Astronomy & Astrophysics

Free access article

A&A 408, 581-599 (2003)
DOI: 10.1051/0004-6361:20030942

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

M. H. Vuong¹, T. Montmerle^{1, 2}, N. Grosso², E. D. Feigelson^{1, 3}, L. Verstraete⁴ and H. Ozawa^{1, 2}

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

(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 ( 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 $N_{\rm H,X}$ . 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 $N_{\rm H,X}$ 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 $A_{J} \la 14$ ( $A_{V} \la 45$ ), and find a best-fit linear relation $N_{\rm H,X}/A_{J} = 5.6$ ( $\pm$ 0.4) $\times$ 10 ²¹ cm ^-2 mag ^-1, adopting standard ISM abundances. The other regions reveal a large dispersion in the $N_{\rm H,X}$ / A_J ratio for each source but for lack of adequate IR data these studies remain limited to moderate extinctions ( $A_{J} \la 1.5$ or $A_{V} \la 5$ ). For $\rho$ Oph, the $N_{\rm H,X}/A_{J}$ ratio is significantly lower ( $\ga$ $2\sigma$ ) 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.

Key words: X-rays: stars -- stars: pre-main sequence -- ISM: clouds -- ISM: dust, extinction -- open clusters and associations: general

Offprint request: M. H. Vuong, vuong@discovery.saclay.cea.fr

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