Gaia21fji is indeed a Young Stellar Object
ATel #15131; Ch. Adami, N. Grosso (LAM-Marseille), M. Dennefeld (IAP-Paris), St. Favard, J. Schmitt, Fr. Huppert, J. C. Brunel (OHP)
on 21 Dec 2021; 14:13 UT
Credential Certification: Michel Dennefeld (dennefel@iap.fr)
Subjects: Optical, Young Stellar Object
Gaia 21fji (= AT2021aftk) was alerted on Nov. 6, 2021 due to a recent brightening (Hodgkin et al.,
TNS Astronomical Transient Report 130737) and noted as "Candidate YSO". It had in fact been recognised as variable much earlier by the ATLAS survey (Tonry et al. 2018, PASP 130, 4505) as ATLAS 16awa.
We obtained a classification spectrum on Dec. 8 with the new Mistral spectrograph mounted on the Cassegrain focus of the Haute-Provence Observatory (OHP) 1.93m telescope. The red setting was in use, covering the range from 6000 to 9800 Angstroems, at a resolution of 2.1 A per pixel and a 2"
(= 4 pixels) N-S slit.
The spectrum displays many strong emission lines above a red continuum: Halpha (with an EW of -114 A), the full series of Paschen lines, from PBeta to P14 (last detected line), the IR Calcium triplet and the OI line at 8446 A. Most notably, the OI 7771 line is not seen, nor any
stellar absorption line. The object is therefore undoubtedly a reddened Young Stellar Object (YSO).
In fact, the position measured by Gaia coincides with the pre-main-sequence star LkHalpha-324SE
in the wider complex L988 discussed by Herbig & Dahm (2006 AJ, 131,1530) and is therefore not
a new candidate. But we see no [OI] 6300 nor the [SII] doublet 6723 A in emission, to an upper limit of 0.5 % of Halpha, which were seen by those authors (but our slit was not oriented along the NW flow).
As we have presently no measure yet of Hbeta, it cannot be decided yet if the recent increase in brightness is due simply to a decrease of obscuration, moving dust clumps in the disc
or another cause. It is to be noted that the ATLAS lightcurve shows a steady increase of more than 2 magnitudes since August 25 this year, and both Gaia and ATLAS lightcurves show that similar excursions have already occured in the past: the object deserves further spectroscopic monitoring.
