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!