Miguel. A. Urbaneja

The quantitative spectral analysis of low resolution Keck LRIS spectra of blue supergiants in the disk of the giant spiral galaxy M81 is used to determine stellar effective temperatures, gravities, metallicities, luminosites, interstellar reddening and a new distance using the Flux-weighted Gravity--Luminosity Relationship (FGLR). Substantial reddening and extinction is found with E(B-V) ranging between 0.13 to 0.38 mag and an average value of 0.26 mag. The distance modulus obtained after individual reddening corrections is 27.7+/-0.1 mag. The result is discussed with regard to recently measured TRGB and Cepheid distances. The metallicities (based on elements such as iron, titanium, magnesium) are supersolar (~0.2 dex) in the inner disk (R<=5kpc) and slightly subsolar (~ -0.05 dex) in the outer disk (R>10 kpc) with a shallow metallicity gradient of 0.034 dex/kpc. The comparison with published oxygen abundances of planetary nebulae and metallicities determined through fits of HST color-magnitude diagrams indicates a late metal enrichment and a flattening of the abundance gradient over the last 5 Gyrs. This might be the result of gas infall from metal rich satellite galaxies. Combining these M81 metallicities with published blue supergiant abundance studies in the Local Group and the Sculptor Group a galaxy mass metallicity-relationship based solely on stellar spectroscopic studies is presented and compared with recent studies of SDSS star forming galaxies.

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.

This collection of papers from the Space Telescope Science Institute Symposium on massive stars addresses the many aspects of astrophysics in which these stars play an important role. Review papers are presented from both observational and theoretical work by world experts in the study of these rare stars. Topics discussed include star formation in the local and distant universe, the feedback effects of the massive stars, mass loss from massive stars, and explosions of massive stars. The combination of papers produces a comprehensive overview of up-to-date research in the field, making this book an invaluable resource for professional researchers and for students of astrophysics.

We present optical ESO time-series and UV archival (FUSE, HST, IUE) spectroscopy of the H-rich central star of He 2-138. Our study targets the central star wind in a very young planetary nebula, and explores physical conditions that may provide clues to the nature of the preceding post-AGB super-wind phases of the star. We provide evidence for a dense, slowly accelerating outflow that is variable on time-scales of hours. Line-synthesis modelling (SEI and CMFGEN) of low and high ionization UV and optical lines is interpreted in terms of an asymmetric, two-component outflow, where high-speed high-ionization gas forms mostly in the polar region. Slower, low ionization material is then confined primarily to a cooler equatorial component of the outflow. A dichotomy is also evident at photospheric levels. We also document temporal changes in the weak photospheric lines of He 2-138, with tentative evidence for a 0.36-day modulation in blue-to-red migrating features in the absorption lines. These structures may betray 'wave-leakage' of prograde non-radial pulsations of the central star. These multi-waveband results on the aspherical outflow of He 2-138 are discussed in the context of current interest in understanding the origin of axi- and point-symmetric planetary nebulae.

The quantitative spectral analysis of medium resolution optical spectra of A and B supergiants obtained with DEIMOS and ESI at the Keck Telescopes is used to determine a distance modulus of 24.93 ± 0.11 mag (968 ± 50 kpc) for the Triangulum Galaxy M33. The analysis yields stellar effective temperatures, gravities, interstellar reddening, and extinction, the combination of which provides a distance estimate via the flux-weighted gravity-luminosity relationship (FGLR). This result is based on an FGLR calibration that is continually being polished. An average reddening of E(B – V) ~ 0.08 mag is found, with a large variation ranging from 0.01 to 0.16 mag, however, demonstrating the importance of accurate individual reddening measurements for stellar distance indicators in galaxies with evident signatures of interstellar absorption. The large-distance modulus found is in good agreement with recent work on eclipsing binaries, planetary nebulae, long-period variables, RR Lyrae stars, and also with Hubble Space Telescope (HST) observations of Cepheids, if reasonable reddening assumptions are made for the Cepheids. Since distances based on the tip of the red giant branch (TRGB) method found in the literature give conflicting results, we have used HST Advanced Camera for Surveys V- and I-band images of outer regions of M33 to determine a TRGB distance of 24.84 ± 0.10 mag, in basic agreement with the FGLR result. We have also determined stellar metallicities and discussed the metallicity gradient in the disk of M33. We find metallicity of Z _☉ at the center and 0.3 Z _☉ in the outskirts at a distance of one isophotal radius. The average logarithmic metallicity gradient is –0.07 ± 0.01 dex kpc^–1. However, there is a large scatter around this average value, very similar to what has been found for the H II regions in M33.

The quantitative analysis of low resolution spectra of A and B supergiants is used to determine a distance modulus of 24.99 +/- 0.10 mag (995 +/- 46 Kpc) to the Local Group galaxy WLM. The analysis yields stellar effective temperatures and gravities, which provide a distance through the Flux weighted Gravity--Luminosity Relationship (FGLR). Our distance is 0.07 mag larger than the most recent results based on Cepheids and the tip of the RGB. This difference is within the 1-sigma overlap of the typical uncertainties quoted in these photometric investigations. In addition, non-LTE spectral synthesis of the rich metal line spectra (mostly iron, chromium and titanium) of the A supergiants is carried out, which allows the determination of stellar metallicities. An average metallicity of -0.87 +/- 0.06 dex with respect to solar metallicity is found.