2,196 research outputs found
Kinematics of a hot massive accretion disk candidate
Characterizing rotation, infall and accretion disks around high-mass
protostars is an important topic in massive star formation research. With the
Australia Telescope Compact Array and the Very Large Array we studied a massive
disk candidate at high angular resolution in ammonia (NH3(4,4) & (5,5)) tracing
the warm disk but not the envelope. The observations resolved at ~0.4''
resolution (corresponding to ~1400AU) a velocity gradient indicative of
rotation perpendicular to the molecular outflow. Assuming a Keplerian accretion
disk, the estimated protostar-disk mass would be high, similar to the
protostellar mass. Furthermore, the position-velocity diagram exhibits
additional deviation from a Keplerian rotation profile which may be caused by
infalling gas and/or a self-gravitating disk. Moreover, a large fraction of the
rotating gas is at temperatures >100K, markedly different to typical low-mass
accretion disks. In addition, we resolve a central double-lobe cm continuum
structure perpendicular to the rotation. We identify this with an ionized,
optically thick jet.Comment: 5 pages, 3 figures, accepted for Astrophysical Journal Letters, a
high-resolution version of the draft can be found at
http://www.mpia.de/homes/beuther/papers.htm
Femtosecond Photoionization of Atoms under Noise
We investigate the effect of incoherent perturbations on atomic
photoionization due to a femtosecond mid-infrared laser pulse by solving the
time-dependent stochastic Schr\"odinger equation. For a weak laser pulse which
causes almost no ionization, an addition of a Gaussian white noise to the pulse
leads to a significantly enhanced ionization probability. Tuning the noise
level, a stochastic resonance-like curve is observed showing the existence of
an optimum noise for a given laser pulse. Besides studying the sensitivity of
the obtained enhancement curve on the pulse parameters, such as the pulse
duration and peak amplitude, we suggest that experimentally realizable
broadband chaotic light can also be used instead of the white noise to observe
similar features. The underlying enhancement mechanism is analyzed in the
frequency-domain by computing a frequency-resolved atomic gain profile, as well
as in the time-domain by controlling the relative delay between the action of
the laser pulse and noise.Comment: 10 pages, 10 figure
Ammonia from cold high-mass clumps discovered in the inner Galactic disk by the ATLASGAL survey
The APEX Telescope Large Area Survey: The Galaxy (ATLASGAL) is an unbiased
continuum survey of the inner Galactic disk at 870 \mu m. It covers +/- 60 deg
in Galactic longitude and aims to find all massive clumps at various stages of
high-mass star formation in the inner Galaxy, particularly the earliest
evolutionary phases. We aim to determine properties such as the gas kinetic
temperature and dynamics of new massive cold clumps found by ATLASGAL. Most
importantly, we derived their kinematical distances from the measured line
velocities. We observed the ammonia (J,K) = (1,1) to (3,3) inversion
transitions toward 862 clumps of a flux-limited sample of submm clumps detected
by ATLASGAL and extracted 13CO (1-0) spectra from the Galactic Ring Survey
(GRS). We determined distances for a subsample located at the tangential points
(71 sources) and for 277 clumps whose near/far distance ambiguity is resolved.
Most ATLASGAL clumps are cold with rotational temperatures from 10-30 K. They
have a wide range of NH3 linewidths, which by far exceeds the thermal
linewidth, as well as a broad distribution of high column densities with an NH3
abundance in the range of 5 to 30 * 10^{-8}. We found an enhancement of clumps
at Galactocentric radii of 4.5 and 6 kpc. The high detection rate (87%)
confirms ammonia as an excellent probe of the molecular content of the massive,
cold clumps revealed by ATLASGAL. A clear trend of increasing rotational
temperatures and linewidths with evolutionary stage is seen for source samples
ranging from 24 \mu m dark clumps to clumps with embedded HII regions. The
survey provides the largest ammonia sample of high-mass star forming clumps and
thus presents an important repository for the characterization of statistical
properties of the clumps and the selection of subsamples for detailed,
high-resolution follow-up studies
Evolution and excitation conditions of outflows in high-mass star-forming regions
Theoretical models suggest that massive stars form via disk-mediated
accretion, with bipolar outflows playing a fundamental role. A recent study
toward massive molecular outflows has revealed a decrease of the SiO line
intensity as the object evolves. The present study aims at characterizing the
variation of the molecular outflow properties with time, and at studying the
SiO excitation conditions in outflows associated with massive YSOs. We used the
IRAM30m telescope to map 14 massive star-forming regions in the SiO(2-1),
SiO(5-4) and HCO+(1-0) outflow lines, and in several dense gas and hot core
tracers. Hi-GAL data was used to improve the spectral energy distributions and
the L/M ratio, which is believed to be a good indicator of the evolutionary
stage of the YSO. We detect SiO and HCO+ outflow emission in all the sources,
and bipolar structures in six of them. The outflow parameters are similar to
those found toward other massive YSOs. We find an increase of the HCO+ outflow
energetics as the object evolve, and a decrease of the SiO abundance with time,
from 10^(-8) to 10^(-9). The SiO(5-4) to (2-1) line ratio is found to be low at
the ambient gas velocity, and increases as we move to high velocities,
indicating that the excitation conditions of the SiO change with the velocity
of the gas (with larger densities and/or temperatures for the high-velocity gas
component). The properties of the SiO and HCO+ outflow emission suggest a
scenario in which SiO is largely enhanced in the first evolutionary stages,
probably due to strong shocks produced by the protostellar jet. As the object
evolves, the power of the jet would decrease and so does the SiO abundance.
During this process, however, the material surrounding the protostar would have
been been swept up by the jet, and the outflow activity, traced by entrained
molecular material (HCO+), would increase with time.Comment: 31 pages, 10 figures and 5 tables (plus 2 figures and 3 tables in the
appendix). Accepted for publication in A&A. [Abstract modified to fit the
arXiv requirements.
Detection of FeO towards SgrB2
We have observed the J=5-4 ground state transition of FeO at a frequency of
153 GHz towards a selection of galactic sources.
Towards the galactic center source SgrB2, we see weak absorption at
approximately the velocity of other features towards this source (62 km
s LSR).
Towards other sources, the results were negative as they were also for
MgOH(3-2) and FeC(6-5). We tentatively conclude that the absorption seen toward
SgrB2 is due to FeO in the hot ( 500 K) relatively low density absorbing
gas known to be present in this line of sight.
This is the first (albeit tentative) detection of FeO or any iron--containing
molecule in the interstellar gas. Assuming the observed absorption to be due to
FeO, we estimate [FeO]/[SiO] to be of order or less than 0.002 and
[FeO]/[H] of order . This is compatible with our negative
results in other sources.
Our results suggest that the iron liberated from grains in the shocks
associated with SgrB2 remains atomic and is not processed into molecular form.Comment: 1 postscrit figure,10 page
Characterization of Infrared Dark Clouds -- NH Observations of an Absorption-contrast Selected IRDC Sample
Despite increasing research in massive star formation, little is known about
its earliest stages. Infrared Dark Clouds (IRDCs) are cold, dense and massive
enough to harbour the sites of future high-mass star formation. But up to now,
mainly small samples have been observed and analysed. To understand the
physical conditions during the early stages of high-mass star formation, it is
necessary to learn more about the physical conditions and stability in
relatively unevolved IRDCs. Thus, for characterising IRDCs studies of large
samples are needed. We investigate a complete sample of 218 northern hemisphere
high-contrast IRDCs using the ammonia (1,1)- and (2,2)-inversion transitions.
We detected ammonia (1,1)-inversion transition lines in 109 of our IRDC
candidates. Using the data we were able to study the physical conditions within
the star-forming regions statistically. We compared them with the conditions in
more evolved regions which have been observed in the same fashion as our sample
sources. Our results show that IRDCs have, on average, rotation temperatures of
15 K, are turbulent (with line width FWHMs around 2 km s), have ammonia
column densities on the order of cm and molecular hydrogen
column densities on the order of cm. Their virial masses are
between 100 and a few 1000 M. The comparison of bulk kinetic and
potential energies indicate that the sources are close to virial equilibrium.
IRDCs are on average cooler and less turbulent than a comparison sample of
high-mass protostellar objects, and have lower ammonia column densities. Virial
parameters indicate that the majority of IRDCs are currently stable, but are
expected to collapse in the future.Comment: 21 pages, 11 figures, 7 tables. Paper accepted for publication in
Astronomy & Astrophysic
A 1.3 cm Line Survey toward Orion KL
Orion KL has served as a benchmark for spectral line searches throughout the
(sub)millimeter regime. The main goal is to systematically study spectral
characteristics of Orion KL in the 1.3 cm band. We carried out a spectral line
survey (17.9 GHz to 26.2 GHz) with the Effelsberg-100 m telescope towards Orion
KL. We find 261 spectral lines, yielding an average line density of about 32
spectral features per GHz above 3. The identified lines include 164
radio recombination lines (RRLs) and 97 molecular lines. A total of 23
molecular transitions from species known to exist in Orion KL are detected for
the first time in the interstellar medium. Non-metastable 15NH3 transitions are
detected in Orion KL for the first time. Based on the velocity information of
detected lines and the ALMA images, the spatial origins of molecular emission
are constrained and discussed. A narrow feature is found in SO2
(), possibly suggesting the presence of a maser line. Column
densities and fractional abundances relative to H2 are estimated for 12
molecules with LTE methods. Rotational diagrams of non-metastable 14NH3
transitions with J=K+1 to J=K+4 yield different results; metastable 15NH3 is
found to have a higher excitation temperature than non-metastable 15NH3,
indicating that they may trace different regions. Elemental and isotopic
abundance ratios are estimated: 12C/13C=63+-17, 14N/15N=100+-51,
D/H=0.0083+-0.0045. The dispersion of the He/H ratios derived from
H/He pairs to H/He pairs is very small, which
is consistent with theoretical predictions that the departure coefficients bn
factors for hydrogen and helium are nearly identical. Based on a non-LTE code
neglecting excitation by the infrared radiation field and a likelihood
analysis, we find that the denser regions have lower kinetic temperature, which
favors an external heating of the Hot Core.Comment: 70 pages, 26 figures, 12 tables, accepted for publication in A&A.
Figs. 1, 2, 8, 9 have been downsize
The Nature of the Molecular Environment within 5 pc of the Galactic Center
We present a detailed study of molecular gas in the central 10pc of the
Galaxy through spectral line observations of four rotation inversion
transitions of NH3 made with the VLA. Updated line widths and NH3(1,1)
opacities are presented, and temperatures, column densities, and masses are
derived. We examine the impact of Sgr A East on molecular material at the
Galactic center and find that there is no evidence that the expansion of this
shell has moved a significant amount of the 50 km/s GMC. The western streamer,
however, shows strong indications that it is composed of material swept-up by
the expansion of Sgr A East. Using the mass and kinematics of the western
streamer, we calculate an energy of E=(2-9)x10^{51} ergs for the progenitor
explosion and conclude that Sgr A East was most likely produced by a single
supernova. The temperature structure of molecular gas in the central ~20pc is
also analyzed in detail. We find that molecular gas has a ``two-temperature''
structure similar to that measured by Huttemeister et al. (2003a) on larger
scales. The largest observed line ratios, however, cannot be understood in
terms of a two-temperature model, and most likely result from absorption of
NH3(3,3) emission by cool surface layers of clouds. By comparing the observed
NH3 (6,6)-to-(3,3) line ratios, we disentangle three distinct molecular
features within a projected distance of 2pc from Sgr A*. Gas associated with
the highest line ratios shows kinematic signatures of both rotation and
expansion. The southern streamer shows no significant velocity gradients and
does not appear to be directly associated with either the circumnuclear disk or
the nucleus. The paper concludes with a discussion of the line-of-sight
arrangement of the main features in the central 10pc.Comment: 51 pages, 16 figures, accepted for publication in ApJ. Due to size
limitations, some of the images have been cut from this version. A complete,
color PS or PDF version can be downloaded from
http://www.astro.columbia.edu/~herrnstein/NH3/paper
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