Peculiar nova M31N 2016-04a: Spectroscopy of the re-brightening
ATel #9394; S. C. Williams (Lancaster), M. J. Darnley (LJMU), A. W. Shafter (SDSU), K. Chinetti (Caltech)
on 22 Aug 2016; 20:06 UT
Credential Certification: Steven Williams (scw@astro.ljmu.ac.uk)
Subjects: Optical, Nova, Transient
M31N 2016-04a (MASTER OT J004528.12+414117.6) was discovered on 2016 Apr 10 (ATel #8950). We obtained a spectrum of the nova on 2016 June 3.19 UT, which showed narrow Balmer, He I and He II emission lines (ATel #9116). The nova recently underwent an optical re-brightening (see CBAT TOCP and ATel #9390).
We obtained a spectrum of M31N 2016-04a during this re-brightening with the SPRAT spectrograph (Piascik et al. 2014) on the 2m Liverpool Telescope (Steele et al. 2004) on 2016 Aug 21.17 UT, four months after discovery. The spectrum shows strong, narrow (Hα FWHM ~ 1000 km/s) Balmer emission (Hα, Hβ, Hγ and Hδ). The spectrum now also clearly shows several Fe II emission lines, including those of the 42, 48 and 49 multiplets. N III (4638 Å) is tentatively detected and helium lines may still be present. The spectrum confirms that M31N 2016-04a is indeed a classical nova in M31 and now shows an Fe II-type spectrum. Having previously showed an He/Nn-type (see Shafter et al. 2011) spectrum, this nova may be considered a `hybrid' nova.
As noted in ATel #9390, the nova is photometrically similar to the very slow novae V723 Cas and V5558 Sgr. We further note that the spectroscopic evolution from narrow Balmer and helium lines to an Fe II-type spectrum described here in M31N 2016-04a was also seen in V5558 Sgr (Tanaka et al. 2011). Further photometric and spectroscopic observations of this peculiar nova as is evolves are encouraged.
