7 research outputs found
The mass spectrum of metal-free Stars resulting from photodissociation feedback: A scenario for the formation of low-mass population III stars
The initial mass function (IMF) of metal-free stars that form in the initial
starburst of massive (virial temperatures >10^4K) metal-free protogalaxies is
studied. In particular, we focus on the effect of H2 photodissociation by
pre-existing stars on the fragmentation mass scale, presumedly determined by
the Jeans mass at the end of the initial free-fall phase, i.e., at the
so-called ``loitering phase,'' characterized by the local temperature minimum.
Photodissociation diminishes the Jeans mass at the loitering phase, thereby
reducing the fragmentation mass scale of primordial clouds. Thus, in a given
cloud, far ultraviolet (FUV) radiation from the first star, which is supposedly
very massive (about 10^3Msun), reduces the mass scale for subsequent
fragmentation. Through a series of similar processes the IMF for metal-free
stars is established. If FUV radiation exceeds a threshold level, the
star-forming clumps collapse solely through atomic cooling. Correspondingly,
the fragmentation scale drops discontinuously from a few time 10Msun to
sub-solar scales. In compact clouds (>1.6kpc for clouds of gas mass 10^8Msun),
this level of radiation field is attained, and sub-solar mass stars are formed
even in a metal-free environment. Consequently, the IMF becomes bi-modal, with
peaks at a few tenths of Msun and a few times 10Msun. The high-mass portion of
the IMF is found to be a very steep function of the stellar mass, xi_high(m)
being proportinal to m^{-5}. Therefore, the typical mass scale of metal-free
stars is significantly smaller than that of the very first stars. Also we study
the thermal instability in collapsing primordial prestellar clumps, and discuss
why the thermal instability occuring during the three-body H2 formation does
not appear to manifest itself in causing further fragmentation of such clumps.Comment: 34 pages, 6 figures, ApJ accepte
Formation of Primordial Stars in a LCDM Universe
We study the formation of the first generation of stars in the standard cold
dark matter model, using a very high-resolution hydordynamic simulations. Our
simulation achieves a dynamic range of 10^{10} in length scale. With accurate
treatment of atomic and molecular physics, it allows us to study the
chemo-thermal evolution of primordial gas clouds to densities up to n =
10^{16}/cc without assuming any a priori equation of state; a six orders of
magnitudes improvement over previous three-dimensional calculations. All the
relevant atomic and molecular cooling and heating processes, including cooling
by collision-induced continuum emission, are implemented. For calculating
optically thick H2 cooling at high densities, we use the Sobolev method. To
examine possible gas fragmentation owing to thermal instability, we compute
explicitly the growth rate of isobaric perturbations. We show that the cloud
core does not fragment in either the low-density or high-density regimes. We
also show that the core remains stable against gravitational deformation and
fragmentation. We obtain an accurate gas mass accretion rate within a 10 Msun
innermost region around the protostar. The protostar is accreting the
surrounding hot gas at a rate of 0.001-0.01 Msun/yr. From these findings we
conclude that primordial stars formed in early minihalos are massive. We carry
out proto-stellar evolution calculations using the obtained accretion rate. The
resulting mass of the first star is M_ZAMS = 60-100 Msun, with the exact mass
dependent on the actual accretion rate.Comment: 27 pages, 13 embedded figures. Revised versio
The Formation of the First Stars in the Universe
In this review, I survey our current understanding of how the very first
stars in the universe formed, with a focus on three main areas of interest: the
formation of the first protogalaxies and the cooling of gas within them, the
nature and extent of fragmentation within the cool gas, and the physics -- in
particular the interplay between protostellar accretion and protostellar
feedback -- that serves to determine the final stellar mass.
In each of these areas, I have attempted to show how our thinking has
developed over recent years, aided in large part by the increasing ease with
which we can now perform detailed numerical simulations of primordial star
formation. I have also tried to indicate the areas where our understanding
remains incomplete, and to identify some of the most important unsolved
problems.Comment: 74 pages, 4 figures. Accepted for publication in Space Science
Review