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Mass loss from stars and galaxies: impact on evolution

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Title: 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 (v8 )
    are in the range
  • 5
    5000 km/s

3
(No Transcript)
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 35 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 stars evolution. Only
    about 1 M? 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 12 M? /yr. This amount is about five
    times larger as compared to the yield of SN.
  • The SFR is about 35 M? /yr in the Galaxy.
    The total amount of the molecular gas is about
    13 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!
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