Photometric and spectroscopic evidence for the EX Lup nature of the ongoing outburst from V1741 Sgr (= Gaia22dtk/ZTF18abfogsw/SPICY 71482)

ATel #15721; Michael A. Kuhn (Caltech), Lynne A. Hillenbrand (Caltech), Michael S. Connelley (U. of Hawaii), Viraj R. Karambelkar (Caltech), Christoffer Fremling (Caltech), Ellen Lee (U. of Hawaii), Tomas Ahumada (Caltech), Rafael S. de Souza (SHAO), Emille E. O. Ishida (U. Clermont Auvergne)
on 26 Oct 2022; 02:01 UT
Credential Certification: Michael Kuhn (mkuhn1@gmail.com)

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Brightening of candidate YSO V1741 Sgr (= SPICY 71482) was reported as a Gaia Alert (Gaia22dtk; Hodgkin et al. 2021) based on a single-epoch (5 Sep 2022) Gaia photometric measurement. This has been confirmed by Zwicky Transient Facility (ZTF) light curves (ZTF18abfogsw) showing the source brightening and becoming bluer from g~18.5 mag to 15 mag (Δg = 3.5 mag) and r~17 mag to 14 mag (Δr = 3 mag). Pre-outburst, the light curves exhibited lower-level variability with standard deviations of ~0.5 mag. The current outburst started in late April 2022 (or shortly before), peaked by mid-July 2022, and has subsequently plateaued, with a 0.5 mag dip in August 2022. Although V1741 Sgr was historically observed exhibiting burst-like variability (Walker 1957; Petit 1958), little attention has been paid to this source in the intervening years.

In follow-up, we obtained spectra from IRTF/SpeX (15 Oct 2022) and Keck/LRIS (19 Oct 2022), showing that the source resembles a rapidly accreting T Tauri-type star. LiI 6707Å absorption W(Li)=0.25Å demonstrates stellar youth. Strong emission features in the optical spectrum include H Balmer and Paschen lines, HeI, and the CaII infrared triplet, with weak FeI and MgI also present. Forbidden lines from [OI], [NII], [CaII], and [FeII] are seen, though there is no [SII]. Lines including NaD, KI, OI, FeII, and MgI, present deeper absorption than expected from normal stellar photospheres and suggest the presence of a wind. In the infrared, which shows a blue continuum, the CO bands are in emission, as are lines from the Paschen and Brackett series, along with HeI 10830A and NaI.

The lightcurve amplitude and rise time, the spectroscopic emission lines, and the presence of wind absorption are all consistent with an EX Lup-type outburst for V1741 Sgr.

V1741 Sgr is ~0.5 degrees from the center of the young star cluster NGC 6530 (= the Lagoon Nebula), and the star's Gaia DR3 parallax = 0.795±0.072 mas (corrected for zero-point offset; Lindegren et al. 2021) and proper motions are statistically consistent with membership in NGC 6530 (Kuhn et al. 2021a). The star was identified as a Class II YSO in the Spitzer/IRAC Candidate YSO (SPICY) catalog (Kuhn et al. 2021b).

ZTF is led by the California Institute of Technology, US, supported by the National Science Foundation under Grant No. AST-2034437, and includes IPAC, US; Los Alamos National Laboratory, US; University of Maryland, US; the University of Wisconsin at Milwaukee, US; University of Washington, US; Oskar-Klein Center of the University of Stockholm, Sweden; DESY and Humboldt University of Berlin, Germany; Weizmann Institute of Science, Israel; and the University System of Taiwan, Taiwan. The infrared Telescope Facility is operated by the University of Hawaii under contract 80HQTR19D0030 with the National Aeronautics and Space Administration. W. M. Keck Observatory is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

V1741 Sgr Figures