37 research outputs found
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
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
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
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&
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
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
Parameterizing the interstellar dust temperature
The temperature of interstellar dust particles is of great importance to
astronomers. It plays a crucial role in the thermodynamics of interstellar
clouds, because of the gas-dust collisional coupling. It is also a key
parameter in astrochemical studies that governs the rate at which molecules
form on dust. In 3D (magneto)hydrodynamic simulations often a simple expression
for the dust temperature is adopted, because of computational constraints,
while astrochemical modelers tend to keep the dust temperature constant over a
large range of parameter space. Our aim is to provide an easy-to-use parametric
expression for the dust temperature as a function of visual extinction () and to shed light on the critical dependencies of the dust temperature on
the grain composition. We obtain an expression for the dust temperature by
semi-analytically solving the dust thermal balance for different types of
grains and compare to a collection of recent observational measurements. We
also explore the effect of ices on the dust temperature. Our results show that
a mixed carbonaceous-silicate type dust with a high carbon volume fraction
matches the observations best. We find that ice formation allows the dust to be
warmer by up to 15% at high optical depths ( mag) in the
interstellar medium. Our parametric expression for the dust temperature is
presented as , where is in units of the Draine (1978) UV fieldComment: 16 pages, 17 figures, 4 tables. Accepted for publication in A&A.
Version 2: the omission of factor 0.921 in equation 4 is correcte
The impact of X-rays on molecular cloud fragmentation and the IMF
Star formation is regulated through a variety of feedback processes. In this
study, we treat feedback by X-rays and discuss its implications. Our aim is to
investigate whether star formation is significantly affected when a star
forming cloud resides in the vicinity of a strong X-ray source. We perform an
Eulerian grid simulation with embedded Lagrangian sink particles of a
collapsing molecular cloud near a massive, 10^7 M_o black hole. The chemical
and thermal changes caused by radiation are incorporated into the FLASH code.
When there is strong X-ray feedback the star forming cloud fragments into
larger clumps whereby fewer but more massive protostellar cores are formed.
Competitive accretion has a strong impact on the mass function and a near-flat,
non-Salpeter IMF results.Comment: 6 pages, 4 figures. Accepted for publication in Astronomy and
Astrophysic
Warm dust surface chemistry in protoplanetary disks : formation of phyllosilicates
Funding: Dr. Thomas Müller, and Dr. Victor Ali-Lagoa. IK,WFT, CR, and PW acknowledge fundingfrom the EU FP7- 2011 under Grant Agreement nr. 284405. CR also acknowl-edges funding by the Austrian Science Fund (FWF), project number P24790.Context. The origin of the reservoirs of water on Earth is debated. The Earth’s crust may contain at least three times more water than the oceans. This crust water is found in the form of phyllosilicates, whose origin probably differs from that of the oceans. Aims. We test the possibility to form phyllosilicates in protoplanetary disks, which can be the building blocks of terrestrial planets. Methods. We developed an exploratory rate-based warm surface chemistry model where water from the gas-phase can chemisorb on dust grain surfaces and subsequently diffuse into the silicate cores. We applied the phyllosilicate formation to a zero-dimensional chemical model and to a 2D protoplanetary disk model (PRODIMO). The disk model includes in addition to the cold and warm surface chemistry continuum and line radiative transfer, photoprocesses (photodissociation, photoionisation, and photodesorption), gas-phase cold and warm chemistry including three-body reactions, and detailed thermal balance. Results. Despite the high energy barrier for water chemisorption on silicate grain surfaces and for diffusion into the core, the chemisorption sites at the surfaces can be occupied by a hydroxyl bond (–OH) at all gas and dust temperatures from 80 to 700 K for a gas density of 2 × 104 cm−3. The chemisorption sites in the silicate cores are occupied at temperatures between 250 and 700 K. At higher temperatures thermal desorption of chemisorbed water occurs. The occupation efficiency is only limited by the maximum water uptake of the silicate. The timescales for complete hydration are at most 105 yr for 1 mm radius grains at a gas density of 108 cm−3. Conclusions. Phyllosilicates can be formed on dust grains at the dust coagulation stage in protoplanetary disks within 1 Myr. It is however not clear whether the amount of phyllosilicate formed by warm surface chemistry is sufficient compared to that found in Solar System objects.PostprintPeer reviewe