207 research outputs found
Observations on the Formation of Massive Stars by Accretion
Observations of the H66a recombination line from the ionized gas in the
cluster of newly formed massive stars, G10.6-0.4, show that most of the
continuum emission derives from the dense gas in an ionized accretion flow that
forms an ionized disk or torus around a group of stars in the center of the
cluster. The inward motion observed in the accretion flow suggests that despite
the equivalent luminosity and ionizing radiation of several O stars, neither
radiation pressure nor thermal pressure has reversed the accretion flow. The
observations indicate why the radiation pressure of the stars and the thermal
pressure of the HII region are not effective in reversing the accretion flow.
The observed rate of the accretion flow, 0.001 solar masses/yr, is sufficient
to form massive stars within the time scale imposed by their short main
sequence lifetimes. A simple model of disk accretion relates quenched HII
regions, trapped hypercompact HII regions, and photo-evaporating disks in an
evolutionary sequence
A Multi-Epoch Study of the Radio Continuum Emission of Orion Source I: Constraints on the Disk Evolution of a Massive YSO and the Dynamical History of Orion BN/KL
We present new 7mm continuum observations of Orion BN/KL with the VLA. We
resolve the emission from the protostar radio Source I and BN at several
epochs. Source I is highly elongated NW-SE, and remarkably stable in flux
density, position angle, and overall morphology over nearly a decade. This
favors the extended emission component arising from an ionized disk rather than
a jet. We have measured the proper motions of Source I and BN for the first
time at 43 GHz. We confirm that both sources are moving at high speed (12 and
26 km/s, respectively) approximately in opposite directions, as previously
inferred from measurements at lower frequencies. We discuss dynamical scenarios
that can explain the large motions of both BN and Source I and the presence of
disks around both. Our new measurements support the hypothesis that a close
(~50 AU) dynamical interaction occurred around 500 years ago between Source I
and BN as proposed by Gomez et al. From the dynamics of encounter we argue that
Source I today is likely to be a binary with a total mass on the order of 20
Msun, and that it probably existed as a softer binary before the close
encounter. This enables preservation of the original accretion disk, though
truncated to its present radius of ~50 AU. N-body numerical simulations show
that the dynamical interaction between a binary of 20 Msun total mass (I) and a
single star of 10 Msun mass (BN) may lead to the ejection of both and binary
hardening. The gravitational energy released in the process would be large
enough to power the wide-angle flow traced by H2 and CO emission in the BN/KL
nebula. Assuming the proposed dynamical history is correct, the smaller mass
for Source I recently estimated from SiO maser dynamics (>7 Msun) by Matthews
et al., suggests that non-gravitational forces (e.g. magnetic) must play an
important role in the circumstellar gas dynamics.Comment: 17 pages, 7 figures, 4 tables, accepted by Ap
Massive star-formation toward G28.87+0.07 (IRAS 18411-0338) investigated by means of maser kinematics and radio to infrared, continuum observations
We used the Very Long Baseline Array (VLBA) and the European VLBI Network
(EVN) to perform phase-referenced VLBI observations of the three most powerful
maser transitions associated with the high-mass star-forming region
G28.87+0.07: the 22.2 GHz HO, 6.7 GHz CHOH, and 1.665 GHz OH lines.
We also performed VLA observations of the radio continuum emission at 1.3 and
3.6 cm and Subaru observations of the continuum emission at 24.5 m. Two
centimeter continuum sources are detected and one of them (named "HMC") is
compact and placed at the center of the observed distribution of HO,
CHOH and OH masers. The bipolar distribution of line-of-sight (l.o.s)
velocities and the pattern of the proper motions suggest that the water masers
are driven by a (proto)stellar jet interacting with the dense circumstellar
gas. The same jet could both excite the centimeter continuum source named "HMC"
(interpreted as free-free emission from shocked gas) and power the molecular
outflow observed at larger scales -- although one cannot exclude that the
free-free continuum is rather originating from a hypercompact \ion{H}{2}
region. At 24.5 m, we identify two objects separated along the north-south
direction, whose absolute positions agree with those of the two VLA continuum
sources. We establish that 90% of the luminosity of the region
(\times10^{5} L_\sun$) is coming from the radio source "HMC", which
confirms the existence of an embedded massive young stellar object (MYSO)
exciting the masers and possibly still undergoing heavy accretion from the
surrounding envelope.Comment: Accepted for publication in Ap
The Early Evolution of Massive Stars: Radio Recombination Line Spectra
Velocity shifts and differential broadening of radio recombination lines are
used to estimate the densities and velocities of the ionized gas in several
hypercompact and ultracompact HII regions. These small HII regions are thought
to be at their earliest evolutionary phase and associated with the youngest
massive stars. The observations suggest that these HII regions are
characterized by high densities, supersonic flows and steep density gradients,
consistent with accretion and outflows that would be associated with the
formation of massive stars.Comment: ApJ in pres
A Search for Small-Scale Clumpiness in Dense Cores of Molecular Clouds
We have analyzed HCN(1-0) and CS(2-1) line profiles obtained with high
signal-to-noise ratios toward distinct positions in three selected objects in
order to search for small-scale structure in molecular cloud cores associated
with regions of high-mass star formation. In some cases, ripples were detected
in the line profiles, which could be due to the presence of a large number of
unresolved small clumps in the telescope beam. The number of clumps for regions
with linear scales of ~0.2-0.5 pc is determined using an analytical model and
detailed calculations for a clumpy cloud model; this number varies in the
range: ~2 10^4-3 10^5, depending on the source. The clump densities range from
~3 10^5-10^6 cm^{-3}, and the sizes and volume filling factors of the clumps
are ~(1-3) 10^{-3} pc and ~0.03-0.12. The clumps are surrounded by inter-clump
gas with densities not lower than ~(2-7) 10^4 cm^{-3}. The internal thermal
energy of the gas in the model clumps is much higher than their gravitational
energy. Their mean lifetimes can depend on the inter-clump collisional rates,
and vary in the range ~10^4-10^5 yr. These structures are probably connected
with density fluctuations due to turbulence in high-mass star-forming regions.Comment: 23 pages including 4 figures and 4 table
Towards a liquid Argon TPC without evacuation: filling of a 6 m^3 vessel with argon gas from air to ppm impurities concentration through flushing
In this paper we present a successful experimental test of filling a volume
of 6 m with argon gas, starting from normal ambient air and reducing the
impurities content down to few parts per million (ppm) oxygen equivalent. This
level of contamination was directly monitored measuring the slow component of
the scintillation light of the Ar gas, which is sensitive to {\it all} sources
of impurities affecting directly the argon scintillation.Comment: 9 pages, 6 figures, to appear in Proc. 1st International Workshop
towards the Giant Liquid Argon Charge Imaging Experiment (GLA2010), Tsukuba,
March 201
(Sub)mm Interferometry Applications in Star Formation Research
This contribution gives an overview about various applications of (sub)mm
interferometry in star formation research. The topics covered are molecular
outflows, accretion disks, fragmentation and chemical properties of low- and
high-mass star-forming regions. A short outlook on the capabilities of ALMA is
given as well.Comment: 20 pages, 7 figures, in proceedings to "2nd European School on Jets
from Young Star: High Angular Resolution Observations". A high-resolution
version of the paper can be found at
http://www.mpia.de/homes/beuther/papers.htm
The RMS survey: ammonia mapping of the environment of massive young stellar objects
We present the results of ammonia observations towards 66 massive star forming regions identified by the Red Midcourse Space Experiment Source survey. We have used the Green Bank Telescope and the K-Band Focal Plane Array to map the ammonia (NH3) (1,1) and (2,2) inversion emission at a resolution of 30 arcsec in 8 arcmin regions towards the positions of embedded massive star formation. We have identified a total of 115 distinct clumps, approximately two-thirds of which are associated with an embedded massive young stellar object or compact H ii region, while the others are classified as quiescent. There is a strong spatial correlation between the peak NH3 emission and the presence of embedded objects. We derive the spatial distribution of the kinetic gas temperatures, line widths, and NH3 column densities from these maps, and by combining these data with dust emission maps we estimate clump masses, H2 column densities and ammonia abundances. The clumps have typical masses of ∼1000 M⊙ and radii ∼0.5 pc, line widths of ∼2 km s−1 and kinetic temperatures of ∼16–20 K. We find no significant difference between the sizes and masses of the star-forming and quiescent subsamples; however, the distribution maps reveal the presence of temperature and line width gradients peaking towards the centre for the star-forming clumps while the quiescent clumps show relatively uniform temperatures and line widths throughout. Virial analysis suggests that the vast majority of clumps are gravitationally bound and are likely to be in a state of global free fall in the absence of strong magnetic fields. The similarities between the properties of the two subsamples suggest that the quiescent clumps are also likely to form massive stars in the future, and therefore provide an excellent opportunity to study the initial conditions of massive pre-stellar and protostellar clumps
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