The remarkable light and colour variability of Small Magellanic Cloud Be stars

¹ Laboratoire d'Astrophysique de Grenoble, Université Joseph Fourier, BP 53, 38041 Grenoble Cedex 9, France e-mail: dewit@obs.ujf-grenoble.fr
² Astronomical Institute, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands
³ SRON Laboratory for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
⁴ Institut d'Astrophysique de Paris, UMR 7095 CNRS, Université Pierre & Marie Curie, 98bis boulevard Arago, 75014 Paris, France

Received: 3 March 2006
Accepted: 12 June 2006

Abstract

Aims.To test if the irregularly variable optical emission of hot stars in the Small Magellanic Cloud is due to variable amounts of Brehmstrahlung by ionized material in the circumstellar environment. The light curve variability of this sample of hot stars (∼200 objects) is described by Mennickent et al. (2002).

Methods.(1) Probing the relation between flux excess in the optical and in the near-infrared J-band using EROS light curves (time span of 5 years) and 2MASS photometry; (2) analyzing the relation between colour and magnitude of the variability by means of an analytical, time-dependent model of emission from ionized material in an outflowing circumstellar disk.

Results.(1) A correlation between optical and near-IR flux excess is found; (2) a bi-valued relation between excess colour and magnitude for ∼100 objects is discovered. It results in a loop structure in a colour–magnitude diagram. Significantly, this loop is traversed in a clockwise sense for ~90% of the stars, and anti-clockwise for the remainder; (3) a simple analytical model of an outflowing disk and a variable mass loss rate is capable of reproducing observed light curves and the bi-valued colour–magnitude relation.

Conclusions.The optical variability of a large fraction of the hot stars is due to variations in the amount of Brehmstrahlung. The circumstellar envelope can contribute up to 50% of the total optical flux. This variability can be interpreted as due to an outflowing ionized CS disk, that evolves into a ring structure. The observed bi-valued colour–magnitude relation is an optical depth effect at the various wavelengths of interest. The loop is traversed clockwise by outflowing matter, but anti-clockwise by infalling material. As the material is generally outflowing, the CS environment of the star is progressively cleared once the star stops losing mass. This study makes use of public OGLE, MACHO and 2MASS data.