593 research outputs found
Detection of interstellar HCS and its metastable isomer HSC: new pieces in the puzzle of sulfur chemistry
We present the first identification in interstellar space of the thioformyl
radical (HCS) and its metastable isomer HSC. These species were detected toward
the molecular cloud L483 thanks to observations carried out with the IRAM 30m
telescope in the 3 mm band. We derive beam-averaged column densities of 7e12
cm-2 for HCS and 1.8e11 cm-2 for HSC, which translate to fractional abundances
relative to H2 of 2e-10 and 6e-12, respectively. Although the amount of sulfur
locked by these radicals is low, their detection allows to put interesting
constraints on the chemistry of sulfur in dark clouds. Interestingly, the
H2CS/HCS abundance ratio is found to be quite low, around 1, in contrast with
the oxygen analogue case, in which the H2CO/HCO abundance ratio is around 10 in
dark clouds. Moreover, the radical HCS is found to be more abundant than its
oxygen analogue, HCO. The metastable species HOC, the oxygen analogue of HSC,
has not been yet observed in space. These observational constraints are
confronted with the outcome of a recent model of the chemistry of sulfur in
dark clouds. The model underestimates the fractional abundance of HCS by at
least one order of magnitude, overestimates the H2CS/HCS abundance ratio, and
does not provide an abundance prediction for the metastable isomer HSC. These
observations should prompt a revision of the chemistry of sulfur in
interstellar clouds.Comment: Accepted for publication in A&A Letter
Nascent bipolar outflows associated with the first hydrostatic core candidates Barnard 1b-N and 1b-S
In the theory of star formation, the first hydrostatic core (FHSC) phase is a
critical step in which a condensed object emerges from a prestellar core. This
step lasts about one thousand years, a very short time compared with the
lifetime of prestellar cores, and therefore is hard to detect unambiguously.
We present IRAM Plateau de Bure observations of the Barnard 1b dense
molecular core, combining detections of H2CO and CH3OH spectral lines and dust
continuum at 2.3" resolution (~ 500 AU). The two compact cores B1b-N and B1b-S
are detected in the dust continuum at 2mm, with fluxes that agree with their
spectral energy distribution. Molecular outflows associated with both cores are
detected. They are inclined relative to the direction of the magnetic field, in
agreement with predictions of collapse in turbulent and magnetized gas with a
ratio of mass to magnetic flux somewhat higher than the critical value, \mu ~ 2
- 7. The outflow associated with B1b-S presents sharp spatial structures, with
ejection velocities of up to ~ 7 kms from the mean velocity. Its dynamical age
is estimated to be ~2000 yrs. The B1b-N outflow is smaller and slower, with a
short dynamical age of ~1000 yrs. The B1b-N outflow mass, mass-loss rate, and
mechanical luminosity agree well with theoretical predictions of FHSC. These
observations confirm the early evolutionary stage of B1b-N and the slightly
more evolved stage of B1b-S.Comment: 6 pages, 3 figure
Investigation of HNCO isomers formation in ice mantles by UV and thermal processing: an experimental approach
Current gas phase models do not account for the abundances of HNCO isomers
detected in various environments, suggesting a formation in icy grain mantles.
We attempted to study a formation channel of HNCO and its possible isomers by
vacuum-UV photoprocessing of interstellar ice analogues containing HO,
NH, CO, HCN, CHOH, CH, and N followed by warm-up, under
astrophysically relevant conditions. Only the HO:NH:CO and HO:HCN
ice mixtures led to the production of HNCO species. The possible isomerization
of HNCO to its higher energy tautomers following irradiation or due to ice
warm-up has been scrutinized. The photochemistry and thermal chemistry of
HO:NH:CO and HO:HCN ices was simulated using the Interstellar
Astrochemistry Chamber (ISAC), a state-of-the-art ultra-high-vacuum setup. The
ice was monitored in situ by Fourier transform mid-infrared spectroscopy in
transmittance. A quadrupole mass spectrometer (QMS) detected the desorption of
the molecules in the gas phase. UV-photoprocessing of
HO:NH:CO/HO:HCN ices lead to the formation of OCN as main
product in the solid state and a minor amount of HNCO. The second isomer HOCN
has been tentatively identified. Despite its low efficiency, the formation of
HNCO and the HOCN isomers by UV-photoprocessing of realistic simulated ice
mantles, might explain the observed abundances of these species in PDRs, hot
cores, and dark clouds
CH2D+, the Search for the Holy Grail
CH2D+, the singly deuterated counterpart of CH3+, offers an alternative way
to mediate formation of deuterated species at temperatures of several tens of
K, as compared to the release of deuterated species from grains. We report a
longstanding observational search for this molecular ion, whose rotational
spectroscopy is not yet completely secure. We summarize the main spectroscopic
properties of this molecule and discuss the chemical network leading to the
formation of CH2D+, with explicit account of the ortho/para forms of H2, H3+
and CH3+. Astrochemical models support the presence of this molecular ion in
moderately warm environments at a marginal level.Comment: 25 pages, 6 Figures Accepted in Journal of Physical Chemistry A. "Oka
Festschrift: Celebrating 45 years of Astrochemistry
A line confusion-limited millimeter survey of Orion KL. III. Sulfur oxide species
We present a study of the sulfur-bearing species detected in a line
confusion-limited survey towards Orion KL performed with the IRAM 30m telescope
in the range 80-281 GHz. The study is part of an analysis of the line survey
divided into families of molecules. Our aim is to derive accurate physical
conditions and molecular abundances in the different components of Orion KL
from observed SO and SO2 lines. First we assumed LTE conditions obtain
rotational temperatures. We then used a radiative transfer model, assuming
either LVG or LTE excitation to derive column densities of these molecules in
the different components of Orion KL. We have detected 68 lines of SO, 34SO,
33SO, and S18O and 653 lines of SO2, 34SO2, 33SO2, SO18O and SO2 v2=1. We
provide column densities for all of them and also upper limits for the column
densities of S17O, 36SO, 34S18O, SO17O and 34SO2 v2=1 and for several
undetected sulfur-bearing species. In addition, we present 2'x2' maps around
Orion IRc2 of SO2 transitions with energies from 19 to 131 K and also maps with
four transitions of SO, 34SO and 34SO2. We observe an elongation of the gas
along the NE-SW direction. An unexpected emission peak appears at 20.5 km/s in
most lines of SO and SO2. A study of the spatial distribution of this emission
feature shows that it is a new component ~5" in diameter, which lies ~4" west
of IRc2. We suggest the emission from this feature is related to shocks
associated to the BN object. The highest column densities for SO and SO2 are
found in the high-velocity plateau (a region dominated by shocks) and in the
hot core. These values are up to three orders of magnitude higher than the
results for the ridge components. We also find high column densities for their
isotopologues in both components. Therefore, we conclude that SO and SO2 are
good tracers, not only of regions affected by shocks, but also of regions with
warm dense gas.Comment: Paper (ref AA/2013/21285) accepted for publication by A&A. 52 Pages,
26 figures, 13 table
Extended warm gas in Orion KL as probed by methyl cyanide
In order to study the temperature distribution of the extended gas within the
Orion Kleinmann-Low nebula, we have mapped the emission by methyl cyanide
(CH3CN) in its J=6_K-5_K, J=12_K-11_K, J=13_K-12_K, and J=14_K-13_K transitions
at an average angular resolution of ~10 arcsec (22 arcsec for the 6_K-5_K
lines), as part of a new 2D line survey of this region using the IRAM 30m
telescope. These fully sampled maps show extended emission from warm gas to the
northeast of IRc2 and the distinct kinematic signatures of the hot core and
compact ridge source components. We have constructed population diagrams for
the four sets of K-ladder emission lines at each position in the maps and have
derived rotational excitation temperatures and total beam-averaged column
densities from the fitted slopes. In addition, we have fitted LVG model spectra
to the observations to determine best-fit physical parameters at each map
position, yielding the distribution of kinetic temperatures across the region.
The resulting temperature maps reveal a region of hot (T > 350 K) material
surrounding the northeastern edge of the hot core, whereas the column density
distribution is more uniform and peaks near the position of IRc2. We attribute
this region of hot gas to shock heating caused by the impact of outflowing
material from active star formation in the region, as indicated by the presence
of broad CH3CN lines. This scenario is consistent with predictions from C-shock
chemical models that suggest that gas-phase methyl cyanide survives in the
post-shock gas and can be somewhat enhanced due to sputtering of grain mantles
in the passing shock front.Comment: 24 pages, 20 figures, accepted for publication in A&
Distribution and genetic variability of Staphylinidae across a gradient of anthropogenically influenced insular landscapes
This paper describes the distribution and genetic variability of rove beetles (Coleoptera Staphylinidae) in anthropogenically influenced insular landscapes. The study was conducted in the Azores archipelago, characterized by high anthropogenic influence and landscape fragmentation. Collections were made in five islands, from eight habitats, along a gradient of anthropogenic influence. The species of Staphylinidae from the Azores collected for this study were widely distributed and showed low habitat fidelity. Rove beetle richness was associated with anthropogenic influence and habitat type, increasing from less to more anthropogenic impacted habitats. However, genetic diversity of profiled species (i.e. with three or more specimens per species/habitat) does not seem affected by anthropogenic influence in the different habitat types, isolation or landscape fragmentation. COI haplotypes were, as a rule, not exclusive to a given island or habitat. High level of genetic divergence and nucleotide saturation was found in closely related morphological designated species, demonstrating possible disparities between currently defined taxonomic units based on morphology and molecular phylogenies of Staphylinidae. This study found evidence of cryptic speciation in the Atheta fungi (Gravenhorst) species complex which had thus far remained undetected. Similar trends were found for Oligota parva Kraatz, Oxytelus sculptus Gravenhorst, Oligota pumilio Kiesenwetter. Previous studies with lower taxonomical resolution may have underestimated the biotic diversity reported in the Azores in comparison to other Macaronesian archipelagos.info:eu-repo/semantics/publishedVersio
Nitrogen isotopic ratios in Barnard 1: a consistent study of the N2H+, NH3, CN, HCN and HNC isotopologues
The 15N isotopologue abundance ratio measured today in different bodies of
the solar system is thought to be connected to 15N-fractionation effects that
would have occured in the protosolar nebula. The present study aims at putting
constraints on the degree of 15N-fractionation that occurs during the
prestellar phase, through observations of D, 13C and 15N-substituted
isotopologues towards B1b. Both molecules from the nitrogen hydride family,
i.e. N2H+ and NH3, and from the nitrile family, i.e. HCN, HNC and CN, are
considered in the analysis. As a first step, we model the continuum emission in
order to derive the physical structure of the cloud, i.e. gas temperature and
H2 density. These parameters are subsequently used as an input in a non-local
radiative transfer model to infer the radial abundances profiles of the various
molecules. Our modeling shows that all the molecules are affected by depletion
onto dust grains, in the region that encompasses the B1-bS and B1-bN cores.
While high levels of deuterium fractionation are derived, we conclude that no
fractionation occurs in the case of the nitrogen chemistry. Independently of
the chemical family, the molecular abundances are consistent with 14N/15N~300,
a value representative of the elemental atomic abundances of the parental gas.
The inefficiency of the 15N-fractionation effects in the B1b region can be
linked to the relatively high gas temperature ~17K which is representative of
the innermost part of the cloud. Since this region shows signs of depletion
onto dust grains, we can not exclude the possibility that the molecules were
previously enriched in 15N, earlier in the B1b history, and that such an
enrichment could have been incorporated into the ice mantles. It is thus
necessary to repeat this kind of study in colder sources to test such a
possibility.Comment: accepted in A&
Discovery of Interstellar Propylene (CH_2CHCH_3): Missing Links in Interstellar Gas-Phase Chemistry
We report the discovery of propylene (also called propene, CH_2CHCH_3) with
the IRAM 30-m radio telescope toward the dark cloud TMC-1. Propylene is the
most saturated hydrocarbon ever detected in space through radio astronomical
techniques. In spite of its weak dipole moment, 6 doublets (A and E species)
plus another line from the A species have been observed with main beam
temperatures above 20 mK. The derived total column density of propylene is 4
10^13 cm^-2, which corresponds to an abundance relative to H_2 of 4 10^-9,
i.e., comparable to that of other well known and abundant hydrocarbons in this
cloud, such as c-C_3H_2. Although this isomer of C_3H_6 could play an important
role in interstellar chemistry, it has been ignored by previous chemical models
of dark clouds as there seems to be no obvious formation pathway in gas phase.
The discovery of this species in a dark cloud indicates that a thorough
analysis of the completeness of gas phase chemistry has to be done.Comment: 13 pages, 2 figures, accepted for publication in ApJ
Aromatic cycles are widespread in cold clouds
We report the detection of large hydrocarbon cycles toward several cold dense
clouds. We observed four sources (L1495B, Lupus-1A, L483, and L1527) in the Q
band (31-50 GHz) using the Yebes 40m radiotelescope. Using the line stack
technique, we find statistically significant evidence of benzonitrile
(CHCN) in L1495B, Lupus-1A, and L483 at levels of 31.8,
15.0, and 17.2, respectively, while there is no hint of
CHCN in the fourth source, L1527. The column densities derived are in
the range (1.7-3.8) cm, which is somewhat below the
value derived toward the cold dense cloud TMC-1. When we simultaneously analyze
all the benzonitrile abundances derived toward cold clouds in this study and in
the literature, a clear trend emerges in that the higher the abundance of
HCN, the more abundant CHCN is. This indicates that aromatic cycles
are especially favored in those interstellar clouds where long carbon chains
are abundant, which suggests that the chemical processes that are responsible
for the formation of linear carbon chains are also behind the synthesis of
aromatic rings. We also searched for cycles other than benzonitrile, and found
evidence of indene (CH), cyclopentadiene (CH), and 1-cyano
cyclopentadiene (1-CHCN) at levels of 9.3, 7.5, and
8.4, respectively, toward L1495B, which shows the strongest signal
from CHCN. The relative abundances between the various cycles detected
in L1495B are consistent -- within a factor of three -- with those previously
found in TMC-1. It is therefore likely that not only CHCN but also
other large aromatic cycles are abundant in clouds rich in carbon chains.Comment: Accepted for publication in A&A Letters. Changes with respect to
previous version: language edited, error in abstract corrected, and title
change
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