Strong linear polarization of V4332 Sagittarii: a dusty disc geometry

Abstract

The eruption of V4332 Sgr was observed in 1994. During the outburst, the object became extremely red, so it is considered as belonging to luminous red transients of the V838 Mon type. It has recently been suggested that the central object in V4332 Sgr is now hidden in a dusty disc and that the photospheric spectrum of this object observed in the optical results from scattering the central star radiation on dust grains in the disc. One expects significant polarization of the spectrum in this case. We investigate this prediction. We present and analyse polarimetric observations of V4332 Sgr in the V and R photometric bands done with the NOT telescope. The optical light of V4332 Sgr is linearly polarized with a degree of ~26% in the V band and ~11% in R. Discussion of the observed polarization leads us to conclude that the photospheric spectrum observed in V4332 Sgr is probably produced by dust scattering not only in the disc but also in the outflow from the object seen in the emission features.