The Near-Infrared Extinction Law and Limits on the Pre–Main-Sequence Population of the ρ Ophiuchi Dark Cloud

Abstract

We describe new techniques to measure the near-infrared extinction law and to place limits on the preÈmain-sequence stellar population of a dark cloud. We analyze JHK imaging data for the central 1 deg2 of the o Ophiuchi cloud core and show that nearly all stars projected onto regions of low CS intensity, K km s~1, are background stars. Most sources at larger CS intensities lie within cloud I CS ¹ 10 material. We use the background stars to derive the slope of the near-IR extinction law, E J~H /E H~K \ 1.57 ^ 0.03. This result is consistent with previous extinction laws but has a factor of 2È3 smaller uncer-tainty. The new o Oph extinction law yields strong constraints on the number of previously undis-covered preÈmain-sequence stars in the cloud, 46 ^ 11, and the number of previously undiscovered young stars with near-IR excesses, 15 ^ 4. Neither limit exceeds the number of known preÈmain-sequence stars in the cloud, D100. Thus, current samples of preÈmain-sequence stars are reasonably complete for K ¹ 14.