54 research outputs found
Dust-to-gas ratios in the starburst regions of luminous infrared galaxies
We investigate the properties of dust and dust-to-gas ratios in different
starburst regions of luminous infrared galaxies (LIGs). We refer to the sample
of seven LIGs recently observed in the mid-infrared by Soifer et al. (2001)
using the Keck telescopes with spatial resolution approaching the diffraction
limit. These seven objects are among the closest LIGs and have been classified
as starburst galaxies from optical spectroscopy. Our goal consists in modelling
the continuum spectral energy distribution (SED) of each galaxy, particularly
in the infrared range. Models are further constrained by observed emission-line
ratios in the optical range. The multi-cloud models consistently account for
the coupled effect of shock, photoionization by hot stars, and diffuse
secondary radiation from the shock-heated gas. Emission from clouds in the
neighbourhood of evolved starbursts and with high shock velocities (~ 500 km/s)
explains both the bremsstrahlung and reradiation from dust in the mid-infrared.
Clouds with lower velocity (~ 100 km/s) and corresponding to younger starbursts
also contribute to both line and continuum spectra. Both low- and high-velocity
clouds are thus present in nearly all the sample galaxies. For all the
galaxies, an old stellar population is revealed by black body emission in the
optical-NIR range. Dust-to-gas ratios vary in different regions of individual
galaxies.Comment: 17 pages, 16 figures, and 12 tables. Accepted for publication in
MNRA
R Aquarii spectra revisited by SUMA
We analyse the optical spectra and the UV spectral evolution of the jets and
of the HII region inside the R Aquarii binary system by the code SUMA which
consistently accounts for shock and photoionization. The temperature of the hot
star results 80,000 K as for a white dwarf. We find that the shock velocity in
the NE jet increased between 1983 and 1989. The spectral evolution between 1989
and 1991 of the SW jet indicates that a larger contribution from low
density-velocity matter affects the 1991 spectra. The evolution of the UV
spectra from 8/11/1980 to 26/5/1991 in the HII region indicates that the
reverse shock is actually a standing shock. The results obtained by modelling
the line spectra are cross-checked by the fit of the continuum SED. It is found
that a black-body temperature of 2800 K reproduces the radiation from the red
giant. A black-body emission component corresponding to 1000 K is emitted by
dust in the surrounding of the red giant. Model calculations confirm that the
radio emission is of thermal origin. We found that the NE jet bulk emission is
at a distance of about 2 (15) cm from the internal system, while the distance
of the SW jet bulk is about 6 (14) cm. The distance of the reverse shock from
the hot source in the internal region is < 9 (13) cm.Comment: 9 pages, MN LaTeX style (including 6 Tables) + 5 PostScript figures.
To appear in The Monthly Notices of the Royal Astronomical Societ
Shock-Generated Turbulence In the Innermost 50 pc of the Galaxy Center
The center of the Milky Way galaxy (MW) is an extreme environment which
contains a massive black hole surrounded by a very dense star cluster, two
other adjacent star clusters, and a giant molecular cloud which would serve as
an incubator to a new generation of stars. The gas and dust in its vicinity are
denser by 2-3 orders of magnitude than in other locations in the MW. This is
also the case with the magnetic field. The kinematics of the gas is
characterized by apparently random, supersonic flows. In this paper we provide
observational evidence for the existence of a supersonic turbulence, most
likely generated by the shock waves. Moreover, the mere existence of turbulence
and its characteristics are shown to be instrumental in testing the validity
and consistency of theoretical modeling of the spectra of the gas filaments.Comment: Talk presented in "Waves and Instabilities in Space and Astrophysical
Plasmas" June 19-24 2011, Eilat, Israel. To appear in "Waves and
Instabilities in Space and Astrophysical Plasmas", P.-L. Sulem and M. Mond,
editors, AIP
The symbiotic system AG Draconis. Soft X-ray bremsstrahlung from the nebulae
The modeling of UV and optical spectra emitted from the symbiotic system AG
Draconis, adopting collision of the winds, predicts soft X-ray bremsstrahlung
from nebulae downstream of the reverse shock with velocities > 150 km/s and
intensities comparable to those of the white dwarf black body flux. At
outbursts, the envelop of debris, which corresponds to the nebula downstream of
the high velocity shocks (700-1000 km/s) accompanying the blast wave, absorbs
the black body soft X-ray flux from the white dwarf, explains the broad
component of the H and He lines, and leads to low optical-UV-X-ray continuum
fluxes. The high optical-UV flux observed at the outbursts is explained by
bremsstrahlung downstream of the reverse shock between the stars. The depletion
of C, N, O, and Mg relative to H indicates that they are trapped into dust
grains and/or into diatomic molecules, suggesting that the collision of the
wind from the white dwarf with the dusty shells, ejected from the red giant
with about 1 year periodicity, leads to the U-band fluctuations during the
major bursts.Comment: 12 pages, 5 figures, 2 tables. New Astronomy, in pres
An analysis of infrared emission spectra from the regions near the Galactic Center
We present consistent modelling of line and continuum IR spectra in the
region close to the Galactic center. The models account for the coupled effect
of shocks and photoionization from an external source. The results show that
the shock velocities range between 65 and 80 km/s, the pre-shock densities
between 1 cm-3 in the ISM to 200 cm-3 in the filamentary structures. The
pre-shock magnetic field increases from 5. 10^{-6} gauss in the surrounding ISM
to ~8. 10^{-5} gauss in the Arched Filaments. The stellar temperatures are
~38000 K in the Quintuplet cluster and ~27000 K in the Arches Cluster. The
ionization parameter is relatively low (<0.01) with the highest values near the
clusters, reaching a maximum >0.01 near the Arches Cluster. Depletion from the
gaseous phase of Si is found throughout the whole observed region, indicating
the presence of silicate dust. Grains including iron, are concentrated
throughout the Arched Filaments. The modelling of the continuum SED in the IR
range, indicates that a component of dust at temperatures of ~100-200 K is
present in the central region of the Galaxy. Radio emission appears to be
thermal bremsstrahlung in the E2-W1 filaments crossing strip, however a
synchrotron component is not excluded. More data are necessary to resolve this
questions.Comment: accepted for publication in the MNRA
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