694 research outputs found
On the stability of nonisothermal Bonnor-Ebert spheres. III. The role of chemistry in core stabilization
Aims. We investigate the effect of chemistry on the stability of starless
cores against gravitational collapse.
Methods. We combine chemical and radiative transfer simulations in the
context of a modified Bonnor-Ebert sphere to model the effect of chemistry on
the gas temperature, and study the effect of temperature changes on core
stability.
Results. We find that chemistry has in general very little effect on the
nondimensional radius which parametrizes the core stability.
Cores that are initially stable or unstable tend to stay near their initial
states, in terms of stability (i.e., constant), as the
chemistry develops. This result is independent of the initial conditions. We
can however find solutions where decreases at late times () which correspond to increased stabilization caused by
the chemistry. Even though the core stability is unchanged by the chemistry in
most of the models considered here, we cannot rule out the possibility that a
core can evolve from an unstable to a stable state owing to chemical evolution.
The reverse case, where an initially stable core becomes ultimately unstable,
seems highly unlikely.
Conclusions. Our results indicate that chemistry should be properly accounted
for in studies of star-forming regions, and that further investigations of core
stability especially with hydrodynamical models are warranted.Comment: 8 pages, 10 figures; accepted for publication in A&
Hydrodynamics with gas-grain chemistry and radiative transfer: comparing dynamical and static models
We quantify if the chemical abundance gradients given by a dynamical model of
core collapse including time-dependent changes in density and temperature
differ greatly from abundances derived from static models, where the density
and temperature structures of the core are kept fixed as the chemistry evolves.
For this study we developed a new one-dimensional spherically symmetric
hydrodynamics code that couples the hydrodynamics equations with a
comprehensive time-dependent gas-grain chemical model, including deuterium and
spin-state chemistry, and radiative transfer calculations to derive
self-consistent time-dependent chemical abundance gradients. We applied the
code to model the collapse of a starless core up to the point when the infall
flow becomes supersonic.
The abundances predicted by the dynamical and static models are almost
identical during the quiescent phase of core evolution, but the results start
to diverge after the onset of core collapse, where the static model
underestimates abundances at high medium density (inner core) and
underestimates them at low density (outer core), and this is clearly reflected
in simulated lines. The static model generally overestimates deuteration, which
is increasingly evident the more D atoms are substituted in the molecule. We
also find that using a limited chemical network, or a limited set of cooling
molecules, may lead to an overestimate of the collapse timescale, and in some
cases may prevent the collapse altogether. In our model, most of the line
cooling near the center of the core is due to HCN, CO, and NO. In conclusion,
the use of a static physical model is not a reliable method of simulating
chemical abundances in starless cores after the onset of gravitational
collapse. The adoption of complex chemistry and a comprehensive set of cooling
molecules is necessary to model the collapse adequately.Comment: Accepted by A&A; 15 pages, 12 figures, 5 tables; arXiv abstract
heavily modified and redacted from the origina
Effect of grain size distribution and size-dependent grain heating on molecular abundances in starless and pre-stellar cores
We present a new gas-grain chemical model to constrain the effect of grain
size distribution on molecular abundances in starless and pre-stellar cores. We
introduce grain-size dependence simultaneously for cosmic-ray (CR)-induced
desorption efficiency and for grain equilibrium temperatures. We keep explicit
track of ice abundances on a set of grain populations. We find that the
size-dependent CR desorption efficiency affects ice abundances in a highly
non-trivial way that depends on the molecule. Species that originate in the gas
phase follow a simple pattern where the ice abundance is highest on the
smallest grains (the most abundant in the distribution). Some molecules, such
as HCN, are instead concentrated on large grains throughout the time evolution,
while others (like ) are initially concentrated on large grains, but
at late times on small grains, due to grain-size-dependent competition between
desorption and hydrogenation. Most of the water ice is on small grains at high
medium density (), where the water
ice fraction, with respect to total water ice reservoir, can be as low as on large (> 0.1 m) grains. Allowing the grain equilibrium
temperature to vary with grain size induces strong variations in relative ice
abundances in low-density conditions where the interstellar radiation field and
in particular its ultraviolet component are not attenuated. Our study implies
consequences not only for the initial formation of ices preceding the starless
core stage, but also for the relative ice abundances on the grain populations
going into the protostellar stage. In particular, if the smallest grains can
lose their mantles due to grain-grain collisions as the core is collapsing, the
ice composition in the beginning of the protostellar stage could be very
different to that in the pre-collapse phase.Comment: Accepted by A&A; 16 pages incl. appendices; abstract abridged to meet
ArXiV requirement
HD depletion in starless cores
Aims: We aim to investigate the abundances of light deuterium-bearing species
such as HD, H2D+ and D2H+ in a gas-grain chemical model including an extensive
description of deuterium and spin state chemistry, in physical conditions
appropriate to the very centers of starless cores. Methods: We combine a
gas-grain chemical model with radiative transfer calculations to simulate
density and temperature structure in starless cores. The chemical model
includes deuterated forms of species with up to 4 atoms and the spin states of
the light species H2, H2+ and H3+ and their deuterated forms. Results: We find
that HD eventually depletes from the gas phase because deuterium is efficiently
incorporated to grain-surface HDO, resulting in inefficient HD production on
grains. HD depletion has consequences not only on the abundances of e.g. H2D+
and D2H+, whose production depends on the abundance of HD, but also on the spin
state abundance ratios of the various light species, when compared with the
complete depletion model where heavy elements do not influence the chemistry.
Conclusions: While the eventual HD depletion leads to the disappearance of
light deuterium-bearing species from the gas phase in a relatively short
timescale at high density, we find that at late stages of core evolution the
abundances of H2D+ and D2H+ increase toward the core edge and the disributions
become extended. The HD depletion timescale increases if less oxygen is
initially present in the gas phase, owing to chemical interaction between the
gas and the dust predecing the starless core phase. Our results are greatly
affected if H2 is allowed to tunnel on grain surfaces, and therefore more
experimental data not only on tunneling but also on the O + H2 surface reaction
in particular is needed.Comment: 14 pages, 12 figures, abstract abridged; accepted for publication in
A &
A study of the -/- ratio in low-mass star forming regions
We use the deuteration of - to probe the physical
parameters of starless and protostellar cores, related to their evolutionary
states, and compare it to the -deuteration in order to
study possible differences between the deuteration of C- and N-bearing species.
We observed the main species -, the singly and doubly
deuterated species - and -, as
well as the isotopologue - toward 10 starless
cores and 5 protostars in the Taurus and Perseus Complexes. We examined the
correlation between the
(-)/(-) ratio and the dust
temperature along with the column density and the CO depletion
factor. The resulting
(-)/(-) ratio is within the
error bars consistent with in all starless cores with detected
-. This also accounts for the protostars except for the
source HH211, where we measure a high deuteration level of . The
deuteration of follows the same trend but is considerably
higher in the dynamically evolved core L1544. Toward the protostellar cores the
coolest objects show the largest deuterium fraction in
-. We show that the deuteration of
- can trace the early phases of star formation and is
comparable to that of . However, the largest
- deuteration level is found toward protostellar cores,
suggesting that while - is mainly frozen onto dust
grains in the central regions of starless cores, active deuteration is taking
place on ice
Nitrogen fractionation towards a pre-stellar core traces isotope-selective photodissociation
Context. Isotopologue abundance ratios are important to understand the evolution of astrophysical objects and ultimately the origins of a planetary system such as our own. With nitrogen being a fundamental ingredient of pre-biotic material, understanding its chemistry and inheritance is of fundamental importance to understand the formation of the building blocks of life.
Aims. We aim to study the 14N/15N ratio in HCN, HNC, and CN across the prototypical pre-stellar core L1544. This study allows us to test the proposed fractionation mechanisms for nitrogen.
Methods. We present here single-dish observations of the ground state rotational transitions of the 13C and 15N isotopologues of HCN, HNC, and CN with the IRAM 30 m telescope. We analyse their column densities and compute the 14N/15N ratio map across the core for HCN. The 15N fractionation of CN and HNC is computed towards different offsets across L1544.
Results. The 15 N-fractionation map of HCN towards a pre-stellar core is presented here for the first time. Our map shows a very clear decrease in the 14N/15N ratio towards the southern edge of L1544, where carbon chain molecules present a peak, strongly suggesting that isotope-selective photodissociation has a strong effect on the fractionation of nitrogen across pre-stellar cores. The 14N/15N ratio in CN measured towards four positions across the core also shows a decrease towards the south-east of the core, while HNC shows the opposite behaviour. We also measured the 12CN/13CN ratio towards four positions across the core.
Conclusions. The uneven illumination of the pre-stellar core L1544 provides clear evidence that 15 N fractionation of HCN and CN is enhanced towards the region more exposed to the interstellar radiation field. Isotope-selective photodissociation of N2 is then a crucial process to understand 15N fractionation, as already found in protoplanetary disks. Therefore, the 15N fractionation in prestellar material is expected to change depending on the environment within which pre-stellar cores are embedded. The 12CN/13CN ratio also varies across the core, but its variation does not affect our conclusions as to the effect of the environment on the fractionation of nitrogen. Nevertheless, the interplay between the carbon and nitrogen fractionation across the core warrants follow-up studies
The incidence of Burkholderia in epiphytic and endophytic bacterial cenoses in hybrid aspen grown on sandy peat
Combined model for , , and spin-state chemistry in molecular clouds
We present a new gas-grain chemical model for the combined isotopic
fractionation of carbon and nitrogen in molecular clouds, in which the isotope
chemistry of carbon and nitrogen is coupled with a time-dependent description
of spin-state chemistry. We updated the rate coefficients of some isotopic
exchange reactions considered in the literature, and present here a set of new
exchange reactions involving molecules substituted in and simultaneously. We apply the model to a series of zero-dimensional
simulations representing a set of physical conditions across a prototypical
prestellar core, exploring the deviations of the isotopic abundance ratios in
the various molecules from the elemental isotopic ratios as a function of
physical conditions and time. We find that the ratio can
deviate from the elemental ratio by up to a factor of several depending on the
molecule, and that there are highly time-dependent variations in the ratios.
The ratio, for example, can obtain values of less than 10
depending on the simulation time. The ratios tend to remain
close to the assumed elemental ratio within ten per cent, with no clear
trends as a function of the physical conditions. Abundance ratios between -containing molecules and +-containing molecules
show somewhat increased levels of fractionation due to the newly included
exchange reactions, though still remaining within a few tens of per cent of the
elemental ratio. Our results imply the existence of
gradients in isotopic abundance ratios across prestellar cores, suggesting that
detailed simulations are required to interpret observations of isotopically
substituted molecules correctly, especially given that the various isotopic
forms of a given molecule do not necessarily trace the same gas layers.Comment: Accepted to A\&A; abstract abridged to meet arXiv requirement
Hoxd11 specifies a program of metanephric kidney development within the intermediate mesoderm of the mouse embryo
AbstractThe mammalian kidney consists of an array of tubules connected to a ductal system that collectively function to control water/salt balance and to remove waste from the organisms' circulatory system. During mammalian embryogenesis, three kidney structures form within the intermediate mesoderm. The two most anterior structures, the pronephros and the mesonephros, are transitory and largely non-functional, while the most posterior, the metanephros, persists as the adult kidney. We have explored the mechanisms underlying regional specific differentiation of the kidney forming mesoderm. Previous studies have shown a requirement for Hox11 paralogs (Hoxa11, Hoxc11 and Hoxd11) in metanephric development. Mice lacking all Hox11 activity fail to form metanephric kidney structures. We demonstrate that the Hox11 paralog expression is restricted in the intermediate mesoderm to the posterior, metanephric level. When Hoxd11 is ectopically activated in the anterior mesonephros, we observe a partial transformation to a metanephric program of development. Anterior Hoxd11+ cells activate Six2, a transcription factor required for the maintenance of metanephric tubule progenitors. Additionally, Hoxd11+ mesonephric tubules exhibit an altered morphology and activate several metanephric specific markers normally confined to distal portions of the functional nephron. Collectively, our data support a model where Hox11 paralogs specify a metanephric developmental program in responsive intermediate mesoderm. This program maintains tubule forming progenitors and instructs a metanephric specific pattern of nephron differentiation
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