Spectroscopic Observation of TCP J19183710+2127170: a Young Stellar Object in an EX Lup-type Outburst

ATel #16177; Kenta Taguchi (Kyoto University), Mitsugu Fujii (Fujii Kurosaki Observatory), Taichi Kato (Kyoto University)
on 9 Aug 2023; 00:48 UT
Credential Certification: Keisuke Isogai (isogai@kusastro.kyoto-u.ac.jp)

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We report the spectroscopic classification and photometric observation of TCP J19183710+2127170. We obtained its spectra by the fiber-fed integral field spectrograph KOOLS-IFU; Matsubayashi+2019) mounted on the 3.8 m Seimei telescope (Kurita+2020) at Okayama Observatory of Kyoto University on 2023-08-02.55 UT using the VPH-blue grism (covering 4100-8500 Å, resolution~500), VPH683 grism (covering 5800-8000 Å, resolution~2000) and by a spectrograph (resolution~1000) on a 0.4 m telescope at Fujii Kurosaki Observatory.

As reported to CBAT, this object was discovered at 12.8 mag (unfiltered) on 2023-07-29.6761 UT by Tadashi Kojima, who mentioned the potential identification this object with ZTFJ191837.29+212722.1. This identification has been confirmed by the ZTF ALeRCE, which recorded the very slow brightening and getting bluer starting from 2023 April or May. No such bright outburst was detected in the ASAS-SN light curve. The ZTF position corresponds to Gaia DR3 4517076551873840384, whose position (J2000) is (19:18:37.292,+21:27:22.1) and parallax is 1.5330±0.0631 mas (distance ~ 0.65 kpc, distance modulus ~ 9.1 mag). From Bayestar2019, the reddening is E(g-r)~0.54 mag. According to Gaia DR3 Part 4. Variability (Gaia Collaboration 2022), this object had been classified as a young stellar object. According to 2MASS and WISE, this object showed an infrared excess in the quiescent phase.

Our spectrum by the Seimei telescope is dominated by a continuum with strong narrow emission lines of Balmer (Hα and Hβ). The Gaussian FWHM of the Hα is ~ 5.9 Å (or 270 km/s). Also, Fe II emission lines (e.g., 4924, 5018, 5169, and 6248 Å) and a possible He I emission line (6678 Å) are seen. The Balmer and Fe II lines are also confirmed by Fujii's spectra on 2023-07-30.502, 2023-08-01.632, and 2023-08-04.552. We propose the classification of the outburst as an EX Lup-type one because the overall spectral features, the time scale of the rising, and the infrared excess in the quiescent phase are common with those of EX Lup (Sicilia-Aguilar et al. 2012) during its 2008 outburst. The Li I absorption line (6707 Å) is undetected in our spectrum probably due to the high temperature during the outburst and/or the limited S/N and resolution of our data.

Our spectra