Studies of young stellar clusters are important tools for understanding the formation and early evolution of stars. By measuring the distributions of masses and ages of stars found in such clusters, we can begin to address some of the following fundamental questions related to the formation of stars. What is the origin of the initial mass function (IMF) ? Are all stars drawn from a universal distribution of masses, similar to that found among field stars in the solar neighborhood, or is each cluster different? Do clusters form in a burst with all members coeval, or do they form more slowly over time? Could star formation have been triggered?
Optical photometric and spectroscopic studies have been the traditional techniques used for inferring the masses, ages, and evolutionary histories of young stars and young stellar clusters during this century (e.g., Walker 1956). Comparisons of observed luminosities and temperatures with theoretical stellar models have yielded the masses and ages of stars in young open clusters with great success. Cohen and Kuhi (1979; hereafter CK) used such techniques to study the masses and ages of pre-main-sequence (PMS) stars in several young clusters including Taurus-Auriga, Ophiuchus, Orion, and NGC 2264. While the stellar populations of some star forming regions are well sampled using optical surveys, many regions are known to contain large numbers of optically invisible IR sources. Dust associated with the molecular cloud material from which stars are born obscures young stellar objects (YSOs) that are deeply embedded in their natal cloud cores. Near-IR surveys which penetrate the obscuring dust have revealed rich clusters of young stars in regions such as Orion, Ophiuchus, and the Monoceros molecular clouds. Only through combining optical and IR studies of star forming regions will we be able to obtain a complete picture of the mass and age distributions of the stellar populations in young clusters.
Recent works have utilized near- and mid-IR photometry in order to derive stellar masses and estimate evolutionary states of optically invisible YSOs forming in embedded clusters. Wilking, Lada, & Young (1989; hereafter WLY) and Greene et al. (1994, hereafter GWAYL) have inferred the evolutionary status of the embedded cluster found in the Ophiuchi cloud (L1688) core (e.g., Grasdalen, Strom, & Strom, 1973; Wilking & Lada, 1983) from the spectral energy distributions (SEDs) of its members. Other work has focussed on combining theoretical PMS evolutionary models with estimates of cluster ages in order to derive stellar masses from IR photometric data (Comeron et al. 1993; GWAYL; Meyer et al. 1995). Comeron et al. (1983) and Strom, Kepner, & Strom (1995; hereafter SKS) have recently applied such techniques in order to derive mass distributions for the Oph cloud core IR population. However these photometric studies are inherently limited since they can not uniquely determine both stellar masses and ages. Either a mass function can be determined based on an assumed cluster age or vice versa.
With the new generation of IR arrays and cryogenic grating spectrographs, it is now possible to use near-IR spectroscopy to obtain reliable photospheric spectral types of optically invisible PMS stars. This permits IR YSOs to be placed in the H-R diagram, allowing the independent measurement of stellar masses and ages. In pioneering work, Casali & Matthews (1992) observed 10 low luminosity Class I and II YSOs in the Ophiuchi cloud. They found that at least three of these YSOs showed K band absorption features indicative of effective temperatures between 3000 and 5000 K. Hodapp & Deane (1993) conducted a near-IR spectroscopic study of YSOs in the L1641 cloud and used these spectra to determine stellar spectral types. They produced an H-R diagram for 12 stars in the cluster, about half of which are optically invisible.
We have embarked on a large scale near-IR spectroscopic study of YSOs in order to characterize better their physical properties and evolutionary phases. This paper, the first in a series, analyzes the stellar characteristics of the YSO cluster embedded in the Oph dark cloud. Subsequent papers will analyze the circumstellar environments of YSOs. Most of the 80 confirmed Oph cluster members are optically invisible due to their extinction (AV ~ 20 mag) by foreground dust within the cloud. Our goals in this paper are two-fold; i) to investigate the age of this embedded cluster and compare it to stellar ages of the larger surrounding region, and ii) to use this age to reexamine the distribution of stellar masses in the core. We describe our new observations in §2 of this paper. We determine the spectral types, luminosities, and extinctions of 34 YSOs in §3. In §4, we independently determine stellar ages and masses, re-examine the work of SKS in light of our new results, and discuss the star forming history of the region. We summarize our results in §5.