PTF14jg: The Remarkable Outburst and Post-burst Evolution of a Previously Anonymous Galactic Star

We report the outbursting source PTF 14jg, which, prior to the onset of its late 2013 eruption, was a faint, unstudied, and virtually uncataloged star. The salient features of the PTF 14jg outburst are (i) projected location near the W4 H ii region and radial velocity consistent with physical association; (ii) a light curve that underwent an ∼6-7 mag optical (R-band) through mid-infrared (L-band) brightening on a timescale of a few months, peaked and then faded by ∼3 mag, but plateaued still >3.5 mag above quiescence by ∼8 months post-peak, lasting to at least 4 yr after eruption; (iii) strong outflow signatures with velocities reaching -530 ; (iv) a low-gravity and broad (∼100-150 FWHM) optical absorption-line spectrum that systematically changes its spectral type with wavelength; (v) lithium; and (vi) ultraviolet and infrared excess. We tentatively identify the outburst as exhibiting characteristics of a young star FU Ori event. However, the burst would be unusually hot, with an absorption spectrum exhibiting high-excitation (∼11,000-15,000 K) lines in the optical and no evidence of CO in the near-infrared, in addition to exhibiting an unusual light curve. We thus also consider alternative scenarios - including various forms of novae, nuclear-burning instabilities, massive star events, and mergers - finding them all inferior to the atypically hot FU Ori star classification. The source eventually may be interpreted as a new category of young star outburst with a larger amplitude and shorter rise time than most FU Ori-like events.