233 research outputs found
Collisional Quenching at Ultralow Energies: Controlling Efficiency with Internal State Selection
Calculations have been carried out for the vibrational quenching of excited
H molecules which collide with Li ions at ultralow energies. The
dynamics has been treated exactly using the well known quantum coupled-channel
expansions over different initial vibrational levels. The overall interaction
potential has been obtained from the calculations carried out earlier in our
group using highly correlated ab initio methods. The results indicate that
specific features of the scattering observables, e.g. the appearance of
Ramsauer-Townsend minima in elastic channel cross sections and the marked
increase of the cooling rates from specific initial states, can be linked to
potential properties at vanishing energies (sign and size of scattering
lengths) and to the presence of either virtual states or bound states. The
suggestion is made that by selecting the initial state preparation of the
molecular partners, the ionic interactions would be amenable to controlling
quenching efficiency at ultralow energies
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
Ion chemistry in the early universe: revisiting the role of HeH+ with new quantum calculations
The role of HeH+ has been newly assessed with the aid of newly calculated
rates which use entirely ab initio methods, thereby allowing us to compute more
accurately the relevant abundances within the global chemical network of the
early universe. A comparison with the similar role of the ionic molecule LiH+
is also presented. Quantum calculations have been carried out for the gas-phase
reaction of HeH+ with H atoms with our new in-house code, based on the negative
imaginary potential method. Integral cross sections and reactive rate
coefficients obtained under the general conditions of early universe chemistry
are presented and discussed. With the new reaction rate, the abundance of HeH+
in the early universe is more than one order of magnitude larger than in
previous studies. Our more accurate findings further buttress the possibility
to detect cosmological signatures of HeH+.Comment: Astronomy and Astrophysics, in pres
The formation of the primitive star SDSS J102915+172927: effect of the dust mass and the grain-size distribution
Understanding the formation of the extremely metal poor star
SDSS-J102915+172927 is of fundamental importance to improve our knowledge on
the transition between the first and second generation of stars in the
Universe. In this paper, we perform three-dimensional cosmological
hydrodynamical simulations of dust-enriched halos during the early stages of
the collapse process including a detailed treatment of the dust physics. We
employ the astrochemistry package \krome coupled with the hydrodynamical code
\textsc{enzo} assuming grain size distributions produced by the explosion of
core-collapse supernovae of 20 and 35 M primordial stars which are
suitable to reproduce the chemical pattern of the SDSS-J102915+172927 star. We
find that the dust mass yield produced from Population III supernovae
explosions is the most important factor which drives the thermal evolution and
the dynamical properties of the halos. Hence, for the specific distributions
relevant in this context, the composition, the dust optical properties, and the
size-range have only minor effects on the results due to similar cooling
functions. We also show that the critical dust mass to enable fragmentation
provided by semi-analytical models should be revised, as we obtain values one
order of magnitude larger. This determines the transition from disk
fragmentation to a more filamentary fragmentation mode, and suggests that
likely more than one single supernova event or efficient dust growth should be
invoked to get such a high dust content.Comment: Accepted on Ap
Formation of carbon-enhanced metal-poor stars in the presence of far ultraviolet radiation
Recent discoveries of carbon-enhanced metal-poor stars like SMSS
J031300.36-670839.3 provide increasing observational insights into the
formation conditions of the first second-generation stars in the Universe,
reflecting the chemical conditions after the first supernova explosion. Here,
we present the first cosmological simulations with a detailed chemical network
including primordial species as well as C, C, O, O, Si, Si, and
Si following the formation of carbon-enhanced metal poor stars. The
presence of background UV flux delays the collapse from to and
cool the gas down to the CMB temperature for a metallicity of
Z/Z=10. This can potentially lead to the formation of lower mass
stars. Overall, we find that the metals have a stronger effect on the collapse
than the radiation, yielding a comparable thermal structure for large
variations in the radiative background. We further find that radiative
backgrounds are not able to delay the collapse for Z/Z=10 or a
carbon abundance as in SMSS J031300.36-670839.3.Comment: submitted to ApJ
Dark-matter halo mergers as a fertile environment for low-mass Population III star formation
While Population III stars are typically thought to be massive, pathways
towards lower-mass Pop III stars may exist when the cooling of the gas is
particularly enhanced. A possible route is enhanced HD cooling during the
merging of dark-matter halos. The mergers can lead to a high ionization degree
catalysing the formation of HD molecules and may cool the gas down to the
cosmic microwave background (CMB) temperature. In this paper, we investigate
the merging of mini-halos with masses of a few 10 M and explore the
feasibility of this scenario. We have performed three-dimensional cosmological
hydrodynamics calculations with the ENZO code, solving the thermal and chemical
evolution of the gas by employing the astrochemistry package KROME. Our results
show that the HD abundance is increased by two orders of magnitude compared to
the no-merging case and the halo cools down to 60 K triggering
fragmentation. Based on Jeans estimates the expected stellar masses are about
10 M. Our findings show that the merging scenario is a potential
pathway for the formation of low-mass stars.Comment: Submitted to MNRA
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