Palomar Gattini-IR discovery and spectroscopic classification of a highly reddened YSO in outburst

ATel #13902; M. Hankins (Caltech), L. A. Hillenbrand (Caltech), K. De (Caltech), A. Tzanidakis (Caltech), M. M. Kasliwal (Caltech), M. Ashley (UNSW), V. Karambelkar (Caltech), R. Soria (NAOC), R. M. Lau (ISAS/JAXA), A. Moore (ANU), E. O. Ofek (Weizmann), J. Soon (ANU), T. Travouillon (ANU) on behalf of the Palomar Gattini-IR team
on 29 Jul 2020; 00:24 UT
Credential Certification: Matthew Hankins (mhankins@caltech.edu)

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We report the discovery and spectroscopic classification of a reddened large amplitude outburst from a highly unusual Young Stellar Object. The source, PGIR20dwf (AT 2020nrs), was found as a large amplitude variable in the Palomar Gattini-IR survey (De et al. 2020, Moore & Kasliwal 2019). Palomar Gattini-IR is a wide-field NIR transient survey scanning the visible Northern sky every two nights to a median J band depth of 15.7 AB mag, although the sensitivity in the galactic plane can be ~1-2 mags less as the observations become confusion limited. PGIR20dwf was first detected in the Gattini data processing pipeline on UT 2020-06-23 at a J magnitude of 12.7 +/- 0.1 mag (AB), at J2000 coordinates of RA 19:25:40.56 Dec +15:07:46.7 Our initial detection of the source from Gattini-IR was after a seasonal gap with the last non-detection on 2019-12-14 to a J band depth of 13.3 mag (AB)- noting that this location is 3.2 degrees off the galactic plane. Optical data from ZTF also detect the source (ZTF19ablttxs) and show brightening from quiescence beginning around this same time. Since plateauing in late June at g ~ 17.1 mag (AB), r ~ 15.4 mag (AB), and J ~ 12.6 mag (AB), the source has exhibited ~0.5 mag oscillations on few week timescales in both the infrared and the optical. A search of the literature reveals this source was identified as a candidate YSO based on Spitzer photometry, but is otherwise not noted as remarkable prior to this recent brightening episode. Archival photometry from PanSTARRS, however, indicates a previous large-amplitude bright state beginning between mid-2012 and mid-2013, suggesting that the source may be a repeating outburst, EX Lup type YSO. As part of follow-up efforts, we obtained an optical spectrum of the source on UT 2020-07-02 using the Double Beam Spectrograph on the Palomar 200-inch telescope. The spectrum shows several common YSO features such as H-alpha and the CaII triplet in emission, as well as several other species NaID, K I, and OI in strong absorption, likely blueshifted due to a wind. The most remarkable aspect of the optical spectrum is the presence of broad TiO and VO bands in emission. The presence of these molecular bands in emission is exceedingly rare for YSOs and has only been observed in a few other cases (e.g., the highly variable accretors PTF10nvg and iPTF15afq as well as a few Class I type SED sources in Taurus). Further follow-up observations are underway and we encourage additional multi-wavelength follow-up observations of the source. We note that there is a close (~1”) optical companion to the SW, as well as another nearby (~2”) infrared source to the E, possibly extended nebular structure, that is apparent in 2MASS K-band as well as Spitzer/GLIMPSE images.