27 research outputs found
Molecular outflow launched beyond the disk edge
One of the long-standing problems of star formation is the excess of angular
momentum of the parent molecular cloud. In the classical picture, a fraction of
angular momentum of the circumstellar material is removed by the
magneto-centrifugally driven disk wind that is launched from a wide region
throughout the disk. In this work, we investigate the kinematics in the
envelope-disk transition zone of the Class I object BHB07-11, in the B59 core.
For this purpose, we used the Atacama Large Millimeter/submillimeter Array in
extended configuration to observe the thermal dust continuum emission
( 1.3 mm) and molecular lines (CO, CO and HCO),
which are suitable tracers of disk, envelope, and outflow dynamics at a spatial
resolution of AU. We report a bipolar outflow that was launched at
symmetric positions with respect to the disk (80~AU in radius), but was
concentrated at a distance of 90--130~AU from the disk center. The two outflow
lobes had a conical shape and the gas inside was accelerating. The large offset
of the launching position coincided with the landing site of the infall
material from the extended spiral structure (seen in dust) onto the disk. This
indicates that bipolar outflows are efficiently launched within a narrow region
outside the disk edge. We also identify a sharp transition in the gas
kinematics across the tip of the spiral structure, which pinpoints the location
of the so-called centrifugal barrier.Comment: 5 pages, 5 figures, Accepted for publication in A&A Letter
Magnetic field in a young circumbinary disk
We use polarization observations of a circumbinary disk to investigate how
the polarization properties change at distinct frequency bands. Our goal is to
discern the main mechanism responsible for the polarization through comparison
between our observations and model predictions. We used ALMA to perform full
polarization observations at 97.5 GHz, 233 GHz and 343.5 GHz. The target is the
Class I object BHB07-11, which is the youngest object in the Barnard 59
protocluster. Complementary VLA observations at 34.5 GHz revealed a binary
system within the disk. We detect an extended and structured polarization
pattern remarkably consistent among all three bands. The distribution of
polarized intensity resembles a horseshoe shape with polarization angles
following this morphology. From the spectral index between bands 3 and 7, we
derive a dust opacity index consistent with maximum grain sizes
larger than expected to produce self-scattering polarization in each band. The
polarization morphology do not match predictions from self-scattering. On the
other hand, marginal correspondence is seen between our maps and predictions
from radiation field assuming the brightest binary component as main radiation
source. Molecular line data from BHB07-11 indicates disk rotation. We produced
synthetic polarization maps from a rotating magnetized disk model assuming
combined poloidal and toroidal magnetic field components. The magnetic field
vectors (i. e., the polarization vectors rotated by 90\degr) are better
represented by a model with poloidal magnetic field strength about 3 times the
toroidal one. The similarity of our polarization patterns among the three bands
provides a strong evidence against self-scattering and radiation fields. On the
other hand, our data are reasonably well reproduced by a model of disk with
toroidal magnetic field components slightly smaller than poloidal ones.Comment: 8 pages, 8 figures, accepted for publication in Astronomy &
Astrophysic
Adaptable Radiative Transfer Innovations for Submillimeter Telescopes (ARTIST)
Submillimeter observations are a key for answering many of the big questions
in modern-day astrophysics, such as how stars and planets form, how galaxies
evolve, and how material cycles through stars and the interstellar medium. With
the upcoming large submillimeter facilities ALMA and Herschel a new window will
open to study these questions. ARTIST is a project funded in context of the
European ASTRONET program with the aim of developing a next generation model
suite for comprehensive multi-dimensional radiative transfer calculations of
the dust and line emission, as well as their polarization, to help interpret
observations with these groundbreaking facilities.Comment: 4 pages, 1 figure; to appear in "IAU Symposium 270: Computational
Star formation", Eds. J. Alves, B. Elmegreen, J. Girart, V. Trimbl
Maser Polarization
Through the observations and the analysis of maser polarization it is possible to measure the magnetic field in several astrophysical environments (e.g., star-forming regions, evolved stars). In particular from the linearly and circularly polarized emissions we can determine the orientation and the strength of the magnetic field, respectively. In these proceedings the implications, on observed data, of the new estimation of the Landé g-factors for the CH3OH maser are presented. Furthermore, some example of the most recent results achieved in observing the polarized maser emission from several maser species will also be reported
A magnetically collimated jet from an evolved star
Planetary nebulae often have asymmetric shapes, which could arise due to
collimated jets from evolved stars before evolution to the planetary nebula
phase. The source of jet collimation in these stars is unknown. Magnetic fields
are thought to collimate outflows that are observed in many other astrophysical
sources, such as active galactic nuclei and proto-stars, although hitherto
there are no direct observations of both the magnetic field direction and
strength in any collimated jet. Theoretical models have shown that magnetic
fields could also be the dominant source of collimation of jet in evolved
stars. Here we report measurements of the polarization of water vapour masers
that trace the precessing jet emanating from the asymptotic giant branch star
W43A at 2.6 kpc from the Sun, which is undergoing rapid evolution into a
planetary nebula. The masers occur in two clusters at opposing tips of the
jets, ~1,000 AU from the star. We find direct evidence that the magnetic field
is collimating the jet.Comment: Published in Nature 440 (March 2nd 2006). High-res figures can be
found at http://www.jb.man.ac.uk/~wouter/papers/w43a/w43a.htm
Molecular line study of the S-type AGB star W Aquilae. ALMA observations of CS, SiS, SiO and HCN
Context. With the outstanding spatial resolution and sensitivity of the
Atacama Large Millimeter/sub-millimeter Array (ALMA), molecular gas other than
the abundant CO can be observed and resolved in circumstellar envelopes (CSEs)
around evolved stars, such as the binary S-type Asymptotic Giant Branch (AGB)
star W Aquilae. Aims. We aim to constrain the chemical composition of the CSE
and determine the radial abundance distribution, the photospheric peak
abundance, and isotopic ratios of a selection of chemically important molecular
species in the innermost CSE of W Aql. The derived parameters are put into the
context of the chemical evolution of AGB stars and are compared with
theoretical models. Methods. We employ one-dimensional radiative transfer
modeling - with the accelerated lambda iteration (ALI) radiative transfer code
- of the radial abundance distribution of a total of five molecular species
(CS, SiS, 30SiS, 29SiO and H13CN) and determine the best fitting model
parameters based on high-resolution ALMA observations as well as archival
single-dish observations. The additional advantage of the spatially resolved
ALMA observations is that we can directly constrain the radial profile of the
observed line transitions from the observations. Results. We derive abundances
and e-folding radii for CS, SiS, 30SiS, 29SiO and H13CN and compare them to
previous studies, which are based only on unresolved single-dish spectra. Our
results are in line with previous results and are more accurate due to
resolution of the emission regions
An SiO Toroid and Wide-angle Outflow associated with the Massive Protostar W75N(B)-VLA2
We have carried out ALMA observations of the massive star-forming region
W75N(B), which contains the massive protostars VLA1, VLA2, and VLA3.
Particularly, VLA2 is an enigmatic protostar associated with a wind-driven
HO maser shell, which has evolved from an almost isotropic outflow to a
collimated one in just 20 years. The shell expansion seemed to be halted by an
obstacle located to the northeast of VLA2. Here we present our findings from
observing the 1.3 mm continuum and HCO and SiO emission lines. Within a
region of ( au) diameter, we have detected 40 compact
mm-continuum sources, three of them coinciding with VLA1, VLA2, and VLA3. While
the HCO emission is mainly distributed in a fragmented structure around the
three massive protostars, but without any of the main HCO clumps spatially
coinciding with them, the SiO is highly concentrated on VLA2, indicating the
presence of very strong shocks generated near this protostar. The SiO emission
is clearly resolved into an elongated structure (; au390 au) perpendicular to the major axis of the wind-driven maser
shell. The structure and kinematics of the SiO emission are consistent with a
toroid and a wide-angle outflow surrounding a central mass of
M, thus supporting previous theoretical predictions regarding the
evolution of the outflow. Additionally, we have identified the expected
location and estimated the gas density of the obstacle that is hindering the
expansion of the maser shell.Comment: To be published in The Astrophysical Journal Letters. Sixteen pages,
seven figures. Updated metadat
Mapping circumstellar magnetic fields of late-type evolved stars with the Goldreich-Kylafis effect: CARMA observations at mm of R Crt and R Leo
Mapping magnetic fields is the key to resolving what remains an unclear
physical picture of circumstellar magnetic fields in late-type evolved stars.
Observations of linearly polarized emission from thermal molecular line
transitions due to the Goldreich-Kylafis (G-K) effect provides valuable insight
into the magnetic field geometry in these sources that is complementary to
other key studies. In this paper, we present the detection of spectral-line
polarization from both the thermal CO line and the SiO
maser line toward two thermal-pulsating (TP-) AGB stars, R Crt and R Leo. The
observed fractional linear polarization in the CO emission is measured as
and for R Crt and R Leo respectively. A
circumstellar envelope (CSE) model profile and the associated parameters are
estimated and used as input to a more detailed modeling of the predicted linear
polarization expected from the G-K effect. The observed thermal line
polarization level is consistent with the predicted results from the G-K model
for R Crt; additional effects need to be considered for R Leo