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

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

Gas and Dust Emission from the Nuclear Region of the Circinus Galaxy

Simultaneous modeling of the line and continuum emission from the nuclear region of the Circinus galaxy is presented. Composite models which include the combined effect of shocks and photoionization from the active center and from the circumnuclear star forming region are considered. The effects of dust reradiation, bremsstrahlung from the gas and synchrotron radiation are treated consistently. The proposed model accounts for two important observational features. First, the high obscuration of Circinus central source is produced by high velocity and dense clouds with characteristic high dust-to-gas ratios. Their large velocities, up to 1500 km\s, place them very close to the active center. Second, the derived size of the line emitting region is well in agreement with the observed limits for the coronal and narrow line region of Circinus.Comment: 36 pages, LaTex (including 4 Tables and 9 figures), $$ removed from Abstract To appear in "The Astrophysical Journal