849 research outputs found
The Infrared Continuum Radiation of NGC1808 : A PAH and Polarisation Study
The paper is devoted to the understanding of the infrared emission of nuclear
regions in galaxies. a) ISO data of NGC1808 are presented: spectro--photometry
from 5.1 to 16.4 mic., a 25"x25" map at 6 mic. and 170mic. photometry. b) The
data are complemented by a polarization measurement at 170 mic. (2.5+/-0.4% at
position angle 94+/-5deg) and a map at 6 mic. In the map, the degree of
polarisation goes up to 20% in the outer regions. We argue that the
polarisation is produced by emission of big grains and exclude very small
grains and PAHs or scattering and extinction. c) The mid infrared spectrum
shows, beside the main emission bands, a so far unknown plateau of PAH features
in the >13 mic.region. d) The total spectrum can be fit under the assumption of
optically thin emission. However, such a model fails to reproduce the 25mic.
point and implies that the mid infrared is due to very small grains and PAHs.
These particles would then also have to be responsible for the 6 mic.
polarisation, which is unlikely. e) To avoid these difficulties, we
successfully turn to a radiative transfer model whose major feature is the
existence of (hot spots) produced by the dust clouds around OB stars. We
demonstrate the decisive influence on the mid infrared spectrum of both the
PAHs and the hot spots.Comment: 11 pages, 6 figures, accepted by A&
Properties of grains derived from IRAS observations of dust
The authors used the results of Infrared Astronomy Satellite (IRAS) observations of diffuse medium dust to develop a theoretical model of the infrared properties of grains. Recent models based entirely on traditional observations of extinction and polarization include only particles whose equilibrium temperatures do not exceed 20 K in the diffuse interstellar medium. These classical grains, for which the authors have adopted the multipopulation model developed by Hong and Greenberg (1980), can explain only the emission in the IRAS 100 micron band. The measurements at shorter wavelengths (12, 25 and 60 microns) require two new particle populations. Vibrational fluorescence from aromatic molecules provides the most likely explanation for the emission observed at 12 microns, with polycyclic aeromatic hydrocarbons (PAHs) containing about 10 percent of cosmic carbon. A simplified model of the emission process shows that PAH molecules can also explain most of the emission measured by IRAS at 25 microns. The authors identified the warm particles responsible for the excess 60 microns emission with small (a approx. equals 0.01 microns) iron grains. A compilation of the available data on the optical properties of iron indicates that the diffuse medium temperature of small iron particles should be close to 50 K and implies that a large, possibly dominant, fraction of cosmic iron must be locked up in metallic particles in order to match the observed 60 microns intensities. The model matches the infrared fluxes typically observed by IRAS in the diffuse medium and can also reproduce the infrared surface brightness distribution in individual clouds. In particular, the combination of iron and classical cool grains can explain the surprising observations of the 60/100 microns flux ratio in clouds, which is either constant or increases slightly towards higher opacities. The presence of metallic grains has significant implications for the physics of the interstellar medium, including catalytic H2 formation, for which iron grains could be the main site; differences in depletion patterns between iron and other refractory elements (Mg, Si); and superparamagnetic behavior of large grains with embedded iron clusters giving rise to the observed high degree of alignment by the galactic magnetic field
Far infrared observations of pre-protostellar sources in Lynds 183
Using ISOPHOT maps at 100 and 200um and raster scans at 100, 120, 150 and
200um we have detected four unresolved far-infrared sources in the high
latitude molecular cloud L183. Two of the sources are identified with 1.3mm
continuum sources found by Ward-Thompson et al. and are located near the
temperature minimum and the coincident column density maximum of dust
distribution. For these two sources, the ISO observations have enabled us to
derive temperatures (about 8.3 K) and masses (about 1.4 and 2.4 solar masses).
They are found to have masses greater than or comparable to their virial masses
and are thus expected to undergo gravitational collapse. We classify them as
pre-protostellar sources. The two new sources are good candidates for
pre-protostellar sources or protostars within L183.Comment: 12 pages, 7 Postscript figures, 1 JPEG figure. Accepted for
publication in Astronomy & Astrophysic
Infrared properties of dust grains derived from IRAS observations
The analysis of several diffuse interstellar clouds observed by the Infrared Astronomy Satellite (IRAS) is presented. The 60/100 micron flux ratios appear to be nearly constant in clouds with up to 1 sup m visual extinction at the center. Observations of a highly regular cloud in Chamaeleon show that the 12/100 micron ratio peaks at an intermediate radial distance and declines towards the center of the cloud. These observations indicate that nonequilibrium emission accounts only for the 12 and 25 micron bands; strong emission observed at the 60 micron band is probably due to equilibrium thermal radiation. The correlation of the 12 micron emission with a red excess observed for a high latitude cloud, L1780, is shown to be consistent with the assumption that both features are due to fluorescence by the same molecular species
Dust Emissivity Variations In the Milky Way
Dust properties appear to vary according to the environment in which the dust
evolves. Previous observational indications of these variations in the FIR and
submm spectral range are scarce and limited to specific regions of the sky. To
determine whether these results can be generalised to larger scales, we study
the evolution in dust emissivities from the FIR to mm wavelengths, in the
atomic and molecular ISM, along the Galactic plane towards the outer Galaxy. We
correlate the dust FIR to mm emission with the HI and CO emission. The study is
carried out using the DIRBE data from 100 to 240 mic, the Archeops data from
550 mic to 2.1 mm, and the WMAP data at 3.2 mm (W band), in regions with
Galactic latitude |b| < 30 deg, over the Galactic longitude range (75 deg < l <
198 deg) observed with Archeops. In all regions studied, the emissivity spectra
in both the atomic and molecular phases are steeper in the FIR (beta = 2.4)
than in the submm and mm (beta = 1.5). We find significant variations in the
spectral shape of the dust emissivity as a function of the dust temperature in
the molecular phase. Regions of similar dust temperature in the molecular and
atomic gas exhibit similar emissivity spectra. Regions where the dust is
significantly colder in the molecular phase show a significant increase in
emissivity for the range 100 - 550 mic. This result supports the hypothesis of
grain coagulation in these regions, confirming results obtained over small
fractions of the sky in previous studies and allowing us to expand these
results to the cold molecular environments in general of the outer MW. We note
that it is the first time that these effects have been demonstrated by direct
measurement of the emissivity, while previous studies were based only on
thermal arguments.Comment: 16 pages, 6 figures, accepted in A&
Dust emission from high latitude cirrus clouds
In order to study dust emission from grains in the interstellar medium, the infrared properties were analyzed in a number of isolated high latitude dust clouds which contain no dominant internal heating sources. The clouds are spatially resolved, have a simple geometry, and are mapped in the IRAS bands at 12, 25, 60, and 100 microns. For a number of these clouds, extinction data (A sub B) were obtained from starcounts. A large part (30 to 50 percent) of the infrared radiation of the clouds in the IRAS wavelength range of 8 to 130 micron is emitted in the short wavelength bands at 12 and 25 micron. The 60/100 micron ratios for the integrated fluxes of the clouds have a typical value of 0.19 + or - 0.05
Far-infrared and molecular line observations of Lynds 183 - studies of cold gas and dust
We have mapped the dark cloud L183 in the far-infrared at 100um and 200um
with the ISOPHOT photometer aboard the ISO satellite. The observations make it
possible for the first time to study the properties of the large dust grains in
L183 without confusion from smaller grains. The observations show clear colour
temperature variations which are likely to be caused by changes in the emission
properties of the dust particles. In the cloud core the far-infrared colour
temperature drops below 12K. The data allow a new determination of the cloud
mass and the mass distribution. The mass within a radius of 10 arcmin from the
cloud centre is 25 Msun. We have mapped the cloud in several molecular lines
including DCO+(2-1) and H13CO+(1-0). These species are believed to be tracers
of cold and dense molecular material and we detect a strong anticorrelation
between the DCO+ emission and the dust colour temperatures. In particular, the
DCO+(2-1) emission is not detected towards the maximum of the 100um emission
where the colour temperature rises above 15K. The H13CO+ emission follows the
DCO+ distribution but CO isotopes show strong emission even towards the 100um
peak. A comparison of the DCO+ and C18O maps shows sharp variations in the
relative intensities of the species. Morphologically the 200um dust emission
traces the distribution of dense molecular material as seen e.g. in C18O lines.
A comparison with dust column density shows that C18O is depleted by a factor
of 1.5 in the cloud core. We present results of R- and B-band starcounts. The
extinction is much better correlated with the 200um than with the 100um
emission. Based on the 200um correlation at low extinction values we deduce a
value of ~17mag for the visual extinction towards the cloud centre.Comment: to be published in A&
Planck Observations of M33
We have performed a comprehensive investigation of the global integrated flux
density of M33 from radio to ultraviolet wavelengths, finding that the data
between 100 GHz and 3 THz are accurately described by a single modified
blackbody curve with a dust temperature of = 21.670.30 K
and an effective dust emissivity index of = 1.350.10,
with no indication of an excess of emission at millimeter/sub-millimeter
wavelengths. However, sub-dividing M33 into three radial annuli, we found that
the global emission curve is highly degenerate with the constituent curves
representing the sub-regions of M33. We also found gradients in
and across the disk of M33, with both
quantities decreasing with increasing radius. Comparing the M33 dust emissivity
with that of other Local Group members, we find that M33 resembles the
Magellanic Clouds rather than the larger galaxies, i.e., the Milky Way and M31.
In the Local Group sample, we find a clear correlation between global dust
emissivity and metallicity, with dust emissivity increasing with metallicity. A
major aspect of this analysis is the investigation into the impact of
fluctuations in the Cosmic Microwave Background (CMB) on the integrated flux
density spectrum of M33. We found that failing to account for these CMB
fluctuations would result in a significant over-estimate of
by 5 K and an under-estimate of by 0.4.Comment: Accepted for publication in MNRA
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