Rotation Periods and Variability of Stars in the Trapezium Cluster

Details Rotation Periods and Variability of Stars in the Trapezium Cluster Rotation Periods and Variability of Stars in the Trapezium Cluster

Oct 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995aj....110.1735e&link_type=abstract

Astronomical Journal v.110, p.1735

Astronomy and Astrophysics

Astronomy

26

Stars: Rotation, Stars: Oscillations, Stars: Early-Type

Scientific paper

Results from a fourth season (1993/94) of monitoring the Trapezium cluster in the Cousins I band at Van Vleck Observatory are reported. Data were obtained on 28 nights with multiple exposures on most. A periodogram technique described by Horne & Baliunas [ApJ, 302, 757 (1986)] based on Scargle's method was used to search for periods. False alarm probabilities must be carefully assessed when the sampling intervals are distributed in the manner of this study. Multiple observations per night allow us to effectively resolve problems with aliasing. Rotation periods are now known for 19 cluster members; ten are confirmations of periods reported by Mandel & Herbst [ApJ, 383, L75 (1991)] and/or Attridge & Herbst [ApJ, 398, L61 (1992)] and nine are newly discovered. 17 of these are likely Type I (cool spot) variables and two are of Type IIp (hot spot). Periods range from 1.68 to 34.5 d and have a bimodal distribution similar to that found for the Orion Nebula cluster as whole. In this limited, but homogeneous sample, we find no trends in angular velocity with stellar mass, stellar age, or presence on an inner disk, as inferred from H - K excess. The existence of a bimodal period distribution is, however, strong evidence in favor of disk-regulated angular momentum evolution. Irregular variables are also common in the Trapezium cluster and we identify 14 that have ranges exceeding 0.5 mag. Most of them exhibit light curves typical of classical T Tauri stars (Type II), but one has a much slower (perhaps FUor-like?) behavior, and three showed eclipse-like drops in brightness. They could be eclipsing binaries but no periods were found, so they are more likely to be Type III irregular variables (UXors). VIJHK photometry demonstrates that the large amplitude variables (LAVs), as a group, have substantial IR excesses, indicative of accretion disks, signifying that they are indeed CTTS. None of these stars are brighter than V = 14, in accord with the failure of spectroscopic surveys to detect CTTS in the Trapezium cluster, heretofore. Our data indicate that both CTTS and WTTS are present in the Trapezium cluster, but that the brightest WTTS are systematically brighter than the brightest CTTS by about 1.6 mag in V and 1.8 mag in I. A simple interpretation of this fact is that the CTTS (at least the LAVs) are of lower mass and cooler than the WTTS in this cluster.

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