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): ρ 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,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,X and A_J for each object, up to unprecedently high extinction. For the ρ Oph dark cloud with a relatively large sample of 20 bona-fide Class III sources, 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. The other regions reveal a large dispersion in the N_H,X/A_J ratio for each source but for lack of adequate IR data these studies remain limited to moderate extinctions (A _J ≲ 1.5 or A_V ≲ 5). For σ Oph, the N _H,X/A_J ratio is significantly lower (≳2σ) 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 ρ 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.