M. Contini

The Shell of the Recurrent Nova T Pyxidis: A Model Based on Shocks

A model for the shell of the recurrent nova T Pyxidis is presented, consistent with the bulk of observational data on this object. Evolutionary calculations of thermonuclear runaways on the surface of an accreting white dwarf that simulate the observed outburst characteristics of this nova—recurrence time, rise time, time of decline, luminosity—provide the mass of the ejected shell, its composition, and the effective temperature and luminosity of the nova in quiescence (for a white dwarf mass of 1.25 M☉, a core temperature of 107 K, and an accretion rate of 10-7 M☉ yr-1). These results are used as input parameters for the SUMA code, which simulates the physical conditions of an emitting gaseous cloud under the combined effects of photoionization and shocks. The shell model involves two emitting regions besides the central star (which is the main UV radiation source), both resulting from compressed gas behind a shock front. The shocks are assumed to be produced by the collision between two extended shells ejected at consecutive outbursts. As a result of the collision, a secondary blast wave propagates outward into the older ejecta, while a reflected (reverse) shock wave propagates back through the new ejecta. Applying the SUMA code to each zone, we find that different lines are emitted in the two zones, and combined, they account for the entire observed line spectrum. The absolute fluxes in different spectral ranges agree with those derived from observations, assuming a distance of ~2 kpc. The derived abundances imply that mixing has taken place between the nova ejecta and interstellar material. Nevertheless, helium is overabundant and N/O is high. The model accounts for the observed structure of the nebula surrounding T Pyx, including the inner shell, the stagnant outer shell, and the extended faint region beyond it.

The continuum and narrow-line region of the narrow-line Seyfert 1 galaxy Mrk 766

We present the first spectroscopic observations in the interval 0.8-4.0 microns, complemented with HST/UV and optical spectroscopy, of the narrow line Seyfert 1 galaxy Mrk766. The NIR spectrum is characterized by permitted lines of HI, HeI, HeII and FeII, and forbidden lines of [SII], [SIII] and [FeII] among others. High ionized species such as [SiIX], [SiX], [SIX] and [MgVII] are also observed. The continuum has a complex shape, with contribution of the central engine, circumnuclear stellar population and dust. This last component is evidenced by the presence of an excess of emission peaking at 2.25 microns, fitted by blackbody function with T_bb=1200K. That temperature is close to the evaporation temperature of graphite grains. As such, it provides strong evidence of hot dust, probably very close to the nucleus. Consistent modeling of the line and broad band continuum spectrum by composite models, which account for the photoionizing flux of the central engine and shocks, shows that the shock velocities are between 100 and 500 km/s, the preshock densities between 100 and 1000 cm^-3 and the radiation fluxes from the active centre between 10^9 and 5x10^12 photons cm^-2 s^-1 eV^-1 at 1 Ryd with spectral indices

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

\alpha_{UV}

The complex structure of low-luminosity active galactic nuclei: NGC 4579

=-1.5 and

The infrared continuum of active galactic nuclei

\alpha_X

Modeling RR Telescopii through the Evolution of the Spectra

=-0.4. Adopting silicon grains, dust-to-gas ratios are between 10^-{6} and 4x10

Dust-to-gas ratios in the starburst regions of luminous infrared galaxies

^{-4}

The Interpretation of the Emission Spectra of the Seyfert 2 Galaxy NGC 7130: Determination of the Active Galactic Nucleus and Starburst Contributions

by mass. The emitting clouds are at an average distance of 160 pc from the centre, with high velocity clouds closer and low velocity clouds farther from the centre. The N/H relative abundance could be twice solar. In constrast, Fe is depleted from the gaseous phase by a factor >2. Ratios of calculated to observed line ratios to Hbeta indicate an average contribution of the broad line region to the observed Hbeta of about 40%.

An analysis of infrared emission spectra from the regions near the Galactic Centre

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.Simultaneous modeling of the line and continuum emission from the nuclear region of the Circinus galaxy is presented. Composite models that 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. Models that fit the continuum energy distribution of Circinus are used to constrain all possible models suitable for the line emission; they are all, together, aimed to converge to a most probable representation of the central emission region of Circinus. 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 dust-to-gas ratios above 10-12. Their large velocities, up to 1500 km s-1, place them very close to the active center. Second, the derived size of the line-emitting region (~13 pc) is well in agreement with the observed limits for the coronal (<10 pc) and narrow-line regions (~30 pc) of Circinus.

The Optical-Ultraviolet Continuum of Seyfert 2 Galaxies

We have modelled the low-luminosity active galactic nucleus (AGN) NGC 4579 by explaining both the continuum and the line spectra observed with different apertures. It was found that the nuclear emission is dominated by an AGN such that the flux from the active centre (AC) is relatively low compared with that of the narrow emission-line region (NLR) of Seyfert galaxies. However, the contribution of a young starburst cannot be neglected, as well as that of shock-dominated clouds with velocities of 100, 300 and 500 km s -1 . A small contribution from an older starburst with an age of 4.5 Myr, probably located in the external nuclear region, is also found. HII regions appear in the extended regions (∼1 kpc), where radiation and shock-dominated clouds with V s = 100 km s -1 prevail. The continuum SED of NGC 4579 is characterized by the strong flux from an old stellar population. Emissions in the radio range show synchrotron radiation from the base of the jet outflowing from the accretion disc within 0.1 pc from the active centre. Radio emission within intermediate distances (10-20 pc) is explained by the bremsstrahlung from gas downstream of low-velocity shocks (V s = 100 km s -1 ) reached by a relatively low radiation flux from the AC. In extended regions (> 100 pc) the radio emission is synchrotron radiation created by the Fermi mechanism at the shock front. The shocks are created by collision of clouds with the jet. All types of emissions observed at different radius from the centre can be reconciled with the presence of the jet.