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Session 5 - Accretion and Outflows in YSOs.
Display session, Wednesday, January 07
Exhibit Hall,

[5.06] Accretion Disk Lifetimes and Stellar Rotation Periods for Young Stars in NGC 2264

R. B. Makidon (UMass, Amherst and STScI), S. E. Strom (UMass), B. Tingley (UMass, Amherst), M. T. Adams (U. Texas, McDonald Obs.), L. Hillenbrand (UC, Berkeley), L. Hartmann, N. Calvet (CfA), B. F. Jones (UC, Santa Cruz)

We present the initial results of a study aimed at: determining the lifetime of the disk accretion phase among low mass pre-main sequence stars; (2) establishing the time dependence of disk mass accretion rates; and (3) further exploring the role played by accretion disks in regulating stellar rotation. Our laboratory for this study is NGC 2264, a young cluster which contains more than 300 proper motion members with ages ranging from 0.1 to 10 Myr and masses ranging from 0.1 to 10 Msun. We diagnose the presence of circumstellar accretion disks from observed ultraviolet excesses, estimate accretion rates from the magnitude of those excesses, and determine stellar rotation periods for more than 200 stars from the analysis of spot-modulated I-band light curves. We find for PMS stars with masses M \le 0.4 Msun: (1) that accretion disk lifetimes can exceed 10 Myr; (2) that accretion rates decay with time (dM/dt \sim M^-n; 0.9 < n < 2); and (3) that disks appear to play a critical role in regulating stellar rotation periods. In particular, PMS stars stars surrounded by accretion disks on average rotate more slowly than their counterparts which show no evidence of such disks: the median rotation period for stars surrounded by disks is 7.91 days, while for stars which lack disks the median period is 3.97 days. However, our results suggest the range of periods (0.5 < P < 30 days) among stars surrounded by disks is considerably larger than reported in previous studies.

The authors would like to thank Dr. Brian Patten for his many contributions to this project. This work was supported by a grant awarded under the NASA Origins of Solar Systems Program.

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