Australia Telescope National Facility

A first optical spectroscopic monitoring of the post--AGB star SAO 96709 = IRAS 07134+1005: pulsation and shock waves.

A. Lèbre, N. Mauron , D. Gillet and D. Barthès

GRAAL, URA CNRS 1368, CC 072, Université Montpellier II, F--34095 Montpellier Cedex, France

Observatoire de Haute Provence, CNRS, F--04870 Saint Michel l'Observatoire, France

We have performed a spectroscopic monitoring of the F5I "post--AGB" star SAO 96709 on the H and NaD spectral regions in order to study atmospheric motions that could be associated with shock wave propagation throughout the stellar atmosphere. Twenty--one H spectra and seventeen NaD spectra were obtained as regularly as possible, over a 14 month interval. We find that the H, NaD, and other line profiles are strongly variable on a timescale of a few days. The 5853.688 Å BaII and 6587.622 Å CI lines also reveal a variation in their radial velocities. Fourier analysis of these variations yields one main period of 27.3 days and three secondary ones of 32.9, 11.9 and 7.1 days. The H profiles and the velocity variations are good evidence for complex atmospheric dynamics that are related to shock wave propagation throughout the atmospheric layers. These observed spectral features are in good agreement with the recently found photometric variability and point to a pulsating nature for SAO 96709 = HD 56126 = IRAS 07134+1005.

Accepted by Astronomy and Astrophysics For preprints, contact lebre@graal.univ-montp2.fr

Detection of C, CN, and NaI D absorption
in the AGB remnant of HD 56126

Eric J. Bakker, L.B.F.M. Waters, Henny J.G.L.M. Lamers, Norman R. Trams,
Frank L.A. Van der Wolf

Astronomical Institute, University of Utrecht, P.O.Box 80.000, NL-3508 TA Utrecht, The Netherlands

SRON Laboratory for Space Research Utrecht, Sorbonnelaan 2, NL-3584 CA Utrecht, The Netherlands

Astronomical Institute, University of Amsterdam, Kruislaan 403, NL-1098 SJ Amsterdam, The Netherlands

SRON Laboratory for Space Research Groningen, P.O.Box 800, NL-9700 AV Groningen, The Netherlands

ESTEC Space Science Department, P.O.Box 299, NL-2200 AG Noordwijk, The Netherlands

We present the detection of molecular absorption lines in the optical spectrum of the post-AGB star HD 56126. The C Phillips (1,0), (2,0), and (3,0); Swan (0,0) and (1,0); and CN Red system (1,0), (2,0), (3,0), and (4,0) bands have been identified. From the identification of the molecular bands we find an expansion velocity of km s independent of excitation condition or molecular specie. On the basis of the expansion velocity, rotational temperatures, and molecular column densities we argue that the line-forming region is the AGB remnant. This is in agreement with the expansion velocity derived from the CO lines. We find column densities of cm and cm, and rotational temperatures of K and K respectively for C and CN.

By studying molecular line absorption in optical spectra of post-AGB stars we have found a new tracer of the AGB remnant. From comparison with the results of CO and IR observations it is possible to obtain information on non-spherical behavior of the AGB remnant. Using different molecules with different excitation conditions it should be possible to study the AGB remnant as a function of the distance to the star, and thus as a function of the evolutionary status of the star on the AGB.

Accepted by A&A main journal For preprints, contact ebakker@astro.as.utexas.edu

Mid-IR and radio images of IC 418: dust in a young planetary nebula

M. Meixner, C.J. Skinner, E. Keto, A. Zijlstra, M. G. Hoare , J. F. Arens, J. G. Jernigan

Dept. of Astronomy, MC 221, University of Illinois, Urbana, IL 61801, U.S.A

Dept. of Astronomy, University of California, Berkeley, CA94720, U.S.A.

Laboratory for Experimental Astrophysics, Lawrence Livermore National Laboratory, P.O.Box 808, Livermore, CA94551-9900, U.S.A.

Institute of Geophysics and Planetary Physics, L-413, Lawrence Livermore National Laboratory, P.O.Box 808, Livermore, CA 94551-9900, U.S.A.

Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, U.S.A.

Lawrence Livermore National Laboratory, L-59, P.O.Box 808, Livermore, CA 94551-9900, U.S.A.

European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching bei München, Germany

Max-Planck-Institute für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany

Space Sciences Laboratory, University of California, Berkeley, CA94720, U.S.A.

We present three new images of the young, carbon rich planetary nebula, IC 418: 11.3m dust emission, a [NeII] 12.8m line emission and 6 cm free-free continuum. All three images show different morphologies. In order to investigate these spatial differences and the mechanisms of dust emission in IC 418, we compare our data to two radiative transfer models with different radial density distributions. Model 1 has a thin shell that drops off as r, resulting in a high density ionized region surrounded by an ionized halo. While, model 2 has a thicker shell that drops off as r resulting in the ionization front stopping in the thick shell and a high density ionized region immediately surrounded by a neutral shell. Both models use a mixture of silicon carbide (SiC) and amorphous carbon (AC) dust grains with dust to gas ratios of and , respectively and the standard power law distribution in sizes (a; 0.005m < a< 0.25m ). Both models reproduce our mid--IR images and mid--IR spectra well suggesting that, while IC 418 has emission features attributed to polycyclic aromatic hydrocarbons (PAHs), the SiC feature and not the PAH feature probably dominates the 11.3m emission. Comparison of our images with broad-band J, H and K images of IC 418 by Hora et al. (1993), suggest that dust emission processes may contribute more near--IR emission than previously thought and that this near-IR ``dust'' emission is contained within the ionized gas region. Both models reproduce the observed spatial distributions of the 11.3m , [NeII], and 6cm emissions which differ because they have different radial excitation gradients in the nebula. However, model 1 better explains all of the many previous observations of IC 418 at different wavelengths. We suggest a three layer onion model for IC 418: a radius high density ionized region surrounded by a 20 radius low density ionized halo, enclosed by a completely photodissociated neutral halo extended beyond a 80 radius. This structure may suggest a mass loss history for IC 418 in which its progenitor AGB star experienced a superwind (M M yr, v11 km s) just before departing the AGB.

Accepted by Astronomy and Astrophysics For preprints, contact meixner@astro.uiuc.edu

Spectroscopy of selected pulsating stars :
the anomalous variable V351 Cep

G.A. Galazutdinov and V.G. Klochkova

State Research Center Special Astrophysical Observatory of Russian Academy of Sciences, Nizhnij Arkhyz, Russia, 357147

The physical parameters: effective temperature , surface gravity log g, microturbulent velocity , abundances of 26 chemical elements, radial velocity - were obtained for 5 phases of the variable star V351 Cep using CCD echelle spectra with spectral resolution R = 25000 in the region . The chemical composition of V351 Cep coincides within the errors with the solar chemical composition and, except for the CNO elements, it coincides with the chemical composition of the young supergiant Per. It is shown that the massive supergiant Per has an essential overabundance of N and deficiency of C, O in comparison with the solar abundances, which is in good agreement with evolutionary status of Per. At the same time, no considerable modifications of the CNO abundances for V351 Cep were found. It evidences for absence of dredge-up H-burning products to the surface of the star.

The luminosity of V351 Cep, , was obtained using the intensity of the infrared triplet OI (near 7774 Å). This value is not consistent with the luminosity obtained from the P-L relation.

The combination of the physical parameters of V351 Cep allows to classify V351 Cep as an anomalous cepheid.

Accepted by Astron. & Astrophys. Transactions For preprints, contact vkloch@sao.stavropol.su

On the growth of carbonaceous grains in circumstellar envelopes

D. Krüger, A. B. C. Patzer, E. Sedlmayr

Institut für Astronomie und Astrophysik, PN 8-1 Hardenbergstrae 36, 10623 Berlin, Federal Republic of Germany

We present a phenomenological description of relevant aspects of the grain surface chemistry on pre-existing carbonaceous seed particles. The production of radical surface sites is calculated by means of modified analogous gas-phase thermodynamic and reaction kinetic data. Our results suggest that surface growth processes mainly take place at sp bonded carbon atoms rather than at the edges of graphitic plates or polycyclic aromatic hydrocarbons and that the resulting grain material will be amorphous diamond-like carbon. Furthermore, we argue that the sticking efficiency of the key growth species CH at radical sites is high even for significant grain drift velocities. The dust growth rate will generally be proportional to the radical surface coverage.

Accepted by Astron. Astrophys. For preprints, contact D.Krueger@physik.TU-Berlin.DE

The Unusual Circumstellar Environment of the Evolved Star, U Equ

Cecilia Barnbaum, Alain Omont and Mark Morris

NRAO, 520 Edgemont Rd, Charlottesville, VA, USA 22901
l'Institut d'Astrophysique de Paris, CNRS, 98 bis Boulevard Arago, 75014 Paris, France
Department of Physics & Astronomy, UCLA, Los Angeles, CA, USA 90095-1562

In the course of a spectroscopic survey to identify cold IRAS stars, we discovered that the evolved star, U Equ, has a very peculiar optical spectrum indicating an unusual circumstellar environment. The moderate resolution (62 Å mm) spectrum has deep, yet unusually thin, molecular absorption features that must arise in cold gas at a large distance from the star. The optical spectrum also has striking molecular emission lines of TiO, AlO, VO possibly due to a fluorescence mechanism. Although the IRAS 25/12 m colors of U Equ are typical of optically thin circumstellar envelopes, the LRS spectrum displays a strong 10 m silicate absorption band, indicating a thick, dusty envelope. Comparing our HO and OH maser observations with profiles in the literature taken in 1987 (HO) and 1990 (OH), we have found a variable velocity pattern and profile shape. We suggest that the star is of a warm spectral type, G to early K, and that it has an edge-on dusty disk or torus and an inner warm molecular circumstellar shell. The luminosity criteria in our moderate-resolution spectra are consistent with that of an evolved giant. The velocity and Galactic latitude are not consistent with an early pre-main sequence evolutionary stage; however, the line-of-sight proximity of U Equ and a bright ribbon of IRAS Galactic cirrus suggest further investigation into the interstellar environment is needed. We present optical spectra, profiles of the HO and OH masers, and J and K band images and photometry.

Accepted by Astron. Astrophys. For preprints, contact cbarnbau@nrao.edu

Distances to Planetary Nebulae BD +303639 and NGC 6572

J. Kawamura and C. Masson

Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 USA

Third-epoch Very Large Array proper-motion measurements on the planetary nebulae and NGC 6572 have been made. Combining these measurements with simple models for the nebulae and using improved analysis, it has been determined that the distance to is and NGC 6572 is . These measurements are consistent with the distances determined from the previous epoch. However, the errors are significantly smaller and are now dominated not by the measurements but by the modeling and the uncertainty in the flux variation. The distances are significantly higher than those determined by statistical methods, with important consequences for calculation of the nebular masses and luminosities.

In addition to their distances, the observations have elucidated the dynamical properties of the objects. First, the expansion of a shell is consistent with the self-similar expansion that is usually assumed for these objects, with the magnitude of the proper motion of a portion of a shell being linearly related to its distance from the center of symmetry. Second, the proper-motion measurements have directly yielded the ages for the nebulae, which are determined to be for and for NGC 6572.

Accepted by Astrophysical Journal. For preprints, contact jkawamura@cfa.harvard.edu

A Survey for 22GHz water maser emission from the Arecibo set of OH/IR stars

D.Engels and B.M.Lewis

Sternwarte der Universität Hamburg, Gojenbergsweg 112, D-21029 Hamburg, Germany

Arecibo Observatory, P.O. Box 995, Arecibo, PR 00613, Puerto Rico

We report here on our search for water masers from a set of 382 OH/IR stars identified at Arecibo from color selected IRAS sources. Maser emission with an integrated flux 10 W m is detected from about half. The detection rate is highest for the bluest stars (90% for IRAS) and decreases to 30% for the reddest. We expect multiple observations, better sensitivity and improved positions to raise the eventual detection rate from this sample to about 75%. Some of the undetected OH/IR stars are proto planetary nebulae; most of the rest are stars with very heavy mass-loss rates.

The morphological characteristics of our spectra are analyzed: this confirms our expectation that they generally evolve into double-peaked profiles at the higher mass loss rates. Emission in the velocity interval which is blue-shifted with respect to the stellar velocity is statistically stronger than emission from the red-shifted interval. The water outflow velocities reach 7515% of the circumstellar expansion velocities exhibited by OH, while flux variations are rarely larger than a factor of 10.

Accepted by Astronomy & Astrophysics Supplement
For preprints, contact D. Engels, e-mail: dengels@hs.uni-hamburg.de

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Interferometric molecular line observations of the
circumstellar envelope(s) around U Camelopardalis

M. Lindqvist, R. Lucas, H. Olofsson, A. Omont, K. Eriksson, B. Gustafsson

Sterrewacht Leiden, Postbus 9513, 2300 RA Leiden, The Netherlands

IRAM, 300 rue de la Piscine, F-38406 St Martin d'Heres Cedex, France

Stockholm Observatory, S-13336, Saltsjöbaden, Sweden

Institut d'Astrophysique de Paris, CNRS, 98 bis bd Arago, F-75014 Paris, France,

Astronomical Observatory, Box 515, S-75120 Uppsala, Sweden

We have observed the circumstellar envelope of the carbon star U Cam in the and lines using the IRAM Plateau de Bure interferometer. There is evidence of a two-envelope structure: an outer extended envelope, possibly a shell, with a radius of , that expands with a velocity of , surrounding an inner envelope with a radius of and an expansion velocity of only . Mass loss rate estimates based on these data alone are uncertain, but they suggest that the mass loss rate during the formation of the outer envelope was higher than during the present mass loss epoch. Thus, we have evidence for a significant variation in the mass loss characteristics of U Cam within the last 10 years.

Accepted by A&A (Letters) For preprints, contact lindqvis@strw.leidenuniv.nl

A search for hot post-AGB stars in the IRAS Point Source Catalog

René D. Oudmaijer

Kapteyn Astronomical Institute, P.O.Box 800, NL-9700 AV Groningen, The Netherlands

Imperial College of Science, Medicine and Technology, Blackett Laboratory, Prince Consort Road,
London, SW7 2BZ, U.K.

In this paper a first step is made to search for hot post-AGB stars in the IRAS Point Source Catalog. By selecting objects that were not detected at the IRAS 12 m passband, stars with cool dust shells were retrieved.

The selection yielded 15 objects, of which 12 have spectral type B, and would appear to be efficient in finding hot stars. However, this result does not necessarily mean that these are old evolved objects. Some stars are associated with dense galactic cirrus and are probably normal massive supergiants, a number of stars are good post-AGB candidates because of their galactic latitudes and their cool, extended infrared emission. The possibility remains that these objects are normal supergiants heating the local interstellar medium.

Three objects appear to be genuine post-AGB stars, as is attested by their spectral and photometric properties. The main conclusion derived from modelling their spectral energy distributions is that the objects evolved off the AGB about 1000 years ago, significantly longer than what is found for other post-AGB stars.

Emission lines have appeared in the spectrum of one of these objects, SAO 243756, over the last 20 years. SAO 243756 bears close resemblance to other objects that have been proposed to have entered the Planetary Nebula phase recently.

Finally, several aspects of the problem of selecting post-AGB stars in general are discussed.

Accepted by A&A For preprints, please contact r.oudmaijer@ic.ac.uk, or write to the London address

Structural and chemical evolution of intermediate-mass asymptotic giant branch stars

Nami Mowlavi

Thesis work conducted at: Université Libre de Bruxelles, Brussels, Belgium

Ph.D dissertation directed by: Prof. Marcel Arnould

Ph.D degree awarded: September 1995

Current address: Observatoire de Genève, CH-1290 Sauverny, Switzerland

Electronic mail: mowlavi@scsun.unige.ch

The thesis studies the structural and chemical evolution of asymptotic giant branch (AGB) stars, with special emphasis on the fluorine production during the thermally pulsing AGB (TP-AGB) phase. For that purpose, a new one-dimensional stellar evolution code has been designed in order to reach the high degree of accuracy in the stellar models required to follow the nucleosynthesis during the TP-AGB phase (Chapter II). The improvements include an extensive nuclear network comprising 47 nuclides from H to S and seed nuclei in order to mimick the s-process nucleosynthesis, and new mesh distribution techniques. Special care is also exercised to correctly handle the growth of convective zones into regions of variable chemical composition.

The structural and chemical evolution of three stars with masses M and metallicities Z (M= ,Z= 0.02), (M= ,Z= 0.02) and (M= ,Z= 0.001) have been followed in detail from the pre-main sequence up to the first dozen thermal pulses. These are described in chapter III, with particular attention to the properties during the AGB phase.

The analysis of the fluorine production in our AGB models is presented in chapter IV. The results reveal that the variations of the level of production during the evolution is very sensitive to the maximum temperature reached at the base of the pulse. Comparison of surface fluorine enhancements with observations is presented. Predictions of surface fluorine abundances in low-metallicity stars, for which no observations are reported yet, are also presented.

The postcript file of the thesis is available upon request at the above e-mail address.