398 research outputs found
Absolute flux measurements for swift atoms
While a torsion balance in vacuum can easily measure the momentum transfer from a gas beam impinging on a surface attached to the balance, this measurement depends on the accommodation coefficients of the atoms with the surface and the distribution of the recoil. A torsion balance is described for making absolute flux measurements independent of recoil effects. The torsion balance is a conventional taut suspension wire design and the Young modulus of the wire determines the relationship between the displacement and the applied torque. A compensating magnetic field is applied to maintain zero displacement and provide critical damping. The unique feature is to couple the impinging gas beam to the torsion balance via a Wood's horn, i.e., a thin wall tube with a gradual 90 deg bend. Just as light is trapped in a Wood's horn by specular reflection from the curved surfaces, the gas beam diffuses through the tube. Instead of trapping the beam, the end of the tube is open so that the atoms exit the tube at 90 deg to their original direction. Therefore, all of the forward momentum of the gas beam is transferred to the torsion balance independent of the angle of reflection from the surfaces inside the tube
Oscillating Starless Cores: The Nonlinear Regime
In a previous paper, we modeled the oscillations of a thermally-supported
(Bonnor-Ebert) sphere as non-radial, linear perturbations following a standard
analysis developed for stellar pulsations. The predicted column density
variations and molecular spectral line profiles are similar to those observed
in the Bok globule B68 suggesting that the motions in some starless cores may
be oscillating perturbations on a thermally supported equilibrium structure.
However, the linear analysis is unable to address several questions, among them
the stability, and lifetime of the perturbations. In this paper we simulate the
oscillations using a three-dimensional numerical hydrodynamic code. We find
that the oscillations are damped predominantly by non-linear mode-coupling, and
the damping time scale is typically many oscillation periods, corresponding to
a few million years, and persisting over the inferred lifetime of gobules.Comment: 7 pages, 7 figures, accepted by Ap
Rotation of the pre-stellar core L1689B
The search for the onset of star formation in pre-stellar cores has focussed
on the identification of an infall signature in the molecular line profiles of
tracer species. The classic infall signature is a double peaked line profile
with an asymmetry in the strength of the peaks such that the blue peak is
stronger. L1689B is a pre-stellar core and infall candidate but new JCMT HCO+
line profile data, presented here, confirms that both blue and red asymmetric
line profiles are present in this source. Moreover, a dividing line can be
drawn between the locations where each type of profile is found. It is argued
that it is unlikely that the line profiles can be interpreted with simple
models of infall or outflow and that rotation of the inner regions is the most
likely explanation. A rotational model is developed in detail with a new 3D
molecular line transport code and it is found that the best type of model is
one in which the rotational velocity profile is in between solid body and
Keplerian. It is firstly shown that red and blue asymmetric line profiles can
be generated with a rotation model entirely in the absence of any infall
motion. The model is then quantitively compared with the JCMT data and an
iteration over a range of parameters is performed to minmize the difference
between the data and model. The results indicate that rotation can dominate the
line profile shape even before the onset of infall.Comment: Accepted by MNRAS, 7 pages, 4 figure
Hot high-mass accretion disk candidates
To better understand the physical properties of accretion disks in high-mass
star formation, we present a study of a 12 high-mass accretion disk candidates
observed at high spatial resolution with the Australia Telescope Compact Array
(ATCA) in the NH3 (4,4) and (5,5) lines. Almost all sources were detected in
NH3, directly associated with CH3OH Class II maser emission. From the remaining
eleven sources, six show clear signatures of rotation and/or infall motions.
These signatures vary from velocity gradients perpendicular to the outflows, to
infall signatures in absorption against ultracompact HII regions, to more
spherical infall signatures in emission. Although our spatial resolution is
~1000AU, we do not find clear Keplerian signatures in any of the sources.
Furthermore, we also do not find flattened structures. In contrast to this, in
several of the sources with rotational signatures, the spatial structure is
approximately spherical with sizes exceeding 10^4 AU, showing considerable
clumpy sub-structure at even smaller scales. This implies that on average
typical Keplerian accretion disks -- if they exist as expected -- should be
confined to regions usually smaller than 1000AU. It is likely that these disks
are fed by the larger-scale rotating envelope structure we observe here.
Furthermore, we do detect 1.25cm continuum emission in most fields of view.Comment: 21 pages, 32 figures, accepted for ApJS. A high-resolution version
can be found at http://www.mpia.de/homes/beuther/papers.htm
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
12mm line survey of the dense molecular gas towards the W28 field TeV gamma-ray sources
We present 12mm Mopra observations of dense molecular gas towards the W28
supernova remnant (SNR) field. The focus is on the dense molecular gas towards
the TeV gamma-ray sources detected by the H.E.S.S. telescopes, which likely
trace the cosmic-rays from W28 and possibly other sources in the region. Using
the NH3 inversion transitions we reveal several dense cores inside the
molecular clouds, the majority of which coincide with high-mass star formation
and HII regions, including the energetic ultra-compact HII region G5.89-0.39. A
key exception to this is the cloud north east of W28, which is well-known to be
disrupted as evidenced by clusters of 1720MHz OH masers and broad CO line
emission. Here we detect broad NH3, up to the (9,9) transition, with linewidths
up to 16 km/s. This broad NH3 emission spatially matches well with the TeV
source HESS J1801-233 and CO emission, and its velocity dispersion distribution
suggests external disruption from the W28 SNR direction. Other lines are
detected, such as HC3N and HC5N, H2O masers, and many radio recombination
lines, all of which are primarily found towards the southern high-mass star
formation regions. These observations provide a new view onto the internal
structures and dynamics of the dense molecular gas towards the W28 SNR field,
and in tandem with future higher resolution TeV gamma-ray observations will
offer the chance to probe the transport of cosmic-rays into molecular clouds.Comment: 18 pages, 10 figures, 5 tables, accepted for publication in MNRAS.
Online appendices containing additional molecular line: fit parameters, maps,
PV plots & spectra, will be available through MNRA
Molecular outflows towards O-type young stellar objects
We have searched for massive molecular outflows in a sample of high-mass star
forming regions, and we have characterised both the outflow properties and
those of their associated molecular clumps. With a sample composed largely of
more luminous objects than previous ones, this work complements analogous
surveys performed by other authors by adding the missing highest luminosity
sources. The sample under study has been selected so as to favour the earliest
evolutionary phases of star formation, and is composed of very luminous objects
(L_bol > 2x10^4 L_sun and up to ~10^6 L_sun), possibly containing O-type stars.
Each source has been mapped in 13CO(2-1) and C18O(2-1) with the IRAM-30m
telescope on Pico Veleta (Spain). The whole sample shows high-velocity wings in
the 13CO(2-1) spectra, indicative of outflowing motions. In addition, we have
obtained outflow maps in 9 of our 11 sources, which display well-defined blue
and/or red lobes. For these sources, the outflow parameters have been derived
from the line wing 13CO(2-1) emission. An estimate of the clump masses from the
C18O(2-1) emission is also provided and found to be comparable to the virial
masses. From a comparison between our results and those found by other authors
at lower masses, it is clear that the outflow mechanical force increases with
the bolometric luminosity of the clump and with the ionising photon rate of the
associated HII regions, indicating that high-mass stars drive more powerful
outflows. A tight correlation between outflow mass and clump mass is also
found. Molecular outflows are found to be as common in massive star forming
regions as in low-mass star forming regions. This, added to the detection of a
few tentative large-scale rotating structures suggests that high-mass stars may
generally form via accretion, as low-mass stars.Comment: 16 pages, 10 figures, accepted by Astronomy and Astrophysic
A high flux source of swift oxygen atoms
A source of swift oxygen atoms is described which has several unique features. A high current ion beam is produced by a microwave discharge, accelerated to 10 keV and the mass selected by a modified Du Pont 21-110 mass spectrometer. The O(+) beam exciting the mass spectrometer is focused into a rectangular shape with an energy spread of less than 1 eV. The next section of the machine decelerates the ion beam into a counterpropagating electron beam in order to minimize space charge effects. After deceleration, the ion beam intersects at 90 deg, a neutral oxygen atom beam, which via resonant charge exchange produces a mixture of O(+) and O. Any remaining O(+) are swept out of the beam by an electric field and differentially pumped away while the desired O beam, collimated by slits, impinges on the target. In situ monitoring of the target surface is done by X-ray photoelectron or Auger spectroscopy. Faraday cups provide flux measurements in the ion sections while the neutral flux is determined by a special torsion balance or by a quadrupole mass spectrometer specially adapted for swift atoms. While the vacuum from the source through the mass spectrometer is maintained by diffusion pumps, the rest of the machine is UHV
The HCO+ emission excess in bipolar outflows
A plausible model is proposed for the enhancement of the abundance of
molecular species in bipolar outflow sources. In this model, levels of HCO+
enhancement are considered based on previous chemical calculations, that are
assumed to result from shock-induced desorption and photoprocessing of dust
grain ice mantles in the boundary layer between the outflow jet and the
surrounding envelope. A radiative transfer simulation that incorporates
chemical variations within the flow shows that the proposed abundance
enhancements in the boundary layer are capable of reproducing the observed
characteristics of the outflow seen in HCO+ emission in the star forming core
L1527. The radiative transfer simulation also shows that the emission lines
from the enhanced molecular species that trace the boundary layer of the
outflow exhibit complex line profiles indicating that detailed spatial maps of
the line profiles are essential in any attempt to identify the kinematics of
potential infall/outflow sources. This study is one of the first applications
of a full three dimensional radiative transfer code which incorporates chemical
variations within the source.Comment: MNRAS, accepted. 10 pages, 6 figure
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