470 research outputs found
Tracing the base of protostellar wind(s) towards the high-mass star forming region AFGL 5142: VLA continuum and VLBA water maser observations
We have conducted phase-reference multi-epoch observations of the 22.2 GHz
water masers using the VLBA and multi-frequency study of the continuum emission
using the VLA towards the high-mass SFR AFGL 5142. The water maser emission
comes from two elongated structures (indicated as Group I and Group II), with
the measured proper motions aligned along the structures' elongation axes. Each
group consists of two (blue- and red-shifted) clusters of features separated by
a few hundreds and thousands of AU respectively for Group I and Group II. The
maser features of Group II have both positions and velocities aligned along a
direction close to the axis of the outflow traced by HCO+ and SiO emission on
angular scales of tens of arcsec. We predict that the maser emission arises
from dense, shocked molecular clumps displaced along the axis of the molecular
outflow. The two maser clusters of Group I are oriented on the sky along a
direction forming a large angle (> 60 degrees) with the axis of the jet/outflow
traced by Group II maser features. We have detected a compact (8.4 and 22 GHz)
continuum source that falls close to the centroid of Group I masers, indicating
that the source ionizing the gas is also responsible for the excitation of the
water masers. The kinematic analysis indicates that the Group I masers trace
outflowing rather than rotating gas, discarding the Keplerian disk scenario
proposed in a previous paper for Group I. Since the axis joining the two maser
clusters of Group II does not cross the position of the continuum source, Group
II masers might be excited by an (undetected) massive YSO, distinct from the
one (pinpointed by the VLA continuum emission) responsible for the excitation
of the Group I masers.Comment: 12 pages, 3 figures, accepted for publication in A&
Unveiling the gas kinematics at 10 AU scales in high-mass star-forming regions (Milliarcsecond structure of 6.7 GHz methanol masers)
This work presents a study of the milliarcsecond structure of the 6.7 GHz
methanol masers at high-velocity resolution (0.09 km s^(-1)) in four high-mass
star-forming regions: G16.59-0.05, G23.01-0.41, IRAS20126+4104, and AFGL5142.
We studied these sources by means of multi-epoch VLBI observations in the 22
GHz water and 6.7 GHz methanol masers, to determine the 3-D gas kinematics
within a few thousand AU from the (proto)star. The present work exploits the
6.7 GHz maser data collected so far to investigate the milliarcsecond structure
of this maser emission at high-velocity resolution. Most of the detected 6.7
GHz maser features present an ordered (linear, or arc-like) distribution of
maser spots on the plane of the sky, together with a regular variation in the
spot LSR velocity (V_LSR) with position. Typical values for the amplitude of
the V_LSR gradients (defined in terms of the derivative of the spot V_LSR with
position) are found to be 0.1-0.2 km s^(-1) mas^(-1). In each of the four
target sources, the orientation and the amplitude of most of the feature V_LSR
gradients remain remarkably stable in time, on timescales of (at least) several
years. We also find that the data are consistent with having the V_LSR
gradients and proper motion vectors in the same direction on the sky,
considered the measurement uncertainties. The time persistency, the ordered
angular and spatial distribution, and the orientation generally similar to the
proper motions, altogether suggest a kinematical interpretation for the origin
of the 6.7 GHz maser V_LSR gradients. This work shows that the organized
motions (outflow, infall, and rotation) revealed by the (22 GHz water and 6.7
GHz methanol) masers on large scales (~100-1000 AU) also persist to very small
(~10 AU) scales.Comment: 14 pages, 7 figures, accepted for publication in Astronomy and
Astrophysics Journa
Momentum-driven outflow emission from an O-type YSO: Comparing the radio jet with the molecular outflow
Aims: We want to study the physical properties of the ionized jet emission in
the vicinity of an O-type young stellar object (YSO), and estimate how
efficient is the transfer of energy and momentum from small- to large-scale
outflows. Methods: We conducted Karl G. Jansky Very Large Array (VLA)
observations, at both 22 and 45 GHz, of the compact and faint radio continuum
emission in the high-mass star-forming region G023.01-00.41, with an angular
resolution between 0.3" and 0.1", and a thermal rms of the order of 10
uJy/beam. Results: We discovered a collimated thermal (bremsstrahlung) jet
emission, with a radio luminosity (L_rad) of 24 mJy kpc^2 at 45 GHz, in the
inner 1000 AU from an O-type YSO. The radio thermal jet has an opening angle of
44 degrees and brings a momentum rate of 8 10^-3 M_sun yr^-1 km/s. By combining
the new data with previous observations of the molecular outflow and water
maser shocks, we can trace the outflow emission from its driving source through
the molecular clump, across more than two order of magnitude in length (500
AU-0.2 pc). We find that the momentum-transfer efficiency, between the inner
jet emission and the extended outflow of entrained ambient gas, is near unity.
This result suggests that the large-scale flow is swept-up by the mechanical
force of the radio jet emission, which originates in the inner 1000 AU from the
high-mass YSO.Comment: 5 pages, 2 figures, 2 tables, accepted by Astronomy & Astrophysic
Unveiling Sources of Heating in the Vicinity of the Orion BN/KL Hot Core as Traced by Highly Excited Inversion Transitions of Ammonia
Using the Expanded Very Large Array, we have mapped the vicinity of the Orion
BN/KL Hot Core with sub-arcsecond angular resolution in seven metastable
inversion transitions of ammonia: (J,K)=(6,6) to (12,12). This emission comes
from levels up to 1500 K above the ground state, enabling identification of
source(s) responsible for heating the region. We used this multi-transition
dataset to produce images of the rotational/kinetic temperature and the column
density of ammonia for ortho and para species separately and on a
position-by-position basis. We find rotational temperature and column density
in the range 160-490 K and (1-4)x10^17 cm^-2, respectively. Our
spatially-resolved images show that the highest (column) density and hottest
gas is found in a northeast-southwest elongated ridge to the southeast of
Source I. We have also measured the ortho-para ratio of ammonia, estimated to
vary in the range 0.9-1.6. Enhancement of ortho with respect to para and the
offset of hot ammonia emission peaks from known (proto)stellar sources provide
evidence that the ammonia molecules have been released from dust grains into
the gas-phase through the passage of shocks and not by stellar radiation. We
propose that the combined effect of Source I's proper motion and its
low-velocity outflow impinging on a pre-existing dense medium is responsible
for the excitation of ammonia and the Orion Hot Core. Finally, we found for the
first time evidence of a slow (5 km/s) and compact (1000 AU) outflow towards
IRc7.Comment: To appear in Astrophysical Journal Letters Special Issue on the EVLA.
8 pages, 4 figure
Infall and outflow within 400 AU from a high-mass protostar. 3-D velocity fields from methanol and water masers in AFLG 5142
Observational signatures of infalling envelopes and outflowing material in
early stages of protostellar evolution, and at small radii from the protostar,
are essential to progress in the understanding of the mass-accretion process in
star formation. In this letter, we report a detailed study of the accretion and
outflow structure around a protostar in the well-known high-mass star-forming
region AFGL 5142. We focus on the mm source MM-1, which exhibits hot-core
chemistry, radio continuum emission, and strong water and methanol masers.
Remarkably, our Very Long Baseline Interferometry (VLBI) observations of
molecular masers over six years provided us with the 3-D velocity field of
circumstellar molecular gas with a resolution of 0.001-0.005 arcseconds and at
radii <0.23 arcseconds (or 400 AU) from the protostar. In particular, our
measurements of methanol maser emission enabled, for the first time, a direct
measurement of infall of a molecular envelope (radius of 300 AU and velocity of
5 km/s) onto an intermediate- to high-mass protostar. We estimate an infall
rate of 0.0006 n_8 Msun/year, where n_8 is the ambient volume density in units
of 10^8 cm-3 (required for maser excitation). In addition, our measurements of
water maser (and radio continuum) emission identify a collimated bipolar
molecular outflow (and ionized jet) from MM-1. The evidence of simultaneous
accretion and outflow at small spatial scales, makes AFGL 5142 an extremely
compelling target for high-angular resolution studies of high-mass star
formation.Comment: 6 pages, 4 figures, 1 table, accepted for publication in A&A Letter
A Feature Movie of SiO Emission 20-100 AU from the Massive Young Stellar Object Orion Source I
We present multi-epoch VLBA imaging of the 28SiO v=1 & v=2, J=1-0 maser
emission toward the massive YSO Orion Source I. Both SiO transitions were
observed simultaneously with an angular resolution of ~0.5 mas (~0.2 AU for
d=414 pc). Here we explore the global properties and kinematics of the emission
through two 19-epoch animated movies spanning 21 months (2001 March 19 to 2002
December 10). These movies provide the most detailed view to date of the
dynamics and temporal evolution of molecular material within ~20-100 AU of a
massive (~>8M_sun) YSO. The bulk of the SiO masers surrounding Source I lie in
an X-shaped locus; emission in the South/East arms is predominantly blueshifted
and in the North and West is predominantly redshifted. In addition, bridges of
intermediate-velocity emission connect the red and blue sides of the emission
distribution. We have measured proper motions of >1000 maser features and find
a combination of radially outward migrations along the four arms and motions
tangent to the bridges. We interpret the SiO masers as arising from a
wide-angle bipolar wind emanating from a rotating, edge-on disk. The detection
of maser features along extended, curved filaments suggests that magnetic
fields may play a role in launching and/or shaping the wind. Our observations
appear to support a picture in which stars with M ~>8 M_sun form via
disk-mediated accretion. However, we cannot rule out that the Source I disk may
have been formed or altered following a close encounter. (Abridged).Comment: Accepted to ApJ (January 2010); a full resolution version along with
two accompanying GIF movies may be found at
http://www.cfa.harvard.edu/kalypso
A study on subarcsecond scales of the ammonia and continuum emission toward the G16.59-0.05 high-mass star-forming region
We wish to investigate the structure, velocity field, and stellar content of
the G16.59-0.05 high-mass star-forming region, where previous studies have
established the presence of two almost perpendicular (NE-SW and SE-NW), massive
outflows, and a rotating disk traced by methanol maser emission. We performed
Very Large Array observations of the radio continuum and ammonia line emission,
complemented by COMICS/Subaru and Hi-GAL/Herschel images in the mid- and
far-infrared (IR). Our centimeter continuum maps reveal a collimated radio jet
that is oriented E-W and centered on the methanol maser disk, placed at the SE
border of a compact molecular core. The spectral index of the jet is negative,
indicating non-thermal emission over most of the jet, except the peak close to
the maser disk, where thermal free-free emission is observed. We find that the
ammonia emission presents a bipolar structure consistent (on a smaller scale)
in direction and velocity with that of the NE-SW bipolar outflow detected in
previous CO observations. After analyzing our previous N2H+(1-0) observations
again, we conclude that two scenarios are possible. In one case both the radio
jet and the ammonia emission would trace the root of the large-scale CO bipolar
outflow. The different orientation of the jet and the ammonia flow could be
explained by precession and/or a non-isotropic density distribution around the
star. In the other case, the N2H+(1-0) and ammonia bipolarity is interpreted as
two overlapping clumps moving with different velocities along the line of
sight. The ammonia gas also seems to undergo rotation consistent with the maser
disk. Our IR images complemented by archival data allow us to derive a
bolometric luminosity of about 10^4 L_sun and to conclude that most of the
luminosity is due to the young stellar object associated with the maser disk.Comment: 11 pages, 12 figures, published in Astronomy and Astrophysic
A Documentary of High-Mass Star Formation: Probing the Dynamical Evolution of Orion Source I on 10-100 AU Scales using SiO Masers
A comprehensive picture of high-mass star formation has remained elusive, in
part because examples of high-mass YSOs tend to be relatively distant, deeply
embedded, and confused with other emission sources. These factors have impeded
dynamical investigations within tens of AU of high-mass YSOs--scales that are
critical for probing the interfaces where outflows from accretion disks are
launched and collimated. Using observations of SiO masers obtained with the VLA
and the VLBA, the KaLYPSO project is overcoming these limitations by mapping
the structure and dynamical/temporal evolution of the material 10-1000 AU from
the nearest high-mass YSO: Radio Source I in the Orion BN/KL region. Our data
include ~40 epochs of VLBA observations over a several-year period, allowing us
to track the proper motions of individual SiO maser spots and to monitor
changes in the physical conditions of the emitting material with time.
Ultimately these data will provide 3-D maps of the outflow structure over
approximately 30% of the outflow crossing time. Here we summarize recent
results from the KaLYPSO project, including evidence that high-mass star
formation is occurring via disk-mediated accretion.Comment: 5 pages; to appear in the proceedings of IAU Symposium 242,
Astrophysical Masers and their Environments, ed. J. Chapman & W. Baa
- …