Session 32 - Stellar Astrophysics II.
Oral session, Monday, January 15
La Condesa, Hilton

## [32.04] Rotational Studies of Naked T Tauri Stars

S. J. Wolk (SUNY at Stony Brook)

Naked T Tauri stars show variability consistent with the existence of starspots on their surface. As the stars rotate, the number of spots in view varies and the rotational period can be measured. This method of period determination has been used on a large set of known classical T Tauri stars (Bouvier et al. 1993, 1995) and ZAMS stars (Prosser et al. 1993,1995). The results of these works and others have lead to an improved understanding of the evolution of angular momentum in PMS stars and disks (Edwards et al. 1994). Here, I discuss a portion of my studies of naked T Tauri stars. NTTs are intermediate in evolutionary status between cTTs and ZAMS stars (although closer the cTTs).

I will specifically discuss sources near \sigma Orionis. Deep ROSAT pointed observations detected nearly 80 sources within 10 arcminutes of \sigma Ori. I have made follow-up photometric observations to determine their color and rotation periods. To date my findings indicate the following:

\begin enumerate

\item 90% of the X-ray sources in this region lie in a well defined region above the main sequence. This indicates that the X-ray sources are both PMS and nearly coeval.

\item The color of the observed variability of these PMS stars is indicative of a wide variety of spot temperatures, filling factors and spot size.

\item The rotation periods of the X-ray sources are on the order of one day \pm half a day.

\item There are many non X-ray sources along with the X-ray sources above the main sequence. This implies that there may be many X-ray quiet PMS stars still undiscovered by X-ray surveys.

\end enumerate

My thesis will include the rotation periods of about 70 the PMS stars in this region and others elsewhere in Orion. Overall, I will increase the number of nTTs with known periods by an order of magnitude.