450 research outputs found
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
Sustainable governance for small desert settlements
'Sustainable governance for small desert settlements: Combining single settlement localism and multi-settlement regionalism', a paper presented to the Desert Knowledge Symposium & Business Showcase, Alice Springs, Northern Territory, Australia, 1-3 November 2006
An infrared study of the double nucleus in NGC3256
We present new resolved near and mid-IR imaging and N-band spectroscopy of
the two nuclei in the merger system NGCA3256, the most IR luminous galaxy in
the nearby universe. The results from the SED fit to the data are consistent
with previous estimates of the amount of obscuration towards the nuclei and the
nuclear star formation rates. However, we also find substantial differences in
the infrared emission from the two nuclei which cannot be explained by
obscuration alone. We conclude that the northern nucleus requires an additional
component of warm dust in order to explain its properties. This suggests that
local starforming conditions can vary significantly within the environment of a
single system.Comment: Accepted for publication (MNRAS
Morphological analysis of the cm-wave continuum in the dark cloud LDN1622
The spectral energy distribution of the dark cloud LDN1622, as measured by
Finkbeiner using WMAP data, drops above 30GHz and is suggestive of a Boltzmann
cutoff in grain rotation frequencies, characteristic of spinning dust emission.
LDN1622 is conspicuous in the 31 GHz image we obtained with the Cosmic
Background Imager, which is the first cm-wave resolved image of a dark cloud.
The 31GHz emission follows the emission traced by the four IRAS bands. The
normalised cross-correlation of the 31 GHz image with the IRAS images is higher
by 6.6sigma for the 12um and 25um bands than for the 60um and 100um bands:
C(12+25) = 0.76+/-0.02 and C(60+100) = 0.64+/-0.01.
The mid-IR -- cm-wave correlation in LDN 1622 is evidence for very small
grain (VSG) or continuum emission at 26-36GHz from a hot molecular phase. In
dark clouds and their photon-dominated regions (PDRs) the 12um and 25um
emission is attributed to stochastic heating of the VSGs. The mid-IR and
cm-wave dust emissions arise in a limb-brightened shell coincident with the PDR
of LDN1622, where the incident UV radiation from the Ori OB1b association heats
and charges the grains, as required for spinning dust.Comment: accepted for publication in ApJ - the complete article with
uncompressed figures may be downloaded from
http://www.das.uchile.cl/~simon/ftp/l1622.pd
Calculating Cross Sections of Composite Interstellar Grains
Interstellar grains may be composite collections of particles of distinct
materials, including voids, agglomerated together. We determine the various
optical cross sections of such composite grains, given the optical properties
of each constituent, using an approximate model of the composite grain. We
assume it consists of many concentric spherical layers of the various
materials, each with a specified volume fraction. In such a case the usual Mie
theory can be generalized and the extinction, scattering, and other cross
sections determined exactly.
We find that the ordering of the materials in the layering makes some
difference to the derived cross sections, but averaging over the various
permutations of the order of the materials provides rapid convergence as the
number of shells (each of which is filled by all of the materials
proportionately to their volume fractions) is increased. Three shells, each
with one layer of a particular constituent material, give a very satisfactory
estimate of the average cross section produced by larger numbers of shells.
We give the formulae for the Rayleigh limit (small size parameter) for
multi-layered spheres and use it to propose an ``Effective Medium Theory''
(EMT), in which an average optical constant is taken to represent the ensemble
of materials.
Multi-layered models are used to compare the accuracies of several EMTs
already in the literature.Comment: 29 pages, 6 figures, accepted for publication in the Astrophysical
Journal (part 1, scheduled in Vol. 526, #1, Nov. 20
The Embedded Super Star Cluster of SBS0335-052
We analyze the infrared (6-100 micron) spectral energy distribution of the
blue compact dwarf and metal-poor (Z=Z_solar/41) galaxy SBS0335-052. With the
help of DUSTY (Ivezic et al. 1999), a program that solves the radiation
transfer equations in a spherical environment, we evaluate that the infrared
(IR) emission of SBS0335-052 is produced by an embedded super-star cluster
(SSC) hidden under 10^5 M_solar of dust, causing 30 mag of visual extinction.
This implies that one cannot detect any stellar emission from the 2x10^6
M_solar stellar cluster even at near-infrared (NIR) wavelengths. The derived
grain size distribution departs markedly from the widely accepted size
distribution inferred for dust in our galaxy (the so-called MRN distribution,
Mathis et al. 1977), but resembles what is seen around AGNs, namely an absence
of PAH and smaller grains, and grains that grow to larger sizes (around 1
micron). The fact that a significant amount of dust is present in such a
low-metallicity galaxy, hiding from UV and optical view most of the star
formation activity in the galaxy, and that the dust size distribution cannot be
reproduced by a standard galactic law, should be borne in mind when
interpreting the spectrum of primeval galaxies.Comment: 32 pages, 3 figures,accepted for publication in A
The Ultraviolet Radiation Environment Around M dwarf Exoplanet Host Stars
The spectral and temporal behavior of exoplanet host stars is a critical
input to models of the chemistry and evolution of planetary atmospheres. At
present, little observational or theoretical basis exists for understanding the
ultraviolet spectra of M dwarfs, despite their critical importance to
predicting and interpreting the spectra of potentially habitable planets as
they are obtained in the coming decades. Using observations from the Hubble
Space Telescope, we present a study of the UV radiation fields around nearby M
dwarf planet hosts that covers both FUV and NUV wavelengths. The combined
FUV+NUV spectra are publically available in machine-readable format. We find
that all six exoplanet host stars in our sample (GJ 581, GJ 876, GJ 436, GJ
832, GJ 667C, and GJ 1214) exhibit some level of chromospheric and transition
region UV emission. No "UV quiet" M dwarfs are observed. The bright stellar
Ly-alpha emission lines are reconstructed, and we find that the Ly-alpha line
fluxes comprise ~37-75% of the total 1150-3100A flux from most M dwarfs; >
10^{3} times the solar value. The F(FUV)/F(NUV) flux ratio, a driver for
abiotic production of the suggested biomarkers O2 and O3, is shown to be ~0.5-3
for all M dwarfs in our sample, > 10^{3} times the solar ratio. For the four
stars with moderate signal-to-noise COS time-resolved spectra, we find UV
emission line variability with amplitudes of 50-500% on 10^{2} - 10^{3} s
timescales. Finally, we observe relatively bright H2 fluorescent emission from
four of the M dwarf exoplanetary systems (GJ 581, GJ 876, GJ 436, and GJ 832).
Additional modeling work is needed to differentiate between a stellar
photospheric or possible exoplanetary origin for the hot (T(H2) \approx
2000-4000 K) molecular gas observed in these objects.Comment: ApJ, accepted. 16 pages, 10 figures. On-line data at:
http://cos.colorado.edu/~kevinf/muscles.htm
Observations of the Sunyaev-Zel'dovich effect at high angular resolution towards the galaxy clusters A665, A2163 and CL0016+16
We report on the first observation of the Sunyaev-Zel'dovich effect with the
Diabolo experiment at the IRAM 30 metre telescope. A significant brightness
decrement is detected in the direction of three clusters (Abell 665, Abell 2163
and CL0016+16). With a 30 arcsecond beam and 3 arcminute beamthrow, this is the
highest angular resolution observation to date of the SZ effect.Comment: 23 pages, 8 figures, 6 tables, accepted to New Astronom
Hubble Space Telescope Transmission Spectroscopy of the Exoplanet HD 189733b: High-altitude atmospheric haze in the optical and near-UV with STIS
We present Hubble Space Telescope optical and near-ultraviolet transmission
spectra of the transiting hot-Jupiter HD189733b, taken with the repaired Space
Telescope Imaging Spectrograph (STIS) instrument. The resulting spectra cover
the range 2900-5700 Ang and reach per-exposure signal-to-noise levels greater
than 11,000 within a 500 Ang bandwidth. We used time series spectra obtained
during two transit events to determine the wavelength dependance of the
planetary radius and measure the exoplanet's atmospheric transmission spectrum
for the first time over this wavelength range. Our measurements, in conjunction
with existing HST spectra, now provide a broadband transmission spectrum
covering the full optical regime. The STIS data also shows unambiguous evidence
of a large occulted stellar spot during one of our transit events, which we use
to place constraints on the characteristics of the K dwarf's stellar spots,
estimating spot temperatures around Teff~4250 K. With contemporaneous
ground-based photometric monitoring of the stellar variability, we also measure
the correlation between the stellar activity level and transit-measured
planet-to-star radius contrast, which is in good agreement with predictions. We
find a planetary transmission spectrum in good agreement with that of Rayleigh
scattering from a high-altitude atmospheric haze as previously found from HST
ACS camera. The high-altitude haze is now found to cover the entire optical
regime and is well characterised by Rayleigh scattering. These findings suggest
that haze may be a globally dominant atmospheric feature of the planet which
would result in a high optical albedo at shorter optical wavelengths.Comment: 14 pages, 14 figures, 4 tables, accepted to MNRAS, revised version
has minor change
On the Correlation Between CO Absorption and Far-Ultraviolet Non-Linear Extinction Toward Galactic OB Stars
A sample of 59 sight lines to reddened Galactic OB stars was examined for
correlations of the strength of the CO Fourth Positive (A - X) absorption band
system with the ultraviolet interstellar extinction curve parameters. We used
archival high-dispersion NEWSIPS IUE spectra to measure the CO absorption for
comparison to parametric fits of the extinction curves from the literature. A
strong correlation with the non-linear far-UV curvature term was found with
greater absorption, normalized to E(B-V), being associated with more curvature.
A weaker trend with the linear extinction term was also found. Mechanisms for
enhancing CO in dust environments exhibiting high non-linear curvature are
discussed.Comment: 10 pages, including 6 figures. LaTeX2e (emulateapj5.sty). To appear
in ApJ, Sep 20, 200
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