784 research outputs found
A Herschel/HIFI Legacy Survey of HF and H2O in the Galaxy: Probing Diffuse Molecular Cloud Chemistry
We combine Herschel observations of a total of 12 sources to construct the
most uniform survey of HF and H2O in our Galactic disk. Both molecules are
detected in absorption along all sight lines. The high spectral resolution of
the Heterodyne Instrument for the Far-Infrared (HIFI) allows us to compare the
HF and H2O distributions in 47 diffuse cloud components sampling the disk. We
find that the HF and H2O velocity distributions follow each other almost
perfectly and establish that HF and H2O probe the same gas-phase volume. Our
observations corroborate theoretical predictions that HF is a sensitive tracer
of H2 in diffuse clouds, down to molecular fractions of only a few percent.
Using HF to trace H2 in our sample, we find that the N(H2O)-to-N(HF) ratio
shows a narrow distribution with a median value of 1.51. Our results further
suggest that H2O might be used as a tracer of H2 -within a factor 2.5- in the
diffuse interstellar medium. We show that the measured factor of ~2.5 variation
around the median is driven by true local variations in the H2O abundance
relative to H2 throughout the disk. The latter variability allows us to test
our theoretical understanding of the chemistry of oxygen-bearing molecules in
the diffuse gas. We show that both gas-phase and grain-surface chemistry are
required to reproduce our H2O observations. This survey thus confirms that
grain surface reactions can play a significant role in the chemistry occurring
in the diffuse interstellar medium n_H < 1000 cm^-3.Comment: 53 pages; 12 figures, accepted for publication in ApJ main journa
The IRAM-30m line survey of the Horsehead PDR: III. High abundance of complex (iso-)nitrile molecules in UV-illuminated gas
Complex (iso-)nitrile molecules, such as CH3CN and HC3N, are relatively
easily detected in our Galaxy and in other galaxies. We constrain their
chemistry through observations of two positions in the Horsehead edge: the
photo-dissociation region (PDR) and the dense, cold, and UV-shielded core just
behind it. We systematically searched for lines of CH3CN, HC3N, C3N, and some
of their isomers in our sensitive unbiased line survey at 3, 2, and 1mm. We
derived column densities and abundances through Bayesian analysis using a large
velocity gradient radiative transfer model. We report the first clear detection
of CH3NC at millimeter wavelength. We detected 17 lines of CH3CN at the PDR and
6 at the dense core position, and we resolved its hyperfine structure for 3
lines. We detected 4 lines of HC3N, and C3N is clearly detected at the PDR
position. We computed new electron collisional rate coefficients for CH3CN, and
we found that including electron excitation reduces the derived column density
by 40% at the PDR position. While CH3CN is 30 times more abundant in the PDR
than in the dense core, HC3N has similar abundance at both positions. The
isomeric ratio CH3NC/CH3CN is 0.15+-0.02. In the case of CH3CN, pure gas phase
chemistry cannot reproduce the amount of CH3CN observed in the UV-illuminated
gas. We propose that CH3CN gas phase abundance is enhanced when ice mantles of
grains are destroyed through photo-desorption or thermal-evaporation in PDRs,
and through sputtering in shocks. (abridged)Comment: Accepted for publication in Astronomy & Astrophysic
Interstellar CH absorption in the diffuse interstellar medium along the sight-lines to G10.6–0.4 (W31C), W49N, and W51
We report the detection of the ground state N, J = 1, 3/2 → 1, 1/2 doublet of the methylidyne radical CH at ~532 GHz and ~536 GHz with
the Herschel/HIFI instrument along the sight-line to the massive star-forming regions G10.6–0.4 (W31C), W49N, and W51. While the molecular
cores associated with these massive star-forming regions show emission lines, clouds in the diffuse interstellar medium are detected in absorption
against the strong submillimeter background. The combination of hyperfine structure with emission and absorption results in complex profiles,
with overlap of the different hyperfine components. The opacities of most of the CH absorption features are linearly correlated with those of CCH,
CN, and HCO^+ in the same velocity intervals. In specific narrow velocity intervals, the opacities of CN and HCO^+ deviate from the mean trends,
giving rise to more opaque absorption features. We propose that CCH can be used as another tracer of the molecular gas in the absence of better
tracers, with [CCH]/[H_2] ~3.2 ± 1.1 × 10^(−8). The observed [CN]/[CH], [CCH]/[CH] abundance ratios suggest that the bulk of the diffuse matter
along the lines of sight has gas densities n_H = n(H) + 2n(H_2) ranging between 100 and 1000 cm^(−3)
Molecular Carbon Chains and Rings in TMC-1
We present mapping results in several rotational transitions of HC3N, C6H,
both cyclic and linear C3H2 and C3H, towards the cyanopolyyne peak of the
filamentary dense cloud TMC-1 using the IRAM 30m and MPIfR 100m telescopes. The
spatial distribution of the cumulene carbon chain propadienylidene H2C3
(hereafter l-C3H2) is found to deviate significantly from the distributions of
the cyclic isomer c-C3H2, HC3N, and C6H which in turn look very similar. The
cyclic over linear abundance ratio of C3H2 increases by a factor of 3 across
the filament, with a value of 28 at the cyanopolyyne peak. This abundance ratio
is an order of magnitude larger than the range (3 to 5) we observed in the
diffuse interstellar medium. The cyclic over linear abundance ratio of C3H also
varies by ~2.5 in TMC-1, reaching a maximum value (13) close to the
cyanopolyyne peak. These behaviors might be related to competitive processes
between ion-neutral and neutral-neutral reactions for cyclic and linear
species.Comment: Accepted for publication in The Astrophysical Journal, part I. 24
pages, including 4 tables, 7 figures, and figure caption
The IRAM-30m line survey of the Horsehead PDR: IV. Comparative chemistry of H2CO and CH3OH
Aims. We investigate the dominant formation mechanism of H2CO and CH3OH in
the Horsehead PDR and its associated dense core. Methods. We performed deep
integrations of several H2CO and CH3OH lines at two positions in the Horsehead,
namely the PDR and dense core, with the IRAM-30m telescope. In addition, we
observed one H2CO higher frequency line with the CSO telescope at both
positions. We determine the H2CO and CH3OH column densities and abundances from
the single-dish observations complemented with IRAM-PdBI high-angular
resolution maps (6") of both species. We compare the observed abundances with
PDR models including either pure gas-phase chemistry or both gas-phase and
grain surface chemistry. Results. We derive CH3OH abundances relative to total
number of hydrogen atoms of ~1.2e-10 and ~2.3e-10 in the PDR and dense core
positions, respectively. These abundances are similar to the inferred H2CO
abundance in both positions (~2e-10). We find an abundance ratio H2CO/CH3OH of
~2 in the PDR and ~1 in the dense core. Pure gas-phase models cannot reproduce
the observed abundances of either H2CO or CH3OH at the PDR position. Both
species are therefore formed on the surface of dust grains and are subsequently
photodesorbed into the gas-phase at this position. At the dense core, on the
other hand, photodesorption of ices is needed to explain the observed abundance
of CH3OH, while a pure gas-phase model can reproduce the observed H2CO
abundance. The high-resolution observations show that CH3OH is depleted onto
grains at the dense core. CH3OH is thus present in an envelope around this
position, while H2CO is present in both the envelope and the dense core itself.
Conclusions. Photodesorption is an efficient mechanism to release complex
molecules in low FUV-illuminated PDRs, where thermal desorption of ice mantles
is ineffective.Comment: 12 pages, 5 tables, 7 figures; Accepted for publication in A&
Sulphur-bearing molecules in diffuse molecular clouds: new results from SOFIA/GREAT and the IRAM 30 m telescope
We have observed five sulphur-bearing molecules in foreground diffuse
molecular clouds lying along the sight-lines to five bright continuum sources.
We have used the GREAT instrument on SOFIA to observe the 1383 GHz transitions of SH towards the star-forming regions W31C,
G29.96-0.02, G34.3+0.1, W49N and W51, detecting foreground absorption towards
all five sources; and the EMIR receivers on the IRAM 30m telescope at Pico
Veleta to detect the HS 1(10)-1(01), CS J=2-1 and SO 3(2)-2(1) transitions.
In nine foreground absorption components detected towards these sources, the
inferred column densities of the four detected molecules showed relatively
constant ratios, with N(SH)/N(HS) in the range 1.1 - 3.0, N(CS)/N(HS)
in the range 0.32 - 0.61, and N(SO)/N(HS) in the range 0.08 - 0.30. The
observed SH/H ratios - in the range (0.5-2.6) - indicate
that SH (and other sulphur-bearing molecules) account for << 1% of the
gas-phase sulphur nuclei. The observed abundances of sulphur-bearing molecules,
however, greatly exceed those predicted by standard models of cold diffuse
molecular clouds, providing further evidence for the enhancement of endothermic
reaction rates by elevated temperatures or ion-neutral drift. We have
considered the observed abundance ratios in the context of shock and turbulent
dissipation region (TDR) models. Using the TDR model, we find that the
turbulent energy available at large scale in the diffuse ISM is sufficient to
explain the observed column densities of SH and CS. Standard shock and TDR
models, however, fail to reproduce the column densities of HS and SO by a
factor of about 10; more elaborate shock models - in which account is taken of
the velocity drift, relative to H, of SH molecules produced by the
dissociative recombination of HS - reduce this discrepancy to a factor
~ 3.Comment: 30 pages, accepted for publication in A&
Atomic Carbon in Galaxies
We present new measurements of the ground state fine-structure line of atomic
carbon at 492 GHz in a variety of nearby external galaxies, ranging from spiral
to irregular, interacting and merging types. In comparison with CO(1-0), the
CI(1-0) intensity stays fairly comparable in the different environments, with
an average value of the ratio of the line integrated areas in Kkm/s of
CI(1-0)/CO(1-0) = 0.2 +/- 0.2. However, some variations can be found within
galaxies, or between galaxies. Relative to CO lines, CI(1-0) is weaker in
galactic nuclei, but stronger in disks, particularly outside star forming
regions. Also, in NGC 891, the CI(1-0) emission follows the dust continuum at
1.3mm extremely well along the full length of the major axis where molecular
gas is more abundant than atomic gas. Atomic carbon therefore appears to be a
good tracer of molecular gas in external galaxies, possibly more reliable than
CO. Atomic carbon can contribute significantly to the thermal budget of
interstellar gas. Cooling due to C and CO amounts typically to 2 x 10^{-5} of
the FIR continuum or 5% of the CII line. However, C and CO cooling reaches 30%
of the gas total, in Ultra Luminous InfraRed Galaxies, where CII is abnormally
faint. Together with CII/FIR, the emissivity ratio CI(1-0)/FIR can be used as a
measure of the non-ionizing UV radiation field in galaxies.Comment: 26 pages, 8 figure
The IRAM-30m line survey of the Horsehead PDR: II. First detection of the l-C3H+ hydrocarbon cation
We present the first detection of the l-C3H+ hydrocarbon in the interstellar
medium. The Horsehead WHISPER project, a millimeter unbiased line survey at two
positions, namely the photo-dissociation region (PDR) and the nearby shielded
core, revealed a consistent set of eight unidentified lines toward the PDR
position. Six of them are detected with a signal-to-noise ratio from 6 to 19,
while the two last ones are tentatively detected. Mostly noise appears at the
same frequency toward the dense core, located less than 40" away. We
simultaneously fit 1) the rotational and centrifugal distortion constants of a
linear rotor, and 2) the Gaussian line shapes located at the eight predicted
frequencies. The observed lines can be accurately fitted with a linear rotor
model, implying a 1Sigma ground electronic state. The deduced rotational
constant value is Be= 11244.9512 +/- 0.0015 MHz, close to that of l-C3H. We
thus associate the lines to the l-C3H+ hydrocarbon cation, which enables us to
constrain the chemistry of small hydrocarbons. A rotational diagram is then
used to infer the excitation temperature and the column density. We finally
compare the abundance to the results of the Meudon PDR photochemical model.Comment: 9 pages, 7 PostScript figures. Accepted for publication in Astronomy
\& Astrophysics. Uses aa LaTeX macro
Tentative Detection of the Nitrosylium Ion in Space
We report the tentative detection in space of the nitrosylium ion, NO.
The observations were performed towards the cold dense core Barnard 1-b. The
identification of the NO =2--1 line is supported by new laboratory
measurements of NO rotational lines up to the =8--7 transition
(953207.189\,MHz), which leads to an improved set of molecular constants: \,MHz, \,kHz, and \,MHz. The profile of the feature assigned to NO exhibits two
velocity components at 6.5 and 7.5 km s, with column densities of and cm, respectively. New
observations of NO and HNO, also reported here, allow to estimate the following
abundance ratios: (NO)/(NO), and
(HNO)/(NO). This latter value provides important constraints
on the formation and destruction processes of HNO. The chemistry of NO and
other related nitrogen-bearing species is investigated by the means of a
time-dependent gas phase model which includes an updated chemical network
according to recent experimental studies. The predicted abundance for NO
and NO is found to be consistent with the observations. However, that of HNO
relative to NO is too high. No satisfactory chemical paths have been found to
explain the observed low abundance of HNO. HSCN and HNCS are also reported here
with an abundance ratio of . Finally, we have searched for NNO,
NO, HNNO, and NNOH, but only upper limits have been obtained for
their column density, except for the latter for which we report a tentative
3- detection.Comment: To appear in the Astrophysical Journal October 20, 201
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