479 research outputs found
Molecular line contamination in the SCUBA-2 450 {\mu}m and 850 {\mu}m continuum data
Observations of the dust emission using millimetre/submillimetre bolometer
arrays can be contaminated by molecular line flux, such as flux from 12CO. As
the brightest molecular line in the submillimetre, it is important to quantify
the contribution of CO flux to the dust continuum bands. Conversion factors
were used to convert molecular line integrated intensities to flux detected by
bolometer arrays in mJy per beam. These factors were calculated for 12CO line
integrated intensities to the SCUBA-2 850 {\mu}m and 450 {\mu}m bands. The
conversion factors were then applied to HARP 12CO 3-2 maps of NGC 1333 in the
Perseus complex and NGC 2071 and NGC 2024 in the Orion B molecular cloud
complex to quantify the respective 12CO flux contribution to the 850 {\mu}m
dust continuum emission. Sources with high molecular line contamination were
analysed in further detail for molecular outflows and heating by nearby stars
to determine the cause of the 12CO contribution. The majority of sources had a
12CO 3-2 flux contribution under 20 per cent. However, in regions of molecular
outflows, the 12CO can dominate the source dust continuum (up to 79 per cent
contamination) with 12CO fluxes reaching \sim 68 mJy per beam.Comment: Accepted 2012 April 19 for publication in MNRAS. 21 pages, 13
figures, 3 table
Star formation in Perseus: III. Outflows
We present a search for outflows towards 51 submillimetre cores in Perseus.
With consistently derived outflow properties from a large homogeneous dataset
within one molecular cloud we can investigate further the mass dependence and
time evolution of protostellar mass loss. Of the 51 cores, 37 show broad
linewings indicative of molecular outflows. In 13 cases, the linewings could be
due to confusion with neighbouring flows but 9 of those sources also have
near-infrared detections confirming their protostellar nature. The total
fraction of protostars in our sample is 65%. All but four outflow detections
are confirmed as protostellar by Spitzer IR detections and only one Spitzer
source has no outflow, showing that outflow maps at this sensitivity are
equally good at identifying protostars as Spitzer. Outflow momentum flux
correlates both with source luminosity and with core mass but there is
considerable scatter even within this one cloud despite the homogeneous
dataset. We fail to confirm the result of Bontemps et al. (1996) that Class I
sources show lower momentum fluxes on average than Class 0 sources, with a KS
test showing a significant probability that the momentum fluxes for both Class
0s and Class Is are drawn from the same distribution. We find that outflow
power may not show a simple decline between the Class 0 to Class I stages. Our
sample includes low momentum flux, low-luminosity Class 0 sources, possibly at
a very early evolutionary stage. If the only mass loss from the core were due
to outflows, cores would last for 10^5-10^8 years, longer than current
estimates of 1.5-4 x 10^5 years for the mean lifetime for the embedded phase.
Additional mechanisms for removing mass from protostellar cores may be
necessary.Comment: 26 pages, 21 figures. Version with full colour figures from
http://www.astro.ex.ac.uk/people/hatchell/RecentPapers/hatchell07_outflows.pd
JCMT in the new era
This is the final version of the article. Available from OUP via the DOI in this record.Star-formation studies continue at the James Clerk Maxwell Telescope under new management, as Jennifer Hatchell and Derek Ward-Thompson report from an RAS meeting in March
The JCMT dense gas survey of the Perseus Molecular Cloud
We present the results of a large-scale survey of the very dense gas in the
Perseus molecular cloud using HCO+ and HCN (J = 4 - 3) transitions. We have
used this emission to trace the structure and kinematics of gas found in pre-
and protostellar cores, as well as in outflows. We compare the HCO+/HCN data,
highlighting regions where there is a marked discrepancy in the spectra of the
two emission lines. We use the HCO+ to identify positively protostellar
outflows and their driving sources, and present a statistical analysis of the
outflow properties that we derive from this tracer. We find that the relations
we calculate between the HCO+ outflow driving force and the Menv and Lbol of
the driving source are comparable to those obtained from similar outflow
analyses using 12CO, indicating that the two molecules give reliable estimates
of outflow properties. We also compare the HCO+ and the HCN in the outflows,
and find that the HCN traces only the most energetic outflows, the majority of
which are driven by young Class 0 sources. We analyse the abundances of HCN and
HCO+ in the particular case of the IRAS 2A outflows, and find that the HCN is
much more enhanced than the HCO+ in the outflow lobes. We suggest that this is
indicative of shock-enhancement of HCN along the length of the outflow; this
process is not so evident for HCO+, which is largely confined to the outflow
base.Comment: 25 pages, 14 figures, 9 table
Spatial Statistics in Star Forming Regions: Testing the Limits of Randomness
This is the author accepted manuscript. the final version is available from OUP via the DOI in this recordObservational studies of star formation reveal spatial distributions of Young Stellar Objects (YSOs) that are ‘snapshots’ of an ongoing star formation process. Using methods from spatial statistics it is possible to test the likelihood that a given distribution process could produce the observed patterns of YSOs. The aim of this paper is to determine the usefulness of the spatial statistics tests Diggle’s G function (G), the ‘free-space’ function (F), Ripley’s K and O-ring for application to astrophysical data. The spatial statistics tests were applied to simulated data containing 2D Gaussian clusters projected on random distributions of stars. The number of stars within the Gaussian cluster and number of background stars were varied to determine the tests’ ability to reject complete spatial randomness (CSR) with changing signal-to-noise. The best performing test was O-ring optimised with overlapping logarithmic bins, closely followed by Ripleys K. The O-ring test is equivalent to the 2-point correlation function. Both F and G (and the minimum spanning tree, of which G is a subset) performed significantly less well, requiring a cluster with a factor of two higher signal-to-noise in order to reject CSR consistently. We demonstrate the tests on example astrophysical datasets drawn from the Spitzer catalogue.Science and Technology Facilities Council (STFC
What can the SEDs of first hydrostatic core candidates reveal about their nature?
The first hydrostatic core (FHSC) is the first stable object to form in
simulations of star formation. This stage has yet to be observed definitively,
although several candidate FHSCs have been reported. We have produced synthetic
spectral energy distributions (SEDs) from 3D hydrodynamical simulations of
pre-stellar cores undergoing gravitational collapse for a variety of initial
conditions. Variations in the initial rotation rate, radius and mass lead to
differences in the location of the SED peak and far-infrared flux. Secondly, we
attempt to fit the SEDs of five FHSC candidates from the literature and five
newly identified FHSC candidates located in the Serpens South molecular cloud
with simulated SEDs. The most promising FHSC candidates are fitted by a limited
number of model SEDs with consistent properties, which suggests the SED can be
useful for placing constraints on the age and rotation rate of the source. The
sources we consider most likely to be in FHSC phase are B1-bN, CB17-MMS,
Aqu-MM1 and Serpens South candidate K242. We were unable to fit SerpS-MM22,
Per-Bolo 58 and Chamaeleon-MMS1 with reasonable parameters, which indicates
that they are likely to be more evolved.Comment: 26 pages, 28 figures. Accepted for publication in MNRA
An observational survey of molecular emission ahead of Herbig-Haro objects
Context. A molecular survey recently performed ahead of HH~2 supports the
idea that the observed molecular enhancement is due to UV radiation from the HH
object. Aims. The aim of the present work is to determine whether all HH
objects with enhanced HCO emission ahead of them also exhibit the same
enhanced chemistry as HH~2. We thus observed several molecular lines at several
positions ahead of five Herbig-Haro objects where enhanced HCO emission was
previously observed. Methods. We mapped the five Herbig-Haro objects using the
IRAM-30 m. For each position we searched for more than one molecular species,
and where possible for more than one transition per species. We then estimated
the averaged beam column densities for all species observed and also performed
LVG analyses to constrain the physical properties of the gas. Results. The
chemically richest quiescent gas is found ahead of the HH~7-11 complex, in
particular at the HH~7-11 A position. In some regions we also detected a high
velocity gas component. We find that the gas densities are always higher than
those typical of a molecular cloud while the derived temperatures are always
quite low, ranging from 10 to 25 K. The emission of most species seems to be
enhanced with respect to that of a typical dense clump, probably due to the
exposure to a high UV radiation from the HH objects. Chemical differentiation
among the positions is also observed. We attempt a very simple chemical
analysis to explain such differentiation.Comment: Accepted by Astronomy and Astrophysics; 17 pages, 8 figure
Signatures of inflow motion in cores of massive star formation: Potential collapse candidates
Using the IRAM 30 m telescope, a mapping survey in optically thick and thin
lines was performed towards 46 high mass star-forming regions. The sample
includes UC H{\sc ii} precursors and UC H{\sc ii} regions. Seventeen sources
are found to show "blue profiles", the expected signature of collapsing cores.
The excess of sources with blue over red profiles ([ -- ]/) is 29% in the HCO =1--0 line, with a probability
of 0.6% that this is caused by random fluctuations. UC H{\sc ii} regions show a
higher excess (58%) than UC H{\sc ii} precursors (17%), indicating that
material is still accreted after the onset of the UC H{\sc ii} phase. Similar
differences in the excess of blue profiles as a function of evolutionary state
are not observed in low mass star-forming regions. Thus, if confirmed for high
mass star-forming sites, this would point at a fundamental difference between
low- and high-mass star formation. Possible explanations are inadequate
thermalization, stronger influence of outflows in massive early cores, larger
gas reserves around massive stellar objects or different trigger mechanisms
between low- and high- mass star formation
- …