95 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
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
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
High-Frequency Polarization Variability from Active Galactic Nuclei
The linear polarization of non-thermal emission encodes information about the structure of the magnetic fields, either from the region where the emission is produced (i.e., the intrinsic polarization angle) and/or from the screens of magnetized plasma that may be located on its way towards Earth (i.e., the effect of Faraday rotation). In addition, the variability timescale of the polarized emission, or its Faraday rotation, can be used to estimate the size of the region where the emission (or the Faraday rotation) originates. The observation of polarized emission from active galactic nuclei (AGN) and, in particular, its time evolution, also provides information about the critical role that magnetic fields may play in the process of jet launching and propagation. In this paper, we review some recent results about polarization variability from the cores of AGN jets, including observations at high spatial resolutions and/or at high radio frequencies
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.
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
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
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
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