698 research outputs found
On the Modified Random Walk for Monte-Carlo Radiation Transfer
Min et al. (2009) presented two complementary techniques that use the
diffusion approximation to allow efficient Monte-Carlo radiation transfer in
very optically thick regions: a modified random walk and a partial diffusion
approximation. In this note, I show that the calculations required for the
modified random walk method can be significantly simplified. In particular, the
diffusion coefficient and the mass absorption coefficients required for the
modified random walk are in fact the same as the standard diffusion coefficient
and the Planck mean mass absorption coefficient.Comment: Accepted for publication in A&
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
Infrared point source variability between the Spitzer and MSX surveys of the Galactic mid-plane
We present a list of 552 sources with suspected variability, based on a
comparison of mid-infrared photometry from the GLIMPSE I and MSX surveys, which
were carried out nearly a decade apart. We were careful to address issues such
as the difference in resolution and sensitivity between the two surveys, as
well as the differences in the spectral responses of the instruments. We
selected only sources where the IRAC 8.0 and MSX 8.28 micron fluxes differ by
more than a factor of two, in order to minimize contamination from sources
where the difference in fluxes at 8 micron is due to a strong 10 micron
silicate feature. We present a subset of 40 sources for which additional
evidence suggests variability, using 2MASS and MIPSGAL data. Based on a
comparison with the variability flags in the IRAS and MSX Point-Source Catalogs
we estimate that at least a quarter of the 552 sources, and at least half of
the 40 sources are truly variable. In addition, we tentatively confirm the
variability of one source using multi-epoch IRAS LRS spectra. We suggest that
most of the sources in our list are likely to be Asymptotic Giant Branch stars.Comment: 47 pages, 12 Figures, 3 Tables, accepted for publication in A
Sky maps without anisotropies in the cosmic microwave background are a better fit to WMAP's uncalibrated time ordered data than the official sky maps
The purpose of this reanalysis of the WMAP uncalibrated time ordered data
(TOD) was two fold. The first was to reassess the reliability of the detection
of the anisotropies in the official WMAP sky maps of the cosmic microwave
background (CMB). The second was to assess the performance of a proposed
criterion in avoiding systematic error in detecting a signal of interest. The
criterion was implemented by testing the null hypothesis that the uncalibrated
TOD was consistent with no anisotropies when WMAP's hourly calibration
parameters were allowed to vary. It was shown independently for all 20 WMAP
channels that sky maps with no anisotropies were a better fit to the TOD than
those from the official analysis. The recently launched Planck satellite should
help sort out this perplexing result.Comment: 11 pages with 1 figure and 2 tables. Extensively rewritten to explain
the research bette
Duration of Star Formation in Galactic Giant Molecular Clouds. I. The Great Nebula in Carina
We present a novel infrared spectral energy distribution (SED) modeling methodology that uses likelihood-based weighting of the model fitting results to construct probabilistic Hertzsprung–Russell diagrams (pHRD) for X-ray-identified, intermediate-mass (2–8 M⊙), pre-main-sequence young stellar populations. This methodology is designed specifically for application to young stellar populations suffering strong, differential extinction (ΔA_V > 10 mag), typical of Galactic massive star-forming regions. We pilot this technique in the Carina Nebula Complex (CNC) by modeling the 1–8 μm SEDs of 2269 likely stellar members that exhibit no excess emission from circumstellar dust disks at 4.5 μm or shorter wavelengths. A subset of ~100 intermediate-mass stars in the lightly obscured Trumpler 14 and 16 clusters have available spectroscopic T_(eff), measured from the Gaia-ESO survey. We correctly identify the stellar temperature in 85% of cases, and the aggregate pHRD for all sources returns the same peak in the stellar age distribution as obtained using the spectroscopic T_(eff). The SED model parameter distributions of stellar mass and evolutionary age reveal significant variation in the duration of star formation among four large-scale stellar overdensities within the CNC and a large distributed stellar population. Star formation began ~10 Myr ago and continues to the present day, with the star formation rate peaking ≾3 Myr ago when the massive Trumpler 14 and 16 clusters formed. We make public the set of 100,000 SED models generated from standard pre-main-sequence evolutionary tracks and our custom software package for generating pHRDs and mass–age distributions from the SED fitting results
Rapid Circumstellar Disk Evolution and an Accelerating Star Formation Rate in the Infrared Dark Cloud M17 SWex
We present a catalog of 840 X-ray sources and first results from a 100 ks
Chandra X-ray Observatory imaging study of the filamentary infrared dark cloud
G014.22500.506, which forms the central regions of a larger cloud complex
known as the M17 southwest extension (M17 SWex). In addition to the rich
population of protostars and young stellar objects with dusty circumstellar
disks revealed by Spitzer Space Telescope archival data, we discover a
population of X-ray-emitting, intermediate-mass pre--main-sequence stars (IMPS)
that lack infrared excess emission from circumstellar disks. We model the
infrared spectral energy distributions of this source population to measure its
mass function and place new constraints on the inner dust disk destruction
timescales for 2-8 stars. We also place a lower limit on the star
formation rate (SFR) and find that it is quite high ( yr), equivalent to several Orion Nebula Clusters in
G14.2250.506 alone, and likely accelerating. The cloud complex has not
produced a population of massive, O-type stars commensurate with its SFR. This
absence of very massive () stars suggests that either (1)
M17 SWex is an example of a distributed mode of star formation that will
produce a large OB association dominated by intermediate-mass stars but
relatively few massive clusters, or (2) the massive cores are still in the
process of accreting sufficient mass to form massive clusters hosting O stars.Comment: 29 pages, 9 figures, accepted to Ap
The youngest massive protostars in the Large Magellanic Cloud
We demonstrate the unique capabilities of Herschel to study very young
luminous extragalactic young stellar objects (YSOs) by analyzing a central
strip of the Large Magellanic Cloud obtained through the HERITAGE Science
Demonstration Program. We combine PACS 100 and 160, and SPIRE 250, 350, and 500
microns photometry with 2MASS (1.25-2.17 microns) and Spitzer IRAC and MIPS
(3.6-70 microns) to construct complete spectral energy distributions (SEDs) of
compact sources. From these, we identify 207 candidate embedded YSOs in the
observed region, ~40% never-before identified. We discuss their position in
far-infrared color-magnitude space, comparing with previously studied,
spectroscopically confirmed YSOs and maser emission. All have red colors
indicating massive cool envelopes and great youth. We analyze four example
YSOs, determining their physical properties by fitting their SEDs with
radiative transfer models. Fitting full SEDs including the Herschel data
requires us to increase the size and mass of envelopes included in the models.
This implies higher accretion rates (greater than or equal to 0.0001 M_sun/yr),
in agreement with previous outflow studies of high-mass protostars. Our results
show that Herschel provides reliable longwave SEDs of large samples of
high-mass YSOs; discovers the youngest YSOs whose SEDs peak in Herschel bands;
and constrains the physical properties and evolutionary stages of YSOs more
precisely than was previously possible.Comment: Main text: 4 pages, 3 figures, 1 table; Online material: 3 figures, 1
table; to appear in the A&A Herschel Special Issu
Star Formation in the Milky Way. The Infrared View
I present a brief review of some of the most recent and active topics of star
formation process in the Milky Way using mid and far infrared observations, and
motivated by the research being carried out by our science group using data
gathered by the Spitzer and Herschel space telescopes. These topics include
bringing together the scaling relationships found in extragalactic systems with
that of the local nearby molecular clouds, the synthetic modeling of the Milky
Way and estimates of its star formation rate.Comment: 12 pages, 9 figures. To apper in "Cosmic-ray induced phenomenology in
star-forming environments: Proceedings of the 2nd Session of the Sant Cugat
Forum of Astrophysics" (April 16-19, 2012), Olaf Reimer and Diego F. Torres
(eds.
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