Wednesday, 14th December at 2.00pm @ JAC

ABSTRACT: We determine and compare the color excess ratios E(J-H)/E(H-Ks) for the rho Ophiuchi, Chamaeleon and Coalsack dark clouds with the J, H, and Ks bands simultaneous camera, SIRIUS, on the IRSF 1.4m telescope at SAAO. Determining the accurate ratios is indispensable for discussing the extinction law. However, previous surveys suffered from uncertainties in transforming one photometric system to another when comparing data on different photometric systems. To overcome the problem, we observe both clouds in exactly the same photometric system on the same telescope, simplifying the comparison of the extinction laws among the three clouds. From our surveys covering about 1.56 square-degree (rho Ophiuchi), 2.77 square-degree (Chamaeleon) and 0.16 square-degree (Coalsack) complete for J = 19.0, H = 18.3 and Ks = 17.0 mag at a 10 sigma limiting magnitude:

(1) We did not find a significant difference in the extinction law E(J-H)/E(H-Ks) between the rho Ophiuchi and Chamaeleon clouds, in contrast to previous work.
(2) From calculations for a model star with the effects of filter and atmosphere transmissions, the differences of the color excess ratios E(J-H)/E(H-Ks) for the two clouds described in the past literature can be attributed to the different photometric systems so far employed.
(3) Among the three clouds, the extinction law for the Coalsack cloud shows a significantly large value. This is the first systematic study showing the difference of the extinction law for these dark clouds.
(4) We also demonstrate a change in the color excess ratio with increasing optical depth, which shows the grain growth appearing toward the inside of the dark clouds.