A multiwavelength study of LS II+34 26: a hot post-AGB star in the process of becoming a planetary nebula

Abstract

We present the results from a multiwavelength analysis of LS II+34 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) 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 70km/s) 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+34 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+34 26, a comparative analysis with LS IV-12 111 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.