1,839 research outputs found
The Near-Infrared Extinction Law in Regions of High Av
We present a spectroscopic study of the shape of the dust-extinction law
between 1.0 and 2.2um towards a set of nine ultracompact HII regions with Av >
15 mag. We find some evidence that the reddening curve may tend to flatten at
higher extinctions, but just over half of the sample has extinction consistent
with or close to the average for the interstellar medium. There is no evidence
of extinction curves significantly steeper than the standard law, even where
water ice is present. Comparing the results to the predictions of a simple
extinction model, we suggest that a standard extinction law implies a robust
upper limit to the grain-size distribution at around 0.1 - 0.3um. Flatter
curves are most likely due to changes in this upper limit, although the effects
of flattening due to unresolved clumpy extinction cannot be ruled out.Comment: 9 pages, 7 figure
Helium and Hydrogen Line Ratios and The Stellar Content of Compact HII Regions
We present observations and models of the behaviour of the HI and HeI lines
between 1.6 and 2.2um in a small sample of compact HII regions. As in our
previous papers on planetary nebulae, we find that the `pure' 1.7007um
4^3D-3^3P and 2.16475um 7^(3,1)G-4^(3,1)F HeI recombination lines behave
approximately as expected as the effective temperature of the central exciting
star(s) increases. However, the 2.058um 2^1P-2^1S HeI line does not behave as
the model predicts, or as seen in planetary nebulae. Both models and planetary
nebulae showed a decrease in the HeI 2^1P-2^1S/HI Br gamma ratio above an
effective temperature of 40000K. The compact HII regions do not show any such
decrease. The problem with this line ratio is probably due to the fact that the
photoionisation model does not account correctly for the high densities seen in
these HII regions, and that we are therefore seeing more collisional excitation
of the 2^1P level than the model predicts. It may also reflect some deeper
problem in the assumed model stellar atmospheres. In any event, although the
normal HeI recombination lines can be used to place constraints on the
temperature of the hottest star present, the HeI 2^1P-2^1S/HI Br gamma ratio
should not be used for this purpose in either Galactic HII regions or in
starburst galaxies, and conclusions from previous work using this ratio should
be regarded with extreme caution. We also show that the combination of the near
infrared `pure' recombination line ratios with mid-infrared forbidden line data
provides a good discriminant of the form of the far ultraviolet spectral energy
distribution of the exciting star(s). From this we conclude that CoStar models
are a poor match to the available data for our sources, though the more recent
WM-basic models are a better fit.Comment: Accepted for publication in MNRA
The RMS Survey: The Bolometric Fluxes and Luminosity Distributions of Young Massive Stars
Context: The Red MSX Source (RMS) survey is returning a large sample of
massive young stellar objects (MYSOs) and ultra-compact (UC) \HII{} regions
using follow-up observations of colour-selected candidates from the MSX point
source catalogue. Aims: To obtain the bolometric fluxes and, using kinematic
distance information, the luminosities for young RMS sources with far-infrared
fluxes. Methods: We use a model spectral energy distribution (SED) fitter to
obtain the bolometric flux for our sources, given flux data from our work and
the literature. The inputs to the model fitter were optimised by a series of
investigations designed to reveal the effect varying these inputs had on the
resulting bolometric flux. Kinematic distances derived from molecular line
observations were then used to calculate the luminosity of each source.
Results: Bolometric fluxes are obtained for 1173 young RMS sources, of which
1069 have uniquely constrained kinematic distances and good SED fits. A
comparison of the bolometric fluxes obtained using SED fitting with trapezium
rule integration and two component greybody fits was also undertaken, and
showed that both produce considerable scatter compared to the method used here.
Conclusions: The bolometric flux results allowed us to obtain the luminosity
distributions of YSOs and UC\HII{} regions in the RMS sample, which we find to
be different. We also find that there are few MYSOs with L
10\lsol{}, despite finding many MYSOs with 10\lsol{} L
10\lsol{}.Comment: 12 pages, 12 figures, 3 tables, accepted to A&A. The full versions of
tables 1 and 2 will be available via the CDS upon publicatio
Neutron and X-ray Scattering Studies of the Lightly-Doped Spin-Peierls System Cu(1-x)Cd(x)GeO3
Single crystals of the lightly-doped spin-Peierls system Cu(1-x)Cd(x)GeO3
have been studied using bulk susceptibility, x-ray diffraction, and inelastic
neutron scattering techniques. We investigate the triplet gap in the magnetic
excitation spectrum of this quasi-one dimensional quantum antiferromagnet, and
its relation to the spin-Peierls dimerisation order parameter. We employ two
different theoretical forms to model the inelastic neutron scattering cross
section and chi''(Q,omega), and show the sensitivity of the gap energy to the
choice of chi''(Q,omega). We find that a finite gap exists at the spin-Peierls
phase transition.Comment: 15 Pages, 7 Figures, Submitted to J. Phys. :Condensed Matte
Doping Dependence of Spin Dynamics in Electron-Doped Ba(Fe1-xCox)2As2
The spin dynamics in single crystal, electron-doped Ba(Fe1-xCox)2As2 has been
investigated by inelastic neutron scattering over the full range from undoped
to the overdoped regime. We observe damped magnetic fluctuations in the normal
state of the optimally doped compound (x=0.06) that share a remarkable
similarity with those in the paramagnetic state of the parent compound (x=0).
In the overdoped superconducting compound (x=0.14), magnetic excitations show a
gap-like behavior, possibly related to a topological change in the hole Fermi
surface (Lifshitz transition), while the imaginary part of the spin
susceptibility prominently resembles that of the overdoped cuprates. For the
heavily overdoped, non-superconducting compound (x=0.24) the magnetic
scattering disappears, which could be attributed to the absence of a hole
Fermi-surface pocket observed by photoemission.Comment: 6 pages, 5 figures, published versio
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
A distance-limited sample of massive star-forming cores from the RMS survey
We analyse C18O (J = 3−2) data from a sample of 99 infrared (IR)-bright massive young stellar objects (MYSOs) and compact H ii regions that were identified as potential molecular-outflow sources in the Red MSX Source survey. We extract a distance-limited (D < 6 kpc) sample shown to be representative of star formation covering the transition between the source types. At the spatial resolution probed, Larson-like relationships are found for these cores, though the alternative explanation, that Larson's relations arise where surface-density-limited samples are considered, is also consistent with our data. There are no significant differences found between source properties for the MYSOs and H ii regions, suggesting that the core properties are established prior to the formation of massive stars, which subsequently have little impact at the later evolutionary stages investigated. There is a strong correlation between dust-continuum and C18O-gas masses, supporting the interpretation that both trace the same material in these IR-bright sources. A clear linear relationship is seen between the independently established core masses and luminosities. The position of MYSOs and compact H ii regions in the mass–luminosity plane is consistent with the luminosity expected from a cluster of protostars when using an ∼40 per cent star formation efficiency and indicates that they are at a similar evolutionary stage, near the end of the accretion phase
The RMS Survey: Ammonia and water maser analysis of massive star forming regions
The Red MSX Source (RMS) survey has identified a sample of ~1200 massive
young stellar objects (MYSOs), compact and ultra compact HII regions from a
sample of ~2000 MSX and 2MASS colour selected sources. We have used the 100 m
Green Bank telescope to search for 22-24 GHz water maser and ammonia (1,1),
(2,2) and (3,3) emission towards ~600 RMS sources located within the northern
Galactic plane. We have identified 308 H2O masers which corresponds to an
overall detection rate of ~50%. Abridged: We detect ammonia emission towards
479 of these massive young stars, which corresponds to ~80%. Ammonia is an
excellent probe of high density gas allowing us to measure key parameters such
as gas temperatures, opacities, and column densities, as well as providing an
insight into the gas kinematics. The average kinetic temperature, FWHM line
width and total NH3 column density for the sample are approximately 22 K, 2
km/s and 2x10^{15} cm^{-2}, respectively. We find that the NH3 (1,1) line width
and kinetic temperature are correlated with luminosity and finding no
underlying dependence of these parameters on the evolutionary phase of the
embedded sources, we conclude that the observed trends in the derived
parameters are more likely to be due to the energy output of the central source
and/or the line width-clump mass relationship. The velocities of the peak H2O
masers and the NH3 emission are in excellent agreement with each other, which
would strongly suggest an association between the dense gas and the maser
emission. Moreover, we find the bolometric luminosity of the embedded source
and the isotropic luminosity of the H2O maser are also correlated. We conclude
from the correlations of the cloud and water maser velocities and the
bolometric and maser luminosity that there is a strong dynamical relationship
between the embedded young massive star and the H2O maser.Comment: 17 pages and 17 figures and 8 tables. Tables\,2 and 5 and full
versions of Figs. 3 and 7 are only available in electronic form at the CDS
via anonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or via
http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A
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