AN OBSERVATIONAL STUDY OF LOW-MASS STAR FORMATION IN NGC 2264
The history of an actively star-forming region, containing the young, open cluster NGC 2264, is examined in this thesis using UBVRIHα photographic plate material. After digitization and removal of the variable background from these plates, automatic detection and photometry algorithms yield magnitudes for a set of candidate low-mass cluster members. Using the photographic plates and a supplementary video camera Hα survey, these candidates are selected on the basis of their exhibiting significant Hα emission, large amplitude variability, or anomalously blue colors. To study the age spread in NGC 2264, a theoretical H-R diagram is constructed. Infrared photometry of 33 candidate members permits an evaluation of the bolometric luminosities of these stars; the unreddened (V - R) and (V - I) colors yield effective temperatures. The theoretical mass tracks and isochrones of Cohen and Kuhi (1979) are adopted and extrapolated to lower luminosities and temperatures. The effects of circumstellar dust and gas shells on the location of the NGC 2264 stars in the H-R diagram are examined. No systematic biases in the derived bolometric luminosities and temperatures are found. The optical and infrared excesses of these stars are found to be uncorrelated, requiring separate mechanisms for their generation. A gaseous envelope is most likely the source of the optical excess; thermal emission from hot dust most probably leads to the observed infrared excesses. An age spread for the low-mass NGC 2264 stars of ≳10⁷ years is indicated. Star formation in NGC 2264 is found to have proceeded sequentially as a function of mass and time. Low-mass star formation began ∼2-3 x 10⁷ years ago, peaked ∼4-5 x 10⁶ years ago, and declined thereafter. Star formation at successively higher masses has started, peaked, and then declined at successively younger ages. No significant differences are seen between the field and NGC 2264 luminosity functions to the faintest levels observed (Mᵥ ≈ +11). There is no evidence for a low-mass turnover in NGC 2264.
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