188 research outputs found
Ortho-H2 and the Age of Interstellar Dark Clouds
International audienceInterstellar dark clouds are the sites of star formation. Their main component, dihydrogen, exists under two states, ortho and para. H2 is supposed to form in the ortho:para ratio (OPR) of 3:1 and to subsequently decay to almost pure para-H2 (OPR = 0.1 is necessary to prevent DCO+ large-scale apparition. We conclude that the inevitable decay of ortho-H2 sets an upper limit of ~6 million years to the age of starless molecular clouds under usual conditions
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
Reactions of N+ (3P) ions with H2 and HD molecules at low temperatures
International audienceContext. This work is motivated by the necessity to take account of both the nuclear spin symmetries of H 2 and the spin-orbit interaction of N + ions in order to investigate gas phase reactions in interstellar chemistry, leading to the formation of nitrogenous and deuterated compounds. Aims. The main objective in this work is to determine the rate coefficients for each possible initial quantum state of the reactants N + (3 P j) + H 2 (J) (and their isotopic variants). Only in this way does it become possible both to analyse experimental data and to develop realistic applications to interstellar chemical models to constrain the gas phase chemistry of ammonia and its isotopologues. Methods. A statistical treatment is presented of state selective reactive collisions involving N + ions in fine structure state j with H 2 or HD molecules in a rotation level J of the ground vibration state, leading either to the production of NH + ions and H in the case of the H 2 reactant, and to the production of either NH + ions or ND + in the case of the HD reactant. The energies of fine structure states (j = 0, 1, 2) of the N + ions are treated on an equal footing with the other energies of internal motions. All fine structure states are considered to be reactive. Results. Cross sections for state-to-state collisions are calculated for collision energies ranging from 0.1–30 meV. These cross sections are then averaged over the kinetic energies of the reactants for each (J, j) to obtain the rate coefficients for a range of kinetic temperatures 10–200 K. The exo/endothermicity of the reactions involving N + (3 P j) + H 2 (J) (and isotopic variants) is derived from the difference ∆E e between the dissociation energies of the electronic molecular potentials of NH + and H 2. The value ∆E e = 101 meV is found to satisfactorily reproduce the experiments performed with ortho-H 2 and to a lesser extent with para-H 2. This value is used to determine the rate coefficient of the N + + HD reaction leading to the formation of ND +. The calculated value is consistent with the available experimental data. Conclusions. The present results allow for the determination of reaction rate coefficients for any given distribution of specific fine structure and rotational state populations of the reactants. In interstellar conditions, where N + is in its 3 P 0 state and para-and ortho-H 2 respectively in J = 0 and J = 1. Our results enable a study of the influence of the ortho/para evolution of molecular hydrogen on the formation of nitrogen compounds
Gas-Grain Modeling of Isocyanic Acid (HNCO), Cyanic Acid (HOCN), Fulminic Acid (HCNO), and Isofulminic Acid (HONC) in Assorted Interstellar Environments
Isocyanic acid (HNCO) is a well-known interstellar molecule. Evidence also exists for the presence of two of its metastable isomers in the interstellar medium: HCNO (fulminic acid) and HOCN (cyanic acid). Fulminic acid has been detected toward cold and lukewarm sources, while cyanic acid has been detected both in these sources and in warm sources in the Galactic Center. Gas-phase models can reproduce the abundances of the isomers in cold sources, but overproduce HCNO in the Galactic Center. Here we present a detailed study of a gas-grain model that contains these three isomers, plus a fourth isomer, isofulminic acid (HONC), for four types of sources: hot cores, the warm envelopes of hot cores, lukewarm corinos, and cold cores. The current model is partially able to rationalize the abundances of HNCO, HOCN, and HCNO in cold and warm sources. Predictions for HONC in all environments are also made
IVOA Recommendation: Simple Spectral Lines Data Model Version 1.0
This document presents a Data Model to describe Spectral Line Transitions in
the context of the Simple Line Access Protocol defined by the IVOA (c.f.
Ref[13] IVOA Simple Line Access protocol) The main objective of the model is to
integrate with and support the Simple Line Access Protocol, with which it forms
a compact unit. This integration allows seamless access to Spectral Line
Transitions available worldwide in the VO context. This model does not provide
a complete description of Atomic and Molecular Physics, which scope is outside
of this document. In the astrophysical sense, a line is considered as the
result of a transition between two energy levels. Under the basis of this
assumption, a whole set of objects and attributes have been derived to define
properly the necessary information to describe lines appearing in astrophysical
contexts. The document has been written taking into account available
information from many different Line data providers (see acknowledgments
section)
The Horsehead mane: Towards an observational benchmark for chemical models
After a discussion about the need for observational benchmark for chemical
models, we explain 1) why the Horsehead western edge is well suited to serve as
reference for models and 2) the steps we are taking toward this goal. We
summarize abundances obtained to date and we show recent results
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
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
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