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Astron. Astrophys. 341, 385-398 (1999)


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Influence of the metallicity and of the irradiation on the structure of accretion disks around massive black holes

Suzy Collin  * 1 and Jean-Marc Huré 1,2

1 Observatoire de Paris, Section de Meudon, Place Janssen, F-92195 Meudon, France
2 Université Paris 7 (Denis Diderot), 2 Place Jussieu, F-75251 Paris Cedex 05, France

Received 25 March 1998 / Accepted 20 October 1998

Abstract

We discuss the structure of the outer regions of accretion disks around massive black holes, up to about one parsec from the center. In the gravitationally stable region, we assume the standard [FORMULA]-prescription for the viscosity but we take into account self-gravitation. Beyond about 104 gravitational radii, self-gravitation is larger than central gravitation, and the disk becomes gravitationally unstable. We then assume that it is maintained in a state of marginal instability such that the Toomre parameter stays of the order of a few units, as proposed by Paczynski (1978). The specificity of this prescription is that the midplane density is imposed as a function of the radius and depends only on the central mass. We demonstrate that the gas metallicity is of fundamental importance for the disk structure by considering two extreme cases: a pure hydrogen-helium mixture (i.e. a primeval gas), and a gas with solar abundances. In particular, the gravitationally unstable regions are optically thin in the case of primeval abundances while they remain optically thick for solar abundances. A consequence is that the midplane temperature is almost constant in the case of primeval abundances, and the disk is "flaring". Irradiation of the disk by the central UV-X source should thus be taken into account. We study the influence of this irradiation on the structure of a flaring disk, as well as in several other cases, and we show that it does not modify strongly the radial profile. We compute the column density of the ionized and atomic layers due to the irradiation, and show that they are much smaller than the column density of the molecular layer. Both for the non irradiated and for the irradiated disk, and for the primeval and for the solar metallicity cases, we give analytical expressions for the scale height and the midplane temperature which can be helpful to approximate the radial profile of an accretion disk in the molecular region. When applied to the masing disk of NGC 4258, these expressions show that the irradiation flux becomes dominant between 0.1 and 1 pc, depending on the value of accretion rate.

Key words: accretion, accretion disks – instabilities – galaxies: active – galaxies: nuclei – galaxies: quasars: general

* Research associate at Institut d'Astrophysique, Paris

Send offprint requests to: Suzy Collin (Observatoire de Meudon)

© European Southern Observatory (ESO) 1999

Online publication: December 4, 1998

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