SOAR follow up of six highly reddened Galactic classical nova candidates

ATel #14928; A. Kawash, E. Aydi, J. Strader, K. V. Sokolovsky, and L. Chomiuk (MSU)
on 21 Sep 2021; 15:30 UT
Credential Certification: Adam Kawash (kawashad@msu.edu)

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We report on spectroscopic follow up of six red, optical transients reported by Gaia Photometric Science Alerts between February 2018 and August 2021: Gaia18ajn, Gaia18cew, Gaia20dfb, Gaia20dfc, Gaia21axf, and Gaia21dwe. The color and the presence of emission lines in the low resolution Gaia spectra indicate that these transients were potentially highly reddened, Galactic classical novae. We obtained low resolution (R ~ 1000) spectra of these transients with the Goodman spectrograph (Clemens et al. 2004, SPIE, 5492, 331) on the 4.1 m SOAR telescope, during the nights of 2020-10-05 (Gaia20dfb) and 2021-10-20 (the rest). We found evidence of past nova eruptions in four of the candidates while no continuum or line emission was detected in the remaining two. Below is a table and brief discussion of our findings.

Name	Discovery Date	Peak Brightness (G mag)
Gaia18ajn	2018-02-07	14.7
Gaia18cew	2018-08-13	14.9
Gaia20dfb	2020-07-09	15.9
Gaia20dfc	2020-07-09	14.9
Gaia21axf	2021-02-16	18.1
Gaia21dwe	2021-08-18	16.3

Gaia18ajn: The SOAR spectrum, taken more than 3 years after discovery, shows a red continuum with very weak emission lines of Halpha and possibly [Fe VI] 5176 A and [Fe VII] 5721 A. The SOAR spectrum, the emission lines in the Gaia spectrum, and the fact that it took more than a year to drop by 6 mags all suggest that the transient was a highly reddened, Galactic classical nova.

Gaia18cew: The SOAR spectrum shows a red continuum and broad emission lines of Halpha and [Fe VII] 6086. The FWHM of Halpha is around 2000 km/s. The SOAR spectrum, the emission lines in the Gaia spectrum, and the fact that it took over two years to decline by 6 mags indicate that the transient was a highly reddened, Galactic classical nova.

Gaia20dfb: The SOAR spectrum, taken around 3 months after discovery, shows a broad Halpha line with a FWHM of more than 2000 km/s. The transient declined by 3 mags in roughly 60 days, and the Gaia spectrum shows emission lines. This indicates that Gaia20dfb was a reddened classical nova.

Gaia20dfc: There was no sign of continuum or emission lines in the 1800 s SOAR spectrum at the location of the transient, observed more than a year after discovery. The transient declined by 4 mags in 55 days, and the Gaia low-resolution spectrum shows emission lines, similar to the previously mentioned classical nova candidates. The emission lines and the color in the Gaia spectrum are consistent with the other candidates, suggesting that this transient was likely a highly reddened classical nova, but a definitive classification based on the SOAR spectrum cannot be made.

Gaia21axf: There were no sign of continuum or emission lines in the 2400 s SOAR spectrum taken around 8 months after discovery. The previous Gaia non-detection at the position of this transient occurred on 2020-10-04, 136 days prior to the discovery date, so the eruption date and peak brightness are highly uncertain. It took about 24 days to decline by one magnitude in the Gaia lightcurve. The lack of flux in the blue side of the Gaia spectrum suggests that this was likely not a nearby dwarf nova outburst and could have been a reddened classical nova observed after peak brightness, but a definitive classification based on the SOAR spectrum cannot be made.

Gaia21dwe: The SOAR spectrum taken around 2 months after discovery, shows a red continuum with strong emission lines of H I, O I lines at 7773 and 8446 A, and C II 7235 A. The FWHM of Halpha is around 1400 km/s. The SOAR spectrum shows this was a highly reddened, Galactic classical nova.