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

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 rho Oph dark cloud, we probe the extinction up to A(J less than or equal to) 14 (A(V) less than or similar to 45), and find a best-fit linear relation N(H,X)/A(J) = 5.6 (+/-0.4) x 10(21) cm(-2) mag(-1), adopting standard ISM abundances. This ratio is significantly lower (greater than or similar to 2sigma) than the galactic value. We show that this difference can be attributed entirely to a difference in metallicity.