Gabriel Gómez Velarde, Lucio Crivellari, Sergio Simón Díaz, Miguel Cerviño Saavedra, Inés Camacho Iñesta, Lee Patrick, Mikola Britvavskiy
Colaboradores del IAC: Sebastián L. Hidalgo Rodríguez, Klaus S. Rübke Zúñiga, Sara Rodríguez Berlanas, Gonzalo Holgado Alijo
J. Puls (Univ. Munich); C. Evans (ROE, RU); A. de Koter (Astronomical Institute, Univ. Utrecht); J.S. Vink (Obs. de Armagh); N. Markova (NAO, Bulgary); D.J. Lennon (ESA, Spain), N.R. Walborn (STScI); R.P. Kudritzki (IoA, Univ. Hawai), M.A. Urbaneja (Univ. Insbruck); F. Najarro, M. García (CAB, Spain); I. Negueruela (Univ. Alicante, Spain); J. Maíz Apellániz (IAA, Spain), N. Langer, N. Castro (Univ. Bonn), S. Clark (Open University), G. Meynet (Univ. Geneva); C. Sabín (Univ. La Serena); D. Calzetti (Univ. Massachussets), M. Godart (Univ. Liege)
This project aims at the observation and analysis of massive stars in nearby galaxies to understand their properties and evolution in different environments, particularly environments with conditions close to those in the early Universe.
Massive stars are the source of extremely energetic phenomena and a primary agent of the chemical and dynamical evolution of galaxies and the Universe. They are also one of the most important candidates for the reionization of the Universe, rendering it optically thin and allowing its observation. These stars are born with at least eight solar masses, they are unavoidably facing a violent death as Supernovae, forming neutron stars and black holes and originating Gamma Ray Bursts. Their evolution proceeds very fast yielding large amounts of nuclear processed material by means of strong stellar winds (loosing up to 90% of their initial mass) and emitting intense radiation fields as high energy photons. Their high masses make them prone to form binary systems that can evolve to high mass X ray binaries and form compact objects.
These processes are dependent on stellar properties like mass, metallicity, rotation, etc., that change as a function of the host galaxy. In order to interpret the radiation coming from far galaxies we have to understand how these properties vary with those of the host galaxy and how they determine the described processes. Therefore we need to study them in a galaxy sample that covers a range of characteristics.
Fortunately, because of their high luminosities, massive stars can be individually studied in nearby galaxies, in which their physics approach that of the early Universe and can be used as standard candles. They can also be observed collectively in intense star forming regions at large distances, even shortly after the reionization of the Universe that they may have originated.
The determination of their stellar parameters and chemical abundances allows a detailed comparison with the predictions of the theory of stellar evolution, but requires a detailed calculation of the emergent spectrum. The stellar atmosphere modelling process is complicated by the severe conditions of NLTE, sphericity and mass loss, which effect is to couple the radiative transfer, statistical equilibrium and hydrodynamic equations in a spherical geometry. Moreover, the problem should be solved by using realistic atomic models. However, if we have these parameters and stellar abundances, we can also compare the determinations of abundances in the interstellar medium of our galaxy and nearby galaxies, and the predictions of the theories of chemical evolution of galaxies.
The analyses of massive stars in the Milky Way and nearby galaxies, within and beyond the Local Group, will provide us with a large amount of information about the structure and evolution of these stars and their host galaxies, under different conditions that can be extrapolated to more distant regions in the Universe. However, these requires the identification of massive stars as such, which forces us to use color-magnitude diagrams and to obtain low-resolution spectra. Moreover, we need to combine the observations at different wavelength ranges to obtain the needed data. Although many stellar parameters can be obtained from the different spectral ranges, the UV is necessary to derive the terminal wind velocities and the optical or IR, for temperatures, gravities and mass-loss rates.
The objectives of the present project are the following:
- Identify massive stars in nearby galaxies, particularly in those with conditions resembling the early Universe.
- Observe and analyze these stars.
- Determine the properties of massive stars in different environments. Correlate their properties with those of the host galaxies.
- Study the variation of the stellar evolution with the initial conditions of the stars (partly determined by the environment).
- To design a reliable numerical treatment to represent the physical behaviour of the different processes inside stellar atmospheres.
The main highlight this year has been the publication of the first spectroscopic atlas of massive stars at metallicities below the SMC. The atlas consists of 18 stars, of which 16 are between spectral types O7.5 and A0. In addition, we have carried out the first quantitative analysis of O stars at these metallicities.