Mass loss from stars and galaxies: impact on evolution

1. Mass loss from stars and galaxies: impact on evolution Tiit Nugis (Tartu Observatory) February 27, 2007

2. Mass loss from stars Mass loss = mass lost by stellar wind (steady outflow of the matter from the surface) or by shell ejections. Nuclear burning in the core (in the central part of the star) decreases the mass of the star by L/c2 per unit time (L is luminosity of the star and c is the speed of light). This rate of the decrease of mass is in most cases lower (much lower) than the mass-loss rates of stellar winds. Mass-loss rates (dM/dt ) of stars in the Galaxy are in the range 10-14 _ 10-3 M/yr Terminal velocities of stellar winds (v∞) are in the range 5 – 5000 km/s

4. Mass loss from stars: impact on stellar evolution Mass loss has a significant effect on the evolution of stars. For massive stars the mass loss is important throughout their evolution. For low mass stars the mass loss is important after the main sequence phase, i.e. in the red giant and AGB phase. The main effects of mass loss are: change in the surface chemistry, lifetime and evolutionary tracks, determination of the end products of evolution, explanation of the presence of circumstellar nebulae.

5. Dependence of stellar evolution on mass-loss rate • The evolutionary tracks and the end products of evolution depend strongly on the mass-loss rate. Two-three times differences in mass-loss rate lead to substantially different evolution tracks (scenarios). • In modern computation codes are used empirical mass-loss rates which have been derived from the observations, but these data are quite uncertain (the factor of uncertainty is 3…5 times). • The determination of the mass-loss rates from the theory is in most cases impossible (the true mechanism which leads to the mass loss is not known or the solution is not unique).

6. The dependence of stellar evolution on Z • The study of first stars and evolutionary scenarios of stars at low-metallicity galaxies has become an actual subject. • The IMF in low metallicity galaxies is expectedly top-heavy and the mean mass of the formed stars is much greater as compared to our Galaxy. • The knowledge of stellar mass-loss rates at low metallicity galaxies is very important for correct prediction of the evolution of these populations of stars. Some new results in this field of research are presented in: ASP Conf. Ser. Vol. 353, Stellar Evolution at Low Metallicity: Mass Loss, Explosions, Cosmology, eds. H. Lamers, N. Langer, T. Nugis and K. Annuk, 2006

7. Mass loss from stars: impact on chemical enrichment • Almost all of the mass that forms a star of a mass greater than about 8 M is returned to the interstellar medium (ISM), in one form or another, by the end of star´s evolution. Only about 1M is left as a compact remnant such as black hole or neutron star. • Stars of lower masses lose most of their mass on the asymptotic giant branch (AGB). • In total, the mass returned to the ISM by stellar winds and by shell ejections of all stars is about 1…2 M/yr. This amount is about five times larger as compared to the yield of SN. • The SFR is about 3…5 M/yr in the Galaxy. The total amount of the molecular gas is about 1…3 billion solar masses. The new generations of stars are formed in the gas which is gradually enriched with heavy elements (stellar wind material is enriched with CNO elements and the SN ejecta with metals).

8. Stellar winds and the mass of protostars Stellar winds are playing an important role in determining the mass of the forming star. In the early phases the evolution is dominated by the interaction between the protostar and the surrounding material which continues to be accreted. Mass of the star is determined by the total mass of the dense interstellar cloud (molecular cloud) and also on the stellar wind intensity. The mass of the star is mainly limited by the dynamical pressure of the wind which stops the accretion.

9. Galactic winds Galactic winds are streams of high speed particles often observed blowing out of galaxies. With speeds of between 300 and 3000 km/s, these winds can either blow material into the halo of the galaxy, or expel the matter from the galaxy completely to mix with the intergalactic medium (IGM). Galactic winds have been observed in X-rays, radio wavelengths, in the IR and also in the optical range.

10. Galactic winds: sources of energy Galactic winds were discovered about thirty years ago. It was believed at first that galactic winds are connected only with the starburst galaxies and AGN. At present it is found a lot of proofs that this phenomenon is quite widespread in the world of galaxies. Recently was discovered the galactic wind from our own Galaxy (ApJ 582, 246, 2003). The sources of energy for the formation and powering of galactic winds are stellar winds, SN explosions and central supermassive BH activity (liberation of the energy of an accreted matter). Mass-loss rates of galactic winds are of the order of of star-formation rates (SFR) in the galaxy. These rates may be as high as 10-1000 M/ yr.

11. The impact of galactic winds on the IGM • Chemical enrichment (pollution) of the intergalactic medium with heavy elements. • Great influence on the halo structure. • The amount of matter expelled from the galaxy is quite large, but what is the exact role of galactic winds in keeping the primordial gas away from the main body of the galaxy is not yet clear. X-ray observations ought to clarify the situation. Most of the radiation may be absorbed by halo matter as is the case of X-rays in the winds of hot massive stars!