SUMMARY:
The cataloged but heretofore unstudied source Gaia 21bty is described as a likely YSO outburst,
with spectroscopic similarity to recently confirmed FU Ori stars.
DETAILS:
The source of interest is located at 17:25:14.19 -37:08:14.1 (2000.) in southern Scorpius.
Images below are DECaps optical, Spitzer/GLIMPSE360 mid-infrared, and WISE mid-infrared.
They are the color renditions available from Aladin, with ~3' FOV.
Gaia 21bty is located in a dark region adjacent to a nebulous area that was
designated as a small bubble by Simpson (2012).
The source was cataloged as a candidate
young stellar object by Marton (2017) and also included in Vioque (2020).
Here is a slightly broader perspective, 12' FOV.
As illustrated in Vizier the spectral energy distribution is that of a typical Class II or perhaps Class I source.
There is a substantial difference between the Spitzer and WISE photometric measurements.
These are separated in time, with WISE later; see lightcurve below.
The optical brightening of this cataloged but otherwise unstudied star
was announced in 2021 as a
Gaia Alert.
The optical burst was a 3 mag brightening that took less than 4.5 months (137 days)
for the source to go from quiescence to its current brightness of G = 17.1 mag, which may or may not be the peak.
The brightening was also captured by NEOWISE in the mid-infrared,
though occurrs at an earlier epoch than in the optical.
The NEOWISE data are not as up-to-date as the Gaia data,
so the full amplitude of the burst likely is not captured in the lightcurve below.
Again note the difference between the earlier Spitzer and first WISE measurements.
The telescopes do have different beam sizes, with WISE larger.
However, WISE measurements are via aperture fitting photometry, which should compensate for any crowding and high nebulosity that would have an affect aperture photometry.
A moderate resolution (R=8000) optical spectrum of Gaia 21bty was obtained on 2021-05-01 UT.
The Halpha profile is double-peaked with a central absorption reaching to the continuum level
and 2.5 times the flux on the red side as the blue side.
The CaII triplet profiles have a clear P Cygni structure with a narrow absorption component.
There is also wind-like absorption in the profiles of KI 7669, 7699 A
and the OI 7773 A triplet.
Other strong absorption features include OI 8446, FeI 8688 A, MgI 8806 A, and FeI 8824 A.
There are indications of the TiO bandheads at 8860 and 8432 A,
but not shorter wavelength TiO.
In the above and other regards,
the far red optical absorption spectrum of Gaia 21bty is strikingly similar
to those of Gaia 18dvy and Gaia 17bpi, both of which are FU Ori type objects.
Comparisons are shown below.
The spectroscopic properties of these sources are distinct from those of regularly accreting
T Tauri-type stars
with low (veiled-photosphere-dominated) to high accretion (continuum-plus-emission-dominated) rates.
It should also be noted that the Halpha and CaII profiles of FU Ori stars do tend to show evolution over the first few years.
Unfortunately the S/N of the Gaia 21bty spectrum is too low in the LiI region to detect that important line.