He Abundance in Planetary Nebulae

Type I planetary nebulae (PNe), defined as those with high He (and N) abundances, generally present bipolar geometries and high stellar temperatures. The main goal of this paper is to check if the empirically derived He overabundance for Type I PNs is real, or if it is a consequence of geometrical effects due to the bipolarity and/or to the ionization stratification due to the high stellar temperature of these objects.

Probing Wolf-Rayet wind structures: helping hands from WR+O binaries

For binaries with well-known orbital parameters we describe methods to restrict some characteristics of the WR star in the binary, such as Te and RWR(τ ≃ 1). The presence of a hot, luminous companion helps to address the question of spherical symmetry of the WR wind and reveal the ionization stratification in the wind. Also, we discuss the restrictions which can be imposed on the wind velocity law(s).

Ionization-stratification in Wolf-Rayet atmospheres

We summarize an analysis of low-resolution IUE archival data for galactic and LMC Wolf-Rayet stars. We examine the correlation between ionization potential with line-width (FWHM) constituting a sample of stars showing a well-defined UV ionization structure.

An empirical study of Wolf-Rayet stellar wind ionization stratification

We present an analysis of high resolution UV (IUE) and optical (INT, AAT and ESO) spectroscopy of eight galactic WN and WC stars. We examine the correlation between ionization potential (IP) and the velocity structure of the stellar winds and find important correlations between line-width, IP and excitation potential (EP). These results provide potential constraints on the velocity law used in models of WR atmospheres.

Deceleration Zones in the Winds of WR and P Cygni Type Stars

Specific features both of continua and of line spectra of WR and P Cygni type stars can probably be explained if an extensive deceleration zone is proposed to exist in their stellar winds. The outflowing matter is first accelerated near the stellar surface, then follows the deceleration of the flow and after that the final acceleration of the outflowing matter takes place (i.e. the wind has an ADA-structure: acceleration-deceleration-acceleration). Such a structure of the wind probably arises due to the multiscattering of photons in the envelope having two detached shells, which are optically thick in resonance lines (these shells can form if ionization stratification is present in the envelope).