365 research outputs found
An Extinction Threshold for Protostellar Cores in Ophiuchus
We have observed continuum emission at lambda = 850 microns over ~4 square
degrees of the Ophiuchus star-forming cloud using SCUBA on the JCMT, producing
a submillimetre continuum map twenty times larger than previous Ophiuchus
surveys. Our sensitivity is 40 mJ/beam, a factor of ~2 less sensitive than
earlier maps. Using an automated identification algorithm, we detect 100
candidate objects. Only two new objects are detected outside the boundary of
previous maps, despite the much wider area surveyed. We compare the
submillimetre continuum map with a map of visual extinction across the
Ophiuchus cloud derived using a combination of 2MASS and R-band data. The total
mass in submillimetre objects is ~ 50 Msun compared with ~ 2000 Msun in
observed cloud mass estimated from the extinction. The submillimetre objects
represent only 2.5% of the cloud mass. A clear association is seen between the
locations of detected submillimetre objects and high visual extinction, with no
objects detected at A_V<7 magnitudes. Using the extinction map, we estimate
pressures within the cloud from P/k ~2x10^5 cm^-3 K in the less-extincted
regions to P/k ~2x10^6 cm^-3 K at the cloud centre. Given our sensitivities,
cold (T_d ~15K) clumps supported by thermal pressure, had they existed, should
have been detected throughout the majority of the map. Such objects may not be
present at low A_V because they may form only where A_V > 15, by some mechanism
(e.g., loss of non-thermal support).Comment: 12 pages, 1 figure. Accepted by Astrophysical Journal Letter
Kompaneets Model Fitting of the Orion-Eridanus Superbubble
Winds and supernovae from OB associations create large cavities in the
interstellar medium referred to as superbubbles. The Orion molecular clouds are
the nearest high mass star-forming region and have created a highly elongated,
20 degree x 45 degree, superbubble. We fit Kompaneets models to the
Orion-Eridanus superbubble and find that a model where the Eridanus side of the
superbubble is oriented away from the Sun provides a marginal fit. Because this
model requires an unusually small scaleheight of 40 pc and has the superbubble
inclined 35 degrees from the normal to the Galactic plane, we propose that this
model should be treated as a general framework for modelling the Orion-Eridanus
superbubble, with a secondary physical mechanism not included in the Kompaneets
model required to fully account for the orientation and elongation of the
superbubble.Comment: 15 pages, 5 figures, 2 tables, accepted by MNRAS, minor grammatical
change
Filamentary Star Formation: Observing the Evolution toward Flattened Envelopes
Filamentary structures are ubiquitous from large-scale molecular clouds (few
parsecs) to small-scale circumstellar envelopes around Class 0 sources (~1000
AU to ~0.1 pc). In particular, recent observations with the Herschel Space
Observatory emphasize the importance of large-scale filaments (few parsecs) and
star formation. The small-scale flattened envelopes around Class 0 sources are
reminiscent of the large-scale filaments. We propose an observationally derived
scenario for filamentary star formation that describes the evolution of
filaments as part of the process for formation of cores and circumstellar
envelopes. If such a scenario is correct, small-scale filamentary structures
(0.1 pc in length) with higher densities embedded in starless cores should
exist, although to date almost all the interferometers have failed to observe
such structures. We perform synthetic observations of filaments at the
prestellar stage by modeling the known Class 0 flattened envelope in L1157
using both the Combined Array for Research in Millimeter-wave Astronomy (CARMA)
and the Atacama Large Millimeter/Submillimeter Array (ALMA). We show that with
reasonable estimates for the column density through the flattened envelope, the
CARMA D-array at 3mm wavelengths is not able to detect such filamentary
structure, so previous studies would not have detected them. However, the
substructures may be detected with CARMA D+E array at 3 mm and CARMA E array at
1 mm as a result of more appropriate resolution and sensitivity. ALMA is also
capable of detecting the substructures and showing the structures in detail
compared to the CARMA results with its unprecedented sensitivity. Such
detection will confirm the new proposed paradigm of non-spherical star
formation.Comment: 9 pages, 10 figures. Accepted by Ap
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