196 research outputs found
Efficient ortho-para conversion of H2 on interstellar grain surfaces
Context: Fast surface conversion between ortho- and para-H2 has been observed
in laboratory studies, and this mechanism has been proposed to play a role in
the control of the ortho-para ratio in the interstellar medium. Observations of
rotational lines of H2 in Photo-Dissociation Regions (PDRs) have indeed found
significantly lower ortho-para ratios than expected at equilibrium. The
mechanisms controlling the balance of the ortho-para ratio in the interstellar
medium thus remain incompletely understood, while this ratio can affect the
thermodynamical properties of the gas (equation of state, cooling function).
Aims: We aim to build an accurate model of ortho-para conversion on dust
surfaces based on the most recent experimental and theoretical results, and to
validate it by comparison to observations of H2 rotational lines in PDRs.
Methods: We propose a statistical model of ortho-para conversion on dust grains
with fluctuating dust temperatures, based on a master equation approach. This
computation is then coupled to full PDR models and compared to PDR
observations. Results: We show that the observations of rotational H2 lines
indicate a high conversion efficiency on dust grains, and that this high
efficiency can be accounted for if taking dust temperature fluctuations into
account with our statistical model of surface conversion. Simpler models
neglecting the dust temperature fluctuations do not reach the high efficiency
deduced from the observations. Moreover, this high efficiency induced by dust
temperature fluctuations is quite insensitive to the values of microphysical
parameters of the model. Conclusions: Ortho-para conversion on grains is thus
an efficient mechanism in most astrophysical conditions that can play a
significant role in controlling the ortho-para ratio.Comment: Accepted in Astronomy & Astrophysic
Surface chemistry in the Interstellar Medium II. formation on dust with random temperature fluctuations
The formation on grains is known to be sensitive to dust
temperature, which is also known to fluctuate for small grain sizes due to
photon absorption. We aim at exploring the consequences of simultaneous
fluctuations of the dust temperature and the adsorbed H-atom population on the
formation rate under the full range of astrophysically relevant
UV intensities and gas conditions. The master equation approach is generalized
to coupled fluctuations in both the grain's temperature and its surface
population and solved numerically. The resolution can be simplified in the case
of the Eley-Rideal mechanism, allowing a fast computation. For the
Langmuir-Hinshelwood mechanism, it remains computationally expensive, and
accurate approximations are constructed. We find the Langmuir-Hinshelwood
mechanism to become an efficient formation mechanism in unshielded photon
dominated region (PDR) edge conditions when taking those fluctuations into
account, despite hot average dust temperatures. It reaches an importance
comparable to the Eley-Rideal mechanism. However, we show that a simpler rate
equation treatment gives qualitatively correct observable results in full cloud
simulations under most astrophysically relevant conditions. Typical differences
are a factor of 2-3 on the intensities of the lines. We
also find that rare fluctuations in cloud cores are sufficient to significantly
reduce the formation efficiency. Our detailed analysis confirms that the usual
approximations used in numerical models are adequate when interpreting
observations, but a more sophisticated statistical analysis is required if one
is interested in the details of surface processes.Comment: 21 pages, 28 figures, accepted in A&
Modélisation du milieu interstellaire sur la Grille
International audienceModélisation du milieu interstellaire sur la Grill
HI-to-H2 Transitions in the Perseus Molecular Cloud
We use the Sternberg et al. (2014) theory for interstellar atomic to
molecular (HI-to-H) conversion to analyze HI-to-H transitions in five
(low-mass) star-forming and dark regions in the Perseus molecular cloud, B1,
B1E, B5, IC348, and NGC1333. The observed HI mass surface densities of 6.3 to
9.2 M pc are consistent with HI-to-H transitions dominated
by HI-dust shielding in predominantly atomic envelopes. For each source, we
constrain the dimensionless parameter , and the ratio ,
of the FUV intensity to hydrogen gas density. We find values from
5.0 to 26.1, implying characteristic atomic hydrogen densities 11.8 to 1.8
cm, for appropriate for Perseus. Our analysis
implies that the dusty HI shielding layers are probably multiphased, with
thermally unstable UNM gas in addition to cold CNM within the 21 cm kinematic
radius.Comment: 5 pages, 2 Figures. Minor improvements suggested by the referee.
Accepted for publication in the Astrophysical Journa
Chemical complexity in the Horsehead photodissociation region
The interstellar medium is known to be chemically complex. Organic molecules
with up to 11 atoms have been detected in the interstellar medium, and are
believed to be formed on the ices around dust grains. The ices can be released
into the gas-phase either through thermal desorption, when a newly formed star
heats the medium around it and completely evaporates the ices; or through
non-thermal desorption mechanisms, such as photodesorption, when a single
far-UV photon releases only a few molecules from the ices. The first one
dominates in hot cores, hot corinos and strongly UV-illuminated PDRs, while the
second one dominates in colder regions, such as low UV-field PDRs. This is the
case of the Horsehead were dust temperatures are ~20-30K, and therefore offers
a clean environment to investigate what is the role of photodesorption. We have
carried-out an unbiased spectral line survey at 3, 2 and 1mm with the IRAM-30m
telescope in the Horsehead nebula, with an unprecedented combination of
bandwidth high spectral resolution and sensitivity. Two positions were
observed: the warm PDR and a cold condensation shielded from the UV field
(dense core), located just behind the PDR edge. We summarize our recently
published results from this survey and present the first detection of the
complex organic molecules HCOOH, CH2CO, CH3CHO and CH3CCH in a PDR. These
species together with CH3CN present enhanced abundances in the PDR compared to
the dense core. This suggests that photodesorption is an efficient mechanism to
release complex molecules into the gas-phase in far-UV illuminated regions.Comment: 15 pages, 7 figures, 7 tables, Accepted in Faraday discussions 16
Calcul intensif pour la modélisation du milieu interstellaire
Calcul intensif pour la modélisation du milieu interstellair
Understanding the temperatures of H3+ and H2 in diffuse interstellar sightlines
The triatomic hydrogen ion H3+ is one of the most important species for the
gas phase chemistry of the interstellar medium. Observations of H3+ are used to
constrain important physical and chemical parameters of interstellar
environments. However, the temperatures inferred from the two lowest rotational
states of H3+ in diffuse lines of sight - typically the only ones observable -
appear consistently lower than the temperatures derived from H2 observations in
the same sightlines. All previous attempts at modelling the temperatures of H3+
in the diffuse interstellar medium failed to reproduce the observational
results. Here we present new studies, comparing an independent master equation
for H3+ level populations to results from the Meudon PDR code for photon
dominated regions. We show that the populations of the lowest rotational states
of H3+ are strongly affected by the formation reaction and that H3+ ions
experience incomplete thermalisation before their destruction by free
electrons. Furthermore, we find that for quantitative analysis more than two
levels of H3+ have to be considered and that it is crucial to include radiative
transitions as well as collisions with H2. Our models of typical diffuse
interstellar sightlines show very good agreement with observational data, and
thus they may finally resolve the perceived temperature difference attributed
to these two fundamental species
Small scale structure in diffuse molecular gas from repeated FUSE and visible spectra of HD 34078
We present preliminary results from an ongoing program devoted to a study of
small scale structure in the spatial distribution of molecular gas. Our work is
based on multi-epoch FUSE and visible observations of HD34078. A detailed
comparison of H2, CH and CH+ absorption lines is performed. No short term
variations are seen (except for highly excited H2) but long-term changes in
N(CH) are clearly detected when comparing our data to spectra taken about 10
years ago.Comment: 4 pages, 2 figures, To appear in the Proceedings of the XVII IAP
Colloquium "Gaseous Matter in Galaxies and Intergalactic Space
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