356 research outputs found
Lifting the Dusty Veil II: A Large-Scale Study of the Galactic Infrared Extinction Law
We combine near-infrared (2MASS) and mid-infrared (Spitzer-IRAC) photometry
to characterize the IR extinction law (1.2-8 microns) over nearly 150 degrees
of contiguous Milky Way midplane longitude. The relative extinctions in 5
passbands across these wavelength and longitude ranges are derived by
calculating color excess ratios for G and K giant red clump stars in contiguous
midplane regions and deriving the wavelength dependence of extinction in each
one. Strong, monotonic variations in the extinction law shape are found as a
function of angle from the Galactic center, symmetric on either side of it.
These longitudinal variations persist even when dense interstellar regions,
known a priori to have a shallower extinction curve, are removed. The
increasingly steep extinction curves towards the outer Galaxy indicate a steady
decrease in the absolute-to-selective extinction ratio (R_V) and in the mean
dust grain size at greater Galactocentric angles. We note an increasing
strength of the 8 micron extinction inflection at high Galactocentric angles
and, using theoretical dust models, show that this behavior is consistent with
the trend in R_V. Along several lines of sight where the solution is most
feasible, A_lambda/A_Ks as a function of Galactic radius is estimated and shown
to have a Galactic radial dependence. Our analyses suggest that the observed
relationship between extinction curve shape and Galactic longitude is due to an
intrinsic dependence of the extinction law on Galactocentric radius.Comment: Accepted to Ap
Structure of the interstellar medium around Cas A
We present a three-year series of observations at 24 microns with the Spitzer
Space Telescope of the interstellar material in a 200 x 200 arcmin square area
centered on Cassiopeia A. Interstellar dust heated by the outward light pulse
from the supernova explosion emits in the form of compact, moving features.
Their sequential outward movements allow us to study the complicated
three-dimensional structure of the interstellar medium (ISM) behind and near
Cassiopeia A. The ISM consists of sheets and filaments, with many structures on
a scale of a parsec or less. The spatial power spectrum of the ISM appears to
be similar to that of fractals with a spectral index of 3.5. The filling factor
for the small structures above the spatial wavenumber k ~ 0.5 cycles/pc is only
~ 0.4%.Comment: 29 pages including 10 figures; accepted by The Astrophysical Journa
Fragmentation Instability of Molecular Clouds: Numerical Simulations
We simulate fragmentation and gravitational collapse of cold, magnetized
molecular clouds. We explore the nonlinear development of an instability
mediated by ambipolar diffusion, in which the collapse rate is intermediate to
fast gravitational collapse and slow quasistatic collapse. Initially uniform
stable clouds fragment into elongated clumps with masses largely determined by
the cloud temperature, but substantially larger than the thermal Jeans mass.
The clumps are asymmetric, with significant rotation and vorticity, and lose
magnetic flux as they collapse. The clump shapes, intermediate collapse rates,
and infall profiles may help explain observations not easily fit by
contemporary slow or rapid collapse models.Comment: 25pp, 20 small eps figures, in press ApJ, April 1, 200
The Infrared Properties of Super Star Clusters: Predictions from Three-Dimensional Radiative Transfer Models
With high-resolution infrared data becoming available that can probe the
formation of high-mass stellar clusters for the first time, models that make
testable predictions of these objects are necessary. We utilize a
three-dimensional radiative transfer code, including a hierarchically clumped
medium, to study the earliest stages of super star cluster evolution. We
explore a range of parameter space in geometric sequences that mimic the
evolution of an embedded super star cluster. The inclusion of a hierarchically
clumped medium can make the envelope porous, in accordance with previous models
and supporting observational evidence. The infrared luminosity inferred from
observations can differ by a factor of two from the true value in the clumpiest
envelopes depending on the viewing angle. The infrared spectral energy
distribution also varies with viewing angle for clumpy envelopes, creating a
range in possible observable infrared colors and magnitudes, silicate feature
depths and dust continua. General observable features of cluster evolution
differ between envelopes that are relatively opaque or transparent to
mid-infrared photons. The [70]-[160] color can be used to determine star
formation efficiency; the Spitzer IRAC/MIPS [8.0]-[24] color is able to
constrain Rin and Rout values; and the IRAC [3.6]-[5.8] color is sensitive to
the fraction of the dust distributed in clumps. Finally, in a comparison of
these models to data of ultracompact HII regions, we find good agreement,
suggesting that these models are physically relevant, and will provide useful
diagnostic ability for datasets of resolved, embedded SSCs with the advent of
high-resolution infrared telescopes like JWST.Comment: ApJ, accepted, to be published in the 729 -1 issue. 17 pages with 18
figure
Discovery of a Wolf-Rayet Star Through Detection of its Photometric Variability
We report the serendipitous discovery of a heavily reddened Wolf-Rayet star
that we name WR142b. While photometrically monitoring a cataclysmic variable,
we detected weak variability in a nearby field star. Low-resolution
spectroscopy revealed a strong emission line at 7100 Ang., suggesting an
unusual object and prompting further study. A spectrum taken with the
Hobby-Eberly Telescope confirms strong HeII emission and a NIV 7112 Ang. line
consistent with a nitrogen-rich Wolf-Rayet star of spectral class WN6. Analysis
of the HeII line strengths reveals no detectable hydrogen in WR142b. A
blue-sensitive spectrum obtained with the Large Binocular Telescope shows no
evidence for a hot companion star. The continuum shape and emission line ratios
imply a reddening of E(B-V)=2.2 to 2.5 mag. If not for the dust extinction,
this new Wolf-Rayet star could be visible to the naked eye.Comment: 15 pages, 6 figures, submitted to the Astronomical Journa
Embedded Star Formation in the Eagle Nebula with Spitzer/GLIMPSE
We present new Spitzer photometry of the Eagle Nebula (M16, containing the
optical cluster NGC 6611) combined with near-infrared photometry from 2MASS. We
use dust radiative transfer models, mid-infrared and near-infrared color-color
analysis, and mid-infrared spectral indices to analyze point source spectral
energy distributions, select candidate young stellar objects (YSOs), and
constrain their mass and evolutionary state. Comparison of the different
protostellar selection methods shows that mid-infrared methods are consistent,
but as has been known for some time, near-infrared-only analysis misses some
young objects. We reveal more than 400 protostellar candidates, including one
massive young stellar object (YSO) that has not been previously highlighted.
The YSO distribution supports a picture of distributed low-level star
formation, with no strong evidence of triggered star formation in the
``pillars''. We confirm the youth of NGC 6611 by a large fraction of
infrared-excess sources, and reveal a younger cluster of YSOs in the nearby
molecular cloud. Analysis of the YSO clustering properties shows a possible
imprint of the molecular cloud's Jeans length. Multiwavelength mid-IR imaging
thus allows us to analyze the protostellar population, to measure the dust
temperature and column density, and to relate these in a consistent picture of
star formation in M16.Comment: 16p preprint - ApJ accepte
ALMA Observations of Supernova 1987A
Supernova (SN) 1987A has provided a unique opportunity to study how SN ejecta evolve in 30 years time scale. We report our ALMA spectral observations of SN 1987A, taken in 2014, 2015 and 2016, with detections of CO, 28SiO, HCO+ and SO, with weaker lines of 29SiO.
We find a dip in the SiO line profiles, suggesting that the ejecta morphology is likely elongated. The difference of the CO and SiO line profiles is consistent with hydrodynamic simulations, which show that Rayleigh-Taylor instabilities causes mixing of gas, with heavier elements much more disturbed, making more elongated structure.
Using 28SiO and its isotopologues, Si isotope ratios were estimated for the first time in SN 1987A. The estimated ratios appear to be consistent with theoretical predictions of inefficient formation of neutron rich atoms at lower metallicity, such as observed in the Large Magellanic Cloud (about half a solar metallicity).
The deduced large HCO+ mass and small SiS mass, which are inconsistent to the predictions of chemical model, might be explained by some mixing of elements immediately after the explosion. The mixing might have made some hydrogen from the envelope to sink into carbon and oxygen-rich zone during early days after the explosion, enabling the formation of a substantial mass of HCO+. Oxygen atoms may penetrate into silicon and sulphur zone, suppressing formation of SiS.
Our ALMA observations open up a new window to investigate chemistry, dynamics and explosive-nucleosynthesis in supernovae
A simple connection between the near- and mid-infrared emission of galaxies and their star-formation rates
We have measured the near-infrared colors and the fluxes of individual pixels
in 68 galaxies common to the Spitzer Infrared Nearby Galaxies Survey and the
Large Galaxy Atlas Survey. Each galaxy was separated into regions of
increasingly red near-infrared colors. In the absence of dust extinction and
other non-stellar emission, stellar populations are shown to have relatively
constant NIR colors, independent of age. In regions of high star formation, the
average intensity of pixels in red-excess regions (at 1.25, 3.6, 4.5, 5.6, 8.0
and 24 micron) scales linearly with the intrinsic intensity of Halpha emission,
and thus with the star-formation rate within the pixel. This suggests that most
NIR-excess regions are not red because their light is being depleted by
absorption. Instead, they are red because additional infrared light is being
contributed by a process linked to star-formation. This is surprising because
the shorter wavelength bands in our study (1.25 micron-5.6 micron) do not probe
emission from cold (10-20 K) and warm (50-100 K) dust associated with
star-formation in molecular clouds. However, emission from hot dust (700-1000
K) and/or Polycyclic Aromatic Hydrocarbon molecules can explain the additional
emission seen at the shorter wavelengths in our study. The contribution from
hot dust and/or PAH emission at 2-5micron and PAH emission at 5.6 and 8.0
micron scales linearly with warm dust emission at 24 micron and the intrinsic
Halpha emission. Since both are tied to the star-formation rate, our analysis
shows that the NIR excess continuum emission and PAH emission at ~1-8 micron
can be added to spectral energy distribution models in a very straight-forward
way, by simply adding an additional component to the models that scales
linearly with star-formation rate.Comment: Accepted to ApJ December 1, 2010, Issue 724 -2. 16 pages, 8 colour
figures, 1 online figure to be available at publicatio
The Physical Conditions in Starbursts Derived from Bayesian Fitting of Mid-IR SEDS: 30 Doradus as a Template
To understand and interpret the observed Spectral Energy Distributions (SEDs)
of starbursts, theoretical or semi-empirical SED models are necessary. Yet,
while they are well-founded in theory, independent verification and calibration
of these models, including the exploration of possible degeneracies between
their parameters, are rarely made. As a consequence, a robust fitting method
that leads to unique and reproducible results has been lacking. Here we
introduce a novel approach based on Bayesian analysis to fit the Spitzer-IRS
spectra of starbursts using the SED models proposed by Groves et al. (2008). We
demonstrate its capabilities and verify the agreement between the derived best
fit parameters and actual physical conditions by modelling the nearby,
well-studied, giant HII region 30 Dor in the LMC. The derived physical
parameters, such as cluster mass, cluster age, ISM pressure and covering
fraction of photodissociation regions, are representative of the 30 Dor region.
The inclusion of the emission lines in the modelling is crucial to break
degeneracies. We investigate the limitations and uncertainties by modelling
sub-regions, which are dominated by single components, within 30 Dor. A
remarkable result for 30 Doradus in particular is a considerable contribution
to its mid-infrared spectrum from hot ({\simeq} 300K) dust. The demonstrated
success of our approach will allow us to derive the physical conditions in more
distant, spatially unresolved starbursts.Comment: 17 pages, 10 figures. Accepted por publication in the Astrophysical
Journa
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