Date:
Tue, 13/04/201012:30-13:30
Location:
Kaplun Bldg, seminar room, 2nd floor
Characterising gravitational instabilities inprotostellar and accretion discs
The development of gravitational instabilities in a disc might be the leading process of angular momentum transport in the early phases of star formation, as well as in other contexts where disc accretion plays a role. In this talk, I will first describe some simple analytical models of
self-gravitating accretion discs. I will then describe some new results that characterise the properties of gravitational perturbations as a function of the gas cooling rate. I will show that self-gravitating instabilities can saturate due to thermal effects at an amplitude proportional to
$1/\sqrt{\Omega t_{\rm cool}}$, where $t_{\rm cool}$ is the cooling time and $\Omega$ the angular frequency in the disc. Such saturation phenomenon can be simply derived by the condition that spiral density waves efficiently dissipate when their phase velocity with respect to the background flow becomes sonic. An important aspect of the process is related to the
possibility of modeling such transport in terms of an effective, local viscosity. I will thus discuss the conditions under which such local description is adequate for the transport induced by gravitaional instablities.
Finally, I will describe some simple models of self-regulated protostellar discs, consistent with the above numerical results. I will then discuss the relevance of such models in the process of the formation of planetesimals. In particular, I will show that planetesimals can form very efficiently early in the evolution of the protostellar disc, but in a spatial region confined to the outermost parts of the disc, roughly coincident with the location of the Kuiper belt in our Solar System. This can have important implications for the dynamics and the observable properties of both protostellar and debris discs.
The development of gravitational instabilities in a disc might be the leading process of angular momentum transport in the early phases of star formation, as well as in other contexts where disc accretion plays a role. In this talk, I will first describe some simple analytical models of
self-gravitating accretion discs. I will then describe some new results that characterise the properties of gravitational perturbations as a function of the gas cooling rate. I will show that self-gravitating instabilities can saturate due to thermal effects at an amplitude proportional to
$1/\sqrt{\Omega t_{\rm cool}}$, where $t_{\rm cool}$ is the cooling time and $\Omega$ the angular frequency in the disc. Such saturation phenomenon can be simply derived by the condition that spiral density waves efficiently dissipate when their phase velocity with respect to the background flow becomes sonic. An important aspect of the process is related to the
possibility of modeling such transport in terms of an effective, local viscosity. I will thus discuss the conditions under which such local description is adequate for the transport induced by gravitaional instablities.
Finally, I will describe some simple models of self-regulated protostellar discs, consistent with the above numerical results. I will then discuss the relevance of such models in the process of the formation of planetesimals. In particular, I will show that planetesimals can form very efficiently early in the evolution of the protostellar disc, but in a spatial region confined to the outermost parts of the disc, roughly coincident with the location of the Kuiper belt in our Solar System. This can have important implications for the dynamics and the observable properties of both protostellar and debris discs.