Spectral Features of YSO Outburst Gaia 21bty
ATel #14590; Lynne A. Hillenbrand (Caltech)
on 2 May 2021; 18:09 UT
Credential Certification: Lynne Hillenbrand (lah@astro.caltech.edu)
Subjects: Infra-Red, Optical, Variables, Young Stellar Object, Pre-Main-Sequence Star
As described at
this URL,
while the nature of the recent mid-infrared and optical brightening of Gaia 21bty
is still ambiguous, spectroscopic evidence of mixed-temperature absorption lines
plus wind-like profiles including P Cygni CaII triplet lines, suggests similarity to young FU Ori type stars.
Gaia 21bty (17:25:14.19 -37:08:14.17) was designated in a
Gaia Alert (Hodgkin 2013)
as a candidate outbursting YSO after the source brightened optically
from G=20.1 on 2020-10-02, to G=17.3 by 2021-02-17.
The amplitude is therefore nearly 3 mag on a timescale of less than 4.5 months (137 days).
Previous variability had been exhibited, ranging between G=20-21 mag.
The NEOWISE (Mainzer et al. 2014) lightcurve also shows a brightening,
but with the mid-infrared rise beginning at an earlier epoch.
Between 2019-08-24 and 2020-08-22 (MJD 58719.9 to 58920.2 to 59083.7)
Gaia 21bty increased in brightness by 0.3 mag in W1 and 0.5 mag in W2.
During this year-long interval, the W1-W2 color also became redder by about 0.2 mag,
after having drifted blueward over the previous two years.
On 2021-05-01 an optical spectrum was taken of Gaia 21bty
with Keck/ESI (Sheinis et al. 2002) at R=8000. An exposure of 1800 sec produced
continuum S/N ~40 at the CaII triplet but only ~5 at Halpha,
with little signal at shorter wavelengths.
The Halpha profile has double-peaked emission, with a central absorption
reaching to the continuum level and a red-to-blue flux ratio of 2.5.
The CaII triplet profiles have a clear P Cygni structure, with the absorption component narrow.
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 no shorter wavelength TiO is apparent.
In the above and other regards,
the far red optical absorption spectrum of Gaia 21bty is strikingly similar
to spectra of Gaia 18dvy (Szegedi-Elek et al., 2020) and Gaia 17bpi (Hillenbrand et al. 2017),
both of which are FU Ori type objects.
The Gaia 21bty spectrum is dissimilar to that of standard K-M type T Tauri stars having
low accretion (veiled-photosphere-dominated) to high accretion (continuum-plus-emission-dominated)
rates.
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).
It is otherwise unstudied, heretofore.
Gaia 21bty figures
