VISION - Vienna Survey in Orion II. Infrared extinction in Orion A
  We have investigated the shape of the extinction curve in the infrared up to
~25 {\mu}m for the Orion A star-forming complex. The basis of this work is
near-infrared data acquired with VISTA, in combination with Pan-STARRS and
mid-infrared Spitzer photometry. We obtain colour excess ratios for eight
passbands by fitting a series of colour-colour diagrams. The fits are performed
using Markov chain Monte Carlo methods, together with a linear model under a
Bayesian formalism. The resulting colour excess ratios are directly interpreted
as a measure of the extinction law. We show that the Orion A molecular cloud is
characterized by flat mid-infrared extinction, similar to many other recently
studied sightlines. Moreover, we find statistically significant evidence that
the extinction law from ~1 {\mu}m to at least ~6 {\mu}m varies across the
cloud. In particular, we find a gradient along galactic longitude, where
regions near the ONC show a different extinction law compared to L1641 and
L1647, the low-mass star-forming sites in the cloud complex. These variations
are of the order of only 3% and are most likely caused by the influence of the
massive stars on their surrounding medium. While the observed general trends in
our measurements are in agreement with model predictions, both well-established
and new dust grain models are not able to fully reproduce our infrared
extinction curve. We also present a new extinction map featuring a resolution
of 1 arcmin and revisit the correlation between extinction and dust optical
depth. This analysis shows that cloud substructure, which is not sampled by
background sources, affects the conversion factor between these two measures.
In conclusion, we argue that specific characteristics of the infrared
extinction law are still not well understood, but Orion A can serve as an
unbiased template for future studies.
