Determination of the gas-to-dust ratio in nearby dense clouds using X-ray absorption measurements

We present a comparison of the gas and dust properties of the dense interstellar matter in six nearby star-forming regions (d < 500 pc): rho Oph, Cha I, R CrA, IC 348, NGC 1333, and Orion. We measure from Chandra and XMM-Newton observations the X-ray absorption toward pre-main sequence stars (PMS) without accretion disks (i.e., Class III sources) to obtain the total hydrogen column density N-H,N-X. For these sources we take from the literature the corresponding dust extinction in the near-infrared, A(J), or when unavailable we derive it from SED fitting using the available DENIS, 2MASS, ISOCAM and other data. We then compare N-H,N-X and A(J) for each object, up to unprecedently high extinction. For the rho Oph dark cloud with a relatively large sample of 20 bona-fide Class III sources, we probe the extinction up to A(J) less than or similar to 14 (A(V) less than or similar to 45), and find a best-fit linear relation N-H,N-X/A(J) = 5.6 (+/-0.4) x 10(21) cm(-2) mag(-1), adopting standard ISM abundances. The other regions reveal a large dispersion in the N-H,N-X/A(J) ratio for each source but for lack of adequate IR data these studies remain limited to moderate extinctions (A(J) less than or similar to 1.5 or A(V) less than or similar to 5). For rho Oph, the N-H,N-X/A(J) ratio is significantly lower (greater than or similar to 2sigma) than the galactic value, derived using the standard extinction curve (R-V = 3.1). This result is consistent with the recent downwards revision of the metallicity of the Sun and stars in the solar vicinity. We find that the rho Oph dense cloud has the same metallicity than the local ISM when assuming that the galactic gas-to-dust ratio remains unchanged. The difference between galactic and local values of the gas-to-dust ratio can thus be attributed entirely to a difference in metallicity.