摘要：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.
