Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996aj....111..283c&link_type=abstract
Astronomical Journal v.111, p.283
Astronomy and Astrophysics
Astronomy
115
Hii Regions, Ism: Individual: Orion Nebula, Stars: Rotation
Scientific paper
Results from the 1992-1993 season of imaging fields in the Orion Nebula Cluster at Van Vleck Observatory are presented. Data were obtained on 525 stars brighter than I˜16 mag and 50 periodic variables were found, of which 27 are newly discovered. Light curves of these spotted stars are displayed. The number of rotation periods known for ONC members, based on four years of monitoring at VVO, now stands at 75 and a comprehensive list is given here. Since candidate objects were selected only by proper motion and position, without reference to spectral, x ray, infrared or other properties, the sample provides a relatively unbiased view of the angular velocity distribution of low-mass stars at an age of around one million years. Twenty-five stars have periods determined in more than one year and in 23 cases they agree to better than 1%. In one case the agreement is within 5% and in one case there is evidence of period doubling. Seventy stars have amplitudes consistent with cool spots; the other five are interpreted as hot spot stars. The frequency distribution of rotation periods is distinctly bimodal, confirming the discovery of Attridge & Herbst [ApJ, 398, L61 (1992)]. About one-third of the stars are rapid rotators with a median period of 2.55 days and dispersion of 0.7 days. The others are slow rotators with a median of 8.3 days, a dispersion of 3.8 days and a tail of very long period stars extending to 34.5 days. Six stars have rotation periods exceeding 12 days, which had been proposed as a limit for T Tauri stars. Our observations support theories of disk-regulated rotational evolution during the pre-main-sequence phase. Slow rotators are interpreted as stars "locked" to their accretion disks and rapid rotators are presumed free of such locking. The gap in the frequency distribution at rotation periods near 4 days is interpreted as a portion of angular velocity space through which contracting stars pass quickly once released from the disk lock. We find a significant difference between the period distributions of the more and less nebulous fields within the ONC; the more nebulous (arguably younger) fields have a larger proportion of slow rotators. Combining the ONC sample with one drawn from the Tau-Aur region, we show that most pre-main-sequence stars more nearly conserve angular velocity than angular momentum as they contract, consistent with predictions of the disk-interaction theories. A large amount of angular momentum loss appears to occur as a result of this process during the early stages of pre-main-sequence evolution.
Choi Philip I.
Herbst William
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