Near-infrared extinction law in the ρ Ophiuchi and chamaeleon dark clouds

We determine and compare the color excess ratios EJ-H/ EH-KS for the rho 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 deg(2) (rho Oph) and 2.77 deg(2) ( Chamaeleon), which are complete for J similar to 19.0, H similar to 18.3, and K-S similar to 17.0 mag at a 10 sigma limiting magnitude, we did not find a significant difference in the extinction law EJ-H/EH-K for 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 EJ-H/EH-K for 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.