302 research outputs found
Hyperfine transitions of 13CN from pre-protostellar sources
Recent quantum mechanical calculations of rate coefficients for collisional
transfer of population between the hyperfine states of 13CN enable their
population densities to be determined. We have computed the relative
populations of the hyperfine states of the N = 0, 1, 2 rotational states for
kinetic temperatures 5 T 20 K and molecular hydrogen densities 1
n(H2) 10 10 cm --3. Spontaneous and induced radiative transitions
were taken into account. Our calculations show that, if the lines are optically
thin, the populations of the hyperfine states, F, within a given rotational
manifold are proportional to their statistical weights, (2F + 1) -- i.e. in
local thermodynamic equilibrium -- over the entire range of densities. We have
re-analysed IRAM 30 m telescope observations of 13CN hyperfine transitions (N =
1 0) in four starless cores. A comparison of these observations
with our calculations confirms that the hyperfine states are statistically
populated in these sources.Comment: MNRAS, Oxford University press, 2015,
http://mnras.oxfordjournals.org/content/452/1/19.full?keytype=ref\&ijkey=CCx468pl8lXgoXx.
\<10.1093/mnras/stv1322\>
Intermittency of interstellar turbulence: extreme velocity-shears and CO emission on milliparsec scale
The condensation of diffuse gas into molecular clouds occurs at a rate driven
largely by turbulent dissipation. This process still has to be caught in action
and characterized. A mosaic of 13 fields was observed in the CO(1-0) line with
the IRAM-PdB interferometer in the translucent environment of two low-mass
dense cores. The large size of the mosaic compared to the resolution (4 arcsec)
is unprecedented in the study of the small-scale structure of diffuse molecular
gas. Eight weak and elongated structures of thicknesses as small as 3 mpc (600
AU) and lengths up to 70mpc are found. These are not filaments because once
merged with short-spacing data, they appear as the sharp edges of larger-scale
structures. Six out of eight form quasi-parallel pairs at different velocities
and different position angles. This cannot be the result of chance alignment.
The velocity-shears estimated for the three pairs include the highest ever
measured far from star forming regions (780 km/s/pc). Because the large scale
structures have sharp edges, with little or no overlap, they have to be thin
CO-layers. Their edges mark a sharp transition between a CO-rich component and
a gas undetected in the CO line because of its low CO abundance, presumably the
cold neutral medium. We propose that these sharp edges are the first
directly-detected manifestations of the intermittency of interstellar
turbulence. The large velocity-shears reveal an intense straining field,
responsible for a local dissipation rate several orders of magnitude above
average, possibly at the origin of the thin CO-layers.Comment: 16 pages, 11 figures, Accepted for publication in Astronomy and
Astrophysic
The 15N-enrichment in dark clouds and Solar System objects
The line intensities of the fundamental rotational transitions of H13CN and
HC15N were observed towards two prestellar cores, L183 and L1544, and lead to
molecular isotopic ratios 140 6 14N/15N 6 250 and 140 6 14N/15N 6 360,
respectively. The range of values reflect genuine spatial variations within the
cores. A comprehensive analysis of the available measurements of the nitrogen
isotopic ratio in prestellar cores show that molecules carrying the nitrile
functional group appear to be systematically 15N-enriched com- pared to those
carrying the amine functional group. A chemical origin for the differential
15N-enhance- ment between nitrile- and amine-bearing interstellar molecules is
proposed. This sheds new light on several observations of Solar System objects:
(i) the similar N isotopic fractionation in Jupiter's NH3 and solar wind N+;
(ii) the 15N-enrichments in cometary HCN and CN (that might represent a direct
inter- stellar inheritance); and (iii) 15N-enrichments observed in organics in
primitive cosmomaterials. The large variations in the isotopic composition of
N-bearing molecules in Solar System objects might then simply reflect the
different interstellar N reservoirs from which they are originating
Intermittency of interstellar turbulence: Parsec-scale coherent structure of intense velocity-shear
Guided by the duality of turbulence (random versus coherent we seek coherent
structures in the turbulent velocity field of molecular clouds, anticipating
their importance in cloud evolution. We analyse a large map (40' by 20')
obtained with the HERA multibeam receiver (IRAM-30m telescope) in a high
latitude cloud of the Polaris Flare at an unprecedented spatial (11") and
spectral (0.05 km/s) resolutions in the 12CO(2-1) line. We find that two
parsec-scale components of velocities differing by ~2 km/s, share a narrow
interface ( pc) that appears as an elongated structure of intense
velocity-shear, ~15 to 30 km/s/pc. The locus of the extrema of
line--centroid-velocity increments (E-CVI) in that field follows this
intense-shear structure as well as that of the 12CO(2-1) high-velocity line
wings. The tiny spatial overlap in projection of the two parsec-scale
components implies that they are sheets of CO emission and that discontinuities
in the gas properties (CO enrichment and/or increase of gas density) occur at
the position of the intense velocity shear. These results disclose spatial and
kinematic coherence between scales as small as 0.03 pc and parsec scales. They
confirm that the departure from Gaussianity of the probability density
functions of E-CVIs is a powerful statistical tracer of the intermittency of
turbulence. They disclose a link between large scale turbulence, its
intermittent dissipation rate and low-mass dense core formation
Dissipative structures of diffuse molecular gas: I - Broad HCO(1-0) emission
Results: We report the detection of broad HCO+(1-0) lines (10 mK < T < 0.5
K). The interpretation of 10 of the HCO+ velocity components is conducted in
conjunction with that of the associated optically thin 13CO emission. The
derived HCO+ column densities span a broad range, , and the inferred HCO+ abundances, , are more than one order of magnitude above
those produced by steady-state chemistry in gas weakly shielded from UV
photons, even at large densities. We compare our results with the predictions
of non-equilibrium chemistry, swiftly triggered in bursts of turbulence
dissipation and followed by a slow thermal and chemical relaxation phase,
assumed isobaric. The set of values derived from the observations, i.e. large
HCO+ abundances, temperatures in the range of 100--200 K and densities in the
range 100--1000 cm3, unambiguously belongs to the relaxation phase. The
kinematic properties of the gas suggest in turn that the observed HCO+ line
emission results from a space-time average in the beam of the whole cycle
followed by the gas and that the chemical enrichment is made at the expense of
the non-thermal energy. Last, we show that the "warm chemistry" signature (i.e
large abundances of HCO+, CH+, H20 and OH) acquired by the gas within a few
hundred years, the duration of the impulsive chemical enrichment, is kept over
more than thousand years. During the relaxation phase, the \wat/OH abundance
ratio stays close to the value measured in diffuse gas by the SWAS satellite,
while the OH/HCO+ ratio increases by more than one order of magnitude.Comment: 14 page
Small-scale dissipative structures of diffuse ISM turbulence: I- CO diagnostics
Observations of translucent molecular gas in CO and CO emission lines, at high spectral and spatial resolutions, evidence different kinds of structures at small scales: (1) optically thin CO emission, (2) optically thick CO emission, visible in CO(1-0), and (3) regions of largest velocity shear in the field, found from a statistical analysis. They are all elongated with high aspect ratio, preferentially aligned with the plane-of-the-sky projection of the magnetic fields. The latter structures coincide with the former, shown to trace gas warmer and more diluted than average. Combining our data to large-scale observations of poorer spatial resolution, we show that the regions of largest velocity shear remain coherent over more than a parsec. These filaments are proposed to be the sites of the intermittent dissipation of turbulence
Cosmic ray induced ionisation of a molecular cloud shocked by the W28 supernova remnant
Cosmic rays are an essential ingredient in the evolution of the interstellar
medium, as they dominate the ionisation of the dense molecular gas, where stars
and planets form. However, since they are efficiently scattered by the galactic
magnetic fields, many questions remain open, such as where exactly they are
accelerated, what is their original energy spectrum, and how they propagate
into molecular clouds. In this work we present new observations and discuss in
detail a method that allows us to measure the cosmic ray ionisation rate
towards the molecular clouds close to the W28 supernova remnant. To perform
these measurements, we use CO, HCO, and DCO millimetre line
observations and compare them with the predictions of radiative transfer and
chemical models away from thermodynamical equilibrium. The CO observations
allow us to constrain the density, temperature, and column density towards each
observed position, while the DCO/HCO abundance ratios provide us with
constraints on the electron fraction and, consequently, on the cosmic ray
ionisation rate. Towards positions located close to the supernova remnant, we
find cosmic ray ionisation rates much larger () than those in
standard galactic clouds. Conversely, towards one position situated at a larger
distance, we derive a standard cosmic ray ionisation rate. Overall, these
observations support the hypothesis that the rays observed in the
region have a hadronic origin. In addition, based on CR diffusion estimates, we
find that the ionisation of the gas is likely due to GeV cosmic rays.
Finally, these observations are also in agreement with the global picture of
cosmic ray diffusion, in which the low-energy tail of the cosmic ray population
diffuses at smaller distances than the high-energy counterpart.Comment: Accepted to A\&
The ionization fraction gradient across the Horsehead edge: An archetype for molecular clouds
The ionization fraction plays a key role in the chemistry and dynamics of
molecular clouds. We study the H13CO+, DCO+ and HOC+ line emission towards the
Horsehead, from the shielded core to the UV irradiated cloud edge, i.e., the
Photodissociation Region (PDR), as a template to investigate the ionization
fraction gradient in molecular clouds. We analyze a PdBI map of the H13CO+
J=1-0 line, complemented with IRAM-30m H13CO+ and DCO+ higher-J line maps and
new HOC+ and CO+ observations. We compare self-consistently the observed
spatial distribution and line intensities with detailed depth-dependent
predictions of a PDR model coupled with a nonlocal radiative transfer
calculation. The chemical network includes deuterated species, 13C
fractionation reactions and HCO+/HOC+ isomerization reactions. The role of
neutral and charged PAHs in the cloud chemistry and ionization balance is
investigated. The detection of HOC+ reactive ion towards the Horsehead PDR
proves the high ionization fraction of the outer UV irradiated regions, where
we derive a low [HCO+]/[HOC+]~75-200 abundance ratio. In the absence of PAHs,
we reproduce the observations with gas-phase metal abundances, [Fe+Mg+...],
lower than 4x10(-9) (with respect to H) and a cosmic-rays ionization rate of
zeta=(5+/-3)x10(-17) s(-1). The inclusion of PAHs modifies the ionization
fraction gradient and increases the required metal abundance. The ionization
fraction in the Horsehead edge follows a steep gradient, with a scale length of
~0.05 pc (or ~25''), from [e-]~10(-4) (or n_e ~ 1-5 cm(-3)) in the PDR to a few
times ~10(-9) in the core. PAH^- anions play a role in the charge balance of
the cold and neutral gas if substantial amounts of free PAHs are present ([PAH]
>10(-8)).Comment: 13 pages, 7 figures, 6 tables. Accepted for publication in A&A
(english not edited
Collisional excitation of singly deuterated ammonia NHD by H
The availability of collisional rate coefficients with H is a
pre-requisite for interpretation of observations of molecules whose energy
levels are populated under non local thermodynamical equilibrium conditions. In
the current study, we present collisional rate coefficients for the NHD /
para--H() collisional system, for energy levels up to (735 K) and for gas temperatures in the range K. The
cross sections are obtained using the essentially exact close--coupling (CC)
formalism at low energy and at the highest energies, we used the
coupled--states (CS) approximation. For the energy levels up to
(215 K), the cross sections obtained through the CS formalism are
scaled according to a few CC reference points. These reference points are
subsequently used to estimate the accuracy of the rate coefficients for higher
levels, which is mainly limited by the use of the CS formalism. Considering the
current potential energy surface, the rate coefficients are thus expected to be
accurate to within 5\% for the levels below , while we estimate
an accuracy of 30\% for higher levels
Detection of the HCNH and HCNH ions in the L1544 pre-stellar core
The L1544 pre-stellar core was observed as part of the ASAI (Astrochemical
Surveys At IRAM) Large Program. We report the first detection in a pre-stellar
core of the HCNH and HCNH ions. The high spectral resolution of the
observations allows to resolve the hyperfine structure of HCNH. Local
thermodynamic equilibrium analysis leads to derive a column density equal to
(2.00.2)10cm for HCNH and
(1.50.5)10cm for HCNH. We also present
non-LTE analysis of five transitions of HCN, three transitions of
HCN and one transition of HNC, all of them linked to the
chemistry of HCNH and HCNH. We computed for HCN, HCN, and HNC a
column density of (2.00.4)10cm,
(3.60.9)cm, and
(3.01.0)10cm, respectively. We used the gas-grain
chemical code Nautilus to predict the abundances all these species across the
pre-stellar core. Comparison of the observations with the model predictions
suggests that the emission from HCNH and HCNH originates in the
external layer where non-thermal desorption of other species was previously
observed. The observed abundance of both ionic species
([HCNH] and
[HCNH], with respect to H) cannot
be reproduced at the same time by the chemical modelling, within the error bars
of the observations only. We discuss the possible reasons for the discrepancy
and suggest that the current chemical models are not fully accurate or
complete. However, the modelled abundances are within a factor of three
consistent with the observations, considering a late stage of the evolution of
the pre-stellar core, compatible with previous observations.Comment: Accepted for publication in MNRAS, 13 pages, 9 figure
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