Lyman $\alpha$ as an Accretion Diagnostic in Young Stellar Objects
Session 41 -- Circumstellar Disks
Display presentation, Thursday, January 13, 9:30-6:45, Salons I/II Room (Crystal Gateway)

## [41.04] Lyman $\alpha$ as an Accretion Diagnostic in Young Stellar Objects

M. R. P\'erez (CSC/IUE-GSFC), C. A. Grady (ARC), P. F. C. Blondel, D. de Winter, P. S. Th\'e (U. Amsterdam)

The surprising and unpredicted detection of strong and highly redshifted Lyman $\alpha$ emission in several young stellar objects (YSOs) by IUE, which has been interpreted as the signature of accreting material in the line of sight, has added a new and important diagnostic to the current theory of accretion. Until recently, previous work on YSOs relied on either subordinate lines of hydrogen or trace species and did not involve the resonance transition of the most cosmically abundant element. Based on high-dispersion images of the short wavelength camera (SWP) of several YSOs has been possible to measure redshifted emission velocities in excess of +800 km/s and mass accretion rates of the order of 10$^{-7}$ $M_{\sun}$/yr. In contrast, chromospheric Lyman $\alpha$ emission is typically symmetric with respect to the geocoronal emission. It has been also possible to detect the Lyman $\alpha$ stellar emission in several low-dispersion SWP images of objects such as RW Aur, HD 31648, and possibly UX Ori and BF Ori. We present results of our ongoing Lyman $\alpha$ survey and discuss correlations of its strength with other emission lines and with additional accretion diagnostics such as infrared excesses, blueing effect, and flux variabilities.