Gas absorption and dust extinction towards the Orion Nebula Cluster
  We characterise the relation between the gas and dust content of the
interstellar medium towards young stellar objects in the Orion Nebula Cluster.
X-ray observations provide estimates of the absorbing equivalent hydrogen
column density N_H based on spectral fits. Near-infrared extinction values are
calculated from intrinsic and observed colour magnitudes (J-H) and (H-K_s) as
given by the VISTA Orion A survey. A linear fit of the correlation between
column density and extinction values A_V yields an estimate of the N_H/A_V
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 N_H/A_V
ratio. Assuming a Galactic extinction law with R_V=3.1 and solar abundances by
Anders & Grevesse (1989), we deduce an N_H/A_V ratio of (1.39 +- 0.14) x 10^21
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
N_H/A_V ratios. Changes in the assumed extinction law and elemental abundances
are demonstrated to have a relevant impact on deduced A_V and N_H values,
respectively. Large systematic uncertainties associated with metal abundances
in the Orion Nebula Cluster represent the primary limitation for the deduction
of a definitive N_H/A_V ratio and the physical interpretation of these results.
