search for them. Indeed, photometric and spectroscopic investigations of the stellar populations in nearby star-forming regions are now sensitive to the detection of objects with masses below the hydrogen burning limit. Deep optical surveys (in the R and I bands) have enabled study of young stars that are still partially embedded in their nascent molecular clouds. When optical photometry is combined with optical spectroscopy, stars can be individually de-reddened and located in conventional Hertzsprung-Russell (H-R) diagrams, from which stellar masses and ages can be derived, leading to the construction of stellar mass and age distributions.
More recently, by combining infrared photometry (in the J, H, and K bands) with infrared spectroscopy, this traditionally optical technique has been successfully employed in regions where most of the stellar population is fully embedded in molecular cloud material. We are thus able to determine stellar masses and ages for stars that are obscured by 10 to 50 magnitudes of interstellar and local circumstellar extinction. Probing into the birthplaces of stars in this way means that we can measure stellar-/substellar-mass distributions for temporally and spatially coherent populations that are unaffected by evolutionary processes. Moreover, rich, dense, extremely young clusters permit identification of complete samples of stellar as well as substellar objects, avoiding the membership ambiguities associated with their identification and study in older open clusters.
Results to date indicate that while several very good substellar candidates do exist in young star-forming regions, the relative numbers of these objects imply a mass spectrum that does not increase from the stellar- into the substellar-mass regime.
However, studies of the stellar populations in star-forming regions are hampered by the effects of high extinction, nebular contamination, source crowding, and circumstellar emission. Thus great care is needed in data analysis and interpretation. Furthermore, the translation from observational quantities (e.g., colors and spectral types) to physical quantities (e.g., masses and ages) depends on accurate understanding both of the intrinsic properties of late-type (M6.5 to M9) stars and of theoretical predictions for pre-main sequence evolution.
At present, large uncertainties remain in stellar colors, bolometric corrections, and temperatures (on the observational side) and in opacities, convection, and the effects of accretion (on the theoretical side). Future observations aimed at narrowing these uncertainties are required in order to make additional progress on the stellar-/substellar-mass distribution in star-forming regions, for example, with large, near-infrared spectroscopic surveys.