I began this dissertation with a discussion of the status of our understanding of the rotational evolution of young stars. The existence of UFRs at the ZAMS is a quandary for the understanding of the theoretical development of young stars. By studying the rotation of PMS stars in Orion OB1a and OB1b, I hoped to gather some insights to the evolution of rotation in young stars.
In Chapter 2, I laid the groundwork for the rotational investigation by performing optical follow-up to X--ray observations in regions of star formation. First, optical photometry showed that the X--ray sources lay in a well-defined locus, about a magnitude and a half above the main sequence. Infrared photometry and optical spectroscopy were available for a small subset of stars for which I was able to measure extinctions and luminosities. The small size of the data set which could be processed this way and the large variations within the dereddened group made age determination difficult, but showed that the Orion OB1b stars are about 0.5 dex older than the Orion OB1a stars. In an effort to understand the incompleteness of my data, I carried out optical photometry on every star in the observed region of the two X--ray fields. I discovered that, in addition to the background stars, there exists a population of X--ray quiet stars which occupy the same region of the color--magnitude diagram as the X--ray sources. While a distance estimate is required to demonstrate that any given star is PMS, follow--up spectroscopy among the brightest of these stars indicates that many of them are PMS. Thus, the photometric techniques may give us a method of identifying all the PMS stars in a given region. It may also provide a method of selecting PMS stars for future investigations which is independent of their physical properties. Additionally, the X--ray quiet PMS population occupies all mass ranges from 1.0 M down to the brown dwarf limit. The magnitude limit on the optical observations was such that it prevented the unambiguous detection of brown dwarfs. The are not data sufficient to prove a change in the slope of the initial mass function at very low mass. The main results of the photometric study include:
Once the PMS nature of the X--ray sources was established, I tested my ability to detect periods on stars observed during the 11 night run at the WISE observatory. I found that the periodogram code needs fairly high signal to noise to work effectively on such a short data set. In addition, the non--sinusoidal nature of the stellar signals makes period detection even more difficult. To make period detection more robust, I describe the criteria which I use to determine the validity of periods which are detected. The main results of the rotational study are:
The data presented here represent the start of the first systematic study of the evolution of low mass stars in the Orion OB1a and OB1b sub--associations. The data provide some of the most convincing evidence to date that the nTTs represent an evolutionary stage between cTTs and ZAMS stars, as well as strong support for disk braking of spin--up. The data have also demonstrated deficiencies with the reliance on X--ray surveys to find nTTs. While more sensitive X--ray pointing will be available in the future using Advanced X--ray Analysis Facility (AXAF), wide--field optical photometry supplemented with multi--object spectroscopy has now surpassed X--rays as the premier method of detecting PMS stars. This new knowledge will allow future work to escape the selection effects which have hampered our understanding of how stars form.
Future work should concentrate on the rotation periods of the non--X--ray detected PMS stars. Many of these stars were subject to monitoring, but these data were excluded from the period searches since there was originally no reason to suspect any unusual behavior by these stars. I will more generally study the recently unveiled population of PMS stars with masses below about 0.5 M as well as the spectral properties of the X--ray quiet PMS stars. Future planned observations also include deeper mapping of regions of star formation in V, R and I bands. Such data may allow us to search for the low end of the IMF and determine whether such an end exists. This would tell us how low mass an object can be and still form like a star or if there is some limit on the minimum mass dictated by the physics of star formation.