Roberto Hugo Mendez

Peculiar CNO photospheric abundances in the central star of NGC 2392

Using new, high signal-to-noise CFHT ESPaDOnS visual spectrograms, and archive IUE and FUSE UV spectrograms, together with state-of-the-art non-LTE hydrodynamical model atmospheres, we have obtained accurate He, C, N, O photospheric abundance determinations in the central stars of NGC 2392, IC 4593, and NGC 6826. We compare with the corresponding nebular abundances, taken from the literature. The central star of NGC 2392 shows high He, N, and very low C, O abundances. We propose that these peculiar abundances must have originated in a common-envelope phase of interaction with a close binary companion. If we assume that the companion is more evolved than the visible central star, this offers a way of solving the old mystery of the discrepant He II Zanstra temperature of NGC 2392.

Stellar Metallicities Beyond the Local Group with VLT/FORS

Luminous BA-type supergiants are the visually brightest stars in spiral and irregular galaxies. Substantial progress in their theoretical modeling has been achieved by the application of sophisticated non-LTE spectrum synthesis. After testing of the method on Local Group objects at high resolution, the next step has been taken. First results from medium-resolution spectroscopy of supergiants in NGC 3621 at a distance of 6.7 Mpc, obtained with the FORS instrument on the VLT, are presented. Fundamental parameters of individual stars are determined at such distances for the first time. This allows the reduction of systematic errors of the Cepheid distance scale by providing information on the metallicity, color excess and extinction of objects adjacent to Cepheids. Moreover, the supergiants themselves will act as an independent distance indicator through the application of the wind momentum-luminosity relationship after a proper empirical calibration.

Identification and Study of Blue Supergiants in Nearby Galaxies

For the past two years we have been using the FORS multi-object spectrograph at the VLT to investigate the blue supergiant populations in nearby spiral galaxies. For the first time we have obtained quantitative spectroscopy to a distance up to 7 Mpc, estimating abundances and wind properties of luminous A-type supergiants. We have begun to test the wind momentum-luminosity relationship as an extragalactic distance indicator beyond the Local Group. Our very preliminary results on single blue supergiants in NGC 3621 and NGC 300 make us look with optimism at the future of the project, especially when looking ahead to the new generation of multi-object spectrographs which will be soon made available at 8 meter-class telescopes.

Unified NLTE Model Atmospheres Including Spherical Extension and Stellar Winds: EUV — Fluxes and the He II Discrepancy in Central Stars of Planetary Nebulae

We present NLTE model atmospheres for hot stars that avoid the artificial division between hydrostatic photosphere and supersonic stellar wind envelope. The models are of spherical geometry and extended over the entire sub- and supersonic atmosphere. They need only stellar effective temperature, gravity, radius and helium abundance as free parameters and are in principle self-consistent. We have calculated a large grid of models for Central Stars of Planetary Nebulae ( CSPN ) in the effective range from 40000 K to 100000 K. The calculated energy distributions are used to investigate the longstanding problem of the ”Zanstra - discrepancy”. It is shown that the increase of EUV flux in the unified models, which is caused by the presence of the wind velocity field in the layers of He II- groundstate continuum formation leads to a much better agreement with the observations than was the case before, when plane-parallel hydrostatic models were used. However, for a significant fraction of the Central Stars investigated ( 45% ) the computed flux is still about 1 dex too small. This remaining discrepancies are attributed to additional EUV — photons generated by shocks in the radiation driven winds ( Pauldrach et al. in prep. ).