Author name code: vitense
ADS astronomy entries on 2022-09-14
author:"Vitense, Erika" OR author:"Boehm-Vitense, Erika"
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Title: The Number of Rotations per Stellar Activity Cycle in G and
K Main Sequence Stars
Authors: Böhm-Vitense, E.; Davenport, J. R. A.
Bibcode: 2011ASPC..448.1077B
Altcode: 2011csss...16.1077B
We find that for G and K main sequence stars, the number of rotations
per stellar activity cycle nc increases inversely as the
rotation period rm prot decreases. The scaling constant C may
vary by a factor of 10 for different types of stars, while nc
may change by as much as 3 orders of magnitude. We show that we can
understand this relation when we consider how the rotation period
varies between the top and the bottom of the convection zone and with
depth of the convection zone in stars of different mass. In general,
faster rotating stars have thinner convection zones. We also show that
the observed sequences in the relation between the rotation period
and the activity cycle period for G and K stars are obtained if the
rotation periods in the dynamo layer and at the surface are correlated,
as is the case if both are due to convective angular momentum transport.
Title: Hyades Morphology and Star Formation
Authors: Böhm-Vitense, Erika
Bibcode: 2007AJ....133.1903B
Altcode:
Perryman and collaborators found that for the Hyades cluster the
fraction of multiple stars increases from G stars to early-A stars. We
discuss here whether this may be a general property of star formation
or whether collisions in the cluster environment change the fractions
of binary stars. A star ring of mainly F and G stars is seen around
the Hyades cluster core, supposedly created by a shock wave due to a
supernova explosion. These ring stars show the same fraction of binary
stars as observed for the F and G stars in the core of the cluster. This
suggests that collisions in the high stellar density cluster center
did not measurably change the multiple-star fractions. There is so
far only evidence of this one supernova explosion in the Hyades. If
this is indeed the only one, then only one massive star was born in
the cluster. There are also fewer than 10 white dwarf descendants of
B stars found in the cluster. Unless many white dwarfs and neutron
stars evaporated from the cluster, the initial mass function for
stellar masses >2 Msolar must have been very steep,
corresponding to a Salpeter function with an exponent of less than -3.3.
Title: Chromospheric Activity in G and K Main-Sequence Stars, and
What It Tells Us about Stellar Dynamos
Authors: Böhm-Vitense, Erika
Bibcode: 2007ApJ...657..486B
Altcode:
For main-sequence G and K stars we study again the empirical relations
between the periods of the activity cycles, Pcyc, and the
rotational periods, prot. We use the high-quality data
selected by Brandenburg, Saar, and Turpin. As found by those authors
``the Pcyc increase proportional to the prot,
along two distinctly different sequences,'' the active ``A'' sequence,
and the inactive ``'I'' sequence with cooler and more slowly rotating
stars. It is found here that along each sequence the number of
rotation periods per activity cycle is nearly the same, but the
numbers are different for the different sequences, indicating that
probably different kinds of dynamos are working for the stars on the
different sequences. The transition from one sequence to the other
occurs at a rotation period of 21 days. The rotation periods then
increase abruptly by about a factor of 2 for the cooler stars. We
suggest that this indicates abruptly increased deep mixing. Along the
I sequence the overall dependence of the Ca II emission line fluxes,
F(Ca II), on rotation and Teff is consistent with F(Ca
II)~T4effp-4/3rot. For
the A-sequence stars the dependence of F(Ca II) on rotation seems to
be stronger than for the I-sequence stars.
Title: The Puzzle of the Metallic Line Stars
Authors: Böhm-Vitense, Erika
Bibcode: 2006PASP..118..419B
Altcode:
In the puzzle of the metallic line (Am) stars, there still seem to be
missing pieces. While the ``normal'' A stars have elemental abundances
close to solar, the classical Am stars show stronger absorption lines
for most heavy elements in their spectra. Elements with ionization
potentials that nearly agree with those of hydrogen or helium have
reduced abundances. The Ca II and Sc II lines are especially weak. The
Am stars have no ultraviolet emission lines. They are binaries that,
with very few exceptions, have rotational velocities vsini lower
than 100 km s-1. Of the main-sequence A stars, 20% to
30% are Am stars. Here we rediscuss previous suggestions that tried
to explain the peculiar line strengths in the Am star spectra. In
particular, we compare the well-studied properties of Hyades A and
Am stars in order to identify reasons that can or cannot explain
the differences. We find that accretion of interstellar material by
A stars with distorted magnetic fields, which are weaker than those
in peculiar A (Ap) stars, has the best chance of explaining the main
characteristics of the peculiar heavy-element abundances in Am star
photospheres. Charge-exchange reactions also seem to be important.
Title: Rotation and Lithium Surface Abundances, Revisited
Authors: Böhm-Vitense, Erika
Bibcode: 2004AJ....128.2435B
Altcode:
For giants in the Hertzsprung gap, the relations between
Teff, vsini, and lithium surface abundances are
reinvestigated and compared with the relations found for Hyades
main-sequence F stars. For the Hyades main-sequence F stars, the vsini
decrease steeply around Teff~6450 K. At the same temperature
the lithium surface abundances show a narrow, deep dip. For most giants
there is also a steep decrease of vsini for Teff around 6450
K. At this temperature the lithium surface abundances of the giants
also decrease steeply and remain low for Teff<6400
K. The changes in rotation and Li surface abundances occur over a
temperature interval of less than 300 K, which for a 2 Msolar
giant corresponds to an age interval of about 106 yr. The
simultaneous steep decreases of rotation velocities and Li surface
abundances indicate that for the giants these changes are due to the
same cause, which we suggest to be deep mixing. It then seems rather
likely that for the Hyades main-sequence F5 V stars the decrease of
rotation and Li surface abundance is also caused by deep mixing. We
suggest that in both cases the changes are related to the merging of
the hydrogen and helium convection zones.
Title: The O VI and C III Lines at 1032 and 977 Å in Hyades F Stars
Authors: Böhm-Vitense, Erika; Robinson, Richard D.; Carpenter,
Kenneth G.
Bibcode: 2004ApJ...606.1174B
Altcode:
We continue our investigations into the mechanisms heating the outer
layers of cool dwarf stars. In this study we specifically seek to
determine whether in the layers with temperatures around 250,000-300,000
K, in which the O VI lines are emitted, the temperatures are determined
by heat conduction from the coronae or by the same processes that
heat the lower temperature regions. To study this we discuss here 22
spectra of Hyades F stars taken by the Far Ultraviolet Spectroscopic
Explorer (FUSE) satellite to study the O VI lines at 1032 Å and the C
III lines at 977 Å and compare them with other lower transition layer
lines, observed with HST and IUE, and with existing X-ray data. For our
targets with B-V>0.4, the X-ray fluxes of single F stars increase,
on average, slowly with increasing B-V, while the O VI line fluxes show
the same steep decrease around B-V=0.43 as previously found for the
lower temperature transition layer lines. For single stars the X-ray
fluxes decrease with increasing vsini, except for the stars with B-V
between 0.418 and 0.455, while for the O VI lines, as for the other
transition layer lines, fluxes increase with increasing vsini, if vsini
is larger than 30 km s-1. For smaller vsini, line fluxes are
independent of vsini. The B-V and vsini dependences of the O VI line
fluxes are then very different from those of the X-ray fluxes. We thus
conclude that for electron temperature Te below 300,000 K,
the transition layers for Hyades F stars are not mainly heated by
heat conduction from their coronae. Based on observations made
with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer (FUSE)
telescope, which is operated for NASA by Johns Hopkins University,
under contract NAS5-32985.
Title: Karl Schwarzschild Lecture: What Hyades F Stars tell us about
Heating Mechanisms in the outer Stellar Atmospheres (With 15 Figures)
Authors: Boehm-Vitense, Erika
Bibcode: 2004RvMA...17....1B
Altcode:
No abstract at ADS
Title: What Is Happening at Spectral Type F5 in Hyades F Stars?
Authors: Böhm-Vitense, Erika; Robinson, Richard; Carpenter, Kenneth;
Mena-Werth, Jose
Bibcode: 2002ApJ...569..941B
Altcode:
Aiming at a better understanding of the mechanisms heating the
chromospheres, transition regions, and coronae of cool stars, we study
ultraviolet, low-resolution Hubble Space Telescope/Space Telescope
Imaging Spectrograph spectra of Hyades main-sequence F stars. We
study the B-V dependence(s) of the chromospheric and transition layer
emission line fluxes and their dependences on rotational velocities. We
find that the transition layer emission line fluxes and also those of
strong chromospheric lines decrease steeply between B-V=0.42 and 0.45,
i.e., at spectral type F5, for which the rotational velocities also
decrease steeply. The magnitude of the line-flux decrease increases
for lines of ions with increasing degree of ionization. This shows
that the line-flux decrease is not due to a change in the surface
filling factor but rather due to a change of the relative importance
of different heating mechanisms. For early F stars with B-V<0.42
we find for the transition layer emission lines increasing fluxes
for increasing vsini, indicating magnetohydrodynamic heating. The
vsini dependence is strongest for the high-ionization lines. On
the other hand, the low chromospheric lines show no dependence on
vsini, indicating acoustic shock heating for these layers. This also
contributes to the heating of the transition layers. The Mg II and Ca
II lines show decreasing fluxes for increasing vsini, as long as vsini
is less than ~40 km s-1. The coronal X-ray emission also
decreases for increasing vsini, except for vsini larger than ~100 km
s-1. We have at present no explanation for this behavior. For
late F stars the chromospheric lines show vsini dependences similar to
those observed for early F stars, again indicating acoustic heating
for these layers. We were unable to determine the vsini dependence
of the transition layer lines because of too few single star
targets. The decrease of emission line fluxes at the spectral type
F5, with steeply decreasing vsini, indicates, however, a decreasing
contribution of magnetohydrodynamic heating for the late F stars. The
X-ray emission for the late F stars increases for increasing vsini,
indicating magnetohydrodynamic heating for the coronae of the late F
stars, different from the early F stars. Based on observations with
the NASA/ESA Hubble Space Telescope obtained at the Space Telescope
Science Institute, which is operated by the Association of Universities
for Research in Astronomy, Incorporated, under NASA contract NAS5-26555.
Title: Mg II Emission Lines of Hyades F Stars
Authors: Böhm-Vitense, Erika; Mena-Werth, Jose; Carpenter, Kenneth
G.; Robinson, Richard D.
Bibcode: 2001ApJ...550..457B
Altcode:
With the Hubble Space Telescope (HST) we have observed Hyades
F stars, using the Space Telescope Imaging Spectrograph (STIS),
in order to get more information about the heating mechanism(s)
for the chromospheres and transition layers and their dependence
on rotation and age. In this paper we study the Mg II lines at 2800
Å. We include earlier observations with the International Ultraviolet
Explorer (IUE) satellite. The Mg II emission lines become observable
for B-V>0.3. The emission line fluxes increase steeply until
B-V~0.40. For single stars there is a steep decrease in flux between
B-V=0.41 and B-V=0.44, similar to the behavior of the Ca II emission
line cores. For larger B-V the Mg II emission line fluxes again
increase, but much more slowly than for the Ca II lines. Generally,
the low point of the emission is reached between B-V=0.43 and B-V=0.45,
i.e., similar to the Ca II emission cores. For the Hyades F stars there
appears to be a difference between the emissions for single stars and
those for binaries. We find that for Hyades stars with surface line
fluxes larger than 106 ergs cm-2 s-1
the emission line fluxes decrease with increasing vsini. For smaller
fluxes they may increase with increasing vsini. We have only three
stars that perhaps show this. We study the flux ratios of the Mg II
k and h lines at 2795.7 and 2802.5 Å in order to determine where
the lines fall on the curve of growth. For the earliest F stars
studied here the ratio is close to 2, as expected for optically thin
lines. Generally, it seems that the optical depths in the line centers
are less than 10. There remain problems in understanding the size of
the line widths. We discuss the interpretation of the Wilson-Bappu
effect. For the Hyades F stars there is a strong dependence of the
line width on the effective temperature. Based on observations with
the NASA/ESA Hubble Space Telescope obtained at the Space Telescope
Science Institute, which is operated by the Association of Universities
for Research in Astronomy. Incorporated, under NASA contract NAS5-26555.
Title: Ultraviolet Emission Lines in BA and Non-BA Giants
Authors: Böhm-Vitense, Erika; Carpenter, Kenneth G.; Robinson,
Richard D.
Bibcode: 2000ApJ...545..992B
Altcode:
With the Hubble Space Telescope (HST) and the Goddard High Resolution
Spectrograph we have observed four barium and three weak barium stars
in the ultraviolet spectral region, together with two nonpeculiar giant
standard stars. An additional suspected Ba star was observed with HST
and the Space Telescope Imaging Spectrograph. In the H-R diagram,
three of the observed Ba stars lie on the same evolutionary tracks
as the Hyades giants. Using International Ultraviolet Explorer (IUE)
spectra of previously studied giants together with our HST spectra,
we investigate whether the chromospheric and transition layer
emission-line spectra of the Ba stars are different from those of
nonpeculiar giants and from those of giants with peculiar carbon and/or
nitrogen abundances. Except for the Ba star HD 46407 and the suspected
Ba star HD 65699, the Ba star and mild Ba star emission-line fluxes are,
for a given effective temperature and for a given luminosity, lower
than those for the nonpeculiar giants observed with IUE. In comparison
with the HST-observed standard stars, the C IV λ1550-to-C II λ1335
line flux ratios are smaller, but not necessarily so in comparison
with all IUE-observed nonpeculiar giants. However, the C IV-to-C II
line flux ratios for the Ba stars decrease with increasing carbon
abundances. This shows that the energy balance in the lower transition
layer is influenced by the carbon abundance. The temperature gradient
appears to be smaller in the C II line-emitting region. There does not
seem to be a difference in chromospheric electron densities for the
Ba and non-Ba stars, though this result is rather uncertain. Based on
observations with the NASA/ESA Hubble Space Telescope obtained at the
Space Telescope Science Institute, which is operated by the Association
of Universities for Research in Astronomy, Inc., under NASA contract
NAS 5-26555.
Title: Masses for Galactic Beat Cepheids
Authors: D'Cruz, Noella L.; Morgan, Siobahn M.; Böhm-Vitense, Erika
Bibcode: 2000AJ....120..990D
Altcode:
Accurate mass determinations for Cepheids may be used to determine
the degree of excess mixing in the interiors of their main-sequence
progenitors: the larger the excess mixing, the larger the luminosity
of the Cepheid of a given mass, or the smaller the mass of a Cepheid
with given luminosity. Dynamical masses determined recently for a
few Cepheid binaries indicate excess mixing somewhat stronger than
that corresponding to the convective overshoot models by Schaller et
al. Beat Cepheids can be used similarly to test main-sequence mixing
in stellar interiors. The period ratios for beat Cepheids depend on
luminosity, Teff, heavy element abundance, and mass. By
comparing pulsational models and the observationally derived luminosity,
Teff, metallicities, and period ratios it is possible
to obtain masses for these stars, the so-called beat masses. With
the old opacities masses much smaller than the evolutionary masses
were obtained. With the new OPAL opacities a beat mass close to the
dynamical mass was obtained for the binary beat Cepheid Y Carinae,
showing that it is now possible to obtain reliable beat masses. In
this paper, we determine beat masses for seven Galactic beat Cepheids
for which photometric and spectroscopic data are available. We find
an average mass around 4.2+/-0.3 Msolar for these stars,
though the actual error limits for each star may be larger mainly
because of uncertainties in E(B-V) and the heavy element abundances. (As
derived spectroscopically, beat Cepheids are in general metal-poor,
with -0.4<~[Fe/H]<~0.0). The relation between the derived beat
masses and the luminosities again indicates excess mixing that is
somewhat larger than that corresponding to the models by Schaller et al.
Title: Do All BA II Stars Have White Dwarf Companions?
Authors: Böhm-Vitense, Erika; Carpenter, Kenneth; Robinson, Richard;
Ake, Tom; Brown, Jeffery
Bibcode: 2000ApJ...533..969B
Altcode:
With the Hubble Space Telescope (HST) and the Goddard High Resolution
Spectrograph (GHRS) we have observed four barium stars, three mild
barium stars, and one weak G-band star in the ultraviolet spectral
region. One barium star was observed with HST and the Space Telescope
Imaging Spectrograph (STIS). The aim was to check the hypothesis
that all these peculiar stars have white dwarf (WD) companions,
which at their asymptotic giant branch phase transferred mass with
peculiar element abundances to the present barium and CH peculiar
stars. Assuming that the ultraviolet continua of the cool giants,
including the barium stars, are generated in their chromospheres and
that the relations between the continua and the emission lines created
in the chromospheres and transition layers are similar in field giants
and barium stars, we found that, indeed, most of our target barium and
weak barium stars appear to have excess flux in the UV when compared
to standard giant stars. For most of the stars the excess flux can
be attributed to WD companions with temperatures between 10,000 and
12,000 K, if the WD mass is about 0.6 Msolar. Cooling times
for the WDs were derived from their effective temperatures and model
calculations by M. Wood. The calculated cooling times are longer than
the lifetimes of the barium stars on the giant branch. For our target
stars the mass transfer therefore happened while they were still on
the main sequence. For two of the mild barium stars and one or perhaps
two barium stars the derived cooling times for the WD companions come
out to be longer than the total evolutionary times of the barium stars
as calculated by Schaller et al. If our derivations are correct (the
error bars are rather large) then either evolutionary models with larger
convective overshoot have to be used for the barium stars or the cooling
times of the white dwarfs have to be revised downward. Possibly an
additional (as yet unknown) cooling mechanism has to be considered? The
weak G-band star HD 165634, which has a carbon underabundance of about
a factor of 10, also appears to have a WD companion. We discuss the
implications of this very low carbon abundance. Based on observations
with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope
Science Institute, which is operated by the Association of Universities
for Research in Astronomy. Incorporated, under NASA contract NAS5-26555.
Title: On VI emission lines in Hyades F stars
Authors: Bohm-Vitense, Erika
Bibcode: 2000fuse.prop.A106B
Altcode:
We propose to obtain additional empirical constraints on the atmospheric
heating mechanisms operating in stellar transition regions (TR)
and coronae. In particular, we will address the question of whether
different heating mechanisms may be working in different layers of a
given star and whether the relative importance of the mechanisms changes
from star to star. The heating mechanisms may depend on Teff,
gravity, rotation, magnetic fields, age, chemical composition, and,
possibly, binarity. In order to identify the heating mechanisms, we need
to disentangle the dependences on all these parameters. We propose to
do this by observing the 1032 & 1038 Å O VI emission lines in a
large sample of main sequence Hyades F stars. These stars are chosen
since they are the brightest cluster stars with strong TR-emission
and because marked changes in the heating mechanisms apparently occur
around spectral type F5, leading to easily-observable effects on our
spectral diagnostics. In addition, the age, chemical composition,
and surface gravity of these stars are essentially the same, removing
dependences on 3 of the stellar parameters. Relations between the
lower-TR and upper-TR emissions, together with their relationships
to the coronal X-ray emission will tell us the relationship between
heating mechanisms in the different layers. We already have reliable
lower-TR emission line (e.g. C IV) measurements for 15 Hyades F stars
observed by IUE and will be obtaining such for 17 additional Hyades
F stars in an approved Cycle 7 HST program. FUSE will provide the
critically needed flux measurements of upper-TR lines (O VI), as well
as of an additional lower-TR line, C III.
Title: The Mass of the Cepheid Binary V636 Scorpii
Authors: Böhm-Vitense, E.; Evans, N. R.; Carpenter, K.; Albrow,
Michael D.; Cottrell, P. L.; Robinson, R.; Beck-Winchatz, B.
Bibcode: 1998ApJ...505..903B
Altcode:
The mass-luminosity relation for Cepheids depends on the degree of
mixing in their main-sequence progenitors. Masses of Cepheids can be
inferred by using different aspects of pulsation theory. These methods
have in the past led to diverging results, showing that something was
wrong in either evolution theory or pulsation theory or both. For some
binary Cepheids dynamical masses, which are independent of pulsation and
evolution theories, can be determined. V636 Sco is one of a handful of
Cepheid binaries whose companions are bright enough in the ultraviolet
that orbital radial velocities can be measured. We have here attempted
to determine the radial velocity of the companion V636 Sco B to the
Cepheid V636 Sco A by means of two Hubble Space Telescope (HST) spectra
taken at phases of minimum and maximum Cepheid orbital velocities. The
ratio of the orbital velocity of the companion and the Cepheid (which
is the inverse of their mass ratio) came out to be 1.25 +/- 0.17. V636
Sco B has a spectral type B9.5 V, for which we expect a stellar mass
of 2.5 M⊙. For the Cepheid we thus determine a mass
of 3.1 +/- 0.4 M⊙, which is surprisingly low. If true,
such a low mass would indicate an extremely large amount of excess
mixing, which is very unlikely. We discuss the likely possibility
that V636 Sco B may itself be a binary with an unseen secondary,
in which case the mass derived from the two measurements discussed
above is not correct. Although the error limits for the mass of each
Cepheid are rather large, the combined mass-luminosity relation for
all Cepheids studied by us so far by means of HST spectra indicates
excess mixing corresponding to core convective overshoot by 0.25 to
about 0.5 pressure scale height in the main-sequence progenitors of
the Cepheids. Based on observations made with the NASA/ESA Hubble
Space Telescope, obtained at the Space Telescope Science Institute,
which is operated by the Association of Universities for Research in
Astronomy, Inc., under NASA contract NASS-26555.
Title: Classical Cepheid Masses: U Aquilae
Authors: Evans, Nancy Remage; Böhm-Vitense, Erika; Carpenter, Kenneth;
Beck-Winchatz, Bernhard; Robinson, Richard
Bibcode: 1998ApJ...494..768E
Altcode:
We have obtained medium-resolution spectra (λ/Δλ ~ 20,000) of the hot
binary companion to the classical Cepheid U Aql with the Goddard High
Resolution Spectrograph on the Hubble Space Telescope (HST). These have
been used to determine the orbital velocity amplitude. Combining this
with the orbital velocity amplitude of the Cepheid from the ground-based
orbit and the mass of the companion inferred from its spectral type, we
measure a mass of the Cepheid of 5.1 +/- 0.7 M⊙. We discuss
the full sample of Cepheids for which we have determined masses with HST
(S Mus, V350 Sgr, Y Car, and U Aql) and also SU Cyg (mass from IUE). The
HST masses are in agreement with the luminosities predicted by recent
evolutionary tracks with moderate overshoot. This comparison, however,
may be altered by reassessment of Cepheid distances based on Hipparcos
parallaxes. Based on observations made with the NASA/ESA Hubble
Space Telescope, obtained at the Space Telescope Science Institute,
which is operated by the Association of Universities for Research in
Astronomy, Inc., under NASA contract NASA-26555.
Title: The Mass of the Classical Cepheid V350 SGR
Authors: Evans, N. R.; Bohm-Vitense, E.; Beck-Winchatz, B.; Carpenter,
K.; Robinson, R.
Bibcode: 1998ASPC..143..313E
Altcode: 1998sigh.conf..313E
No abstract at ADS
Title: The Mass of the Beat Cepheid Y Carinae
Authors: Bohm-Vitense, E.; Beck-Winchatz, B.; Evans, N. R.; Carpenter,
K. G.; Robinson, R.
Bibcode: 1998ASPC..143..317B
Altcode: 1998sigh.conf..317B
No abstract at ADS
Title: The Mass of the Classical Cepheid S MUSCAE
Authors: Evans, N. R.; Bohm-Vitense, E.; Beck-Winchatz, B.; Carpenter,
K.; Robinson, R.
Bibcode: 1998ASPC..143..309E
Altcode: 1998sigh.conf..309E
No abstract at ADS
Title: The Dynamical and Beat Masses of the Beat Cepheid Y Carinae
Authors: Bohm-Vitense, E.; Evans, N. R.; Carpenter, K.; Winchatz,
B. Beck-; Morgan, S.; Robinson, R.
Bibcode: 1998ASPC..135..280B
Altcode: 1998hcsp.conf..280B
The mass-luminosity relation for Cepheids depends on the degree
of excess mixing in their main sequence progenitors. The mass
determination for Cepheids with their known luminosities therefore
determines the degree of excess mixing in massive main sequence
stars. We have determined the dynamical mass of several Cepheids with
blue companions. Here we discuss the beat Cepheid Y Carinae. By means
of HST, GHRS spectra we measured the radial velocity of the B9.5 V
companion Y Car B at phases near minimum and maximum orbital radial
velocities. The orbital velocity amplitude ratio between the Cepheid
and the hot companion and thereby the mass ratio comes out to be 1.51
0.5, leading to a mass of 3.8 1 1.2 solar masses. Taking the 3.8 solar
masses at face value this indicates excess mixing corresponding to
convective overshoot by about 1 pressure scale height, (see Bertelli et
al.1986), but the error limits are too large for a firm conclusion. The
beat masses, determined from the period ratios for the beat Cepheids,
have puzzled astronomers for a long time (see Cox 1980) because they
came out around 1 to 2 solar masses, when the Cox-Tabor opacities
were used for the model calculations. Moskalik et al.(1992) showed
that beat masses around 4 to 5 solar masses can now be derived, if
the new OPAL opacities are used instead. With the determination of
the dynamical mass for Y Car A we can now check the validity of the
OPAL opacities more quantitatively than was possible before. Using the
model calculations incorporating the OPAL opacities, and the observed
effective temperature of Y Carinae we determine for its beat mass a
possible range of 3.75 < M/Ms < 4.0, in very good agreement with
the dynamical mass. This supports the validity of the OPAL opacities,
and also supports the conclusion about the high degree of excess mixing
in the main sequence progenitor of Y Carinae A.
Title: The dynamical mass of the beat cepheid Y carinae and stellar
opacities.
Authors: Bohm-Vitense, E.; Evans, N. R.; Carpenter, K.; Morgan, S.;
Beck-Winchatz, B.; Robinson, R.
Bibcode: 1997AJ....114.1176B
Altcode:
The beat Cepheid, Y Carinae A, has a B9 V companion, Y Car
B. The primary period P0 of the Cepheid is 3.64 days
and the secondary, P1, is 2.56 days. Its period ratio
P1/P0 is thus 0.703. Y Car is the only beat
Cepheid known to be a binary and thus offers us the unique opportunity
to determine the dynamical mass for a beat Cepheid. We have determined
its mass by measuring the orbital velocity amplitude of the hot
companion Y Car B using the Goddard High-Resolution Spectrograph (GHRS)
with the G200M grating on the Hubble Space Telescope. When combined
with the ground-based orbital velocity amplitude of the Cepheid
and the mass of the companion, the implied mass of the Cephieid
is M=3.8±1.2Msun. With the Cepheid luminosity given
by the period-luminosity relation, this mass, taken at face value,
indicates excess mixing in the main sequence progenitor corresponding
to convective overshoot by about 0.9 pressure scale height, however,
the large error bars prevent a firm conclusion. As shown by Simon the
period ratio for beat Cepheids depends sensitively on the opacities. For
models calculated with Cox-Tabor opacities the period ratios for beat
Cepheids indicate masses between one and two Msun. Models
calculated with the new Livermore OPAL opacities on the other hand
indicate masses around 4 solar masses. The good agreement of the beat
mass with the dynamical mass, determined here for Y Car, provides a
confirmation that the OPAL opacities are a significant improvement
over the Cox-Tabor (1976) opacities.
Title: Chromospheres and transition layers in Hyades and Pleiades
F stars
Authors: Boehm-Vitense, Erika
Bibcode: 1997hst..prop.7389B
Altcode:
This proposal seeks to obtain additional empirical constraints on the
heating mechanisms for stellar chromospheres, transition layers and
coronae. These mechanisms may depend on effective temperature, gravity,
rotation, magnetic fields, age, chemical abundances, and possibly
binarity. Thus, we need to disentangle the dependences on all these
parameters if we want to identify the heating mechanisms. We propose to
study main sequence F stars in the Hyades and Pleiades. For each cluster
two parameters are constant: age and chemical abundances. Gravity
is also essentially constant. We still have to deal with at least 3
remaining parameters. This means we need to observe as many stars as
feasible. The comparison of the Hyades and Pleiades stars will tell
us the age dependence for the young stars. The verification of any age
dependence, separately from the one on rotation, will show the influence
of parameters other than rotation and those related to convection,
which is not expected to depend on age. Main sequence F stars are the
brightest cluster stars with strong chromospheric and transition layer
emission. Changes in the heating mechanism occur around spectral type
F5. The related changes in chromospheric and transition layer emission,
and the steeply decreasing rotational velocities may hold the clue to
the identification of the heating mechanisms.
Title: The Mass of the Classical Cepheid V350 SGR
Authors: Evans, N. R.; Bohm-Vitense, E.; Carpenter, K.; Beck-Winchatz,
B.; Robinson, R.
Bibcode: 1997PASP..109..789E
Altcode:
Two medium resolution spectra of the hot companion of the Cepheid V350
Sgr have been obtained with the Goddard High Resolution Spectrograph
aboard the Hubble Space Telescope. Comparison of these spectra (in the
wavelength range 1840 to 1880 \AA) with the spectrum of $\alpha$ Lyr
shows that V350 Sgr B has a projected rotation velocity which may be as
high as 150 km sec$^{-1}$. The velocity difference between the spectra
of V350 Sgr B at two orbital phases is measured to be -23.1 $\pm$ 3.8
km sec$^{-1}$. The error is dominated by the aperture centering. When
combined with the orbital velocity variation of the Cepheid derived
from the ground-based orbit and the mass of the companion deduced
from IUE spectra, the mass of the Cepheid is found to be 5.2 $\pm$
0.9 M$\sun$. The observed mass--luminosity combination of V350 Sgr A
is a good match to recent evolutionary calculations which use moderate
convective overshoot near the main sequence, however, the blue loops
do not extend to temperatures as hot as the Cepheid. (SECTION: Stars)
Title: White dwarf companions of Barium and CH peculiar Stars
Authors: Boehm-Vitense, Erika
Bibcode: 1997hst..prop.7753B
Altcode: 1997hst..prop.4085B
In 1980 McClure et al.{ApJ 238, L35} discovered that all Ba stars show
radial velocity variations indicative of their binary nature. These
authors already suspected that the companions are white dwarfs. White
dwarf companions were indeed detected for the Ba star zeta Cap and the
mild Ba star zeta Cyg {Boehm-Vitense 1980, ApJ 239, L79} by means of
IUE observations. Evidence for a high temperature radiation field was
seen in the spectra of the mild Ba stars HR1016, 16Ser and 56UMa. If
present, the white dwarf companions are too faint to be seen with
IUE. We would like to look for them now with HST. It seems to us to be
very important to check whether indeed all Ba stars have white dwarf
companions. If so this makes it highly probable that the peculiar
abundances are due to mass transfer from a very luminous companion to
the present Ba star. After losing its envelope the luminous companion
was left as the present white dwarf. This opens up the possibility
that other peculiar abundances seen in other peculiar stars like the
CH stars or perhaps also the weak G band stars might also be due to
mass transfer. On the other hand if the white dwarfs are not found for
all Ba stars we may have to look for other processes, that could bring
to the surface of these stars material which has experienced the slow
neutron capture process.
Title: The Mass of the Classical Cepheid S Muscae
Authors: Böhm-Vitense, Erika; Remage Evans, Nancy; Carpenter, Kenneth;
Beck-Winchatz, Bernhard; Robinson, Richard
Bibcode: 1997ApJ...477..916B
Altcode:
A good determination of the mass-luminosity relation for evolved stars
on blue loops can determine the degree of excess mixing in the interiors
of their main-sequence companions. In this study we determine the
dynamical mass of the Cepheid binary S Muscae. This can be combined
with its known luminosity and be fitted on evolutionary tracks to
determine the amount of mixing in the main-sequence progenitor. Using the Goddard High Resolution Spectrograph on the Hubble Space
Telescope we have measured the orbital radial velocity changes for the
companion of the Cepheid S Mus. Spectra taken at minimum and maximum
orbital velocities were cross-correlated. The velocity difference was
measured to be 30.6 +/- 0.4 km s-1. The difference for the
orbital velocities of the Cepheid for the same phases was determined
to be 26.9 km s-1 +/- 0.4 km s-1. This gives
a velocity ratio of 1.14 +/- 0.02, which gives us the mass ratio for
the companions. Adding possible centering errors of the target in the
entrance apertures the error limit would be increased to +/-0.06. The derived spectral types of the companion S Mus B range from B3
V to B5 V depending on the criterion used to determine it, with an
average spectral type B3.8 V. Using the average spectral type and the
main-sequence mass-spectral type relation from Andersen & Harmanec
we find for S Mus B a mass of 5.2 +/- 0.2 M⊙. With the
newly determined mass ratio the mass for the Cepheid S Mus A comes out
to be 5.9+0.7-0.6 M⊙. Taking
the mass of 5.9 M⊙ at face value and adopting the absolute
visual magnitude of Mv = -4.29 (log L/L⊙ =
3.62) for the Cepheid S Mus this indicates mixing in its main-sequence
progenitor slightly in excess of the one assumed for the Maeder and
Meynet evolutionary tracks. The present uncertainties in mass and
luminosity prevent, however, a firm conclusion about the exact degree
of mixing. Based on observations made with the NASA/ESA Hubble
Space Telescope, obtained at the Space Telescope Science Institute,
which is operated by the Association of Universities for Research in
Astronomy, Inc., under NASA Contract No. NASS-26555.
Title: The First Steps of the Extragalactic Distance Ladder
Authors: Bohm-Vitense, Erika
Bibcode: 1997AJ....113...13B
Altcode:
Recent distance determinations by means of the Cepheid period-magnitude
relations are based on a true distance modulus of m-M=18.5 to the
Large Magellanic Cloud. It has further been assumed that for the LMC
Cepheids the average E(B-V)=0.10 and that the ratio of interstellar
extinction Av in the visual to E(B-V), i.e., Rv,
is 3.1. In order to determine the distance to the LMC with its lower
than solar metallicity, it is also assumed that the period-magnitude
relation is independent of metallicity. Here we reexamine the different
assumptions, i.e., the reddening and distance to the LMC and to the
Galactic Cepheids. We also study the dependence of the period-magnitude
relations for different heavy element abundances. In that process we
rediscuss the distances to the LMC, to M31 and the Small Magellanic
Cloud. Adopting the theoretical result, that the period-luminosity
relation is independent of metallicity, we derive for the V and B bands
the dependence of the period-magnitude relations on Z. The use of these
relations changes the determination of the interstellar extinction
for the three M3 1 fields, studied by Freedman and Madore, and yields
very good agreement for the distance moduli of the three fields, namely
m-M=24.59±0.02, or (m-M)M31-(m-M)LMC=6.09± 0.02,
where the error limits refer only to the internal agreement for the
three fields. We adopt a foreground reddening of E(B-V)=0.08 for M31 and
estimate an average E(B-V)=0.18 for the LMC Cepheids. With a distance
modulus to the Pleiades, m-M=5.57, we derive a distance modulus to the
LMC of m-M=18. 11 andm-M=18.84±0.1 for the SMC. These error limits are
only estimates. We also point out a way to get a rough estimate about
large metallicity differences of Cepheids observed in external galaxies.
Title: New Results on Cepheid Masses
Authors: Evans, N. R.; Bohm-Vitense, E.; Carpenter, K.; Robinson,
R.; Beck-Winchatz, B.
Bibcode: 1996AAS...189.9608E
Altcode: 1996BAAS...28.1399E
Masses for Cepheid variable stars can be measured by combining the
orbital velocity amplitude for the Cepheid (from a ground-based orbit)
with the orbital velocity amplitude of a hot main sequence companion
(observed in the ultraviolet from satellites such as IUE and HST) and
the mass of the companion (inferred from from the ultraviolet energy
distribution). Observations of 5 binary systems are now completed
or in progress with the Goddard High Resolution Spectrograph on the
Hubble Space Telescope. Recently completed observations of U Aql lead
to a mass of 5.1 +/- 1.1 Msun . We will discuss the results for S Mus,
V350 Sgr, U Aql, and Y Car, and the constraints they place on stellar
evolution calculations. As would be expected, some of the B companions
have high rotational velocities, decreasing the accuracy with which
their orbital velocities can be measured. The preliminary conclusion
from the 4 HST targets and SU Cyg (mass from IUE observations) is that
a weighted mean indicates no convective overshoot but the mode (which
reflects the HST results better) agrees with the modest overshoot
used in the Geneva evolutionary calculations. Financial Support was
provided by a NASA grant GO-4541-01 to EB--V and GO-4541.02 to KGC,
a grant from the Natural Sciences and Engineering Council, Canada to
NRE, from the AXAF Science Center NASA Contract NAS8-39073.
Title: Cepheid Masses
Authors: Boehm-Vitense, Erika
Bibcode: 1996hst..prop.6504B
Altcode: 1996hst..prop.2915B
This is a proposal for the fourth year of an originally threeyear
proposal which was accepted for cycles 3, 4, 5 and 6.The aim is to
determine Cepheid masses for binaries withCepheids. For 2 decades
the Cepheid ``mass problem" haspersisted: Mass determinations from
standard evolutionarytracks and those from pulsation theory gave
conflictingresults. The luminosity of a Cepheid with a given mass
dependssensitively on the amount of excess mixing above the core ofthe
main sequence progenitor. Hence a good mass determinationfor a Cepheid
with known luminosity measures the amount ofconvective overshoot. This
knowledge is important for theinterpretation of HR diagrams of populous
clusters in the LMC,especially for age determinations, and also for
theunderstanding of mixing processes in stars. IUE observationshave
revealed a number of Cepheids with blue companions,whose orbits have
now been determined by groundbasedobservations. We propose to measure
the orbital radialvelocities of altogether 5 companions for wavelengths
shorterthan 2000A. The ratio of the orbital velocity amplitudes forthe
binary provides the mass ratio for the stars. Theeffective temperature
of the main sequence companion andthereby its mass is determined from
its ultraviolet spectrumand energy distribution. With good GHRS spectra
the velocityratio and thereby the mass ratio can be determined with
anaccuracy of +/-10%.
Title: White Dwarf Companions of Barium and CH Peculiar Stars
Authors: Boehm-Vitense, Erika
Bibcode: 1996hst..prop.6594B
Altcode: 1996hst..prop.3005B
In 1980 McClure et al.(ApJ 238, L35) discovered that all Bastars show
radial velocity variations indicative of theirbinary nature. These
authors already suspected that thecompanions are white dwarfs. White
dwarf companions wereindeed detected for the Ba star zeta Cap and the
mild Ba starzeta Cyg (Boehm-Vitense 1980, ApJ 239, L79) by means of
IUEobservations. Evidence for a high temperature radiation fieldwas seen
in the spectra of the mild Ba stars HR1016, 16Ser and56UMa. If present,
the white dwarf companions are too faint tobe seen with IUE. We would
like to look for them now with HST.It seems to us to be very important
to check whether indeedall Ba stars have white dwarf companions. If so
this makes ithighly probable that the peculiar abundances are due to
masstransfer from a very luminous companion to the present Bastar. After
losing its envelope the luminous companion wasleft as the present white
dwarf. This opens up the possibilitythat other peculiar abundances
seen in other peculiar starslike the CH stars or perhaps also the weak
G band stars mightalso be due to mass transfer. On the other hand if
the whitedwarfs are not found for all Ba stars we may have to look
forother processes, that could bring to the surface of thesestars
material which has experienced the slow neutron captureprocess.
Title: The Mass of the Cepheid S Muscae.
Authors: Bohm-Vitense, E.; Evans, N. R.; Beck-Winchatz, B.; Carpenter,
K.; Robinson, R.
Bibcode: 1995AAS...18710205B
Altcode: 1995BAAS...27.1428B
Using the Goddard High Resolution Spectrograph on the Hubble Space
Telescope we have measured the orbital radial velocity changes for
the companion of the Cepheid S Muscae. The observations were done
at minimum and maximum orbital velocities. The two spectra were
cross-correlated. The velocity difference was measured to be 30.6 +/-
0.4 km/s. The difference for the orbital velocities of the Cepheid
for the same phases was determined to be 26.9 +/-0.4 km/s. This gives
a mass ratio of 1.14 +/-0.06. For a spectral type between B3.5 V and
B5 V for the companion S Mus B its mass is 5.0 +/- 0.3 M(sun). With
the mass ratio of 1.14 the mass of the Cepheid, with a period of 9.6
days, comes out to be 5.7 +/- 0.4 solar masses. This compares with a
pulsational mass of 5.5 solar masses. Taking the 5.7 solar masses for
the Cepheid at face value its absolute visual magnitude of M(V)=-4.29
indicates a high degree of excess interior mixing in its main sequence
progenitor corresponding to convective overshoot by one pressure
scale height. Even for the upper mass limit the excess mixing has to
correspond to convective overshoot by 1/2 pressure scale height.
Title: White Dwarf Companions to Hyades F Stars
Authors: Bohm-Vitense, Erika
Bibcode: 1995AJ....110..228B
Altcode:
We have studied archival, short-wavelength, low resolution IUE spectra
of Hyades F stars in an effort to detect white dwarf companions, which
can give us information about the IMF for the Hyades duster Weidemann
et al. pointed out that we have not detected the expected number of
white dwarfs in the Hyades. They suspected that the missing white dwarfs
have evaporated from the cluster This cannot have happened with white
dwarf companions to A and F stars. We estimate that for the 27 observed
Hyades F stars we should have seen 3 white dwarfs while we found none.
Title: Cepheid Masses
Authors: Boehm-Vitense, Erika
Bibcode: 1995hst..prop.5854B
Altcode: 1995hst..prop.2327B
This is a proposal for the third {and fourth} year of a three year
proposal which was accepted for the last two years. The aim is to
determine Cepheid masses for binaries with Cepheids. For 2 decades
the Cepheid ``mass problem" has persisted: Mass determinations from
standard evolutionary tracks and those from pulsation theory gave
conflicting results. The luminosity of a Cepheid with given mass
depends sensitively on the amount of convective overshoot above the
core of the main sequence progenitor. Hence a good mass determination
for a Cepheid with known luminosity measures the amount of convective
overshoot. This knowledge is important for the interpretation of
HR diagrams of populous clusters in the LMC and especially for age
determinations. It is also necessary for the understanding of mixing
processes in stars. IUE observations have revealed a number of Cepheid
binaries with blue companions whose orbits have now been determined
by groundbased observations. We propose to measure the orbital radial
velocities of altogether 5 blue Cepheid companions for wavelengths
shorter than 2000 A. The ratio of the orbital velocity amplitudes
for the binary provides the mass ratio for the stars. The effective
temperature for the companion is determined from its ultraviolet
energy distribution. For main sequence stars this also determines its
mass. With good GHRS spectra the velocity ratio and thereby the mass
ratio can be determined with an accuracy of +/- 10%
Title: A dip in the CaII H and K emission line fluxes for Hyades
F stars.
Authors: Bohm-Vitense, E.
Bibcode: 1995A&A...297L..25B
Altcode:
The Hyades chromospheric emission line fluxes for the Ca H and K
lines show a minimum at B-V=0.45, very close to the minimum for the
Li surface abundances. No anomalies have been reported for Ca surface
abundances in Hyades F stars, though published abundance analyses may
have missed stars at this B-V. We do, therefore not think that the dip
in the CaII emission is due to diffusion (as believed to be the case
for Li) but rather suspect that it is a chromospheric effect. Only on
the branch of the dip with declining fluxes for increasing B-V (which
means for B-V=0.43 and 0.44) a correlation of the CaII emission with
v sini is seen.
Title: Studies with IUE spectra: Stellar chromospheres and transition
layers, and Cepheid Binaries
Authors: Bohm-Vitense, Erika
Bibcode: 1995wub..rept.....B
Altcode:
During the last years the work under this grant (NSG 5398) on IUE
spectra was mainly concerned with two topics: the study of stellar
chromospheres and transition layers; and the study of Cepheid companions
in order to derive the masses of the Cepheids. Both are reported here.
Title: The Lithium Content and Other Properties of F2-G5 Giants in
the Hertzsprung Gap
Authors: Wallerstein, George; Bohm-Vitense, Erika; Vanture, Andrew D.;
Gonzalez, Guillermo
Bibcode: 1994AJ....107.2211W
Altcode:
As stars of 2-5 solar mass evolve across the Hertzsprung Gap they
should first deplete their surface lithium by convective dilution
and then, when convection penetrates deeper, begin to bring CN
processed material to their surfaces. To investigate this process
we have observed 52 giants, 25 of which have known C/N ratios, for
their Li abundances. After eliminating four stars that may actually
be dwarfs and including the two components of Capella analyzed by
Pilachowski and Sowell we have compared our (Li/Fe) ratios with
models of Swenson. For stars showing v sin i greater than 50 km/s
we find (Li/Fe) to be unaffected by mixing for B - V less than 0.7
as predicted. For stars cooler than B - V = 0.7 both v sin i and
(Li/Fe) drop to smaller values. For the sharp lined stars (v sin i
less than 50 km/s) we find a drop in Li between B - V = 0.45 and 0.60
which cannot be understood in terms of dilution by convection. Various
possible causes of such an early depletion or dilution of surface Li
are discussed including diffusion at the base of the convection zone,
mass loss possibly enhanced by pulsation, and magnetic activity as in
the magnetic A and B type stars. The models of Richer & Michaud
(1993) with diffusion point toward a satisfactory solution. A few
giants with low v sin i values stand out with much higher than expected
(Li/Fe) values despite their cool effective temperatures. We do not
understand why those stars have not depleted their lithium as have
most giants of similar color. The correlation of (N/C) with (Li/Fe)
follows expectations in so far as almost all stars with enhanced (N/C)
have depleted their Li as well.
Title: The Two period-Luminosity Relations for Population I Cepheids
Authors: Bohm-Vitense, E.
Bibcode: 1994AJ....107..673B
Altcode:
We summarize the evidence that most Population I Cepheids with periods
less than ~8 days pulsate in the first overtone mode. Fundamental mode
and first overtone pulsators must follow different period-luminosity (P-
L) relations. We demonstrate these different relations for different
stellar systems, especially for the calibrating Cepheids in clusters
and for Cepheids in the LMC, the SMC, M31, and IC 4182.
Title: Cepheid Masses - Cyc 4 High - Part 2
Authors: Boehm-Vitense, Erika
Bibcode: 1994hst..prop.5673B
Altcode: 1994hst..prop.2114B
For 2 decades the "Cepheid mass problem" has persisted: Mass
determinations from standard evolutionary tracks and those from
pulsation theory gave conflicting values. The luminosity of a Cepheid of
given mass depends sensitively on the amount of convective overshoot
above the core of the main sequence progenitor. Hence a good mass
determination for the Cepheid with known luminosity will measure the
amount of convective core overshoot. This knowledge is important
for interpretation of HR diagrams of populous clusters in the LMC
and especially for age determinations. It is also necessary for the
understanding of the mixing processes in stars. Program 5357 obtained
the GHRS spectra for this program. This is a follow-on part 2 to obtain
the calibration spectra to measure the radial velocity of the 2 primary
targets observed in 5357.
Title: Emission Lines in the Long-Period Cepheid L Carinae
Authors: Bohm-Vitense, Erika; Love, Stanley G.
Bibcode: 1994ApJ...420..401B
Altcode:
For the Cepheid l Carinae, with a pulsation period of 35.5 days, we
have studied the emission-line fluxes as a function of pulsational phase
in order to find out whether we see chromospheric and transition-layer
emission due to an outward-moving shock. All emission lines show a steep
increase in flux shortly before maximum light, suggestive of a shock
moving through the surface layers. The large ratio of C IV to C II line
fluxes shows that these are not transition-layer lines. During maximum
light the large ratio of the large ratio of the C IV to C II line
fluxes also suggests that we see emission from a shock with velocities
greater than 100 km/s such that C IV emission can be excited. With such
velocities mass outflow appears possible. The variations seen in the
MG II line profiles show that there is an external absorption over a
broad velocity band independent of the pulsation phase. We attribute
this absorption to a circumstellar 'shell.' This 'shell' appears to
be seen also as spatially extended emission in the O I line at 1300 A,
which is probably excited by resonance with Ly beta.
Title: Cepheid Masses -CYC4-HIGH
Authors: Boehm-Vitense, Erika
Bibcode: 1994hst..prop.5357B
Altcode: 1994hst..prop.1803B
For 2 decades the "Cepheid mass problem" has persisted: Mass
determinations from standard evolutionary tracks and those from
pulsation theory gave conflicting values. The luminosity of a Cepheid of
given mass depends sensitively on the amount of convective overshoot
above the core of the main sequence progenitor. Hence a good mass
determination for the Cepheid with known luminosity will measure the
amount of convective core overshoot. This knowledge is important
for interpretation of HR diagrams of populous clusters in the LMC
and especially for age determinations. It is also necessary for the
understanding of the mixing processes in stars. IUE observations have
revealed a number of Cepheid binaries with blue companions, whose orbits
have now been determined by groundbased observations. We propose to
measure the orbital radial velocities of 5 blue Cepheid companions on
GHRS spectra for wavelengths shorter than 2000 A. The ratios of the
orbital velocities for the binaries provide the mass ratios for the
stars. The effective temperature of the companion can be determined from
its energy distribution. For main sequence stars this also determines
its mass. With GHRS spectra the orbital velocity ratio and thereby
the mass ratio can be determined with an accuracLy of +/- 10 %.
Title: Silicon Abundances in Population I Giants
Authors: Bohm-Vitense, Erika
Bibcode: 1993AJ....106.2510B
Altcode:
Silion to carbon abundance ratios for Population I giants were
determined from emission lines originating in the transition layers
between stellar chromospheres and coronae. For effective temperatures
larger than 6200 K we find a group of stars for which the silicon
to carbon abundance ratio appears to be increased. These stars
are presumably descendents from AP stars on Am stars with increased
surface silicon to carbon abundance ratios. Around B-V approximately
equal to 0.45 this anomaly disappears as is to be expected due to the
increased depth of the convection zone and therefore deeper mixing
which dilutes the surface overabundances. Unexplained is the apparent
increased silicon to carbon abundance ratio observed for several stars
cooler than 5100 K. RS CVn and related stars do not show this increased
abundance ratio.
Title: Detection of a White Dwarf Compensation to the Hyades Stars
HD 27483
Authors: Boehm-Vitense, Erika
Bibcode: 1993AJ....106.1113B
Altcode:
We observed with IUE a white dwarf (WD) companion to the Hyades F6 V
binary stars HD 27483. This system is known to be a close binary of
two nearly equal stars with an orbital period of 3.05 days. Our IUE
observations revealed the presence of a third star, a white dwarf
with an effective temperature of 23,000 +/- 1000 K and a mass of
approximately 0.6 solar mass. Its presence in the Hyades cluster with
a known age permits me to derive the mass of its progenitor, which must
have been about 2.3 solar masses. The presence of the white dwarf in a
binary system opens the possibility that some of the envelope material,
which was expelled by the WD progenitor, may have been collected by
the F6 stars. We may thus be able to study abundance anomalies of the
WD progenitor with known mass on the surface of the F6 companions.
Title: Cepheids, Distance Indicators and Tests of Stellar Evolution
Theory
Authors: Bohm-Vitense, E.
Bibcode: 1993AAS...182.5801B
Altcode: 1993BAAS...25..896B
No abstract at ADS
Title: Book-Review - Introduction to Stellar Astrophysics
Authors: Bohm-Vitense, E.; De Greve, J. P.
Bibcode: 1993SSRv...64..172B
Altcode:
No abstract at ADS
Title: Ultraviolet Studies of Cepheids
Authors: Bohm-Vitense, Erika
Bibcode: 1993npsp.conf..387B
Altcode: 1993IAUCo.139..387B
No abstract at ADS
Title: Book-Review - Introduction to Stellar Astrophysics - V.2 -
Stellar Atmospheres
Authors: Bohm-Vitense, E.; Trimble, V.
Bibcode: 1993ComAp..16..278B
Altcode:
No abstract at ADS
Title: Transition Layers of Hyades F Stars
Authors: Bohm-Vitense, Erika H.
Bibcode: 1993iue..prop.4487B
Altcode:
We would like to observe most Hyades F main sequence stars in order to
determine the dependence of transition layer emission on temperature,
rotation, binarity and Li abundances. In order to study the dependence
on any one parameter one has to keep all other parameters constant. For
the Hyades cluster age and metal abundance are constant. The Hyades
cluster is the nearest one for which the F stars are bright enough to be
observed with IUE in the SWP wavelength region. At the same time there
are enough F stars to study the dependence on one parameter with all
other parameters being nearly the same. In the F star region rotation
decreases when going from early to late F stars. Simon and Drake
suggest that the heating mechanism changes for B-V around 0.45. The
chromospheric emission appears not to vary with time for stars earlier
than F7 while it is variable for stars later than F8. Michaud suggests
depletion of He for early F stars but not for late F stars. Boesgaard
and Tripicco find a minimum in the Li abundances for stars with Teff
around 6700 K. We want to study the changes (or not) in the transition
layer emission related to all these events. This is last year's proposal
which was accepted for two years, but the committee failed to assign
shifts for the second year.
Title: Absorption-Line Profiles in a Companion Spectrum of a
Mass-losing Cool Supergiant
Authors: Rodrigues, Liliya L.; Boehm-Vitense, Erika
Bibcode: 1992ApJ...401..695R
Altcode:
Cool star winds can best be observed in resonance absorption lines seen
in the spectrum of a hot companion, due to the wind passing in front
of the blue star. We calculated absorption line profiles that would
be seen in the ultraviolet part of the blue companion spectrum. Line
profiles are derived for different radial dependences of the cool
star wind and for different orbital phases of the binary. Bowen and
Wilson find theoretically that stellar pulsations drive mass loss. We
therefore apply our calculations to the Cepheid binary S Muscae which
has a B5V companion. We find an upper limit for the Cepheid mass loss
of M less than or equal to 7 x 10 -10 solar mass per year
provided that the stellar wind of the companion does not influence
the Cepheid wind at large distances.
Title: Detection of a White Dwarf in a Visual Binary System
Authors: Boehm-Vitense, Erika
Bibcode: 1992AJ....104.1539B
Altcode:
The F6 giant HD 160365 was detected to have a white dwarf companion
about 8 arcsec south of the star. The UV energy distribution observed
with IUE shows that the white dwarf has an effective temperature of
23,000 +/- 2000 K. If log g = 8 the Lya profile indicates an effective
temperature around 24,500 K. Using the theoretical models by Wesemael et
al. (1980) one finds a visual magnitude of m(V) about 16.5. For T(eff)
= 24,500 K one expects for a white dwarf a luminosity of log L/L(solar)
about 1.3 and M(V) about 10.67. This gives a distance modulus for the
system of m(V) - M(V) = 5.83 and an absolute magnitude M(V)= 0.3 for
the giant.
Title: Determination of nitrogen to carbon abundance ratios from
transition layer emission lines
Authors: Boehm-Vitense, Erika
Bibcode: 1992uwse.reptQ....B
Altcode:
We have finished studying the nitrogen to carbon abundance ratios
for stars with different effective temperatures Teff
and luminosities using transition layer emission lines and using
spectra available in the IUE archives. The N/C abundance ratio
determinations using transition layer emission lines are as accurate
as the photospheric abundance determinations as found by comparison of
results obtained by both methods for the same stars. Our measurements
confirm photospheric abundance determinations in regions of the HR
diagram where they can be obtained. Our studies have extended the
temperature range to higher temperatures. They have shown the exact
positions in the HR diagram where the mixing due to the outer convection
zones reaches deep enough to bring nuclear processed material to
the surface. This occurs at effective temperatures which are higher
by delta log Teff approximately 0.04 or roughly 400 K
than expected theoretically. Since the depth of the convection zone
increases rapidly with decreasing Teff this may indicate
considerable overshoot beyond the lower boundary of the convection
zone. Our N/C abundance ratio determinations from transition layer
emission lines have confirmed that the actual enrichment observed
for some cool giants is larger than expected theoretically, again
indicating a larger degree of mixing in several stars either from below
or from above. For the supergiants it probably indicates overshoot
above the convective core in the progenitor main sequence stars. For
the more massive giants this may also be the case, though we did not
find a correlation between delta log N/C and the absolute magnitudes,
but these are rather uncertain. As byproducts of these studies we also
found anomalies in Si/C and N/C abundance ratios for F giants which
can be understood as the relict of surface abundance changes for their
main sequence progenitors due to diffusion. This anomaly disappears for
G giants, for which the depths of the convection zones are apparently
deep enough to wipe out these element separations (Bohm-Vitense 1992).
Title: Cepheid Masses -CYC3-HIGH
Authors: Boehm-Vitense, Erika
Bibcode: 1992hst..prop.4541B
Altcode:
For 2 decades the "Cepheid mass problem" has persisted: Mass
determinations from standard evolutionary tracks and those from
pulsation theory gave conflicting values. The luminosity of a Cepheid of
given mass depends sensitively on the amount of convective overshoot
above the core of the main sequence progenitor. Hence a good mass
determination for the Cepheid with known luminosity will measure the
amount of convective core overshoot. This knowledge is important
for interpretation of HR diagrams of populous clusters in the LMC
and especially for age determinations. It is also necessary for the
understanding of the mixing processes in stars. IUE observations have
revealed a number of Cepheid binaries with blue companions, whose orbits
have now been determined by groundbased observations. We propose to
measure the orbital radial velocities of 5 blue Cepheid companions on
GHRS spectra for wavelengths shorter than 2000 A. The ratios of the
orbital velocities for the binaries provide the mass ratios for the
stars. The effective temperature of the companion can be determined from
its energy distribution. For main sequence stars this also determines
its mass. With GHRS spectra the orbital velocity ratio and thereby
the mass ratio can be determined with an accuracLy of +/- 10 %.
Title: Carbon and Nitrogen Abundances Determined from Transition
Layer Lines
Authors: Boehm-Vitense, Erika; Mena-Werth, Jose
Bibcode: 1992ApJ...390..253B
Altcode:
The possibility of determining relative carbon, nitrogen, and silicon
abundances from the emission-line fluxes in the lower transition layers
between stellar chromospheres and coronae is explored. Observations
for main-sequence and luminosity class IV stars with presumably
solar element abundances show that for the lower transition layers
Em = BT-gamma. For a given carbon abundance the constants
gamma and B in this relation can be determined from the C II and C IV
emission-line fluxes. From the N V and S IV lines, the abundances of
these elements relative to carbon can be determined from their surface
emission-line fluxes. Ratios of N/C abundances determined in this way
for some giants and supergiants agree within the limits of errors with
those determined from molecular bands. For giants, an increase in the
ratio of N/C at B-V of about 0.8 is found, as expected theoretically.
Title: Book-Review - Introduction to Stellar Astrophysics - V.3 -
Stellar Structure and Evolution
Authors: Bohm-Vitense, E.
Bibcode: 1992JBAA..102Q.109B
Altcode:
No abstract at ADS
Title: Rotation and Transition Layer Emission in Cool Giants
Authors: Boehm-Vitense, Erika
Bibcode: 1992AJ....103..608B
Altcode:
Gray (1981, 1982) found that field giants with T(eff) less than about
5500 K experience a steep decrease in rotational velocities coupled
with a decrease in transition layer emission. This decrease may be
attributable to fast magnetic braking or to redistribution of angular
momentum for rapidly increasing depths of the convection zones if these
rotate with depth independent specific angular momentum. Additional
arguments in favor of the latter interpretation are presented. The
increase of N/C abundances due to deep mixing occurs at the same point
as the decrease in v sin i. On the other hand, the ratios of the C
IV to C II emission line fluxes decrease at this point indicating
smaller contributions of MHD wave heating. The X-ray fluxes decrease
at nearly the same T(eff). Thus, no observations are found which would
indicate larger magnetic activity which could lead to fast magnetic
braking. Theory predicts a rapid increase in the convection zone
depth at the T(eff) where the decrease in v sin i is observed. This
can explain the observed phenomena.
Title: Introduction to Stellar Astrophysics. Vol.3: Stellar structure
and evolution
Authors: Boehm-Vitense, Erika
Bibcode: 1992itsa.book.....B
Altcode:
The present volume on current physical understanding of the
internal structure of stars gives attention to basic principles while
stressing comparisons with observations capable of elucidating stellar
evolution. Treatments are conducted of the hydrostatic and thermal
equilibriums, opacities, convective instability and energy transport,
the depths of the outer convection zones, stellar energy-generation
processes, the basic stellar-structure equations, the influence of
convection zones on stellar structure, main-sequence star models, the
evolutionary paths of low mass and massive stars, the late stages of
stellar evolution, pulsating stars, and the Cepheid mass problem.
Title: Introduction to stellar astrophysics. Volume 3. Stellar
structure and evolution.
Authors: Böhm-Vitense, Erika
Bibcode: 1992isa..book.....B
Altcode:
Introduction; Hydrostatic equilibrium; Thermal equilibrium;
The opacities; Convective instability; Theory of convective energy
transport; Depths of outer convection zones; Energy generation in stars;
Basic stellar structure equations; Homologous stars in radiative
equilibrium; Influence of convection zones on stellar structure;
Calculation of stellar models; Models for main sequence stars;
Evolution of low mass stars; Evolution of massive stars; Late stages
of stellar evolution; Observational tests of stellar evolution theory;
Pulsating stars; The cepheid mass problem; Star formation; Problems;
Bibliography; Index.
Title: Introduction to Stellar Astrophysics
Authors: Böhm-Vitense, Erika
Bibcode: 1992isa3.book.....B
Altcode:
This book is the final one in a series of three texts which together
provide a modern, complete and authoritative account of our present
knowledge of the stars. It discusses the internal structure and
the evolution of stars, and is completely self-contained. There is
an emphasis on the basic physics governing stellar structure and
the basic ideas on which our understanding of stellar structure is
based. The book also provides a comprehensive discussion of stellar
evolution. Careful comparison is made between theory and observation,
and the author has thus provided a lucid and balanced introductory text
for the student. As for volumes 1 and 2, volume 3 is self-contained
and can be used as an independent textbook. The author has not only
taught but has also published many original papers in this subject. Her
clear and readable style should make this text a first choice for
undergraduate and beginning graduate students taking courses in
astronomy and particularly in stellar astrophysics.
Title: Transition Layers of Hyades F Stars
Authors: Bohm-Vitense, Erika H.
Bibcode: 1992iue..prop.4205B
Altcode:
We would like to observe most Hyades F main sequence stars in order to
determine the dependence of transition layer emission on temperature,
rotation, binarity and Li abundances. In order to study the dependence
on any one parameter one has to keep all other parameters constant. For
the Hyades cluster age and metal abundance are constant. The Hyades
cluster is the nearest one for which the F stars are bright enough to be
observed with IUE in the SWP wavelength region. At the same time there
are enough F stars to study the dependence on one parameter with all
other parameters being nearly the same. In the F star region rotation
decreases when going from early to late F stars. Simon and Drake
suggest that the heating mechanism changes for B-V around 0.45. The
chromospheric emission appears not to vary with time for stars earlier
than F7 while it is variable for stars later than F8. Michaud suggests
depletion of He for early F stars but not for late F stars. Boesgaard
and Tripicco find a minimum in the Li abundances for stars with Teff
around 6700 K. We want to study the changes (or not) in the transition
layer emission related to all these events.
Title: Silicon abundances in population I giants
Authors: Boehm-Vitense, Erika
Bibcode: 1992STIN...9233658B
Altcode:
Silicon to carbon abundance ratios for population I giants were
determined from emission lines originating in the transition layers
between stellar chromospheres and coronae. For effective temperatures
larger than 6200 K we find a group of stars with increased silicon to
carbon but normal nitrogen to carbon abundance ratios. These stars are
presumably descendents from Ap stars with increased surface silicon
to carbon abundance ratios. For G stars this anomaly disappears as is
to be expected due to the increased depth of the convection zone and
therefore deeper mixing which dilutes the surface overabundances. The
disappearance of the abundance anomalies proves that the anomalous
abundances observed for the F giants are indeed only a surface
phenomenon. It also proves that the same holds for their progenitors,
the Ap and Am stars, as has been generally believed. Unexplained is
the increased silicon to carbon abundance ratio observed for several
stars cooler than 5100 L. RS CVn and related stars do not show this
increased abundance ratio. There are also some giants which appear to
be enriched in carbon, perhaps due to a helium flash with some mixing
if the star is a clump star.
Title: Ultraviolet studies of Cepheids
Authors: Boehm-Vitense, Erika
Bibcode: 1992STIN...9311073B
Altcode:
We discuss whether with new evolutionary tracks we still have a problem
fitting the Cepheids and their evolved companions on the appropriate
evolutionary tracks. We find that with the Bertelli et al. tracks
with convective overshoot by one pressure scale height the problem is
essentially removed, though somewhat more mixing would give a better
fit. By using the results of recent nonlinear hydrodynamic calculations,
we find that we also have no problem matching the observed pulsation
periods of the Cepheids with those expected from their new evolutionary
masses, provided that Cepheids with periods less than 9 days are
overtone pulsators. We investigate possible mass loss of Cepheids from
UV studies of the companion spectrum of S Mus and from the ultraviolet
spectra of the long period Cepheid l Carinae. For S Mus with a period
of 9.6 days we derive an upper limit for the mass loss of M less than
10-9 solar mass, if a standard velocity law is assumed for
the wind. For l Carinae with a period of 35.5 days we find a probable
mass loss of M is approximately 10-5+/-2 solar mass.
Title: Heating Mechanism(s) for Transition Layers in Giants
Authors: Bohm-Vitense, Erika; Mena-Werth, Jose
Bibcode: 1991ApJ...378..718B
Altcode:
The emission-line fluxes of lines originating in the lower parts of
the transition layers between stellar chromospheres and coronas are
studied. Simon and Drake (1989) suspect different heating mechanisms
for 'hot' and cool stars. Changes in the flux ratios for the C IV to
C II emission lines support this suspicion. Large C IV/C II line flux
ratios appear to be indicative of magnetically controlled heating. A
correlation between excess continuum flux around 1950 A and C II
emission-line fluxes are confirmed for the cooler giants (late F and
cooler). Excess continuum flux correlates positively with large C
IV/C II line flux ratio. The excess continuum flux corresponds to an
increase in temperature by several hundred degrees in layers with a
mean optical depth of about 0.03. For chromospherically active stars
these layers experience a mechanical flux deposition of the order
of 1 percent of the total radiative flux. This flux is tentatively
identified as an MHD wave flux similar to Alfven waves.
Title: Report on carbon and nitrogen abundance studies
Authors: Boehm-Vitense, Erika
Bibcode: 1991STIN...9211934B
Altcode:
The aim of the proposal was to determine the nitrogen to carbon
abundance ratios from transition layer lines in stars with different
Teff and luminosities. The equations which give the
surface emission line fluxes and the measured ratio of the NV to
CIV emission line fluxes are presented and explained. The abundance
results are compared with those of photospheric abundance studies
for stars in common with the photospheric investigations. The results
show that the analyses are at least as accurate as the photospheric
determinations. These studies can be extended to F and early G
stars for which photospheric abundance determinations for giants
are hard to do because molecular bands become too weak. The abundance
determination in the context of stellar evolution is addressed. The N/C
abundance ratio increases steeply at the point of evolution for which
the convection zone reaches deepest. Looking at the evolution of the
rotation velocities v sin i, a steep decrease in v sin i is related to
the increasing depth of the convection zone. It is concluded that the
decrease in v sin i for Teff less than or approximately
= 5800 K is most probably due to the rearrangement of the angular
momentum in the stars due to deep convective mixing. It appears that
the convection zone is rotating with nearly depth independent angular
momentum. Other research results and ongoing projects are discussed.
Title: Book-Review - Introduction to Stellar Astrophysics - V.1 -
Basic Stellar Observations and Data - Vol. - Stellar Atmospheres
Authors: Bohm-Vitense, E.; Stoll, D.; Stahlberg, J.
Bibcode: 1991AN....312..276B
Altcode:
No abstract at ADS
Title: Book-Review - Introduction to Stellar Astrophysics - VOL.1 -
Basic Stellar Observations and Data
Authors: Bohm-Vitense, E.; Doyle, J. G.
Bibcode: 1991IrAJ...20...50B
Altcode:
No abstract at ADS
Title: Absorption Line Profiles in a Companion Spectrum of a Mass
Losing Cool Supergiant
Authors: Rodrigues, L. L.; Böhm-Vitense, E.
Bibcode: 1991BAAS...23..910R
Altcode:
No abstract at ADS
Title: Transition Layers of F and Early G Giants
Authors: Bohm-Vitense, Erika H.
Bibcode: 1991iue..prop.3932B
Altcode:
Giants are especially well suited to study the dependence of transition
layer stratifications on v sini and log Teff because they are a group
of stars of nearly the same mass, age and chemical composition. We
want to observe additional F and early G giants in order to better
understand which heating mechanism(s) are responsible for the heating
of the transition layers. We also want to determine N/C and Si/C
abundance ratios which appear to be abnormal for several F giants in
contradiction to theoretical expectations.
Title: Emission lines in the long period Cepheid L Carinae
Authors: Boehm-Vitense, Erika; Love, Stanley G.
Bibcode: 1991STIN...9234060B
Altcode:
For the Cepheid (l) Carinae with a pulsation period of 35.5 days we have
studied the emission line fluxes as a function of pulsational phase
in order to find out whether we see chromosphere and transition layer
emission or whether we see emission due to an outward moving shock. All
emission lines show a steep increase in flux shortly before maximum
light suggestive of a shock moving through the surface layers. The
large ratio of the C IV to C II line fluxes shows that these are not
transition layer lines. During maximum light the large ratio of the C
IV to C II line fluxes also suggests that we see emission from a shock
with velocities greater than 100 km/sec such that C IV emission can
be excited. With such velocities mass outflow appears possible. The
variations seen in the Mg II line profiles show that there is an
internal absorption over a broad velocity band independent of the
pulsational phase. We attribute this absorption to a circumstellar
'shell'. This 'shell' appears to be seen also as spatially extended
emission in the O I line at 1300 angstrom, which is probably excited
by resonance with Ly beta.
Title: Mass Loss of B3 to B6 Main Sequence Stars
Authors: Bohm-Vitense, Erika H.
Bibcode: 1991iue..prop.3937B
Altcode:
Mass Loss for early type stars down to B2 has been observed in CIV,
SiIV and Si III lines. In these lines nothing has been seen for stars
later than B2. This does not exclude the possibility that winds may
he observed in lower ionization resonance lines like those of Fe II,
Mn II or Si II. We want to study whether such cool stellar winds can
be detected for single, slowly rotating B3 to B6 main sequence, non
Be stars.
Title: Ultraviolet Gas Absorption and Dust Extinction toward M8
Authors: Boggs, Don; Bohm-Vitense, Erika
Bibcode: 1990ApJ...358..441B
Altcode:
Interstellar absorption lines are analyzed using high-resolution IUE
spectra of 11 stars in the young cluster NGC 6530 located in the M8
region. High-velocity clouds at -35 km/s and -60 km/s are seen toward
all cluster stars. The components arise in gases that are part of large
interstellar bubbles centered on the cluster and driven by stellar
winds of the most luminous members. Absorption lines of species of
different ionization states are separated in velocity. The velocity
stratification is best explained as a 'champagne' flow of ionized
gas away from the cluster. The C IV/Si IV ratios toward the hotter
cluster members are consistent with simple photoionization models if
the gas-phase C/Si ratio is increased by preferential accretion onto
dust grains. High ion column densities in the central cluster decline
with distance from W93, suggesting that radiation from a hot source
near W93 has photoionized gas in the central cluster.
Title: Book-Review - Introduction to Stellar Astrophysics VOL.2
Stellar Atmospheres
Authors: Bohm-Vitense, E.
Bibcode: 1990Sci...249...24B
Altcode:
No abstract at ADS
Title: Introduction to Stellar Astrophysics
Authors: Böhm-Vitense, Erika
Bibcode: 1990isa2.book.....B
Altcode:
Volume 2 contains the basic physical ideas and laws used in the study
of the outer layers of a star including stellar magnitudes, spectra
and temperatures, radiative transfer in a stellar atmosphere, line
formation, the spectrum of hydrogen, spectral analysis, and available
structural components.
Title: Book-Review - Introduction to Stellar Astrophysics - V.1 -
Basic Stellar Observations and Data
Authors: Bohm-Vitense, E.
Bibcode: 1990Sci...247..584B
Altcode:
No abstract at ADS
Title: Transition Layer Emission in Giants
Authors: Bohm-Vitense, E.; Mena-Werth, J.
Bibcode: 1990ASPC....9..128B
Altcode: 1990csss....6..128B
No abstract at ADS
Title: UV Extinction Law and CIV, SIIV Column Densities in NGC 6530
Authors: Bohm-Vitense, Erika
Bibcode: 1990iue..prop.3657B
Altcode:
We discovered that in the central part of the young galactic cluster
NGC 6530 there is a strong correlation between the C IV and Si IV
column densities and the strength of the interstellar extinction bump
at 2175 A. Both depend on the distance from the star W 93, which is
close to the center of the high stellar density part of the cluster. In
the absence of any evidence for high velocity gas we conclude that a
strong UV radiation field is responsible for the ionization to CIV and
for the changes in the UV extinction law. The BIV star W93 is too cool
to photoionize CIII, however. An unseen neighbor or companion to W 93
must be the source of the UV radiation. While the correlation between
extinction bump and distance from W 93 is based on about 10 stars,
the correlation between extinction bump and C IV, Si IV column density
is based on four stars only. The data refer mainly to the southeast
part of the cluster and to stars southeast of W 93. We would like to
extend our studies to more stars and to different directions from W 93.
Title: Book-Review - Introduction to Stellar Astrophysics - Basic
Stellar Observations and Data
Authors: Bohm-Vitense, E.
Bibcode: 1990AstQ....7..188B
Altcode:
No abstract at ADS
Title: Book-Review - Introduction to Stellar Astrophysics - Volume
2 - Stellar Atmospheres
Authors: Bohm-Vitense, E.
Bibcode: 1990AstQ....7..251B
Altcode:
No abstract at ADS
Title: Absorption line profiles in a companion spectrum of a mass
losing cool supergiant
Authors: Rodrigues, Liliya L.; Boehm-Vitense, Erika
Bibcode: 1990STIN...9234230R
Altcode:
Cool star winds can best be observed in resonance absorption lines seen
in the spectrum of a hot companion, due to the wind passing in front
of the blue star. We calculated absorption line profiles that would
be seen in the ultraviolet part of the blue companion spectrum. Line
profiles are derived for different radial dependences of the cool
star wind and for different orbital phases of the binary. Bowen and
Wilson find theoretically that stellar pulsations drive mass loss. We
therefore apply our calculations to the Cepheid binary S Muscae which
has a B5V companion. We find an upper limit for the Cepheid mass loss
of M less than or equal to 7 x 10-10 solar mass per year
provided that the stellar wind of the companion does not influence
the Cepheid wind at large distances.
Title: The Dynamical Mass of S MUSCAE
Authors: Bohm-Vitense, Erika; Clark, M.; Cottrell, P. L.; Wallerstein,
George
Bibcode: 1990AJ.....99..353B
Altcode:
The amplitudes and phase relations of pulsational velocities of
the Cepheid S Muscae, measured for lines originating at different
atmospheric depths, are investigated. A mass M(A) = 4.4 + or - 0.5 solar
masses is found for the Cepheid; however, the Cepheid is the evolved
star and should have a mass larger than that of its companion. If the
measured velocity differences are too low by 2.5 km/s on the average,
then the mass ratio M(B)/M(A) may be decreased to 0.94, giving
an estimated upper limit to the Cepheid mass of 6.1 solar masses,
consistent with the Cepheid being the more evolved star. Considering
all the uncertainties, it is concluded that the mass of the Cepheid
is between 4.6 and 6.1 solar masses.
Title: UV observations of NGC 205
Authors: Wilcots, Eric M.; Hodge, Paul W.; Eskridge, Paul B.;
Boehm-Vitense, Erika; Bertola, Francesco
Bibcode: 1989uwsa.rept.....W
Altcode:
Low resolution IUE observations of the dwarf elliptical galaxy NGC
205 show that the UV spectral energy distribution (SED) of the galaxy
is relatively flat. Spectra centered on the nucleus and on a region
north of the nucleus show evidence of recent bursts of star formation
which contribute strongly to the UV spectral energy distribution. The
UV spectra was fit with a composite spectrum based on a Miller-Scalo
initial mass function, an underlying older population (modelled using
the UV spectrum of 47 Tuc), and an extinction based on a SMC-like
extinction cure. This fit implies that the total mass of young stars
(with M equal to or greater than 1 solar mass) in the galaxy is
approx. 7x10(5) solar mass, which can be compared to the total mass of
globular cluster like stars in the galaxy of approx. 8x10(7) solar mass.
Title: Introduction to Stellar Astrophysics
Authors: Böhm-Vitense, Erika
Bibcode: 1989isa1.book.....B
Altcode:
This textbook introduction to the basic elements of fundamental
astronomy and astrophysics serves as a foundation for understanding
the structure, evolution, and observed properties of stars. The first
half of the book explains how stellar motions, distances, luminosities,
colors, radii, masses and temperatures are measured or derived. The
author then shows how data of these sorts can be arranged to classify
stars through their spectra. Stellar rotation and stellar magnetic
fields are introduced. Stars with peculiar spectra and pulsating stars
also merit special attention. The endpoints of stellar evolutions are
briefly described. There is a separate chapter on the Sun and a final
one on interstellar absorption. The usefulness of this text is enhanced
by the inclusion of problems for students, tables of astronomical
constants, and a selective bibliography. This is an excellent textbook
for undergraduate and beginning graduate students studying astronomy
and astrophysics.
Title: Problems with the Baade-Wesselink Method
Authors: Boehm-Vitense, E.; Garnavich, P.; Lawler, M.; Mena-Werth,
J.; Morgan, S.; Peterson, E.; Temple, S.
Bibcode: 1989ApJ...343..343B
Altcode:
The discrepancy noted in radii obtained by the Baade-Wesselink method
when different colors are used to determine the effective temperatures
is explored. The discrepancy is found to be due to an inconsistency
in the applied temperature-color calibrations. The assumption of the
maximum likelihood method that beta (the effective temperature + 0.1
times the bolometric correction) is a linear function of the color
is valid for the B-V and V-I colors, but not for the V-R colors. It
is suggested that the errors introduced by the nonlinearity in the
relation between beta and the V-R colors will produce radii which are
too large. The radii derived from the V-B colors appear to be too small.
Title: Ultraviolet Extinction Differences within NGC 6530
Authors: Boggs, Don; Bohm-Vitense, Erika
Bibcode: 1989ApJ...339..209B
Altcode:
Ultraviolet extinction curves are presented for 14 stars in the
very young cluster NGC 6530. The variation of the short-wavelength
extinction is consistent with that expected based on random errors
alone, although the weakest extinction is found only toward the cluster
center. Variations significantly larger than the measurement uncertainty
are found in the area, width, and central peak position of the 2200 A
extinction bump. These parameters are largest in the cluster center
and decrease systematically outwards. Physical processes capable of
modifying the grains in the H II region containing the cluster are
discussed.
Title: Problems with the Baade-Wesselink Method
Authors: Böhm-Vitense, E.; Garnavich, P.; Lawler, M.; Mena-Werth,
J.; Morgan, S.; Peterson, E.; Temple, S.
Bibcode: 1989upsf.conf..250B
Altcode: 1989IAUCo.111..250B
No abstract at ADS
Title: Introduction to Stellar Astrophysics. Vol.1: Basic stellar
observations and data; Vol.2: Stellar atmospheres
Authors: Boehm-Vitense, Erika
Bibcode: 1989itsa.book.....B
Altcode:
Volume 1: The global properties of stars and the observational
techniques used to determine them are examined in an introduction for
undergraduate students. Chapters are devoted to stellar positions,
proper motions, brightness, color-magnitude diagrams, luminosities,
angular radii, and effective temperatures. Also considered are stellar
masses and radii, spectral classification, population II stars,
stellar rotation, magnetic fields, peculiar spectra, pulsating stars,
explosive stars, the sun, and interstellar absorption. Diagrams, graphs,
sample images and spectra, tables of numerical constants, and a set of
problems are provided. Volume 2: The basic principles used in the study
of the outer layers of a star are discussed. The subjects addressed
include stellar magnitudes, colors, and spectra; temperature estimates
for stars; radiative transfer; depth dependence of the source function;
the continuous absorption coefficient; the influence of the nongreyness
of the absorption coefficient; pressure stratification; theory of line
formation; hydrogen lines; spectrum analysis; nonlocal thermodynamic
equilibrium; the hydrogen convection zone; stellar chromospheres,
transition layers, and coronae; and stellar winds.
Title: Introduction to Stellar Astrophysics
Authors: Böhm-Vitense, Erika
Bibcode: 1989ESASP.290.....B
Altcode: 1989isa..book.....B
No abstract at ADS
Title: The Companion of the RR Lyrae Star TV Boo
Authors: Bohm-Vitense, Erika
Bibcode: 1989iue..prop.3380B
Altcode:
In her study of the RR Lyrae star TV Boo, Claudia Oliveira at the
University of British Columbia, measured over a period of 3 days a mean
radial velocity of Vr = -110 km/sec, while measurements made a few
years earlier had shown V, - -30 km/sec. These measurements indicate
orbital motions of a binary. Combining all available measurements an
orbital period around 60 days is suggested. The color-color plot of
V-I versus B-V is abnormal and indicates a blue companion. This must
then be either a white dwarf or a horizontal branch star on the very
blue end with decreasing Mv. This then is an extremely interesting
binary which for the first time can give direct information about the
masses of such stars. We want to observe the blue companion of this RR
Lyrae star in order to determine its properties and possibly measure
its orbital velocity.
Title: UV Continua of Cluster OB Stars in the Wing of the SMC
Authors: Bohm-Vitense, Erika
Bibcode: 1989iue..prop.3377B
Altcode:
Many investigators have found that the UV color temperatures of galactic
O stars are generally well below the effective temperatures of the
stars as inferred from optical line spectra and Zanstra colors. The
difference is so severe that even the earliest O stars have UV continua,
normalized at V, no brighter than early B main sequence stars. The
effects of two possible explanations for the relatively cool continua,
wind blanketing and non-LTE line blanketing, should be dependent on
the stellar metal content. In particular, the winds of early-type
stars with lower metallicity should be weaker and less effective at
scattering radiation back onto the photosphere. The net effect should
be reduced surface heating and consequently steeper UV continua than
stars of galactic abundance. We propose to measure the UV continua of
main sequence O and B stars in two lightly reddened clusters in the
extreme wing of the SMC. Our observations will allow a precise test
of these theories. With a much lower metallicity in the SMC compared
to the Galaxy, line- and wind blanketing effects should be greatly
reduced. If neither of these mechanisms are shaping the UV continua,
the color temperatures of the O stars should be as cool as the B
stars. Restricting our target stars to a single lightly reddened
association will limit the uncertainties in dereddening the spectra.
Title: Variable Bump Extinction in NGC6530
Authors: Bohm-Vitense, Erika
Bibcode: 1989iue..prop.3372B
Altcode:
We have found that the area and width of the 2200 A ultraviolet
extinction bump varies dramatically from star to star within the young
cluster NGC 6530. The bump is strongest toward stars in the center
of the cluster and systematically weakens with distance from center
appearing to reach an asymptotic state. Because the highest temperature
stars in our sample are generally farther from the center this may also
mean that the extinction bump is dependent on stellar temperature. It is
very unfortunate that five stars whose positions and/or temperatures
make them well suited for separating the effects of distance and
temperature have only low quality long wavelength spectra. We want to
reobserve those stars and obtain spectra of new targets advantageously
located within the cluster to study whether the extinction is dependent
on distance from the center or on stellar temperature.
Title: Ultraviolet Extinction in the Outer Galaxy
Authors: Bohm-Vitense, Erika
Bibcode: 1989iue..prop.3373B
Altcode:
We propose to test whether the empirical trend of increasing
far-ultraviolet extinction and decreasing 2200 A bump strength with
decreasing metallicity seen among the solar neighborhood, LMC, and
SMC is also valid within the Milky Way. Recent re-examinations of the
average extinction curve in the LMC suggest that the dependence may
not be as strong as originally thought; stars outside of the atypical
30 Doradus region exhibit extinction remarkably similar to that of
diffuse regions in the Milky Way. To test the validity of the trend
requires observations of the extinction in other systems of different
metallicities. Long exposure IUE spectra of luminous stars in M31 and
M33 have been used to estimate the extinction in these galaxies but the
low flux levels and uncertainties of the true stellar intrinsic colors
make the results susceptible to large errors. To study extinction
in systems of low metal content does not require observations of
extragalactic stars, however. With the galactic abundance gradient,
a metallicity comparable to that in the LMC is reached in only three
or four kiloparsecs from the solar circle. We intend to measure the
extinction curves toward several stars at this distance or beyond. The
low nearby extinction toward much of the outer Galaxy ensures that
the extinction of the stars is dominated by distant dust. The use
of relatively bright and nearby stars will allow us to measure the
extinction in an unexplored metal poor environment with unusually
high precision.
Title: Dynamical Masses for V636 SCO
Authors: Bohm-Vitense, Erika
Bibcode: 1989iue..prop.3368B
Altcode:
For the Cepheid V636 Scorpii we want to determine the pulsational radial
velocities for the spectral lines seen in the LWR region of the IUE
Observatory. These measurements are essential for the determination
of the dynamical masses for this binary. On LWR IUE high resolution
images of V636 Sco the spectrum of the Cepheid can be studied for
lambda > 2400 A while the one for the companion can be studied
for lambda < 2400 A for minimum light of the Cepheid. This fact
permits the measurement of the difference in radial velocity for the two
components of the binary system. After subtraction of the pulsational
velocity of the Cepheid the difference of the orbital velocities is
obtained. With the orbital velocity of the Cepheid known from optical
studies the velocity ratio and thereby the mass ratio of the binary
components can be determined. So far the pulsational velocities for
the Cepheid measured in the optical region have been used in this
procedure. Observations show, however, that pulsational velocities
are different for different layers of the atmosphere. Subtraction of
the pulsational velocities determined for the absorption lines in the
optical region, when we should subtract the pulsational velocities
for the UV region, may cause large errors in the difference of the
orbital velocities and their ratios. We therefore have during the lIth
period redetermined for S Mus the pulsational velocities for those high
layers in which the lines in the LWR region are formed. This requires
measurements for many phases of the pulsational cycle. We want to do
the same for V636 Sco.
Title: Introduction to stellar astrophysics. Vol. 2. Stellar
atmospheres.
Authors: Böhm-Vitense, E.
Bibcode: 1989isa2.book.....B
Altcode:
This comprehensive textbook conveys the basic physical ideas and laws
in the study of the outer layers of a star. This complete account
first introduces stellar magnitudes, spectra and temperatures. This
is followed by a full discussion of radiative transfer in a stellar
atmosphere, which leads to descriptions of line formation, the spectrum
of hydrogen, and spectral analysis. Finally the structural components
that are accessible, such as the convection zone, chromosphere,
corona, and mass outflow are described. Contents: 1. Stellar
magnitudes and stellar colors. 2. Stellar spectra. 3. Temperature
estimates for stars. 4. Basics about radiative transfer. 5. Radiative
transfer in stellar atmospheres. 6. The depth dependence of the
source function. 7. The continuous absorption coefficient. 8. The
influence of the non-greyness of the absorption coefficient. 9. The
pressure stratification. 10. Theory of line formation. 11. The
hydrogen lines. 12. Spectrum analysis. 13. Basics about non-local
thermodynamic equilibrium. 14. The hydrogen convection zone. 15. Stellar
chromospheres, transition layers, and coronae. 16. Stellar winds.
Title: Effects of SN 1987A on the Interstellar Medium around it
Authors: Bohm-Vitense, Erika; Boggs, Don; Morgan, Siobahn
Bibcode: 1989AJ.....97..131B
Altcode:
It is shown that the interstellar spectrum from SN 1987A in the LMC
shows a rather strong component in C IV and Si IV, and possibly a weak
component in N V, at a velocity of about 300 + or - 10 Km/s. In the
spectra of the neighboring stars HD38268 and R123, a component at this
velocity is only seen in low-ionization lines, not in the C IV, or N V
lines. The possibility is studied that SN 1987A ionized a measurable
fraction of this 300 Km/s cloud within less than one day since the
Si IV, C IV, and possibly, N V interstellar lines are seen only
about one day after the first light from the supernova explosion was
detected. This may be the case, but only if the supernova temperature at
outburst was larger than 50,000 K, depending on the density assumed for
the interstellar cloud. Also, H and He must have been preionized once.
Title: ISM Chemical Abundances in Two Intermediate-Velocity Clouds
in the Line of Sight to SN 1987A
Authors: Morgan, Siobahn; Bohm-Vitense, Erika
Bibcode: 1988AJ.....96.1373M
Altcode:
The earliest IUE high-resolution spectra of SN 1987A have been studied
and reveal the presence of several clouds in the line of sight to the
LMC. In particular, there are two clouds with radial velocities of ~
130 km s^-1^ and ~ 180 km s^-1^. These clouds' velocities are between
those of Galactic clouds at 0-80 km s^-1^ and those of LMC gas at ~270
km s^-1^. Chemical-abundance determinations may help to determine the
origin and location of these clouds. Curve-of-growth analysis and 21
cm observations show that they may be underabundant in heavy elements
by about a factor of 2 as compared to solar abundances. No depletion
indicative of grain formation can be seen.
Title: UV Observations of NGC 205
Authors: Wilcots, E. M.; Bohm-Vitense, E.; Hodge, P. W.; Eskridge, P.
Bibcode: 1988BAAS...20.1039W
Altcode:
No abstract at ADS
Title: ISM chemical abundances along the line-of-sight to SN 1987A
Authors: Morgan, Siobahn; Boehm-Vitense, Erika
Bibcode: 1988ESASP.281b.207M
Altcode: 1988IUES....1..207M; 1988uvai....2..207M; 1988IUE88...2..207M
The IUE high resolution spectra of SN 1987A were studied. Absorption
lines of 2 clouds with velocities of 130 km/sec and 180 km/sec
(heliocentric) are seen in addition to the local galactic gas and the
Large Magellanic Cloud (LMC) gas. Curve of growth analysis shows that
these clouds may be depleted in heavy elements when compared to the
local ISM and the LMC ISM. There is no indication of depletion due
to grain formation. There seems to be no way to determine whether the
clouds are part of the galactic halo or part of the LMC's halo.
Title: Carbon and nitrogen abundance determinations from transition
layer lines and mixing in stars
Authors: Boehm-Vitense, Erika; Mena-Werth, Jose
Bibcode: 1988ESASP.281a.381B
Altcode: 1988duvb.conf..381B; 1988uvai....1..381B
For red giants a smooth increase in the nitrogen to carbon abundance
ratio for increasing B-V as is expected for the first dredge up phase
when the outer convection zone deepens is found. An average increase
in the nitrogen to silicon ratio for B-V = 0.6 which goes back to
almost solar values for cool giants with B - V approximately 1.0
is reported. It looks as if Si would be enriched for deeper mixing
contrary to expectations from standard evolution theory.
Title: The energy input mechanism into the lower transition regions
between stellar chromospheres and coronae
Authors: Boehm-Vitense, Erika
Bibcode: 1988ESASP.281a.315B
Altcode: 1988uvai....1..315B
The ratio of the emission line fluxes for the C II and C IV lines in
the lower transition regions (T = 30,000 to 100,000 K) between stellar
chromospheres and transition layers is shown to depend mainly on the
temperature gradient in the line emitting regions which can therefore
be determined from this line ratio. From the observed constant (within
the limits of observational error) ratio of the emission line fluxes
of the C II (1335 A) and C IV (1550 A) lines it is concluded that
the temperature gradients in the lower transition layers are similar
for the large majority of stars independently of Teff, L,
and degree of activity. This means that the temperature dependence
of the damping length for the mechanical flux must be the same for
all these stars. Since for different kinds of mechanical fluxes the
dependence of the damping length on gas pressure and temperature is
quite different, it is concluded that the same heating mechanism must
be responsible for the heating of all the lower transition layers
of these stars, regardless of their chromospheric activity. Only the
amount of mechanical flux changes. The T Tauri stars are exceptions:
their emission lines are probably mainly due to circumstellar material.
Title: Are Most Short-Period Cepheids Overtone Pulsators?
Authors: Bohm-Vitense, Erika
Bibcode: 1988ApJ...324L..27B
Altcode:
Data given and analyzed recently by Coulson et al. (1986) and Gieren
(1982) provide a consistent set of Wesselink radii and Wesselink
masses, for which the internal error bars appear to be small enough
that a quantitative discussion appears to be warranted. When it is
assumed that all Cepheids pulsate in the fundamental mode, a steep
discontinuity in mass is found for periods around 6.5 days. When
short-period Cepheids are interpreted as first-overtone pulsators as
required theoretically (Christy, 1966; Stobie, 1969), the discontinuity
disappears and the mass-period relation passes through the bump and
beat masses for Cepheids.
Title: Long Term Variations of AM Stars
Authors: Bohm-Vitense, Erika
Bibcode: 1988iue..prop.3045B
Altcode:
We would like to reobserve some selected Am stars which we originally
observed in 1979 and 1980 in order to study long term time variations
in the UV. Such variations have been observed earlier to occur
in the optical spectral region for some of them (15 Vul, tau UMa)
If temperature changes are responsible for the optical variations
than larger variations in the energy distribution are expected in the
ultraviolet. Our original observations were proposed because we wanted
to study such variations. We would now like 10 reobserve some of the
stars observed earlier in order to check whether any changes in the
uv energy distribution or in the details of the spectra have occurred
during the time span of 6-8 years. If we do not make the observations
now we mail never get a chance to actually do the investigation which
we set out to do 10 years ago.
Title: Blue Companions of Supergiants
Authors: Bohm-Vitense, Erika
Bibcode: 1988iue..prop.3054B
Altcode:
We would like to observe blue companions of supergiants in star
clusters. For binaries in clusters we know the distance with some
accuracy. IUE observations of blue companions of supergiants therefore
tell their absolute magnitudes, so we can decide whether we are dealing
with giants or main sequence stars. Because of the short evolution
times along the giant branch the supergiants and evolved companions
must have essentially the same mass. They then outline two points
on an evolutionary track. For other apparently giant stars in the
cluster we can never be sure about their membership. The luminosity
difference between a supergiant and a giant of the same mass depends on
the degree of mixing in the interior of the main sequence progenitor
star, which can therefore be determined empirically by studying the
luminosity difference between the supergiant and a giant companion. As
all supergiants in a cluster must have essentially the same mass
a dynamical mass determination for a supergiant in a cluster with
Cepheids also determines the mass of the Cepheid.
Title: Masses for Four Cepheids
Authors: Bohm-Vitense, Erika
Bibcode: 1988iue..prop.3049B
Altcode:
Velocities from IUE high dispersion spectra have provided mass ratios
between a Cepheid and a blue companion for the systems SIT Cyg,
S Mus, and V636 Sco. The proposed IUE high dispersion observations
will produce a mass ratio for a new system, U Aql, with a percentage
error as low as 14%, depending on the orbital velocity amplitude of
the companion. They will also provide a second optimal spectrum for
both S Mus and V636 Sco which will confirm the previous results at the
second velocity extremum, reduce the uncertainty in the mass ratio
by sqrt2 to 14%, and also provide an error estimate from comparison
with the previous spectrum. For AW Per a spectrum will provide a
preliminary mass function. confirmation that it is not a triple system,
and information about the rotational velocity for a system which can
be resolved by Hubble Space Telescope. These results, together with
the SU Cyg results, are the only mass determinations for Cepheids to
decide between the discordant evolutionary and pulsational masses.
Title: Emission Lines of the Long Period Copheid L Carinae
Authors: Bohm-Vitense, Erika
Bibcode: 1988iue..prop.3051B
Altcode:
We propose to observe the time dependence of the ultraviolet emission
line fluxes in the Cepheid l Carinae. Previous observations by Schmidt
and Parsons have shown that l Carinae appears to be the only Cepheid
observed so far to show the high ionization lines up to C IV. This
Cepheid therefore appears to be the best star to study the rise and
decay of emission lines of different stages of ionization. During
the phases of increasing emission we should be able to see the phase
delay for the emission of high ionization line, as compared to low
ionization lines, corresponding to the increasing amount of heating
and the propagation of the ionization front through the surface layers
when the transition region is built up. The amount of heating measures
the amount of energy being deposited. If the emission is related
to the onset of convection during the cool phases of the pulsation,
as seems to be the case. then the rise and decay of the ultraviolet
emission lines will be able to teach us a lot about the timescales
for the generation of the mechanical flux heating the chromosphere and
transition laver. This in turn will tell us something about the nature
of this flux and about the time dependence of the developing convection.
Title: High Resolution Studies of F and G Star Transition Layer Lines
Authors: Bohm-Vitense, Erika
Bibcode: 1988iue..prop.3048B
Altcode:
The question, which heating mechanism is responsible for the heating
of the transition layers and stellar coronae. is still an open one. We
have now developed a theory for stellar transition layers which are
homogeneous. at least in sectors. which does permit to determine from
the observed emission measures the amount of mechanical energy input in
the lower transition layer and determine the boundary condition Pe(T)
for the upper transition layer. The mechanical flux and the damping
length for this flux for the upper part of the transition layer can be
determined from the emission measures in the upper transition zone and
from the coronal temperature. or, if no emission measures for the upper
transition zone are available the emission measure determined from the
coronal X-ray emission can be used instead. though less reliably. Using
this method we found that the lower transition layers of the sun and
Procyon are probably heated by shock wave heating. The upper transition
layer of Procyon also seems to be heated by shock waves. while in the
sun in the upper part of the transition layer the damping length is
too long to be consistent with shock wave heating. We want to study
whether and where in the late F star region a transition from shock
wave heating to MHD wave heating of the upper transition layer occurs.
Title: Dynamical Masses for the Two Cepheids S Mus and V636 Sco
Authors: Bohm-Vitense, Erika
Bibcode: 1988iue..prop.3065B
Altcode:
For the Cepheids S Muscae and V636 Scorpii we want to determine the
pulsational radial velocities for the spectral lines seen in the LWR
region of the IUE Observatory. These measurements are essential for
the determination of the dynamical masses for these binaries. On LWR
IUE high resolution images of S Mus and V636 Sco the spectrum of the
Cepheid can be studied for lambda > 2500 A while the one for the
companion can be studied for lambda < 2500 A. This fact permits
the measurement of the difference in radial velocity for the two
components of the binary system. After subtraction of the pulsational
velocity of the Cepheid the difference of the orbital velocities is
obtained. With the orbital velocity of the Cepheid known from optical
studies the velocity ratio and thereby the mass ratio of the binary
components can be determined. So far the pulsational velocities for
the Cepheid measured in the optical region have been used in this
procedure. Observations show, however, that pulsational velocities
are different for different layers of the atmosphere. Subtraction of
the pulsational velocities determined for the absorption lines in the
optical region, when we should subtract the pulsational velocities
for the uv region, may cause large errors in the difference of the
orbital velocities and their ratios. We therefore want to determine
the pulsational velocities for those high layers in which the lines in
the LWR region are formed. This requires measurements for many phases
of the pulsational cycle.
Title: A Nonspherically Symmetric Model for the Peculiar A Star
Alpha 2 CVn
Authors: Bohm-Vitense, Erika; van Dyk, Schuyler D.
Bibcode: 1987AJ.....93.1527B
Altcode:
Observations show that in the optical region the peculiar A star
α2CVn has a flatter energy distribution during maximum light
than during minimum light. This indicates that during maximum light we
see a lower-temperature region, but necessarily larger surface area,
than during minimum light. This suggests a nonspherically symmetric
star, which is oblate with respect to the magnetic axis and which is
cooler at the magnetic poles than at the magnetic equator. The authors
have studied the light variations of such nonspherically symmetric
oblique rotator models. They find for an oblate ellipsoid with an axial
ratio of 0.92 and a temperature difference of about 1000K between the
poles and the equator that the overall variations of the optical and
the ultraviolet energy distributions can be well understood.
Title: Theory of Transition-Layer Emission Measures and Coronae
Authors: Bohm-Vitense, Erika
Bibcode: 1987ApJ...317..750B
Altcode:
The basic equations describing the energy equilibria, the conductive
heat flux, and the temperature stratifications for stellar transition
layers and coronae with 'open' field lines are summarized. The
temperature dependence of the emission measures for lines originating in
different temperature regions of the transition zone is determined. It
is found that the stellar transition regions consist of two basically
different parts: the lower part where mechnical energy input is balanced
by the radiative losses, and the upper part where the mechanical energy
input is balanced by the divergence of the conductive flux and radiative
losses. In the lower part, the temperature stratification is determined
by an equilibrium between mechanical flux input and radiative energy
losses. The coronal temperatures increase with increasing mechanical
flux and damping length in the upper transition zone.
Title: MG II Line Profiles in W VIR
Authors: Bohm-Vitense, Erika
Bibcode: 1987iue..prop.2731B
Altcode:
On low resolution spectra the Mg II emission lines at 2800 A of W
Vir nearly always show the same central intensities, regardless of
the pulsational cycle. This suggests that the Mg II emission lines do
not originate in the stellar photosphere or chromosphere but rather in
detached circumstellar material possibly indicating mass loss for this
prototype population no. II Cepheid. During minimum light, however,
the emission lines at 2800 A appears much weakened, an additional
emission line appears at 2828 A. We would like to take high resolution
IUE spectra with the long wavelength camera in order to study the Mg II
line profiles and to see whether they share the photospheric velocities
or at least the pulsational velocity cycle or whether they originate in
detached circumstellar material. If so this would be a definite sign
of mass loss by at least one population II Cepheid. If all population
II Cepheids turn out to lose mass during their pulsationally unstable
phases of stellar evolution this may possibly be a way to transform
red giants into horizontal branch stars.
Title: Mass Loss in Population I Cepheids
Authors: Bohm-Vitense, Erika
Bibcode: 1987iue..prop.2742B
Altcode:
It has been proposed by several authors that Cepheids lose mass during
their pulsational cycle. due to outwards propagating shockwaves. The
S Mus system offers a unique opportunity to check this hypothesis
directly. The Cepheid has a main sequence companion with a temperature
of about 17,700K. which means a spectral type of B4 V. This star is
hot enough that the ultraviolet circumstellar and stellar wind lines
can be observed, if present, with high resolution SWP images. On the
other hand, it is cool enough that it is not a stellar wind source
itself. The apparent visual magnitude of the companion is inferred
to be mv ~ 7. The Cepheid has E(B-V) = 0.27. The orbital period
is 506 days. so the companion is close enough that circumstellar
material and stellar wind material still has high enough densities
to be observable in the spectrum of the companion. There is no other
known system with such favorable properties. Stellar wind lines can be
recognized by their velocities and widths and variations during the
orbital cycle. Circurnstellar lines, resting in the binary system,
can be distinguished from interstellar lines by means of their
velocity difference. We want to search for evidence of mass loss
of the Cepheid by studying the companion spectrum during different
orbital and pulsational phases, and see whether lines, normally seen
in interstellar gas, can be seen to vary, which would show that they
are circumstellar. The equivalent widths of the lines will show the
amount of circumstellar material. The width or wavelength shift of
the lines determines the velocity field.
Title: A possible solution to the cepheid mass problem?
Authors: Bohm-Vitense, Erika
Bibcode: 1987LNP...274..159B
Altcode: 1987stpu.conf..159B
With new, smaller distances of the Cepheids, as determined recently by
Schmidt (1984) and by Böhm-Vitense (1985), smaller pulsational masses
are obtained than previously. Giant companions of Cepheids show that the
luminosities of the Cepheids are too large in comparison with those of
the giants. If increased mixing, for instance by convective overshoot
at the boundary of the convective core during the main sequence stage,
is responsible for this, then we expect an increase in the luminosity
of the Cepheids of a given mass by approximately a factor of 4 as
compared to conventional evolution calculations. Taking into account
both of these effects we find good agreement between the corrected
evolutionary masses, the pulsational masses, the dynamical masses,
the giant companion masses and the Wesselink masses. The bump masses
are only slightly smaller than the other masses.
Title: Blue Companions of Long Period Cepheids
Authors: Bohm-Vitense, Erika
Bibcode: 1987iue..prop.2730B
Altcode:
We want to search for giant companions of very short and very long
period Cepheids. These permit a check on the relative luminosities
of giants and their Cepheid companions. which should fit on the same
evolutionary tracks. For Cepheids with periods around 9 days we found
a discrepancy between the observed relative luminosities and the
theoretically expected values. This discrepancy might be explained by
additional, probably convective overshoot, mixing at the boundaries
of the convective cores of main sequence massive stars. If this is
indeed the explanation we might probably expect that the mixing is
mass dependent and that therefore the discrepancy between giant and
Cepheid luminosities will be different for longer and shorter period
Cepheids. We want to check this suspicion. If such an effect is seen
it tells us about the importance of convective overshoot mixing for
stars with different masses. In order to find Cepheids with giant
companions we want to look at Cepheids whose Wesselink masses are
especially low. Generally, blue giant companions contribute more
light in the visual spectral region than do blue main sequence
companions. "Wesselink" masses determined for Cepheids with blue
companions give masses which are too low because of the influence of
the companion on the optical colors. For giants this effect should be
quite noticeable.
Title: Masses for Four Cepheids
Authors: Bohm-Vitense, Erika
Bibcode: 1987iue..prop.2732B
Altcode:
Velocities from IUE high dispersion spectra have provided mass
ratios between a Cepheid and a blue companion for the systems SU Cyg,
S Mus, and V636 Sco. The proposed IUE high dispersion observations
will produce a mass ratio for a new system, U Aql. with a percentage
error as low as 14%. depending on the orbital velocity amplitude of
the companion. They will also provide a second optimal spectrum for
both S Mus and V636 Sco which will confirm the previous results at
the second velocity extremum, reduce the uncertainty in the mass ratio
by sqrt 2 to 14%. and also provide an error estimate from comparison
with the previous spectrum. For AW Per a spectrum will provide a
preliminary mass function. confirmation that it is not a triple system,
and information about the rotational velocity for a system which can
be resolved by Hubble Space Telescope. These results, together with
the SU Cyg results, are the only mass determinations for Cepheids to
decide between the discordant evolutionary and pulsational masses.
Title: Intrinsically Variable Stars
Authors: Bohm-Vitense, Erika; Querci, Monique
Bibcode: 1987ASSL..129..223B
Altcode: 1987euwi.book..223B
The characteristics of intrinsically variable stars are examined,
reviewing the results of observations obtained with the IUE satellite
since its launch in 1978. Selected data on both medium-spectral-class
pulsating stars (Delta Cep stars, W Vir stars, and related groups) and
late-type variables (M, S, and C giants and supergiants) are presented
in spectra, graphs, and tables and described in detail. Topics addressed
include the calibration of the the period-luminosity relation, Cepheid
distance determination, checking stellar evolution theory by the giant
companions of Cepheids, Cepheid masses, the importance of the hydrogen
convection zone in Cepheids, temperature and abundance estimates for
Population II pulsating stars, mass loss in Population II Cepheids,
SWP and LWP images of cold giants and supergiants, temporal variations
in the UV lines of cold stars, C-rich cold stars, and cold stars with
highly ionized emission lines.
Title: Emission Measures and Heating Mechanisms for Stellar Transition
Regions and Coronae
Authors: Bohm-Vitense, E.
Bibcode: 1987IAUS..122..359B
Altcode:
In order to determine the heating mechanisms for stellar transition
regions and coronae the author tries to determine the damping lengths
for the mechanical flux(es) responsible for the heating. For the
lower part of the transition regions (30,000 < T ≤ 100,000K) the
damping lengths are consistent with shockwave damping. This appears to
be also true for the upper part of the transition region in Procyon,
while for the upper part of the solar transition region the damping
length is much larger.
Title: The Cepheid Mass Problem and Cepheid Binaries
Authors: Bohm-Vitense, E.
Bibcode: 1986ApJ...303..262B
Altcode:
Existing mass determinations for Cepheids with different periods are
examined. Wesselink masses are independent of the adopted distance
scale. For short periods (less than 6 days) they follow the sequence
of evolutionary masses. For periods longer than 10 days they are lower
by up to a factor of 2. The lower mass branch joins up with the bump
masses. The new pulsational masses agree with the Wesselink masses
for periods longer than 6 days. Cepheid masses determined by means of
their giant companions also agree with the Wesselink masses and the
new pulsational masses. While the error bars are large, the derived
dynamical masses determined for S Mus and V636 Sco also agree with
the low Wesselink and giant companion masses.
Title: A Simple Explanation for the Linksy-Haisch Boundary Line for
Transition Layers
Authors: Bohm-Vitense, E.
Bibcode: 1986ApJ...301..297B
Altcode:
It is found that, for stars with low gravitational acceleration,
transition layers between chromosphere and corona cannot form, because
the chromospheres are so extended that the mechanical energy flux
decreases faster than the square of the electron density. There is
not enough flux left to lead to a steep temperature increase in the
transition layer and corona. If the disipation length lambda for the
mechanical energy flux is the same for all stars, the boundary line
for transition layers would be expected to coincide with a line of g
= const in the H-R diagram. The numerial value for this g depends on
the dissipation length. A comparison with the observed boundary line
shows that the dissipation length is not the same for all stars but
increases roughly as (Teff/g) exp 0.93.
Title: The Effects of Metallicity on Stellar Winds
Authors: Bohm-Vitense, Erika
Bibcode: 1986iue..prop.2417B
Altcode:
A fundamental prediction of line driven wind theory is that the mass
loss rate of hot stars should scale with the metallicity. Initial
studies with the IUE, however, have found that the mass loss
rates in the metal deficient Magellanic Clouds are similar to
the galactic rates. We propose to test this prediction with low
resolution observations of galactic O and B stars with different
abundances. The target stars are associated with high and low abundance
H II regions. The metallicity range is quite large and the stars are
all expected to show unsaturated P Cygni profiles. A mass loss rate
difference as predicted by radiatively driven wind theory will be
readily detectable if present. If we find no dependence on z then
a serious difficulty with radiatively wind theory will have to be
addressed.
Title: Periodic Light Variations of Alpha-Squared CVN
Authors: Bohm-Vitense, Erika
Bibcode: 1986iue..prop.2405B
Altcode:
Visual observations of the Ap star alpha^2 CVn show a steeper energy
distribution during minimum light than at maximum. A steeper energy
distribution means a higher temperature. The star must therefore show a
region of higher temperature during minimum light, which is possible
only if we see a smaller area at this phase than during maximum
light. We have made model calculations for the light variations of a
nonspherically symmetric star, which shows qualitative agreement with
the observations in the uv and in the visual. We would like to check
and improve our model calculations by detailed comparison with high
and low resolution IUE spectra which will supplement existing optical
spectra. We want to supplement existing IUE spectra to get complete
phase coverage.
Title: Age Dependence of the Boundary Line for Chromospheric Emission
in the HR Diagram
Authors: Bohm-Vitense, Erika
Bibcode: 1986iue..prop.2403B
Altcode:
We want to observe late A stars in the Hyades and Pleiades in order
to see whether the boundary line for the onset of convection changes
for younger stars. This is suggested by the changing appearance of the
B-V gap for different clusters. The extrapolation of observed emission
line fluxes for early F stars shows that the C IV and perhaps the C
II lines are expected to be strong enough to be observable in spite
of the increasing photospheric radiation if indeed the young late A
stars have chromospheres and transition layers like the early F stars.
Title: Blue companions of cepheids.
Authors: Bohm-Vitense, E.; Proffitt, C.
Bibcode: 1985ApJ...296..175B
Altcode:
Twenty-one Cepheids, known or suspected to have blue companions, were
studied with the International Ultraviolet Explorer satellite. For
13 of them, companions were indeed seen, though they were generally
fainter in the UV than expected. For four Population I Cepheids,
the suspected companions were not seen. For none of the Population II
Cepheids could a companion be detected. The effective temperatures and
luminosities of the companions which could be observed are discussed,
and the positions of Cepheids and companions in the T(eff) luminosity
diagrams are compared with positions expected from stellar evolution
calculations.
Title: Theory of the Transition Layer Emission Measures
Authors: Böhm-Vitense, E.
Bibcode: 1985BAAS...17..847B
Altcode:
No abstract at ADS
Title: Cepheid distances from blue main-sequence companions.
Authors: Bohm-Vitense, E.
Bibcode: 1985ApJ...296..169B
Altcode:
The author determines the absolute visual magnitudes of main-sequence
Cepheid companions from their effective temperatures. These are
obtained by comparing the measured relative energy distributions with
model atmosphere energy distributions. Assuming an average galactic
extinction law, it is found that the distance moduli for the Cepheids
should be smaller by Δ(mv-Mv) = -0.5 as compared
to the Sandage-Tammann relation.
Title: Detection of a compact companion of the mild barium star
ksi1 Ceti.
Authors: Bohm-Vitense, E.; Johnson, H. R.
Bibcode: 1985ApJ...293..288B
Altcode:
In the present paper, the observation of a white dwarf companion of the
mild Ba star Xi-1 Ceti (= 65 Ceti = HR 649 = HD 13611) is reported,
taking into account also the properties of the mild Ba star and of
its companion. The UV spectrum of Xi-1 Ceti is discussed along with an
interpretation of this spectrum. Attention is given to the effective
temperature of the companion, the absorption bands in the spectrum,
the radius and mass of the Xi-1 Ceti companion, and questions regarding
the obscuration of the companion by the atmosphere of the Ba star. It is
found that the overall energy distribution of the Xi-1 Ceti companion
can best be matched with a 14,000 K DA white dwarf of log g = 8 or
less. However, the absolute intensity is too small and would require
a radius too small and a mass too large for such a gravity.
Title: The unexpected ultraviolet variability of Herbig-Haro object 1.
Authors: Brugel, E. W.; Bohm, K. H.; Shull, J. M.; Bohm-Vitense, E.
Bibcode: 1985ApJ...292L..75B
Altcode:
The line fluxes of the C IV 1550 A and semiforbidden C III 1909 A
emission lines in HH 1 are noted to have monotonically decreased by
factors of 4-6 between 1979 and 1983, despite a lack of indications
of optical range changes. The thickness of the shocked layers and
preshock density can be estimated on the basis of these rapid changes,
with results suggesting a clumpy medium that leads to truncated shock
waves with column densities insufficient for the development of complete
recombination zones.
Title: Ultraviolet studies of stars in the populous cluster NGC 2100
in the Large Magellanic Cloud.
Authors: Bohm-Vitense, E.; Hodge, P.; Proffitt, C.
Bibcode: 1985ApJ...292..130B
Altcode:
The authors have studied the ultraviolet energy distributions of the B
stars in the LMC cluster NGC 2100. The ultraviolet extinction law was
studied for the LMC cluster. The Teff-luminosity diagrams
were determined. The brightest stars in the clusters are evolved stars
on the "horizontal" supergiant branch. No P Cygni profiles indicating
mass loss were recognized for these supergiants.
Title: Cepheid masses and cepheid binaries.
Authors: Böhm-Vitense, E.
Bibcode: 1985BAAS...17..559B
Altcode:
No abstract at ADS
Title: Cepheid Masses and Cepheid Binaries
Authors: Böhm-Vitense, E.
Bibcode: 1985BAAS...17Q.559B
Altcode: 1985BAAS...17Z.559B
No abstract at ADS
Title: Ultraviolet Observations of the System Containing the Cepheid
SU Cyg
Authors: Evans, N. R.; Bohm-Vitense, E.; Bolton, C. T.
Bibcode: 1985BAAS...17..559E
Altcode: 1985BAAS...17R.559R
No abstract at ADS
Title: W Vir
Authors: Bohm-Vitense, Erika
Bibcode: 1985iue..prop.2295B
Altcode:
No abstract at ADS
Title: The UV extinction laws in two very young clusters, NGC 6530
in the Galaxy and NGC 2100 in the LMC.
Authors: Bohm-Vitense, E.; Hodge, P.; Boggs, D.
Bibcode: 1984NASCP2349..187B
Altcode: 1984IUE84......187B; 1984fiue.rept..187B
The UV extinction for a number of O and B stars in the galactic
cluster NGC 6530 was studied along with the extinction law for several
stars in the LMC cluster NGC 2100. Distinct differences were found
for different stars in the same cluster. In NGC 6530 a correlation
of the extinction law with TEFF or with position in the
cluster was observed. In NGC 2100 the 2200 A absorption appears to be
much stronger in the center of the cluster. The interstellar gas line
absorption for the stars in NGC 6530 was also studied. No correlation
with the variations in the UV extinction law was found so far.
Title: Blue companions of Cepheids.
Authors: Bohm-Vitense, E.; Proffitt, C.
Bibcode: 1984NASCP2349..344B
Altcode: 1984fiue.rept..344B; 1984IUE84......344B
Nineteen Cepheids, known or suspected to have blue companions, have
been observed. For 11 of them, blue companions were indeed seen,
though many of them were fainter in the UV than suspected. For four
Population I Cepheids the suspected companions were not seen. For
none of the Population II Cepheids could a companion be detected. For
the observed companions the authors have determined Teff,
luminosities and masses from their position in the HR diagram.
Title: The ultraviolet spectra of the O and B stars in the young
galactic cluster NGC 6530.
Authors: Boehm-Vitense, E.; Hodge, P.; Boggs, D.
Bibcode: 1984ApJ...287..825B
Altcode:
The UV spectra between 1200 and 3000 A of stars in the young galactic
cluster NGC 6530 and the surrounding association are studied. From
the UBV colors and empirical as well as theoretical calibrations,
the T(eff) and L for those stars which follow a sequence in the H-R
diagram corresponding to the main sequence are determined. From a
comparison with theoretical evolutionary tracks, the age of the cluster
is estimated to be 5 + or - 2 x 10 to the 6th yr, with a very small
scatter for the different stars. The UV extinction is determined for
the stars from a comparison of theoretical model energy distributions
for the stellar T(eff)s and the observed energy distributions. The
stellar wind lines are studied, and strong stellar winds are found
for bolometric magnetidues less than -8.
Title: Ultraviolet observations of Population II Cepheids.
Authors: Bohm-Vitense, E.; Proffitt, C.; Wallerstein, G.
Bibcode: 1984NASCP2349..348B
Altcode: 1984IUE84......348B; 1984fiue.rept..348B
The two Population II Cepheids, ST Pup and W Vir, have nearly the same
length of period and B-V colors, yet spectral types are very different,
indicating large differences in metal abundances. The authors have
determined metal abundances, Teff and the color excess from
the observed discontinuities at 1700 Å, at 2600 Å, the ultraviolet to
visual colors, and the 2400 Å absorption band. The observations for
ST Pup were made shortly after maximum light. For ST Pup the authors
found E(B-V) = 0.2, Teff = 6600±100K at 0.5 days after
maximum. A metal abundance of [A/H] = -1.7±0.2 was determined. For
W Vir a metal abundance of [A/H] ≡ -0.9 is suggested.
Title: The white dwarf companion of the mild Ba star
ξ1Cet.
Authors: Bohm-Vitense, E.; Proffitt, C.; Johnson, H.
Bibcode: 1984NASCP2349..293B
Altcode: 1984IUE84......293B; 1984fiue.rept..293B
The mild Ba star ξ1Cet was found to have a hot
companion. The absolute intensities and the relative energy distribution
shows that it is a DA white dwarf with broad absorption bands around
1400 and 1650 Å. The temperature is determined to be Teff
= 13000±1000K.
Title: Ultraviolet analysis of the peculiar F supergiant HD 112374.
Authors: Bohm-Vitense, E.; Proffitt, C.
Bibcode: 1984NASCP2349..352B
Altcode: 1984IUE84......352B; 1984fiue.rept..352B
The authors have studied the ultraviolet energy distribution of the
metal-poor supergiant HD 112347. They need a temperature Teff
= 5500±100K, log g = -0.3±0.3 and a metal deficiency of log Z/Z_sun;
= -0.7 in order to find agreement between theoretical and observed
ultraviolet energy distributions with a reddening of E(B-V) ≤ 0.1
consistent with its galactic latitude of +36°.
Title: Ultraviolet studies of the young populous cluster NGC 2100
in the LMC.
Authors: Bohm-Vitense, E.; Hodge, P.; Proffitt, C.
Bibcode: 1984NASCP2349..191B
Altcode: 1984IUE84......191B; 1984fiue.rept..191B
Stars in the populous young cluster NGC 2100 were observed with IUE
in the low resolution mode. Color excesses, effective temperatures
and luminosities were determined for eight stars in the cluster. A
comparison of observed and model atmosphere energy distributions
shows that the Nandy et al. (1981) average LMC extinction curve does
not give good matches for the cluster stars. For most of the cluster
stars no hump in the extinction at 2200 A is observed and too little
flux is observed between 1400 and 2000 A. Masses between 15 and 30
solar masses were found for the stars and an age of about 6 million
years. Two stars, b27 and C1, may have an age of up to 12 million years.
Title: The changing ultraviolet spectrum of Herbig-Haro object No. 1.
Authors: Bohm, K. H.; Bohm-Vitense, E.; Brugel, E. W.
Bibcode: 1984NASCP2349..167B
Altcode: 1984IUE84......167B; 1984fiue.rept..167B
IUE spectra of H-H1 taken in March 1982 and December 1983 show
steady qualitative changes of the short wavelength emission line
spectrum. Earlier IUE studies (in 1979 and 1980) had shown the typical
"high excitation object" ultraviolet spectrum with the C IV 1550
and the C III] 1909 lines being very strong. These line fluxes have
steadily decreased and are presently not detectable on a 4-1/2 hour
exposure. While the spectrum looked very similar to that of H-H2
in 1980, it now gives more the impression of the spectrum of a low
excitation H-H object with a possible presence of fluorescent lines
from the H2 Lyman bands.
Title: Stellar winds in the young galactic cluster NGC 6530.
Authors: Bohm-Vitense, E.; Hodge, P.
Bibcode: 1984NASCP2349..223B
Altcode: 1984fiue.rept..223B; 1984IUE84......223B
The authors have studied line profiles for O stars on or close to
the main sequence in the young galactic cluster NGC 6530. P Cygni
profiles are seen for Teff > 36000K. Different stars show,
however, different lines and different outflow velocities. The degree
of ionization in the wind appears to depend on Teff.
Title: Ultraviolet analysis of the peculiar supergiant HD 112374 =
HR 4912.
Authors: Bohm-Vitense, E.; Proffitt, C.
Bibcode: 1984PASP...96..897B
Altcode:
The ultraviolet energy distribution of the metal-poor supergiant
HD 112374 is analyzed based on observations from the International
Ultraviolet Explorer (IUE) satellite for the region between 1200
and 2000 A. A discontinuity was found in the UV spectra at 2600 A
which confirmed the low-abundance of heavy elements found by Luck et
al. (1983). Values for effective temperature and log g in HD112374
were consistent with the star being a very luminous Population II
semi-regular variable. The full observational results are presented
in a table.
Title: Ultraviolet studies of O and B stars in the LMC cluster NGC
2100, the SMC cluster NGC 330 and the Galactic cluster NGC 6530
Authors: Boehm-Vitense, E.; Hodge, P.
Bibcode: 1984IAUS..108...59B
Altcode:
High-resolution and low-resolution IUE spectra of O and B stars in the
LMC cluster NGC 2100, the SMC cluster NGC 330, and the young Galactic
cluster NGC 6530 are investigated. Temperatures and luminosities are
determined. In the LMC and SMC clusters, the most luminous stars are
evolved stars on the horizontal supergiant branch, while in NGC 6530
the stars are all still on the main sequence. Extinction laws were
determined. They confirm the known differences between LMC and Galactic
extinctions. No mass loss was detected for the evolved B stars in the
LMC and SMC clusters, while the high-luminosity stars in NGC 6530 show
P Cygni profiles.
Title: G and Early K Giant UV Continua and Early Emission Line
Intensities
Authors: Bohm-Vitense, Erika
Bibcode: 1984iue..prop.1766B
Altcode:
The continuum flux around 1950A for cool giants is different for
otherwise apparently similar stars. For the Hyades this flux is
correlated with the chromospheric emission line intensities. We want to
study this correlation for other giants. We want to study the systematic
changes in emission line and continuum flux along the G to K giant
sequence in order to understand the abrupt changes in continuum flux,
CIV emission and v(r) sin i observed to occur at spectral types G5III.
Title: Ultraviolet Spectra of Gamma Boo Stars
Authors: Bohm-Vitense, Erika
Bibcode: 1984iue..prop.1761B
Altcode:
Lambda Boo stars show abnormally weak metallic lines for their spectral
type, if this is determined from the ratio of the CaII K to hydrogen
line strength, yet they are no population II stars as shown by their low
space velocities and by their apparently large rotational velocities
which are around 100 km/sec. (Baschek and Searle,.1969). Spectrum
analysis shows a metal deficiency except that OI has normal abundances
and CI seems to be actually overabundant (Saschek et al 1983). The
UV spectra suggest that another light source is contributing to the
continuous light. It appears that the strong CI lines seen in the Lambda
Boo uv spectrum may be formed in a shell of carbon rich material. We
want to observe additional lambda Boo stars in order to see whether this
interpretation fits all lambda Boo stars. We would like to re-observe
lambda Boo in order to see whether time variations in the CI lines
can be observed, which would support the interpretation as envelope
lines. We want to study in detail the binary ADS 3910 which contains
one A Boo star and one apparently normal B8V star (Sargent, 1966).
Title: Interstellar Gas and Dust Absorptions Near NGC 6530
Authors: Bohm-Vitense, Erika
Bibcode: 1984iue..prop.1772B
Altcode:
In our study of the extinction law for 0 and early B stars in the
young galactic clusters NGC 6530 we found a rather strong dependence of
extinction on T(eff) (see Fig. 1). Since the highest temperature stars
are also farthest away from the center of the cluster (see Fig. 2)
this may also mean that the extinction law changes with distance from
the cluster center. We want to study which is the true correlation
by studying stars with the same T(eff) but at different distances
from the Cluster center. We also want to study the interstellar gas,
which is concentrated near the Cluster in order to see whether the gas
properties change in a systematic way in accordance with the observed
changes in the extinction law.
Title: Variations of POP. II Cepheid UV Energy Distributions
Authors: Bohm-Vitense, Erika
Bibcode: 1984iue..prop.1768B
Altcode:
We want to observe the changes of the ultraviolet energy distribution
of the population II Cepheid kappa Pav and the suspected population II
Cepheid SZ Tau. The observations of the continuum energy distributions
will give us log Z/Z(sun) and the change in Teff as a function of
pulsational phase. We would also like to follow the properties of
the outward moving shock by observing the changes in the emission
lines. For the previously observed population II Cepheids ST Pup and
W Vir, we saw emission lines develop shortly after maximum light. No
more observing time was available to follow the changes. For the
bright Cepheids kappa Pav and SZ Tau we will be able to follow the
evolution of these emission lines. The ratio of the SiIII] to CIII]
lines at 1892 and 1909A (both observed in ST Pup and W Vir) will give
us the change in densities for the outward moving shock. Perhaps we
will be able to determine rough values for the velocities and in that
way probe the outer atmospheres of the Cepheids.
Title: Ap Stars with Short-Period Optical Variability
Authors: Bohm-Vitense, Erika
Bibcode: 1984iue..prop.1935B
Altcode:
No abstract at ADS
Title: Search for White Dwarf Companions of BA Stars Evolving up
the Giant Branch
Authors: Bohm-Vitense, Erika
Bibcode: 1984iue..prop.1775B
Altcode:
We want to look for a correlation between temperatures of white dwarf
companions of Ba stars and evolutionary state of the Ba stars. We expect
to find generally cooler white dwarfs for cooler (i.e., further evolved)
Ba stars. Only for the brightest (i.e., generally closest) Ba stars do
we have a chance to see those cool white dwarf companions. Only mild Ba
stars can be found bright enough. We therefore want to look for white
dwarf companions in bright mild Ba stars of advancing spectral types.
Title: Dynamical Masses for Population I and Population II Cepheids
Authors: Bohm-Vitense, Erika
Bibcode: 1984iue..prop.1764B
Altcode:
We would like to obtain additional LWR 1) high resolution spectra of
Pop I Cepheid binaries in order to determine the orbital velocities of
the companions. (Actually, the velocity difference between Cepheid and
companion). The velocities of the Cepheids are known from ground-based
observation. The ratio of the velocities and thereby the mass ratio of
the stars can then be determined. The spectral type of the companion
is determined from the measured SWP spectrum energy distribution and
yields the mass of the companion. The mass of the Cepheid can thereby
be found. We can thereby check pulsational and evolutionary masses. We
would like to extend our observations to the peculiar Pop II binary
Cepheid AU Peg, whose orbital elements have recently been determined.
Title: The problem of the barium stars.
Authors: Bohm-Vitense, E.; Nemec, J.; Proffitt, C.
Bibcode: 1984ApJ...278..726B
Altcode:
The authors report on ultraviolet observations of barium stars
and other cool stars with peculiar element abundances. Those
observations attempted to find hot white dwarf companions. Among
six real barium stars studied, only ζ Cap was found to have a white
dwarf companion. Among seven mild, or marginal, barium stars studied,
at least three were found to have hot subluminous companions. It is
likely that all of them have white dwarf companions.
Title: The ultraviolet spectrum of a strongly reddened,
high-excitation Herbig-Haro object.
Authors: Bohm, K. H.; Bohm-Vitense, E.
Bibcode: 1984ApJ...277..216B
Altcode:
Short- and long-wavelength IUE spectra of the rather strongly reddened,
high radial velocity Herbig-Haro object HH 32A have been obtained. The
object is well known for its large positive radial velocity and for
its unusually large velocity dispersion. In consequence of the large
ultraviolet extinction, the spectra are faint. The semiforbidden C III
1909 and Mg II 2800 emission lines are, however, clearly detected, and
their fluxes can be measured with reasonable accuracy. C IV 1550 and
Si III 1892 are very probably present. The continuum can be detected
convincingly in the wavelength range 1500-1900 A. In other regions it
is just barely detectable. The continuum has an energy distribution
which is probably very similar to that of HH 1 and HH 2H. If an average
galactic extinction curve is used, the line fluxes for semiforbidden C
III 1909 and C IV 1550 indicate a shock velocity of at least 140 km/s
which is higher than for HH 1 or HH 2. If a Theta Ori-type extinction
curve is used instead, ultraviolet line ratios are obtained which are
rather similar to those in HH 1 and HH 2.
Title: Chromospheres, Transition Regions, and Coronas
Authors: Boehm-Vitense, E.
Bibcode: 1984Sci...223..777B
Altcode:
The increase in temperature outward from the surface of a stellar
photosphere can be understood by looking at the local energy
balance. The relatively high-density stellar photosphere is cooled
effectively by radiative energy loss penetrating the optically thin
corona. For the low-density chromosphere and corona, if the energy
input cannot be balanced by radiative energy losses, the temperature
will rise steeply, possibly up to 1 million degrees or more. Coronal
heating and emission appear to be strongly influenced by magnetic
fields, leading to large differences in x-ray emission for otherwise
similar stars. Comparatively small variations are seen in the overall
chromospheric emission of stars. Chromospheres are probably mainly
heated by shock-wave energy dissipation, modified by magnetic fields.
Title: On the Origin of the Barium Stars
Authors: Bohm-Vitense, E.
Bibcode: 1984IAUS..105..191B
Altcode:
The author shows that Ba stars cannot originate from single stars, they
can only be formed in binaries by mass transfer. The companion must then
have been an evolved star with log L/L_sun; ≥ 3.2, requiring a radius
larger than 0.5 AU, explaining the long periods of the observed Ba star
binaries. White dwarf companions with Teff ≈ 12000K have
been seen for the nearest Ba stars. For Ba stars at larger distances
companions can only be detected if they have Teff ≥ 20000K.
Title: IUE observations of weak G-band stars.
Authors: Parthasarathy, M.; Sneden, C.; Bohm-Vitense, E.
Bibcode: 1984PASP...96...44P
Altcode:
High- and low-resolution ultraviolet spectra of several weak G-band
giants have been obtained with the IUE satellite, to derive Be
abundances and to search for degenerate companions. The abundances of
Be in these stars are about log epsilon (Be) approximately -0.5. These
values are in good agreement with the Be abundances of Hyades giants,
and are consistent with standard theories of post-main-sequence Be
depletion in stars. However, the Be abundances do not fit with the large
Li abundances in weak G-band stars. Post-main-sequence production of
Li or element segregation in main-sequence stars may explain the high
Li abundance in weak G-band stars. No direct evidence is seen for the
presence of hot subluminous companions. The peculiar abundances in
these stars probably are not due to binary mass transfer.
Title: The Puzzle of the UV Continua of the Hyades Giants
Authors: Bohm-Vitense, Erika
Bibcode: 1984LNP...193..273B
Altcode: 1984csss....3..273B
No abstract at ADS
Title: Cepheid Companions and the Masses of Cepheids
Authors: Bohm-Vitense, E.; Borutzki, S.; Harris, H.
Bibcode: 1984IAUS..105..449B
Altcode:
The authors have observed in the ultraviolet the hot companions of the
Cepheids SV Per, RW Cam, SY Nor and KN Cen. The study of the absolute
and relative intensities reveals that all, except the companion for
KN Cen are evolved stars which should fit on almost the same mass
track as the Cepheid. The authors find however that with generally
accepted reddening values the companions of at least SV Per and RW
Cam are too faint. Either the Cepheid loops are more luminous than
presently calculated or the reddening is larger than presently accepted.
Title: Search for white dwarf companions of cool stars with peculiar
element abundances
Authors: Boehm-Vitense, E.
Bibcode: 1984STIN...8511923B
Altcode:
A search for a white dwarf companions of cool stars with peculiar
element abundances was undertaken. One additional star the xi Cet,
was found with a white dwarf companion. It was found that HR 1016,
56Uma, 16 Ser, have high excitation emission lines which indicate a
high temperature object in the system. It is suggested that since these
indications for high temperature companions were seen for all nearby
Ba stars, it is highly probable that all Ba stars have white dwarf
companions, and that the peculiar element abundances seen in the Ba
stars are due to mass transfer. Observations, arguments and conclusions
are presented. White dwarf companions were not found. Together with
the Li and Be abundances and the chromospheric emission line spectra
in these stars were studied. No white dwarf companions were seen for
subgiant CH stars.
Title: Book-Review - Be-Stars
Authors: Jaschek, M.; Groth, H. G.; Bohm-Vitense, E.
Bibcode: 1984ApL....24..125J
Altcode:
No abstract at ADS
Title: EINSTEIN observations of three classical cepheids.
Authors: Bohm-Vitense, E.; Parsons, S. B.
Bibcode: 1983ApJ...266..171B
Altcode:
The authors have looked for X-ray emission from the classical Cepheids
δ Cep, β Dor, and ζ Gem during phases when the later two stars
show emission in low excitation chromospheric lines. No X-ray flux
was detected except possibly from ζ Gem at phase 0.26. Derived upper
limits are in line with emission flux or upper limits obtained for
other F and G supergiants.
Title: An attempt to determine stellar Lyman-Alpha emission-line
fluxes for F stars with different metal abundances.
Authors: Bohm-Vitense, E.; Woods, J.
Bibcode: 1983ApJ...265..331B
Altcode:
We have studied stellar Lyα fluxes for F stars of different metal
abundances which are presumably older the more metal deficient they
are. We describe the correction procedure for the geocoronal emission
and our estimate of the correction for interstellar absorption. We find
stronger than average Lyα emission for the Hyades stars and for the two
close binaries HR 1354 and α Tri. For single non-Hyades stars we
find increasing Lyα emission for decreasing metal abundances. Assuming
that the total hydrogen emission, including H and H - continua and
Lyman lines, is twice the amount of the Lyα emission, and that the
metallic emission, including Mg II h and k and Ca II H and K lines,
is 4 times the Mg II k emission, we find that the total chromospheric
energy loss, and therefore also energy input, is nearly independent
of metal abundance and therefore presumably independent of age, for
ages larger than a few times 109 years.
Title: White Dwarf Companions and Chromospheres of Stars with Peculiar
Element Abundances
Authors: Bohm-Vitense, Erika
Bibcode: 1983iue..prop.1408B
Altcode:
During our studies of BaII and mild Ba stars we found 3 hot
companions. An additional one was found by Schindler, et al. With
these 4 cases we can start to do some preliminary statistics in order
to guide us with further observations. We find that the Ba stars With
companions have all been classified as luminosity class II or higher. We
also find that 3 of the 4 have late G spectral types while most of
the Ba stars have K types. Also, 3 of the 4 are mild Ba stars. They
are, however, not the same 3 stars. We would like to check whether
these correlations persist in further studies. Several mild Ba stars
are fairly bright. For these we may have a good chance to see the
companions. We would therefore like to observe preferentially mild
Ba stars with G spectral classification and luminosity class 11 or
higher. If the frequency of companions for these stars is indeed high,
we may find enough to study the relation between companion temperatures
and the properties of the peculiar abundance stars.
Title: Chromospheric Emission of Close Binaries
Authors: Bohm-Vitense, Erika
Bibcode: 1983iue..prop.1400B
Altcode:
It is generally believed that rapid rotation of stars enhances
chromospheric emission. Studies of G stars have led to this
conclusion. Our studies of Mg II k emission in early F stars only
shows that close binaries show increased emission while single rapidly
rotating stars with v(r) sin i > 40 km/sec do not. For G stars
close binaries are usually the only ones that rotate at a measurable
rate. Rapid rotation and binary nature are correlated. For early F
stars this is not the case. We therefore propose to study early F star
binaries of known periods and different rotational velocities in order
to decide on the cause of enhanced emission: either tidal effects in
close binaries or rapid rotation.
Title: Lambda Bootis Stars
Authors: Bohm-Vitense, Erika
Bibcode: 1983iue..prop.1586B
Altcode:
No abstract at ADS
Title: Ultraviolet radiation from the environment of the
Cohen-Schwartz star.
Authors: Bohm, K. H.; Bohm-Vitense, E.
Bibcode: 1982ApJ...263L..35B
Altcode:
No abstract at ADS
Title: The ultraviolet continuous and emission line spectra of the
Herbig-Haro objects HH 2 and HH 1.
Authors: Boehm-Vitense, E.; Cardelli, J. A.; Nemec, J. M.; Boehm, K. H.
Bibcode: 1982ApJ...262..224B
Altcode:
Recent studies of the continuous spectrum of Herbig-Haro (HH) objects
at optical and near-infrared wavelengths and the observation of
continuous radiation in the ultraviolet have shown an unexpectedly
steep increase of the flux toward shorter wavelengths. The present
investigation provides the results of ultraviolet observations of HH
2. The obtained data are compared with the HH 1 data. It is found that
HH 2 has an ultraviolet continuous and emission-line spectrum which
is similar to that of HH 1. The UV line spectrum of HH 2H indicates
an even somewhat larger ionization than does the HH 1 spectrum. As
in HH1, the UV emission-line spectrum shows a much higher degree of
ionization than that derived from the optical spectrum. Consequently,
the same difficulty arises as in the case of HH 1. The complete UV
plus optical spectrum cannot be explained by a single plane-parallel
shock-wave model.
Title: The interstellar absorption-line spectrum of mu. Oph.
Authors: Cardelli, J.; Boehm-Vitense, E.
Bibcode: 1982ApJ...262..213C
Altcode:
UV interstellar lines have been measured on high-resolution, long-
and short-wavelength IUE spectra of the B8 V star Mu Oph. Column
densities for the observed atoms and ions have been determined as well
as turbulent velocities. The interstellar spectrum of Mu Oph is similar
to the ones for Rho Oph and Zeta Oph. The ionization equilibria of
several elements give consistent limits for the electron density. The
C I line arising from different fine-structure levels are studied
to yield estimates on the physical conditions in the cloud. Relative
depletion of elements in the cloud seen in the interstellar spectrum
of Mu Oph follows the same pattern as seen in the interstellar spectra
of Zeta Oph and six other stars in the Rho Oph cloud complex.
Title: Suggested UV spectral classification criteria for A stars
Authors: Boehm-Vitense, E.
Bibcode: 1982ESASP.182..121B
Altcode: 1982uvsc.conf..121B
Criteria for determining effective temperature and electron
density independently of abundances, for A, late B, and F stars are
given. Comparison of IUE data with standard classification techniques,
shows that for late B and A stars, temperature classification by UV
spectra is more sensitive than classification using visual spectra. The
long wavelength wing of the Lyman is recommended for these stars. For
A and F stars, the ratio of the Mg (2580 A) and Mg+ (2800A) resonance
lines gives a good temperature/electron density (ne) criterion which
is nearly independent of metal abundance (Z). The SiI discontinuities
give an excellent temperature classification for A and F stars, but it
is dependent on Z and ne. If the absolute intensities of either of the
Mg lines are added to the criteria, three classification criteria for
a three dimensional classification, according to temperature, ne and Z,
are obtained.
Title: The MG II resonance lime emission at 2800 angstrom in stars
with different rotational velocities and different metal abundances.
Authors: Boehm-Vitense, E.
Bibcode: 1982ApJ...258..628B
Altcode:
In many investigations, a study of the Mg II 2800 A emission was
conducted in order to derive more information about the correlation of
chromospheric emission with the properties of the convection zone. It
has been suggested by Ayres and Linsky (1980) that there is a positive
correlation between rotation and chromospheric emission at least
for binary stars. The present investigation is concerned with such a
correlation between rotation and chromospheric emission. Since large
differences in rotational velocities are observed for F stars for which
also strong chromospheric emission is observable, this particular study
deals mainly with F stars. On the basis of observations of the Mg II
emission in F and G stars, it is found that the emission is abnormally
strong in close binary F stars. This may be explained by the generation
of a large differential rotation due to tidal effects which may give
very efficient dynamo generation of a surface magnetic field.
Title: Effective temperatures of A and F stars.
Authors: Boehm-Vitense, E.
Bibcode: 1982ApJ...255..191B
Altcode:
Effective temperatures of late A and early F stars are determined from
the observed fluxes in the visual at 1900 A and 1420 A. The observed
ratios are compared with those calculated by Kurucz (1979). A correction
of the theoretical fluxes at 1900 A brings the effective temperatures
obtained from different ratios into reasonable agreement. The effective
temperatures determined in this way for late A stars agree well with
those obtained from the optical region. For F stars, however, the
effective temperatures obtained from the UV are found to be higher
than those obtained from the optical region if radiative equilibrium
models are used for the comparison. It is thought that this discrepancy
may derive from the effects of temperature, pressure, and absorption
coefficient inhomogeneities caused by convection.
Title: Mg II k emission lines in stars with different rotational
velocities and metal abundances.
Authors: Böhm-Vitense, E.
Bibcode: 1982SAOSR.392B..71B
Altcode: 1982csss....2...71B
No abstract at ADS
Title: Chromospheric, transition layer and X-ray emission for stars
with different rotational velocities.
Authors: Boehm-Vitense, E.
Bibcode: 1982NASCP2238..247B
Altcode: 1982auva.nasa..247B; 1982NASCP2338..247B; 1982IUE82......247B
In agreement with previous findings for the MgII k line emission in F
stars an increase of Lya and transition layer emission with increasing
Vr sin i, if vr sin i greater than 30 km/sec. was
not found. For Vr sin i 30 km/sec., the measured line
intensities are consistent with an increase in emission with increasing
Vr sin i. Such a relation between emission and rotation for
single stars is also in agreement with X-ray observations. For the young
F stars in the Hyades we find generally enhanced emission independently
of rotational velocities. The enhancement is most pronounced for low
excitation lines.
Title: Binary F Stars
Authors: Bohm-Vitense, Erika
Bibcode: 1982iue..prop.1252B
Altcode:
No abstract at ADS
Title: X-Ray and Ultraviolet Observations of Classical Cepheids
Authors: Bohm-Vitense, E.; Parsons, S. B.; Schmidt, E. G.
Bibcode: 1982pccv.conf..238B
Altcode:
No abstract at ADS
Title: Ultraviolet spectra of Herbig-Haro objects and of the
environment of the Cohen-Schwartz star.
Authors: Boehm, K. H.; Boehm-Vitense, E.; Cardelli, J. A.
Bibcode: 1982NASCP2238..223B
Altcode: 1982IUE82......223B; 1982auva.nasa..223B; 1982NASCP2338..223B
Observations in both spectral regions were used in order to determine
the continuous energy distribution and the emission line fluxes for
H-H 2. The continuous spectrum is similar to that in H-H 1 and F lambda
increases rapidly towards shorter wavelengths. This statement is found
to be qualitatively correct for all obvious choices of the ultraviolet
extinction curve if we use the E(B-V) value determined by the use of
the S II method. The origin of the continuum remains enigmatic. The
emission line spectrum of H-H 2 shows an even somewhat higher degree
of ionization than the spectrum of H-H 1, indicating an even larger
discrepancy between ionization information from optical data and
that contained in the UV spectra. The immediate environment of the
Cohen-Schwartz star emits a continuous spectrum similar to that of a
H-H-object but increasing even more steeply towards shorter wavelengths.
Title: Search for & Study of Hot Companions of Cepheids
Authors: Bohm-Vitense, Erika
Bibcode: 1982iue..prop.1058B
Altcode:
Studies of the track of cepheid colors in the two color diagram during
their pulsational cycle reveals that some cepheids show an additional
blue light source, which is presumably due to a blue companion roughly
two or three magnitudes fainter than the cepheid in the visual. During
one exploratory shift we have taken low resolution short wavelength
IUE spectra of 5 suspected binaries. Four of them did indeed show
the spectrum of a hot companion. We suggest to take well exposed
spectra of these companions in order to determine spectral type and
luminosities. We also want to search for hot companions in other
suspected cepheid binaries.
Title: Hr 8752
Authors: Bohm-Vitense, Erika
Bibcode: 1982iue..prop.1230B
Altcode:
No abstract at ADS
Title: Chromospheres & White Dwarf Companions of Peculiar
Abundance Stars
Authors: Bohm-Vitense, Erika
Bibcode: 1982iue..prop.1062B
Altcode:
It is proposed to continue to observe chromospheres and transition
region emission of stars with peculiar element abundances. At the same
time we want to continue the search for white dwarf companions of these
stars. Since the studies of McClure et al. 1980 it is suspected that
many if not all of the peculiar element abundance stars have white dwarf
companions and that mass exchange has led to the exposure of the deeper
layers of the stars in which peculiar abundances have been generated
by nuclear reactions. This would especially explain the large nitrogen
abundance and may explain the large Li abundance seen in weak G band
stars, and the large C abundance seen in CH stars. While cool white
dwarf companions may be too faint to be visible an active chromosphere
may still indicate their presence. A study of the relative emission
line intensities will confirm the peculiar element abundances and
exclude peculiar surface temperature stratifications as a possible
explanation for anomalous molecular line intensities.
Title: Observation of Ultraviolet Light VAriations of POP II Cepheids
Authors: Bohm-Vitense, Erika
Bibcode: 1982iue..prop.1055B
Altcode:
We want to study the changes of the ultraviolet energy distributions
in Pop II cepheids, and see whether emission lines during maximum
light can be detected. The gradient of the energy distributions for
longer wavelengths (lambda >1900A) together with studies of the
discontinuities at 2600A and at 1700A (for the brighter stars) will
permit to determine the metal abundances and the effective temperatures
as a function of phase. For k Pav we will in connection with radial
velocity measurements be able to determine radii. Comparision with
stellar evolution calculations will yield masses for these stars. A
comparison of spectra for W Vir and ST Pup both with periods of 17^d
but vastly different spectral types will be very interesting. The
metal abundances are expected to be very different.
Title: Chromospheric, transition layer and coronal emission of metal
deficient stars.
Authors: Boehm-Vitense, E.
Bibcode: 1982NASCP2238..231B
Altcode: 1982auva.nasa..231B; 1982NASCP2338..231B; 1982IUE82......231B
It is shown that while MgII k line emission decreases for metal
deficient stars, the Ly alpha emission increases. The sum of
chromospheric hydrogen and metallic emission appears to be independent
of metal abundances. The total chromospheric energy loss is estimated
to be 0.0004 Fbol. The chromospheric energy input does not
seem to decrease for increasing age. The transition layer emission
is reduced for metal deficient stars, but it is not known whether
the reduction is larger than can be explained by curve of growth
effects only. Coronal X-ray emission was measured for 4 metal deficient
stars. Within a 12 limit it could still be consistent with the emission
of solar abundance stars.
Title: The ultraviolet spectrum of Herbig-Haro object 1.
Authors: Boehm, K. H.; Bohm-Vitense, E.; Brugel, E. W.
Bibcode: 1981ApJ...245L.113B
Altcode:
Continuum and emission line fluxes for HH 1 in the range of 1200-1300
A have been determined using IUE spectra, and it is found that the
luminosity of HH 1 in this wavelength interval is slightly larger than
1 solar luminosity, or 20 times greater than for the wavelength range
3200-11,000 A. If averaged over intervals of 100 A, it is shown that the
UV continuum rises almost monotonically toward shorter wavelengths over
the entire interval 1300-3000 A. The UV continuum energy distribution
cannot be explained by a T Tauri stellar continuum that is scattered
by dust. Emission line identifications and flux measurements strongly
confirm the statement that the high-ionization emission lines are too
strong to be explained by shock-wave models derived from the emission
line spectrum in the interval 3700-11,000 A. A possible explanation
for this fact is discussed.
Title: Interstellar absorption in the MgII resonance line K2 and
H2 emissions.
Authors: Boehm-Vitense, E.
Bibcode: 1981ApJ...244..504B
Altcode:
High-resolution (0.2 A) IUE spectra for the long wavelength range
(1800-3000 A) have been studied. It is shown that narrow interstellar
Mg II lines are seen in the center of the k2 and h2 emissions from
nearby stars with large rotational velocities. For all observed stars,
the radial velocity of the central k3 absorption component in the
rest system of the star is strongly correlated with the mirror image
of the radial velocity of the stars; this shows that a major fraction
if not all of the k3 absorption is due to interstellar absorption in
the solar neighborhood. The violet to red asymmetry of the k2 emission
also correlates with the radial velocities of the star; this shows that
the shift of k3 is due to the velocity shift of the local interstellar
cloud with respect to the star.
Title: The ultraviolet energy distributions of late A stars.
Authors: Boehm-Vitense, E.
Bibcode: 1981ApJ...244..938B
Altcode:
Observed late A star energy distributions for the wavelength range
1400-2500 A are compared. No difference is found between energy
distributions of Am stars and those of normal slowly rotating A
stars. The fluxes of rapidly rotating stars, however, appear to be
increased for wavelengths smaller than 1530 A; this cannot be understood
as an effect of pole heating or reduced gravity. In addition, the
comparison of the UV energy distributions with model atmosphere energy
distributions of Kurucz indicates some problems with the theoretical
Si I absorption edges at 1530 A.
Title: Far-ultraviolet energy distributions of the metal-poor A
stars HD 109995 and HD 161817.
Authors: Boehm-Vitense, E.
Bibcode: 1981ApJ...243..213B
Altcode:
Low-resolution IUE spectra at wavelengths between 1300 and 3400 A of
the metal-poor stars HD 109995 (A1p) and HD 161817 (A4p) have been
compared with model-atmosphere energy distributions computed by Kurucz
(1979). Good overall agreement is found. Effective temperatures,
metal abundances, and angular diameters could be determined. Assuming
an absolute visual magnitude of 0.7, the previously determined gravity
log = 3 yields masses of 0.5 solar masses for both stars. It is found
that the theoretical UBV colors calculated earlier agree reaonably
well with the ones observed for these stars.
Title: The chromospheric and transition layer emission of stars with
different metal abundances.
Authors: Boehm-Vitense, E.
Bibcode: 1981NASCP2171..303B
Altcode: 1981uviu.nasa..303B; 1981NASCP3171..303B; 1980IUE80......303B
Preliminary results on observations of chromospheric and transition
layer emission of stars with different metal abundances are
reported. Metal deficient stars generally show reduced emission in
the Mg II resonance lines and also in the other chromospheric and
transition layer emission lines. This is interpreted as showing that
energy fluxes other than acoustic fluxes must at least be coresponsible
for the coronal and transition layer heating.
Title: University of Washington, Seattle, Washington 98195. Report.
Authors: Böhm-Vitense, E.
Bibcode: 1981BAAS...13..470B
Altcode:
No abstract at ADS
Title: The effective temperature scale
Authors: Boehm-Vitense, E.
Bibcode: 1981ARA&A..19..295B
Altcode:
It is found that the agreement between the radiative equilibrium model
atmosphere Teff (B-V) relation and observed direct determinations
is generally rather good, which indicates that it may be possible to
understand the structure of most atmospheres reasonably well. There are
still some problems in the main-sequence O and F stars and in the M star
regions. The red giants and supergiants also pose some problems. It is
not known whether Rayleigh scattering alone can explain the red colors
for giants around 3600 K; model atmosphere colors for such stars have
not been published. The metal-poor giants seem to be well represented
by the model calculations; for the metal-poor F stars there are still
some uncertainties.
Title: Outlook for ultraviolet astronomy.
Authors: Boehm-Vitense, E.
Bibcode: 1981NASCP2171....3B
Altcode: 1981NASCP3171....3B; 1980IUE80........3B; 1981uviu.nasa....3B
A brief overview of galactic and extragalactic research is given
with emphasis on the problems of temperature determination, chemical
abundance determination, and the question about the energy sources
for the high temperature regions. Stellar astronomy, stellar winds,
and the interstellar medium are among the topics covered.
Title: The chromospheric Mg II k2 emission in stars with
different rotational velocities and different metal abundances.
Authors: Böhm-Vitense, E.
Bibcode: 1981PASP...93Q.545B
Altcode:
No abstract at ADS
Title: Cepheid Variables
Authors: Bohm-Vitense, Erika
Bibcode: 1981iue..prop..870B
Altcode:
No abstract at ADS
Title: The white dwarf companion of the Ba II star zeta Cap.
Authors: Boehm-Vitense, E.
Bibcode: 1981NASCP2171..455B
Altcode: 1980IUE80......455B; 1981uviu.nasa..455B; 1981NASCP3171..455B
The Ba II star zeta Cap has a white dwarf companion. Its T
eff is determined to be 22000 K, its mass is approximately
one solar mass. The importance of this finding for the explanation of
abundance peculiarities is discussed.
Title: Search for White Dwarf Companions of Stars with Peculiar
Process & CNO Abundances
Authors: Bohm-Vitense, Erika
Bibcode: 1981iue..prop..722B
Altcode:
We would like to continue our search for white dwarf companions of
Ba, CH strong, CH weak and CN strong stars. The Ba II star zeta Cap
was observed to have a white dwarf companion. The weak Ba II star
zeta Cyg appears to have a somewhat cooler WD companion. HD 65699
(BA II) and HR 2392 (Ba II) also show indications of companions but
additional long exposures have to be taken to verify this. If mass
exchange between binaries is responsible for the visibility of the
excess s process elements we may speculate that mass exchange might
also be responsible for the CH and CN peculiarities. We would also
like to obtain and study a high resolution short wavelength spectrum
of the Barium star zeta Cap which was observed to have a hot white
dwarf companion, The emission line profiles will probably tell us the
origin of these strong emission lines and give us information about
circumstellar material which is probably present as judged from the
strong central reversals in the Mg II emission lines. At the same time
the chromospheric and transition layer emissions of these peculiar
abundance stars will be studied. Peculiar CNO abundances might be
verified by their chromospheric and transition layer line intensities
as compared to normal stars
Title: A comparison of the MG resonance lines in AM and non AM stars
of similar temperatures.
Authors: Boehm-Vitense, E.
Bibcode: 1980A&A....92..219B
Altcode:
A comparison of the Mg II resonance lines in Am and none Am stars
of similar effective temperatures does not show any measurable
differences. The Mg I resonance lines may be weaker in the Am stars. The
mechanism reducing the Ca II K line intensities in Am stars does not
work for the Mg II k and h lines.
Title: The white dwarf companion of the barium star ZET Cap.
Authors: Boehm-Vitense, E.
Bibcode: 1980ApJ...239L..79B
Altcode:
It is shown that the barium star Zeta Cap has a white dwarf companion
with a mass of approximately 1.0 solar masses and an effective
temperature of 23,000 K. Strong emission lines and very deep reversals
in the Mg II emission line cores indicate circumstellar gas which
may be due to mass transfer in the system. The Ba star HD 65699 also
shows the very deep central reversals in very strong Mg II emission
cores. The weak spectra of the barium stars Zeta Cyg, HD 65699, and HD
116713 probably show excessive UV radiation also, but further studies
are necessary.
Title: The boundary line in the H-R diagram for stellar chromospheres
and the theory of convection.
Authors: Boehm-Vitense, E.; Dettmann, T.
Bibcode: 1980ApJ...236..560B
Altcode:
Ultraviolet emission-line spectra of A, F, and early G stars have been
observed with the International Ultraviolet Explorer. For supergiants,
classical chromospheric and transition-layer emission is seen only on
the red side of the Cepheid instability strip. For luminosity classes
III-V, chromospheric emission can be detected for spectral types F2
and later. For none of the A stars was normal chromospheric emission
detected, regardless of their rotational velocities or peculiarities
(i.e., Am or Ap).
Title: Dependence of Stellar Chromospheres on Metal Abundance
Rotation, and Age
Authors: Bohm-Vitense, Erika
Bibcode: 1980iue..prop..412B
Altcode:
It is proposed to study the stellar chromospheric emission and its
dependence on metal abundances, rotation and age. If the emission is
enhanced by rapid rotation of the stars it will indicate that acoustic
heating will be unimportant for the formation of chromospheres and
coronae since the acoustic flux generation should not depend on
rotation. We want to study variability in the chromospheric emission
of some supergiants. We also want to determine luminosity differences
between normal stars and metallic line stars. These measurements
will also provide bolometric corrections for these stars. For stars
with known angular diameters effective temperatures can be determined
directly. This study will show whether different surface metal abundance
is the only major difference between metallic line A stars and normal
A stars as suggested by the diffusion hypothesis.
Title: Summary - Stellar Turbulence
Authors: Bohm-Vitense, E.
Bibcode: 1980LNP...114..300B
Altcode: 1980IAUCo..51..300B; 1980sttu.coll..300B
No abstract at ADS
Title: Detection of a white dwarf in a visual binary system
Authors: Boehm-Vitense, Erika
Bibcode: 1980STIN...9311075B
Altcode:
The F6 giant HD 160365 was detected to have a white dwarf companion
about 8 arcsec south of the star. The UV energy distribution observed
with International Ultraviolet Explorer (IUE) shows that the white
dwarf has an effective temperature of 23,000 +/- 2,000 K. If log g =
8 the Ly(alpha) profile indicates an effective temperature around
24,500 K. Using the theoretical models, one finds a visual magnitude
of mv is approximately 16.5. For Teff = 24,500 K
one expects for a white dwarf a luminosity of log L/solar luminosity is
approximately -1.3 and MV is approximately 10.67. This gives
a distance modulus for the system of mv - MV =
5.83 and an absolute magnitude Mv = 0.3 for the giant.
Title: The influence of the helium abundance on the appearance of
late-type spectra.
Authors: Boehm-Vitense, E.
Bibcode: 1979ApJ...234..521B
Altcode:
The paper studies the dependence of atomic, ionic, and molecular lines
on the helium abundance in stars in which the continuous absorption
is due to H(-) and the electrons are supplied by the metals, which
are assumed to be mainly ionized. These conditions will approximately
apply to subgiants and red giants. Possible applications to red giant
studies in Omega Cen, M22, 47 Tuc, NGC 2420, and Draco are discussed.
Title: On the energy distribution in Sirius B.
Authors: Boehm-Vitense, E.; Dettmann, T.; Kapranidis, S.
Bibcode: 1979ApJ...232L.189B
Altcode:
A spectrum of Sirius B in the wavelength region 1100-1900 A taken
with the IUE satellite shows an absolute flux and spectral energy
distribution in the continuum in agreement with theoretical white-dwarf
models with T(eff) of 26,000 K plus or minus 1000 K for log g equals
8.65 and R equals 5.08 x 10 to the 8th cm. The Lyman-alpha profile is
also in agreement with these parameters. No obvious emission lines in
the spectrum of Sirius B or A which could indicate the presence of a
chromosphere or corona in either of the stars are observed.
Title: Is "Sirius C" the X-ray Source?
Authors: Böhm-Vitense, E.
Bibcode: 1979BAAS...11..465B
Altcode:
No abstract at ADS
Title: University of Washington, Astronomy Department, Seattle,
Washington 98195. Report.
Authors: Böhm-Vitense, E.
Bibcode: 1979BAAS...11..372B
Altcode:
No abstract at ADS
Title: Classical Stellar Chromospheres and Dependence on Teff,
and Chemical Abundances
Authors: Bohm-Vitense, Erika
Bibcode: 1979iue..prop..171B
Altcode:
I would like to continue to observe stars which appear in the HR diagram
close to the dividing line between the convective and non-convective
stars. Since convection is supposed to be a necessary condition for
the formation of chromospheres we should be able to determine the
boundary line for convection from the boundary line for the existence
of classical chromospheres. I would like to continue to investigate
whether the phenomenon of metallic line stars can in any way be related
to the onset of convection and thereby to the presence or absence
of chromospheres. We also want to observe a few manganese stars in
order to check if any effects of the surface convection zone due to He
ionization could be detected in the UV spectrum of Mn stars. I would
like to extend the study of the onset of convection to the region
of the He rich hot white dwarfs for which convection is expected
for temperatures up to 30,000 K. Of special interest would be the
observation of some dwarf novae. I would like to study chromospheres
and transition layers of Pop II stars, red giants and subdwarfs and
compare those to Pop I stars. Knowing the chromospheric spectra of
field Pop II stars, I would like to analyse the integrated spectra of
globular clusters and their center to limb variation. One might hope to
be able to determine the population and perhaps the break off of the
lower main sequence which appears to depend on the metal abundance of
the cluster. The study of the globular clusters will also reveal the
presence or absence of hot blue objects and their distribution in the
cluster. If the lambda 2200 absorption could be detected this may give
some information about the presence of interstellar dust and gas in
the clusters, which is expected to be there but has not yet been found.
Title: Observing with IUE.
Authors: Böhm-Vitense, E.
Bibcode: 1979Mercu...8...29B
Altcode:
No abstract at ADS
Title: The Energy Distribution of Sirius B
Authors: Boehm-Vitense, E.
Bibcode: 1979wdvd.coll..114B
Altcode: 1979IAUCo..53..114B
The origin of the X-ray emission observed from the DA white dwarf Sirius
B is investigated on the basis of low-resolution short wavelength IUE
spectrum. Following a correction for the influence of scattered light
from Sirius A, the intensity distribution of Sirius B is obtained
as a function of wavelength. Comparison of the continuous energy
distribution with model atmosphere energy distributions indicates an
effective temperature of 25,500 K, which is not high enough to explain
the observed X rays as normal photospheric radiation. In addition,
with the exception of an unusual feature in the neighborhood of the C I
1657 line, no clear evidence is found of emission lines indicative of
a corona, chromosphere or disk, and the origin of the X rays remains
unknown.
Title: Spectral Classification from a Theoretical Point of View
Authors: Bohm-Vitense, E.
Bibcode: 1979RA......9..309B
Altcode: 1979IAUCo..47..309B
Conditions are studied under which a two-dimensional spectral
classification according to effective temperature and electron density
is possible and when three- or four-dimensional spectral classification
is necessary because the classification criteria are unavoidably
influenced by the metal and helium abundances. For late-type stars,
where molecular absorption becomes important for the continuum, a seven-
or higher-dimensional classification may be necessary. At this point
spectral classification may not be useful any more. It is found in
general that a derivation of the gravitational acceleration from the
spectroscopically determined electron density is possible only if the
helium abundance is known.
Title: Scanner observations of Am and Ap stars.
Authors: Böhm-Vitense, E.; Johnson, P.
Bibcode: 1978ApJ...225..514B
Altcode:
We have rescanned the three metallic4ine stars 15 Vul, T UMa, and 60
Leo and find that we cannot reproduce the scanner results of Baschek
and Oke for these stars. We conclude that these Am stars are variable
on a time scale longer than weeks, possibly decades. The variations
cannot be explained by a variable back-warming effect. Since all three
Am stars which we rescanned are variable, we suspect that most Am stars
may be variable on similar time scales. Subject headings: convection -
spectrophotometry - stars: metallic-line - stars: peculiar A - stars:
variables
Title: Energy distribution in main-sequence A and F stars.
Authors: Boehm-Vitense, E.
Bibcode: 1978ApJ...223..509B
Altcode:
Scanner observations of A and F main-sequence stars are examined in
order to check whether the observed continuum energy distributions
match scaled Bilderberg models and whether some influence of rotation
on convection can be detected. The continuum energy distributions of
A and F field stars and of stars in different clusters with different
rotational velocities are compared; the clusters include the Hyades,
Praesepe, Coma, Alpha Persei, and the Pleiades. A comparison with
radiative-equilibrium model-atmosphere computations reveals some UV
flux reduction for many stars with B-V greater than 0.14 and a strong
UV flux reduction for all stars with B-V greater than 0.22, regardless
of rotational velocity. The energy distributions are found to match
the scaled Bilderberg models, and this is attributed to a reduced
temperature gradient in convectively unstable layers. It is shown that
rotation does not impede convection, but even seems to enhance it,
contrary to expectations.
Title: Ultraviolet Observations of A and F Stars
Authors: Bohm Vitense, Erika
Bibcode: 1978iue..prop....6B
Altcode:
There are two main points of interest for which the IUE would be very
helpful (1) line identification in A Stars. High resolution spectra
of Vega / Sirius and --Cyg would permit the line identification. (2)
Termination of Hydrogen Convection. The observation of the effects of
stellar chromospheres may give us information about the termination
point of the hydrogen convection.
Title: Scanner observations of main-sequence A and F stars.
Authors: Böhm-Vitense, E.; Johnson, P.
Bibcode: 1977ApJS...35..461B
Altcode:
In order to understand the observed UB V colors of main-sequence
A and F stars, and especially to understand the B - V gap observed
for field stars and some clusters, we have made scanner observations
of main-sequence stars with 0.20 < B - V < 0.45 in different
clusters and with different Vr sin i. The results are given in Tables
2-7. Subject heading: spectrophotometry
Title: Observations bearing on the theory of stellar convection. II
Authors: Boehm-Vitense, E.
Bibcode: 1977LNP....71...63B
Altcode: 1977stco.coll...63B; 1977IAUCo..38...63B; 1977psc..conf...63B
Difficulties in determining the ratio of mixing length to pressure
scale height from observations of stars and from stellar evolutionary
tracks are discussed. An attempt is made to show that information
about convection can sometimes be obtained from direct and indirect
measurements of velocity fields as well as from observations of
temperature inhomogeneities. The effect of convective energy transport
on the continuous energy distribution of F stars is considered. Scanner
observations of various field and cluster stars are examined which
apparently indicate that convection generally becomes efficient for
B-V of at least 0.22, while fast rotation causes an earlier onset of
efficient convection.
Title: Weshalb sind magnetische Sterne magnetisch?
Authors: Böhm-Vitense, E.
Bibcode: 1977Umsch..77..355B
Altcode:
No abstract at ADS
Title: Neue Beobachtungsergebnisse für Am-Sterne
Authors: Böhm-Vitense, E.; Johnson, P.
Bibcode: 1977MitAG..42..137B
Altcode:
No abstract at ADS
Title: About the Proper Choice of the Characteristic Length in the
Convection Theory
Authors: Bohm-Vitense, Erika; Nelson, George Driver
Bibcode: 1976ApJ...210..741B
Altcode:
It is known that convection sets in rather abruptly for stars with Teff
< Tb, where Tb depends on the luminosity. The boundary line Tb(L)
goes through the H-R diagram from late A or early F main-sequence stars
to late F or early G supergiants. In the mixing length approach to the
theory of convection the exact theoretical position of this boundary
temperature depends on the assumptions about the size of the mixing
length 1. We make the assumption that the red boundary of the Cepheid
instability strip agrees with the line Tb(L). If we also assume 1 = n
pressure scale height H, with n being the same constant for all stars,
we find 1 H to give the correct Tb(L). If we assume the region of large
instability to determine 1, the relation 1 R2 also gives approximately
the correct Tb(L). We do not think, however, that the latter relation
has any physical significance. Subject headings: convection - stars:
Cepheids - stars: chromosphere - stars: interiors
Title: On the Interpretation of the AM Phenomenon
Authors: Böhm-Vitense, E.
Bibcode: 1976paps.coll..633B
Altcode: 1976IAUCo..32..633B
No abstract at ADS
Title: Model stellar atmospheres and heavy element abundances.
Authors: Boehm-Vitense, E.
Bibcode: 1975psae.book...21B
Altcode:
Stars whose atmospheric structure depends in an observable way on the
heavy element abundances are considered. Such stars have effective
temperatures which do not exceed 8,000 K. The stellar temperature
stratification is examined and a description of the gas and electron
pressures is presented. Aspects concerning the energy distribution in
the continuum are explored, taking into account the Balmer discontinuity
and the ultraviolet continuum. The line absorption is discussed along
with the UBV colors, the temperature calibrations, the bolometric
correction, and questions related to convection and metal abundances.
Title: The Gap in the Two-Color Diagram of Main-Sequence Stars
Authors: Bohm-Vitense, E.; Canterna, R.
Bibcode: 1974ApJ...194..629B
Altcode:
in the two-color diagrams of main-sequence stars a gap of A(B - V) 0.07
is expected at B - V 0.25 due to the abrupt onset of convection. This
gap is observed for field stars. it is also indicated for many clusters;
however, it appears at different B - V and is less pronounced the more
it is shifted to the red. Since in these clusters the stars on the blue
side of the gap show large rotation, we interpret the shift as showing
that large rotational velocities shift the onset of convection to lower
Te . The abrupt decrease of the rotational velocities occurs at still
lower Teff when convective energy transport becomes important in the
helium ionization zones. Subject headings: convection - interiors,
stellar - open clusters - rotation, stellar
Title: Masses and luminosities of population II cepheids.
Authors: Böhm-Vitense, E.; Szkody, P.; Wallerstein, G.; Iben, I., Jr.
Bibcode: 1974ApJ...194..125B
Altcode:
Mean luminosities, effective temperatures, and radii of 11 Population
II Cepheids are derived from three color photometry and, in some
cases, radial velocities. From a comparison of the observed and the
theoretical period-radius relation masses of 0.55 I 0.05 are derived
for all Population II Cepheids except the variables No. 1 and 2 in
M13, which appear to be less massive and possibly helium-rich. The
period-luminosity relation is also discussed. Subject headings: Cepheids
and W Virginis stars - globular clusters - luminosities - mass loss
Title: UBVr Colors for Population II Giants
Authors: Bohm-Vitense, Erika; Szkody, Paula
Bibcode: 1974ApJ...193..607B
Altcode:
The UBVr colors for intermediate Population II giants with metal
abundance Z = 1/10 of the solar metal abundance Z0 are computed from
radiative equilibrium models. Line blanketing and Rayleigh scattering
are taken into account. The colors are given for models with 4000 <
Tgtf < 8000 K and 4.6 < L/L0 < 1800. The relation between B -
V and Tgff is determined for different luminosities. Graphs for Tgff
and U - B as functions of B - V for different luminosities and metal
abundances are given and compared. It is shown that the U - B excess is
not a simple function of the metal abundance alone but depends on B - V
and L also. A simple method is described by which luminosity, effective
temperature, and metal abundance can be determined simultaneously
from B - V, U - B, and Mv. Subject headings: abundances, stellar -
photometry - Population II stars
Title: The luminosities of population II cepheids.
Authors: Böhm-Vitense, E.
Bibcode: 1974ApJ...188..571B
Altcode:
A method is described for the distance determination of pulsating
stars from measured colors and radial velocities. it follows the line
of thought given by Baade and Wesselink. This method does not require
any calibration by conventional distance determinations once the depth
variation of the expansion velocities in the pulsating stars is known
from theory. A period-luminosity relation for Population ii stars with
periods between 17.3 and 28.5 days is derived from the stars W Vir, TW
Cap, and star No. 42 in MS. Subject headings: Cepheids and W Virginis
stars - luminosities
Title: On the Excitation of β Cephei Pulsation.
Authors: Bohm-Vitense, E.
Bibcode: 1973BAAS....5..427B
Altcode:
No abstract at ADS
Title: The interpretation of the two-color and color-magnitude
diagrams of M15 and M92.
Authors: Böhm-Vitense, E.; Szkody, P.
Bibcode: 1973ApJ...184..211B
Altcode:
From the theoretical relation between U - B, B - Vand Teff, g for
Population II stars and the observed color-magnitude diagrams, we
determine the Teff and L values for the red giants and horizontal-branch
stars in M15 and M92. Several possibilities for the stellar masses, the
metal abundances, and distance moduli are considered. The "empirical"
(luminosity, )-diagrams are compared with theoretical evolutionary
tracks. The comparison suggests that Z(M15) < Z(M92), the age t(M15)
> t(M92), and the masses M(M15) < M(M92), while possibly Y(M15)
> Y(M92). The average values found for both clusters are log Z = -4 +
0.3, Y = 0.25 + 0.03, t10 = 1.36 t 0.05, MIM0 = 0.85 i 0.05. Subject
headings: abundances, stellar - globular clusters - luminosities -
RR Lyrae stars
Title: The UBVr colors of extreme Pop II giants.
Authors: Böhm-Vitense, E.
Bibcode: 1973A&A....24..447B
Altcode:
Summary. The UB Colors of Pop II giants with metal abundances 1/100 and
1/1000 times the solar metal abundances are computed. For these stars
the Rayleigh scattering is extremely important for the U fluxes. The
mean intensity j, the source function for the Rayleigh scattering,
has been determined, considering not only the continuous absorption
of hydrogen and the metals but also the line absorption. The UBYr
colors are given in the range 42000 Teff 77000 and 4.6 < L/L0 <
1800. From this the relation between B - V and Teff is determined for
different luminosities and also the ultraviolet excess, which for a
given B - V depends on the gravity, g, as well as on the metal abundance
Z Key words: colors - Population II giants - Rayleigh scattering -
source function
Title: Line Effects on the Radiative Acceleration in Supergiant Stars
Authors: Bohm-Vitense, E.
Bibcode: 1973ApJ...181..379B
Altcode:
The radiative acceleration g is studied for supergiants with Teff 8000
K. Nongray effects, including line absorption, raise g by a factor
of at least 6-in comparison with the gray case-at the surface of the
stars. The luminosity limit for stable stars set by g will probably
be lowered by this factor in comparison with earlier estimates
based on electron scattering only. No instability is found for red
supergiants. Subject headings: atmospheres, stellar - late-type stars -
luminous stars - mass loss - opacities
Title: The theoretical relation between UBV colors and Teff
and luminosity for extreme pop. II giants.
Authors: Boehm-Vitense, E.
Bibcode: 1973BAAS....5..266B
Altcode:
No abstract at ADS
Title: The UBVr Colors of Supergiants
Authors: Bohm-Vitense, E.
Bibcode: 1972A&A....17..335B
Altcode:
The atmospheres of supergiants are studied in the range 40000<
T65< 9 000 . The influence of radiation pressure and spherical
effects are discussed. The observed colors of supergiants are obtained
with radiative equilibrium and hydrostatic models. A calibration of
B- V in terms of T1ff is given. The Tgff obtained here for G and
K supergiants are about 10% lower than adopted so far. Key words:
supergiants - radiation pressure - sphericity - microturbulence -
effective temperatures
Title: Surface Temperatures and the Curves of Growth for Population
I and Population II Stars
Authors: Bohm-Vitense, E.
Bibcode: 1972A&A....16...81B
Altcode:
The dependence of the curve of growth on metal abundance is studied
by comparing the solar curve of growth with that of a star with the
same effective temperature and gravity but a metal content reduced
by a factor 100 and also with that of a giant with reduced metal
abundance. The T(r) turned out to be essentially the same for the models
with the same T , therefore the curves of growth (computed for Fe 1
and Fe H and Doppler broadening only) are also the same for the stars
with the same and g but different metal abundances. A theoretical
discussion of this result is given. Key words: curves of growth -
metal abundance - surface temperature
Title: Convection and Metal Abundance
Authors: Bohm-Vitense, E.
Bibcode: 1971A&A....14..390B
Altcode:
The influence of the metal abundance Z on the structure of the
convection zone is studied. The velocities in the convective zones
decrease and the depths of the convection zones increase with decreasing
metal abundance. For lower metal content the transition from radiative
to convective atmospheres therefore occurs at higher effective
temperatures. Key words: convection - microturbulence - metal abundance
Title: The UBVr colors of main sequence stars.
Authors: Böhm-Vitense, E.
Bibcode: 1970A&A.....8..283B
Altcode:
The UBVr colors of main sequence stars with spectral types between A
0 and K 0 are computed into account theoretical line blanketing. The
observed colors require turbulent velocities between 2 and 4 km/s
provided the solar microturbulence is 2 km/s as assumed here. For
late A and F stars good agreement with observations is obtained if
scaled solar "Bilderberg" models are used, showing that convection
in these stars is more effective than indicated by the local mixing
length theory. The scarcity of stars with 0.2 < B - V <0.4 can
be undsrsod if convection sets in abruptly for T 8Ooo . The relation
between B - V and T is derived for B - V 0.90. Key words: colors -
convection - F stars - microturbulence - Sun
Title: Convection in A and F stars and metallicism.
Authors: Böhm-Vitense, E.
Bibcode: 1970A&A.....8..299B
Altcode:
Theoretical spectra of radiative and convective A and F stars with
a given colour B - V are compared. The Hv lines are essentially the
same. The Ca U K line is weakened in the convective star. Temperature
inhomogeneities further weaken the K line. The possibility is discussed
that the metallic line stars may be the convective version of late A
stars. Key words: F stars - convection - temperature inhomogeneities -
line profiles - metallic line stars
Title: The UBV Colors of A, F, and G Main-Sequence Stars
Authors: Böhm-Vitense, Erika
Bibcode: 1970BAAS....2R.297B
Altcode:
No abstract at ADS
Title: Stellar Analysis by Means of the Hydrogen Lines
Authors: Böhm-Vitense, E.
Bibcode: 1970saac.book..107B
Altcode:
No abstract at ADS
Title: A simple approximation forJ v (σ
v ,κ v ,τ v ,B v ),
and its application for temperature-correction procedures
Authors: Böhm-Vitense, Erika
Bibcode: 1969Ap&SS...4..233B
Altcode:
For the case of isotropic coherent scattering plus absorption a simple
expression is given (Equation 12) to compute the mean intensity of the
radiationJ v (as a function of optical depth τ v
) if the scattering coefficient σ v , the absorption
coefficientK v and the Planck functionB v
are given as a function of depth. In general the accuracy of this
approximation is of the order of a few percent. A fairly simple
temperature-correction procedure for the case when scattering is
important is described.
Title: The Pulsar NP 0532 and the Implications of the Oblique-Rotator
Hypothesis
Authors: Böhm-Vitense, Erika
Bibcode: 1969ApJ...156L.131B
Altcode:
The light curve of the pulsar NP 0532 can be reproduced with the
oblique-rotator hypothesis, pro- vided that the light emission is
strongly concentrated at two opposite spots on the star and is confined
to a narrow cone
Title: Line statistics for solar type stars.
Authors: Böhm-Vitense, E.
Bibcode: 1969JQSRT...9.1167B
Altcode:
No abstract at ADS
Title: 36 et 12 Session mixte (Joint Meeting)
Authors: Böhm-Vitense, E.
Bibcode: 1968IAUTB..13..198B
Altcode:
No abstract at ADS
Title: On the Atmospheres of Helium Stars
Authors: Böhm-Vitense, Erika
Bibcode: 1967ApJ...150..483B
Altcode:
The appearance of the spectra of helium stars as a function of hydrogen
abundance is investigated theoretically for dwarfs (log g = 4 5)
and giants (log g = 2) with effective temperatures Teff = 7350 , 9900
`and 129000 K For normal C, N, and 0 abundances, the He and the He I
absorptions appear to be the most important absorption mechanisms for
very low hydrogen abundances. H becomes weakened for hydrogen abundances
less than 0 85 X 10-2 by numbers of atoms The metal line intensities
increase. The colors of extreme helium stars are somewhat redder than
for normal stars The helium convection zone becomes important for the
stars with Teff > 99000 K.
Title: ON the Absolute Magnitude and Effective Temperature of the
Hydrogen-Poor Star HD 30353
Authors: Danziger, I. J.; Wallerstein, George; Böhm-Vitense, Erika
Bibcode: 1967ApJ...150..239D
Altcode:
Interstellar polarization and reddening data for stars near HD 30353
were used to obtain a reddening excess E(B - V) = 0.35 and an absolute
visual magnitude of -3.2. An effective temperature of 100000 K for HD
30353 was obtained by fitting photoelectric scans of the continuum to
fluxes obtained from the model atmosphere of a hydrogen-poor star.
Title: The variations of stellar magnetic fields and the oblique
rotator hypothesis
Authors: Böhm-Vitense, E.
Bibcode: 1967maco.conf..179B
Altcode:
No abstract at ADS
Title: Der "Crossover" Effekt bei α CVn und HD 98088
Authors: Böhm-Vitense, E.
Bibcode: 1967ZA.....67....1B
Altcode:
No abstract at ADS
Title: The peculiar a stars and the oblique rotator hypothesis
Authors: Böhm-Vitense, E.
Bibcode: 1967mamt.book...97B
Altcode:
No abstract at ADS
Title: Outer Convection Zones in Stars of Different Spectral Types
and Luminosities
Authors: Böhm-Vitense, E.
Bibcode: 1966IAUTB..12..547B
Altcode:
No abstract at ADS
Title: Zur Interpretation der Geschwindigkeits- und
Intensitätsvariationen der Linien von α2C Vn
Authors: Böhm-Vitense, E.
Bibcode: 1966ZA.....64..326B
Altcode:
No abstract at ADS
Title: A Differential Equation for the Solution of the Non-LTE Line
Transfer Problem
Authors: Böhm-Vitense, E.
Bibcode: 1965SAOSR.174..165B
Altcode:
No abstract at ADS
Title: An Integral Equation for the Temperature Correction in a
Nongray Atmosphere
Authors: Böhm-Vitense, E.
Bibcode: 1964SAOSR.167...99B
Altcode:
No abstract at ADS
Title: Introductory Remarks
Authors: Böhm-Vitense, E.
Bibcode: 1964SAOSR.167....3B
Altcode:
No abstract at ADS
Title: The Spectrum of γ Pegasi Between 3300 and 3030 Å.
Authors: Böhm-Vitense, Erika; Pyper, Diane M.; Wallerstein, George
Bibcode: 1964ApJ...140..807B
Altcode:
No abstract at ADS
Title: The Correlation Between Metal Deficiency of Stars and Their
Distance from the Galactic Plane.
Authors: Böhm-Vitense, Erika; Holweger, H.; Kohl, K.
Bibcode: 1963ApJ...138..604B
Altcode:
No abstract at ADS
Title: On the Beat Phenomenon in β Cephei Stars
Authors: Böhm-Vitense, Erika
Bibcode: 1963PASP...75..154B
Altcode:
No abstract at ADS
Title: Berechnung der Temperaturschichtung nichtgrauer Atmosphären
im Strahlungsgleichgewicht. Mit 3 Textabbildungen
Authors: Böhm-Vitense, E.
Bibcode: 1963ZA.....57..241B
Altcode:
No abstract at ADS
Title: Interpretation der Phasenbeziehungen zwischen Geschwindigkeits-
und Leuchtkraftkurve bei δ Cephei-Sternen. Mit 2 Textabbildungen
Authors: Böhm-Vitense, E.
Bibcode: 1962ZA.....56...53B
Altcode:
No abstract at ADS
Title: Theory of the hydrogen convection zone
Authors: Böhm-Vitense, E.
Bibcode: 1960IAUS...12..338B
Altcode:
No abstract at ADS
Title: Convection and granulation: Preview on granulation -
Observational studies - Observations
Authors: Böhm-Vitense, E.
Bibcode: 1960IAUS...12..330B
Altcode: 1960IAUS...12Q.....
No abstract at ADS
Title: Zur Interpretation des Intensitätsabfalls am äuβeren
Sonnenrand. Mit 1 Textabbildung
Authors: Böhm, K. -H.; Böhm-Vitense, E.
Bibcode: 1960ZA.....50...69B
Altcode:
No abstract at ADS
Title: Über die Spektren einiger Metalliniensterne. Mit 10
Textabbildungen
Authors: Böhm-Vitense, E.
Bibcode: 1960ZA.....49..243B
Altcode:
No abstract at ADS
Title: Über die Wasserstoffkonvektionszone in Sternen verschiedener
Effektivtemperaturen und Leuchtkräfte. Mit 5 Textabbildungen
Authors: Böhm-Vitense, E.
Bibcode: 1958ZA.....46..108B
Altcode:
No abstract at ADS
Title: Extragalactic Nebulae Close to the Galactic Plane
Authors: Böhm-Vitense, Erika
Bibcode: 1956PASP...68..430B
Altcode:
No abstract at ADS
Title: The Widths of the Lines of Fe, Si, o, and N in the Spectrum
of Gamma Pegasi.
Authors: Böhm-Vitense, Erika; Struve, Otto
Bibcode: 1956ApJ...123..228B
Altcode: 1956ApJ...123..228S
The unblended lines of 011 and N II in Pegasi are systematically
broader than those of Si Ir, Si III, Si Iv, and especially Fe III. This
phenomenon is explained in terms of thermal motions of the atoms. But
there remains a number of abnormally broad lines whose half-widths
cannot be accounted for by the present theory.
Title: Variations in the Spectrum of 89 Herculis
Authors: Böhm-Vitense, Erika
Bibcode: 1956PASP...68...57B
Altcode:
No abstract at ADS
Title: Hydrogen in the Solar Chromosphere and Photosphere
Authors: Böhm-Vitense, Erika
Bibcode: 1955PASP...67...21B
Altcode:
No abstract at ADS
Title: Die Sonnenchromosphäre. Mit 11 Textabbildungen
Authors: Böhm-Vitense, E.
Bibcode: 1955ZA.....36..145B
Altcode:
No abstract at ADS
Title: The Spectrum of Beta Lyrae.
Authors: Böhm-Vitense, Erika
Bibcode: 1954ApJ...120..271B
Altcode:
The B5 absorption component of p Lyrae shows rapid changes in radial
velocity between successive cycles and also during a single cycle
when the exposures are separated by 1 or 2 hours. The absorption line
Fe 114351 yields an average rotational velocity of the order of 4( 50
km/sec. This line appears to be slightly broader during the elongations
than near secondary eclipse. If this difference in rotational broadening
is attributed to the ellipticity of the B8 component, the resulting
ratio of the axes should be somewhat less than the photometric value of
1.33. From the weakening of the equivalent width of the same line at
elongation, as compared to the equivalent width measured at secondary
minimum, a value of approximately 20 per cent is obtained for the
fraction of the light at maximum which is caused by the secondary
component and the gaseous stream.
Title: Über die Temperatur- und Druckschichtung der
Sonnenatmosphäre. Mit 6 Textabbildungen
Authors: Böhm-Vitense, E.
Bibcode: 1954ZA.....34..209B
Altcode:
No abstract at ADS
Title: The Spectrum of Beta Lyrae
Authors: Vitense, Erika
Bibcode: 1953PASP...65..206V
Altcode:
No abstract at ADS
Title: Die Wasserstoffkonvektionszone der Sonne. Mit 11
Textabbildungen
Authors: Vitense, E.
Bibcode: 1953ZA.....32..135V
Altcode:
No abstract at ADS
Title: Der Aufbau von Sternatmosphären : I. kontinuierliche
Absorption und Streuung als Funktion von Druck und Temperatur;
II. über die mittleren Zustandsgrößen und spektralen Eigenschaften
von Sternatmosphären in Abhängigkeit von Effektivtemperatur und
Schwerebeschleunigung Title: Der Aufbau von Sternatmosphären
: I. kontinuierliche Absorption und Streuung als Funktion von
Druck und Temperatur; II. über die mittleren Zustandsgrößen und
spektralen Eigenschaften von Sternatmosphären in Abhängigkeit von
Effektivtemperatur und Schwerebeschleunigung Title: The structure
of stellar atmospheres : I. continuous absorption and scattering
as a function of pressure and temperature; II. On the mean state
variables and spectral properties of stellar atmospheres as a function
of effective temperature and gravity acceleration;
Authors: Böhm-Vitense, Erika Helga Ruth
Bibcode: 1951PhDT........70B
Altcode:
No abstract at ADS
Title: Der Aufbau der Sternatmosphären. IV. Teil. Kontinuierliche
Absorption und Streuung als Funktion von Druck und Temperatur. Mit
16 Textabbildungen
Authors: Vitense, Erika
Bibcode: 1951ZA.....28...81V
Altcode:
No abstract at ADS
Title: Über die mittleren Zustandsgröβen und spektralen
Eigenschaften von Sternatmosphären in Abhängigkeit von
Effektivemperatur und Schwerebeschleunigung. Mit 13 Textabbildungen
Authors: Vitense, E.
Bibcode: 1951ZA.....29...73V
Altcode:
No abstract at ADS