Narrow-band images and spatially-resolved spectroscopy of hydrogen-poor planetary nebulae
M. A. Guerrero, A. Manchado
Instituto de Astrofísica de Canarias, E-38200 La Laguna (Tenerife), Spain
We present new CCD narrow-band images and spatially-resolved intermediate-dispersion spectroscopy of the hydrogen-poor nebulae A 30 and A 58. These data have allowed us to obtain information on the physical properties and chemical abundances of the inner knots and outer envelopes of A 30 and A 58 as a basis for the discussion of their implications in the evolutionary theories. In particular, hydrogen emission from the central knots has been carefully measured and, consequently, the rates of conversion of hydrogen into helium have been established with accuracy.
The rates found indicate that almost all the original hydrogen (between 75% and 95%) has been burnt into helium. The chemical segregation among the knots of A 30 previously suggested by Jacoby & Ford (1983) is confirmed. The A 58 inner knot, which is spatially resolved in two different components, shows evidence of excitation by shocks. Helium abundances and the N/O ratio in the outer envelope of this nebula are very high, therefore indicating that it corresponds to the evolution of an intermediate-mass stars.
An analysis of the chemical abundances, including other objects of the same class, gives some hints that He/H and O/H are anti-correlated. The time elapsed between the formation of the outer envelopes and the ejection of the central knots also seems to be related with the central knots chemical abundances.
Accepted for publication in Astrophysical Journal, Dec. 1, 1996 issue, Vol. 471 Preprints can be obtained by contacting Martín A. Guerrero (mar@ll.iac.es) or via anonymous ftp on suncc.ll.iac.es then cd /pub/A3058
KI emission from envelopes around N-type stars - Spectroscopic observations and interpretations.
Bengt Gustafsson 
, Kjell Eriksson ,
Dan Kiselman 
, Nils Olander 
and Hans Olofsson 
Uppsala Astronomical Observatory, Box 515, S-751 20 Uppsala, Sweden
The Royal Swedish Academy of Sciences, Stockholm Observatory,
S-133 36 Saltsjöbaden, Sweden
Dept. of Physics and Mathematics, Mid Sweden University,
S-851 70 Sundsvall, Sweden
Stockholm Observatory, S-133 36 Saltsjöbaden, Sweden
Circumstellar envelopes around three bright N-type stars, R Scl, X TrA, and V Aql have been detected in emission in resonance lines from KI. This radiation, which is most probably scattered photospheric radiation, was first found spectroscopically, but has later been imaged with coronographic and polarimetric techniques. In the present paper, which is the first in a series, the spectroscopic KI observations are discussed.
>From the observations of the KI 769.9 nm emission we find systemic and expansion velocities in fair agreement with those obtained from the CO millimetre lines. We find a decline of the emission with distance from the star, in rough agreement with the assumption of a constant expansion velocity, mass-loss rate and KI abundance. Our mass loss rate estimates from the KI line observations agree rather well with those obtained from CO (ranging from 1/4 to 1/1 of the CO mass loss), which suggests that a considerable fraction of the potassium stays neutral through the envelope. This puts strong upper limits on the photoionizing chromospheric UV emission from the stars. Some indirect indications that the envelopes have inhomogeneous structures, clumps, are discussed.
Accepted for publication in Astronomy and Astrophysics For preprints, contact kjell.eriksson@astro.uu.se
Circumstellar H 
from SN 1994D and future
Type Ia supernovae: an observational test of progenitor models
Robert J. Cumming ,
Peter Lundqvist ,
Linda J. Smith ,
Max Pettini
and David L. King
Stockholm Observatory, S-133 36 Saltsjöbaden, Sweden
Department of Physics & Astronomy, University College
London, London WC1E 6BT, UK
Royal Greenwich Observatory, Madingley Road, Cambridge
CB3 0EZ, UK
Searching for the presence of circumstellar material is currently the
only direct way to discriminate between the different types of
possible progenitor systems for Type Ia supernovae. We have therefore
looked for narrow H in a high-resolution spectrum of the
normal Type Ia supernova 1994D taken 10 days before maximum and only
6.5 days after explosion. We derive an upper limit of
2.0 
10 
erg s 
cm 
for an unresolved
emission line at the local H II region velocity. To estimate
the limit this puts on wind density, we have made time-dependent
photoionization calculations. Assuming spherical symmetry we find an
upper limit of 
M 
yr
for a wind speed of 10 km s . This limit can exclude only the
highest-mass-loss-rate symbiotic systems as progenitors. We discuss
the effect of asymmetry and assess the relative merits of early
optical, radio and X-ray limits in constraining mass loss from Type Ia
progenitors. We find that X-ray observations can probably provide the
most useful limits on the progenitor mass loss, while high-resolution
optical spectroscopy offers our only chance of actually identifying
circumstellar hydrogen.
Accepted for publication in Monthly Notices of the Royal
Astronomical Society, 1996 July.
Preprints can be obtained by
contacting robert@astro.su.se
or via WWW on
http://www.astro.su.se/ 
robert/Paper/94D.html
or via anonymous ftp on
ftp://www.astro.su.se/pub/supernova/clspk96_94D_MN/clspk96_94D_MN.ps.gz
Spectroscopic investigation of the PPN candidate AICMi = IRAS07331+0021
V.G.Klochkova, V.E.Panchuk
State Research Center Special Astrophysical Observatory, Nizhnij Arkhyz, 357147 Russia
Chemical composition (26 elements) of the cool supergiant AICMi was studied
by the model atmosphere method using echelle spectra taken with the 6m
telescope in 1993-95. Metallicity of the star turns to be by an order of
magnitude lower than solar, the abundance of -process elements
(Na, Mg, Al, Si) is on the average 
, the mean
abundance of heavy s-process elements (La, Ce, Pr, Nd) is 
. Together with the radial velocity determination results this permits
us to classify AICMi with halo population or with population of the old
(thick) disk. The spectra obtained in 1994 and 1995 contain titanium oxide
bands which allowed the outermost AICMi atmosphere layer characteristics to
be estimated. The presence of emission components in hydrogen lines of Balmer
and Paschen series is indicative of mass loss with varying rate.
Accepted for publication in the Bull. Special Astrophysical Observ., vol.41, 1996 Preprints can be obtained by contacting Valentina Klochkova (valenta@alba.sao.ru
The evolved central star of the planetary nebula ESO 166-PN 21
M. Peña1 , M. T. Ruiz , P. Bergeron , S. Torres-Peimbert and S. Heathcote
Instituto de Astronom´ia, UNAM, Apdo. Postal 70 264, México D.F. 04510, México
Depto. de Astronom´ia, Universidad de Chile, Casilla 36-D, Santiago, Chile
Département de Physique, Université de Montréal, C.P. 6128, Succ. Centre-Ville, Montréal, Québec, Canada, H3C 3J7
Cerro Tololo Inter-American Observatory, NOAO, Casilla 603, La Serena, Chile
Optical and UV spectrophotometric data of the nebula and the central
star of the planetary nebula ESO 166-PN 21 are presented. The analysis
of the nebular lines confirms that it is a He- and N-rich PN, with
He/H = 
and N/O = 
. The oxygen abundance
is 12 + log O/H = 
. A distance of 
kpc is
derived for the nebula. The central star is very faint and blue, with an
apparent magnitude 
mag and a dereddened color index
= -0.38 mag. It shows faint wide H and He absorption lines
typical of a DAO star. By modeling the line profiles we derived
T 
K, log g = 
and
log He/H = 
for
the star. The position of the star in a HR diagram compared with
evolutionary tracks indicates a stellar mass of 0.55 M . The
bolometric correction derived from the model atmosphere is -5.6 mag
which, combined with the mass, yields an absolute visual magnitude
M 
= 6.95, a luminosity of 22 L and a distance of
pc,
in good agreement with the nebular distance. Therefore, ESO 166-PN 21
central star is among the hottest and most helium-rich DAO stars and it is
one of the most evolved PN nuclei known, similar to the central stars of
S216 and NGC7293. A kinematical age of 16100 yr is deduced for the
nebula which is lower by about two orders of magnitude than the age of the
central star. The possibility that this object is a member of a close
binary system is suggested.
Accepted by Astronomy and Astrophysics For preprints, contact miriam@astroscu.unam.mx
Infrared Light Curves of Carbon-Rich Variables
Patricia Whitelock
South African Astronomical Observatory, PO Box 9, 7935 Observatory, South Africa
Long-term trends in the infrared, JHKL, light curves of
various carbon variables are described. Some stars show multiple
periodicities, e.g. the semi-regular variables R Scl and GM CMa; others,
particularly the Miras with moderately thick dust-shells, show more erratic
long-term changes. The light curves for R For, which have been intensively
monitored over 20 years, show a pattern which is reminiscent of that seen
for R CrB stars. This pattern is superimposed on regular large-amplitude Mira
pulsations. The multi-periodic and erratic behaviour of these stars is
compared with the predictions from various models.
keywords: Mira variables; Semi-regular variables; mass-loss; dust; infrared.
To appear in: Proceedings of IAU Sym 177 Carbon Stars Ed. R Wing (Kluwer) For preprints, contact paw@saao.ac.za
Long-term UBV(RI) 
monitoring of 12 southern hemisphere
Long Period Variables
P. de Laverny 
, H. Geoffray , L. Jorda and M. Kopp 
Uppsala Astronomical Observatory, Box 515, S-751 20 Uppsala, Sweden.
GRAAL, Université Montpellier II, F-34095 Montpellier Cedex 05,
France.
ESO, Casilla 19001, Santiago 19, Chile.
ARPEGES, Observatoire de Paris, place J. Janssen, 92195 Meudon Principal
Cedex, France.
DAEC, Observatoire de Paris, place J. Janssen, 92195 Meudon Principal
Cedex,France.
A program of UBV(RI) photometric observations of twelve southern
hemisphere Long Period Variables (LPV or Miras) has been carried out
at the European Southern Observatory.
Each star was observed on at least 11 occasions up to 29
over a 4-year period.
A determination of their spectral types from the (V-R) and (R-I)
colours was also performed using the
method described by Celis (1986b). We therefore
present for the first time spectral
type variations together with visible and
colour light-curves for at least three successive cycles.
New photometric parameters as visible and spectral type extrema are
given. Spectral type variations are found in the range 1.3 - 4.5
subclasses.
The variations during a cycle and from cycle-to-cycle are discussed and
short-term declines with large amplitude are reported for R Oct and
RY Hyi.
We finally estimate the distance of these stars using a
( 
,P, Spectral type) relation. It is shown that
rather
good distances (mean error less than 30 %) can be determined
if the period of the stars is known and good UBVRI data are collected
at the maximum of luminosity.
Accepted by Astronomy & Astrophysics Suppl. Ser. Preprints can be obtained by contacting laverny@astro.uu.se
From the tip of the AGB towards a planetary: a hydrodynamical simulation
Detlef Schönberner, Jürgen Stahlberg, Mathias Steffen, Konstantinos Kifonidis, Thomas Blöcker
Astrophysikalisches Institut Potsdam, Potsdam, Germany
Based on the mass-loss description developed by Blöcker (1995, A&A, 297,
727) we present the first attempt to compute the dynamical response of a
stellar wind envelope for the last 
years of the AGB evolution and the
following 5000 years of post-AGB evolution for an initially 3 M
stellar model. For the cool envelope we used a two-component 1D
hydrodynamical code which computes the radiation pressure on the dust grains
and takes the variable coupling between these grains and the gas explicitly
into account. The grains are either carbon or oxygen based and of single,
spherical shape. At the end of the AGB evolution the density and velocity
structure of the wind envelope reflects the previous mass-loss history and
differs considerably from that of stationary outflows. No significant
differences between carbon and oxygen-rich envelopes are found.
The post-AGB phase has been modelled with a newly developed code that
computes the radiation hydrodynamics of interacting winds with fully
time-dependent heating and cooling processes, using our previously gained
final AGB wind envelope as initial structure. The development of
observationally well-known structures of planetaries are the natural
consequences of the temporal changes of the surface and wind
parameters of the now 0.6 M central star. We show in particular
that the often found double-shell structures
are entirely formed by hydrodynamical effects and have nothing to do with the
mass-loss history along the upper AGB, contrary to what is often stated in
the literature.
Invited paper at IAU Symposium 177, ``The Carbon Star Phenomenon'', Antalya, Turkey, May 27-31, 1996 For preprints, contact DeSchoenberner@aip.de
Mid-Infrared Imaging of the Bipolar Nebulae AFGL 618, AFGL 2688, and AFGL 915
Joseph L. Hora , Lynne K. Deutsch , William F. Hoffmann ,
& Giovanni G. Fazio
Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
Astronomy Dept., Boston University, 725 Commonwealth Ave., Boston,
MA 02152, USA
Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
Smithsonian Astrophysical Observatory, 60 Garden St., Cambridge,
MA 02138, USA
We have obtained nearly diffraction-limited images of three bipolar
proto-planetary nebulae: AFGL 618, AFGL 2688 (the ``Egg nebula"), and
AFGL 915 (the ``Red Rectangle"). Images were taken at unidentified
infrared (UIR) emission feature wavelengths and at several continuum
wavelengths in the 10 and 20 
m atmospheric windows. In all three
nebulae the emission is dominated by a central point source. In AFGL 618,
evidence for a slight (0.1 - 0.2 arcsec) extension of the core in the
E-W direction is seen. In AFGL 2688 and AFGL 915, the cores are extended
and fainter emission is detected several arcsec from the central peaks.
In AFGL 2688, the mid-IR emission is extended in the same direction as the
main optical and near-IR lobes. In AFGL 915, the nebula is seen to follow
the optical and near-IR bicone morphology. The ``spikes'' that have been
observed at 2 m and give the nebula its rectangular appearance are
also visible at 10 m. The UIR feature emission is spatially
separate from the central source and is enhanced along the walls of the
bicone. In all three nebulae, the direction of largest spatial extension
is along the major axis as defined by the optical and near-infrared
morphology. Therefore the mid-infrared emission appears to be tracing
material in the bipolar outflow regions, rather than detecting structure
related to an equatorial density enhancement that presumably is
collimating the flow.
Accepted by Astronomical Journal
Preprints can be obtained by
contacting hora@galileo.ifa.hawaii.edu
or via WWW on http://www.ifa.hawaii.edu/faculty/hora/jlh/preprints.html
or via anonymous ftp on
ftp://hubble.ifa.hawaii.edu/pub/hora/ppnppnet.ps.gz
On the Numerical Treatment and Dependence of the Third Dredge-Up Phenomenon
C. A. Frost and J. C. Lattanzio
Department of Mathematics, Monash University, Vic. 3168, Australia
We present results of an investigation into the behaviour of the base of the convective envelope of models of AGB stars during third dredge-up. We find that the extent, and even presence, of third dredge-up depends critically on the treatment of convection within a stellar structure calculation.
Accepted by Astrophysical Journal Letters (Volume 473, December 10th issue) Preprints can be obtained by contacting caf@thala.maths.monash.edu.au
On the pulsation mode of Mira variables: Evidence from the LMC
P. R. Wood and K. M. Sebo
Mount Stromlo and Siding Spring Observatories
Private Bag, Weston Creek PO, ACT 2611, Australia
Recent angular diameter measurements for Mira variables suggest that the radii
of these stars are very large and consistent with pulsation in the first
overtone mode rather than the fundamental mode (Haniff et al. 1995). On the
other hand, nonlinear pulsation models of Mira variables suggest that the
observed pulsation velocity amplitudes can only be achieved during fundamental
mode pulsation, at least for stellar masses 
2.0M . Here, we
present
some new observations of long period variables (LPVs) in the LMC which show
that the LPVs lie on two (K,logP) sequences, one sequence being the
well-known Mira sequence and the other being a sequence parallel to the Mira
sequence but separated from it by 
logP 0.35. The LPVs on the
Mira sequence have a wide range of amplitudes (0.1 < I < 3) while
those on the second sequence have relatively small amplitudes ( I <
0.5). The previously known LPVs of large amplitude ( I > 0.5) in the
LMC lie almost always on the Mira sequence. Theoretical models of LPVs predict
a ratio of fundamental to first or second overtone period of logP
0.3-0.4, and overtone pulsators are expected to have smaller limiting
amplitudes than fundamental mode pulsators. Hence, the above observations can
be easily understood if the LPVs on the Mira sequence are fundamental mode
pulsators while LPVs on the second sequence are overtone pulsators. A second
test of the pulsation mode is obtained by computing pulsation periods for model
stars on the LMC old giant branch and comparing these periods with those of
observed Mira variables. Once again, the fundamental mode pulsators have
periods consistent with those seen in the LMC Miras while the overtone periods
are too short. The above results strongly suggest that Mira variables are
fundamental mode pulsators.
Accepted by MNRAS Preprints can be obtained by contacting wood@mso.anu.edu.au or via anonnymous ftp on ftp://mso.anu.edu.au/pub/wood/lpvmodes.tar.gz
Big grains in the Red Rectangle?
M. Jura , J. Turner , and S. P. Balm
Department of Physics and Astronomy, University of California, Los
Angeles, CA 90095-1562, USA
current address: Amundsen-Scott South Pole Station, Antarctica
We report VLA observations at 3.6 cm, 2 cm and 1.3 cm of the Red Rectangle
and H spectroscopy of HD 44179, the central binary star in the
nebulosity. Emission is detected and all three radio wavelengths, and the
source is resolved at 2 cm and 1.3 cm. Convolved to the same beam, the
radio spectrum of the inner 0".76 0".49 is
characteristic of thermal emission from ionized gas which becomes
optically thin near 2 cm. The diameter of the bulk of the ionized gas
producing this radio emission is between 1.3 10 
cm
( 10 AU) and 2 10 
cm ( 100 AU); in this
region we estimate that 10 
cm 
n 
4
10 cm . The H profile has both a wide
(200 km s full width zero intensity) plateau which is probably
associated with a small circumstellar region (< 10 
cm or
1 AU) and a spike (FWHM 
20 km s ) which we
suggest is produced in the extended ionized gas ( 
10 cm)
detected at radio wavelengths.
We also detect low surface brightness radio emission at 2 cm and 1.3 cm,
but not at 3.6 cm, that is extended well beyond 1" from the source.
This extended radio emission has a spectral index between 1.3 cm and 3.6 cm
greater than 3.2, and is therefore produced by grains. Our VLA data also
indicate that the previously-observed mm-continuum flux cannot result from
ionized gas and therefore is emitted by dust. Although uncertain, it is
possible that the cm and mm wavelength continuum arise from the same dust
in which case there is probably little frequency variation in the
emissivity of the dust between 1.3 cm and 0.13 cm. We propose that there
is a an orbiting, long-lived gravitationally bound disk of dust grains with
radii 0.02 cm.
Accepted by Astrophysical Journal Preprints can be obtained by contacting jura@clotho.astro.ucla.edu
The mass and temperature distribution in the protoplanetary nebula
M1-92: CO interferometric observations
V. Bujarrabal , J. Alcolea , R. Neri , and M. Grewing
Observatorio Astronómico Nacional (IGN), Apartado
1143, E-28800 Alcalá de Henares, Spain
IRAM, 300 rue de la Piscine, F-38406 St Martin d'Hères, France
We present high-resolution CO J=1-0 maps of the protoplanetary
nebula M1-92, Minkowski's Footprint, obtained with the IRAM
interferometer at Plateau de Bure. Previous 
CO J=1-0 maps and single
dish observations of the J=1-0, J=2-1, and J=3-2 transitions in both
isotopic substitutions are also discussed. The cartography confirms the
axial symmetry and complex structure already found from CO data. The
gas velocity presents a dominant axial component that increases in absolute
value from the center, up to a (deprojected) velocity of 70 km s .
Most of the observed emission is at velocities clearly above the expected AGB
expansion kinematics. This fact, together with a remarkable continuity
found along the axis in the structure and velocity of the nebula, is
interpreted as showing that the present CO nebula has been shaped by
momentum transport from the fast post-AGB flow to the rest of the (AGB)
envelope, probably by means of a bow-like shock.
The comparison of the different transitions is used to deduce the physical
conditions in the molecular gas. Most of the observed (post-shock) material
is found to have a low temperature of about 15 K, indicating that the
cooling processes are efficient after the passage of the shock. Typical
densities of 3 10 - 2 10 cm are deduced, corresponding
to a total gas mass of about 1 solar mass. This high value shows
that most of the nebular material is molecular and probed by the CO
observations. Therefore, our maps effectively represent the disruption of
the AGB envelope by the passage of a shock, allowing a
description of the physical conditions present in the nebula during this
evolutionary phase.
Accepted by Astronomy and Astrophysics Preprints can be obtained by contacting bujarrabal@oan.es or via WWW on http://www.oan.es/preprints/
Is UU Herculis a post-AGB star?
V.G.Klochkova, V.E.Panchuk, E.L.Chentsov
State Research Center Special Astrophysical Observatory, Nizhnij Arkhyz, 357147 Russia
In order to understand the evolutionary status of the anomalous supergiant
UUHer, the prototype of the class of variable supergiants located at high
galactic latitudes, we obtained several high-resolution spectra of this
star, with the 6m telescope, over 5 years. This material was used for a
search of possible temporal variations of the radial velocity at the
different depths in the photosphere and for studying the chemical
composition. The average radial velocity 
suggests that UUHer belongs to the old population of the Galaxy.
No systematic dependence of the velocity on depth of the line formation
layer or on ionization and excitation potential is observed. The radial
velocity of the 
absorption differs strongly from the
average photospheric velocity.
The iron abundance in the photosphere of UUHer is significantly lower than
that of the Sun: 
. The enhancement of nitrogen
relatively to iron content 
in combination
with the
carbon underabundance 
suggests that only a
first dredge-up episode occurred. The Na content is normal relatively to
iron, therefore there is no evidence for dredging-up of Ne-Na cycle products.
The heavy s-process metals Y, Ba are depleted relative to H and Fe, which
again implies that the third dredge-up did not occur.
>From the high luminosity ( 
), the large radial
velocity and the chemical abundance pattern, we conclude that UU Her is a
low-mass halo star, but not a post-AGB star.
Accepted for publication in the Astronomy & Astrophysics Preprints can be obtained by contacting Valentina Klochkova (valenta@alba.sao.ru) or via WWW on http://www.sao.ru/publ/UUHer.html
Spectrophotometry of Planetary Nebulae. III. IIDS Observations of Compact Nebulae
J.B. Kaler , K.B. Kwitter , R.A. Shaw and L. Browning
Dept. of Astronomy, Univ. of Illinois, Urbana, IL 61801, USA
Dept. of Astronomy, Williams College, Williamstown, MA 01267, USA
Space Telescope Science Institute, 3700 San Martin Dr., Baltimore,
MD 21218, USA
We give spectral line intensities, electron temperatures, electron densities, and chemical compositions for 25 compact planetary nebulae observed with the IIDS at Kitt Peak. Several (Ha 3-75, He 2-15, K 3-89, M 1-16, M 2-52, and M 3-28) are found to be especially rich in helium and/or nitrogen (He/H > 0.15; N/O > 1). The results minimally confirm the correlation between N/O and He/H, but also confirm the intrinsic scatter in the relation, as N-rich nebulae can be unenriched in He and vice versa.
Accepted by Pub. ASP. Preprints can be obtained by contacting kaler@astro.uiuc.edu
Nucleosynthesis of light elements inside thermally pulsing AGB stars. I: the case of intermediate-mass stars
M. Forestini and C. Charbonnel
Laboratoire d'Astrophysique, Observatoire de Grenoble,
Université Joseph Fourier, BP 53, F-38041 Grenoble Cedex 9,
France
Laboratoire d'Astrophysique de Toulouse, CNRS UMR 5572,
Toulouse, France
The structural and nucleosynthetic evolution of 3, 4, 5, 6 and 
stars with two metallicities ( 
and 0.02) has been computed in
detail, from the early pre-main sequence phase up to the thermally pulsing
(TP) AGB phase or the onset of off-center carbon burning. Typically 10 to
20 thermal pulses have been followed for each TP-AGB object. This
homogeneous and quite large set of models allows us to present an overview
of the thermal pulse properties as well as of the nucleosynthesis
accompanying the TP-AGB phase of intermediate-mass stars.
More specifically, after a brief description of the previous evolutionary
stages, predictions are given for the isotopic ratios involving C, N, O,
Ne, Mg, Al and Si. Also the surface abundances of 
,
and 
are reported. As the asymptotic phase of
the thermal pulses has been reached for each star, we also indicate how
these abundances will probably evolve until the stars completely loose
their envelope, by including the evolution of the nucleosynthesis itself.
This article, in its paper form, has been shortened at a level of roughly 60 % as required by directives coming from the A&A editors. The complete article (50 pages containing 37 figures) is only available in electronic form.
Accepted by Astronomy and Astrophysics Supplement Series Preprints can be obtained by contacting forestin@gag.observ-gr.fr or via WWW on http://gag.observ-gr.fr/liens/starevol/evol.html
Chemical composition of optically bright post-AGB stars
Hans Van Winckel
Instituut voor Sterrenkunde, Celestijnenlaan 200B, 3001 Heverlee, Belgium
We present a detailed LTE chemical analysis of 10 optically bright F-type post-AGB objects on the basis of the analysis of high-resolution optical spectra and compare the results with similar objects discussed in the literature.
The iron content is low on average, and so confirms the old and hence low-mass nature of the supergiants, with a noticable exception of HD 95767.
We emphasize the fact that the chemical patterns observed are very diverse: several different classes can be distinguished. Only a minor fraction of the objects are conform to standard post third dregde-up theory. Only in HD 187885 (Van Winckel et al., 1996), HD 56126 (Klochkova, 1995) and HD 158616 (this paper) is there conclusive chemical evidence that they occur in a post-AGB evolutionary phase : a high total CNO abundance, for HD 187885 a supersolar He content and--above all--a large overabundance of s-process elements.
The other objects, together with other well studied high galactic latitude F-supergiants, display no s-process enhancement but even depletion in some cases. The high N abondance and the mildly enhanced total CNO abundance indicate that the atmospheres of these objects contain a mixture of CNO-cycled material and He-burning products. For some sources, however, this enhancement of the total CNO abundance is barely significant.
HD 107369, the only object in our sample with neither H 
,
emission nor
observed IR excess, displays also unique chemical patterns among our sample
stars ( a C deficiency coupled with a moderate Fe depletion of [Fe/H] = -
1.1). This star is the only object in our sample showing similar chemical
patterns to the metal poor B stars at high galactic latitude (Conlon et
al., 1993). Our chemical analysis does therefore not point to an
evolutionary connection between the dusty high-latitude supergiants and the
metal-poor B stars, but rather suggests that the latter evolve from stars
such as HD 107369.
Accepted by Astronomy and Astrophysics. Preprints can be obtained by contacting hans@ster.kuleuven.ac.be
Really Cool Stars at the Galactic Center
R. D. Blum , K. Sellgren, & D. L. DePoy
JILA, University of Colorado, Campus Box 440, Boulder, CO,
80309, USA
Department of Astronomy, The Ohio State University, 174 W. 18th
Ave., Columbus, Oh, 43210, USA
New and existing K-band spectra for 19 Galactic center late-type stars have been analyzed along with representative spectra of disk and bulge M giants and supergiants. Absorption strengths for strong atomic and molecular features have been measured. The Galactic center stars generally exhibit stronger absorption features centered near Na I (2.206 microns) and Ca I (2.264 microns) than representative disk M stars at the same CO absorption strength.
Based on the absolute K-band magnitudes and CO and h2o absorption strengths for the Galactic center stars and known M supergiants and asymptotic giant branch (AGB) stars, we conclude that only IRS 7 must be a supergiant. Two other bright stars in our Galactic center sample are likely supergiants as well. The remaining bright, cool stars in the Galactic center that we have observed are most consistent with being intermediate mass/age AGB stars. We identify five of the Galactic center stars as long period variables based on their K-band spectral properties and associated photometric variability. Estimates of initial masses and ages for the GC stars suggest multiple epochs of star formation have occurred in the Galactic center over the last 7-100 Myr.
Accepted by AJ Preprints can be obtained by contacting rblum@casa.colorado.edu or via anonymous ftp on ftp://degobah.colorado.edu/rblum/RCS2/
Dust formation in winds of long-period variables
IV. Atmospheric dynamics and mass loss
S. Höfner and E.A. Dorfi
Institut für Astronomie der Universität Wien,
Türkenschanzstraße 17, A-1180 Wien, Austria
We have calculated radiation-hydrodynamical models of the atmosphere and
circumstellar dust shell of C-rich long-period variables which include a
detailed description of the dust formation process. We discuss the
time-dependent behaviour of the circumstellar envelope (e.g. multiperiodicity)
and its relevance for observational properties like near-IR light curves
and investigate the time-averaged mass loss characteristics
of a sample of models satisfying radius-luminosity-mass
and period-luminosity relations. The results can be summarized as follows:
The dependence of the mass loss rate on stellar parameters predicts
a strong increase of mass loss as stars evolve along the AGB.
The models presented agree reasonably well with mean mass loss-period
relations deduced from observations of Mira stars and
the observed scatter of mass loss rates for a given period can be understood
in terms of pulsation amplitude or non-linearities of the wind mechanism.
The wind velocities show a good correlation with the quantity
which characterizes the
strength of radiation pressure on dust relative to gravitation.
Accepted by Astronomy and Astrophysics For preprints, contact hoefner@astro.ast.univie.ac.at
First Announcement
A UMIST/CCP7 Workshop on
DUST AND MOLECULES IN EVOLVED STARS
24-27 March 1997
UMIST, Manchester - UK
(Organisers: I. Cherchneff & T.J. Millar)
Aim of the workshop: ----------
Late stages of evolution of low- and high-mass stars are characterized by very different circumstellar environments, but observations of these objects at optical, infrared and millimetre wavelengths often reveal the presence of the same type of solid compounds and molecular species. The aim of this workshop is to gather scientists working on late stages of stellar evolution to discuss the nature and the role of dust and molecules in evolved objects and to address and answer fundamental questions such as how the same type of solid compounds and molecular species can form in very different astrophysical environments.
A large fraction of the Infrared Space Observatory ISO mission is dedicated to the study of evolved objects and the workshop will be an opportunity to present new and exciting ISO results on circumstellar matter related to late stages of stellar evolution.
Invited reviews: --------
If you are interested in participating, please contact :
Dr. Isabelle Cherchneff
Department of Physics
UMIST
P.O. Box 88
Manchester M60 1QD - United Kingdom
imc@ast.ma.umist.ac.uk
by September 30, 1996.