The Eruption of the Candidate Young Star ASASSN-15qi

TL;DR

This paper characterizes a sudden, energetic outburst of the young star ASASSN-15qi in 2015, revealing explosive gas ejection and wind phenomena that challenge typical accretion burst models.

Contribution

It provides detailed spectral and photometric analysis of the outburst, highlighting unique explosive features and temporal evolution that differ from standard young star outburst explanations.

Findings

Sudden brightening less than one day in optical brightness.

Spectral evidence of explosive gas ejection at 1000 km/s.

No warm dust emission detected in mid-infrared spectrum.

Abstract

Outbursts on young stars are usually interpreted as accretion bursts caused by instabilities in the disk or the star-disk connection. However, some protostellar outbursts may not fit into this framework. In this paper, we analyze optical and near-infrared spectra and photometry to characterize the 2015 outburst of the probable young star ASASSN-15qi. The $\sim 3.5$ mag brightening in the $V$ band was sudden, with an unresolved rise time of less than one day. The outburst decayed exponentially by 1 mag for 6 days and then gradually back to the pre-outburst level after 200 days. The outburst is dominated by emission from $\sim10,000$ K gas. An explosive release of energy accelerated matter from the star in all directions, seen in a spectacular cool, spherical wind with a maximum velocity of 1000 km/s. The wind and hot gas both disappeared as the outburst faded and the source returned to its quiescent F-star spectrum. Nebulosity near the star brightened with a delay of 10-20 days. Fluorescent excitation of H$_2$ is detected in emission from vibrational levels as high as $v=11$, also with a possible time delay in flux increase. The mid-infrared spectral energy distribution does not indicate the presence of warm dust emission, although the optical photospheric absorption and CO overtone emission could be related to a gaseous disk. Archival photometry reveals a prior outburst in 1976. Although we speculate about possible causes for this outburst, none of the explanations are compelling.