124 research outputs found
The molecular clump towards the eastern border of SNR G18.8+0.3
The eastern border of the SNR G18.8+0.3, close to an HII regions complex, is
a very interesting region to study the molecular gas that it is probably in
contact with the SNR shock front. We observed the aforementioned region using
the Atacama Submillimeter Telescope Experiment (ASTE) in the 12CO J=3-2, 13CO
J=3-2, HCO+ J=4-3, and CS J=7-6 lines with an angular resolution of 22". To
complement these observations, we analyzed IR, submillimeter and radio
continuum archival data. In this work, we clearly show that the radio continuum
"protrusion" that was early thought to belong to the SNR is an HII regions
complex deeply embedded in a molecular clump. The new molecular observations
reveal that this dense clump, belonging to an extended molecular cloud that
surrounds the SNR southeast border, is not physically in contact with SNR
G18.8+0.3, suggesting that the SNR shock front have not yet reached it or maybe
they are located at different distances. We found some young stellar objects
embedded in the molecular clump, suggesting that their formation should be
approximately coeval with the SN explosion.Comment: Accepted for publication in A&A (Sept. 7, 2012
Herschel observations of EXtra-Ordinary Sources: The Terahertz spectrum of Orion KL seen at high spectral resolution
We present the first high spectral resolution observations of Orion KL in the
frequency ranges 1573.4 - 1702.8 GHz (band 6b) and 1788.4 - 1906.8 GHz (band
7b) obtained using the HIFI instrument on board the Herschel Space Observatory.
We characterize the main emission lines found in the spectrum, which primarily
arise from a range of components associated with Orion KL including the hot
core, but also see widespread emission from components associated with
molecular outflows traced by H2O, SO2, and OH. We find that the density of
observed emission lines is significantly diminished in these bands compared to
lower frequency Herschel/HIFI bands.Comment: Accepted for publication in the Herschel HIFI special issue of
Astronomy and Astrophysics Letters, 5 pages, 3 figure
Variations in the Galactic star formation rate and density thresholds for star formation
The conversion of gas into stars is a fundamental process in astrophysics and
cosmology. Stars are known to form from the gravitational collapse of dense
clumps in interstellar molecular clouds, and it has been proposed that the
resulting star formation rate is proportional to either the amount of mass
above a threshold gas surface density, or the gas volume density. These
star-formation prescriptions appear to hold in nearby molecular clouds in our
Milky Way Galaxy's disk as well as in distant galaxies where the star formation
rates are often much larger. The inner 500 pc of our Galaxy, the Central
Molecular Zone (CMZ), contains the largest concentration of dense, high-surface
density molecular gas in the Milky Way, providing an environment where the
validity of star-formation prescriptions can be tested. Here we show that by
several measures, the current star formation rate in the CMZ is an
order-of-magnitude lower than the rates predicted by the currently accepted
prescriptions. In particular, the region 1 deg < l < 3.5 deg, |b| < 0.5 deg
contains ~10^7 Msun of dense molecular gas -- enough to form 1000 Orion-like
clusters -- but the present-day star formation rate within this gas is only
equivalent to that in Orion. In addition to density, another property of
molecular clouds, such as the amplitude of turbulent motions, must be included
in the star-formation prescription to predict the star formation rate in a
given mass of molecular gas.Comment: 17 pages, 6 figures, submitted MNRA
Dynamically Influenced Molecular Clouds in the Nucleus of NGC 6946: Variations in the CO Isotopic Line Ratios
We present high resolution (~5'') maps of the J = 1 - 0 transitions of
^{13}CO and C^{18}O towards the nucleus of NGC 6946, made with the Owens Valley
Millimeter Array. The images are compared with existing ^{12}CO(1-0) maps to
investigate localized changes in gas properties across the nucleus. As compared
to ^{12}CO, both ^{13}CO and C^{18}O are more confined to the central ring of
molecular gas associated with the nuclear star formation; that is, ^{12}CO is
stronger relative to ^{13}CO and C^{18}O away from the nucleus and along the
spiral arms. The ^{12}CO(1-0)/^{13}CO(1-0) line ratio reaches very high values
of >40. We attribute the relative ^{13}CO weakness to a rapid change in the
interstellar medium from dense star forming cores in a central ring to diffuse,
low density molecular gas in and behind the molecular arms. This change is
abrupt, occurring in less than a beamsize (90 pc), about the size of a giant
molecular cloud. Column densities determined from ^{13}CO(1-0), C^{18}O(1-0),
and 1.4 mm dust continuum all indicate that the standard Galactic conversion
factor, X_{CO}, overestimates the amount of molecular gas in NGC 6946 by
factors of ~3-5 towards the central ring and potentially even more so in the
diffuse gas away from the central starburst. We suggest that the nuclear bar
acts to create coherent regions of molecular clouds with distinct and different
physical conditions. The ^{12}CO(1-0)/^{13}CO(1-0) line ratio in galactic
nuclei can be a signpost of a dynamically evolving ISM.Comment: 38 pages, 9 figures. Accepted to the Astronomical Journa
Testing grain surface chemistry : a survey of deuterated formaldehyde and methanol in low-mass Class 0 protostars
Context : Despite the low cosmic abundance of deuterium (D/H ~ 1e-5), large
degrees of deuterium fractionation in molecules are observed in star forming
regions with enhancements that can reach 13 orders of magnitude, which current
models have difficulties to account for.
Aims : Multi-isotopologue observations are a very powerful constraint for
chemical models. The aim of our observations is to understand the processes
forming the observed large abundances of methanol and formaldehyde in low-mass
protostellar envelopes (gas-phase processes ? chemistry on the grain surfaces
?) and better constrain the chemical models. Methods : Using the IRAM 30m
single-dish telescope, we observed deuterated formaldehyde (HDCO and D2CO) and
methanol (CH2DOH, CH3OD, and CHD2OH) towards a sample of seven low-mass class 0
protostars. Using population diagrams, we then derive the fractionation ratios
of these species (abundance ratio between the deuterated molecule and its main
isotopologue) and compare them to the predictions of grain chemistry models.
Results : These protostars show a similar level of deuteration as in
IRAS16293-2422, where doubly-deuterated methanol -- and even triply-deuterated
methanol -- were first detected. Our observations point to the formation of
methanol on the grain surfaces, while formaldehyde formation cannot be fully
pined down. While none of the scenarii can be excluded (gas-phase or grain
chemistry formation), they both seem to require abstraction reactions to
reproduce the observed fractionations.Comment: 21 pages, 12 figures, accepted by A&
Physical structure of the envelopes of intermediate-mass protostars
Context: Intermediate mass protostars provide a bridge between low- and
high-mass protostars. Furthermore, they are an important component of the UV
interstellar radiation field. Despite their relevance, little is known about
their formation process. Aims: We present a systematic study of the physical
structure of five intermediate mass, candidate Class 0 protostars. Our two
goals are to shed light on the first phase of intermediate mass star formation
and to compare these protostars with low- and high-mass sources. Methods: We
derived the dust and gas temperature and density profiles of the sample. We
analysed all existing continuum data on each source and modelled the resulting
SED with the 1D radiative transfer code DUSTY. The gas temperature was then
predicted by means of a modified version of the code CHT96. Results: We found
that the density profiles of five out of six studied intermediate mass
envelopes are consistent with the predictions of the "inside-out" collapse
theory.We compared several physical parameters, like the power law index of the
density profile, the size, the mass, the average density, the density at 1000
AU and the density at 10 K of the envelopes of low-, intermediate, and
high-mass protostars. When considering these various physical parameters, the
transition between the three groups appears smooth, suggesting that the
formation processes and triggers do not substantially differ
The structure of the NGC1333-IRAS2 protostellar system on 500 AU scales: an infalling envelope, a circumstellar disk, multiple outflows, and chemistry
This paper investigates small-scale structures of dense gas and dust around
the low-mass protostellar binary NGC1333-IRAS2 using millimeter-wavelength
aperture-synthesis observations from the OVRO and BIMA interferometers. The
detected 3 mm continuum emission is consistent with models of the envelope
around IRAS2A, based on previously reported submillimeter continuum images,
down to the 3", or 500 AU, resolution of the interferometer data. Our data
constrain the contribution of an unresolved point source to 22 mJy. Within the
accuracy of the parameters describing the envelope model, the point source flux
has an uncertainty by up to 25%. We interpret this point source as a cold disk
of mass \gtrsim 0.3 M_\odot. The same envelope model also reproduces
aperture-synthesis line observations of the optically thin isotopic species
C34S and H13CO+. The more optically thick main isotope lines show a variety of
components in the protostellar environment: N2H+ is closely correlated with
dust concentrations as seen at submillimeter wavelengths and is particularly
strong toward the starless core IRAS2C. We hypothesize that N2H+ is destroyed
through reactions with CO that is released from icy grains near the
protostellar sources IRAS2A and B. CS, HCO+, and HCN have complex line shapes
apparently affected by both outflow and infall. In addition to the east-west
jet from IRAS2A, a north-south velocity gradient near this source indicates a
second, perpendicular outflow. This suggests the presence of a binary companion
within 0.3" (65 AU) from IRAS2A as driving source of this outflow. Alternative
explanations of the velocity gradient such as rotation in a circumstellar
envelope or a single, wide-angle outflow are less likely.Comment: 16 pages including figures. Accepted for publication in A&
The molecular environment of the massive star forming region NGC 2024: Multi CO transition analysis
NGC 2024, a sites of massive star formation, have complex internal structures
caused by cal heating by young stars, outflows, and stellar winds. These
complex cloud structures lead to intricate emission line shapes. The goal of
this paper is to show that the complex line shapes of 12 CO lines in NGC 2024
can be explained consistently with a model, whose temperature and velocity
structure are based on the well-established scenario of a PDR and the Blister
model. We present velocity-resolved spectra of seven CO lines ranging from J=3
to J=13, and we combined these data with CO high-frequency data from the ISO
satellite. We find that the bulk of the molecular cloud associated with NGC
2024 consists of warm (75 K) and dense (9e5 cm-3) gas. An additional hot (~ 300
K) component, located at the interface of the HII region and the molecular
cloud, is needed to explain the emission of the high-J CO lines. Deep
absorption notches indicate that very cold material (20 K) exists in front of
the warm material, too. A temperature and column density structure consistent
with those predicted by PDR models, combined with the velocity structure of a
Blister model, appropriately describes the observed emission line profiles of
this massive star forming region. This case study of NGC 2024 shows that, with
physical insights into these complex regions and careful modeling, multi-line
observations of CO can be used to derive detailed physical conditions in
massive star forming regions.Comment: 10 pages, 5 figures, accepted by A&A for publicatio
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