55 research outputs found
The characteristic stellar mass as a function of redshift
We present a model for the star formation process during the initial collapse
of dark matter haloes at redshifts z=0-30. We derive a simple expression for
the characteristic stellar mass scale during this initial burst of star
formation. In our picture, this characteristic scale reflects both the minimum
temperature to which the gas can cool (determined by the metallicity and the
temperature of the cosmic microwave background) and the pressure of overlying
baryons in the collapsing halo. This prescription reproduces both the large
mass scales found in simulations of Population III star formation and the near
solar values observed for star formation at low redshift.Comment: 8 pages, 4 figures, accepted for publication in MNRA
The Spectrum of GRB 930131 (``Superbowl Burst'') from 20 keV to 200 MeV
We have constructed a broad-band spectrum for GRB 930131 (the ``Superbowl
Burst''), ranging from 20 keV to 200 MeV, by combining spectral information
from the Gamma Ray Observatory's BATSE, COMPTEL and EGRET instruments. We
present general methods for combining spectra from different time intervals
obtained by the same instrument as well as for combining spectra from the same
time interval taken by different instruments. The resulting spectrum is
remarkably flat (in nu F_nu-space) up to high energies. We find that the
spectral shape can be successfully fitted by the shocked synchrotron emission
model of Tavani. We present evidence that the flatness of the spectrum at high
energies is not due to spectral time-variability.Comment: ApJ accepted, 10 pages, 1 table, 3 figure
Low-energy Population III supernovae and the origin of extremely metal-poor stars
Some ancient, dim, metal-poor stars may have formed in the ashes of the first
supernovae (SNe). If their chemical abundances can be reconciled with the
elemental yields of specific Population III (Pop III) explosions, they could
reveal the properties of primordial stars. But multidimensional simulations of
such explosions are required to predict their yields because dynamical
instabilities can dredge material up from deep in the ejecta that would
otherwise be predicted to fall back on to the central remnant and be lost in
one-dimensional (1D) models. We have performed two-dimensional (2D) numerical
simulations of two low-energy Pop III SNe, a 12.4 Msun explosion and a 60 Msun
explosion, and find that they produce elemental yields that are a good fit to
those measured in the most iron-poor star discovered to date, SMSS
J031300.36-670839.3 (J031300). Fallback on to the compact remnant in these weak
explosions accounts for the lack of measurable iron in J031300 and its low
iron-group abundances in general. Our 2D explosions produce higher abundances
of heavy elements (atomic number Z > 20) than their 1D counterparts due to
dredge-up by fluid instabilities. Since almost no Ni is ejected by these weak
SNe, their low luminosities will prevent their detection in the near-infrared
with the James Webb Space Telescope and future 30-m telescopes on the ground.
The only evidence that they ever occurred will be in the fossil abundance
record.Comment: Accepted to MNRA
Formation and evolution of primordial protostellar systems
We investigate the formation of the first stars at the end of the cosmic dark
ages with a suite of three-dimensional, moving mesh simulations that directly
resolve the collapse of the gas beyond the formation of the first protostar at
the centre of a dark matter minihalo. The simulations cover more than 25 orders
of magnitude in density and have a maximum spatial resolution of 0.05 R_sun,
which extends well below the radius of individual protostars and captures their
interaction with the surrounding gas. In analogy to previous studies that
employed sink particles, we find that the Keplerian disc around the primary
protostar fragments into a number of secondary protostars, which is facilitated
by H2 collisional dissociation cooling and collision-induced emission. The
further evolution of the protostellar system is characterized by strong
gravitational torques that transfer angular momentum between the secondary
protostars formed in the disc and the surrounding gas. This leads to the
migration of about half of the secondary protostars to the centre of the cloud
in a free-fall time, where they merge with the primary protostar and enhance
its growth to about five times the mass of the second most massive protostar.
By the same token, a fraction of the protostars obtain angular momentum from
other protostars via N-body interactions and migrate to higher orbits. On
average, only every third protostar survives until the end of the simulation.
However, the number of protostars present at any given time increases
monotonically, suggesting that the system will continue to grow beyond the
limited period of time simulated here.Comment: 19 pages, 13 figures, accepted for publication in MNRAS, movies of
the simulations may be downloaded at http://www.mpa-garching.mpg.de/~tgrei
The Formation of the First Low-Mass Stars From Gas With Low Carbon and Oxygen Abundances
The first stars in the Universe are predicted to have been much more massive
than the Sun. Gravitational condensation accompanied by cooling of the
primordial gas due to molecular hydrogen, yields a minimum fragmentation scale
of a few hundred solar masses. Numerical simulations indicate that once a gas
clump acquires this mass, it undergoes a slow, quasi-hydrostatic contraction
without further fragmentation. Here we show that as soon as the primordial gas
- left over from the Big Bang - is enriched by supernovae to a carbon or oxygen
abundance as small as ~0.01-0.1% of that found in the Sun, cooling by
singly-ionized carbon or neutral oxygen can lead to the formation of low-mass
stars. This mechanism naturally accommodates the discovery of solar mass stars
with unusually low (10^{-5.3} of the solar value) iron abundance but with a
high (10^{-1.3} solar) carbon abundance. The minimum stellar mass at early
epochs is partially regulated by the temperature of the cosmic microwave
background. The derived critical abundances can be used to identify those
metal-poor stars in our Milky Way galaxy with elemental patterns imprinted by
the first supernovae.Comment: 14 pages, 2 figures (appeared today in Nature
Magnetar-powered supernovae in two dimensions. II. Broad-line supernovae Ic
Nascent neutron stars with millisecond periods and magnetic fields in excess
of Gauss can drive highly energetic and asymmetric explosions known
as magnetar-powered supernovae. These exotic explosions are one theoretical
interpretation for supernovae Ic-BL which are sometimes associated with long
gamma-ray bursts. Twisted magnetic field lines extract the rotational energy of
the neutron star and release it as a disk wind or a jet with energies greater
than 10 erg over sec. What fractions of the energy of the
central engine go into the wind and the jet remain unclear. We have performed
two-dimensional hydrodynamical simulations of magnetar-powered supernovae (SNe)
driven by disk winds and jets with the CASTRO code to investigate the effect of
the central engine on nucleosynthetic yields, mixing, and light curves. We find
that these explosions synthesize less than 0.05 Msun of Ni and that this mass
is not very sensitive to central engine type. The morphology of the explosion
can provide a powerful diagnostic of the properties of the central engine. In
the absence of a circumstellar medium these events are not very luminous, with
peak bolometric magnitudes due to low Ni production.Comment: Accepted to Ap
Formation of the First Supermassive Black Holes
We consider the physical conditions under which supermassive black holes
could have formed inside the first galaxies. Our SPH simulations indicate that
metal-free galaxies with a virial temperature ~10^4 K and with suppressed H2
formation (due to an intergalactic UV background) tend to form a binary black
hole system which contains a substantial fraction (>10%) of the total baryonic
mass of the host galaxy. Fragmentation into stars is suppressed without
substantial H2 cooling. Our simulations follow the condensation of ~5x10^6
M_sun around the two centers of the binary down to a scale of < 0.1pc. Low-spin
galaxies form a single black hole instead. These early black holes lead to
quasar activity before the epoch of reionization. Primordial black hole
binaries lead to the emission of gravitational radiation at redshifts z>10 that
would be detectable by LISA.Comment: 11 pages, 9 figures, revised version, ApJ in press (October 10, 2003
Cosmic Renaissance: The First Sources of Light
I review recent progress in understanding the formation of the first stars
and quasars. The initial conditions for their emergence are given by the now
firmly established model of cosmological structure formation. Numerical
simulations of the collapse and fragmentation of primordial gas indicate that
the first stars formed at redshifts z ~ 20 - 30, and that they were
predominantly very massive, with M_* > 100 M_sun. Important uncertainties,
however, remain. Paramount among them is the accretion process, which builds up
the final stellar mass by incorporating part of the diffuse, dust-free envelope
into the central protostellar core. The first quasars, on the other hand, are
predicted to have formed later on, at z ~ 10, in more massive dark matter
halos, with total masses, ~ 10^8 M_sun, characteristic of dwarf galaxies.Comment: 16 pages, 7 figures, invited review, to appear in PASP, Feb. 200
The First Supernova Explosions: Energetics, Feedback, and Chemical Enrichment
We perform three-dimensional smoothed particle hydrodynamics simulations in a
realistic cosmological setting to investigate the expansion, feedback, and
chemical enrichment properties of a 200 M_sun pair-instability supernova in the
high-redshift universe. We find that the SN remnant propagates for a Hubble
time at z = 20 to a final mass-weighted mean shock radius of 2.5 kpc (proper),
roughly half the size of the HII region, and in this process sweeps up a total
gas mass of 2.5*10^5 M_sun. The morphology of the shock becomes highly
anisotropic once it leaves the host halo and encounters filaments and
neighboring minihalos, while the bulk of the shock propagates into the voids of
the intergalactic medium. The SN entirely disrupts the host halo and terminates
further star formation for at least 200 Myr, while in our specific case it
exerts positive mechanical feedback on neighboring minihalos by
shock-compressing their cores. In contrast, we do not observe secondary star
formation in the dense shell via gravitational fragmentation, due to the
previous photoheating by the progenitor star. We find that cooling by metal
lines is unimportant for the entire evolution of the SN remnant, while the
metal-enriched, interior bubble expands adiabatically into the cavities created
by the shock, and ultimately into the voids with a maximum extent similar to
the final mass-weighted mean shock radius. Finally, we conclude that dark
matter halos of at least M_vir > 10^8 M_sun must be assembled to recollect all
components of the swept-up gas.Comment: 16 pages, 14 figures, published in Ap
Local radiative feedback in the formation of the first protogalaxies
The formation of the first galaxies is influenced by the radiative feedback
from the first generations of stars. This feedback is manisfested by the
heating and ionization of the gas which lies within the H II regions
surrounding the first stars, as well as by the photodissociation of hydrogen
molecules within the larger Lyman-Werner (LW) bubbles that surround these
sources. Using a ray-tracing method in three-dimensional cosmological
simulations, we self-consistently track the formation of, and radiative
feedback from, individual stars in the course of the formation of a
protogalaxy. We compute in detail the H II regions of each of these sources, as
well as the regions affected by their molecule-dissociating radiation. We
follow the thermal, chemical, and dynamical evolution of the primordial gas, as
it becomes incorporated into the protogalaxy. While the IGM is, in general,
optically thick to LW photons only over physical distances of > 30 kpc at
redshifts z < 20, the high molecule fraction that is built up in relic H II
regions and their increasing volume-filling fraction renders even the local IGM
optically thick to LW photons over physical distances of the order of a few
kiloparsecs. We find that efficient accretion onto Population III relic black
holes may occur after ~ 60 Myr from the time of their formation, by which time
the photo-heated relic H II region gas can cool and re-collapse into the 10^6
M_solar minihalo which hosts the black hole. Also, Pop II.5 stars, postulated
to have masses of the order of 10 M_solar, can likely form from this
re-collapsing relic H II region gas. Overall, we find that the local radiative
feedback from the first generations of stars suppresses the star formation rate
by only a factor of, at most, a few.Comment: 29 pages, 7 figures; ApJ in pres
- …