Externally-Driven Gravitational Torques on Disk/Star Systems
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**Session 68 -- Star Formation**
*Oral presentation, Thursday, 10:30-12:00, Zellerbach Playhouse Room*

## [68.01] Externally-Driven Gravitational Torques on Disk/Star Systems

*E.C. Ostriker (UCB)*
Using linear perturbation theory, I investigate the torques
between a centrifugally-supported gas/dust disk surrounding a
young star and a second star. I
consider both the cases of a binary companion orbiting beyond the disk,
and a passing member of a stellar cluster. Extending the Goldreich-Tremaine
theory of resonant wave excitation, I develop a theory for
excitation of one-armed disturbances in the broad near-inner
resonance of a Keplerian potential. These waves can be important when
excitation of $m\ge 2$-armed waves is impossible (or much reduced).
This occurs in a bound system when an annulus is cleared in the disk
around the binary companion, like the gaps seen near moons in
Saturn's rings.
The external torque on the disk is negative, encouraging
accretion. I compute numerically and analytically
accretion timescales $t_{\rm acc}$ in the disk due to
near-resonantly driven waves. For the example of a
$0.5 {\rm M}_{\sun}$ disk in a solar-mass scale binary with separation 100 AU,
$t_{\rm acc}
\sim 10^7$ yrs, with $t_{\rm acc}$ proportional to
the binary period.

For unbound systems, I investigate the angular momentum $L$ and energy
transferred
during parabolic perturber passages, which are
most common for
typical disk sizes and cluster velocity dispersions.
I compute the torques for arbitrary inclination angle and longitude of the
pertuber's orbit by integrating
over all horizontal and vertical resonances. I present both numerical
computations and analytic formulae
for cases with closest perturber approach $x_{\rm min}$ beyond two disk
radii $R_{\rm D}$. I find that $\Delta L$ transferred to the disk is
negative, inducing accretion.
The fractional
angular momentum removed from a disk is $\sim 10\%$ at
$x_{\rm min}/R_{\rm D}=2$, averaged over orbit orientations.
But the applied torque drops off exponentially with
increasing $x_{\rm min}/R_{\rm D}$, due to the mismatch of high disk
rotation frequency and low perturber orbital frequency.
When the incoming perturber's orbit is polar or retrograde, energy is removed
from the orbit and given to the disk, permitting capture.
The probability of capture is quite small, however,
because the positive energy transferred to the disk in an encounter
is a tiny fraction of the typical kinetic energy of a star in a cluster.

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