659 research outputs found
A multiple system of high-mass YSOs surrounded by disks in NGC7538 IRS1
NGC7538 IRS1 is considered the best high-mass accretion disk candidate around
an O-type young star in the northern hemisphere. We investigated the 3D
kinematics and dynamics of circumstellar gas with very high linear resolution,
from tens to 1500 AU, with the ultimate goal of building a comprehensive
dynamical model for this YSO. We employed four different observing epochs of
EVN data at 6.7 GHz, spanning almost eight years, which enabled us to measure,
besides line-of-sight (l.o.s.) velocities and positions, also l.o.s.
accelerations and proper motions of methanol masers. In addition, we imaged
with the JVLA-B array highly-excited ammonia inversion lines, from (6,6) to
(13,13), which enabled us to probe the hottest molecular gas very close to the
exciting source(s). We found five 6.7 GHz maser clusters which are distributed
over a region extended N-S across ~1500 AU and are associated with three peaks
of the radio continuum. We proposed that these maser clusters identify three
individual high-mass YSOs, named IRS1a, IRS1b, and IRS1c. We modeled the maser
clusters in IRS1a and IRS1b in terms of edge-on disks in centrifugal
equilibrium. In the first case, masers may trace a quasi-Keplerian thin disk,
orbiting around a high-mass YSO, IRS1a, of up to 25 solar masses. This YSO
dominates the bolometric luminosity of the region. The second disk is both
massive (<16 Msun within ~500 AU) and thick, and the mass of the central YSO,
IRS1b, is constrained to be at most a few solar masses. In summary, we present
compelling evidence that NGC7538 IRS1 is not forming just one single high-mass
YSO, but consists of a multiple system of high-mass YSOs, which are surrounded
by accretion disks, and are probably driving individual outflows. This new
model naturally explains all the different orientations and disk/outflow
structures proposed for the region in previous models.Comment: 34 pages, 18 figures, accepted for publication in Astronomy &
Astrophysic
Outflow Structure and Velocity Field of Orion Source I: ALMA Imaging of SiO Isotopologue Maser and Thermal Emission
Using Science Verification data from the Atacama Large
Millimeter/Submillimeter Array (ALMA), we have identified and imaged five
rotational transitions (J=5-4 and J=6-5) of the three silicon monoxide
isotopologues 28SiO v=0, 1, 2 and 29SiO v=0 and 28Si18O v=0 in the frequency
range from 214 to 246 GHz towards the Orion BN/KL region. The emission of the
ground-state 28SiO, 29SiO and 28Si18O shows an extended bipolar shape in the
northeast-southwest direction at the position of Radio Source I, indicating
that these isotopologues trace an outflow (~18 km/s, P.A. ~50deg, ~5000 AU in
diameter) that is driven by this embedded high-mass young stellar object (YSO).
Whereas on small scales (10-1000 AU) the outflow from Source I has a
well-ordered spatial and velocity structure, as probed by Very Long Baseline
Interferometry (VLBI) imaging of SiO masers, the large scales (500-5000 AU)
probed by thermal SiO with ALMA reveal a complex structure and velocity field,
most likely related to the effects of the environment of the BN/KL region on
the outflow emanating from Source I.
The emission of the vibrationally-excited species peaks at the position of
Source I. This emission is compact and not resolved at an angular resolution of
~1.5" (~600 AU at a distance of 420 pc). 2-D Gaussian fitting to individual
velocity channels locates emission peaks within radii of 100 AU, i.e. they
trace the innermost part of the outflow. A narrow spectral profile and spatial
distribution of the v=1 J=5-4 line similar to the masing v=1 J=1-0 transition,
provide evidence for the most highly rotationally excited (frequency > 200 GHz)
SiO maser emission associated with Source I known to date. The maser emission
will enable studies of the Source I disk-outflow interface with future ALMA
longest baselines.Comment: Accepted to A&A, 11 pages, 13 figure
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&
ALMA and VLA observations of recombination lines and continuum toward the Becklin-Neugebauer object in Orion
Compared to their centimeter-wavelength counterparts, millimeter
recombination lines (RLs) are intrinsically brighter and are free of pressure
broadening. We report observations of RLs (H30alpha at 231.9 GHz, H53alpha at
42.9 GHz) and the millimeter and centimeter continuum toward the
Becklin-Neugebauer (BN) object in Orion, obtained from the Atacama Large
Millimeter/submillimeter Array (ALMA) Science Verification archive and the Very
Large Array (VLA). The RL emission appears to be arising from the
slowly-moving, dense (Ne=8.4x10^6 cm^-3) base of the ionized envelope around
BN. This ionized gas has a relatively low electron temperature (Te<4900 K) and
small (<<10 km s^-1) bulk motions. Comparing our continuum measurements with
previous (non)detections, it is possible that BN has large flux variations in
the millimeter. However, dedicated observations with a uniform setup are needed
to confirm this. From the H30alpha line, the central line-of-sight LSR velocity
of BN is 26.3 km s^-1.Comment: To appear in Astronomy and Astrophysics as a Letter to the editor.
Corrections to mm fluxes. Discussion about flux variability shortened.
Physical properties of ionized gas remain the same. Table 1 to main text
rather than online only. Language edite
Accretion and outflow structures within 1000 AU from high-mass protostars with ALMA longest baselines
Understanding the formation of massive stars is one of the unsolved problems
in modern astronomy. The main difficulty is that the intense radiation from the
high-luminosity stars and the thermal pressure from the resulting ionized gas
(both insignificant for low-mass stars) may be able to reverse the accretion
flow and prevent the star from accreting fresh material. Such feedback effects
can naturally be mitigated if accretion proceeds through discs, which is the
established mechanism to form sun-like stars. However, recent 3D MHD
simulations have shown that accretion on 1000 au scales is through filaments
rather than a large disc. This theoretical prediction has never been confirmed
via observations owing to the poor linear resolution of previous studies (>1000
au). Here we present the first observational evidence that mass assembly in
young high-mass stars forming in protoclusters is predominantly asymmetric and
disordered. In particular, we observed the innermost regions around three
deeply embedded high-mass protostars with very high spatial resolution (~100
au). We identified multiple massive (several solar masses), warm (50-150
Kelvin) filamentary streamers pointing onto the central sources, which we
interpret as multi-directional accretion channels. These structures inhibit the
formation of a large, steady disc. Nevertheless, the identification of fast
collimated outflows in the three observed systems indicates that (non-steady)
compact discs may be present (we measure upper limits on their radii of <80 for
one object and <350 astronomical units for the remaining two objects). Our
finding contrasts with the simplified classic paradigm of an ordered (and
stable) disc/jet system and provides an experimental confirmation of a
multi-directional and unsteady accretion model for massive star formation
supported by recent 3D (magneto)hydrodynamic simulations.Comment: Submitted to Nature on Dec 19 2017, transferred to Nature Astronomy
after review on February 8 2018, rejected after a recommendation for
acceptance by one reviewer, and a more critical report by a second reviewer.
To be submitted to ApJ. Comments from colleagues (even critical ones) are
welcom
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
A Keplerian disk around Orion Source I, a ~15 Msun YSO
We report ALMA long-baseline observations of Orion Source I (SrcI) with
resolution 0.03-0.06" (12-24 AU) at 1.3 and 3.2 mm. We detect both continuum
and spectral line emission from SrcI's disk. We also detect a central weakly
resolved source that we interpret as a hot spot in the inner disk, which may
indicate the presence of a binary system. The high angular resolution and
sensitivity of these observations allow us to measure the outer envelope of the
rotation curve of the HO line, which gives a mass
Msun. We detected several other lines that more closely
trace the disk, but were unable to identify their parent species. Using
centroid-of-channel methods on these other lines, we infer a similar mass.
These measurements solidify SrcI as a genuine high-mass protostar system and
support the theory that SrcI and the Becklin Neugebauer Object were ejected
from the dynamical decay of a multiple star system 500 years ago, an
event that also launched the explosive molecular outflow in Orion.Comment: Accepted to ApJ. Data at https://zenodo.org/record/1213350, source
repository at https://github.com/keflavich/Orion_ALMA_2016.1.00165.
Orion Source I's disk is salty
We report the detection of NaCl, KCl, and their Cl and K
isotopologues toward the disk around Orion SrcI. About 60 transitions of these
molecules were identified. This is the first detection of these molecules in
the interstellar medium not associated with the ejecta of evolved stars. It is
also the first ever detection of the vibrationally excited states of these
lines in the ISM above v = 1, with firm detections up to v = 6. The salt
emission traces the region just above the continuum disk, possibly forming the
base of the outflow. The emission from the vibrationally excited transitions is
inconsistent with a single temperature, implying the lines are not in LTE. We
examine several possible explanations of the observed high excitation lines,
concluding that the vibrational states are most likely to be radiatively
excited via rovibrational transitions in the 25-35 {\mu}m (NaCl) and 35-45
{\mu}m (KCl) range. We suggest that the molecules are produced by destruction
of dust particles. Because these molecules are so rare, they are potentially
unique tools for identifying high-mass protostellar disks and measuring the
radiation environment around accreting young stars.Comment: Accepted to ApJ. Analysis code at
https://github.com/keflavich/Orion_ALMA_2016.1.00165.S, paper source at
https://github.com/keflavich/SaltyDisk, and data at
https://zenodo.org/record/121335
First detection of CS masers around a high-mass young stellar object, W51 e2e
We report the discovery of maser emission in the two lowest rotational
transitions of CS toward the high-mass protostar W51 e2e with ALMA and the
JVLA. The masers from CS J=1-0 and J=2-1 are neither spatially nor spectrally
coincident (they are separated by ~150 AU and ~30 km/s), but both appear to
come from the base of the blueshifted outflow from this source. These CS masers
join a growing list of rarely-detected maser transitions that may trace a
unique phase in the formation of high-mass protostars.Comment: Accepted to A
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