466 research outputs found
Ejection of a Low Mass Star in a Young Stellar System in Taurus
We present the analysis of high angular resolution VLA radio observations,
made at eleven epochs over the last 20 years, of the multiple system T Tauri.
One of the sources (Sb) in the system has moved at moderate speed (5-10 km/s),
on an apparently elliptical orbit during the first 15 years of observations,
but after a close (< 2 AU) encounter with the source Sa, it appears to have
accelerated westward to about 20 km/s in the last few years. Such a dramatic
orbital change most probably indicates that Sb has just suffered an ejection -
which would be the first such event ever detected. Whether Sb will ultimately
stay on a highly elliptical bound orbit, or whether it will leave the system
altogether will be known with about five more years of observations.Comment: 4 pages, accepter in ApJ Letter
The Detection of Cold Dust in Cas A: Evidence for the Formation of Metallic Needles in the Ejecta
Recently, Dunne et al. (2003) obtained 450 and 850 micron SCUBA images of
CasA, and reported the detection of 2-4 M_sun of cold, 18K, dust in the
remnant. Here we show that their interpretation of the observations faces
serious difficulties. Their inferred dust mass is larger than the mass of
refractory material in the ejecta of a 10 to 30 M_sun star. The cold dust model
faces even more difficulties if the 170 micron observations of the remnant are
included in the analysis, decreasing the cold dust temperature to ~ 8K, and
increasing its mass to > 20 M_sun. We offer here a more plausible
interpretation of their observation, in which the cold dust emission is
generated by conducting needles with properties that are completely determined
by the combined submillimeter and X-ray observations of the remnant. The
needles consist of metallic whiskers with <1% of embedded impurities that may
have condensed out of blobs of material that were expelled at high velocities
from the inner metal-rich layers of the star in an asymmetric explosion. The
needles are collisionally heated by the shocked gas to a temperature of 8K.
Taking the destruction of needles into account, a dust mass of only 1E-4 to
1E-3M_sun is needed to account for the observed SCUBA emission. Aligned in the
magnetic field, needles may give rise to observable polarized emission. The
detection of submillimeter polarization will therefore offer definitive proof
for a needle origin for the cold dust emission. Supernovae may still be proven
to be important sources of interstellar dust, but the evidence is still
inconclusive.Comment: 18 pages including 4 figures. Accepted for publication in the ApJ.
Missing reference adde
VLBA determination of the distance to nearby star-forming regions II. Hubble 4 and HDE 283572 in Taurus
The non-thermal 3.6 cm radio continuum emission from the naked T Tauri stars
Hubble 4 and HDE 283572 in Taurus has been observed with the Very Long Baseline
Array (VLBA) at 6 epochs between September 2004 and December 2005 with a
typical separation between successive observations of 3 months. Thanks to the
remarkably accurate astrometry delivered by the VLBA, the trajectory described
by both stars on the plane of the sky could be traced very precisely, and
modeled as the superposition of their trigonometric parallax and uniform proper
motion. The best fits yield distances to Hubble 4 and HDE 283572 of 132.8 +/-
0.5 and 128.5 +/- 0.6 pc, respectively. Combining these results with the other
two existing VLBI distance determinations in Taurus, we estimate the mean
distance to the Taurus association to be 137 pc with a dispersion (most
probably reflecting the depth of the complex) of about 20 pc.Comment: 21 pages, 4 figues, accepted in ApJ (Dec 20, 2007 issue
Detection of doubly-deuterated methanol in the solar-type protostar IRAS16293-2422
We report the first detection of doubly-deuterated methanol (CHD2OH), as well
as firm detections of the two singly-deuterated isotopomers of methanol (CH2DOH
and CH3OD), towards the solar-type protostar IRAS16293-2422. From the present
multifrequency observations, we derive the following abundance ratios:
[CHD2OH]/[CH3OH] = 0.2 +/- 0.1, [CH2DOH]/[CH3OH] = 0.9 +/- 0.3, [CH3OD]/[CH3OH]
= 0.04 +/- 0.02. The total abundance of the deuterated forms of methanol is
greater than that of its normal hydrogenated counterpart in the circumstellar
material of IRAS16293-2422, a circumstance not previously encountered.
Formaldehyde, which is thought to be the chemical precursor of methanol,
possesses a much lower fraction of deuterated isotopomers (~ 20%) with respect
to the main isotopic form in IRAS16293-2422. The observed fractionation of
methanol and formaldehyde provides a severe challenge to both gas-phase and
grain-surface models of deuteration. Two examples of the latter model are
roughly in agreement with our observations of CHD2OH and CH2DOH if the
accreting gas has a large (0.2-0.3) atomic D/H ratio. However, no gas-phase
model predicts such a high atomic D/H ratio, and hence some key ingredient
seems to be missing.Comment: 5 pages, 3 figure
First detection of triply-deuterated methanol
We report the first detection of triply-deuterated methanol, with 12 observed
transitions, towards the low-mass protostar IRAS 16293-2422, as well as
multifrequency observations of 13CH3OH, used to derive the column density of
the main isotopomer CH3OH. The derived fractionation ratio [CD3OH]/[CH3OH]
averaged on a 10'' beam is 1.4%. Together with previous CH2DOH and CHD2OH
observations, the present CD3OH observations are consistent with a formation of
methanol on grain surfaces, if the atomic D/H ratio is 0.1 to 0.3 in the
accreting gas. Such a high atomic ratio can be reached in the frame of
gas-phase chemical models including all deuterated isotopomers of H3+.Comment: Accepted by A&
Tidal foces as a regulator of star formation in Taurus
Only a few molecular clouds in the Solar Neighborhood exhibit the formation
of only low-mass stars. Traditionally, these clouds have been assumed to be
supported against more vigorous collapse by magnetic fields. The existence of
strong magnetic fields in molecular clouds, however, poses serious problems for
the formation of stars and of the clouds themselves. In this {\em Letter}, we
review the three-dimensional structure and kinematics of Taurus --the archetype
of a region forming only low-mass stars-- as well as its orientation within the
Milky way. We conclude that the particularly low star-formation efficiency in
Taurus may naturally be explained by tidal forces from the Galaxy, with no need
for magnetic regulation or stellar feedback.Comment: Minor changes. 5 pages. Accepted by MNRA
The solar type protostar IRAS16293-2422: new constraints on the physical structure
Context: The low mass protostar IRAS16293-2422 is a prototype Class 0 source
with respect to the studies of the chemical structure during the initial phases
of life of Solar type stars. Aims: In order to derive an accurate chemical
structure, a precise determination of the source physical structure is
required. The scope of the present work is the derivation of the structure of
IRAS16293-2422. Methods: We have re-analyzed all available continuum data
(single dish and interferometric, from millimeter to MIR) to derive accurate
density and dust temperature profiles. Using ISO observations of water, we have
also reconstructed the gas temperature profile. Results: Our analysis shows
that the envelope surrounding IRAS16293-2422 is well described by the Shu
"inside-out" collapsing envelope model or a single power-law density profile
with index equal to 1.8. In contrast to some previous studies, our analysis
does not show evidence of a large (>/- 800 AU in diameter) cavity. Conclusions:
Although IRAS16293-2422 is a multiple system composed by two or three objects,
our reconstruction will be useful to derive the chemical structure of the large
cold envelope surrounding these objects and the warm component, treated here as
a single source, from single-dish observations of molecular emission
VLBA determination of the distance to nearby star-forming regions I. The distance to T Tauri with 0.4% accuracy
In this article, we present the results of a series of twelve 3.6-cm radio
continuum observations of T Tau Sb, one of the companions of the famous young
stellar object T Tauri. The data were collected roughly every two months
between September 2003 and July 2005 with the Very Long Baseline Array (VLBA).
Thanks to the remarkably accurate astrometry delivered by the VLBA, the
absolute position of T Tau Sb could be measured with a precision typically
better than about 100 micro-arcseconds at each of the twelve observed epochs.
The trajectory of T Tau Sb on the plane of the sky could, therefore, be traced
very precisely, and modeled as the superposition of the trigonometric parallax
of the source and an accelerated proper motion. The best fit yields a distance
to T Tau Sb of 147.6 +/- 0.6 pc. The observed positions of T Tau Sb are in good
agreement with recent infrared measurements, but seem to favor a somewhat
longer orbital period than that recently reported by Duchene et al. (2006) for
the T Tau Sa/T Tau Sb system.Comment: 24 pages, 3 pages, AASTEX format, accepted for publication in Ap
Monitoring the Large Proper Motions of Radio Sources in the Orion BN/KL Region
We present absolute astrometry of four radio sources in the
Becklin-Neugebauer/Kleinman-Low (BN/KL) region, derived from archival data
(taken in 1991, 1995, and 2000) as well as from new observations (taken in
2006). All data consist of 3.6 cm continuum emission and were taken with the
Very Large Array in its highest angular resolution A configuration. We confirm
the large proper motions of the BN object, the radio source I (GMR I) and the
radio counterpart of the infrared source n (Orion-n), with values from 15 to 26
km/s. The three sources are receding from a point between them from where they
seem to have been ejected about 500 years ago, probably via the disintegration
of a multiple stellar system. We present simulations of very compact stellar
groups that provide a plausible dynamical scenario for the observations. The
radio source Orion-n appeared as a double in the first three epochs, but as
single in 2006. We discuss this morphological change. The fourth source in the
region, GMR D, shows no statistically significant proper motions. We also
present new, accurate relative astrometry between BN and radio source I that
restrict possible dynamical scenarios for the region. During the 2006
observations, the radio source GMR A, located about 1' to the NW of the BN/KL
region, exhibited an increase in its flux density of a factor of ~3.5 over a
timescale of one hour. This rapid variability at cm wavelengths is similar to
that previously found during a flare at millimeter wavelengths that took place
in 2003.Comment: Accepted for publication in Ap
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