123 research outputs found
Dust/gas correlations from Herschel observations
DOI: 10.1051/0004-6361/201014575Context: Previous Spitzer and IRAS observations of the LMC suggest an excess of FIR emission with respect to the gas surface density traced by 12CO rotational emission lines and H i 21 cm emission. This so-called “FIR excess” is especially noticeable near molecular clouds in the LMC, and has usually been interpreted as indicating the presence of a self-shielded H2 component not traced by CO in the envelopes of molecular clouds. Aims: Based on Herschel HERITAGE observations taken as part of the science demonstration phase, we examine the correlation between gas and dust surface densities at higher resolution than previously achieved. We consider three additional possible causes for the FIR excess: X factor, FIR dust emissivity, and gas-to-dust ratio variations between the diffuse and dense phases of the ISM. Methods: We examine the structure of NT80 and NT71, two molecular clouds detected in the NANTEN 12CO survey of the LMC. Dust surface density maps were derived from the HERITAGE data. The gas phase is traced by MAGMA 12CO and ATCA+Parkes H i 21 cm observations of the LMC. These data provide unprecedented resolution (1') to examine the structure of molecular clouds. The dust emissivity, gas-to-dust ratio, and X factor required to match the dust and gas surface densities are derived, and their correlations with the dust surface density are examined.
Results: We show that the dust surface density is spatially correlated with the atomic and molecular gas phases. The dust temperature is consistently lower in the dense phase of the ISM than in the diffuse phase. We confirm variations in the ratio of FIR emission to gas surface density derived from H i and CO observations. There is an excess of FIR emission, spatially correlated with regions of intermediate H i and dust surface densities (AV = 1-2), and little or no CO. While there is no significant trend in the dust emissivity or gas-to-dust ratio with dust surface density, the X factor is enhanced at AV = 1-2. We conclude that H2 envelopes not traced by CO and X factor variations close to the CO boundary may be more likely to cause these deviations between FIR emission and gas surface density than gas-to-dust ratio or emissivity variations.We acknowledge financial support from the NASA
Herschel Science Center (NHSC), J.P.L. contracts #1381522, and #1381650. Part of this research was conducted at the Jet Propulsion Laboratory, California Institute of Technology under contract with the National Aeronautics and Space Administration. We thank the support from the European Space Agency, PACS and SPIRE teams, Herschel Science Center, and NHSC (B. Ali, K. Xu). M.R. is supported by FONDECYT No1080335 and FONDAP No15010003
On Dust Extinction of Gamma-ray Burst Host Galaxies
Although it is well recognized that gamma-ray burst (GRB) afterglows are
obscured and reddened by dust in their host galaxies, the wavelength-dependence
and quantity of dust extinction are still poorly known. Current studies on this
mostly rely on fitting the afterglow spectral energy distributions (SEDs) with
template extinction models. The inferred extinction (both quantity and
wavelength-dependence) and dust-to-gas ratios are often in disagreement with
that obtained from dust depletion and X-ray spectroscopy studies. We argue that
this discrepancy could result from the prior assumption of a template
extinction law. We propose an analytical formula to approximate the GRB host
extinction law. With the template extinction laws self-contained, and the
capability of revealing extinction laws differing from the conventional ones,
it is shown that this is a powerful approach in modeling the afterglow SEDs to
derive GRB host extinction.Comment: 9 pages, 4 figures; The Astrophysical Journal, in press (2008 Oct 1
issue
On Silicon Carbide Grains as the Carrier of the 21 Micron Emission Feature in Post-Asymptotic Giant Branch Stars
The mysterious 21mu emission feature seen in 12 proto-planetary nebulae
(PPNe) remains unidentified since its first detection in 1989. Over a dozen of
candidate materials have been proposed within the past decade, but none of them
has received general acceptance. Very recently, silicon carbide (SiC) grains
with impurities were suggested to be the carrier of this enigmatic feature,
based on recent laboratory data that doped SiC grains exhibit a resonance at
\~21mu. This proposal gains strength from the fact that SiC is a common dust
species in carbon-rich circumstellar envelopes. However, SiC dust has a strong
vibrational band at ~11.3mu. We show in this Letter that in order to be
consistent with the observed flux ratios of the 11.3mu feature to the 21mu
feature, the band strength of the 21mu resonance has to be very strong, too
strong to be consistent with current laboratory measurements. But this does not
yet readily rule out the SiC hypothesis since recent experimental results have
demonstrated that the 21mu resonance of doped SiC becomes stronger as the C
impurity increases. Further laboratory measurements of SiC dust with high
fractions of C impurity are urgently needed to test the hypothesis of SiC as
the carrier of the 21mu feature.Comment: 14 pages, 3 figures, accepted for publication in ApJ
On the Unusual Depletions toward Sk 155, or What Are the Small Magellanic Cloud Dust Grains Made of?
The dust in the Small Magellanic Cloud (SMC), an ideal analog of primordial
galaxies at high redshifts, differs markedly from that in the Milky Way by
exhibiting a steeply rising far-ultraviolet extinction curve, an absence of the
2175 Angstrom extinction feature, and a local minimum at ~12 micron in its
infrared emission spectrum, suggesting the lack of ultrasmall carbonaceous
grains (i.e. polycyclic aromatic hydrocarbon molecules) which are ubiquitously
seen in the Milky Way. While current models for the SMC dust all rely heavily
on silicates, recent observations of the SMC sightline toward Sk 155 indicated
that Si and Mg are essentially undepleted and the depletions of Fe range from
mild to severe, suggesting that metallic grains and/or iron oxides, instead of
silicates, may dominate the SMC dust. However, in this Letter we apply the
Kramers-Kronig relation to demonstrate that neither metallic grains nor iron
oxides are capable of accounting for the observed extinction; silicates remain
as an important contributor to the extinction, consistent with current models
for the SMC dust.Comment: 12 pages, 3 figures; The Astrophysical Journal Letters, in pres
Phase Functions and Light Curves of Wide Separation Extrasolar Giant Planets
We calculate self-consistent extrasolar giant planet (EGP) phase functions
and light curves for orbital distances ranging from 0.2 AU to 15 AU. We explore
the dependence on wavelength, cloud condensation, and Keplerian orbital
elements. We find that the light curves of EGPs depend strongly on wavelength,
the presence of clouds, and cloud particle sizes. Furthermore, the optical and
infrared colors of most EGPs are phase-dependent, tending to be reddest at
crescent phases in and . Assuming circular orbits, we find that at
optical wavelengths most EGPs are 3 to 4 times brighter near full phase than
near greatest elongation for highly-inclined (i.e., close to edge-on) orbits.
Furthermore, we show that the planet/star flux ratios depend strongly on the
Keplerian elements of the orbit, particularly inclination and eccentricity.
Given a sufficiently eccentric orbit, an EGP's atmosphere may make periodic
transitions from cloudy to cloud-free, an effect that may be reflected in the
shape and magnitude of the planet's light curve. Such elliptical orbits also
introduce an offset between the time of the planet's light curve maximum and
the time of full planetary phase, and for some sets of orbital parameters, this
light curve maximum can be a steeply increasing function of eccentricity. We
investigate the detectability of EGPs by proposed space-based direct-imaging
instruments.Comment: submitted to Astrophysical Journa
Cosmic Needles versus Cosmic Microwave Background Radiation
It has been suggested by a number of authors that the 2.7K cosmic microwave
background (CMB) radiation might have arisen from the radiation from Population
III objects thermalized by conducting cosmic graphite/iron needle-shaped dust.
Due to lack of an accurate solution to the absorption properties of exceedingly
elongated grains, in existing literature which studies the CMB thermalizing
process they are generally modelled as (1) needle-like spheroids in terms of
the Rayleigh approximation; (2) infinite cylinders; and (3) the antenna theory.
We show here that the Rayleigh approximation is not valid since the Rayleigh
criterion is not satisfied for highly conducting needles. We also show that the
available intergalactic iron dust, if modelled as infinite cylinders, is not
sufficient to supply the required opacity at long wavelengths to obtain the
observed isotropy and Planckian nature of the CMB. If appealing to the antenna
theory, conducting iron needles with exceedingly large elongations (10^4)
appear able to provide sufficient opacity to thermalize the CMB within the iron
density limit. But the applicability of the antenna theory to exceedingly thin
needles of nanometer/micrometer in thickness needs to be justified.Comment: 13 pages, 4 figures; submitted to ApJ
The Spectral Energy Distribution of Dust Emission in the Edge-on spiral galaxy NGC 4631 as seen with Spitzer and the James Clerk Maxwell telescope
We explore variations in dust emission within the edge-on Sd spiral galaxy NGC 4631 using 3.6-160 μm Spitzer Space Telescope data and 450-850 μm JCMT data with the goals of understanding the relation between PAHs and dust emission, studying the variations in the colors of the dust emission, and searching for possible excess submillimeter emission compared to what is expected from dust models extrapolated from far-infrared wavelengths. The 8 μm PAH emission correlates best with 24 μm hot dust emission on 1.7 kpc scales, but the relation breaks down on 650 pc scales, possibly because of differences in the mean free paths between photons that excite the PAHs and photons that heat the dust and possibly because the PAHs are destroyed by the hard radiation fields within some star formation regions. The ratio of 8 μm PAH emission to 160 μm cool dust emission appears to vary as a function of radius. The 70 μm/160 μm and 160 μm/450 μm flux density ratios are remarkably constant even though the surface brightnesses vary by factors of 25, which suggests that the emission is from dust heated by a nearly uniform radiation field. Globally, we find an excess of 850-1230 μm emission relative to what would be predicted by dust models. The 850 μm excess is highest in regions with low 160 μm surface brightnesses, although the magnitude depends on the model fit to the data. We rule out variable emissivity functions or ~4 K dust as the possible origins of this 850 μm emission, but we do discuss the other possible mechanisms that could produce the emission
On Ultrasmall Silicate Grains in the Diffuse Interstellar Medium
The abundance of both amorphous and crystalline silicates in very small
grains is limited by the fact that the 10 micron silicate emission feature is
not detected in the diffuse ISM. On the basis of the observed IR emission
spectrum for the diffuse ISM, the observed ultraviolet extinction curve, and
the 10 micron silicate absorption profile, we obtain upper limits on the
abundances of ultrasmall (a < 15 Angstrom) amorphous and crystalline silicate
grains.
Contrary to previous work, as much as ~20% of interstellar Si could be in a <
15 Angstrom silicate grains without violating observational constraints. Not
more than ~5% of the Si can be in crystalline silicates (of any size).Comment: Submitted to ApJ Letters, 11 pages, 4 figures, Late
Detections of the 2175 \AA Dust Feature at 1.4z 1.5 from the Sloan Digital Sky Survey
The strongest spectroscopic dust extinction feature in the Milky Way, the
broad absorption bump at 2175 \AA, is generally believed to be caused by
aromatic carbonaceous materials -- very likely a mixture of Polycyclic Aromatic
Hydrocarbon (PAH) molecules, the most abundant and widespread organic molecules
in the Milky Way galaxy. In this paper we report identifications of this
absorption feature in three galaxies at which
produce intervening MgII absorption toward quasars discovered by the Sloan
Digital Sky Survey (SDSS). The observed spectra can be fit using Galactic-type
extinction laws, characterized by parameters [R_V, E(B-V)] ~ [0.7, 0.14], [1.9,
0.13], and [5.5, 0.23], respectively, where R_V=A_V/E(B-V) is the
total-to-selective extinction ratio, E(B-V) = A_B-A_V is the color-excess.
These discoveries imply that the dust in these distant quasar absorption
systems is similar in composition to that of Milky Way, but with a range of
different grain size distributions. The presence of complex aromatic
hydrocarbon molecules in such distant galaxies is important for both
astrophysical and astrobiological investigations.Comment: 20 pages, 4 figures, accepted for publication in ApJ July 10 issu
- …