Document Type

Dissertation

Degree

Doctor of Philosophy

Major

Physics, Astrophysics

Date of Defense

12-10-2013

Graduate Advisor

Bruce A. Wilking

Co-Advisor

John Schmitt

Committee

Susan Kashubeck-West, Ph.D.

Michael Meyer

Alexey Yamilov

Abstract

The ρ Ophiuchi and Serpens molecular clouds are sites of low mass star formation. The goals of this study were to identify young stellar objects (YSOs) and estimate ages and masses in order to infer an initial mass function (IMF), investigate disk evolution, and determine the star-forming history. Optical spectroscopic surveys of an unbiased sample of candidate young stars have been completed. Optical images taken in different photometric bands were used to create color-magnitude diagrams from which sources were selected for spectroscopic observation. In combination with published data, 135 association members in ρ Ophiuchi and 63 association members plus 16 possible members in Serpens have been identified based on the presence of Hα in emission, lithium absorption, X-ray emission, a mid-infrared excess, and/or reflection nebulosity. Effective temperatures and bolometric luminosities were compared with theoretical tracks and isochrones for pre-main-sequence stars to estimate ages and masses. Both regions have similar median ages, supporting the idea that star formation is a relatively fast process. In ρ Ophiuchi, no age spread was found. In Serpens, an age spread of 1-5 Myrs was found; it could not be determined if this age spread was intrinsic or a result of contamination from foreground young stars. Consistent with these ages similar circumstellar disk frequencies were found. In ρ Ophiuchi, an IMF consistent with the field star IMF for YSOs with masses >0.2 M⊙ was inferred. In Serpens, the IMF was in agreement with the field star IMF for M>1 M⊙. Previous studies of these regions have been biased towards particular stages in the star formation process. This study has provided an unbiased sample of pre-main sequence objects, necessary to obtain a complete picture of star formation.

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