The 15273 \AA\ diffuse interstellar band in the dark cloud Barnard 68

TL;DR

This study uses near-infrared spectroscopy to detect and analyze the diffuse interstellar band at 15273 Å in the dark cloud Barnard 68, revealing its carrier's distribution and properties in dense interstellar environments.

Contribution

It demonstrates the feasibility of using NIR DIBs to probe dense clouds and provides new insights into the carrier's density variation within Barnard 68.

Findings

DIB equivalent width increases with visual extinction

The DIB carrier's volume density is lower than expected in dense regions

The rate of DIB strength increase is flatter than in diffuse ISM

Abstract

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.