We have performed a detailed oxygen abundance analysis of 23 metal-poor (-3.0 [Fe/H] -0.3) unevolved halo stars and one giant through the OH bands in the near UV, using high-resolution echelle spectra. Oxygen is found to be overabundant with respect to iron in these stars, with the [O/Fe] ratio increasing from 0.6 to 1 between [Fe/H] = -1.5 and

We present spatially resolved echelle spectroscopy, obtained at high spectral resolution, for 15 multiple-shell planetary nebulae. Most exhibit faint detached halos (IC 1295, MA 3, M 2-2, M 2-40, NGC 6804, NGC 6826, NGC 6884, NGC 6891, NGC 7662, PM 1-295, and Vy 2-3). Furthermore, we have included some with attached shells (IC 1454, K 1-20, K 3-73

The Cometary Knots in the Helix Nebula have shown themselves to be formed of a head which is a photoionized surface, optically thick to Lyman continuum radiation, on the central star facing sides of neutral clumps and a cometary tails photionized by the diffuse radiation field. In this study we will present the results of a photoionization model of

In this paper we address the problem of the non-LTE (NLTE) inversion of line profiles by means of a nonlinear least-squares minimization procedure combined with very efficient multilevel transfer methods. Our approach is based on the concept of response functions, which measure the first-order response of the emergent profiles to changes in the

Daily measurements of the solar aureole were made at the Vacuum Newton Telescope (/D=40 cm) at the Teide Observatory (Tenerife) for various airmasses. We use these measurements to understand how the aureole is produced and to extrapolate visible measurements to the infrared range (1.6 /μm). This information will be used to correct sunspot

We present observations and analyses of some of the most massive stars in the Milky Way, showing the potential and limits of current model atmospheres, and the consequences for the evolution theory of massive stars. We then show how these techniques can be applied to very bright stars in M31 and M33 to derive radial abundance gradients and get