WTP 10aaauow: discovery of a new FU Ori outburst towards the RCW 49 star-forming region in NEOWISE data

Abstract

Large-amplitude accretion outbursts in young stars are expected to play a central role in proto-stellar assembly. Outburst identification historically has taken place using optical techniques, but recent, systematic infrared searches are enabling their discovery in heavily dust-obscured regions of the Galactic plane. Here, we present the discovery of WTP 10aaauow, a large-amplitude mid-infrared (MIR) outburst identified in a systematic search of NEOWISE data using new image subtraction techniques. The source is located towards the RCW 49 star-forming region, and estimated to be at a distance of ≈4 kpc via Gaia parallax measurement. Concurrent with the MIR brightening, the source underwent a ≳ 5 mag outburst in the optical and near-infrared (NIR) bands, reaching a peak luminosity of ≈260 L⊙ in 2014-2015, followed by a slow decline over the next 7 years. Analysis of the pre- and post-outburst spectral energy distributions reveal a pre-outburst stellar photosphere at a temperature of 3600 − 4000 K, surrounded by a likely two-component dust structure similar to a flat-spectrum or Class I type YSO. We present optical and NIR spectroscopy that show a GK-type spectrum in the optical bands exhibiting complex line profiles in strong absorption features, and evidence for a wind reaching a terminal velocity of ≈400 km s−1. The NIR bands are characterized by a cooler M-type spectrum exhibiting a forest of atomic and molecular features. All together, the spectra demonstrate that WTP 10aaauow is an FU Ori type outburst. Ongoing systematic infrared searches will continue to reveal the extent of this population in the Galactic disk.