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

¹ Service d'Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette, France
² Laboratoire d'Astrophysique de Grenoble, Université Joseph Fourier, BP 53, 38041 Grenoble Cedex 9, France
³ Department of Astronomy & Astrophysics, 525 Davey Laboratory, Pennsylvania State University, University Park, PA 16802, USA
⁴ Institut d'Astrophysique Spatiale, Bât. 121, Université de Paris XI, 91405 Orsay Cedex, France

Corresponding author: M. H. Vuong, vuong@discovery.saclay.cea.fr

Received: 17 March 2003
Accepted: 18 June 2003

Abstract

We present a comparison of the gas and dust properties of the dense interstellar matter in six nearby star-forming regions ( 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  (), and find a best-fit linear relation (± 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 ( or ). For ρ  Oph, the  ratio is significantly lower () than the galactic value, derived using the standard extinction curve (). 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.