646 research outputs found
Determining dust temperatures and masses in the Herschel era: The importance of observations longward of 200 micron
Context. The properties of the dust grains (e.g., temperature and mass) can be derived from fitting far-IR SEDs (â„100 ÎŒm). Only with SPIRE on Herschel has it been possible to get high spatial resolution at 200 to 500 ÎŒm that is beyond the peak (~160 ÎŒm) of dust emission in most galaxies.
Aims. We investigate the differences in the fitted dust temperatures and masses determined using only 200 ÎŒm data (new SPIRE observations) to determine how important having >200 ÎŒm data is for deriving these dust properties.
Methods. We fit the 100 to 350 ÎŒm observations of the Large Magellanic Cloud (LMC) point-by-point with a model that consists of a single temperature and fixed emissivity law. The data used are existing observations at 100 and 160 ÎŒm (from IRAS and Spitzer) and new SPIRE observations of 1/4 of the LMC observed for the HERITAGE key project as part of the Herschel science demonstration phase.
Results. The dust temperatures and masses computed using only 100 and 160 ÎŒm data can differ by up to 10% and 36%, respectively, from those that also include the SPIRE 250 & 350 ÎŒm data. We find that an emissivity law proportional to λ^(â1.5) minimizes the 100â350 ÎŒm fractional residuals. We find that the emission at 500 ÎŒm is ~10% higher than expected from extrapolating the fits made at shorter wavelengths. We find the fractional 500 ÎŒm excess is weakly anti-correlated with MIPS 24 ÎŒm flux and the total gas surface density. This argues against a flux calibration
error as the origin of the 500 Όm excess. Our results do not allow us to distinguish between a systematic variation in the wavelength dependent emissivity law or a population of very cold dust only detectable at λ ℠500 Όm for the origin of the 500 Όm excess
Cold dust clumps in dynamically hot gas
Aims. We present clumps of dust emission from Herschel observations of the Large Magellanic Cloud (LMC) and their physical and statistical
properties. We catalog cloud features seen in the dust emission from Herschel observations of the LMC, the Magellanic type irregular galaxy
closest to the Milky Way, and compare these features with Hi catalogs from the ATCA+Parkes Hi survey.
Methods. Using an automated cloud-finding algorithm, we identify clouds and clumps of dust emission and examine the cumulative mass distribution
of the detected dust clouds. The mass of cold dust is determined from physical parameters that we derive by performing spectral energy distribution
fits to 250, 350, and 500 ÎŒm emission from SPIRE observations using dust grain size distributions for graphite/silicate in low-metallicity
extragalactic environments.
Results. The dust cloud mass spectrum follows a power law distribution with an exponent of Îł = â1.8 for clumps larger than 4 Ă 10^2 M_â and is
similar to the Hi mass distribution. This is expected from the theory of ISM structure in the vicinity of star formation
Long-Wavelength Excesses in Two Highly Obscured High-Mass X-Ray Binaries: IGR J16318â4848 and GX 301â2
We present evidence for excess long-wavelength emission from two high-mass X-ray binaries, IGR J16318-4848 and GX 301-2, that show enormous obscuration (N_H â 10^(23)-10^(24) cm^(-2)) in their X-ray spectra. Using archival near- and mid-infrared data, we show that the spectral energy distributions of IGR J16318-4848 and GX 301-2 are substantially higher in the mid-infrared than their expected stellar emission. We successfully fit the excesses with ~1000 K blackbodies, which suggests that they are due to warm circumstellar dust that also gives rise to the X-ray absorption. However, we need further observations to constrain the detailed properties of the excesses. This discovery highlights the importance of mid-infrared observations for understanding highly obscured X-ray binaries
Detection of Far-Infrared Water Vapor, Hydroxyl, and Carbon Monoxide Emissions from the Supernova Remnant 3C 391
We report the detection of shock-excited far-infrared emission of H2O, OH,
and CO from the supernova remnant 3C 391, using the ISO Long-Wavelength
Spectrometer. This is the first detection of thermal H2O and OH emission from a
supernova remnant. For two other remnants, W~28 and W~44, CO emission was
detected but OH was only detected in absorption. The observed H2O and OH
emission lines arise from levels within ~400 K of the ground state, consistent
with collisional excitation in warm, dense gas created after the passage of the
shock front through the dense clumps in the pre-shock cloud. The post-shock gas
we observe has a density ~2x10^5 cm^{-3} and temperature 100-1000 K, and the
relative abundances of CO:OH:H2O in the emitting region are 100:1:7 for a
temperature of 200 K. The presence of a significant column of warm H2O suggests
that the chemistry has been significantly changed by the shock. The existence
of significant column densities of both OH and H2O, which is at odds with
models for non-dissociative shocks into dense gas, could be due to
photodissociation of H2O or a mix of fast and slow shocks through regions with
different pre-shock density.Comment: AASTeX manuscript and 4 postscript figure
Molecular and Ionic shocks in the Supernova Remnant 3C391
New observations of the supernova remnant 3C391 are in the H2 2.12 micron and
[Fe II] 1.64 micron narrow-band filters at the Palomar 200-inch telescope, and
in the 5-15 micron CVF on ISOCAM. Shocked H2 emission was detected from the
region 3C391:BML, where broad millimeter CO and CS lines had previously been
detected. A new H2 clump was confirmed to have broad CO emission, demonstrating
that the near-infrared H2 images can trace previously undetected molecular
shocks. The [Fe II] emission has a significantly different distribution, being
brightest in the bright radio bar, at the interface between the supernova
remnant and the giant molecular cloud, and following filaments in the radio
shell. The near-infrared [Fe II] and the mid-infrared 12-18 micron filter
images are the first images to reveal the radiative shell of 3C391. The
mid-infrared spectrum is dominated by bright ionic lines and H2 S(2) through
S(7). There are no aromatic hydrocarbons associated with the shocks, nor is
their any mid-infrared continuum, suggesting that macromolecules and very small
grains are destroyed. Comparing 3C391 to the better-studied IC443, both
remnants have molecular- and ionic-dominated regions; for 3C391, the
ionic-dominated region is the interface into the giant molecular cloud, showing
that the main bodies of giant molecular clouds contain significant regions with
densities 100 to 1000/cm^3 and a small filling factor with higher-density. The
molecular shocked region resolves into 16 clumps of H2 emission, with some
fainter diffuse emission but with no associated near-infrared continuum
sources. One of the clumps is coincident with a previously-detected OH 1720 MHz
maser. These clumps are interpreted as a cluster of pre-stellar, dense
molecular cores that are presently being shocked by the supernova blast wave
Discovery of Broad Molecular lines and of Shocked Molecular Hydrogen from the Supernova Remnant G357.7+0.3: HHSMT, APEX, Spitzer and SOFIA Observations
We report a discovery of shocked gas from the supernova remnant (SNR)
G357.7+0.3. Our millimeter and submillimeter observations reveal broad
molecular lines of CO(2-1), CO(3-2), CO(4-3), 13CO (2-1) and 13CO (3-2), HCO^+
and HCN using HHSMT, Arizona 12-Meter Telescope, APEX and MOPRA Telescope. The
widths of the broad lines are 15-30 kms, and the detection of such broad lines
is unambiguous, dynamic evidence showing that the SNR G357.7+0.3 is interacting
with molecular clouds. The broad lines appear in extended regions (>4.5'x5').
We also present detection of shocked H2 emission in mid-infrared but lacking
ionic lines using the Spitzer IRS observations to map a few arcmin area. The H2
excitation diagram shows a best-fit with a two-temperature LTE model with the
temperatures of ~200 and 660 K. We observed [C II] at 158um and high-J
CO(11-10) with the GREAT on SOFIA. The GREAT spectrum of [C II], a 3 sigma
detection, shows a broad line profile with a width of 15.7 km/s that is similar
to those of broad CO molecular lines. The line width of [C~II] implies that
ionic lines can come from a low-velocity C-shock. Comparison of H2 emission
with shock models shows that a combination of two C-shock models is favored
over a combination of C- and J-shocks or a single shock. We estimate the CO
density, column density, and temperature using a RADEX model. The best-fit
model with n(H2) = 1.7x10^{4} cm^{-3}, N(CO) = 5.6x10^{16} cm^{-2}, and T = 75
K can reproduce the observed millimeter CO brightnesses.Comment: 19 pages, 22 figure
Spitzer Observations of Molecular Hydrogen in Interacting Supernova Remnants
With Spitzer IRS we have obtained sensitive low-resolution spectroscopy from
5 to 35 microns for six supernova remnants (SNRs) that show evidence of shocked
molecular gas: Kes 69, 3C 396, Kes 17, G346.6-0.2, G348.5-0.0 and G349.7+0.2.
Bright, pure-rotational lines of molecular hydrogen are detected at the shock
front in all remnants, indicative of radiative cooling from shocks interacting
with dense clouds. We find the excitation of H2 S(0)-S(7) lines in these SNRs
requires two non-dissociative shock components: a slow, 10 km/s C- shock
through clumps of density 10^6 cm^-3, and a faster, 40-70 km/s C- shock through
a medium of density 10^4 cm^-3. The ortho-to-para ratio for molecular hydrogen
in the warm shocked gas is typically found to be much less than the LTE value,
suggesting that these SNRs are propagating into cold quiescent clouds.
Additionally a total of thirteen atomic fine-structure transitions of Ar+,
Ar++, Fe+, Ne+, Ne++, S++, and Si+ are detected. The ionic emitting regions are
spatially segregated from the molecular emitting regions within the IRS slits.
The presence of ionic lines with high appearance potential requires the
presence of much faster, dissociative shocks through a lower density medium.
The IRS slits are sufficiently wide to include regions outside the SNR which
permits emission from diffuse gas around the remnants to be separated from the
shocked emission. We find the diffuse molecular hydrogen gas projected outside
the SNR is excited to a temperature of 100 to 300 K with a warm gas fraction of
0.5 to 15 percent along the line of sight.Comment: Accepted to Ap J and scheduled for 2009 April 1 v694 issue. LaTeX, 27
pages with 11 figure
Grain Survival in Supernova Remnants and Herbig-Haro Objects
By using the flux ratio [FeII]8617/[OI]6300, we demonstrate that most of the
interstellar dust grains survive in shocks associated with supernova remnants
and Herbig-Haro objects. The [FeII]/[OI] flux ratio is sensitive to the
gas-phase Fe/O abundance ratio, but is insensitive to the ionization state,
temperature, and density of the gas. We calculate the [FeII]/[OI] flux ratio in
shocks, and compare the results with the observational data. When only 20% of
iron is in the gas phase, the models reproduce most successfully the
observations. This finding is in conflict with the current consensus that
shocks destroy almost all the grains and 100% of metals are in the gas phase.
We comment on previous works on grain destruction, and discuss why grains are
not destroyed in shocks.Comment: 8 pages (AASTex v5.0), 3 figures. To be published in ApJ Letters
(accepted 3/10/2000
- âŠ