The Interstellar Environment of our Galaxy

We review the current knowledge and understanding of the interstellar medium of our galaxy. We first present each of the three basic constituents - ordinary matter, cosmic rays, and magnetic fields - of the interstellar medium, laying emphasis on their physical and chemical properties inferred from a broad range of observations. We then position the different interstellar constituents, both with respect to each other and with respect to stars, within the general galactic ecosystem.Comment: 39 pages, 12 figures (including 3 figures in 2 parts

The effects of stellar winds of fast-rotating massive stars in the earliest phases of the chemical enrichment of the Galaxy

We use the growing data sets of very-metal-poor stars to study the impact of stellar winds of fast rotating massive stars on the chemical enrichment of the early Galaxy. We use an inhomogeneous chemical evolution model for the Galactic halo to predict both the mean trend and scatter of C/O and N/O. In one set of models, we assume that massive stars enrich the interstellar medium during both the stellar wind and supernovae phases. In the second set, we consider that in the earliest phases (Z <10^-8), stars with masses above 40 Msun only enrich the interstellar medium via stellar winds, collapsing directly into black holes. We predict a larger scatter in the C/O and N/O ratios at low metallicities when allowing the more massive fast-rotating stars to contribute to the chemical enrichment only via stellar winds. The latter assumption, combined with the stochasticity in the star formation process in the primordial Galactic halo can explain the wide spread observed in the N/O and C/O ratios in normal very-metal-poor stars. For chemical elements with stellar yields that depend strongly on initial mass (and rotation) such as C, N, and neutron capture elements, within the range of massive stars, a large scatter is expected once the stochastic enrichment of the early interstellar medium is taken into account. We also find that stellar winds of fast rotators mixed with interstellar medium gas are not enough to explain the large CNO enhancements found in most of the carbon-enhanced very-metal-poor stars. In particular, this is the case of the most metal-poor star known to date, HE 1327-2326, for which our models predict lower N enhancements than observed when assuming a mixture of stellar winds and interstellar medium. We suggest that these carbon-enhanced very metal-poor stars were formed from almost pure stellar wind material, without dilution with the pristine interstellar medium.Comment: 10 pages, 7 figures, accepted for publication in A&

Cosmic-ray driven dynamo in the medium of irregular galaxy

We investigate the cosmic ray driven dynamo in the interstellar medium of irregular galaxy. The observations (Chyzy et al. 2000, 2003) show that the magnetic field in irregular galaxies is present and its value reaches the same level as in spiral galaxies. However the conditions in the medium of irregular galaxy are very unfavorable for amplification the magnetic field due to slow rotation and low shearing rate. In this work we present numerical model of the interstellar medium in irregular galaxies. The model includes magnetohydrodynamical dynamo driven by cosmic rays in the interstellar medium provided by random supernova explosions. We describe models characterized by different shear and rotation. We find that even slow galactic rotation with low shearing rate gives amplification of the magnetic field. Simulations have shown that high amount of the magnetic energy flow out off the simulation region becoming an efficient source of intergalactic magnetic fields.Comment: 2 pages, 2 figures, To be published in "Cosmic Magnetic Fields: From Planets, to Stars and Galaxies", K.G. Strassmeier, A.G. Kosovichev & J.E. Beckman, eds., Proc. IAU Symp. 259, CU

The Influence of Stellar Wind Variability on Measurements of Interstellar O VI Along Sightlines to Early-Type Stars

A primary goal of the FUSE mission is to understand the origin of the O VI ion in the interstellar medium of the Galaxy and the Magellanic Clouds. Along sightlines to OB-type stars, these interstellar components are usually blended with O VI stellar wind profiles, which frequently vary in shape. In order to assess the effects of this time-dependent blending on measurements of the interstellar O VI lines, we have undertaken a mini-survey of repeated observations toward OB-type stars in the Galaxy and the Large Magellanic Cloud. These sparse time series, which consist of 2-3 observations separated by intervals ranging from a few days to several months, show that wind variability occurs commonly in O VI (about 60% of a sample of 50 stars), as indeed it does in other resonance lines. However, in the interstellar O VI $\lambda$1032 region, the O VI $\lambda$1038 wind varies only in $\sim$30% of the cases. By examining cases exhibiting large amplitude variations, we conclude that stellar-wind variability {\em generally} introduces negligible uncertainty for single interstellar O VI components along Galactic lines of sight, but can result in substantial errors in measurements of broader components or blends of components like those typically observed toward stars in the Large Magellanic Cloud. Due to possible contamination by discrete absorption components in the stellar O VI line, stars with terminal velocities greater than or equal to the doublet separation (1654 km/s) should be treated with care.Comment: Accepted for publication in the Astrophysical Journal Lette

On the origin of the helium-rich population in Omega Centauri

To study the possible origin of the huge helium enrichment attributed to the stars on the blue main sequence of Omega Centauri, we make use of a chemical evolution model that has proven able to reproduce other major observed properties of the cluster, namely, its stellar metallicity distribution function, age-metallicity relation and trends of several abundance ratios with metallicity. In this framework, the key condition to satisfy all the available observational constraints is that a galactic-scale outflow develops in a much more massive parent system, as a consequence of multiple supernova explosions in a shallow potential well. This galactic wind must carry out preferentially the metals produced by explosive nucleosynthesis in supernovae, whereas elements restored to the interstellar medium through low-energy stellar winds by both asymptotic giant branch (AGB) stars and massive stars must be mostly retained. Assuming that helium is ejected through slow winds by both AGB stars and fast rotating massive stars (FRMSs), the interstellar medium of Omega Centauri's parent galaxy gets naturally enriched in helium in the course of its evolution.Comment: 10 pages, 5 figures, accepted for publication in MNRA