A New Derivation of the Extinct ion-to-CO Column Density Ratio in the Chamaeleon I Dark Cloud

Abstract

We have analyzed new datasets of 13CO (J = 1−0) and C18O (J = 1−0) emission obtained with NANTEN and visual extinction, AV, derived from DENIS J-band observations in the Chamaeleon I dark cloud. The column densities of both molecular isotopes, N(13CO) and N(C18O, have been derived assuming LTE. Both N(13CO) and N(C18O are linearly correlated with AV in the range AV≲ 10 mag. We established the relationships N(13CO) (cm-2)=(1.2± 0.1)× 1015AV(mag)+(0.7± 0.3)× 1015 and N(C18O) (cm-2)=(3.5± 0.3)× 1014AV(mag)-(5.7± 1.3)× 1014. In a region of clustered T Tauri stars, however, both the AV-N(13CO) and AV-N(C18O) relationships are by a factor of ∼ 2 steeper than the above. This may be mainly due to underestimates of the AV values, and the J-band star count might deserve a more careful examination for possible underestimates of associated young stars. Compared with the relationships for the other clouds, the slope of the AV-N(13CO) relationship is relatively shallower and that of the AV-N(C18O) relationship in a non-cluster forming region is slightly steeper. These facts suggest that cloud-to-cloud variation is significant in the AV-N(CO) relationships.