Watching the Stars go `Round and `Round

Scott Joseph Wolk

Abstract:

I present results from several seasons of photometric observations of X--ray sources in the Orion OB1a and OB1b star formation regions. The initial goal was to measure the rotational periods of several dozen pre--main sequence (PMS) stars. Since the early 1980s, X--rays have been found to be the premier method of identifying PMS stars, especially in highly confused regions. I first describe previous work in rotational studies, beginning with spectrophotographic observations of v sin i through the modern use of charge coupled devices to study the rotational modulation of spotted stars. The optical properties of 105 X--ray sources observed as part of my thesis research are discussed in detail next. Based on optical colors, I find that the X--ray sources in both regions lie along a well defined locus above the main sequence. Infrared and optical data confirm that 90% of the X--ray sources are PMS and about 10% of the PMS stars may have disks. Between V magnitudes 12 and 16, I find that the X--ray sources account for 80% of the pre--main sequence population, as determined by location in the H--R diagram. This photometric study also reveals a population of PMS stars to which the X--ray surveys were insensitive. These PMS stars range in mass from Solar mass objects, down to very close to the brown dwarf limit. This photometric method, augmented by spectra, may supplant X--rays as the premier method for identification of PMS sources in regions of star formation. It may also provide a method for observing stars at the low mass end of the initial mass function.
With the PMS nature of these X--ray sources well characterized, I examine the periodic nature of only the stars which were associated with X--ray sources. I use simulated data to demonstrate that for stars with perfectly sinusoidal behavior, accurate periods can be found in a cases of signal to noise greater than 2. that the
For more realistic stars, which do not vary in brightness (along our line--of--sight) exactly as a sine wave, the signal to noise must be higher by a factor of two or more. I also discuss the expected color changes due to spot modulation and find that the allowed variations cover a very limited range in phase space. I find rotation periods for five stars at confidence of greater than 99.9/ greater than 99/ at much lower confidence.
Finally, the relation between the data presented here and other data is discussed. The sampling rate of the data presented here allows for detection of shorter periods than previously reported for T Tauri stars. I note the bimodal distribution of rotation periods which other authors have reported. I further note that the ratio of slow rotators to fast rotators seems to change as a function of the age of the star forming region. Rotation period and various observables are compared. I find weak correlations between slow rotation and high variability and between slow rotation and IR color excess. Both these correlations support the hypothesis that stars with disks are slow rotators. However, there is also a large fraction of slow rotators which do not show evidence for disks.

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Scott J. Wolk
Mon Nov 25 15:41:03 EST 1996