2003A&A...408..581V

We present a comparison of the gas and dust properties of the dense interstellar matter in six nearby star-forming regions (d<500pc): 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)x10²¹cm^–2mag^–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 ρ 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.