Stellar Populations of Deeply Embedded Young Clusters: Near--Infrared Spectral Classification
Session 91 -- Young Stars and Young Clusters
Oral presentation, Wednesday, 11, 1995, 10:00am - 11:30am

## [91.05D] Stellar Populations of Deeply Embedded Young Clusters: Near--Infrared Spectral Classification

Michael R. Meyer (Five College Astronomy Department, University of MAssachusetts)

Deeply embedded young clusters offer a unique opportunity to study stellar evolution at the very earliest phases. I describe a program aimed at studying young stellar objects (YSOs) found in such clusters through the use of near--infrared spectroscopy. Over the past 18 months, we have carried out an extensive survey of fundamental MK spectral standard stars in the J-- and H--bands spanning luminosity classes I--II, III, and IV--V from spectral types O5 through M5 at a resolving power of R=3500. I summarize the results of this survey and present a preliminary two--dimensional classification scheme derived from H--band spectra. We have also observed a set of well--studied optically--visible young stars in the Taurus molecular cloud in the H-- and K--bands in order to quantify the effects of accretion activity on near--infrared classification spectroscopy of young stellar objects. Our sample covers a range of spectral types and disk accretion rates including classical and weak--lined T Tauri stars. Results suggest that spectral types derived from infrared spectra agree with their optically derived counterparts to within a few subclasses for late--type stars. Further, weak--lined T Tauri star spectra closely resemble dwarf star spectra although they appear to have somewhat lower surface gravities. Such data can be used to calculate the level of infrared excess emission in classical T Tauri stars. Finally, we apply this technique to YSOs found in the deeply embedded infrared clusters associated with the NGC2024 nebula in Orion and the $\rho$ Ophiuchi molecular cloud. By placing these optically--invisible sources in an HR diagram we can estimate their masses and ages based on comparison with theoretical PMS evolutionary models. The goal of this program is to understand the origin of the initial mass function by observing emergent mass spectra for isolated star forming events occuring in individual molecular cloud cores.