The little-studied cluster Berkeley 90. II. The foreground ISM

TL;DR

This study investigates the interstellar medium towards the Berkeley 90 cluster, revealing multiple clouds with distinct velocities and densities, and classifies diffuse interstellar bands based on their environmental conditions.

Contribution

It provides detailed measurements of ISM absorption lines and classifies DIBs in a new sightline, enhancing understanding of ISM structure and DIB carriers.

Findings

Two distinct interstellar clouds with different velocities and densities.

Identification and classification of DIBs based on cloud properties.

Detection of a high-velocity redshifted component in one sightline.

Abstract

Context: Nearly one century after their discovery, the carrier(s) of Diffuse Interstellar Bands is/are still unknown and there are few sightlines studied in detail for a large number of DIBs. Aims: We want to study the ISM sightlines towards LS III +46 11 and LS III +46 12, two early-O-type stellar systems, and LS III +46 11 B, a mid-B-type star. The three targets are located in the stellar cluster Berkeley 90 and have a high extinction. Methods: We use the multi-epoch high-S/N optical spectra presented in paper I (Ma\'iz Apell\'aniz et al. 2015), the extinction results derived there, and additional spectra. Results: We have measured equivalent widths, velocities, and FWHMs for a large number of absorption lines in the rich ISM spectrum in front of Berkeley 90. The absorbing ISM has at least two clouds at different velocities, one with a lower column density (thinner) in the K I lines located away from Berkeley 90 and another one with a higher column density (thicker) associated with the cluster. The first cloud has similar properties for both O-star sightlines but the second one is thicker for LS III +46 11. The comparison between species indicate that the cloud with a higher column density has a denser core, allowing us to classify the DIBs in a sigma-zeta scale, some of them for the first time. The LS III +46 12 sightline also has a high-velocity redshifted component.