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Astron. Astrophys. 326, 1103-1110 (1997)

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A multiwavelength study of LS II+340 26: a hot post-AGB star in the process of becoming a planetary nebula *

P. García-Lario 1, 2, M. Parthasarathy 3, 4, D. de Martino 5, L. Sanz Fernández de Córdoba 2, R. Monier 4, A. Manchado 6 and S.R. Pottasch 7

1 Leiden Observatory. PO Box 9513. 2300 RA Leiden, The Netherlands
2 Laboratorio de Astrofísica Espacial y Física Fundamental. INTA. Apartado de Correos 50727. E-28080 Madrid, Spain
3 Indian Institute of Astrophysics. I-560034 Bangalore, India
4 CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France
5 Osservatorio Astronomico di Capodimonte. Via Moiariello, 16. I-80131 Naples, Italy
6 Instituto de Astrofísica de Canarias, E-38200 La Laguna, Tenerife, Spain
7 Kapteyn Astronomical Institute, Postbus 800, 9700 AV Groningen, The Netherlands

Received 26 March 1997 / Accepted 29 May 1997

Abstract

We present the results from a multiwavelength analysis of LS II+340 26, recently identified as a low mass post-AGB star and not a massive population I B-type star, as previously thought. We confirm that the central star is a carbon-poor post-AGB star surrounded by a very low excitation and compact nebula.

Spectroscopic monitoring carried out since 1991 reveals variations which suggest that this star has had a mass loss episode in the period 1993-1995. The asymmetric profiles in a few absorption lines in high resolution optical spectra indicate the presence of complex motions in the outer layers of the atmosphere. The radial velocity variations (several tens of km s-1) observed in spectra taken in a single epoch can be attributed to stellar pulsation. For a few lines stronger variations over the years (up to 70 km s-1) can be explained if these lines are formed in the outflow.

The anomalous extinction observed in the UV suggests that part of the reddening is of circumstellar origin and that the standard interstellar extinction law is not applicable. On the other hand, the absence of a significant near infrared excess in LS II+340 26 suggests that the mass loss enhancement corresponds to a short-lived episode of modest intensity.

Although non-LTE effects prevent the accurate determination of the atmospheric parameters and abundances of LS II+340 26, a comparative analysis with LS IV-120 111 indicates that both stars are very similar. Both are identified as low mass carbon-poor hot post-AGB stars belonging to the halo population of our Galaxy.

Key words: Planetary nebulae: LS II+34026 – stars: LS II+34026 – stars: AGB and post-AGB – stars: abundances – stars: mass loss

* Based partially on observations collected with the International Ultraviolet Explorer (IUE) at the Villafranca Satellite Tracking Station of the European Space Agency, Spain

Send offprint requests to: P. García-Lario

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© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998
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