Diffuse interstellar bands in Upper Scorpius: probing variations in the DIB spectrum due to changing environmental conditions^⋆,⋆⋆

¹ Radboud University Nijmegen, Toernooiveld 1, Postbus 9010, 6500 GL Nijmegen, The Netherlands
e-mail: nick.cox@ster.kuleuven.be
² Instituut voor Sterrenkunde, K.U. Leuven, Celestijnenlaan 200D, bus 2401, 3001 Leuven, Belgium
³ Astronomical Institute “Anton Pannekoek”, Universiteit van Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
⁴ Kapteyn Astronomical Institute, Rijksuniversiteit Groningen, Postbus 800, 9700 AV Groningen, The Netherlands
⁵ Astrobiology Group, Leiden Institute of Chemistry, Leiden University, Einsteinweg 55, 2300 RA Leiden, The Netherlands

Received: 7 March 2008
Accepted: 28 July 2011

Abstract

Aims. We study the effects of local environmental conditions affecting the diffuse interstellar band (DIB) carriers within the Upper Scorpius subgroup of the Sco OB2 association. The aim is to reveal how the still unidentified DIB carriers respond to different physical conditions prevailing in interstellar clouds, in order to shed light on the origin of the DIB carriers.

Methods. We obtained optical spectra with FEROS on the ESO 1.52 m telescope at La Silla, Chile, and measured the equivalent widths of five DIBs (at 5780, 5797, 6196, 6379, and 6613 Å) as well as those of absorption lines of di-atomic molecules (CH, CH⁺, CN) and atoms (K i, Ca i) towards 89 targets in the direction of Upper Scorpius. We construct a simple radiative transfer and chemical network model of the diffuse interstellar medium (ISM) sheet in front of Upp Sco to infer the effective radiation field.

Results. By measuring the DIB and molecular spectrum of diffuse clouds towards 89 sightlines in the Upper Scorpius region, we have obtained a valuable statistical dataset that provides information on the physical conditions that influence the band strengths of the DIBs. Both the interstellar radiation field strength, I_UV, and the molecular hydrogen fraction, f_H2, have been derived for 55 sightlines probing the Upp Sco ISM. We discuss the relations between DIB strengths, CH and CH⁺ line strengths, E_(B−V), I_UV, and f_H2. The ratio between the 5780 and 5797 Å DIBs reveals a (spatial) dependence on the local environment in terms of cloud density and exposure to the interstellar radiation field, reflecting the molecular nature of these DIB carriers.