AAS 195th Meeting, January 2000
Session 79. Young Stars and Clusters
Display, Friday, January 14, 2000, 9:20am-6:30pm, Grand Hall

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[79.04] Protoplanetary Disks in Young Binaries: Testing Coplanarity

A. Donar (Wesleyan University), E. L. N. Jensen (Swarthmore College), R. D. Mathieu (U. Wisconsin-Madison)

We present K-band (2.2 \mum) polarimetry that resolves 18 T Tauri binaries and one triple system in the Taurus and Ophiuchus star forming regions. We observed systems with minimum projected separations of ~1\arcsec, which roughly corresponds to 125--140 AU, in order to determine the relative orientation of the circumstellar disks in each binary system. These binaries all have infrared excesses, suggesting that each system harbors at least one circumstellar disk. Scattered light from these disks is polarized, allowing us to deduce the position angle of the disk on the sky from the position angle of polarization even though our observations do not resolve the disks themselves. We detected measurable polarization (typically 0.5% to 2%, with typical uncertainty 0.1%) from both stars in 16 of the binaries observed. We find that in approximately 70% of these binaries, the two stars' polarization position angles are within 30\arcdeg, inconsistent with their being drawn from a random distribution. In most cases where the interstellar polarization through the cloud is known, its position angle is different from that measured for the binaries, indicating that our observations are indeed tracing disk orientations. Thus, our observations suggest that disks in wide binaries are preferentially aligned with each other. This provides constraints on possible binary formation mechanisms, favoring those (such as disk fragmentation or late fragmentation of a collapsing cloud) that produce aligned disks. If the common inclination of the disks in these binaries is a tracer of the binary orbital plane, then our results also bear on the stability of planetary orbits, suggesting that planetary systems in wide binaries may be stable over billion-year timescales, long enough for life to evolve.

This work was supported by the W.M. Keck Foundation through the Keck Northeast Astronomy Consortium, and by NSF's ``Life in Extreme Environments'' program through grant AST 9714246.

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