2,652 research outputs found
IR Dust Bubbles: Probing the Detailed Structure and Young Massive Stellar Populations of Galactic HII Regions
We present an analysis of wind-blown, parsec-sized, mid-infrared bubbles and
associated star-formation using GLIMPSE/IRAC, MIPSGAL/MIPS and MAGPIS/VLA
surveys. Three bubbles from the Churchwell et al. (2006) catalog were selected.
The relative distribution of the ionized gas (based on 20 cm emission), PAH
emission (based on 8 um, 5.8 um and lack of 4.5 um emission) and hot dust (24
um emission) are compared. At the center of each bubble there is a region
containing ionized gas and hot dust, surrounded by PAHs. We identify the likely
source(s) of the stellar wind and ionizing flux producing each bubble based
upon SED fitting to numerical hot stellar photosphere models. Candidate YSOs
are also identified using SED fitting, including several sites of possible
triggered star formation.Comment: 37 pages, 17 figure
Velocity dependence of friction and Kramers relaxation rates
We study the influence of the velocity dependence of friction on the escape
of a Brownian particle from the deep potential well (,
is the barrier height, is the Boltzmann constant, is the
bath temperature). The bath-induced relaxation is treated within the Rayleigh
model (a heavy particle of mass in the bath of light particles of mass
) up to the terms of the order of ,
. The term is equivalent to the Fokker-Planck
dissipative operator, and the term is responsible for the
velocity dependence of friction. As expected, the correction to the Kramers
escape rate in the overdamped limit is proportional to and is
small. The corresponding correction in the underdamped limit is proportional to
and is not necessarily small. We thus suggest that
the effects due to the velocity-dependent friction may be of considerable
importance in determining the rate of escape of an under- and moderately damped
Brownian particle from a deep potential well, while they are of minor
importance for an overdamped particle
Probing discs around massive young stellar objects with CO first overtone emission
We present high resolution (R~50,000) spectroastrometry over the CO 1st
overtone bandhead of a sample of seven intermediate/massive young stellar
objects. These are primarily drawn from the red MSX source (RMS) survey, a
systematic search for young massive stars which has returned a large, well
selected sample of such objects. The mean luminosity of the sample is
approximately 5 times 10^4 L_\odot, indicating the objects typically have a
mass of ~15 solar masses. We fit the observed bandhead profiles with a model of
a circumstellar disc, and find good agreement between the models and
observations for all but one object. We compare the high angular precision
(0.2-0.8 mas) spectroastrometric data to the spatial distribution of the
emitting material in the best-fitting models. No spatial signatures of discs
are detected, which is entirely consistent with the properties of the
best-fitting models. Therefore, the observations suggest that the CO bandhead
emission of massive young stellar objects originates in small-scale disks, in
agreement with previous work. This provides further evidence that massive stars
form via disc accretion, as suggested by recent simulations.Comment: Accepted for publication in MNRA
Ionization of clusters in intense laser pulses through collective electron dynamics
The motion of electrons and ions in medium-sized rare gas clusters (1000
atoms) exposed to intense laser pulses is studied microscopically by means of
classical molecular dynamics using a hierarchical tree code. Pulse parameters
for optimum ionization are found to be wavelength dependent. This resonant
behavior is traced back to a collective electron oscillation inside the charged
cluster. It is shown that this dynamics can be well described by a driven and
damped harmonic oscillator allowing for a clear discrimination against other
energy absorption mechanisms.Comment: 4 pages (4 figures
The RMS Survey: Far-Infrared Photometry of Young Massive Stars
Context: The Red MSX Source (RMS) survey is a multi-wavelength campaign of
follow-up observations of a colour-selected sample of candidate massive young
stellar objects (MYSOs) in the galactic plane. This survey is returning the
largest well-selected sample of MYSOs to date, while identifying other dust
contaminant sources with similar mid-infrared colours including a large number
of new ultra-compact (UC)HII regions. Aims:To measure the far-infrared (IR)
flux, which lies near the peak of the spectral energy distribution (SED) of
MYSOs and UCHII regions, so that, together with distance information, the
luminosity of these sources can be obtained. Methods:Less than 50% of RMS
sources are associated with IRAS point sources with detections at 60 micron and
100 micron, though the vast majority are visible in Spitzer MIPSGAL or IRAS
Galaxy Atlas (IGA) images. However, standard aperture photometry is not
appropriate for these data due to crowding of sources and strong spatially
variable far-IR background emission in the galactic plane. A new technique
using a 2-dimensional fit to the background in an annulus around each source is
therefore used to obtain far-IR photometry for young RMS sources.
Results:Far-IR fluxes are obtained for a total of 1113 RMS candidates
identified as young sources. Of these 734 have flux measurements using IGA 60
micron and 100 micron images and 724 using MIPSGAL 70 micron images, with 345
having measurements in both data sets.Comment: 10 pages, 10 figures, 2 Tables, accepted to A&A. A full version of
table 1 is available from the lead author or at the CDS upon publicatio
Near-infrared integral field spectroscopy of Massive Young Stellar Objects
We present medium resolution () -band integral field
spectroscopy of six MYSOs. The targets are selected from the RMS survey, and we
used the NIFS on the Gemini North telescope. The data show various spectral
line features including Br, CO, H, and \mbox{He\,{\sc i}}. The
Br line is detected in emission in all objects with
-- 200 kms. V645 Cyg shows a high-velocity
P-Cygni profile between -800 kms and -300 kms. We performed
three-dimensional spectroastrometry to diagnose the circumstellar environment
in the vicinity of the central stars using the Br line. We measured the
centroids of the velocity components with sub-mas precision. The centroids
allow us to discriminate the blueshifted and redshifted components in a roughly
east--west direction in both IRAS 18151--1208 and S106 in Br. This lies
almost perpendicular to observed larger scale outflows. We conclude, given the
widths of the lines and the orientation of the spectroastrometric signature,
that our results trace a disc wind in both IRAS 18151--1208 and S106. The CO
absorption lines at low transitions are detected in IRAS
18151--1208 and AFGL 2136. We analysed the velocity structure of the neutral
gas discs. In IRAS 18151--1208, the absorption centroids of the blueshifted and
redshifted components are separated in a direction of north-east to south-west,
nearly perpendicular to that of the larger scale jet. The
position-velocity relations of these objects can be reproduced with central
masses of 30 M_{\sun} for IRAS 18151--1208 and 20 M_{\sun} for AFGL 2136.
We also detect CO bandhead emission in IRAS 18151--1208, S106 and
V645 Cyg. The results can be fitted reasonably with a Keplerian rotation model,
with masses of 15, 20 and 20 M_{\sun} respectively.Comment: 17 pages, 10 figures, accepted by MNRA
Expanding e-MERLIN with the Goonhilly Earth Station
A consortium of universities has recently been formed with the goal of using
the decommissioned telecommunications infrastructure at the Goonhilly Earth
Station in Cornwall, UK, for astronomical purposes. One particular goal is the
introduction of one or more of the ~30-metre parabolic antennas into the
existing e-MERLIN radio interferometer. This article introduces this scheme and
presents some simulations which quantify the improvements that would be brought
to the e-MERLIN system. These include an approximate doubling of the spatial
resolution of the array, an increase in its N-S extent with strong implications
for imaging the most well-studied equatorial fields, accessible to ESO
facilities including ALMA. It also increases the overlap between the e-MERLIN
array and the European VLBI Network. We also discuss briefly some niche science
areas in which an e-MERLIN array which included a receptor at Goonhilly would
be potentially world-leading, in addition to enhancing the existing potential
of e-MERLIN in its role as a Square Kilometer Array pathfinder instrument.Comment: 7 pages, 3 figures, to appear in the proceedings of "Astronomy with
megastructures: Joint science with the E-ELT and SKA", 10-14 May 2010, Crete,
Greece (Eds: Isobel Hook, Dimitra Rigopoulou, Steve Rawlings and Aris
Karastergiou
Massive expanding torus and fast outflow in planetary nebula NGC 6302
We present interferometric observations of CO and CO =21
emission from the butterfly-shaped, young planetary nebula NGC 6302. The high
angular resolution and high sensitivity achieved in our observations allow us
to resolve the nebula into two distinct kinematic components: (1) a massive
expanding torus seen almost edge-on and oriented in the North-South direction,
roughly perpendicular to the optical nebula axis. The torus exhibits very
complex and fragmentated structure; (2) high velocity molecular knots moving at
high velocity, higher than 20 \kms, and located in the optical bipolar lobes.
These knots show a linear position-velocity gradient (Hubble-like flow), which
is characteristic of fast molecular outflow in young planetary nebulae. From
the low but variable CO/CO =21 line intensity ratio we
conclude that the CO =21 emission is optically thick over much of
the nebula. Using the optically thinner line CO =21 we estimate a
total molecular gas mass of 0.1 M, comparable to the ionized gas
mass; the total gas mass of the NGC 6302 nebula, including the massive ionized
gas from photon dominated region, is found to be 0.5 M. From
radiative transfer modelling we infer that the torus is seen at inclination
angle of 75 with respect to the plane of the sky and expanding at
velocity of 15 \kms. Comparison with recent observations of molecular gas in
NGC 6302 is also discussed.Comment: 24 pages, 7 figures, accepted for publication in Astrophysical
Journa
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