Astron. Astrophys. 327, 1215-1221 (1997)

The diffuse interstellar bands at 5797, 6379 and 6613Å

Ionization properties of the carriers ^*

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

1</sup> Solar System Division, ESA Space Science Department, ESTEC/SO, PB 299, 2200 AG Noordwijk, The Netherlands
² Observatoire de Strasbourg, 11 rue de l'Université, F-67000 Strasbourg, France
³ SRON Groningen, P.O. Box 800, 9700 AV Groningen, The Netherlands
⁴ Astronomical Inst."Anton Pannekoek", Univ. of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
⁵ Leiden Observatory, P.O. Box 9513, 2300 RA Leiden, The Netherlands
⁶ Institut d'Astrophysique Spatiale, CNRS, Bat 121, Campus d'Orsay, F-91405 Orsay, France

Received 5 May 1997 / Accepted 11 July 1997

Abstract

We present a study of the behaviour and ionization properties of three narrow Diffuse Interstellar Bands (DIBs) at 5797, 6379 and 6613 Å. In all three DIBs substructures have recently been detected, indicating large gaseous molecular carriers. Studying DIBs in regions with drastically different physical properties in terms of UV flux and density enables us to monitor the behaviour of the carriers and hence to constrain their nature. We observed these three DIBs along 40 different lines-of-sight (35 program stars and 5 standard stars) consisting of HII regions, dark clouds, molecular clouds and reflection nebulae. The DIB variations at low reddening are explained by a new model of photoionization equilibrium of the DIB carriers. This model takes into account the penetration depth of UV ionizing photons throughout the cloud. The slope of the variation of DIB strength as a function of reddening thus allows us to estimate the effective ionization potentials of the carriers. Following this new analysis, the carriers of the 5797 and 6613 Å DIBs would have ionization potentials above 10 eV, reminiscent of large PAHs or fullerenes which have a single positive charge. The estimated ionization potential (7-9 eV) of the 6379 Å DIB seems to indicate a large neutral carrier.

Key words: ISM: molecules ISM: molecular clouds, HII regions molecular processes line: idendification

* Based on observations with OHP 1.93m Telescope and Elalie spectrograph.

Send offprint requests to: P. Sonnentrucker

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Contents

• 1. Introduction
• 2. Observations
  • 2.1. Data acquisition
  • 2.2. Equivalent widths
• 3. Results and discussion
  • 3.1. Environmental behaviour
  • 3.2. Dehydrogenation versus ionization
  • 3.3. The ionization equilibrium model
  • 3.4. Ionization potential and DIB variation vs reddening
  • 3.5. Comparison with observations
  • 3.6. Sources of errors and limits of the model
• 4. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998
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