Spectroscopic Classification of AT 2018htr as a Likely LBV Outburst with SOAR and Photometric Observations with Swope

ATel #12166; M. R. Siebert, C. D. Kilpatrick, C. Rojas-Bravo, R. J. Foley (UCSC), J. J. Hermes (UNC), A. Campillay (LCO)
on 4 Nov 2018; 04:48 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Ryan Foley (foley@ucsc.edu)

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We report spectral observations, made on 2018 Nov 4 UT, of AT 2018htr, discovered by DLT40 in NGC 0045, with the Goodman spectrograph (wavelength range 3800 - 8950 A) on the SOAR telescope. The spectrum is blue with Balmer and He I P-Cygni features and several strong absorption features including Ca H&K and several Fe II lines. The spectrum is generically similar to "cool" LBV outbursts such as UGC 2773 OT-1 (Smith et al., 2010, AJ, 193, 1451; Foley et al., 2011, ApJ, 732, 32). The spectrum is also similar to the Type IIn SN 1994W (Sollerman et al., 1998, ApJ, 493, 933). Correcting for the small redshift of NGC 0045 (cz = 467 km/s; Monnier Ragaigne et al., 2003, A&A, 408, 465), we find that the minimum of the H-alpha absorption for AT 2018htr is blueshifted by -1240 km/s. The absorption feature extends to approximately -2100 km/s.

We also report photometric observations, also made on 2018 Nov 4 UT, of AT2018htr with the Swope telescope (+ Direct imager). At 20181104.02 UT, we measure g = 19.4 +/- 0.1 mag, fainter than the observation made by DLT40 two days earlier (20181102.07 UT), where they report a clear magnitude of 17.5 +/- 1.1 mag. To test if AT 2018htr was fading quickly, we obtained another epoch (at 20181104.10 UT), but it has not faded significantly between the two epochs.

Using a Tully-Fisher distance to NGC 0045 of 9.2 Mpc (Karachentsev et al., 2013, AJ, 145, 101) or alternatively a TRGB distance of 6.6 Mpc (Tully et al., 2013, AJ, 146, 86), we find that the transient is currently M_g = -10.5 or -9.8 mag, respectively. This is fainter than most LBV outbursts and similar to the quiescent luminosity of some LBVs. It is significantly fainter than any SN IIn, and we therefore consider AT 2018htr likely an LBV or other stellar outburst.

We thank the SOAR observatory staff, particularly C. Corco, P. Ugarte, and SOAR director J. Elias for rapid execution of this ToO observation under NOAO program 2018B-0315.