The Gas-to-Dust Ratio and Metallurgy of Nearby Dark Clouds Probed by X-ray Absorption Measurements

Abstract

We present a comparison of the gas and dust properties of nearby dark clouds. The method is to measure from Chandra and XMM—Newton observations the X-ray absorption toward pre-main sequence stars without accretion disks (i.e., Class III sources) to obtain the total hydrogen column density N _H,X. For these X-ray emitting sources we take from the literature the corresponding dust extinction in the near—infrared, A _J. We then compare N _H,X and A _J for each object, up to unprecedently high extinction. For the only region for which the data are adequate, the ρ Oph dark cloud, we probe the extinction up to A _J≲14 (A _V≲45), and find a best-fit linear relation N _H,X/A _J=5.6(±0.4)×10²¹ cm⁻² mag⁻¹, adopting standard ISM abundances. This ratio is significantly lower (≳2σ) than the galactic value. We show that this difference can be attributed entirely to a difference in metallicity. (This contribution is based on Vuong et al., in press in A&A: astro-ph/0306447.)