The impact of metallicity on massive star evolution

Zusammenfassung

The accurate characterization of the physical properties of massive stars in very low-metallicity environments has important implications in our interpretation of the primitive Universe. The first population of stars, which are thought to have been very massive and extremely metal-poor, has been proposed as the responsible source of the re-ionization of the early Universe. Moreover, very low-metallicity young massive stars have been also suggested to be key agents for the formation and chemical evolution of young galaxies. In fact, the peak of the star formation history of the Universe is observed at a metallicity of about ∼ 1/10Z⊙. Given its relative close proximity, the SMC galaxy remains today as the reference for the metal-poor Universe. Other galaxies beyond the SMC present a significant decrease in metallicity (down to ∼ 1/10Z⊙), while still allowing us to resolve their stellar populations, although their larger distances trans- late in a considerable increase in the required collecting areas and observing times. Until this thesis, only a few spectral studies of individual blue massive stars were performed in galaxies down to ∼ 1/7Z⊙, such as IC1613, WLM and NGC3109. However, the sub-SMC studies have found discrepancies between the observations and the predictions of the radiative driven wind theory for their metallicities. Our group has found in the UV range indications that it is necessary to fully characterize the oxygen and iron abundances of these galaxies in order to accurately obtain their metal content, as the winds of massive stars are mainly driven by iron and iron-group elements. Most metallicity estimates are calculated by scaling oxygen abundances, and an abundance pattern between oxygen and iron different to the solar pattern, could explain the observed discrepancies. Most oxygen estimates in IC1613 (the closest low-metallicity galaxy below that of the SMC) are derived from nebular measurements, which are subject to intrinsic uncertainties. Very few stellar abundances have been measured in this galaxy. In this thesis, we carry out a program to analyze higher resolution spectra (R ∼ 2000) of early-B type stars in IC1613 to obtain their physical parameters and abundances. From our results, we confirm the low oxygen abundance of IC1613, finding also a minor gradient in the spatial distribution of oxygen. We recover higher values when moving away from the HI cavity. The comparison of the carbon and nitrogen abundances with models suggest that the low-metallicity regime seems to further favor higher initial rotational velocities. However, we note that our stars are currently slow rotators and we do not find indications of chemically homogeneous evolution in our sample. Our Teff results suggest that there is not a significant effect of metallicity on the effective temperature of early-B supergiants and giants in the range 1/7−1Z⊙. Moreover, we place our sample in the Wind Momentum-Luminosity diagram (WLR), extending the study of the modified wind momentum to lower luminosities. Our values rather suggest a continuity in the O- and B-types WLR, although the result considering only the B-types suggests also a change in the WLR slope from the low modified wind momentum values obtained for the most luminous B-supergiants. In addition, we target, for the first time, blue massive stars at even lower metallicities. We start a program in the iron-poor Sextans A galaxy, in order to identify the young massive stellar population. Our study has produced the first OB-type stars catalog in Sextans A. Moreover, we perform a basic spectral analysis of our O-type star sample, producing the first Teff and logg estimates at ∼ 1/10Z⊙. We also perform a second program in this galaxy, to extend the number of known early massive stars and to later study the physical properties of these stars in this very-low iron environment. We have refined the criteria to select the OB-type candidates, and between the two programs, we have identified a total of 11 O- and 14 early-B type stars. Lastly, our two programs have added time resolution in the study of the star formation history of Sextans A, revealing a fourth star-formation region.