Australia Telescope National Facility

Semiregular variables of types SRa and SRb. Circumstellar CO emission of an oxygen-rich sample

F. Kerschbaum, H. Olofsson, J. Hron

Institut für Astronomie, Türkenschanzstraße 17, A-1180 Wien, Austria

Stockholms Observatorium, S-13336 Saltsjöbaden, Sweden

O-rich Semiregular Variables (SRVs) of types SRa and SRb have been observed in the CO(J=1--0) and (J=2--1) lines using the SEST, La Silla, Chile, and the 20-m telescope at Onsala, Sweden. In total 22 detections (13 are new ones) and 1 tentative detection can be reported among the 48 stars observed. In 7 cases detections are precluded because of strong interference from interstellar CO emission. The majority of the detected objects, covering both small and longer periods, are weak in CO, i.e. they are low mass-loss rate objects (yr), and have, with only a few exceptions, envelopes with small expansion velocities (the mean value is 8 km/s). However, in this respect their properties are very similar to those of bright O--rich Mira variables, whose pulsational periods are on the average 2--3 times higher.

A comparison between stellar and circumstellar properties shows that the gas expansion velocity does not depend on the stellar effective temperature, nor on the period. Likewise, we find no correlation between mass-loss rate and period, but there might be a weak dependence of the former on the stellar effective temperature.

Accepted by A&A, 1995 Preprints can be obtained by contacting kerschbaum@astro.ast.univie.ac.at

Irregular variables of type Lb. New JHKLM-photometry for 160 stars

F. Kerschbaum, C. Lazaro, P. Habison

Institut für Astronomie, Türkenschanzstraße 17, A-1180 Wien, Austria

Dpto. de Astrofísica, Fac. de Física, Universidad de La Laguna, 38200-La Laguna, Tenerife, Spain

Instituto de Astrofísica de Canarias, 38200-La Laguna, Tenerife, Spain

This paper presents new near infrared observations of 160 Irregular variables of type Lb in the JHKLM filter bands. These measurements are supplemented by data for additional 56 stars taken from the literature. In total 220 datasets are available because of some multiple observations. From our sample, 216 stars have near infrared (NIR) photometry now.

Our sample of visually bright Lb-variables displays very similar infrared properties when compared with SRa- and SRb-variables. Derived from NIR-two colour diagrams the oxygen-rich Lbs seem to have intermediate atmospheric conditions between Miras and normal giants. There may be a sligthly larger ``contamination'' with non AGB-giants than in the case of the semiregulars. Using only our IR-colours the S- and the Carbon-stars among the Lbs again are undistinguishable from SR-variables of the same chemistry.

Accepted by A&AS, 1996 Preprints can be obtained by contacting kerschbaum@astro.ast.univie.ac.at

Abundances in the symbiotic star AG Draconis: the barium-symbiotic connection

V.V. Smith and K. Cunha and A. Jorissen and H.M.J. Boffin

Department of Astronomy and McDonald Observatory, University of Texas, Austin, TX 78712, USA

Observatório Nacional, Rua General José Cristino 77, 20921 Sao Cristovão, Rio de Janeiro, Brazil

Institut d'Astronomie et d'Astrophysique, Université Libre de Bruxelles, Campus de la Plaine C.P. 226, Bd du Triomphe, B-1050 Bruxelles, Belgium

Department of Earth and Planetary Sciences, Kobe University, Rokkoudai machi 1-1, Nada ku, 657 Kobe shi, Japan

An abundance analysis of the yellow symbiotic system AG Draconis reveals it to be a metal-poor K-giant ([Fe/H] = ) which is enriched in the heavy s-process elements. This star thus provides a link between the symbiotic stars and the binary barium and CH stars which are also s-process enriched. These binary systems, which exhibit overabundances of the heavy elements, owe their abundance peculiarities to mass transfer from thermally-pulsing asymptotic giant branch stars, which have since evolved to become white-dwarf companions of the cool stars we now view as the chemically peculiar primaries. A comparison of the heavy-element abundance distribution in AG Dra with theoretical nucleosynthesis calculations shows that the s-process is defined by a relatively large neutron exposure (=1.3 mb), while an analysis of the rubidium abundance suggests that the s-process occurred at a neutron density of about 2 x 10 cm. The derived spectroscopic orbit of AG Dra is similar to the orbits of barium and CH stars. Because the luminosity function of low-metallicity K giants is skewed towards higher luminosities by about 2 magnitudes relative to solar-metallicity giants, it is argued that the lower metallicity K giants have larger mass-loss rates. It is this larger mass-loss rate that drives the symbiotic phenomena in AG Dra and we suggest that the other yellow symbiotic stars are probably low-metallicity objects as well.

Accepted by Astronomy & Astrophysics Preprints can be obtained

via anonymous ftp on
ftp://maxwell.as.utexas.edu/pub/verne/agdra.ps

Fluorine production in intermediate-mass stars

N. Mowlavi , A. Jorissen and M. Arnould

Institut d'Astronomie et d'Astrophysique, Université Libre de Bruxelles, C.P.226, Boulevard du Triomphe, B-1050 Bruxelles, Belgium

Observatoire de Genève, CH-1290 Sauverny, Switzerland

The production during the first dozen thermal pulses of AGB stars with masses M and metallicities Z (M = , Z = 0.02), (M = , Z = 0.02) and (M = , Z = 0.001) is investigated on grounds of detailed stellar models and of revised rates for and . These calculations confirm an early expectation that is produced in AGB thermal pulses. They also enlarge substantially these previous results by showing 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. These variations are analyzed in detail, and are shown to result from a subtle balance between different nuclear effects (mainly production or destruction in a pulse, and synthesis during the interpulse), possibly superimposed on dilution effects in more or less extended pulse convective tongues.

Our calculations, as most others, do not predict the third dredge-up self-consistently. When parametrized, it appears that our models of intermediate-mass AGB stars are able to account only for the lowest overabundances observed in solar-metallicity MS, S and C stars. That conclusion is expected to hold true for low-mass stars when fluorine production results from secondary only. Massive AGB stars, on the other hand, are not expected to build up large surface F abundances. Therefore, the large fluorine overabundance reported for the super Li-rich star WZ Cas (where hot bottom burning is supposed to be operating) remains unexplained so far. Our results for the (, ) star indicate that fluorine surface overabundances can also be expected in low-metallicity stars provided that third dredge-ups occur after the early cool pulses. The relative increase in the surface / ratio is, however, lower in the low-metallicity than in the solar-metallicity star. No observations are reported yet for these stars, and are urgently called for.

Accepted by A&A Preprints can be obtained by contacting mowlavi@scsun.unige.ch

or via anonnymous ftp through ftp obsftp.unige.ch, cd /pub/mowlavi, get fluor.ps.gz
or on ftp://obsftp.unige.ch/pub/mowlavi, file fluor.ps.gz

Thermodynamical properties of stellar matter:
I. Equation of state for stellar interiors

W. Stolzmann and T. Blöcker

Institut für Astronomie und Astrophysik, D-24098 Kiel, Germany, supas051@astrophysik.uni-kiel.d400.de
Astrophysikalisches Institut Potsdam, Telegrafenberg A27, D-14473 Potsdam, Germany, tbloecker@aip.de

Considering the Helmholtz free energy which provides detailed knowledge of the charged many-particle physics we present an analytical formulation of the equation of state (EOS) for fully ionized matter.

The nonideal effects due to exchange and correlation interactions of charged particles are described by Padé approximants, which cover a wide range of densities and temperatures. The formulae are valid at any electron degeneracy, for a broad region of Coulomb coupling and are applicable to any chemical mixture. Relativistic generalizations are taken into account for the most predominant astrophysical conditions.

Finally, we present results for H, He, C and O plasmas and give comparisons with other EOS approaches.

Accepted by Astronomy & Astrophysics. For preprints, contact supas051@astrophysik.uni-kiel.d400.de

Interferometric observations of 43 and 86 GHz SiO masers in RCassiopeiae

F. Colomer, A. Baudry, D.A. Graham, R.S. Booth, P. de Vicente, T.P. Krichbaum, J. Gómez-González, C. Schalinski

Onsala Space Observatory, Chalmers University of Technology, S-439 92 Onsala, Sweden.

Observatoire de l'Université Bordeaux 1, CNRS URA no. 352, BP 89, Avenue P. Semirot, F-33270 Floirac, France.

Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany.

Observatorio Astronómico Nacional, Apartado 1143, E-28800 Alcalá de Henares, Spain.

IRAM, Núcleo Central, Av. Divina Pastora 7, E-18012 Granada, Spain; and Voie 10, Domaine Universitaire, F-38406 St-Martin-d'Heres, France.

We have undertaken interferometric studies of the SiO maser emission from the nearby Mira-type star RCassiopeiae. The SiO transition at 43 GHz was observed with a European very long baseline (VLB) interferometer, and the SiO transition at 86 GHz with the IRAM connected interferometer on Plateau de Bure. These observations show a complex spatial/velocity SiO emission pattern, and reveal the presence of both compact and extended SiO emission. The similarities found in our 43 and 86 GHz maps may indicate that both emissions originate from the same volume of gas. Confirmation of such result would provide some constraint on the SiO pumping mechanisms.

Accepted by Astronomy & Astrophysics on February 22, 1996 (in press) Preprints can be obtained by contacting colomer@oan.es

Circumstellar Envelopes of J-type Carbon Stars

S. Lorenz-Martins

Observatório Nacional - CNPq - DAGE Rua José Cristino, 77 São Cristóvão CEP 20 921 - 400 - Rio de Janeiro - Brasil

Observatoire de la Côte d'Azur, CNRS URA 1361, B.P. 229, F-06304 Nice - France

The modelling of ``normal'' J-type carbon stars (without silicate features) has been performed, considering two kind of grains simultaneously: amorphous carbon (A.C.) and silicon carbide (SiC). The temperature of central stars and some characteristics of circumstellar shell as extinction opacities and its extension were determined by fitting the flux curves. The SiC/A.C. ratio as well as the energy distributions and temperature law have been studied. The results show that these J-type carbon stars have thin envelopes (with extinction opacities about 0.02) and intermediate SiC/A.C. ratios. Based on these results two alternative scenarios are discussed: (1) Normal J-type carbon stars would be in an intermediate group between Groups I and II introduced by Lorenz-Martins & Lefèvre (1994). (2) J-type stars (normal and peculiar) could form an alternative evolutive sequence, which differs from that proposed for ordinary carbon stars.

Accepted by A&A main journal For preprints, contact silvia@on.br

The asymmetric bipolar flow in OH231.8+4.2

J. Alcolea, V. Bujarrabal and C. Sánchez Contreras

Observatorio Astronómico Nacional (IGN), Apartado 1143, E-28800 Alcalá de Henares, Spain.
(j.alcolea,bujarrabal,sanchez@oan.es

We present CO and CO microwave line observations of the protoplanetary nebula OH231.8+4.2. The extent of the emission is comparable to the optical nebula, the CO lines showing a large total range of observed velocity that exceeds 300 kms. The velocity of the molecular gas shows a very strong and well defined gradient along the nebular axis. In the south lobe, coincident with the bow-shaped structure identified in optical images, we detect an extension of the CO nebula which has no counterpart in the north. In this region, the CO emitting gas is flowing in the axial direction with (deprojected) velocities up to 330 kms. The total mass probed by CO is large, 0.5 -- 1 , probably being the dominat component of the nebular material. A mass of at least 0.1 is found to be in axial expansion at more than 30 kms. It is argued that this massive fast wind is the remnant of the past AGB mass loss process, accelerated by the presently active bipolar ejections. Mass and momentum (in the axial direction) are calculated for the different spectral features. We estimate that the kinematic time needed to shape the present nebula (from the AGB precursor) is 900 yr.

Accepted by Astronomy and Astrophysics For preprints, contact j.alcolea@oan.es

Detection of HCN (J=39-38) from CRL2688 and CRL618 and of NH(5,4) toward CRL618

Truong-Bach, D. Graham, and Nguyen-Q-Rieu

Observatoire de Paris-Meudon, DEMIRM, URA336 du CNRS, F92195 Meudon principal Cedex, France

Max-Planck-Institut fr Radioastronomie, Auf dem Hügel 69, D-5300 Bonn 1, Germany

Our previous discovery of the HCN molecule in CRL2688 via the J=43-42 transition has prompted us to search for other transitions. We report here our detection of the HCN J=39-38 emission from the CRL2688 and CRL618 nebulae, using the 100-m Effelsberg telescope. Toward CRL2688, the line is double peaked like the stronger J=43-42 one. Toward CRL618, the line is tentatively detected; and a reversal ammonia line, non-metastable transition (5,4), appears in the spectrum. This absorption line is due to the ammonia emission in the west side of the fast wind which is absorbed by the cooler outer HII region emitting the radio continuum. We define the hot clump region extending from -30 to the systemic velocity -21 km/s and which emits non-metastable and metastable lines. Our previously described envelope model of CRL2688 was applied to both HCN J=39-38 and J=43-42 observed line profiles to derive the HCN abundance and other physical parameters of the envelope.

Accepted by Astronomy & Astrophysics Preprints can be obtained by contacting tbach@obspm.fr

Methanol in the circumstellar envelope of IRC+10216

William B. Latter and S. B. Charnley

Space Science Division, MS 245-3, NASA Ames Research Center, Moffett Field, CA 94035

Astronomy Department, University of California, Berkeley, CA 94720 and Space Science Division, MS 245-3, NASA Ames Research Center, Moffett Field, CA 94035

Millimeter-wave observations of the extremely carbon-rich circumstellar envelope of IRC+10216 show several emission lines that we identify with rotational transitions of methanol. We describe our observations and the reasoning leading to this identification. We argue that shock chemistry, ion-molecule reactions, and grain surface reactions might all be potential sources of circumstellar methanol. Each of these pathways either require or produce HO in the envelope of IRC+10216.

Accepted by Astrophysical Journal Letters For preprints, contact latter@dusty.arc.nasa.gov

HD 133656 : a new high-latitude supergiant

Hans Van Winckel, René D. Oudmaijer, Norman R. Trams

Instituut voor Sterrenkunde, K.U.Leuven, Celestijnenlaan 200B, 3001 Heverlee, Belgium

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.

ISO Science Operations Centre, Astrophysics Division, Villafranca del Castillo Satellite Tracking Station, P.O. Box 50727, 28080, Madrid, Spain

In the course of our study of post-Asymptotic Giant Branch objects, we discovered that the seventh magnitude A supergiant HD 133656 has an infrared excess emission due to cool circumstellar dust, and that its photospheric abundance pattern is population II like. We present a detailed abundance study of the object, and discuss its nature in terms of post-AGB evolution : its galactic latitude, cold IR-excess, luminosity, abundances and photometric behaviour indicate HD 133656 to be a descendant of the F-type high-latitude supergiants. The deficiencies of the s-process elements are more difficult to understand in a post-AGB evolutinary scenario.

Accepted by A&A For preprints, please contact hans@ster.kuleuven.ac.be, or write to his address