The aim of this project is determining the angular momentum history of Pre Main Sequence (PMS) stars in young clusters. Assuming a conservation of angular momentum, as these stars contract during formation, they should all be rotating rapidly after a very short amount of time (~2 Myrs). However, rotation periods of young cluster members show that most of these stars rotate much more slowly than expected. For many years, the precise mechanism which prevents these stars from spinning up has been debated, with no conclusive observational evidence to support one mechanism over another.
The prevailing theory is that the young stars, which possess strong magnetic fields due to their rapid rotation, interact with their circumstellar disks through these magnetic fields. The transfer of angular momentum from the star to the disk would increase the orbital velocity of the disk material and prevent the star from spinning up as it contracts. However, other mechanisms internal to the star, such as differential rotation, could cause the same effect. If an internal mechanism is the cause of the angular momentum drain, then all of the models of PMS star evolution, which, using observational data such as stellar brightness, calculate important characteristics including stellar radii and ages, would be incorrect since they assume an external angular momentum loss mechanism.
The theory that star-disk interaction causes the angular momentum loss which regulates PMS star rotaion periods can be tested by measuring rotation periods and disk indicators of members of young star clusters. Rotation periods may be obtained by photometrically monitoring (currently typically in the I band, and possibly in the J band with the future network) these PMS stars. Again, because of their strong, active magnetic fields, PMS stars have many surface features (star spots). As these features rotate in and out of view, they cause the star's brightness to modulate at the rotation period.
Once rotation periods are obtained, the star-disk interaction theory predicts that rapidly rotating stars should not have a disk, while slow rotators should. Until recently, however, measuring the presence of a disk reliably has been tricky. Spitzer Space Telescope mid-infrared observations have allowed the first accurate means of disk identification sensitive enough to provide an unbiased sample of disk identification for a statistically significant number of PMS stars in young clusters with known rotation periods. By combining rotation periods from ground-based photometry with Spitzer mid-Infrared (IR) disk identification, one can search for the predicted correlation between rotation period and the presence of a circumstellar disk.