Upper limit on the gas density in the β Pictoris system

The effect of gas drag on dust dynamics

¹ Stockholm Observatory, Albanova Universitetcentrum, 10691 Stockholm, Sweden e-mail: philippe.thebault@obspm.fr
² Observatoire de Paris, Section de Meudon, 92195 Meudon Principal Cedex, France
³ Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands
⁴ Laboratoire d'Astrophysique de l'Observatoire de Grenoble, BP 53, 38041 Grenoble Cedex 9, France

Received: 22 December 2004
Accepted: 16 February 2005

Abstract

We investigate the effect of gas drag on the dynamics of the dust particles in the edge-on β Pictoris disc to derive an upper limit on the mass of gas in this system. Our study is motivated by the large uncertainties on the amount of gas in the β Pictoris disc currently found in the literature. The dust particles are assumed to originate from a colliding annulus of planetesimals peaking around 100 AU from the central star. We consider the various gas densities that have been inferred from independent observing techniques and we discuss their impact on dust dynamics and on the disc profile in scattered light along the midplane. We show that the observed scattered light profile of the disc cannot be properly reproduced if the hygrogen gas number density at 117 AU exceeds 10⁴ cm^-3. This corresponds to an upper limit on the total gas mass of about and thus to a gas to dust mass ratio smaller than 1. Our approach therefore provides an independent diagnostic of gas depletion in the β Pictoris system relative to the dust disc. Such an approach could also be used to constrain the gas content of recently identified systems like the edge-on disc around AU Mic.