The Relationship between the Optical Depth of the 9.7 micron Silicate Absorption Feature and Infrared Differential Extinction in Dense Clouds

TL;DR

This study investigates the relationship between the 9.7 micron silicate absorption feature's optical depth and infrared extinction in dense clouds, revealing that in such environments, the feature does not reliably indicate dust column density due to grain growth effects.

Contribution

It provides new observational evidence that the correlation between silicate feature optical depth and extinction breaks down in dense clouds, highlighting grain growth as a key factor.

Findings

Silicate feature optical depth does not increase monotonically with extinction in dense clouds.

Most dense cloud lines of sight show lower tau_9.7 than expected from diffuse ISM correlation.

Grain coagulation likely causes the deviation from the diffuse ISM relationship.

Abstract

We have examined the relationship between the optical depth of the 9.7 micron silicate absorption feature (tau_9.7) and the near-infrared color excess, E(J-Ks) in the Serpens, Taurus, IC 5146, Chameleon I, Barnard 59, and Barnard 68 dense clouds/cores. Our data set, based largely on Spitzer IRS spectra, spans E(J-Ks)=0.3 to 10 mag (corresponding to visual extinction between about 2 and 60 mag.). All lines of sight show the 9.7 micron silicate feature. Unlike in the diffuse ISM where a tight linear correlation between the 9.7 micron silicate feature optical depth and the extinction (Av) is observed, we find that the silicate feature in dense clouds does not show a monotonic increase with extinction. Thus, in dense clouds, tau_9.7 is not a good measure of total dust column density. With few exceptions, the measured tau_9.7 values fall well below the diffuse ISM correlation line for E(J-Ks) > 2 mag (Av >12 mag). Grain growth via coagulation is a likely cause of this effect.