77 research outputs found
The impact of metallicity and X-rays on star formation
Star formation is regulated through a variety of feedback processes. In this
study, we treat feedback by metal injection and a UV background as well as by
X-ray irradiation. Our aim is to investigate whether star formation is
significantly affected when the ISM of a proto-galaxxy enjoys different
metallicities and when a star forming cloud resides in the vicinity of a strong
X-ray source. We perform cosmological Enzo simulations with a detailed
treatment of non-zero metallicity chemistry and thermal balance. We also
perform FLASH simulations with embedded Lagrangian sink particles of a
collapsing molecular cloud near a massive, 10^{7} M\odot, black hole that
produces X-ray radiation. We find that a multi-phase ISM forms for metallicites
as small as 10^{-4} Solar at z = 6, with higher (10^{-2}Z\odot) metallicities
supporting a cold ( 10^{3} cm^{-3}) phase at higher (z =
20) redshift. A star formation recipe based on the presence of a cold dense
phase leads to a self-regulating mode in the presence of supernova and
radiation feedback. We also find that when there is strong X-ray feedback a
collapsing cloud fragments into larger clumps whereby fewer but more massive
protostellar cores are formed. This is a consequence of the higher Jeans mass
in the warm (50 K, due to ionization heating) molecular gas. Accretion
processes dominate the mass function and a near-flat, non-Salpeter IMF results.Comment: Proceedings IAU Symposium No. 270, 2010. 4 pages, 5 figure
Star formation near an obscured AGN: Variations in the initial mass function
The conditions that affect the formation of stars in radiatively and
mechanically active environments are quite different than the conditions that
apply to our local interstellar neighborhood. In such galactic environments, a
variety of feedback processes can play a significant role in shaping the
initial mass function (IMF). Here, we present a numerical study on the effects
of an accreting black hole and the influence of nearby massive stars to a
collapsing, 800 M_sun, molecular cloud at 10 pc distance from the black hole.
We parametrize and study radiative feedback effects of hard X-rays emanating
from the black hole broad line region, increased cosmic ray rates due to
supernovae in starbursts, and strong UV radiation produced by nearby massive
stars. We also investigate the importance of shear from the supermassive,
10^6-10^8 M_sun, black hole as the star-forming cloud orbits around it. We find
that thermal pressure from X-rays compresses the cloud, which induces a high
star formation rate early on, but reduces the overall star formation efficiency
to about 7% due to gas depletion by evaporation. We see that the turn-over mass
of the IMF increases up to a factor of 2.3, M_turn = 1-1.5 M_sun, for the model
with the highest X-ray flux (160 erg s^-1 cm^-2), while the high-mass slope of
the IMF becomes Gamma > -1. This results in more high mass stars and a
non-Salpeter IMF. Cosmic rays penetrate deeply into the cloud and increase the
gas temperature (50-200 K), which leads to a reduced formation efficiency of
low mass stars. High cosmic ray rates increase the average mass of stars,
thereby shifting the turn-over mass to higher values, i.e., up to several solar
masses. Due to this process, the onset of star formation is also delayed. We
conclude that the IMF inside active galaxies is different than the one obtained
from local environments.Comment: 25 pages, 17 figure
Interplay of gas and ice during cloud evolution
During the evolution of diffuse clouds to molecular clouds, gas-phase
molecules freeze out on surfaces of small dust particles to form ices. On dust
surfaces, water is the main constituent of the icy mantle in which a complex
chemistry is taking place. We aim to study the formation pathways and the
composition of the ices throughout the evolution of diffuse clouds. For this
purpose, we use time-dependent rate equations to calculate the molecular
abundances in both gas phase and on solid surfaces (onto dust grains). We fully
consider the gas-dust interplay by including the details of freeze-out,
chemical and thermal desorption, as well as the most important photo-processes
on grain surfaces. The difference in binding energies of chemical species on
bare and icy surfaces is also incorporated into our equations. Using the
numerical code FLASH, we perform a hydrodynamical simulation of a
gravitationally bound diffuse cloud and follow its contraction. We find that
while the dust grains are still bare, water formation is enhanced by grain
surface chemistry which is subsequently released into the gas phase, enriching
the molecular medium. The CO molecules, on the other hand, tend to freeze out
gradually on bare grains. This causes CO to be well mixed and strongly present
within the first ice layer. Once one monolayer of water ice has formed, the
binding energy of the grain surface changes significantly and an immediate and
strong depletion of gas-phase water and CO molecules occur. While hydrogenation
converts solid CO into formaldehyde (HCO) and methanol (CHOH), water
ice becomes the main constituent of the icy grains. Inside molecular clumps
formaldehyde is more abundant than water and methanol in the gas phase owing
its presence in part to chemical desorption.Comment: 19 pages, 10 figures, 9 tables, 23 equations. Accepted for
publication Astronomy & Astrophysics. In version 3: Language edit, added
gas-phase reaction tables, title has change
The impact of freeze-out on collapsing molecular clouds
Atoms and molecules, and in particular CO, are important coolants during the
evolution of interstellar star-forming gas clouds. The presence of dust grains,
which allow many chemical reactions to occur on their surfaces, strongly
impacts the chemical composition of a cloud. At low temperatures, dust grains
can lock-up species from the gas phase which freeze out and form ices. In this
sense, dust can deplete important coolants. Our aim is to understand the
effects of freeze-out on the thermal balance and the evolution of a
gravitationally bound molecular cloud. For this purpose, we perform 3D
hydrodynamical simulations with the adaptive mesh code FLASH. We simulate a
gravitationally unstable cloud under two different conditions, with and without
grain surface chemistry. We let the cloud evolve until one free-fall time is
reached and track the thermal evolution and the abundances of species during
this time. We see that at a number density of 10 cm most of the CO
molecules are frozen on dust grains in the run with grain surface chemistry,
thereby depriving the most important coolant. As a consequence, we find that
the temperature of the gas rises up to 25 K. The temperature drops once
again due to gas-grain collisional cooling when the density reaches a
few10 cm. We conclude that grain surface chemistry not only
affects the chemical abundances in the gas phase, but also leaves a distinct
imprint in the thermal evolution that impacts the fragmentation of a
star-forming cloud. As a final step, we present the equation of state of a
collapsing molecular cloud that has grain surface chemistry included.Comment: Increased the number of significant digits in EQ 2. It mattered.
Accepted for publication in MNRAS letter
Chemical fractionation of deuterium in the protosolar nebula
Understanding gas-grain chemistry of deuterium in star-forming objects may
help to explain their history and present state. We aim to clarify how
processes in ices affect the deuterium fractionation. In this regard, we
investigate a Solar-mass protostellar envelope using an astrochemical
rate-equation model that considers bulk-ice chem- istry. The results show a
general agreement with the molecular D/H abundance ratios observed in low-mass
protostars. The simultaneous processes of ice accumulation and rapid synthesis
of HD on grain surfaces in the prestellar core hampers the deuteration of icy
species. The observed very high D/H ratios exceeding 10 per cent, i.e., super-
deuteration, are reproduced for formaldehyde and dimethyl ether, but not for
other species in the protostellar envelope phase. Chemical transformations in
bulk ice lower D/H ratios of icy species and do not help explaining the
super-deuteration. In the protostellar phase, the D2O/HDO abundance ratio was
calculated to be higher than the HDO/H2O ratio owing to gas-phase chemistry.
Species that undergo evaporation from ices have high molecular D/H ratio and a
high gas-phase abundance.Comment: 11 pages, 4 tables, 6 figures; +3 figures in appendix. Accepted for
publication in MNRA
Dust as interstellar catalyst I. Quantifying the chemical desorption process
Context. The presence of dust in the interstellar medium has profound
consequences on the chemical composition of regions where stars are forming.
Recent observations show that many species formed onto dust are populating the
gas phase, especially in cold environments where UV and CR induced photons do
not account for such processes. Aims. The aim of this paper is to understand
and quantify the process that releases solid species into the gas phase, the
so-called chemical desorption process, so that an explicit formula can be
derived that can be included into astrochemical models. Methods. We present a
collection of experimental results of more than 10 reactive systems. For each
reaction, different substrates such as oxidized graphite and compact amorphous
water ice are used. We derive a formula to reproduce the efficiencies of the
chemical desorption process, which considers the equipartition of the energy of
newly formed products, followed by classical bounce on the surface. In part II
we extend these results to astrophysical conditions. Results. The equipartition
of energy describes correctly the chemical desorption process on bare surfaces.
On icy surfaces, the chemical desorption process is much less efficient and a
better description of the interaction with the surface is still needed.
Conclusions. We show that the mechanism that directly transforms solid species
to gas phase species is efficient for many reactions.Comment: Accepted for publication in A&
Dust temperature and time-dependent effects in the chemistry of photodissociation regions
When studying the chemistry of PDRs, time dependence becomes important as
visual extinction increases, since certain chemical timescales are comparable
to the cloud lifetime. Dust temperature is also a key factor, since it
significantly influences gas temperature and mobility on dust grains,
determining the chemistry occurring on grain surfaces. We present a study of
the dust temperature impact and time effects on the chemistry of different
PDRs, using an updated version of the Meijerink PDR code and combining it with
the time-dependent code Nahoon. We find the largest temperature effects in the
inner regions of high PDRs, where high dust temperatures
favour the formation of simple oxygen-bearing molecules (especially that of
O), while the formation of complex organic molecules is much more efficient
at low dust temperatures. We also find that time-dependent effects strongly
depend on the PDR type, since long timescales promote the destruction of
oxygen-bearing molecules in the inner parts of low PDRs,
while favouring their formation and that of carbon-bearing molecules in high
PDRs. From the chemical evolution, we also conclude that, in
dense PDRs, CO is a late-forming ice compared to water ice, and confirm a
layered ice structure on dust grains, with HO in lower layers than CO.
Regarding steady state, the PDR edge reaches chemical equilibrium at early
times (10 yr). This time is even shorter (10 yr) for high
PDRs. By contrast, inner regions reach equilibrium much
later, especially low PDRs, where steady state is reached at
10-10 yr.Comment: 24 pages, 15 figures, 9 table
The asymmetric radio structure and record jet of giant quasar 4C 34.47
Giant double-lobed radio source 4C34.47 displays a straight one-sided jet,
measuring a record length of 380kpc, in its double-lobed radio structure.
Assuming an intrinsically symmetric two-sided jet structure the radio source
jet axis must be at least 33 degrees away from the sky plane, that is within 57
degrees from the line of sight. The radio polarization properties indicate that
this giant source has largely outgrown the depolarizing halo generally
associated with the host galaxies of powerful radio sources. The measured small
depolarization asymmetry is nevertheless in accordance with its inferred
orientation. All data for this giant radio source are in agreement with its
preferred orientation as predicted within the unification scheme for powerful
radio sources. Seen under a small aspect angle the radio source is large but
not excessively large. The global properties of 4C34.47 do not differ from
other giant (old) FR2 radio sources: it is a slowly expanding low-luminosity
radio source.Comment: Accepted for publication in Astronomy and Astrophysic
The first frost in the Pipe Nebula
Spectroscopic studies of ices in nearby star-forming regions indicate that
ice mantles form on dust grains in two distinct steps, starting with polar ice
formation (H2O rich) and switching to apolar ice (CO rich). We test how well
the picture applies to more diffuse and quiescent clouds where the formation of
the first layers of ice mantles can be witnessed. Medium-resolution
near-infrared spectra are obtained toward background field stars behind the
Pipe Nebula. The water ice absorption is positively detected at 3.0 micron in
seven lines of sight out of 21 sources for which observed spectra are
successfully reduced. The peak optical depth of the water ice is significantly
lower than those in Taurus with the same visual extinction. The source with the
highest water-ice optical depth shows CO ice absorption at 4.7 micron as well.
The fractional abundance of CO ice with respect to water ice is 16+7-6 %, and
about half as much as the values typically seen in low-mass star-forming
regions. A small fractional abundance of CO ice is consistent with some of the
existing simulations. Observations of CO2 ice in the early diffuse phase of a
cloud play a decisive role in understanding the switching mechanism between
polar and apolar ice formation.Comment: 17 pages, 8 figures, accepted by A&
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