A study of the stellar population in the LYNDS 1641 dark cloud. I - The IRAS catalog sources

Abstract

We discuss the properties of 123 sources identified in the IRAS Point Source Catalog and located within the boundaries of the nearest giant molecular cloud, Lynds 1641 (d ~ 480 pc). A search of Palomar Schmidt B, R, and I plates for optical counterparts to these sources allows us to identify 63 candidates, while near-infrared "snapshots" centered on each IRAS source reveal 40 coincident sources (of which 16 lack optical counterparts); 30 objects are visible only at the IRAS bands. New and extant optical and near-infrared photometry are combined to provide spectral energy distributions (SEDs) for these objects. SED slopes are then used to sort the observed infrared spectral energy distributions into three classes: class I objects with flat or rising spectra; class II objects with spectra intermediate in slope between a flat and blackbody spectrum; and class III objects with spectra similar to those of blackbodies. We find L1641 to contain a much larger percentage of class 1 sources than does the nearby Taurus-Auriga star-forming complex. The steep infrared spectra characterizing the L1641 sources appear to result from their fucation in high-optical-depth "cores"; typical cores in L1641 have higher optical depths than do their counterparts in Taurus-Auriga. Recent theories predict that such high-optical-depth cores represent a natural outcome of the star formation process in regions where gravitational forces dominate magnetic forces during the initial cloud and stellar collapse phases, whereas in regions where magnetic forces dominate, low-opacity cores form. These predictions appear to be borne out by the core opacity estimates reported here and by recent comparisons of cloud morphology and magnetic fle1d geometry for L1641 and Taurus-Auriga. Photometric and spectroscopic observations of the subset of IRAS-selected sources with optical counterparts provide the basis for locating these stars in the H-R diagram. The resulting diagram suggests that either (a) most stars in L1641 were formed no longer than 3 X 10^6^ yr ago or (b) that older stars lack infrared excesses of sufficient size to render them detectable as IRAS sources.