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From: Richard Alexander  rda@strw.leidenuniv.nl 
To: gcnews@aoc.nrao.edu
Subject: New paper (MNRAS, in press)

\title{Binary formation and mass function variations in fragmenting discs 
with short cooling times}
\author{R.D.Alexander (1,2), P.J.Armitage (2,3) and J.Cuadra (3)}

\inst{1 - Sterrewacht Leiden, Universiteit Leiden, Niels Bohrweg 2, 2300 
RA, Leiden, 
the Netherlands
2 - JILA, University of Colorado, Boulder, CO 80309-0440, USA
3 - Department of Astrophysical and Planetary Sciences, University of 
Colorado, Boulder, CO 80309-0391, USA
}

\begin{abstract}
Accretion discs at sub-pc distances around supermassive black holes are likely 
to cool rapidly enough that self-gravity results in fragmentation. Here, we 
use high-resolution hydrodynamic simulations of a simplified disc model to 
study how the outcome of fragmentation depends upon numerical resolution and 
cooling time, and to investigate the incidence of binary formation within 
fragmenting discs.  We investigate a range of cooling times, from the 
relatively long cooling time-scales that are marginally unstable to 
fragmentation down to highly unstable cooling on a time-scale that is shorter 
than the local dynamical time. The characteristic mass of fragments decreases 
with reduced cooling time, though the effect is modest and dependent upon 
details of how rapidly bound clumps radiate. We observe a high incidence of 
capture binaries, though we are unable to determine their final orbits or 
probability of survival. The results suggest that faster cooling in the parent 
disc results in an increased binary fraction, and that a high primordial 
binary fraction may result from disc fragmentation.  We discuss our results in 
terms of the young massive stars close to the Galactic Centre, and suggest 
that observations of some stellar binaries close to the Galactic Centre remain 
consistent with formation in a fragmenting accretion disc.
\end{abstract}
%arxiv:0807.1731