59 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
CH3OH and H2O maser associations at very high angular resolution
6.7 and 12.2 GHz CH_{3}OH (methanol) and 22.2 GHz H_{2}O masers are believed to be good tracers of the earliest phases of high-mass star formation. Interferometric and VLBI (Very Long Baseline Interferometry) observations have shown that water masers are predominantly associated with the innermost portions of the jets/outflows emerging from (proto-)stellar objects. On the other hand, the astrophysical environment traced by the 6.7 GHz (and the associated 12.2 GHz) CH_{3}OH masers is still to be more precisely determined. So far, most high-resolution studies have focused either on CH_{3}OH or on H_{2}O masers and little is known on their connection, wehereas it would be important to study both types of maser emission in the same object. The goal of our long-term project is to perform interferometric and VLBI observations of H_{2}O and CH_3OH masers towards a selected sample of high-mass YSOs where both maser types have been detected. This work presents preliminary results obtained for a few objects of our sample, and discusses possible implications
Hot Ammonia around O-type Young Stars. I. JVLA imaging of Ammonia (6,6) to (14,14) in NGC7538 IRS1
To constrain theoretical models of high-mass star formation, observational
signatures of mass accretion in O-type forming stars are desirable. Using the
JVLA, we have mapped the hot and dense molecular gas in the hot core NGC7538
IRS1, with 0.2'' angular resolution, in seven metastable (J=K) inversion
transitions of ammonia: (J,K)=(6,6), (7,7), (9,9), (10,10), (12,12), (13,13),
and (14,14). These lines arise from energy levels between ~400 K and ~1950 K
above the ground state, and are observed in absorption against the HC-HII
region associated with NGC7538 IRS1. With a 500 AU linear resolution, we
resolve the elongated North-South ammonia structure into two compact
components: the main core and a southernmost component. Previous observations
of the radio continuum with a 0.08'' (or 200 AU) resolution, resolved in turn
the compact core in two (northern and southern) components. These features
correspond to a triple system of high-mass YSOs IRS1a, IRS1b, and IRS1c
identified with VLBI measurements of methanol masers. The velocity maps of the
compact core show a clear velocity gradient in all lines, which is indicative
of rotation in a (circumbinary) envelope, containing ~40 solar masses
(dynamical mass). In addition, we derived physical conditions of the molecular
gas: rotational temperatures ~280 K, ammonia column densities ~1.4-2.5 x 10^19
cm-2, H_2 volume densities ~3.5-6.2 x 10^10 cm-3, and a total gas mass in the
range of 19-34 solar masses, for the main core. We conclude that NGC7538 IRS1
is the densest hot molecular core known, containing a rotating envelope which
hosts a multiple system of high-mass YSOs, possibly surrounded by accretion
disks. Future JVLA observations in the A-configuration are needed to resolve
the binary system in the core and may allow to study the gas kinematics in the
accretion disks associated with individual binary members.Comment: 16 pages, 8 figures, accepted for publication in Astronomy &
Astrophysic
A Double-Jet System in the G31.41+0.31 Hot Molecular Core
This work presents a detailed study of the gas kinematics towards the "Hot
Molecular Core" (HMC) G31.41+0.31 via multi-epoch VLBI observations of the H2O
22 GHz and CH3OH 6.7 GHz masers, and single-epoch VLBI of the OH 1.6 GHz
masers. Water masers present a symmetric spatial distribution with respect to
the HMC center, where two nearby (0.2" apart), compact, VLA sources (labeled
"A" and "B") are previously detected. The spatial distribution of a first group
of water masers, named "J1", is well fit with an elliptical profile, and the
maser proper motions mainly diverge from the ellipse center, with average speed
of 36 km s-1. These findings strongly suggest that the "J1" water maser group
traces the heads of a young (dynamical time of 1.3 10^3 yr), powerful (momentum
rate of ~0.2 M_sun yr-1 km s-1), collimated (semi-opening angle ~10 deg) jet
emerging from a MYSO located close (within 0.15") to the VLA source "B". Most
of the water features not belonging to "J1" present an elongated (about 2" in
size), NE--SW oriented (PA = 70 deg), S-shape distribution, which we denote
with the label "J2". The elongated distribution of the "J2" group and the
direction of motion, approximately parallel to the direction of elongation, of
most "J2" water masers suggests the presence of another collimated outflow,
emitted from a MYSO near the VLA source "A". The orientation of the "J2" jet
agrees well with that (PA = 68 deg) of the well-defined V_LSR gradient across
the HMC revealed by previous interferometric, thermal line observations.
Furthermore, the "J2" jet is powerful enough to sustain the large momentum
rate, 0.3 M_sun yr-1 km s-1, estimated assuming that the V_LSR gradient
represents a collimated outflow. These two facts lead us to favour the
interpretation of the V_LSR gradient across the G31.41+0.31 HMC in terms of a
compact and collimated outflow.Comment: 23 pages, 7 figures, accepted for publication in Astronomy &
Astrophysic
3D Gas Dynamics from Methanol Masers observed with the EVN reveals Rotating Disks around O-type Young Stars
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A 10- YSO with a Keplerian disk and a nonthermal radio jet
We previously observed the star-forming region G16.590.05 through
interferometric observations of both thermal and maser lines, and identified a
high-mass young stellar object (YSO) which is surrounded by an accretion disk
and drives a nonthermal radio jet. We performed high-angular-resolution (beam
FWHM ~0.15") 1.2-mm continuum and line observations towards G16.590.05 with
the Atacama Large Millimeter Array (ALMA). The main dust clump, with size
~10 au, is resolved into four relatively compact (diameter ~2000 au)
millimeter (mm) sources. The source harboring the high-mass YSO is the most
prominent in molecular emission. By fitting the emission profiles of several
unblended and optically thin transitions of CHOCH and CHOH, we
derived gas temperatures inside the mm-sources in the range 42--131 K, and
calculated masses of 1--5 . A well-defined Local Standard of Rest
velocity (Vlsr) gradient is detected in most of the high-density molecular
tracers at the position of the high-mass YSO, pinpointed by compact 22-GHz
free-free emission. This gradient is oriented along a direction forming a large
(~70 degree) angle with the radio jet, traced by elongated 13-GHz continuum
emission. The butterfly-like shapes of the P-V plots and the linear pattern of
the emission peaks of the molecular lines at high velocity confirm that this
Vlsr gradient is due to rotation of the gas in the disk surrounding the
high-mass YSO. The disk radius is ~500 au, and the Vlsr distribution along the
major axis of the disk is well reproduced by a Keplerian profile around a
central mass of 102 . The position of the YSO is offset by >~
0.1" from the axis of the radio jet and the dust emission peak. To explain this
displacement we argue that the high-mass YSO could have moved from the center
of the parental mm source owing to dynamical interaction with one or more
companions.Comment: 16 pages, 12 figures, accepted by Astronomy & Astrophysics, Main
Journa
VLBI observations of H2O masers towards high-mass Young Stellar Objects
We have conducted multi-epoch VLBI observations of the 22.2 GHz water masers towards three massive star forming regions (Sh 2-255 IR, WB89-234, AFGL 5142). For the maser fetures persistent in time, accurate values of the proper motions are derived. The comparison of the VLBI 22.2 GHz maps with the highest-resolution images of the sources Sh 2-255 IR and WB89-234 in several thermal tracers, may suggest that the water masers are most likely tracing the inner portion of the molecular outflows detected at much larger-scales. This interpretation is also confirmed by the results obtained by fitting to the data two different kinematical models, a Keplerian disk and a conical outflow. Towards AFGL 5142 the water maser features are distributed in two different structures, whose axes are nearly perpendicular to each other. A possible interpretation is that the maser emission is tracing the disk/jet system nearby the forming YSO
3D velocity fields from methanol and water masers in an intermediate-mass protostar
We report multi-epoch VLBI observations of molecular masers towards the high-mass star forming region AFGL 5142, leading to the determination of the 3D velocity field of circumstellar molecular gas at radii 3OH maser emission enabled, for the first time, a direct measurement of infall of a molecular envelope on to an intermediate-mass protostar (radius of 300 AU, velocity of 5 km s-1, and infall rate of 6 × 10-4 n 8 M ⊙ yr-1, where n 8 is the ambient volume density in units of 108 cm-3). In addition, our measurements of H2O maser (and radio continuum) emission revealed 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
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