541 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
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
The Snow Border
Context. The study of the snow line is an important topic in several domains
of astrophysics, and particularly for the evolution of proto-stellar
environments and the formation of planets. Aims. The formation of the first
layer of ice on carbon grains requires low temperatures compared to the
temperature of evaporation (T > 100 K). This asymmetry generates a zone in
which bare and icy dust grains coexist. Methods. We use Monte-Carlo simulations
to describe the formation time scales of ice mantles on bare grains in
protostellar disks and massive protostars environments. Then we analytically
describe these two systems in terms of grain populations subject to infall and
turbulence, and assume steady-state. Results. Our results show that there is an
extended region beyond the snow line where icy and bare grains can coexist, in
both proto-planetary disks and massive protostars. This zone is not negligible
compared to the total size of the objects: on the order of 0.4 AU for
proto-planetary disks and 5400 AU for high-mass protostars. Times to reach the
steady-state are respectively es- timated from 10^2 to 10^5 yr. Conclusions.
The presence of a zone, a so-called snow border, in which bare and icy grains
co- exist can have a major impact on our knowledge of protostellar
environments. From a theoretical point of view, the progression of icy grains
to bare grains as the temperature increases, could be a realistic way to model
hot cores and hot corinos. Also, in this zone, the formation of planetesimals
will require the coagulation of bare and icy grains. Observationally, this zone
allows high abundances of gas phase species at large scales, for massive
protostars particularly, even at low temperatures (down to 50 K).Comment: accepted in A&
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&
Effective grain surface area in the formation of molecular hydrogen in interstellar clouds
In the interstellar clouds, molecular hydrogens are formed from atomic
hydrogen on grain surfaces. An atomic hydrogen hops around till it finds
another one with which it combines. This necessarily implies that the average
recombination time, or equivalently, the effective grain surface area depends
on the relative numbers of atomic hydrogen influx rate and the number of sites
on the grain. Our aim is to discover this dependency. We perform a numerical
simulation to study the recombination of hydrogen on grain surfaces in a
variety of cloud conditions. We use a square lattice (with a periodic boundary
condition) of various sizes on two types of grains, namely, amorphous carbon
and olivine. We find that the steady state results of our simulation match very
well with those obtained from a simpler analytical consideration provided the
`effective' grain surface area is written as , where, is
the actual physical grain area and is a function of the flux of atomic
hydrogen which is determined from our simulation. We carry out the simulation
for various astrophysically relevant accretion rates. For high accretion rates,
small grains tend to become partly saturated with and and the
subsequent accretion will be partly inhibited. For very low accretion rates,
the number of sites to be swept before a molecular hydrogen can form is too
large compared to the actual number of sites on the grain, implying that
is greater than unity.Comment: 8 pages, 5 figures in eps forma
Pore evolution in interstellar ice analogues: simulating the effects of temperature increase
Context. The level of porosity of interstellar ices - largely comprised of
amorphous solid water (ASW) - contains clues on the trapping capacity of other
volatile species and determines the surface accessibility that is needed for
solid state reactions to take place. Aims. Our goal is to simulate the growth
of amorphous water ice at low temperature (10 K) and to characterize the
evolution of the porosity (and the specific surface area) as a function of
temperature (from 10 to 120 K). Methods. Kinetic Monte Carlo simulations are
used to mimic the formation and the thermal evolution of pores in amorphous
water ice. We follow the accretion of gas-phase water molecules as well as
their migration on surfaces with different grid sizes, both at the top growing
layer and within the bulk. Results. We show that the porosity characteristics
change substantially in water ice as the temperature increases. The total
surface of the pores decreases strongly while the total volume decreases only
slightly for higher temperatures. This will decrease the overall reaction
efficiency, but in parallel, small pores connect and merge, allowing trapped
molecules to meet and react within the pores network, providing a pathway to
increase the reaction efficiency. We introduce pore coalescence as a new solid
state process that may boost the solid state formation of new molecules in
space and has not been considered so far.Comment: 9 pages, 8 figures Accepted for publication in A&
Interstellar ices as witnesses of star formation: selective deuteration of water and organic molecules unveiled
Observations of star forming environments revealed that the abundances of
some deuterated interstellar molecules are markedly larger than the cosmic D/H
ratio of 10-5. Possible reasons for this pointed to grain surface chemistry.
How- ever, organic molecules and water, which are both ice constituents, do not
enjoy the same deuteration. For example, deuterated formaldehyde is very
abundant in comets and star forming regions, while deuterated water rarely is.
In this article, we explain this selective deuteration by following the
formation of ices (using the rate equation method) in translucent clouds, as
well as their evolu- tion as the cloud collapses to form a star. Ices start
with the deposition of gas phase CO and O onto dust grains. While reaction of
oxygen with atoms (H or D) or molecules (H2) yields H2O (HDO), CO only reacts
with atoms (H and D) to form H2CO (HDCO, D2CO). As a result, the deuteration of
formaldehyde is sensitive to the gas D/H ratio as the cloud undergoes
gravitational collapse, while the deuteration of water strongly depends on the
dust temperature at the time of ice formation. These results reproduce well the
deuterium fractionation of formaldehyde observed in comets and star forming
regions and can explain the wide spread of deuterium fractionation of water
observed in these environments.Comment: 4 pages, 3 figures, Accepted in ApJ letter; Astrophysical Journal
LET26536R1 201
Water formation on bare grains: When the chemistry on dust impacts interstellar gas
Context. Water together with O2 are important gas phase ingredients to cool
dense gas in order to form stars. On dust grains, H2 O is an important
constituent of the icy mantle in which a complex chemistry is taking place, as
revealed by hot core observations. The formation of water can occur on dust
grain surfaces, and can impact gas phase composition. Aims. The formation of
molecules such as OH, H2 O, HO2, H2 O2, as well as their deuterated forms and
O2 and O3 is studied in order to assess how the chemistry varies in different
astrophysical environments, and how the gas phase is affected by grain surface
chemistry. Methods. We use Monte Carlo simulations to follow the formation of
molecules on bare grains as well as the fraction of molecules released into the
gas phase. We consider a surface reaction network, based on gas phase
reactions, as well as UV photo-dissociation of the chemical species. Results.
We show that grain surface chemistry has a strong impact on gas phase
chemistry, and that this chemistry is very different for different dust grain
temperatures. Low temperatures favor hydrogenation, while higher temperatures
favor oxygenation. Also, UV photons dissociate the molecules on the surface,
that can reform subsequently. The formation-destruction cycle increases the
amount of species released into the gas phase. We also determine the time
scales to form ices in diffuse and dense clouds, and show that ices are formed
only in shielded environments, as supported by observations.Comment: Accepted in A&
Molecular hydrogen formation in the interstellar medium
We have developed a model for molecular hydrogen formation under
astrophysically relevant conditions. This model takes fully into account the
presence of both physisorbed and chemisorbed sites on the surface, allows
quantum mechanical diffusion as well as thermal hopping for absorbed H-atoms,
and has been benchmarked versus recent laboratory experiments on H2 formation
on silicate surfaces. The results show that H2 formation on grain surface is
efficient in the interstellar medium up to some 300K. At low temperatures
(<100K), H2 formation is governed by the reaction of a physisorbed H with a
chemisorbed H. At higher temperatures, H2 formation proceeds through reaction
between two chemisorbed H atoms. We present simple analytical expressions for
H2 formation which can be adopted to a wide variety of surfaces once their
surfaces characteristics have been determined experimentally.Comment: 4 pages, 1 figur
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