VLT spectropolarimetry of the optical transient in NGC 300^*

Evidence of asymmetry in the circumstellar dust

¹ European Organization for Astronomical Research in the Southern Hemisphere (ESO), K. Schwarzschild-str. 2, 85748 Garching b. München, Germany e-mail: fpatat@eso.org
² Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej, 2100 Copenhagen, Denmark
³ Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Padova, v. Osservatorio n.5, 35122 Padua, Italy
⁴ Queen's University - Belfast, BT7 1NN, Northern Ireland, UK
⁵ Fundación Galileo Galilei – INAF, Telescopio Nazionale Galileo, 38700 Santa Cruz de la Palma, Tenerife, Spain
⁶ Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Catania, v. S. Sofia 78, 95123 Catania, Italy

Received: 7 August 2009
Accepted: 2 October 2009

Abstract

Aims. We study possible signs of asymmetry in the bright optical transient in NGC 300, to obtain independent information about the explosion mechanism, the progenitor star and its circumstellar environment.

Methods. Using VLT-FORS1, we obtained low-resolution optical linear spectropolarimetry of NGC 300 OT2008-1 on two epochs, 48 and 55 days after the discovery, covering the spectral range 3600–9330 Å.

Results. The data exhibit a continuum polarization at a very significant level. At least two separate components are identified. The first is characterized by both a strong wavelength dependency and a constant position angle ( degrees), which is parallel to the local spiral arm of the host galaxy. The latter is aligned along a completely different position angle (). While the former is identified as arising in the interstellar dust associated with NGC 300, the latter is most likely caused by continuum polarization by dust scattering in the circumstellar environment. No line depolarization is detected in correspondence to the most intense emission lines, disfavoring electron scattering as the source of intrinsic polarization. This implies that there is a very small deviation from symmetry in the continuum-forming region. Given the observed level of intrinsic polarization, the transient must be surrounded by a significant amount of dust (≥), asymmetrically distributed within a few thousand AU. This probably implies that one or more asymmetric outflow episodes took place during the past history of the progenitor.