909 research outputs found
Multiple protostellar systems. II. A high resolution near-infrared imaging survey in nearby star-forming regions
(abridged) Our project endeavors to obtain a robust view of multiplicity
among embedded Class I and Flat Spectrum protostars in a wide array of nearby
molecular clouds to disentangle ``universal'' from cloud-dependent processes.
We have used near-infrared adaptive optics observations at the VLT through the
H, Ks and L' filters to search for tight companions to 45 Class I and Flat
Spectrum protostars located in 4 different molecular clouds (Taurus-Auriga,
Ophiuchus, Serpens and L1641 in Orion). We complemented these observations with
published high-resolution surveys of 13 additional objects in Taurus and
Ophiuchus. We found multiplicity rates of 32+/-6% and 47+/-8% over the 45-1400
AU and 14-1400 AU separation ranges, respectively. These rates are in excellent
agreement with those previously found among T Tauri stars in Taurus and
Ophiuchus, and represent an excess of a factor ~1.7 over the multiplicity rate
of solar-type field stars. We found no non-hierarchical triple systems, nor any
quadruple or higher-order systems. No significant cloud-to-cloud difference has
been found, except for the fact that all companions to low-mass Orion
protostars are found within 100 AU of their primaries whereas companions found
in other clouds span the whole range probed here. Based on this survey, we
conclude that core fragmentation always yields a high initial multiplicity
rate, even in giant molecular clouds such as the Orion cloud or in clustered
stellar populations as in Serpens, in contrast with predictions of numerical
simulations. The lower multiplicity rate observed in clustered Class II and
Class III populations can be accounted for by a universal set of properties for
young systems and subsequent ejections through close encounters with unrelated
cluster members.Comment: 15 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
Molecular Tracers of Embedded Star Formation in Ophiuchus
In this paper we analyze nine SCUBA cores in Ophiuchus using the
second-lowest rotational transitions of four molecular species (12CO, 13CO,
C18O, and C17O) to search for clues to the evolutionary state and
star-formation activity within each core. Specifically, we look for evidence of
outflows, infall, and CO depletion. The line wings in the CO spectra are used
to detect outflows, spectral asymmetries in 13CO are used to determine infall
characteristics, and a comparison of the dust emission (from SCUBA
observations) and gas emission (from C18O) is used to determine the fractional
CO freeze-out.
Through comparison with Spitzer observations of protostellar sources in
Ophiuchus, we discuss the usefulness of CO and its isotopologues as the sole
indicators of the evolutionary state of each core. This study is an important
pilot project for the JCMT Legacy Survey of the Gould Belt (GBS) and the
Galactic Plane (JPS), which intend to complement the SCUBA-2 dust continuum
observations with HARP observations of 12CO, 13CO, C18O, and C17O J = 3 - 2 in
order to determine whether or not the cold dust clumps detected by SCUBA-2 are
protostellar or starless objects.
Our classification of the evolutionary state of the cores (based on molecular
line maps and SCUBA observations) is in agreement with the Spitzer designation
for six or seven of the nine SCUBA cores. However, several important caveats
exist in the interpretation of these results, many of which large mapping
surveys like the GBS may be able to overcome to provide a clearer picture of
activity in crowded fields.Comment: 43 pages including 19 postscript figures. Accepted for publication in
the PAS
Linear Temperature Variation of the Penetration Depth in YBCO Thin Films
We have measured the penetration depth on thin films from transmission at 120, 330 and 510~GHz,
between 5 and 50~K. Our data yield simultaneously the absolute value and the
temperature dependence of . In high quality films
exhibits the same linear temperature dependence as single crystals, showing its
intrinsic nature, and . In a lower quality one, the
more usual dependence is found, and . This
suggests that the variation is of extrinsic origin. Our results put the
-wave like interpretation in a much better position.Comment: 12 pages, revtex, 4 uuencoded figure
Two Bipolar Outflows and Magnetic Fields in a Multiple Protostar System, L1448 IRS 3
We performed spectral line observations of CO J=2-1, 13CO J=1-0, and C18O
J=1-0 and polarimetric observations in the 1.3 mm continuum and CO J=2-1 toward
a multiple protostar system, L1448 IRS 3, in the Perseus molecular complex at a
distance of ~250 pc, using the BIMA array. In the 1.3 mm continuum, two sources
(IRS 3A and 3B) were clearly detected with estimated envelope masses of 0.21
and 1.15 solar masses, and one source (IRS 3C) was marginally detected with an
upper mass limit of 0.03 solar masses. In CO J=2-1, we revealed two outflows
originating from IRS 3A and 3B. The masses, mean number densities, momentums,
and kinetic energies of outflow lobes were estimated. Based on those estimates
and outflow features, we concluded that the two outflows are interacting and
that the IRS 3A outflow is nearly perpendicular to the line of sight. In
addition, we estimated the velocity, inclination, and opening of the IRS 3B
outflow using Bayesian statistics. When the opening angle is ~20 arcdeg, we
constrain the velocity to ~45 km/s and the inclination angle to ~57 arcdeg.
Linear polarization was detected in both the 1.3 mm continuum and CO J=2-1. The
linear polarization in the continuum shows a magnetic field at the central
source (IRS 3B) perpendicular to the outflow direction, and the linear
polarization in the CO J=2-1 was detected in the outflow regions, parallel or
perpendicular to the outflow direction. Moreover, we comprehensively discuss
whether the binary system of IRS 3A and 3B is gravitationally bound, based on
the velocity differences detected in 13CO J=1-0 and C18O J=1-0 observations and
on the outflow features. The specific angular momentum of the system was
estimated as ~3e20 cm^2/s, comparable to the values obtained from previous
studies on binaries and molecular clouds in Taurus.Comment: ApJ accepted, 20 pages, 2 tables, 10 figure
Infall models of Class 0 protostars
We have carried out radiative transfer calculations of infalling, dusty
envelopes surrounding embedded protostars to understand the observed properties
of the recently identified ``Class 0'' sources. To match the far-infrared peaks
in the spectral energy distributions of objects such as the prototype Class 0
source VLA 1623, pure collapse models require mass infall rates
\sim10^{-4}\msunyr. The radial intensity distributions predicted by
such infall models are inconsistent with observations of VLA 1623 at sub-mm
wavelengths, in agreement with the results of Andre et al. (1993) who found a
density profile of rather than the expected gradient. To resolve this conflict, while still invoking
infall to produce the outflow source at the center of VLA 1623, we suggest that
the observed sub-mm intensity distribution is the sum of two components: an
inner infall zone, plus an outer, more nearly constant-density region. This
explanation of the observations requires that roughly half the total mass
observed within 2000 AU radius of the source lies in a region external to the
infall zone. The column densities for this external region are comparable to
those found in the larger Oph A cloud within which VLA 1623 is embedded. The
extreme environments of Class 0 sources lead us to suggest an alternative or
additional interpretation of these objects: rather than simply concluding with
Andre et al. that Class 0 objects only represent the earliest phases of
protostellar collapse, and ultimately evolve into older ``Class I'' protostars,
we suggest that many Class 0 sources could be the protostars of very dense
regions. (Shortened)Comment: 22 pages, including 3 PostScript figures, accepted for publication in
The Astrophysical Journa
Star formation triggered by the Galactic HII region RCW 120: First results from the Herschel Space Observatory
By means of different physical mechanisms, the expansion of HII regions can
promote the formation of new stars of all masses. RCW 120 is a nearby Galactic
HII region where triggered star formation occurs. This region is well-studied -
there being a wealth of existing data - and is nearby. However, it is
surrounded by dense regions for which far infrared data is essential to obtain
an unbiased view of the star formation process and in particular to establish
whether very young protostars are present. We attempt to identify all Young
Stellar Objects (YSOs), especially those previously undetected at shorter
wavelengths, to derive their physical properties and obtain insight into the
star formation history in this region. We use Herschel-PACS and -SPIRE images
to determine the distribution of YSOs observed in the field. We use a spectral
energy distribution fitting tool to derive the YSOs physical properties.
Herschel-PACS and -SPIRE images confirm the existence of a young source and
allow us to determine its nature as a high-mass (8-10 MSun) Class 0 object
(whose emission is dominated by a massive envelope) towards the massive
condensation 1 observed at (sub)-millimeter wavelengths. This source was not
detected at 24 micron and only barely seen in the MISPGAL 70 micron data.
Several other red sources are detected at Herschel wavelengths and coincide
with the peaks of the millimeter condensations. SED fitting results for the
brightest Herschel sources indicate that, apart from the massive Class 0 that
forms in condensation 1, young low mass stars are forming around RCW 120. The
YSOs observed on the borders of RCW 120 are younger than its ionizing star,
which has an age of about 2.5 Myr.Comment: 5 pqges, 3 figures, accepted by A&A (Special issue on the Herschel
first results
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