3,954 research outputs found
Primordial star formation: relative impact of H2 three-body rates and initial conditions
Population III stars are the first stars in the Universe to form at z=20-30
out of a pure hydrogen and helium gas in minihalos of 10^5-10^6 M .
Cooling and fragmentation is thus regulated via molecular hydrogen. At
densities above 10^8 cm, the three-body H2 formation rates are
particularly important for making the gas fully molecular. These rates were
considered to be uncertain by at least a few orders of magnitude. We explore
the impact of new accurate three-body H2 formation rates derived by Forrey
(2013) for three different minihalos, and compare to the results obtained with
three-body rates employed in previous studies. The calculations are performed
with the cosmological hydrodynamics code ENZO (release 2.2) coupled with the
chemistry package KROME (including a network for primordial chemistry), which
was previously shown to be accurate in high resolution simulations. While the
new rates can shift the point where the gas becomes fully molecular, leading to
a different thermal evolution, there is no trivial trend in how this occurs.
While one might naively expect the results to be inbetween the calculations
based on Palla et al. (1983) and Abel et al. (2002), the behavior can be close
to the former or the latter depending on the dark matter halo that is explored.
We conclude that employing the correct three-body rates is about as equally
important as the use of appropriate initial conditions, and that the resulting
thermal evolution needs to be calculated for every halo individually.Comment: 10 pages, 9 figures, A&A, 561, A13 (2014
Episodic accretion in binary protostars emerging from self-gravitating solar mass cores
Observations show a large spread in the luminosities of young protostars,
which are frequently explained in the context of episodic accretion. We here
test this scenario using numerical simulations following the collapse of a
solar mass molecular cloud using the GRADSPH code, varying the strength of the
initial perturbations and the temperature of the cores. A specific emphasis of
this paper is to investigate the role of binaries and multiple systems in the
context of episodic accretion, and to compare their evolution to the evolution
in isolated fragments. Our models form a variety of low mass protostellar
objects including single, binary and triple systems with binaries more active
in exhibiting episodic accretion than isolated protostars. We also find a
general decreasing trend for the average mass accretion rate over time,
suggesting that the majority of the protostellar mass is accreted within the
first 10^5 years. This result can potentially help to explain the surprisingly
low average luminosities in the majority of the protostellar population.Comment: 16 pages, 13 figures, 4 tables. Accepted for publication with A&
Probing high-redshift quasars with ALMA. I. Expected observables and potential number of sources
(abridged) We explore how ALMA observations can probe high-redshift galaxies
in unprecedented detail. We discuss the main observables that are excited by
the large-scale starburst, and formulate expectations for the chemistry and the
fluxes in the center of active galaxies, where chemistry may be driven by the
absorption of X-ray photons. We show that such X-ray dominated regions (XDRs)
should be large enough to be resolved with ALMA, and predict the expected
amount of emission in CO and various fine-structure lines. We discuss how such
XDRs can be distinguished from a strong starburst on the same spatial scales
based on the CO line SED. Our models are compared to known sources like NGC
1068 and APM 08279. We also analyze the properties of the z=6.42 quasar SDSS
J114816.64+525150.3, and find that the observed emission in CO, [CII] and [CI]
requires a dense warm and a low-density cold gas component. We estimate the
expected number of sources at redshifts higher than 6, finding that one could
expect one black hole with solar masses per arcmin.Comment: 15 pages, 17 figures, accepted by A&
Magnetic Field Amplification in Young Galaxies
The Universe at present is highly magnetized, with fields of the order of a
few 10^-5 G and coherence lengths larger than 10 kpc in typical galaxies like
the Milky Way. We propose that the magnetic field was amplified to this values
already during the formation and the early evolution of the galaxies.
Turbulence in young galaxies is driven by accretion as well as by supernova
(SN) explosions of the first generation of stars. The small-scale dynamo can
convert the turbulent kinetic energy into magnetic energy and amplify very weak
primordial magnetic seed fields on short timescales. The amplification takes
place in two phases: in the kinematic phase the magnetic field grows
exponentially, with the largest growth on the smallest non-resistive scale. In
the following non-linear phase the magnetic energy is shifted towards larger
scales until the dynamo saturates on the turbulent forcing scale. To describe
the amplification of the magnetic field quantitatively we model the
microphysics in the interstellar medium (ISM) of young galaxies and determine
the growth rate of the small-scale dynamo. We estimate the resulting saturation
field strengths and dynamo timescales for two turbulent forcing mechanisms:
accretion-driven turbulence and SN-driven turbulence. We compare them to the
field strength that is reached, when only stellar magnetic fields are
distributed by SN explosions. We find that the small-scale dynamo is much more
efficient in magnetizing the ISM of young galaxies. In the case of
accretion-driven turbulence a magnetic field strength of the order of 10^-6 G
is reached after a time of 24-270 Myr, while in SN-driven turbulence the dynamo
saturates at field strengths of typically 10^-5 G after only 4-15 Myr. This is
considerably shorter than the Hubble time. Our work can help to understand why
present-day galaxies are highly magnetized.Comment: 13 pages, 8 figures; A&A in pres
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