The 15273 \AA\ diffuse interstellar band in the dark cloud Barnard 68
  High obscuration of background stars behind dark clouds precludes the
detection of optical diffuse interstellar bands (DIBs) and hence our knowledge
of DIB carriers in these environments. Taking advantage of the reduced
obscuration of star-light in the near-infrared (NIR) we used one of the
strongest NIR DIBs at 15273 AA to probe the presence and properties of its
carrier throughout the nearby interstellar dark cloud Barnard 68. Equivalent
widths (EW) have been measured for different ranges of visual extinction AV,
using VLT-KMOS H-band (1.46-1.85 micron) moderate-resolution (R = 4000) spectra
of 43 stars situated behind the cloud. To do so we fitted the data using
synthetic stellar spectra from the APOGEE project and TAPAS synthetic telluric
transmissions appropriate for the observing site and time period. The results
show an increase of DIB EW with increasing AV. However, the rate of increase is
much flatter than expected from the EW-AV quasi-proportionality established for
this DIB in the Galactic diffuse interstellar medium. Based on a simplified
inversion assuming sphericity, it is found that the volume density of the DIB
carrier is 2.7 and 7.9 times lower than this expected average value in the
external and central regions of the cloud which have n(H)= 0.4 and 3.5 x 105
cm3, respectively. Further measurements with multiplex NIR spectrographs should
allow detailed modeling of such an edge effect of this DIB and other bands and
help to clarify its actual origin.
