Gas absorption and dust extinction towards the Orion Nebula Cluster⋆
1 Department for AstrophysicsUniversity
of Vienna, Türkenschanzstraße
17, 1180 Vienna, Austria
2 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
3 Department of Astronomy and Astrophysics, 525 Davey Laboratory, Pennsylvania State University, University Park, PA 16802, USA
4 INAF-Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
Accepted: 31 May 2016
Aims. We characterise the relation between the gas and dust content of the interstellar medium towards young stellar objects in the Orion Nebula Cluster.
Methods. X-ray observations provide estimates of the absorbing equivalent hydrogen column density NH based on spectral fits. Near-infrared extinction values are calculated from intrinsic and observed colour magnitudes (J − H) and (H − Ks) as given by the VISTA Orion A survey. A linear fit of the correlation between column density and extinction values AV yields an estimate of the NH/AV ratio. We investigate systematic uncertainties of the results by describing and (if possible) quantifying the influence of circumstellar material and the adopted extinction law, X-ray models, and elemental abundances on the NH/AV ratio.
Results. Assuming a Galactic extinction law with RV = 3.1 and solar abundances by Anders & Grevesse (1989, Geochim. Cosmochim. Acta, 53, 197), we deduce an NH/AV ratio of (1.39 ± 0.14) × 1021 cm-2 mag-1 for Class III sources in the Orion Nebula Cluster where the given error does not include systematic uncertainties. This ratio is consistent with similar studies in other star-forming regions and approximately 31% lower than the Galactic value. We find no obvious trends in the spatial distribution of NH/AV ratios. Changes in the assumed extinction law and elemental abundances are demonstrated to have a relevant impact on deduced AV and NH values, respectively. Large systematic uncertainties associated with metal abundances in the Orion Nebula Cluster represent the primary limitation for the deduction of a definitive NH/AV ratio and the physical interpretation of these results.
Key words: dust, extinction / X-rays: ISM / infrared: ISM / stars: pre-main sequence
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© ESO, 2016