Astron. Astrophys. 326, 822-830 (1997)

Diffuse Interstellar Bands in single clouds:
new families and constraints on the carriers

J. Cami ^{1, 2, 3}, P. Sonnentrucker ^{4, 5}, P. Ehrenfreund ^{3, 6} and B.H. Foing <sup>4, 6

1</sup> SRON Groningen, P.O. Box 800, 9700 AV Groningen, The Netherlands
² Astronomical Inst. "Anton Pannekoek",Univ. of Amsterdam,Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
³ Leiden Observatory, P.O. Box 9513, 2300 RA Leiden, The Netherlands
⁴ Solar System Division, ESA Space Science Department, ESTEC/SO, 2200 AG Noordwijk, The Netherlands
⁵ Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France
⁶ Institut d'Astrophysique Spatiale, CNRS, Bat 121, Campus d'Orsay, F-91405 Orsay, France

Received 27 March 1997 / Accepted 22 May 1997

Abstract

We present a survey of diffuse interstellar bands (DIBs) in single clouds. High resolution and high S/N observations of DIBs towards lines of sight representing very different environments allow to investigate mutual correlations in a large consistent sample of DIBs. The classification of DIBs into families is reviewed and refined according to these results. We find that the selected 44 DIBs are due to different carriers. Two isolated families of DIBs ( 5797 et al., 4501 et al.) are found, which are mutually slightly anticorrelated. The behaviour of DIBs with respect to the local UV field is investigated using hydrogen column densities to estimate the strength of the UV field, resulting in the hypothesis that most of the DIB carriers are molecules sensitive to photo-ionization. With this interpretation, the apparent division of interstellar clouds into , and Orion types can be understood as a sequence in strength of the UV field. Differences in strength of the UV field between different lines-of-sight are due to the so-called "skin-effect", the effective shielding of UV radiation by the outer layers of interstellar clouds.

Key words: ISM: molecules - ISM: dust - molecular data

Send offprint requests to: P. Ehrenfreund

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Contents

• 1. Introduction
• 2. Observations
  • 2.1. Data acquisition
  • 2.2. Measurements
  • 2.3. Description of the star sample
  • 2.4. The final DIB-sample
• 3. Families of DIBs?
  • 3.1. Why single clouds?
  • 3.2. Mutual DIB correlations: errors, bias and confidence.
  • 3.3. Interpretation of the mutual correlations.
• 4. The nature of the DIB-carriers
  • 4.1. The ionization hypothesis
  • 4.2. The and dichotomy: skin and flesh of clouds.
  • 4.3. DIBs and UV extinction curve
• 5. Conclusion
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: October 15, 1997
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