VLT spectropolarimetry of the optical transient in NGC 300. Evidence of asymmetry in the circumstellar dust

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 (68.6 ± 0.3 degrees), which is parallel to the local spiral arm of the host galaxy. The latter is aligned along a completely different position angle (151.3 ± 0.4). 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 (≥4 × 10 ^-5~M_⊙), 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.

Based on observations made with ESO Telescopes at Paranal Observatory under Program ID 281.D-5016.