282 research outputs found
Deuteration as an evolutionary tracer in massive-star formation
Theory predicts, and observations confirm, that the column density ratio of a
molecule containing D to its counterpart containing H can be used as an
evolutionary tracer in the low-mass star formation process. Since it remains
unclear if the high-mass star formation process is a scaled-up version of the
low-mass one, we investigated whether the relation between deuteration and
evolution can be applied to the high-mass regime. With the IRAM-30m telescope,
we observed rotational transitions of N2D+ and N2H+ and derived the deuterated
fraction in 27 cores within massive star-forming regions understood to
represent different evolutionary stages of the massive-star formation process.
Results. Our results clearly indicate that the abundance of N2D+ is higher at
the pre-stellar/cluster stage, then drops during the formation of the
protostellar object(s) as in the low-mass regime, remaining relatively constant
during the ultra-compact HII region phase. The objects with the highest
fractional abundance of N2D+ are starless cores with properties very similar to
typical pre-stellar cores of lower mass. The abundance of N2D+ is lower in
objects with higher gas temperatures as in the low-mass case but does not seem
to depend on gas turbulence. Our results indicate that the N2D+-to-N2H+ column
density ratio can be used as an evolutionary indicator in both low- and
high-mass star formation, and that the physical conditions influencing the
abundance of deuterated species likely evolve similarly during the processes
that lead to the formation of both low- and high-mass stars.Comment: Accepted by A&AL, 4 pages, 2 figures, 2 appendices (one for Tables,
one for additional figures
The L1157-B1 astrochemical laboratory: testing the origin of DCN
L1157-B1 is the brightest shocked region of the large-scale molecular
outflow, considered the prototype of chemically rich outflows, being the ideal
laboratory to study how shocks affect the molecular gas. Several deuterated
molecules have been previously detected with the IRAM 30m, most of them formed
on grain mantles and then released into the gas phase due to the shock. We aim
to observationally investigate the role of the different chemical processes at
work that lead to formation the of DCN and test the predictions of the chemical
models for its formation. We performed high-angular resolution observations
with NOEMA of the DCN(2-1) and H13CN(2-1) lines to compute the deuterated
fraction, Dfrac(HCN). We detected emission of DCN(2-1) and H13CN(2-1) arising
from L1157-B1 shock. Dfrac(HCN) is ~4x10 and given the uncertainties, we
did not find significant variations across the bow-shock. Contrary to HDCO,
whose emission delineates the region of impact between the jet and the ambient
material, DCN is more widespread and not limited to the impact region. This is
consistent with the idea that gas-phase chemistry is playing a major role in
the deuteration of HCN in the head of the bow-shock, where HDCO is undetected
as it is a product of grain-surface chemistry. The spectra of DCN and H13CN
match the spectral signature of the outflow cavity walls, suggesting that their
emission result from shocked gas. The analysis of the time dependent gas-grain
chemical model UCL-CHEM coupled with a C-type shock model shows that the
observed Dfrac(HCN) is reached during the post-shock phase, matching the
dynamical timescale of the shock. Our results indicate that the presence of DCN
in L1157-B1 is a combination of gas-phase chemistry that produces the
widespread DCN emission, dominating in the head of the bow-shock, and
sputtering from grain mantles toward the jet impact region.Comment: Accepted for publication in A&A. 7 pages, 5 Figures, 1 Tabl
Temperature and kinematics of protoclusters with intermediate and high-mass stars: the case of IRAS 05345+3157
We have mapped at small spatial scales the temperature and the velocity field
in the protocluster associated with IRAS 05345+3157, which contains both
intermediate-/high-mass protostellar candidates and starless condensations, and
is thus an excellent location to investigate the role of massive protostars on
protocluster evolution. We observed the ammonia (1,1) and (2,2) inversion
transitions with the VLA. Ammonia is the best thermometer for dense and cold
gas, and the observed transitions have critical densities able to trace the
kinematics of the intracluster gaseous medium. The ammonia emission is extended
and distributed in two filamentary structures. The starless condensations are
colder than the star-forming cores, but the gas temperature across the whole
protocluster is higher (by a factor of ~1.3-1.5) than that measured typically
in both infrared dark clouds and low-mass protoclusters. The non-thermal
contribution to the observed line broadening is at least a factor of 2 larger
than the expected thermal broadening even in starless condensations, contrary
to the close-to-thermal line widths measured in low-mass quiescent dense cores.
The NH3-to-N2H+ abundance ratio is greatly enhanced (a factor of 10) in the
pre--stellar core candidates, probably due to freeze-out of most molecular
species heavier than He. The more massive and evolved objects likely play a
dominant role in the physical properties and kinematics of the protocluster.
The high level of turbulence and the fact that the measured core masses are
larger than the expected thermal Jeans masses indicate that turbulence likely
was an important factor in the initial fragmentation of the parental clump.Comment: 13 pages (with Appendix), 11 figure
The NH2D/NH3 ratio toward pre-protostellar cores around the UCHII region in IRAS 20293+3952
The deuterium fractionation, Dfrac, has been proposed as an evolutionary
indicator in pre-protostellar and protostellar cores of low-mass star-forming
regions. We investigate Dfrac, with high angular resolution, in the cluster
environment surrounding the UCHII region IRAS 20293+3952. We performed high
angular resolution observations with the IRAM Plateau de Bure Interferometer
(PdBI) of the ortho-NH2D 1_{11}-1_{01} line at 85.926 GHz and compared them
with previously reported VLA NH3 data. We detected strong NH2D emission toward
the pre-protostellar cores identified in NH3 and dust emission, all located in
the vicinity of the UCHII region IRAS 20293+3952. We found high values of
Dfrac~0.1-0.8 in all the pre-protostellar cores and low values, Dfrac<0.1,
associated with young stellar objects. The high values of Dfrac in
pre-protostellar cores could be indicative of evolution, although outflow
interactions and UV radiation could also play a role.Comment: 5 pages, 3 figures. Accepted for publication in Astronomy and
Astrophysics Letter
Broad N2H+ emission towards the protostellar shock L1157-B1
We present the first detection of N2H+ towards a low-mass protostellar
outflow, namely the L1157-B1 shock, at about 0.1 pc from the protostellar
cocoon. The detection was obtained with the IRAM 30-m antenna. We observed
emission at 93 GHz due to the J = 1-0 hyperfine lines. The analysis of the
emission coupled with the HIFI CHESS multiline CO observations leads to the
conclusion that the observed N2H+(1-0) line originates from the dense (> 10^5
cm-3) gas associated with the large (20-25 arcsec) cavities opened by the
protostellar wind. We find a N2H+ column density of few 10^12 cm-2
corresponding to an abundance of (2-8) 10^-9. The N2H+ abundance can be matched
by a model of quiescent gas evolved for more than 10^4 yr, i.e. for more than
the shock kinematical age (about 2000 yr). Modelling of C-shocks confirms that
the abundance of N2H+ is not increased by the passage of the shock. In summary,
N2H+ is a fossil record of the pre-shock gas, formed when the density of the
gas was around 10^4 cm-3, and then further compressed and accelerated by the
shock.Comment: ApJ, in pres
First interferometric study of enhanced N-fractionation in NH: the high-mass star-forming region IRAS 05358+3543
Nitrogen (N) fractionation is used as a tool to search for a link between the
chemical history of the Solar System and star-forming regions. A large
variation of N/N is observed towards different astrophysical
sources, and current chemical models cannot reproduce it. With the advent of
high angular resolution radiotelescopes it is now possible to search for
N-fractionation at core scales. We present IRAM NOEMA observations of the J=1-0
transition of NH, NNH and NNNH towards
the high-mass protocluster IRAS 05358+3543. We find N/N ratios
that span from 100 up to 220 and these values are lower or equal
than those observed with single-dish observations towards the same source.
Since N-fractionation changes across the studied region, this means that it is
regulated by local environmental effects. We find also the possibility, for one
of the four cores defined in the protocluster, to have a more abundant
NNH with respect to NNNH. This is another indication
that current chemical models may be missing chemical reactions or may not take
into account other mechanisms, like photodissociation or grain surface
chemistry, that could be important.Comment: 19 pages, 8 figures, 6 tables, 3 appendices Accepted in Monthly
Notices of the Royal Astronomical Society Letter
Dense gas in IRAS 20343+4129: an ultracompact HII region caught in the act of creating a cavity
The intermediate- to high-mass star-forming region IRAS 20343+4129 is an
excellent laboratory to study the influence of high- and intermediate-mass
young stellar objects on nearby starless dense cores, and investigate for
possible implications in the clustered star formation process. We present 3 mm
observations of continuum and rotational transitions of several molecular
species (C2H, c-C3H2, N2H+, NH2D) obtained with the Combined Array for Research
in Millimetre-wave Astronomy, as well as 1.3 cm continuum and NH3 observations
carried out with the Very Large Array, to reveal the properties of the dense
gas. We confirm undoubtedly previous claims of an expanding cavity created by
an ultracompact HII region associated with a young B2 zero-age main sequence
(ZAMS) star. The dense gas surrounding the cavity is distributed in a filament
that seems squeezed in between the cavity and a collimated outflow associated
with an intermediate-mass protostar. We have identified 5 millimeter continuum
condensations in the filament. All of them show column densities consistent
with potentially being the birthplace of intermediate- to high-mass objects.
These cores appear different from those observed in low-mass clustered
environments in sereval observational aspects (kinematics, temperature,
chemical gradients), indicating a strong influence of the most massive and
evolved members of the protocluster. We suggest a possible scenario in which
the B2 ZAMS star driving the cavity has compressed the surrounding gas,
perturbed its properties and induced the star formation in its immediate
surroundings.Comment: 17 pages, 13 figures. Accepted for publication in Monthly Notices of
the Royal Astronomical Society (Main Journal
Widespread Molecular Outflows in the Infrared Dark Cloud G28.37+0.07: Indications of Orthogonal Outflow-Filament Alignment
We present ALMA CO(2-1) observations toward a massive infrared dark cloud
G28.37+0.07. The ALMA data reveal numerous molecular (CO) outflows with a wide
range of sizes throughout the cloud. Sixty-two 1.3 mm continuum cores were
identified to be driving molecular outflows. We have determined the position
angle in the plane-of-sky of 120 CO outflow lobes and studied their
distribution. We find that the distribution of the plane-of-sky outflow
position angles peaks at about 100 degree, corresponding to a concentration of
outflows with an approximately east-west direction. For most outflows, we have
been able to estimate the plane-of-sky angle between the outflow axis and the
filament that harbors the protostar that powers the outflow. Statistical tests
strongly indicate that the distribution of outflow-filament orientations is
consistent with most outflow axes being mostly orthogonal to their parent
filament in 3D. Such alignment may result from filament fragmentation or
continuous mass transportation from filament to the embedded protostellar core.
The latter is suggested by recent numerical studies with moderately strong
magnetic fields.Comment: 4 figures, 1 table, accepted by Ap
Core Emergence in a Massive Infrared Dark Cloud: A Comparison Between Mid-IR Extinction and 1.3 mm Emission
Stars are born from dense cores in molecular clouds. Observationally, it is
crucial to capture the formation of cores in order to understand the necessary
conditions and rate of the star formation process. The {\it Atacama Large
Mm/sub-mm Array} (ALMA) is extremely powerful for identifying dense gas
structures, including cores, at mm wavelengths via their dust continuum
emission. Here we use ALMA to carry out a survey of dense gas and cores in the
central region of the massive () Infrared Dark Cloud (IRDC)
G28.37+0.07. The observation consists of a mosaic of 86 pointings of the
12m-array and produces an unprecedented view of the densest structures of this
IRDC. In this first paper about this data set, we focus on a comparison between
the 1.3 mm continuum emission and a mid-infrared (MIR) extinction map of the
IRDC. This allows estimation of the "dense gas" detection probability function
(DPF), i.e., as a function of the local mass surface density, , for
various choices of thresholds of mm continuum emission to define "dense gas".
We then estimate the dense gas mass fraction, , in the central
region of the IRDC and, via extrapolation with the DPF and the known
probability distribution function, to the larger-scale surrounding regions,
finding values of about 5\% to 15\% for the fiducial choice of threshold. We
argue that this observed dense gas is a good tracer of the protostellar core
population and, in this context, estimate a star formation efficiency per
free-fall time in the central IRDC region of 10\%, with
approximately a factor of two systematic uncertainties.Comment: 11 pages, 4 figures, 1 table, accepted by ApJL, comments welcom
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