VLT J-to M 0 -band adaptive optics observations of the Circinus galaxy on parsec scales resolve a central bright K s -band source with a FWHM size of 1:9 AE 0:6 pc. This source is only visible at wavelengths longward of 1.6 m and coincides in position with the peak of the [Si vii] 2.48 m coronal line emission. With respect to the peak of the central optical emission, the source is shifted by $0B15 (2.8 pc) to the southeast. Indeed, the K s -band source defines the vertex of a fairly collimated beam that extends for $10 pc and is seen in both continuum light shortward of 1.6 m and in H line emission. The source also lies at the center of a $19 pc size [Si vii] ionization bicone. Identifying this source as the nucleus of Circinus, its size is compatible with a putative parsec-scale torus. Its spectral energy distribution, characterized by a prominent narrow peak, is compatible with a dust temperature of 300 K. Hotter dust within a 1 pc radius of the center is not detected. The active galactic nucleus (AGN) luminosity required to heat this dust is in the range of X-ray luminosities that have been measured toward the central source. This in turn supports the existence of highly obscuring material, with column densities of 10 24 cm À2 , that must be located within 1 pc of the core.
A model of AG Peg is presented, focusing on the physical conditions in the emission nebulae. The model accounts in a consistent way for photoionization by the star and ionization by shocks. The SUMA code is used in the calculations of the spectra. We show that two regions contribute to the UV and optical line spectra. The broad lines are emitted from photoionized gas surrounding the hot star, while narrow lines are emitted by shocked gasÈwhich is also reached by the ionizing Ñux from the hot starÈnear the red giant.At an early phase (/ \ 2.33) the emitting region is between the stars but already very close to the red giant. The nebula surrounding the hot star is not exactly symmetric. At a later phase (/ \ 7.05) the shock created by "" head-on-back ÏÏ collision of the winds propagates outward and slightly accelerates throughout the giant atmosphere characterized by a decreasing slope of the density. The strong shock caused by head-on collision of the winds from the two stars cannot be recognized in the UV-optical line spectrum.The spectral energy distribution of the continuum is well Ðtted by blackbody emissions from the stars. The Ðt of the optical-UV observed continuum at the early phase presents some problems connected with the reddening correction of the data. On the other hand, the Ðt of the data at a later phase is consistent with the parameters deduced by the modeling of the system. The far-IR data are well Ðtted by reradiation by dust, which could not be recognized without modeling, and indicate a dust-to-gas ratio similar to the Galactic one.
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