Astron. Astrophys. 348, 557-569 (1999)

On the HR 4796 A circumstellar disk ^*

J.C. Augereau ¹, A.M. Lagrange ¹, D. Mouillet ¹, J.C.B. Papaloizou ² and P.A. Grorod <sup>1

1</sup> Laboratoire d'Astrophysique de l'Observatoire de Grenoble, Université J. Fourier / CNRS, B.P. 53, F-38041 Grenoble Cedex 9, France
² Astronomy Unit, School of Mathematical Sciences, Queen Mary and Westfield College, Mile End Road, London E1 4NS, UK

Received 18 December 1998 / Accepted 14 June 1999

Abstract

We investigate in details the properties of the disk surrounding the 8 Myr old star HR 4796 A, one of the few stars bringing precious clues to better understand the scenario which lead to planetary system formation. We propose a model able to reproduce all the available observations: the full spectral energy distribution from the mid-infrared to the millimeter wavelengths, resolved scattered light and thermal emission observations. We show that the circumstellar matter splits into two dust components: a cold annulus, peaked at 70 AU from the star, made of ISM-like grains (amorphous composition, porosity 0.6) larger than 10 µm and a population of hot dust close to the star (at about 9 AU) made of comet-like grains (crystalline composition, porosity 0.97). Both dust populations are highly collisional and the grain size distribution in the cold annulus is found to be cut-off by radiation pressure. At 70 AU, bodies as large as a few meters are required to fit the data leading to a minimum disk mass of a few Earth masses and to a gas to dust ratio less than 1. We discuss aftewards some implications on the disk structure and effects of larger bodies.

Key words: stars: circumstellar matter stars: individual: HR 4796 A

* Based on observations collected at the European Southern Observatory, La Silla, Chile

Send offprint requests to: J.C. Augereau

Correspondence to: augereau@obs.ujf-grenoble.fr

© European Southern Observatory (ESO) 1999

Online publication: July 26, 1999

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