997), plus the
self-similar nature of the overall problem, allow a semianalytical
treatment. The inflow occurs in an inside-out manner similar to that
which applies in the collapse of the unmagnetized singular isothermal
sphere (Shu 1977). These two cases (singular sphere and disk) bracket
the range of possible collapse behaviors expected for the family
of isopedic singular isothermal toroids described by Li & Shu
(1996b). Although the strong magnetic fields dilute the effects of
self-gravity in the isopedic isothermal disk, they do not prevent
its outer parts (envelope) from falling onto the central condensed
object (protostar) at a fixed infall rate Ṁ, even when the field is
perfectly frozen to the matter. Indeed, the higher densities supported
by the fields in the equilibrium state increase Ṁ during collapse
in comparison with the unmagnetized case. The larger effective
speed of sound due to magnetization produces a smaller effect. The
flattened geometry enforced by the strong magnetic fields introduces
a complication: the appearance of an outwardly propagating shock wave
that runs ahead of the region of infall (also studied by Tsai &
Hsu 1995 in a different context). We discuss the implications of
our results for the magnetic-flux problem and for the formation of
centrifugally supported disks in the presence of rotation.
---------------------------------------------------------
Title: Protostellar cosmic rays and extinct radioactivities
Authors: Shang, Hsien; Shu, Frank; Lee, Typhoon; Glassgold, Alfred E.
1997IAUS..182P.312S Altcode:
A magnetocentrifugally driven X-wind from the inner edge of the solar
nebula around the young sun may process dustballs into the CAIs and
chondrules found in chondritic meteorites (\cite{Shu-Shang-Lee). Time
variability of the magnetic configuration could trigger flares,
accelerating cosmic rays that spallogenically produce the short-lived
radionuclides inferred for CAIs and chondrules.
---------------------------------------------------------
Title: Protostellar X-rays, Jets, and Bipolar Outflows
Authors: Shu, Frank H.; Shang, Hsien
1997IAUS..182..225S Altcode:
We review the theory of x-winds in young stellar objects
(YSOs). In particular, we consider how a model where the central
star does not corotate with the inner edge of the accretion disk
may help to explain the enhanced emission of X-rays from embedded
protostars. We argue, however, that the departure from corotation is
not large, so a mathematical formulation that treats the long-term
average state as steady and axisymmetric represents a useful
approximation. Magnetocentrifugally driven x-winds of this description
collimate into jets, and their interactions with the surrounding
molecular cloud cores of YSOs yield bipolar molecular outflows.
---------------------------------------------------------
Title: X-rays and fluctuating X-winds from protostars.
Authors: Shu, F. H.; Shang, H.; Glassgold, A. E.; Lee, T.
1997Sci...277.1475S Altcode:
Protostars emit more X-rays, hard and soft, than young Sun-like stars
in more advanced stages of formation. The X-ray emission becomes
harder and stronger during flares. The excess X-rays may arise as a
result of the time-dependent interaction of an accretion disk with the
magnetosphere of the central star. Flares produced by such fluctuations
have important implications for the X-wind model of protostellar jets,
for the flash-heating of the chondrules found in chondritic meteorites,
and for the production of short-lived radioactivities through the
bombardment of primitive rocks by solar cosmic rays.
---------------------------------------------------------
Title: C. C. Lin's Influence on Modern Ideas in Star Formation
Authors: Shu, F. H.
1997tcca.conf...21S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Annual review of astronomy and astrophysics. Vol. 35.
Authors: Burbidge, G.; Sandage, A.; Shu, F. H.
1997ARA&A..35.....B Altcode:
The following topics are discussed: Eta Carinae, solar variable
radiation, luminous supersoft X-ray sources, extragalactic distance
scale, low mass stars and brown dwarfs, dense photodissociation
regions, high-velocity clouds, low surface brightness galaxies,
supernova spectra, compact groups of galaxies, faint blue galaxies,
AGN variability, abundance ratios and Galactic chemical evolution,
mixing in stars, parsec-scale jets, and galactic bulges.
---------------------------------------------------------
Title: Molecules in star formation.
Authors: Shu, F. H.
1997IAUS..178...19S Altcode: 1996IAUS..178...19S
The author reviews current ideas and models in the problem of star
formation from molecular cloud cores that are relatively isolated from
the influences of other forming stars. He discusses the time scales,
flow dynamics, and density and temperature structures applicable
to each of the four stages of the entire process: (1) formation
of a magnetized cloud core by ambipolar diffusion and evolution
to a pivotal state of gravomagneto catastrophe; (2) self-similar
collapse of the pivotal configuration and the formation of protostars,
disks, and pseudo-disks; (3) onset of a magnetocentrifugally driven,
lightly ionized wind from the interaction of an accretion disk and the
magnetosphere of the central star, and the driving of bipolar molecular
outflows; (4) evolution of pre-main-sequence stars surrounded by dusty
accretion disks. For each of these stages and processes, he considers
the characteristics of the molecular diagnostics needed to investigate
the crucial aspects of the observational problem.
---------------------------------------------------------
Title: A Quantitative Model for Bipolar Molecular Outflows
Authors: Li, Z.; Shu, F. H.
1997ASPC..121..757L Altcode: 1997IAUCo.163..757L; 1997apro.conf..757L
No abstract at ADS
---------------------------------------------------------
Title: Magnetized Singular Isothermal Toroids
Authors: Li, Zhi-Yun; Shu, Frank H.
1996ApJ...472..211L Altcode:
We consider the end states for a quasi-magnetostatic phase of
evolution of molecular cloud cores by ambipolar diffusion. The
models yield good initial states for fully dynamical collapse to
isolated protostellar systems. These pivotal transition states
are self-gravitating, magnetized, isothermal masses of gas in
which the density scales with spherical radius r as r<SUP>-2</SUP>
and the magnetic field as r<SUP>-1</SUP>. The dependences of these
quantities with angle θ satisfy the constraints of magnetostatic
equilibrium. Under these circumstances, we find a linear sequence
of possible pivotal states, each member characterized by a separate
value of the differential mass-to-flux ratio in dimensionless form
= λ≡2πG<SUP>½</SUP>dM/dΦ≥1. In general, the pivotal states
are toroids, with the density distribution on a circle of constant
r in the meridional plane declining from a maximum value on the
magnetic equator, θ = π/2, to zero over the magnetic poles, θ =
0 and θ = π. For λ≫ 1, the pivotal configurations approach the
unmagnetized singular isothermal sphere, with volume density p =
(a<SUP>2</SUP>/2πG)r<SUP>-2</SUP>. For λ close to 1, the pivotal
configurations flatten to a thin disk, with surface density Σ =
[(1 + H<SUB>0</SUB>)a2/πG]r<SUP>-2</SUP>, where H<SUB>0</SUB> is
a number ≫ 1. We comment on the implications of these results for
observations and other theoretical investigations.
---------------------------------------------------------
Title: Toward an astrophysical theory of chondrites.
Authors: Shang, H.; Shu, F. H.; Lee, T.
1996NASCP3343..191S Altcode: 1996ASPC..122..191S; 1996NASCP3339..191S
The authors have proposed a theory where chondritic bodies form by
compaction of the processed solids with the ambient nebular dust
comprising the matrices after their reentry at great distances from
the original launch radius.
---------------------------------------------------------
Title: Externally induced evaporation of young stellar disks in Orion.
Authors: Johnstone, D.; Hollenbach, D.; Shu, F.
1996NASCP3343...55J Altcode: 1996NASCP3339...55J; 1996ASPC..122...55J
The authors propose a model for the evaporation of disks around
young low-mass stars by external sources of high energy photons. Two
evaporation techniques are possible. Lyman continuum radiation can
ionize hydrogen at the disk surface powering a steady thermal ionized
disk-wind or FUV radiation can heat the disk through photo-electric
grain processes powering a slower thermal neutral disk-wind. Applying
these two models to the evaporating objects in the Trapezium produces
a satisfactory solution to both the mass-loss rate and size of the
ionized envelopes.
---------------------------------------------------------
Title: Interaction of Wide-Angle MHD Winds with Flared Disks
Authors: Li, Zhi-Yun; Shu, Frank H.
1996ApJ...468..261L Altcode:
A magnetocentrifugal mechanism probably underlies the narrow,
high-speed, optical jets associated with young stellar objects. Recent
theoretical calculations of jet formation in the X-wind model
suggest the co-existence of a wide-angle component surrounding the
well-collimated jet. We consider whether the predicted wide-angle
component can reveal itself through interaction with an underlying
flared accretion disk. We use the method of characteristics to calculate
numerically the properties of the resulting shockwave. Because the
frozen-in magnetic field reduces the effective Mach number of the flow,
the shock is too weak at a distance of 10-100 AU to produce [O I]
λ6300 or [S II] λ6717/6731 emission. Unless disks have obstructing
features more extreme than conventional flaring (e.g., their own wind),
the uncollimated component of an X-wind will remain optically hidden
until it impacts material well off the disk plane at larger distances
from the central star (e.g., an infalling envelope or the ambient
molecular cloud).
---------------------------------------------------------
Title: Kinematic Diagnostics of Disks around Young Stars: CO Overtone
Emission from WL 16 and 1548C27
Authors: Najita, Joan; Carr, John S.; Glassgold, Alfred E.; Shu,
Frank H.; Tokunaga, Alan T.
1996ApJ...462..919N Altcode: 1995astro.ph.12109N
We report high spectral resolution observations of the CO
vibrational overtone emission from the young stellar object 1548C27;
our observations include both the v = 2-0 and v = 5-3 band head
regions. These data and similar observations of the young stellar
object WL 16, reported in a previous contribution to this journal,
provide some of the most compelling evidence to date for the existence
of inner disks around young stars. We describe the simple procedure
that we use to synthesize band head emission from disks including the
effect of thermal dissociation of CO and non-LTE excitation of the
vibrational levels. Using this spectral synthesis procedure to extract
the kinematics and physical properties of the emitting gas from the
overtone data, we show how these high signal-to-noise ratio data are
also powerful probes of the stellar and inner disk properties of these
systems. Our modeling is consistent with the identification of WL 16
and 1548C27 as Herbig AeBe stars with stellar masses of approximately
2 and 4 M<SUB>sun</SUB>, respectively. Thus, the kinematic signature
of rotating disks in the overtone spectra of these sources provides
strong support for the role of accretion disks in the formation of
intermediate-mass stars. For both WL 16 and 1548C27, we interpret our
modeling results as indicating that the overtone emission arises from a
temperature inversion region in the inner disk atmosphere. We also find
evidence for suprathermally broadened lines and are able to place useful
constraints on the radial temperature and column density distributions
of the CO line formation region of the disk atmosphere. Given these
deduced properties, we discuss the constraints that our observations
place on the physical processes responsible for the overtone emission
in these sources.
---------------------------------------------------------
Title: Toward an Astrophysical Theory of Chondrites
Authors: Shu, Frank H.; Shang, Hsien; Lee, Typhoon
1996Sci...271.1545S Altcode:
The chondrules, calcium-aluminum-rich inclusions (CAIs), and rims in
chondritic meteorites could be formed when solid bodies are lifted
by the aerodynamic drag of a magnetocentrifugally driven wind out
of the relative cool of a shaded disk close to the star into the
heat of direct sunlight. For reasonable self-consistent parameters
of the bipolar outflow, the base and peak temperatures reached by
solid bodies resemble those needed to melt CAIs and chondrules. The
process also yields a natural sorting mechanism that explains the size
distribution of CAIs and chondrules, as well as their fine-grained
and coarse-grained rims. After reentry at great distances from the
original launch radius, the CAIs, chondrules, and their rims would
be compacted with the ambient nebular dust comprising the matrices,
forming the observed chondritic bodies.
---------------------------------------------------------
Title: Magnetocentrifugally Driven Flows from Young Stars and
Disks. V. Collimation into Jets: Erratum
Authors: Shu, Frank H.; Najita, Joan; Ostriker, Eve C.; Shang, Hsien
1996ApJ...459L..43S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: An X-Wind Scenario for Early Solar System Irradiation
Authors: Lee, T.; Shu, F. H.; Glassgold, A.; Shang, H.
1996M&PSA..31R..79L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Externally induced evaporation of young stellar disks in Orion.
Authors: Johnstone, D.; Shu, F.; Hollenbach, D.
1996JRASC..90T.319J Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Magnetocentrifugally Driven Flows from Young Stars and
Disks. V. Asymptotic Collimation into Jets
Authors: Shu, Frank H.; Najita, Joan; Ostriker, Eve C.; Shang, Hsien
1995ApJ...455L.155S Altcode:
We consider the asymptotic behavior of magnetocentrifugally driven
X-winds, taking into proper account the pressure balance across the
interface between the open stellar field lines of the dead zone devoid
of matter and the open field lines of the wind. At large distances
from the source the outflow collimates into jets along the rotation
axis. Being almost cylindrically symmetric even close to the source
of the outflow, the density distribution appears more jetlike than
the collection of streamlines, which always contains a few members
that skim over the surface of the disk.
---------------------------------------------------------
Title: Magnetocentrifugally Driven Flows from Young Stars and
Disks. IV. The Accretion Funnel and Dead Zone
Authors: Ostriker, Eve C.; Shu, Frank H.
1995ApJ...447..813O Altcode:
We formulate the time-steady, axisymmetric problem of stellar
magnetospheric inflow of gas from a surrounding accretion disk. The
computational domain is bounded on the outside by a surface of given
shape containing the open field lines associated with an induced
disk wind. The mechanism for this wind has been investigated in
previous publications in this journal. Our zeroth-order solution
incorporates an acceptable accounting of the pressure balance between
the magnetic field lines loaded with accreting gas (funnel flow) and
those empty of matter (dead zone). In comparison with previous models,
our funnel-flow/dead-zone solution has the following novel features:
(1) Because of a natural tendency for the trapped stellar magnetic flux
to pinch toward the corotation radius R<SUB>x</SUB> (X-point of the
effective potential), most of the interesting magnetohydrodynamics is
initiated within a small neighborhood of R<SUB>x</SUB> (X-region), where
the Keplerian angular speed of rotation in the disk equals the spin
rate of the star. (2) Unimpeded funnel flow from the inner portion of
the X-region to the star can occur when the amount of trapped magnetic
flux equals or exceeds 1.5 times the unperturbed dipole flux that would
lie outside R<SUB>x</SUB> in the absence of an accretion disk. (3). Near
the equatorial plane, radial infall from the X-point is terminated at a
"kink" point R<SUB>k</SUB> = 0.74R<SUB>x</SUB> that deflects the flow
away from the midplane, mediating thereby between the field topology
imposed by a magnetic fan of trapped flux at R<SUB>x</SUB> and the
geometry of a strong stellar dipole. (4) The excess angular momentum of
accretion that would otherwise spin up the star rapidly is deposited
by the magnetic torques of the funnel flow into the inner portion
of the X-region of the disk. (5) An induced disk wind arises in the
outer portion of the .X-region, where the stellar field lines have been
blown open, and removes whatever excess angular momentum that viscous
torques do not transport to the outer disk. (6) The interface between
open field lines loaded with outflowing matter (connected to the disk)
and those not loaded (connected to the star) forms a "helmet streamer,"
along which major mass-ejection and reconnection events may arise
in response to changing boundary conditions (e.g., stellar magnetic
cycles), much the way that such events occur in the active Sun. (7)
Pressure balance across the dead-zone/wind interface will probably yield
an asymptotically vertical (i.e., "jetlike") trajectory for the matter
ejected along the helmet streamer, but mathematical demonstration of
this fact is left for future studies. (8) In steady state the overall
balance of angular momentum in the star/disk/ magnetosphere system
fixes the fractions, f and 1 - f, of the disk mass accretion rate into
the X-region carried away, respectively, by the wind and funnel flows.
---------------------------------------------------------
Title: The Birth of Sunlike Stars
Authors: Shu, Frank H.
1995mcsf.conf...97S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: AAS Position on the Space Station
Authors: Shu, Frank H.
1994Sci...265.1789S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Magnetocentrifugally Driven Flows from Young Stars and
Disks. I. A Generalized Model
Authors: Shu, Frank; Najita, Joan; Ostriker, Eve; Wilkin, Frank;
Ruden, Steven; Lizano, Susana
1994ApJ...429..781S Altcode:
We propose a generalized model for stellar spin-down, disk accretion,
and truncation, and the origin of winds, jets, and bipolar outflows
from young stellar objects. We consider the steady state dynamics of
accretion of matter from a viscous and imperfectly conducting disk
onto a young star with a strong magnetic field. For an aligned stellar
magnetosphere, shielding currents in the surface layers of the disk
prevent stellar field lines from penetrating the disk everywhere except
for a range of radii about pi = R<SUB>x</SUB>, where the Keplerian
angular speed of rotation Omega<SUB>x</SUB> equals the angular speed of
the star Omega<SUB>*</SUB>. For the low disk accretion rates and high
magnetic fields associated with typical T Tauri stars, R<SUB>x</SUB>
exceeds the radius of the star R<SUB>*</SUB> by a factor of a few,
and the inner disk is effectively truncated at a radius R<SUB>t</SUB>
somewhat smaller than R<SUB>x</SUB>. Where the closed field lines
between R<SUB>t</SUB> and R<SUB>x</SUB> bow sufficiently inward, the
accreting gas attaches itself to the field and is funneled dynamically
down the effective potential (gravitational plus centrifugal) onto the
star. Contrary to common belief, the accompanying magnetic torques
associated with this accreting gas may transfer angular momentum
mostly to the disk rather than to the star. Thus, the star can spin
slowly as long as R<SUB>x</SUB> remains significantly greater than
R<SUB>*</SUB>. Exterior to R<SUB>x</SUB> field lines threading the
disk bow outward, which makes the gas off the mid-plane rotate at
super-Keplerian velocities. This combination drives a magnetocentrifugal
wind with a mass-loss rate M<SUB>w</SUB> equal to a definite fraction
f of the disk accretion rate M<SUB>D</SUB>. For high disk accretion
rates, R<SUB>x</SUB> is forced down to the stellar surface, the star
is spun to breakup, and the wind is generated in a manner identical
to that proposed by Shu, Lizano, Ruden, & Najita in a previous
communication to this journal. In two companion papers (II and III),
we develop a detailed but idealized theory of the magnetocentrifugal
acceleration process.
---------------------------------------------------------
Title: Magnetocentrifugally Driven Flows from Young Stars and
Disks. II. Formulation of the Dynamical Problem
Authors: Shu, Frank H.; Najita, Joan; Ruden, Steven P.; Lizano, Susana
1994ApJ...429..797S Altcode:
We formulate the dynamical problem of a cool wind centrifugally driven
from the magnetic interface of a young star and an adjoining Keplerian
disk. We examine the situation for mildly accreting T Tauri stars that
rotate slowly as well as rapidly accreting protostars that rotate near
break-up. In both cases a wind can be driven from a small X-region just
outside the stellar magnetopause, where the field lines assume an open
geometry and are rooted to material that rotates at an angular speed
equal both to the local Keplerian value and to the stellar angular
speed. Assuming axial symmetry for the ideal magnetohydrodynamic
flow, which requires us to postpone asking how the (lightly ionized)
gas is loaded onto field lines, we can formally integrate all the
governing equations analytically except for a partial equation that
describes how streamlines spread in the meridional plane. Apart
from the difficulty of dealing with PDEs of mixed type, finding the
functional forms of the conserved quantities along streamlines -
the ratio beta of magnetic field to mass flux, the specific energy
H of the fluid in the rotating frame, and the total specific angular
momentum J carried in the matter and the field - constitutes a standard
difficulty in this kind of (Grad-Shafranov) formalism. Fortunately,
because the ratio of the thermal speed of the mass-loss regions to
the Keplerian speed of rotation of the interface constitutes a small
parameter epsilon, we can attack the overall problem by the method of
matched asymptotic expansions. This procedure leads to a natural and
systematic technique for obtaining the relevant functional dependences
of beta, H, and J. Moreover, we are able to solve analytically for the
properties of the flow emergent from the small transsonic region driven
by gas pressure without having to specify the detailed form of any
of the conserved functions, beta, H, and J. This analytical solution
provides inner boundary conditions for the numerical computation in
a companion paper by Najita & Shu of the larger region where the
main acceleration to terminal speeds occurs.
---------------------------------------------------------
Title: Magnetocentrifugally Driven Flows from Young Stars and
Disks. III. Numerical Solution of the Sub-Alfvenic Region
Authors: Najita, Joan R.; Shu, Frank H.
1994ApJ...429..808N Altcode:
We construct steady, axisymmetric, numerical models of the sub-Alfvenic
regions of cool winds driven by the X-celerator mechanism. We find
that smooth acceleration to speeds of the order of the escape speed
accompanied by substantial collimation of the flow can be achieved
within the Alfven surface, located characteristically at several
stellar radii. We apply the nondimensional results to winds which
emerge from the equator of magnetized YSOs rotating at breakup and
from circumstellar disks interacting with the magnetospheres of slowly
rotating T Tauri stars. Stellar magnetic fields of kilogauss strength
can drive wind mass-loss rates of 10<SUP>-6</SUP> solar mass/year from
rapidly accreting YSOs and 10<SUP>-8</SUP> solar mass/year from slowly
accreting T Tauri stars. The X-celerator mechanism can accelerate
winds from these systems to velocities of hundreds of km/sec within
a few stellar radii in the case of embedded YSOs and approximately 10
stellar radii in the case of T Tauri stars.
---------------------------------------------------------
Title: Photoevaporation of Disks around Massive Stars and Application
to Ultracompact H II Regions
Authors: Hollenbach, David; Johnstone, Doug; Lizano, Susana; Shu, Frank
1994ApJ...428..654H Altcode:
Young massive stars produce sufficient Lyman continuum photon luminosity
Phi<SUB>i</SUB> to significantly affect the structure and evolution
of the accretion disks surrounding them. A nearly static, ionized,
isothermal 10<SUP>4</SUP> K atmosphere forms above the neutral disk for
disk radii r less than r<SUB>g</SUB> = 10<SUP>15</SUP> M<SUB>1</SUB>
cm, where M<SUB>*</SUB> = 10 solar mass M<SUB>1</SUB> is the stellar
mass. For r approximately greater than r<SUB>g</SUB> the diffuse
field created by hydrogen recombinations to the ground state in the
photoionized gas above the disk produces a steady evaporation at the
surface of the disk, and this H II gas flows freely out to the ISM
(the 'disk wind'). The detailed structure depends on the mass-loss
rate dot-M<SUB>w</SUB> of the fast, approximately greater than 1000
km/sec, stellar wind from the massive star. A critical mass-loss
rate dot-M<SUB>cr</SUB> is defined such that the ram pressure of the
stellar wind equals the thermal pressure of the H II atmosphere at
r<SUB>g</SUB>. In the weak stellar wind solution, dot-M<SUB>w</SUB>
less than dot-M<SUB>cr</SUB>, the diffuse photons from the atmosphere
above r<SUB>g</SUB> produce a photoevaporative mass-loss rate from
the disk at r approximately greater than r<SUB>g</SUB> of order 1 x
10<SUP>-5</SUP>(Phi<SUB>49</SUB>)<SUP>1/2</SUP>(M<SUB>1</SUB>)<SUP>1/2</SUP>
solar mass/year, where Phi<SUB>i</SUB> = 10<SUP>49</SUP>
Phi<SUB>49</SUB>/sec. The resulting slow (10 to 50 km/sec) ionized
outflow, which persists for approximately greater than 10<SUP>5</SUP>
year for disk masses M<SUB>d</SUB> approximately 0.3 M<SUB>*</SUB>,
may explain the observational characteri stics of unresolved,
ultracompact H II regions. In the strong stellar wind solution,
dot-M<SUB>w</SUB> greater than dot-M<SUB>cr</SUB>, the ram pressure
of the stellar wind blows down the atmosphere for r less than
r<SUB>g</SUB> and allows the stellar photons to penetrate to greater
radii and smaller heights. A slow, ionized outflow produced mainly
by diffuse photons is again created for r less than r<SUB>g</SUB>;
however, it is now dominated by the flow at r<SUB>w</SUB>(greater than
r<SUB>g</SUB>), the radius at which the stellar wind ram pressure
equals the thermal pressure in the evaporating flow. The mass-loss
rate from the disk is of order 6 x 10<SUP>-5</SUP>dot-M<SUB>w-6</SUB>
v<SUB>w8</SUB>(Phi <SUB>49</SUB>)<SUP>-1/2</SUP> solar mass/year,
where dot-M<SUB>w-6</SUB> = M<SUB>w</SUB>/10<SUP>-6</SUP> solar
mass/year and v<SUB>w8</SUB> = v<SUB>w</SUB>/1000 km/sec is the
stellar wind velocity. The resulting outflow, which also persists
for approximately greater than 10<SUP>5</SUP> year may explain many
of the more extended (r approximately greater than 10<SUP>16</SUP> cm)
ultracompact H II regions. Both the weak-wind and the strong-wind models
depend entirely on stellar parameters Phi<SUB>i</SUB>, M<SUB>*</SUB>,
dot-M<SUB>w</SUB>) and are independent of disk parameters as long as an
extended r much greater than (r<SUB>g</SUB>), neutral disk exists. We
compare both weak-wind and strong-wind model results to the observed
radio free-free spectra and luminosities of ultracompact H II regions
and to the interesting source MWC 349.
---------------------------------------------------------
Title: Collapse of Magnetized Molecular Cloud Cores. I. Semianalytical
Solution
Authors: Galli, Daniele; Shu, Frank H.
1993ApJ...417..220G Altcode:
In this paper we follow the evolution of an unstable magnetized cloud
core modeled with the density distribution of a singular isothermal
sphere and threaded by a uniform magnetic field. We include neutral-ion
slip, and we solve the equations by an expansion about the known
self-similar problem without magnetism. We find that the magnetic
field does not significantly modify the standard rate of mass infall
because of two offsetting effects: the Lorentz force that impedes
gravitational collapse, and the increased characteristic speed that
causes the initiation of infall to travel outward faster (as a fast
magnetohydrodynamic wave rather than an acoustic wave). Strong magnetic
pinching forces deflect infalling gas toward the equatorial plane to
form a flattened disequilibrium structure ("pseudodisk') around the
central proto star. The perturbative approach allows us to calculate
analytically the dependence of the radius of the pseudodisk at small
times on the physical parameters of the problem when a dimensionless
coefficient of order unity is supplied by a separate numerical
calculation for the nonlinear flow in the inner region (Paper II).
---------------------------------------------------------
Title: Collapse of Magnetized Molecular Cloud Cores. II. Numerical
Results
Authors: Galli, Daniele; Shu, Frank H.
1993ApJ...417..243G Altcode:
In a previous paper (Paper I) we presented a perturbative analysis of
the collapse of a molecular cloud core threaded by an ordered magnetic
field, obtaining a semianalytical solution applicable over a moderate
range of temporal and spatial scales. In the present paper we supplement
this analysis with a numerical solution of the magnetohydrodynamic
(MHD) equations that include the effects of ambipolar diffusion,
valid in the region where magnetic effects dominate the dynamics of
the collapse. We focus on the formation of a flattened disequilibrium
structure ("pseudodisk") around the central protostar. The numerical
solution gives dimensionless values for the radius of the pseudodisk
as a function of time. Combined with the analytical scaling laws
found in Paper I, these results provide in the small time limit
a simple power-law expression for the dimensional radius of the
pseudodisk as a function of the initial magnetic field B<SUB>0</SUB>
and effective sound speed a of the unstable molecular cloud core. We
tabulate in nondimensional form the velocity, density, and magnetic
fields as functions of the radius, polar angle, and time for two
values (χ = 11.3 and ∞) of the ion-neutral coupling constant. We
apply the results to the density and magnetic field structures on the
astronomically interesting scale of a few hundred to a few thousand AU
around protostars with mass in the range 0.57-2.0 M<SUB>sun</SUB>. The
resultant magnetic field topology causes us to speculate on the
importance of neutral-ion slip, ohmic dissipation, and reconnection
in the overall problems of the loss of flux and the isolation of
the magnetic fields in the pseudodisk (and smaller centrifugal disk)
from their interstellar origins. We conclude by comparing our results
with observations of flattened dense structures around young stellar
objects in various stages of evolution.
---------------------------------------------------------
Title: Photoevaporation of the Solar Nebula and the Formation of
the Giant Planets
Authors: Shu, Frank H.; Johnstone, Doug; Hollenbach, David
1993Icar..106...92S Altcode:
We review the prevailing theories for the formation of the jovian
planets and comment that they do not provide a natural explanation
for the oft-noted subdivision into two separate classes: the gas-rich
giants, Jupiter and Saturn, and the gas-poor giants, Uranus and
Neptune. To account for the observed differences in envelope mass
relative to core mass, the conventional discriminants would seem to
require special timing for the phases: assemblage of the protoplanetary
cores, runaway accretion of gas, and the dispersal of nebular
gases. We propose a discriminant that relies on photoevaporation by
Lyman continuum photons of the outer parts of the disk as the primary
hydrogen (and helium) loss mechanism. We show that Saturn's orbit
constitutes the natural transitional radius between gas retainage
and loss in this sort of picture and that the evaporative wind in
the Uranus and Neptune regions would have been large enough to get
rid of the hydrogen and helium gas before these planets assembled a
critical core mass for runaway gas accretion, if the primitive Sun had
an enhanced extreme ultraviolet luminosity for a duration comparable
to those estimated for classical T Tauri stars.
---------------------------------------------------------
Title: Photoevaporation of disks around newly formed stars.
Authors: Johnstone, D.; Hollenbach, D.; Shu, F. H.
1993JRASC..87..192J Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Inner-Disk and Stellar Properties of the Young Stellar
Object WL 16
Authors: Carr, John S.; Tokunaga, Alan T.; Najita, Joan; Shu, Frank
H.; Glassgold, Alfred E.
1993ApJ...411L..37C Altcode:
We present kinematic evidence for a rapidly rotating circumstellar
disk around the young stellar object WL 16, based on new
high-velocity-resolution data of the v = 2-0 CO bandhead emission. A
Keplerian disk provides an excellent fit to the observed profile and
requires a projected velocity for the CO-emitting region of roughly
250 km/s at the inner radius and 140 km/s at the outer radius, giving
a ratio of the inner to the outer radius of about 0.3. We show that
satisfying the constraints imposed by the gas kinematics, the observed
CO flux, and the total source luminosity requires the mass of WL 16
to lie between 1.4 and 2.5 solar mass. The inner disk radius for the
CO emission must be less than 8 solar radii.
---------------------------------------------------------
Title: Photoevaporation of Disks around Newly Formed Stars
Authors: Johnstone, D.; Hollenbach, D.; Shu, F. H.
1993AAS...182.3704J Altcode: 1993BAAS...25..854J
Newly formed stars produce sufficient Lyman continuum luminosity phi to
significantly alter the structure and evolution of the accretion disk
surrounding them. In the absence of a stellar wind, a nearly static,
photoionized, 10(4) K, disk atmosphere, with a scale height that
increases with r(3/2) , forms inside the gravitational radius r_g ~
10(14) (M_* / M_⊙) cm where M_* is the mass of the central star. This
ionized atmosphere is maintained by both the direct radiation from the
central star and the diffuse field produced in the disk atmosphere
by the significant fraction of hydrogen recombinations directly to
the ground state. Beyond r_g the material evaporated from the disk
is capable of escaping from the system and produces an ionized disk
wind. The mass-loss due to this disk wind peaks at r_g. The inclusion
of a stellar wind into the basic picture reduces the height of the inner
disk atmosphere and introduces a new scale radius r_w where the thermal
pressure of the material evaporated from the disk balances the ram
pressure in the wind. In this case the mass-loss due to the disk wind
peaks at r_w and is enhanced over the no-wind case. The photoevaporation
of disks around newly formed stars has significance to both the UCHII
problem and the dispersal of solar-type nebulae. High mass stars
are intrinsically hot and thus yield sufficient Lyman luminosity to
create disk mass-loss rates of order 2 times 10(-5) phi_ {49}(1/2)
M_⊙ yr(-1) , where phi_ {49} = phi /(10(49) Lyman continuum photons
s(-1) ) even without a stellar wind. This wind which will last for ~
10(5) yrs if the disk mass is M_d ~ 0.3 M_*, yields sizes, emission
measures and ages consistent with observations of UCHIIs. On the other
end of the stellar scale, many newly formed low-mass stars are known
to have enhanced extreme ultraviolet luminosity suggested to be due to
boundary layer accretion. Assuming that the sun had such an enhanced
Lyman luminosity phi ~ 10(41) s(-1) , for ~ 3times 10(7) yrs it is
possible to remove all of the gas beyond the orbit of Saturn, r_g for
the sun, associated with the minimum solar nebula. This process also
has implications for the formation of the giant planets.
---------------------------------------------------------
Title: Book-Review - the Physics of Astrophysics - V.2 - Gas Dynamics
Authors: Shu, F. H.; Pringle, J. E.
1993Obs...113...85S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Parker Instability in a Realistic Gravitational Field
Authors: Giz, Ahmet T.; Shu, Frank H.
1993ApJ...404..185G Altcode:
The paper examines Parker's instability in a disk with a realistic
vertical structure, but without including the effects of rotation and
shear in the horizontal direction. In addition to the continuum modes
found by previous authors for the idealized case when the vertical
gravitational field is taken to be a simple step function, the
possibility for discrete modes whose power is more spatially confined
in z is discovered. When these discrete modes prove unstable, they
favor condensations that are placed antisymmetrically with respect to
the midplane, a feature found also in numerical simulations carried
into the nonlinear regime by Matsumoto et al. (1988, 1990). For the
continuum modes, the characteristic length scale for instability is
typically half of the conventionally estimated value, yielding growth
rates that are approximately double previous estimates.
---------------------------------------------------------
Title: The Collapse of Clouds and the Formation and Evolution of
Stars and Disks
Authors: Shu, Frank; Najita, Joan; Galli, Daniele; Ostriker, Eve;
Lizano, Susana
1993prpl.conf....3S Altcode:
We consider the interrelationships among the structure of molecular
clouds; the collapse of rotating cloud cores; the formation of stars
and disks; the origin of molecular outflows, protostellar winds, and
highly collimated jets; the birth of planetary and binary systems;
and the dynamics of star/disk/satellite interactions. Our discussion
interweaves theory with the results of observations that span from
millimeter wavelengths to X-rays.
---------------------------------------------------------
Title: The Inner Disk and Stellar Properties of the Young Stellar
Object WL 16
Authors: Carr, J. S.; Tokunaga, A. T.; Najita, J.; Shu, F. H.;
Glassgold, A. E.
1993AAS...18111605C Altcode: 1993BAAS...25Q.734C
We present kinematic evidence for a rapidly rotating circumstellar
disk around the young stellar object WL 16, based on new high velocity
resolution data of the v = 2-0 CO bandhead emission. A Keplerian disk
provides an excellent fit to the observed profile, with a projected
velocity v sin i = 140 km s(-1) at the outer disk edge and about 250
km s(-1) at the inner edge. The disk must extend to within one stellar
radius or less of the stellar surface. Constraints on the stellar mass
of WL 16 and on other properties of the inner disk are discussed.
---------------------------------------------------------
Title: Photoevaporation of Disks Around Massive Stars and Ultracompact
HH Regions
Authors: Hollenbach, D.; Johnstone, D.; Shu, F.
1993ASPC...35...26H Altcode: 1993msli.conf...26H
No abstract at ADS
---------------------------------------------------------
Title: Near-resonant Excitation and Propagation of Eccentric Density
Waves by External Forcing
Authors: Ostriker, Eve C.; Shu, Frank H.; Adams, Fred C.
1992ApJ...399..192O Altcode:
An overview is presented of the astronomical evidence that relatively
massive, distended, gaseous disks form as a natural by-product of
the process of star formation, and also the numerical evidence that
SLING-amplified eccentric modes in the outer parts of such disks can
drive one-armed spiral density waves in the inner parts by near-resonant
excitation and propagation. An ordinary differential equation (ODE) of
the second order that approximately governs the nonlocalized forcing
of waves in a disk satisfying Lindblad resonance almost everywhere
is derived. When transformed and appended with an extra model term,
this ODE implies, for free waves, the usual asymptotic results of the
WKBJ dispersion relationship and the propagation Goldreich-Tremaine
(1978) formula for the resonant torque exerted on a localized Lindblad
resonance. An analytical solution is given for the rate of energy
and angular momentum transfer by nonlocalized near-resonant forcing
in the case when the disk has power-law dependences on the radius of
the surface density and temperature.
---------------------------------------------------------
Title: Books-Received - the Physics of Astrophysics - V.2 - Gas
Dynamics
Authors: Shu, F. H.
1992JBAA..102..230S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Book-Review - the Physics of Astrophysics - V.1&2 -
Radiation and Gas Dynamics
Authors: Shu, F. H.; Hughes, P. A.
1992Natur.357..122S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Book-Review - Radiation - the Physics of Astrophysics - V.1
Authors: Shu, F. H.; Harper, G.
1992JBAA..102..108S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Book-Review - the Physics of Astrophysics - V.2 - Gas Dynamics
Authors: Shu, F. H.
1992Sci...256Q.253S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Book-Review - the Physics of Astrophysics - V.1 - Radiation
Authors: Shu, F. H.
1992JBAA..102Q..62S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The physics of astrophysics. Volume II: Gas dynamics.
Authors: Shu, F. H.
1992pavi.book.....S Altcode:
Volume II is a self-contained textbook, and is not dependent on Volume
I (see 53.003.096). It can be used as the text for a separate,
one-semester course on its subject matter, which includes the
interactions of matter and radiation, and electromagnetic fields of
macroscopic scale in both the strongly collisional and collisionless
regimes. It covers such fields as single-fluid theory, including
radiative processes; waves, shocks, and fronts; magnetohydrodynamics
and plasma physics; as well as their applications to such topics as
self-gravitating spherical masses, accretion disks, spiral density
waves, star formation, and dynamo theory.
---------------------------------------------------------
Title: Physics of Astrophysics, Vol. II
Authors: Shu, Frank H.
1992phas.book.....S Altcode:
Presented in two volumes, The Physics of Astrophysics is ideally
suited for a year-long astrophysics course for university seniors
and first-year graduate students. The first volume deals with the
emission, absorption, and scattering of radiation by matter, as well
as covering related topics such as radiative transfer, statistical
physics, classical electrodynamics, and atomic and molecular structure.
---------------------------------------------------------
Title: Star Formation and the Nature of Bipolar Outflows
Authors: Shu, Frank H.; Ruden, Steven P.; Lada, Charles J.; Lizano,
Susana
1991ApJ...370L..31S Altcode:
This paper presents a simple physical model for the bipolar molecular
outflows that frequently accompany star formation. The model forges an
intrinsic link between the bipolar flow phenomenon and the process of
star formation, and it helps to explain many of the systematics known
for existing sources.
---------------------------------------------------------
Title: Physics of Astrophysics, Vol. II: Gas Dynamics
Authors: Shu, Frank
1991pagd.book.....S Altcode:
This upper-level text deals with interactions of matter and radiation,
electromagnetic fields, single-fluid theory, waves, fronts,
magnetohydrodynamics, and plasma physics.
---------------------------------------------------------
Title: The Formation of Low Mass Stars: Theory
Authors: Shu, Frank H.
1991ASIC..342..365S Altcode: 1991psfe.conf..365S
Overview Bimodal Star Formation The Bipolar Outflow Phase: Observations
Rotating, Magnetized, Molecular Cloud Cores Protostar Formation from
Collapsing Cloud Cores Infrared Appearance of Rotating Protostellar
Objects Protostar Formation By Disk Accretion Stellar Winds and Bipolar
Flows: Theory Revealed T Tauri Stars The Disks Inferred for T Tauri
Stars Binary Stars and Planetary Systems
---------------------------------------------------------
Title: The physics of astrophysics. Volume 1: Radiation.
Authors: Shu, F. H.
1991pav..book.....S Altcode:
This textbook deals with the emission, absorption, and scattering of
radiation by matter, as well as radiative transfer, statistical physics,
classical electrodynamics, and atomic and molecular structure. The
volume include problem sets designed to bring the student to the level
of competence in calculation currently expected in astronomy.
---------------------------------------------------------
Title: Physics of Astrophysics, Vol. I
Authors: Shu, Frank H.
1991phas.book.....S Altcode: 1991QB461.S448.....
Presented in two volumes, The Physics of Astrophysics is ideally
suited for a year-long astrophysics course for university seniors
and first-year graduate students. The first volume deals with the
emission, absorption, and scattering of radiation by matter, as well
as covering related topics such as radiative transfer, statistical
physics, classical electrodynamics, and atomic and molecular structure.
---------------------------------------------------------
Title: Physics of Astrophysics: Volume I Radiation
Authors: Shu, Frank
1991par..book.....S Altcode:
This upper-level text deals with the emission, absorption, and
scattering of radiation by mater, as well as radiative transfer,
statistical physics, classical electrodynamics, and atomic and molecular
structure.
---------------------------------------------------------
Title: Book-Review - the Physics of Astrophysics - V.1 - Radiation
Authors: Shu, F. H.
1991AstQ....8..262S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Ekman Pumping in a Rotating Protostar
Authors: Galli, D.; Shu, F. H.
1991ASIC..340...33G Altcode: 1991amey.conf...33G
No abstract at ADS
---------------------------------------------------------
Title: Star Formation - a Theoretician's View
Authors: Shu, Frank H.
1991ASPC...20...23S Altcode:
The present evaluation of the hypothesis that the birth of a sunlike
star occurs in four stages, with a critical bipolar outflow phase,
notes that observed molecular outflows cannot be initiated from
circumstellar disks and must instead represent swept-up shells
of gas that are driven by strong protostellar winds. The star can
resolve its growing angular-momentum problem only by rejecting a
fraction of the material that is fed through the disk in a powerful,
magnetocentrifugally-driven wind. Empirical properties measured for
well-collimated outflow sources can be accounted for by the interaction
of this anisotropic wind with the ambient molecular cloud core.
---------------------------------------------------------
Title: Thermal Structure of Neutral Winds from Young Stellar Objects
Authors: Ruden, Steven P.; Glassgold, Alfred E.; Shu, Frank H.
1990ApJ...361..546R Altcode:
The physical processes that control the thermal structure of lightly
ionized winds from cool protostars are discussed. Attention is
concentrated on the hydrogen gas, and the heating, cooling, and chemical
processes that affect the neutral and ionic species of atomic and
molecular hydrogen are examined. Warm silicate dust may condense out of
the cooling wind and may heat the gas through collisions. Singly ionized
sodium atoms, which do not recombine for the mass-loss rates considered,
set a lower limit to the ionization fraction in the wind. Magnetic
fields, which are presumed to accelerate the wind, couple directly to
the ionic component of the gas and transfer momentum and energy to
the neutral component through collisions. This process of ambipolar
diffusion is found to be the dominant source of heat input to the gas.
---------------------------------------------------------
Title: SLING Amplification and Eccentric Gravitational Instabilities
in Gaseous Disks
Authors: Shu, Frank H.; Tremaine, Scott; Adams, Fred C.; Ruden,
Steven P.
1990ApJ...358..495S Altcode:
An analytical description is presented of the modal mechanisms relevant
to a recently discovered type of eccentric gravitational instability in
nearly Keplerian disks. A quantum condition is derived which accurately
predicts the pattern speeds for these modes. The growth rates for the
modes are determined, and it is shown that the mode can grow when the
disk is safely stable to axisymmetric disturbances. The case of marginal
stability for the outside edge is discussed, and the implications of
the results for the formation of binary companions and/or giant planets
within disks associated with young stellar objects are considered.
---------------------------------------------------------
Title: Star formation and the lunar initiative
Authors: Shu, Frank H.
1990AIPC..207..161S Altcode: 1990am...proc..161S
This paper considers the opportunities offered by proposed lunar
observatories for studies of the origin of stars and related
objects. We summarize the principal advantages to be gained by
astronomical observations from the Moon. We then give a brief outline
of the working hypothesis that present-day star formation in the Galaxy
involves four conceptually distinct stages. We conclude by emphasizing
that the greatest strides forward in understanding will probably come
from interferometric investigations that can be carried out with at
least 10 milliarcsecond angular resolution at submillimeter and far
infrared wavelengths, and 10 microarcsecond resolution at ultraviolet,
optical, and near infrared wavelengths.
---------------------------------------------------------
Title: The Formation of Sunlike Stars
Authors: Lada, Charles J.; Shu, Frank H.
1990Sci...248..564L Altcode:
Understanding how stars like the sun formed constitutes one of the
principal challenges confronting modern astrophysics. In recent years,
advances in observational technology, particularly at infrared and
millimeter wave-lengths, have produced an avalanche of critical data
and unexpected discoveries about the process of star formation, which
is blocked from external view at optical and shorter wavelengths by an
obscuring blanket of interstellar dust. Fueled by this new knowledge,
a comprehensive empirical picture of stellar genesis is beginning to
emerge, laying the foundations for a coherent theory of the birth of
sunlike stars.
---------------------------------------------------------
Title: The Dense Interstellar Medium and the Birthplaces of Ob-Stars
Authors: Shu, Frank H.
1990arpf.symp..179S Altcode: 1990arpf.conf..179S
A review is presented of Stromgren's research on the relationships
among atomic, ionized, and molecular hydrogen in the Galaxy and the
spatial distribution and kinematics of OB stars. Particular attention
is given to the development of density-wave theory, the implication
that the pattern of the formation of OB stars in a spiral galaxy should
rotate at a constant angular speed, which differs generally from the
mean material speed of the interstellar gas. Other theories involving
the mechanical support of molecular clouds; triggered star formation;
and the relationships among H2, H I, and H II are discussed.
---------------------------------------------------------
Title: Eccentric Gravitational Instabilities in Nearly Keplerian Disks
Authors: Adams, Fred C.; Ruden, Steven P.; Shu, Frank H.
1989ApJ...347..959A Altcode:
The growth of global gravitational instabilities in young stellar
objects (YSOs) with associated circumstellar disks is studied. The
possibility that the accretion ultimately owes its origin to the growth
of spiral gravitational instabilities is explored. The results indicate
that YSO disks will be unstable to the growth of eccentric distortions
which have growth rates comparable to the orbital frequency at the outer
edge of the disk. Thus, the distortions grow on nearly a dynamical
time scale. Perturbations with m = 1 force the star to move from the
center of mass and thereby transfer angular momentum to the stellar
orbit. Depending on whether or not an axisymmetric stability parameter
Q barrier exists near the corotation radius of the disturbance, this
coupling may lead to mass accretion or to the formation of a binary
companion from the disk, or both.
---------------------------------------------------------
Title: Molecular Cloud Cores and Bimodal Star Formation
Authors: Lizano, Susana; Shu, Frank H.
1989ApJ...342..834L Altcode:
The phenomenon of bimodal star formation is reviewed in the context
of supercritical and subcritical states for molecular clouds that
are supported against their self-gravitation by magnetic fields. The
governing set of equations is derived subject to the quasi-static
and axisymmetric approximations. The method of numerical solution and
tests of the resultant computer code are outlined. The results of the
evolutionary calculations are discussed, emphasizing time scales,
masses, and typical sizes of modeled cores that can be compared
with observations. For a fixed mass, it is found that the level or
turbulent support determines whether a dense core forms or not. This
is used to generalize the concept of a critical mass to account for
the contributions of turbulence and thermal pressures to the support
of a cloud.
---------------------------------------------------------
Title: The Role of Dust in Star Formation - Part Four - Nebular
Disks around Young Stellar Objects
Authors: Shu, F. H.; Adams, F. C.; Lizano, S.
1989eidr.proc..262S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Eccentric gravitational instabilities in nearly Keplerian
discs.
Authors: Ruden, S. P.; Adams, F. C.; Shu, F. H.
1989dad..conf..119R Altcode:
There is growing evidence for circumstellar discs associated
with young stellar objects. Motivated by observational evidence
suggesting that these discs produce significant luminosity,
L<SUB>D</SUB>≡L<SUB>*</SUB>, and have moderate masses,
M<SUB>D</SUB>≡M<SUB>*</SUB>, the authors explore the possibility
that the accretion mechanism ultimately owes its origin to the growth
of spiral gravitational instabilities. As a start, they study the
growth and structure of linear, global, gravitational disturbances in
star/disc systems.
---------------------------------------------------------
Title: The Role of Dust in Star Formation - Part Two - Radiative
Transfer in Dust Envelopes
Authors: Shu, F. H.; Adams, F. C.; Lizano, S.
1989eidr.proc..233S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Role of Dust in Star Formation - Part Three - Spectral
Evolution of Protostars
Authors: Shu, F. H.; Adams, F. C.; Lizano, S.
1989eidr.proc..242S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Role of Dust in Star Formation - Part One - Overview of
the Problem of Star Formation
Authors: Shu, F. H.; Adams, F. C.; Lizano, S.
1989eidr.proc..213S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Eccentric Gravitational Instabilities in Nearly Keplerian Disks
Authors: Adams, F. C.; Ruden, S. P.; Shu, F. H.
1988BAAS...20.1069A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Thermal Structure of Neutral Winds from Young Stellar Objects
Authors: Ruden, S. P.; Shu, F. H.; Glassgold, A. E.
1988BAAS...20.1068R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Mass Loss from Rapidly Rotating Magnetic Protostars
Authors: Shu, Frank H.; Lizano, Susana; Ruden, Steven P.; Najita, Joan
1988ApJ...328L..19S Altcode:
The authors propose that bipolar outflows from young stellar objects
originate from a protostar rotating at breakup at its equator because
it is being spun up by an adjoining accretion disk. Mass outflow at an
appreciable fraction of the infall rate from a surrounding molecular
cloud core onto the star and disk can be driven centrifugally if the
protostar has a sufficiently strong magnetic field. The expansion
of the flow toward the rotational poles may provide a collimation
mechanism for focusing an ordinary stellar wind into optical jets.
---------------------------------------------------------
Title: Neutral Stellar Winds That Drive Bipolar Outflows in Low-Mass
Protostars
Authors: Lizano, Susana; Heiles, Carl; Rodriguez, Luis F.; Koo,
Bon-Chul; Shu, Frank H.; Hasegawa, T.; Hayashi, S.; Mirabel, I. F.
1988ApJ...328..763L Altcode:
Using the Arecibo radio telescope at the 21 cm line of atomic
hydrogen, the authors detected a neutral atomic wind in the bipolar
flow source HH 7-11. The H I gas reaches velocities up to ≡170 km
s<SUP>-1</SUP> and has an associated mass-loss rate from the protostar
of ≡3×10<SUP>-6</SUP>M_sun;yr<SUP>-1</SUP>, which suffices to drive
the observed CO bipolar flow. The 21 cm line profile indicates the
H I wind to be decelerating. The deceleration of the atomic wind may
arise from interaction with the surrounding molecular cloud. Evidence
exists that ambient molecular material has been entrained in the
decelerating H I wind.At the Nobeyama 45 m telescope the authors
detected HCO<SUP>+</SUP> at velocities intermediate between the H I
wind and the CO bipolar outflow. The authors also discuss more ambiguous
evidence in H I and in OH for a decelerating neutral stellar wind from
L1551 IRS 5.
---------------------------------------------------------
Title: Book-Review - Galactic Dynamics
Authors: Binney, J.; Tremaine, S.; Shu, F.
1988Natur.333..219B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Stellar systems in theory
Authors: Shu, Frank
1988Natur.333..219S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Disks of T Tauri Stars with Flat Infrared Spectra
Authors: Adams, Fred C.; Lada, Charles J.; Shu, Frank H.
1988ApJ...326..865A Altcode:
The authors model the energy distributions of T Tauri stars with flat
infrared spectra by assuming that they have spatially thin disks with
unorthodox radial gradients of temperature. These sources differ from
more common T Tauri stars in that the extreme shallowness of their
spectral slope cannot be plausibly explained by conventinal ideas
(e.g., Keplerian accretion or reprocessing of starlight by a slightly
flared disk). Derived model parameters show that the disks associated
with the flat-spectrum sources must contain intrinsic luminosity in
addition to the energy intercepted and reprocessed from the central
star; i.e., the disks must be active. Self-gravity may provide one
possible source for this activity - the spectral limits and estimates of
the disk masses derived for three systems (T Tau, DG Tau, and HL Tau)
are close to the theoretical values that would make the self-gravity
of the disks dynamically important.
---------------------------------------------------------
Title: Applied mathematics, fluid mechanics,
astrophysics. Proceedings.
Authors: Benney, D. J.; Shu, F. H.; Chi, Yuan
1988amfm.conf.....B Altcode:
Contents: 1. Applied mathematics. 2. Stability &
turbulence. 3. General fluid mechanics. 4. Plasma physics. 5. Galactic
astrophysics. 6. General astrophysics.
---------------------------------------------------------
Title: Infrared Spectra of Young Stellar Objects
Authors: Adams, F. C.; Shu, F. H.
1988LNP...297..164A Altcode: 1988ctc..conf..164A
The authors present models of the spectral energy distributions for
an evolutionary sequence of young stellar objects, from protostars to
pre-main-sequence stars. The protostellar theory, characterized by a
central star and disk embedded within an infalling envelope of dust
and gas, can explain the observed infrared spectra of embedded IRAS
sources associated with molecular cloud cores. Next it is shown that T
Tauri stars with near- and mid-infrared excesses can be understood as
young stars surrounded by nebular disks. The disks in T Tauri systems
are found in two varieties: passive disks which have no intrinsic
luminosity and merely intercept and re-radiate stellar photons, and
active disks which have appreciable intrinsic luminosity.
---------------------------------------------------------
Title: SIRTF and Star Formation
Authors: Shu, Frank H.
1988ApL&C..27..105S Altcode: 1988ApL....27..105S
The author discusses four problems in the field of star formation
that can be attacked to advantage with SIRTF: (1) the patterns of star
formation in spiral galaxies, (2) the physical mechanism for bimodal
star formation, (3) the nature of bipolar outflows from young stellar
objects, and (4) the birth of brown dwarfs. In each case, SIRTF can
provide the crucial combination of high angular resolution with great
sensitivity over a broad range of wavelengths that is needed to address
the relevant issues.
---------------------------------------------------------
Title: The evolution of molecular clouds.
Authors: Shu, F. H.; Lizano, S.
1988imgh.conf...65S Altcode:
This paper is also published in Astrophys. Lett. Commun. See 45.131.143.
---------------------------------------------------------
Title: The Evolution of Molecular Clouds
Authors: Shu, Frank H.; Lizano, Susana
1988ApL&C..26..217S Altcode: 1988ApL....26..217S
The problem of the structure and evolution of molecular clouds is
reviewed, with particular emphasis given to the relationship with
star formation. The basic hypothesis is that magnetic fields are
the primary agents for supporting molecular clouds, although damped
Alfven waves may play an important role in the direction parallel
to the field lines. This picture naturally leads to a conception of
'bimodal star formation'. It is proposed that high-mass stars form
from the overall gravitational collapse of a supercritical cloud,
whereas low-mass stars form from small individual cores that slowly
condense by ambipolar diffusion from a more extended envelope until
they pass the brink of graviational instability and begin to collapse
dynamically from 'inside-out'. The evidence that the infall stage of
protostellar evolution is terminated by the development of a powerful
stellar wind is reviewed.
---------------------------------------------------------
Title: Winds from Low Mass Protostars
Authors: Shu, Frank H.; Lizano, Susana; Adams, Fred C.; Ruden,
Steven P.
1988ASSL..148..105S Altcode: 1988pmls.conf..105S
In its last stages, star formation in molecular clouds includes the
onset of a stellar wind that helps to clear away the surrounding
placenta of gas and dust, thereby making the young stellar object
optically visible. The authors discuss new observational evidence
that the emerging wind is largely neutral and atomic in low-mass
protostars. They then suggest a simple theoretical mechanism for the
generation of such powerful neutral winds.
---------------------------------------------------------
Title: Beginning and End of a Low-Mass Protostar
Authors: Shu, Frank H.; Lizano, Susana; Adams, Fred C.; Ruden,
Steven P.
1988ASIC..241..123S Altcode: 1988felm.conf..123S
No abstract at ADS
---------------------------------------------------------
Title: The Evolution of Molecular Clouds
Authors: Shu, Frank H.; Lizano, Susana
1988inma.conf...65S Altcode:
This paper is also published in Astrophys. Lett. Commun. See 45.131.143.
---------------------------------------------------------
Title: The Disks of T Tauri Stars with Flat Infrared Spectra
Authors: Adams, F. C.; Shu, F. H.; Lada, C. J.
1987BAAS...19.1096A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Voyager Photometry of Saturn's A Ring
Authors: Dones, L.; Cuzzi, J. N.; Shu, F. H.
1987BAAS...19..881D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Low Luminosity Sources
Authors: Shu, Frank H.
1987NASCP2466..743S Altcode: 1987sfig.conf..743S
The author summarized certain aspects of the conference. He shares
this task with another colleague thereby breaking the task into more
manageable proportions. The author covers the low luminosity sources. He
begins his review with a summary of some major themes of the conference
and ends with a few speculations on possible theoretical mechanisms.
---------------------------------------------------------
Title: On Statistical Mechanical Theories of Violent Relaxation
Authors: Shu, Frank H.
1987ApJ...316..502S Altcode:
The criticisms of Madsen (1987) directed against the author's
particulate description for the statistical mechanics of violent
relaxation are answered. To Madsen's argument that the particulate
approach violates Liouville's theorem, it is answered that the
theorem is necessarily violated on a macroscopic level if the
concept of violent relaxation is to have sensible meaning. The use
of macrocells and microcells in the particulate approach to describe
stars and elementary particles is discussed, and the argument against
the particulate approach based on constraints on maximum phase-space
density are considered.
---------------------------------------------------------
Title: Planetary ring dynamics and morphology.
Authors: Cuzzi, Jeffrey N.; Durisen, Richard H.; Shu, Frank H.
1987pggp.rept..125C Altcode:
Evidence for a moonlet belt in the region between Saturn's close-in
moonrings Pandora and Prometheus is discussed. It is argued that
little-known observations of magnetospheric electron density by Pioneer
11 imply substantial, ongoing injections of mass into the 2000 km region
which surrounds the F ring. A hypothesis is presented that these events
result naturally from interparticle collisions between the smaller
members of an optically thin belt of moonlets. Also discussed is work
on Uranus ring structure and photometry, image processing and analysis
of the Jonian ring strucure, photometric and structural studies of
the A ring of Saturn, and improvements to an image processing system
for ring studies.
---------------------------------------------------------
Title: Heating of molecular cloud cores.
Authors: Lizano, Susana; Shu, Frank H.
1987RMxAA..14..587L Altcode:
Following the work of Chu (1983), the present treatment of the role of
ambipolar diffusion in the heating of molecular cloud cores calculates
the quasistatic evolution of a plane-parallel self-gravitating slab of
slightly ionized gas due to the ambipolar diffusion of the field. The
temperature at each point of the cloud is computed in light of the
most important heating and cooling processes; the results obtained are
found to partially explain the thermal differences between the cores
forming low- and high-mass stars.
---------------------------------------------------------
Title: Star formation in molecular clouds: observation and theory.
Authors: Shu, Frank H.; Adams, Fred C.; Lizano, Susana
1987ARA&A..25...23S Altcode:
Star-formation (SF) processes occurring on the scale of giant molecular
clouds (10 to the 6th solar masses and 10 to the 20th cm) or smaller are
discussed, reviewing the results of recent theoretical and observational
investigations. Topics examined include the origin of stellar masses;
bimodal SF; initial mass functions; binary stars, bound clusters, and
hierarchical fragmentation; and the efficiency of SF. The properties of
molecular clouds and the origin of substructures in molecular clumps
are explored in detail, and consideration is given to gravitational
collapse and protostars, bipolar outflows from young stellar objects,
visible young stellar objects, and the implications for binary-star
and planetary-system formation.
---------------------------------------------------------
Title: Star formation in molecular cloud cores
Authors: Shu, Frank H.; Lizano, Susana; Adams, Fred C.
1987IAUS..115..417S Altcode:
A scenario for star formation is described based on the assumption that
magnetic fields play an important dynamical role in the interstellar
medium. Particular attention is given to the evolution of a molecular
cloud which is initially subcritical but which has a mass (supported
by a combination of magnetic fields and sub-Alfvenic 'turbulence')
that much exceeds the Jeans mass computed on the basis of the average
density and temperature. It is argued that the magnetic support of such
an object will automatically lead to the production of many small cores.
---------------------------------------------------------
Title: Formation and heating of molecular cloud cores.
Authors: Lizano, Susana; Shu, Frank H.
1987ASIC..210..173L Altcode: 1987ppic.proc..173L
The origin of molecular cloud cores and their evolution into
star-forming regions are discussed, reviewing the results of
recent theoretical investigations. Numerical results are presented
graphically, and it is argued that the existence and properties of
self-gravitating clouds can be explained most easily by the action of
magnetic fields. Supercritical and subcritical cloud regimes (where the
cloud mass is greater or less, respectively, than the mass at which
cloud self-gravity can overcome magnetic-field support) are defined
and related to two types of star formation: (1) highly efficient star
formation and (2) inefficient formation of low-mass stars. Mechanism (1)
is shown to favor the formation of high-mass stars whenever significant
core heating occurs (e.g., via ambipolar diffusion).
---------------------------------------------------------
Title: Spectral Evolution of Young Stellar Objects
Authors: Adams, Fred C.; Lada, Charles J.; Shu, Frank H.
1987ApJ...312..788A Altcode:
An evolutionary sequence, from protostars to pre-main sequence stars,
for the classification of young stellar objects is derived by comparing
the predictions of the theoretical protostar models of Adams and Shu
(AS, 1986) with the morphological classification scheme of Lada and
Wilking (1984). It is shown that the AS models adequately explain the
emergent spectral energy distributions of unidentified objects with
negative spectral indices in the mid-IR and near-IR in both Taurus and
Ophiuchus. If the infalling dust envelope is then completely removed,
the spectra of the underlying stars and nebular disks used by AS
provide a natural explanation for the near-IR and mid-IR excesses and
the positive spectral indices of embedded T Tauri stars. It is found
that the addition of a simple physical model for residual dust envelopes
can reproduce the far-IR excesses found in some of these T Tauri stars.
---------------------------------------------------------
Title: Star formation and the circumstellar matter of young stellar
objects.
Authors: Shu, Frank H.; Adams, Fred C.
1987IAUS..122....7S Altcode:
The authors propose that the formation of low mass stars in molecular
clouds takes place in four stages. The first stage is the formation
of slowly rotating cloud cores through the slow leakage of magnetic
(and turbulent) support by ambipolar diffusion. The second phase
begins when a condensing cloud core passes the brink of instability
and collapses dynamically from "inside-out", building up a central
protostar and nebular disk. The emergent spectral energy distributions
of theoretical models in the infall stage are in close agreement with
those of recently found infrared sources with steep spectra. As the
rotating protostar gains mass, deuterium will eventually ignite in
the central regions and drive the star nearly completely convective if
its mass is less than about 2 M_sun;. This initiates the next step of
evolution - the bipolar outflow phase - in which a stellar wind pushes
outward and breaks through the infalling envelope. The initial breakout
is likely to occur along the rotational poles, leading to collimated
jets and bipolar outflows. The intense stellar wind eventually widens to
sweep out gas in nearly all 4π steradian, revealing the fourth stage
- a T Tauri star with a surrounding remnant nebular disk. Radiation
from a disk adds an infrared excess to the expected spectral energy
distribution of the revealed source.
---------------------------------------------------------
Title: Selected papers of C. C. Lin. Volume 1: Fluid mechanics and
applied mathematics. Volume 2: Astrophysics.
Authors: Benney, D. J.; Shu, F. H.; Chi, Yuan
1987spcc.book.....B Altcode:
These two volumes of the Selected papers of C. C. Lin are compiled on
the occasion of his retirement from active service. The introducing
sections entitled "Recollections and comments" which portray the
development of C. C. Lin's thoughts on various subjects and place events
in historical perspective are given by himself. The selected papers
are grouped according to subject. In Vol. 1, the papers belonging to
fluid mechanics and applied mathematics are divided into five topics:
stability theory, turbulence, general fluid dynamics and aerodynamics,
mathematical physics, and essays on applied mathematics. Vol. 2 contains
Lin's work in astrophysics, largely concerning the development of the
density wave theory for the spiral structure in galaxies. A complete
list of C. C. Lin's publications is given in Vol. 2.
---------------------------------------------------------
Title: Why do Molecular Clouds Give Birth to Objects like Stars
Authors: Shu, F. H.
1986PASP...98.1099S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Studies of Star Formation with SIRTF
Authors: Shu, F. H.
1986BAAS...18.1033S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Infrared Spectra of Rotating Protostars
Authors: Adams, F. C.; Shu, F. H.
1986ApJ...308..836A Altcode:
Earlier calculations of the infrared emission expected from stars in
the process of being made are corrected to include the most important
observable effects of rotation and generalized. An improved version
of the spherical model of a previous paper is developed, and the
corresponding emergent spectral energy distributions are calculated
for the theoretically expected mass infall rate in the cores of cool
and quiescent molecular clouds. The dust grain opacity model and the
temperature profile parameterization are improved. It is shown that the
infrared spectrum of the IRAS source 04264+2426, which is associated
with a Herbig-Haro object, can be adequately represented in terms of a
rotating and accreting protostar. This strengthens the suggestion that
collimated outflows in young stellar objects originate when a stellar
wind tries to emerge and reverse the swirling pattern of infall which
gave birth to the central star.
---------------------------------------------------------
Title: Nonlinear Density Waves in Saturn's Rings: Theory
Authors: Dones, L.; Shu, F. H.; Cuzzi, J. N.; Lissauer, J. J.; Lane,
A. L.; Yuan, C.
1986BAAS...18..839D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Molecular Cloud Cores and Star Formation
Authors: Shu, Frank H.
1986copr.conf..219S Altcode:
The author examines the problem of how stars form from the great
reservoirs of cold matter in giant molecular clouds.
---------------------------------------------------------
Title: Theory of Protostellar Objects
Authors: Shu, F. H.
1986LNP...255...10S Altcode: 1986rhsc.conf...10S; 1986IAUCo..89...10S
Many problems in the theory of star formation are amenable to a
complementary attack in which the analytical approach is used to reduce
the governing equations to a form amenable to efficient numerical
solution. The author concentrates here on the semi-analytical methods
which can be used to reduce the governing equations to a form amenable
to quick and accurate numerical solution.
---------------------------------------------------------
Title: Nonlinear spiral density waves - Viscous damping
Authors: Shu, F. H.; Dones, L.; Lissauer, J. J.; Yuan, C.; Cuzzi, J. N.
1985ApJ...299..542S Altcode:
The authors calculate the viscous damping of nonlinear density waves
in Saturn's rings using various kinetic formulations to estimate
the collisional change (δP/δt)<SUB>c</SUB> of the vertically
integrated pressure tensor P in a particulate disk. The pressure
tensor is computed as a function of two parameters: (1) the ratio of
the average collision frequency to the epicyclic frequency, and (2)
a measure of how close streamlines are to crossing when perturbed
by nonaxisymmetric forces. When parameters approximately correct for
the density waves excited by Mimas's 5:3 resonance in the A ring are
used, the authors obtain a satisfactory fit with the observed profile,
providing one uses a coefficient of restitution that is appropriate
to crystalline ice and providing one uses an extreme Krook formulation
to evaluate (δP/δt)<SUB>c</SUB>. When a similar calculation is made
for the density waves in the B ring excited by the comparably strong
2:1 resonance of the so-called co-orbital satellites, the results are
more mixed. It is speculated that the densities in the actual peaks of
the density waves in Saturn's B ring are limited, not by viscosity, but
by the condition of excluded volumes when ice balls are packed tightly
against one another. Finally, the authors discuss the problems of mass
and angular momentum transport in a disk with externally deposited
resonant torques, and of opening gaps and maintaining sharp edges in
planetary rings.
---------------------------------------------------------
Title: Infrared emission from protostars.
Authors: Adams, F. C.; Shu, F. H.
1985ApJ...296..655A Altcode:
The authors use an approximate technique to calculate the temperature
profiles, and thereby the emergent spectral energy distributions,
of protostars which derive all their luminosity by infall of a gas
and dust envelope. The formulation is general, but the application is
to protostars with spherical symmetry. It is shown that the emergent
spectrum in the infrared from the dust envelope is insensitive to the
details assumed for the temperature profile as long as allowance is
made for a transition from optically thick to optically thin conditions
and luminosity conservation is enforced at an inner shell and an outer
shell. An atlas of emergent spectral energy distributions is presented
for a grid of values of the instantaneous mass M of the protostar
and the mass infall rate M. The theoretical spectra consistently
produce too little mid-infrared radiation in comparison with the
observed sources, especially those which are in a fairly late stage
of protostellar evolution.
---------------------------------------------------------
Title: Infrared Spectra of Rotating Protostars
Authors: Adams, F. C.; Shu, F. H.
1985BAAS...17..863A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The collisional dynamics of particulate disks
Authors: Shu, F. H.; Stewart, G. R.
1985Icar...62..360S Altcode:
We use a Krook equation, modified to allow collisions to be inelastic,
to describe the dynamics of a particulate disk. By a simple heuristic
argument, we compute the effective collision rate in a disk of spherical
particles with a power-law distribution of sizes. For Saturn's rings,
the effective collision rate for momentum transport is substantially
lower than that conventionally estimated on the basis of an observed
optical depth at visual wavelengths. We then discuss how the vertically
integrated set of moment equations may be closed without the need to
discard the third-order moments at the outset; our formulation allows
for the possibility of a bent disk. In the limit that the collision
frequency is much larger than the orbit frequency, we recover the
usual Navier-Stokes equations of viscous hydrodynamics for a thin
disk, with an explicit expression for the shear viscosity. For an
unperturbed disk, we can solve the krook equation directly, without
any assumptions about the magnitude of the collision frequency. Our
analytical results, for an unperturbed disk, are in good agreement with
the treatments of Hämeen-Anttila, of Goldreich and Tremaine, and of
Borderies, Goldreich, and Tremaine, using a Boltzmann description for
a collection of identical spheres (assumed to be smooth so that the
rotational and translational degrees of freedom do not couple). As
a final application of the method, we generalize the formation to
include the effects of gravitational scattering. This generalization
is not crucial for many applications in planetary rings, but it may
be important for the discussion of gas clouds in the disk of a spiral
galaxy, and it is probably central to the accumulation of planets from
smaller bodies in the primitive solar nebula.
---------------------------------------------------------
Title: Density Waves in Saturn's Rings: Nonlinear Damping Theory
and Photometry
Authors: Dones, L.; Cuzzi, J. N.; Shu, F. H.; Lissauer, J. J.; Lane,
A. L.; Yuan, C.
1985BAAS...17..717D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Formation of preplanetary disks from the collapse of
rotating molecular cloud cores
Authors: Cassen, P.; Shu, F. H.; Terebey, S.
1985pggp.rept..143C Altcode:
Solutions that describe the collapse of a molecular cloud core that
is initially in unstable equilibrium, embedded within an envelope of
uniform density, and rotating at the same rate as the envelope are
given. Hydrodynamic equations, including self gravity, are deduced to a
set of ordinary differential equations, which are solved by the method
of matched asymptotic expansions. Results of these calculations are:
(1) the range of stellar masses derived seems to correspond to realistic
ranges of observed stellar masses and interstellar cloud parameters,
(2) the proper measure of dissipation rate is the ratio of accretion
time to viscous diffusion time, and (3) the pressure distribution
on the surface of an accreting protostar is nonuniform in a way that
favors the channeling of a stellar wind into a bipolar flow directed
along the rotation axis.
---------------------------------------------------------
Title: Nonlinear spiral density waves - an inviscid theory
Authors: Shu, F. H.; Yuan, C.; Lissauer, J. J.
1985ApJ...291..356S Altcode:
It is pointed out that the theory of spiral density waves, invented
to explain the spiral structure of disk galaxies, has also been found
useful for the study of planetary rings. The linear theory is by now
well developed, while the nonlinear theory is less complete. Analytical
calculations which include self-gravitation have, so far, obtained
results only in the slightly nonlinear regime, or have concentrated on
partial effects which are not of primary importance to the physical
problem at hand. In the present paper, it is attempted to remedy
these shortcomings. The simplest asymptotic ordering which can still
yield useful results is adopted. Attention is given to the reduction
to a nonlinear integral equation in a single variable, the use of the
Wentzel-Kramers-Brillouin-Jeffreys theory, and the replacement of an
equation by another which is easier to handle numerically.
---------------------------------------------------------
Title: Star formation in molecular clouds
Authors: Shu, F. H.
1985IAUS..106..561S Altcode:
The problem of star formation in spiral galaxies is addressed, with
attention to the length scales involved. The phenomenon spans at least
twelve orders of magnitude, from 10 to the 23rd cm in size to 10 to
the 11th cm. It is concluded that: (1) The distribution of newly born
stars in the Galaxy depends on the origin of giant-molecular-cloud
complexes. For individual complexes, the mechanism of Parker's
instability behind galactic shocks is favored. (2) Magnetic fields
help to support the clumps of molecular gas making up a complex against
gravitation collapse. On a timescale of 10 million years, these fields
slip by ambipolar diffusion relative to the neutral gas, leading to
the formation of dense cloud cores. (3) When to cores collapse, they
usually give rise to low-mass stars. (4) A powerful stellar wind shuts
off the accretion flow and determines the mass of the new star. The
stellar wind is driven in low-mass stars by a release of energy of
differential rotation when the entire protostar becomes convectively
unstable at the onset of deuterium burning.
---------------------------------------------------------
Title: Protostellar disks and star formation.
Authors: Cassen, P.; Shu, F. H.; Terebey, S.
1985prpl.conf..448C Altcode:
The status of theoretical work on protostellar disks is
reviewed. Accretion disk theory and its application to models of the
solar nebula and protostellar disks are discussed. A unified view of the
process of star formation is presented, starting from the evolution of
molecular clouds, and leading naturally to the formation of protostellar
disks. The models used to describe this process are idealized, but are
believed to provide good prototypes that well represent the essential
hydromagnetic phenomena involved in star and disk formation. Several
possible evolutionary paths and final configurations are qualitatively
discussed, showing how the outcomes depend on the relative efficiencies
of various angular momentum transport processes.
---------------------------------------------------------
Title: The collapse of the cores of slowly rotating isothermal clouds
Authors: Terebey, S.; Shu, F. H.; Cassen, P.
1984ApJ...286..529T Altcode:
The authors present here a semianalytic perturbational solution for
the collapse of a slowly rotating cloud core. The initial equilibrium
state is exact and corresponds to the uniformly rotating analogue of
the singular isothermal sphere. Star (and disk) formation proceeds
because the equilibrium is unstable to core collapse. The evolution
in time can be followed by performing a perturbational analysis
on the known similarity solution for the nonrotating case. The
hydrodynamic equations, including self-gravity, can be reduced to a
set of linear ordinary differential equations, which is solved by the
method of matched asymptotic expansions. These calculations provide
a self-consistent description of the dynamical collapse of rotating
molecular cloud cores and a framework for the study of the formation
of stars and nebular disks.
---------------------------------------------------------
Title: Infrared Emission from Protostars
Authors: Shu, F. H.; Adams, F.
1984BAAS...16..973S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Damping of Nonlinear Density Waves in Saturn's Rings
Authors: Dones, L.; Shu, F. H.; Lissauer, J. J.; Cuzzi, J. N.; Yuan, C.
1984BAAS...16R.922D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Book-Review - the Physical Universe
Authors: Shu, F. H.; Smith, R. C.
1984Obs...104..101S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Viscosity in Saturn's Rings
Authors: Lissauer, J. J.; Shu, F. H.; Cuzzi, J. N.
1984plri.coll..385L Altcode: 1984prin.conf..385L; 1984IAUCo..75..385L
The authors discuss the technique of estimating the viscosity in
Saturn's rings from the damping rate of waves observed to be propagating
within the rings. They present a new method that considers the damping
of spiral bending waves, a diagnostic which is largely free of major
complications. The authors deduce a kinematic viscosity of 260 (+150,
-100) cm<SUP>2</SUP>/s for the middle of the A ring where bending
waves have been excited by the 5:3 vertical resonance with Mimas.
---------------------------------------------------------
Title: Waves in planetary rings
Authors: Shu, F. H.
1984prin.conf..513S Altcode:
The Voyager spacecraft revealed the rings of Saturn to have an
unexpected richness of structure. Many of the observed features have
now been identified as collective effects arising from the self-gravity
of the ring material. These effects include spiral density waves and
spiral bending waves, the main topics of this review. Both kinds of
waves were first discussed in the astronomical literature in connection
with the dynamics and structure of spiral galaxies, and the present
discussion contrasts the similarities and differences between the disks
of galaxies and planetary rings. After developing the theory of free
and forced waves of both types, the way in which the observed waves can
be used as diagnostics to obtain crucial parameters that characterize
the physical state of the rings is discussed.
---------------------------------------------------------
Title: POST-USE REVIEW: The Physical Universe: An Introduction
to Astronomy
Authors: Shu, Frank; Edwards, Suzan; Greenstein, George
1984AmJPh..52...91S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Formation of Cool Stars from Cloud Cores
Authors: Shu, Frank H.; Terebey, Susan
1984LNP...193...78S Altcode: 1984csss....3...78S
We consider models of the processes by which a molecular cloud
acquires dense cores, a (magnetized) rotating core collapses to give
a protostar plus nebular disk, and a powerful stellar wind sets in
to reverse the accretion flow and reveal the central object as a
pre-main-sequence star. At each stage, we rely on a combination of
theory and observation to fix the basic parameters of the model. We show
that core formation in a molecular cloud is an inevitable byproduct
of ambipolar diffusion in a magnetized self-gravitating medium of low
fractional ionization. We find that the gravitational collapse of a
uniformly-rotating isothermal core, which possesses a 1/r<SUP>2</SUP>
density profile in its inner parts, has simple analytic properties. And
we propose that strong stellar winds in T Tauri stars represent a phase
of readjustment in the angular momentum distribution after deuterium
burning drives convection throughout a strongly differentially-rotating
protostar. We conclude that the major missing link in this picture is
the evolutionary behavior of massive nebular disks that may accumulate
around protostars. Otherwise, there seems to be a satisfying connection
between the cloud cores observed by molecular-line radio astronomers
and the active stellar atmospheres of young stars studied by optical
and x-ray astronomers.
---------------------------------------------------------
Title: Ambipolar diffusion in self-gravitating isothermal layers
Authors: Shu, F. H.
1983ApJ...273..202S Altcode:
Quasi-magnetohydrostatic and local ionization equilibria are assumed
in the present formulation and solution of the drift of magnetic
field and ions embedded in a self-gravitating layer of neutral
isothermal gas. The introduction of Lagrangian coordinates referred
to the neutral gas allows this problem to be reduced to a nonlinear
diffusion equation for the magnetic field, whose dimensionless form
involves no parameters other than those introduced by the initial
values. In the shape-invariant solution, the magnetic field at each
surface density point in the neutral fluid decays as the inverse square
root of the elapsed time. Explicit estimates are given, as a function
of the initial magnetic to neutral gas pressure in a natural family of
cases, for the amount of time that must pass before the shape-invariant
solution becomes a good approximation for actual behavior. The results
obtained are interpreted physically.
---------------------------------------------------------
Title: Nonlinear Density Waves in Saturn's Rings
Authors: Shu, F. H.; Yuan, C.; Lissauer, J. J.
1983BAAS...15..959S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Rotating Cloud Collapse: 10<SUP>18</SUP> cm to 10<SUP>11</SUP>
cm
Authors: Terebey, S.; Shu, F. H.; Cassen, P. M.
1983BAAS...15..922T Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Book-Review - the Physical Universe - an Introduction to
Astronomy
Authors: Shu, F.; Hughes, D. W.
1983Natur.302..186S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Book-Review - the Physical Universe - an Introduction to
Astronomy
Authors: Shu, F. H.; Miles, H.
1983JBAA...93...97S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Bending waves in Saturn's rings
Authors: Shu, F. H.; Cuzzi, J. N.; Lissauer, J. J.
1983Icar...53..185S Altcode:
We investigate certain brightness variations seen in Saturn's A
ring and find them to be due to vertical corrugations of the local
ring plane caused by a spiral bending wave. This wave is resonantly
excited by Mimas and propagates inward via the collective gravity of
the ring particles. B. A. Smith et al. [ Science212, 163-191 (1981)]
had previously associated vertical relief with this feature due to
its observed azimuthal variations and its proximity to an inclination
resonance with Mimas. We develop the theory of forced bending waves,
some aspects of which have been treated in the galactic context by
C. Hunter and A. Toomre [ Astrophys. J.155, 747-776 (1969)] and by
G. Bertin and J.W.-K. Mark [ Astron. Astrophys.88, 289-297 (1980)]. Our
theory is in good agreement with the observations. In particular,
the presence of these bending waves may resolve the conflict between
ground-based estimates of 1-2 km for the global ring thickness [e.g.,
A. Brahic and B. Sicardy, Nature289, 447-450 (1981)] and Voyager stellar
occultation measurements of <200 m for the local ring thickness
[A. L. Lane et al., Science215, 537-543 (1982); E. A. Marouf and
G. L. Tyler, Science217, 243-245 (1982)].
---------------------------------------------------------
Title: Book-Review - the Physical Universe
Authors: Shu, F.; Fraknoi, A.
1983Mercu..12S..88S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Expanding Universe and the Largescale Geometry of Spacetime
Authors: Shu, F.
1983Mercu..12..162S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Role of Rotation in the Formation of Stars and Planetary
Systems
Authors: Terebey, S.; Shu, F. H.; Cassen, P. M.
1982BAAS...14..639T Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Book-Review - the Physical Universe
Authors: Shu, F. H.
1982JBAA...93R..45S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Physical Universe
Authors: Shu, Frank H.
1982phyn.book.....S Altcode: 1982puai.book.....S; 1982phun.book.....S
This is a truly astonishing book, invaluable for anyone with an interest
in astronomy and surely the bargain of the year.--Physics Bulletin
---------------------------------------------------------
Title: Book-Review - the Physical Universe - Introduction to Astronomy
Authors: Shu, F. H.
1982Natur.298R.310S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Erratum: 'Mass, angular momentum and energy transfer in
close binary stars' [Ann. Rev. Astron. Astrophys., Vol. 19, 277 -
293 (1981)].
Authors: Lubow, S. H.; Shu, F. H.
1982ARA&A..20.....L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Book-Review - the Physical Universe
Authors: Shu, F. H.; Trimble, V.
1982S&T....64..342S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Book-Review - the Physical Universe - a Introduction to
Astronomy
Authors: Shu, F. H.
1982S&T....64Q.152S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The evolution of protostars. III - The accretion envelope
Authors: Stahler, S. W.; Shu, F. H.; Taam, R. E.
1981ApJ...248..727S Altcode:
The radiation gas dynamics of the rapidly inflowing material during
the main accretion phase of protostellar evolution is considered. The
momentum and energy transfer by radiation and matter, the dissociation
of the molecular gas, and the thermochemical destruction of graphite
grains are followed in detail. Considerable physical insight is
gained by taking advantage of the vastly disparate time and length
scales associated with the various processes. Different computational
techniques are used in different portions of the accretion flow:
the optically thick dust envelope, the thermalization layer of
stellar photons, the opacity gap, and the radiative precursor (if
one exists). The evolution of the envelope of a protostellar model
whose hydrostatic core was described in an earlier communication
is summarized.
---------------------------------------------------------
Title: Density waves in Saturn's rings
Authors: Cuzzi, J. N.; Lissauer, J. J.; Shu, F. H.
1981Natur.292..703C Altcode:
Certain radial brightness variations in the outer Cassini division
of Saturn's rings may be spiral density waves driven by Saturn's
large moon Iapetus, in which case a value of ~16 g cm<SUP>-2</SUP>
for the surface density is calculated in the region where the waves are
seen. The kinematic viscosity in the same region is ~170 cm<SUP>2</SUP>
s<SUP>-1</SUP> and the vertical scale height of the ring is estimated
to be a maximum of ~40 m.
---------------------------------------------------------
Title: Moonlets in Saturn's rings?
Authors: Lissauer, J. J.; Shu, F. H.; Cuzzi, J. N.
1981Natur.292..707L Altcode:
The brightness structure within Cassini's division in Saturn's rings
is explained in terms of perturbations produced by moonlets embedded
within an optically thin disk of smaller ring particles. The moonlets
exert gravitational torques on neighbouring ring particles and create
gaps; diffusion acts to fill the gaps. A new explanation is offered
for the inner edge of the Cassini division being located at the 2:1
resonance with Mimas.
---------------------------------------------------------
Title: Mass, angular momentum, and energy transfer in close binary
stars
Authors: Shu, F. H.; Lubow, S. H.
1981ARA&A..19..277S Altcode:
The paper surveys some aspects of the dynamics and energetics of
close binary stars which have heretofore not received a detailed
review. Attention is directed to those processes of mass, angular
momentum, and energy transfer that are thought to have potentially
important consequences for the evolutionary history of close binary
systems. The presentation, which proceeds mainly along theoretical
lines, considers in turn detached binaries, semidetached binaries,
and contact binaries.
---------------------------------------------------------
Title: The evolution of protostars. II - The hydrostatic core
Authors: Stahler, S. W.; Shu, F. H.; Taam, R. E.
1980ApJ...242..226S Altcode:
A computational scheme is developed for the evolution of an accreting
protostellar core. Novel techniques are introduced to account for the
dynamical addition of mass and to follow the disparate thermal responses
in different parts of the star. Numerical results are presented from two
evolutionary calculations that begin with very different distributions
of the specific entropy. The two sequences converge rapidly in time,
justifying a posteriori the assumption that the final results are
insensitive to the detailed mechanism for the formation of the core.
---------------------------------------------------------
Title: Why extended radio doubles are found in elliptical galaxies
Authors: Sparke, L. S.; Shu, F. H.
1980ApJ...241L..65S Altcode:
Elliptical galaxies rotate very slowly, and this rotation affects the
way in which gas lost from stars finds its way into the nucleus. It
is suggested that the dynamical nature of this flow is the reason why
extended double radio sources occur in elliptical galaxies, rather
than in spirals. A specific computed example is presented to illustrate
the principle, and some observational consequences are discussed.
---------------------------------------------------------
Title: The evolution of protostars. I - Global formulation and results
Authors: Stahler, S. W.; Shu, F. H.; Taam, R. E.
1980ApJ...241..637S Altcode:
A new formulation for the problem of protostar evolution is developed on
the basis of a method that involves the division of the global problem
into a set of more manageable subproblems. Jump conditions of the
radiative accretion shock which joins the hydrostatic mass-gaining
core to the dynamic inner cloud envelope are derived. Standard
integration schemes yield the high accuracy needed to resolve complex
spatial structures which span many orders of magnitude in density
and temperature. It is shown that a 1-solar-mass protostar ends its
main accretion phase moderately high up in the H-R diagram; this star
begins its pre-main-sequence phase of quasi-static contraction on a
convective Hayashi track.
---------------------------------------------------------
Title: On various criticisms of the contact discontinuity model
Authors: Shu, F. H.; Lubow, S. H.; Anderson, L.
1980ApJ...239..937S Altcode:
We discuss various criticisms raised recently by Lucy and Wilson,
by Papaloizou and Pringle, and by Smith, Robertson, and Smith against
the contact discontinuity model for contact binaries. In the process
we discover a promising means by which the filled fraction occupied by
the common envelope could, in principle, be determined mechanistically
for a contact binary of given total mass, angular momentum, initial
chemical composition, and age.
---------------------------------------------------------
Title: The origin and lifetime of giant molecular cloud complexes
Authors: Blitz, L.; Shu, F. H.
1980ApJ...238..148B Altcode:
From the available observational and theoretical evidence it is argued
that the mass of molecular gas in the Galaxy has been considerably
overestimated and that the ages of the giant molecular complexes do
not exceed a few times 10-million years. An expression derived for
the collisional time scale for clumps in a complex has a maximum value
of 10-million yr. It is argued that the formation of giant complexes
by random collisional agglomeration of small molecular clouds is
incompatible with several firm observational results. The Parker
instability is discussed as a possible formation mechanism which can
explain many of the observed properties of the complexes.
---------------------------------------------------------
Title: Rotation broadening and the shapes of W Ursae Majoris Stars
Authors: Anderson, L.; Raff, M.; Shu, F. H.
1980IAUS...88..485A Altcode:
Rotation broadening functions are extracted from the spectra of W
Ursae Majoris Stars. A fast Fourier transform is used to deconvolve
photographic spectra, covering some 500 A including dozens of strong
lines, with equivalent spectra from non-rotating stars of similar
spectral type. The resulting rotation functions contain information
about global features such as the shape of the stellar surface
(e.g. mass ratio and degree of contact), gravity brightening and
limb darkening. Preliminary data on the stars VW Cep and ER Vul are
presented. The rotation function of the former reveals the presence
of the third component found visually by Heintz (1975), while that of
the latter shows it to be detached and have mass ratio 0.9.
---------------------------------------------------------
Title: Protostars - a New Approach
Authors: Stahler, S. W.; Shu, F. H.
1980BAAS...12..439S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Theories of contact binary stars
Authors: Shu, F. H.
1980IAUS...88..477S Altcode:
We review and contrast two current theories for the structure of
contact binary stars: discontinuity theory and thermal relaxation
oscillation theory. We find that the two theories are complementary
with the crucial theoretical issue to be resolved being the secular
stability of the temperature inversion layer. Critical observational
tests remain to be performed.
---------------------------------------------------------
Title: A theoretical atlas of light curves and rotation broadening
functions for contact binary stars.
Authors: Anderson, L.; Shu, F. H.
1979ApJS...40..667A Altcode:
Bolometric light curves and rotation broadening functions of contact
binaries are computed for a grid of values of mass ratio, filled
fraction, and orbital inclination, assuming three different extreme
combinations of gravity brightening and limb darkening. The results
are presented in a standard graphical format which is readily usable
for comparisons with observations.
---------------------------------------------------------
Title: On the structure of contact binaries. IV. High-mass models.
Authors: Lubow, S. H.; Shu, F. H.
1979ApJ...229..657L Altcode:
Interior models for zero-age contact binaries of high total mass are
constructed on the basis of the contact discontinuity hypothesis. The
contact discontinuity in systems with a total mass greater than about
4 solar masses is shown to occur on the side of the primary rather
than the secondary. Results for two models, one with masses of 4 +
2 solar masses and the other with 8 + 4 solar masses, are presented,
along with the properties of the corresponding single stars. These
results indicate that the most significant change introduced by the
contact configuration is the appearance of a convection zone beneath
the contact discontinuity at the Roche lobe of the primary. A proof is
given that zero-age main-sequence radiative stars in thermal equilibrium
cannot be expanded to fill their Roche lobes in a contact binary.
---------------------------------------------------------
Title: On the structure of contact binaries. III. Mass and energy
flow.
Authors: Shu, F. H.; Lubow, S. H.; Anderson, L.
1979ApJ...229..223S Altcode:
A variety of topics dealing with the mass and energy flow in contact
binaries is examined. It is argued that the contact discontinuity
hypothesis proposed in earlier communications is both necessary
and sufficient to resolve Kuiper's (1941) paradox concerning the
existence of contact binaries with unequal components. A detailed
qualitative picture of the maintenance by fluid flow of the contact
and weak discontinuities of zero-order models in presented. This
discussion supplies a description of the mechanism by which the
interior luminosities are redistributed to give common envelope
luminosities. It also provides order-of-magnitude estimates for the
thickness of the transition layer and mixing region that constitute
the actual structure of the contact and weak discontinuities. The gas
dynamics of mass loss that occurs if stellar evolution should cause
a synchronously rotating common surface to try to expand beyond the
outer critical surface is calculated. The role of various processes
in the problem of the evolution of contact binaries is considered.
---------------------------------------------------------
Title: Rotation Broadening Functions of Selected W Ursae Majoris
Stars.
Authors: Anderson, L.; Raff, M.; Shu, F. H.
1979BAAS...11..438A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The extrapolated central surface brightness of galaxies.
Authors: Allen, R. J.; Shu, F. H.
1979ApJ...227...67A Altcode:
Disney's (1976) suggestion is reexamined that selection effects
dominate Fish's (1964) law concerning the potential energy-mass
relation for elliptical galaxies and Freeman's (1970) law concerning
the extrapolated central surface brightness of disk galaxies. It is
found that selection effects may indeed have influenced the samples
discussed by Fish and Freeman, but not in the sense described by
Disney. In particular, it is argued that the data set does not show a
bias toward large galaxies, but there is a bias against the faint end
of the distribution of central surface brightnesses. It is therefore
suggested that the observed paucity of ellipticals and spirals with very
high extrapolated central surface brightnesses is probably real. Hence
Fish's and Freeman's laws should be reformulated to state that very
few elliptical and spiral galaxies have extrapolated central isophotes
in B which are brighter than, respectively, 12.0 mag per square arcsec
and 19.5 mag per square arcsec.
---------------------------------------------------------
Title: On the statistical mechanics of violent relaxation.
Authors: Shu, F. H.
1978ApJ...225...83S Altcode:
This paper reexamines the foundations of Lynden-Bell's (1967)
statistical-mechanical discussion of violent relaxation in collisionless
stellar systems. It is argued that Lynden-Bell's formulation in terms
of a continuum description introduces unnecessary complications,
and a more conventional formulation in terms of particles is
considered. The exclusion principle discovered by Lynden-Bell is found
to be quantitatively important only at phase densities where two-body
encounters are no longer negligible. Since the dynamical basis for
the exclusion principle vanishes in such cases anyway, Lynden-Bell
statistics always reduces in practice to Maxwell-Boltzmann statistics
when applied to stellar systems. Lynden-Bell also found the equilibrium
distribution function generally to be a sum of Maxwellians with velocity
dispersions dependent on the phase density at star formation. It is
shown that this difficulty vanishes in the particulate description for
an encounterless stellar system as long as stars of different masses
are initially well mixed in phase space.
---------------------------------------------------------
Title: On the role of photospheric convection in W Ursae Majoris
stars.
Authors: Anderson, L.; Shu, F. H.
1978ApJ...221..926A Altcode:
We correct the derivation of the source function in the atmospheres
of contact binaries given in an earlier communication by Anderson and
Shu. This correction affects the cases when convection is present in
the photosphere. In our new treatment photospheric convection is more
efficient for reducing limb darkening. This result does not, however,
modify the numerical examples considered in our earlier paper. Subject
headings: convection - stars: W Ursae Majoris
---------------------------------------------------------
Title: Hot Gas in the Galaxy: how Extensive is It?
Authors: Shu, F. H.
1978IAUS...77..139S Altcode:
Observational and theoretical evidence is reviewed concerning the
possible extent of hot gas at a temperature of the order of 1 million
K in the Galaxy. Consequences of hot gas filling a large fraction
of interstellar space for galactic shocks are considered. H I radio
observations are discussed along with O VI observations in the UV,
soft X-ray observations, and theoretical estimates of the filling
factor of hot gas at 1 million K. A series of future radio and soft
X-ray observations of external spiral galaxies is proposed.
---------------------------------------------------------
Title: On the structure of contact binaries. II. Zero-age models.
Authors: Lubow, S. H.; Shu, F. H.
1977ApJ...216..517L Altcode:
Zero-age models of contact binaries of roughly solar composition are
constructed on the basis of contact discontinuity hypothesis. With
this formulation, systems with common radiative envelopes can be
constructed as well as systems with common convective envelopes. Two
models with total masses, respectively, of 1.5 and 3 solar masses
are presented explicitly; both binary models have a mass ratio chosen
equal to 0.5. The properties of the interior structure of these models
are compared with the properties of zero-age single stars which have
masses corresponding to the individual components. The predictions of
the theory are compared with the empirical period-color relationship
found by Eggen (1961, 1967) for W Ursae Majoris stars. The agreement
with observations is satisfactory.
---------------------------------------------------------
Title: On the light curves of W Ursae Majoris stars.
Authors: Anderson, L.; Shu, F. H.
1977ApJ...214..798A Altcode:
We develop a physical theory for the light curves of contact binaries
based on the assumption that the dynamical time scale is very short
in comparison with the thermal time scale at the base of the common
envelope. In contrast with the case for common radiative envelopes,
the flux distribution in common convective envelopes does not exhibit
any effect of gravity brightening. Combined with a unified treatment
of reflection and limb darkening, this result produces Wtype light
curves for W UMa stars of spectral type later than F5 if the orbit
is inclined by less than approximately 70 90o. The sign of the
effect is in rough accord with the observations, but some discrepancy
remains concerning the magnitude of the effect. We speculate that the
interaction between rapid rotation and convection may contribute to
the remaining discrepancy; it may also produce the asymmetry and time
variability which are observed in some light curves. Subject headings:
stars: binaries - stars: W Ursae Majoris
---------------------------------------------------------
Title: Self-similar collapse of isothermal spheres and star formation.
Authors: Shu, F. H.
1977ApJ...214..488S Altcode:
Similarity solutions are obtained which describe the gravitational
collapse of isothermal spheres that originated as gas clouds not
far removed from the condition of marginal stability. It is argued
that the similarity solution of Larson and Penston (1969) for the
stages before core formation is physically artificial, but the
gasdynamic flow subsequent to core formation exhibits self-similar
properties. Similarity solutions are determined for the collapse of
singular isothermal spheres, minus solutions without critical points
are obtained by imposing the condition that the fluid velocities are
negligible at the 'initial instant', and an expansion-wave collapse
solution is evaluated. The results are illustrated with a numerical
example roughly corresponding to conditions appropriate for Bok
globules or the central regions of a nonmagnetic molecular cloud. Two
possible applications of the solutions are discussed: analyzing the
stability to gravitational fragmentation of collapsing pressure-free
gas spheres and determining the amount of energy radiated away during
protostar formation.
---------------------------------------------------------
Title: Theory of Close Binary Stars.
Authors: Shu, F. H.
1977BAAS....9..318S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On the structure of contact binaries. I. The contact
discontinuity.
Authors: Shu, F. H.; Lubow, S. H.; Anderson, L.
1976ApJ...209..536S Altcode:
The problem of the interior structure of contact binaries is reviewed,
and a simple resolution of the difficulties which plague the theory
is suggested. It is proposed that contact binaries contain a contact
discontinuity between the lower surface of the common envelope and the
Roche lobe of the cooler star. This discontinuity is maintained against
thermal diffusion by fluid flow, and the transition layer is thin to
the extent that the dynamical time scale is short in comparison with
the thermal time scale. The idealization that the transition layer has
infinitesimal thickness allows a simple formulation of the structure
equations which are closed by appropriate jump conditions across
the discontinuity. The further imposition of the standard boundary
conditions suffices to define a unique model for the system once the
chemical composition, the masses of the two stars, and the orbital
separation are specified.
---------------------------------------------------------
Title: Zero-Age Models of Contact Binaries.
Authors: Lubow, S. H.; Shu, F. H.
1976BAAS....8Q.520L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Self-Similar Collapse of Isothermal Spheres and Star Formation.
Authors: Shu, F. H.
1976BAAS....8Q.547S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On the Light Curves of W Ursae Majoris Stars.
Authors: Anderson, L.; Shu, F. H.
1976BAAS....8R.521A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Gas dynamics of semidetached binaries. II. The vertical
structure of the stream.
Authors: Lubow, S. H.; Shu, F. H.
1976ApJ...207L..53L Altcode:
The letter considers the three-dimensional dynamics of the stream in
semidetached binaries undergoing Roche-lobe overflow. Results of an
earlier communication are generalized to include the dynamic effects
in the direction perpendicular to the orbital plane. It is found
that the scale height of the stream characteristically exceeds its
corresponding hydrostatic value by a significant factor because the
inertia of the gas prevents it from responding instantaneously to
the changing gravitational field. This effect is important for the
interpretation of observations relating to stream-disk impacts in
cataclysmic variables and in binary X-ray sources of low total mass.
---------------------------------------------------------
Title: Mass Transfer in Semi-Detached Binaries
Authors: Shu, F. H.
1976IAUS...73..253S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Gas dynamics of semidetached binaries.
Authors: Lubow, S. H.; Shu, F. H.
1975ApJ...198..383L Altcode:
The gas dynamics of semidetached binary systems is analyzed in the
context of the Roche model. It is shown that the steady flow can be
formulated in terms of a problem with multiple length scales if it is
assumed that the contact component rotates synchronously and the flow
occurs isothermally with the thermal speed being a small fraction of the
relative orbital speed of the two stars. This concept and semianalytical
methods are used to demonstrate that material escapes from the surface
of the contact component in a highly anisotropic stellar wind, that
the wind throttles into a narrow stream of material whose width is a
function of the orbital separation and whose density is a function of
the mass transfer rate and the relative orbital speed of the two stars,
and that the stream results in the formation of a disk of material
of prescribed size orbiting the detached component if the latter is
smaller than a specified size.
---------------------------------------------------------
Title: Density wave theory and the classification of spiral galaxies.
Authors: Roberts, W. W., Jr.; Roberts, M. S.; Shu, F. H.
1975ApJ...196..381R Altcode:
Axisymmetric models of disk galaxies taken together with the density
wave theory allow one to distinguish and categorize spiral galaxies
by means of two fundamental galactic parameters: the total mass of
the galaxy, divided by a characteristic dimension, and the degree of
concentration of mass toward the galactic center. These two parameters
govern the strength of the galactic shocks in the interstellar gas and
the geometry of the spiral wave pattern. In turn, the shock strength
and the theoretical pitch angle of the spiral arms play a major role in
determining the degree of development of spiral structure in a galaxy
and its Hubble type. The application of these results to 24 external
galaxies demonstrates that the categorization of galaxies according
to this theoretical framework correlates well with the accepted
classification of these galaxies within the observed sequences of
luminosity class and Hubble type.
---------------------------------------------------------
Title: Star Formation in Spiral Galaxies (Review)
Authors: Shu, Frank H.
1975dgs..conf..309S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Parker Instability in Differentially-rotating Disks
Authors: Shu, F. H.
1974A&A....33...55S Altcode:
Summary. We investigate Parker's instability for a differentially
rotating system comprised of thermal gas, magnetic field, and
cosmic-ray particles. The rotation axis coincides with the direction
of the vertical gravity, and the rotation is modelled to occur with
linear shear. The general initial-value problem is formulated, and
the condition for normal modes is obtained from this formulation. A
dispersion relation is obtained for the limiting case when the
growth rate (or frequency) of the wave is large in comparison with the
kinematic shear rate. This dispersion relation suffices to show, in the
absence of dissipation, that no finite amount of shear and rotation
can ever completely stabilize Parker's mode although the growth rate
of certain Fourier components can be materially reduced. Eigenvalues
and eigenfunctions are computed analytically for a number of limiting
cases of interest, and a few numerical examples are given. The effect
of rotation is to suppress waves which are very long in the horizontal
directions; the full effects of shear are more difficult to assess -
numerical methods are suggested for future work. Subsidiary issues
examined in this study are (i) the derivation of an alternative
equation, valid in the nonlinear regime and for arbitrary geometries,
for the usual fluid equation adopted to describe the behavior of the
cosmic-ray pressure, (ii) the distinction between environments under
which Parker's mode of instability is likely to lead to convection,
to cosmic-ray inflation, or to gas drainage downward to form dense
clumps of matter, (iii) an explanation of the physical reasonableness
of normal mode solutions with finite energy densities at infinity and
the relation of such solutions to the initial-value problem. Key words:
interstellar medium - instability - magnetic field - cosmic-rays -
differential rotation
---------------------------------------------------------
Title: On the Formation of Interstellar Cloud Complexes, OB
Associations and Giant H II Regions
Authors: Mouschovias, T. Ch.; Shu, F. H.; Woodward, P. R.
1974A&A....33...73M Altcode:
Summary. We propose that large cloud complexes, OB associations, and
giant H ii regions form as a result of the initiation of the magnetic
Rayleigh-Taylor instability in the interstellar medium by the passage of
a galactic shock. Total masses of about 106 M0, formation of unbound
systems, and alignment along spiral arms with typical separation
of about 1 kpc are natural consequences of this point of view. In
addition, we argue that the enhancement of synchrotron radiation to
be expected in the compression zones of spiral galaxies has hitherto
been considerably overestimated. Key words: instability - spiral arms -
cloud complexes - giant H ii regions
---------------------------------------------------------
Title: On the Strength of the Galactic Shock Wave and the Degree of
Development of Spiral Structure
Authors: Roberts, W. W., Jr.; Roberts, M. S.; Shu, F. H.
1974IAUS...58..439R Altcode: 1974fdg..symp..439R
No abstract at ADS
---------------------------------------------------------
Title: Density-Wave Theory of Spiral Structure
Authors: Shu, F. H.
1974ASIC....6..219S Altcode: 1974inme.conf..219S
No abstract at ADS
---------------------------------------------------------
Title: Nonlinear Gaseous Density Waves and Galactic Shocks
Authors: Shu, Frank H.; Milione, Vincenzo; Roberts, William W., Jr.
1973ApJ...183..819S Altcode:
We follow the perturbations in the flow of interstellar gas which
result from steady forcing by spiral gravitational fields of various
strengths. The density response is quite nonlinear even if the
amplitude of the spiral field maintained by the disk stars is only a
small fraction of the basic axisymmetric field. An analytical study
of the properties of slightly nonlinear flows yields certain results
which are qualitatively similar to those found numerically for fully
nonlinear flows. Galactic shocks arise naturally, indeed necessarily,
if the strength of the underlying spiral gravitational field exceeds a
certain critical value. The breadth of the zone of high gas compression
depends critically on whether the Dopplershifted phase-velocity of
the stellar density wave is greater than or less than the "effective
acoustic speed" of the gas. In the former case, very narrow compression
zones result; in the latter, quite broad zones. This distinction may
explain why some galaxies have narrow optical arms while others have
broad optical arms. In addition, a certain range of values for the
intrinsic frequency of the wave gives rise to ultraharmonic resonances
which can introduce secondary compressions of the interstellar gas. This
result may relate directly to the origin of the Carina spiral feature
in our own Galaxy as well as to the phenomena of branches, spurs,
and feathers which are often seen in external spiral galaxies. Subject
headings: galactic structure - interstellar matter
---------------------------------------------------------
Title: Spiral structure, dust clouds, and star formation.
Authors: Shu, F. H.
1973AmSci..61..524S Altcode: 1974AmSci..61..524S
No abstract at ADS
---------------------------------------------------------
Title: On the Genetic Relation Between Interstellar Clouds and
Dust Clouds
Authors: Shu, F. H.
1973IAUS...52..257S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Role of Gaseous Dissipation in Density Waves of Finite
Amplitude
Authors: Roberts, W. W., Jr.; Shu, F. H.
1972ApL....12...49R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Galactic Shocks in an Interstellar Medium with Two Stable
Phases
Authors: Shu, Frank H.; Milione, Vincenzo; Gebel, William; Yuan, C.;
Goldsmith, D. W.; Roberts, W. W.
1972ApJ...173..557S Altcode:
Quasi-steady flows of interstellar gas in a spiral gravitational field
are followed for the purpose of investigating galactic shocks and
the resultant processes of the formation of stars and interstellar
clouds. \N'7e model the interstellar medium with two stable phases
in which thermal balance is maintained through heating by low-energy
cosmic rays The problem, including transitions between the two phases,
is given a general formulation but is solved in an approximation which
ignores the difference in fluid velocities of the two phases. We also
assume that the cosmic-ray flux is uniform in circles about the center
of the Galaxy and that the relative abundances of the chemical elements
are "normal." For a spiral gravitational field with strength equal to
5 percent that of the axisymmetric field at 10 kpc from the galactic
center, the density rat at and minimum compressions is 9: 1 for the
inter- cloud medium while it is 40:1 for the gas in a typical cloud
During the decompression phase of the flow, a small percentage of the
mass of the clouds evaporates to become intercloud material, but this
small amount is recovered in the shock As a by-product of phase ,
the properties of the clouds in the regions between spiral arms are
such as to make their detection in 21-cm absorption very difficult. In
the absence of the cloud phase, we determine the thickness of the shock
layer in intercloud medium to be typically 50 pc. An interstellar cloud
immersed as a test particle in the intercloud medium experiences a
dynamic rather than a quasi-static compression as it passes through the
shock layer. The critical mass for the gravitational collapse of a cloud
is ieduced by a large factor because of the compression in the shock.
---------------------------------------------------------
Title: On the Density-Wave Theory of Galactic
Spirals. III. Comparisons with External Galaxies
Authors: Shu, Frank H.; Stachnik, Robert V.; Yost, Jonathan C.
1971ApJ...166..465S Altcode:
We undertake a semiempirical study of the spiral patterns of three model
galaxies constructed from the observed rotation curves of I33, M51,
and M8t. Consistent with the proposal that density waves are initiated
in the outer regions of a galaxy, we find a good fit to result for
the observed spiral structure if we choose a pattern speed equal to
the rotation speed where the distribution of H ii regions is seen
to end. The circumferential bands containing the most prominent H ii
regions are located in qualitative agreement with a mechanism in which
star formation is triggered by spiral galactic shocks. A suggestion
is made for M51 for the coexistence of short and long trailing waves
traveling with the same pattern speed.
---------------------------------------------------------
Title: On a Mass-Energy Angular Momentum Relation for Spiral Galaxies.
Authors: Gezari, D.; Mongillo, M.; Shu, F. H.
1971BAAS....3..238G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Galactic Shocks in an Interstellar Medium with Two Stable
Phases.
Authors: Shu, F. H.; Gebel, W.; Milione, V.; Goldsmith, D. W.
1971BAAS....3..246S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Density wave theory of spiral structure.
Authors: Lin, C. C.; Shu, F. H. -S.
1971agr.....2..235L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On the Spiral Structure of M33, M51, and M81.
Authors: Shu, Frank H.; Stachnik, Robert V.; Yost, Jonathan C.
1970BAAS....2Q.344S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On the Density-Wave Theory of Galactic Spirals. I. Spiral
Structure as a Normal Mode of Oscillation
Authors: Shu, Frank H.
1970ApJ...160...89S Altcode:
An exact formulation of the linearized problem, including appropriate
boundary conditions, is de- veloped to explore whether extensive
galactic density waves of spiral form are permissible normal modes of
oscillation for a stellar disk. An "anti-spiral theorem," of the type
reported previously by Lynden- Bell and Ostriker for neutral modes
in a gaseous disk, holds here with limited validity-namely, when-
ever the effects of stellar resonances can be ignored
---------------------------------------------------------
Title: On the Density-Wave Theory of Galactic Spirals. II. The
Propagation of the Density of Wave Action
Authors: Shu, Frank H.
1970ApJ...160...99S Altcode:
The properties of galactic density waves are studied in the WKBJ
approximation. In the lowest order of approximation, we reproduce the
dispersion relation reported by Lin and Shu in an earlier communi-
cation. In the next order, we demonstrate explicitly that the density
of "wave action" is transported with the group velocity derived by
Toomre. Some general implications are drawn for mechanisms proposed
for the origin of spiral structure
---------------------------------------------------------
Title: The Propogation and Absorption of Spiral Density Waves
Authors: Shu, F. H.
1970IAUS...38..323S Altcode: 1970ssg..conf..323S
No abstract at ADS
---------------------------------------------------------
Title: Models of Partially Relaxed Stellar Disks
Authors: Shu, Frank H.
1969ApJ...158..505S Altcode:
The proposal by Lynden~Be11 that violently changing gravitational
fields serve as the primary mecha- nism of relaxation in a galaxy of
stars is reexamined for disk galaxies. The point of view adopted is
that the oniy relaxation mechanism operative for stars in the early
life of such galaxies is an axisymmetric form of the Jeans instability
discussed by Toomre. The most probable form of the distribution function
which results for a disk of infinitesimal thickness is obtained from
statistical considerations. An asymptot- ic method of solution based
on this distribution is developed for the construction of galactic
models from observed rotation curves. The distribution of stars with
low peculiar velocities follows Schwarzschild's law whereas that of
stars with high velocities shows the asymmetry in galactic longitude
discussed by Oort
---------------------------------------------------------
Title: Erratum: on the Spiral Structure of Disk
Galaxies. III. Comparison with Observations
Authors: Lin, C. C.; Yuan, C.; Shu, Frank H.
1969ApJ...156..797L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On the Spiral Structure of Disk Galaxies. III. Comparison
with Observations
Authors: Lin, C. C.; Yuan, C.; Shu, Frank H.
1969ApJ...155..721L Altcode:
The density-wave theory of galactic spirals is developed in a form
slightly more general than that outlined by Lin and Shu in an earlier
short communication. Only self-sustained waves are studied in this
paper, and the problem of the origin of the spiral structure is
barely discussed. The implications of the theory are examined, both
in general terms and in detail. The conclusions are compared with
observa- tions. Specifically, we consider (1) the distribution of
atomic hydrogen in the Galaxy, (2) the systematic motion of the gas,
(3) the distribution of young stars and other optical objects, and (4)
the migration of moderately young stars. Good agreements are obtained
in all cases if we adopt a pattern speed of about 13 km sec' kpc', and a
spiral gravitational field equal to about 5 per cent of the symmetrical
field. General discussions are also given on (a) the structure of the
magnetic field and its role on the systematic motion of the gas, (b) the
role of the density wave in the process of star formation, and (c) the
distribution of liii regions as revealed by the 109a radio observations
---------------------------------------------------------
Title: On the Effects of Finite Disk Thickness and Gas Content on
Spiral Structure.
Authors: Shu, Frank H.
1968AJS....73R.201S Altcode:
The density wave theory of spiral structure in disk galaxies
outlined previously by Lin and Shu (Astrophys. J. 140, 646, 1964;
Proc. Nati. Acad. Scj. 55, 229,1966; Proc. IAU-URSI, Symp. No. 31,1967)
is extended to include the effects of finite disk thickness and the
presence of a fair amount of interstellar gas. In the theory based on
a model of infinitesimal thickness, there exists a decided discrepancy
between the theoretically predicted values of the stellar velocity
dispersion required for stability and those actually observed in our own
Galaxy. With the consideration of the effect of the finite thickness of
the disks of stars and gas, this discrepancy disappears. The numerical
form of the resultant dispersion relation between wave frequency and
wavelength for spiral~waves does not differ appreciably from that
obtained on the basis of the simple theory. The relative participation
of gas and stars in the spiral structure of our own Galaxy is found
to be about the same in the solar vicinity. In the interior parts,
stars will play the more important role. Frank H. Shu is currently at
the State University of New York, Stony Brook.
---------------------------------------------------------
Title: The Dynamics and Large-Scale Structure of Spiral Galaxies.
Authors: Shu, Frank Hsia-San
1968PhDT.........1S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Density waves in disk galaxies.
Authors: Lin, C. C.; Shu, F. H.
1967IAUS...31..313L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On the Spiral Structure of Disk Galaxies, II. Outline of a
Theory of Density Waves
Authors: Lin, C. C.; Shu, Frank H.
1966PNAS...55..229L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Gravitational Collapse of a Uniform Spheroid.
Authors: Lin, C. C.; Mestel, L.; Shu, F. H.
1965ApJ...142.1431L Altcode:
A uniform, non-rotating, pressure-free spheroid is supposed to collapse
gravitationally from rest. It is shown that the initial eccentricity is
steadily increased by the anisotropic gravitational field: an initially
oblate spheroid tends toward a disk, and an initially prolate spheroid
toward a spindle. Numerical results are computed for a series of
initial eccentricities.
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Title: On the Spiral Structure of Disk Galaxies.
Authors: Lin, C. C.; Shu, Frank H.
1964ApJ...140..646L Altcode:
It is shown that gravitational instability is a plausible basis for the
formation of the spiral pattern in disk galaxies An explicit asymptotic
formula is obtained for the form of the spiral. It gives reasonable
numerical results for the galaxy, and qualitatively satisfactory trends
for normal spirals of various types.
