Evaporation of Dusty Molecular Clouds in AGN

Details Evaporation of Dusty Molecular Clouds in AGN Evaporation of Dusty Molecular Clouds in AGN

Dec 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994aas...185.2003p&link_type=abstract

American Astronomical Society, 185th AAS Meeting, #20.03; Bulletin of the American Astronomical Society, Vol. 26, p.1342

Astronomy and Astrophysics

Astronomy

Scientific paper

We have modeled the evaporation of dusty molecular clouds exposed to the strong ionizing continuum of an Active Galactic Nucleus. This is important to unified models of AGN which include a torus of dusty molecular clouds within a parsec or so of the active nucleus. Deep within an evaporating cloud, where the UV/soft X-ray optical depth is high, the gas is mostly molecular and is close to thermal equilibrium at =~ 10(3) K. This molecular core is surrounded by a layer of partially ionized atomic gas at ~ 10(4) K. At some radius the ionization parameter in the cloud reaches the critical value where it can no longer maintain this ~ 10(4) K equilibrium. At this point the cloud makes the transition to a fully ionized, X-ray heated wind. The dust grains are carried along with this wind until they are destroyed by sputtering. We model the wind as a spherically symmetric, steady state, trans-sonic flow with zero pressure at infinity. We take into account the effects of dust opacity including radiation force. We find evaporation lifetimes t_evap =~ 1.2times 10(5) (F/10(7) ergs cm(-2) s(-1})({0.19)) (M/0.1 M_sun)(0.40) yr , Where F is the ionizing flux incident on the cloud and M is the mass of the cloud. This is comparable to the drift time-scale due to loss of orbital energy from inelastic cloud-cloud collisions in the torus but is longer than the typical orbital time-scale. Therefore, clouds on eccentric orbits will approach the active nucleus much closer than the inner edge of the torus set by cloud evaporation. The cloud itself is primarily pressurized by radiation pressure and therefore has an ionization structure independent of cloud parameters. We calculate the set of emission lines produced by evaporating clouds and discuss their use as diagnostics of the torus.

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