Astron. Astrophys. 337, 808-814 (1998)

An attempt to detect the dust disk of Vega by photopolarimetry, and constraints on the grain size ^{* **}

N. Mauron ¹ and H. Dole <sup>2

1</sup> Equipe d'Astrophysique CC 72, ISTEEM-CNRS-UM2, Place Bataillon, F-34095 Montpellier Cedex 05, France (mauron@merlin.graal.univ-montp2.fr)
² Institut d'Astrophysique Spatiale, CNRS & Université de Paris-Sud, Batiment 121, F-91405 Orsay Cedex, France (dole@ias.fr)

Received 22 December 1997 / Accepted 12 June 1998

Abstract

We report on a first attempt to detect Vega's disk through optical scattered light by using linear photopolarimetry. Polarization measurements on the aureole of Vega were carried out between 7" and 30" from the star with a 10" diameter hole and down to polarization level of 10^-4. No signal reliably attributable to circumstellar dust was detected, and an upper limit to the polarized surface brightness of the disk is derived. This upper limit for Vega's disk is about 200 times lower than the polarized brightness observed around  Pic, at an angular offset of 15" from the stars.

The upper limit is also compared to a simple model, in which one assumes a plausible total dust mass of 2 10^-8 , and a pole-on oriented disk with a typical radius of 20" as favoured by far-infrared and submillimetric experiments. We also suppose that the grains are spherical (Mie) particles. Our analysis can exclude that a major part of dust mass would consist of grains of 0.01-0.3 µm, as in the interstellar medium. If a single size is assumed, the observational upper limit favours radii of at least 5-10 µm or larger. If a size distribution including large particles ( 300 µm) is assumed, the data suggests that only a very small fraction ( 1/1000) of the dust mass is in 0.01-0.3 µm grains.

Key words: polarization circumstellar matter stars: individual: Vega

* Based on observations made at the 2-m telescope of Pic-du-Midi, operated by Observatoire Midi Pyrénées (Centre National de la Recherche Scientifique & Université Paul Sabatier de Toulouse, France)
** The appendix is only available in the electronical form of this paper

Send offprint requests to: N. Mauron

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Contents

• 1. Introduction
• 2. Instrumentation
• 3. Observing method and difficulties
• 4. Observational results
• 5. Discussion and comparison with models
• 6. Conclusions and suggestions
• Acknowledgements
• Appendix A: more details and comments on observations and results
  • A.1. About on-star measurements of Vega and of polarization standards
  • A.2. Additional comments on off-star observations
• References

© European Southern Observatory (ESO) 1998

Online publication: August 27, 1998
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