Astron. Astrophys. 364, 633-640 (2000)

Models of scattered light in UXORs

A. Natta ¹ and B.A. Whitney <sup>2

1</sup> Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
² Space Science Institute, 3100 Marine St., Suite A353, Boulder, CO 80303-1058, USA

Received 13 June 2000 / Accepted 2 August 2000

Abstract

This paper offers an interpretation of the photometric and polarimetric variability of UXORs where the star is surrounded by an optically thick, flared circumstellar disk similar to pre-main-sequence disks. A screen of dust sporadically obscures the stellar radiation, causing a minimum of the stellar light. Using a Monte Carlo code developed by Whitney & Hartmann (1992) we compute the polarization and colors of the observed radiation, and compare it to the available observations. The agreement is remarkably good. We find that the UXOR phenomenon occurs for systems seen in a well-defined range of inclinations, roughly between and 65^o-68^o. About 1/2 of the optically visible Herbig Ae stars should be UXORs, i.e., have deep photometric minima accompanied by a large increase in the polarization fraction, provided that screens can form. The results are not sensitive to the disk parameters, as long as the disk intercepts about 20% of the stellar radiation. The screens causing the light minima have sizes of the order of 1-few stellar radii, optical depth 3-5, and contain relatively small grains. We find a good fit to the observations with a MRN distribution with average radius 0.03 µm. The lack of UXORs with low polarization in deep minima can be understood if the screens are confined in a region close to the disk plane. However, the nature and origin of the screens remain open questions.

Key words: stars: pre-main sequence stars: circumstellar matter stars: variables: general

Send offprint requests to: A. Natta (natta@arcetri.astro.it)

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Contents

• 1. Introduction
• 2. Models
• 3. Results
• 4. Discussion
  • 4.1. Grain properties
  • 4.2. Disk properties
  • 4.3. Disk orientation
  • 4.4. The obscuring screen
• 5. Summary and conclusions
• Acknowledgements
• Appendices
  • A: scattered-light contribution from a spherical cloud
• References

© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001
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