Explosion of an LBV star in the galaxy UGC 8246

TL;DR

This study documents the 2013-2014 explosion of a luminous blue variable star in galaxy UGC 8246, revealing spectral features, ejecta structure, and light echo evidence through spectroscopy and photometry.

Contribution

First detailed spectroscopic and photometric analysis of an LBV star explosion in UGC 8246, identifying bipolar ejecta and light echo phenomena.

Findings

Spectral profiles show composite triple components in emission lines.

Evidence of bipolar ejecta and a surrounding gaseous envelope.

Detection of a light echo propagating after the outburst.

Abstract

We present the results of spectroscopy and CCD photometry of the intermediate-luminosity red transient PSN J13100734+3410514 in the galaxy UGC 8246 performed in February and June 2014 with the Russian 6-meter telescope and SCORPIO spectral camera. Our CCD photometry was continued with the Special Astrophysical Observatory's 1-m telescope till November 2014. The star was discovered in late December 2013 at visual brightness 17.6 mag, which corresponded to the absolute magnitude Mv ~ -12.7, and was identified as a supernova impostor. Spectra taken at the visual brightness level of 19.5 mag show composite triple profiles in the H-alpha and H-beta emission lines. We explain the main component of the profiles as radiation from a photoionized extensive gaseous envelope formed by the stellar wind of the progenitor before the outburst. The other two components are treated as radiation from bipolar ejecta. In Balmer line profiles, there is an evidence for a light echo propagating in the surrounding medium after the outburst. Our spectra contain emissions of He I, Na I, Mg I, numerous Fe II emissions, the strongest of which have P Cyg profiles. There are also [O II], [O III], and [S II] emissions of an H II region associated with the transient. The emissions of the region are superimposed on the star spectrum. The light curves show rapid decline and color reddening. Our observations confirm that the UGC 8246 transient was an explosion of a high-mass LBV star.