2006ApJ...640..373N

We determine and compare the color excess ratios E_J–H/E_H–KSfor the ρ Ophiuchi and Chamaeleon dark clouds with the J-, H-, and K_s-band simultaneous camera, SIRIUS, on the IRSF 1.4 m telescope at SAAO. Determining accurate ratios is indispensable for discussions of the extinction law. However, previous surveys suffered from uncertainties in transforming one photometric system to another when comparing data on different systems. To overcome the problem, we observe both clouds in exactly the same photometric system on the same telescope, greatly simplifying the comparison of the extinction laws between two clouds. From our surveys covering about 1.56 deg² (ρ Oph) and 2.77 deg² (Chamaeleon), which are complete for J∼19.0, H∼18.3, and K_S∼17.0 mag at a 10 σ limiting magnitude, we did not find a significant difference in the extinction law E_J–H/E_H–KSfor the two clouds, in contrast to previous work. From calculations of a model star with the effects of filter and atmosphere transmissions, the differences of the color excess ratios E_J–H/E_H–Kfor two clouds described in the past literature can be attributed to the different photometric systems so far employed. We also note a change of the color excess ratio that occurs with increasing optical depth.