He II Emission Line Became the Most Intense Line in the Optical Spectrum of Nova V1405 Cas

ATel #16496; Gesesew R. Habtie (S. N. Bose National Center for Basic Sciences), R. Das (S. N. Bose National Center for Basic Sciences)
on 1 Mar 2024; 11:25 UT
Credential Certification: Gesesew Reta Habtie (gesesew@bose.res.in)

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We report that He II 4685.7 Ang became the strongest emission line in integrated intensity, based on the observed optical spectra obtained using the Himalayan Faint Object Spectrograph Camera (HFOSC) attached to the 2-m Himalayan Chandra Telescope (HCT) at the Indian Astronomical Observatory (IAO) on 2023 November 23.75 UT (+981 days after the outburst; 2021 March 18.4236 UT). We calculated the observed integrated relative intensity ratios of the lines He II 4685.48, [Fe VI] 5714.99, and [Fe VII] 6086.29 to H I 6563.00 as 1.315, 0.543, and 0.690, respectively. In ATel #16089, C. E. Woodward et al. reported that the observed integrated relative intensity ratios for these same lines on day +833.22 are 0.548, 0.581, and 0.670, respectively. This shows that the intensity of the He II line exceptionally strengthened, unlike the other two, which didn't show significant variation in intensity. The strong emission feature, the He II 4685.48 line, was reported to have disappeared on March 19.785 UT, the day after its observation on March 18.684 UT ( ATel #14476, ATel #14614, ATel #14471, Atel #14472). However, it reappeared and became the most intense feature across the entire optical wavelength range on 2023 November 23.75 UT. In addition to the above mentioned lines, we also calculated the ratio of other lines' integrated intensities to that of H I 6563.00 as follows: (H I 4861 / H I 6562.80) = 0.855; (H I 4340.22 / H I 6563.00) = 0.804; ([Fe VI] 4969 / H I 6562.80) = 0.507; ([Ca V] 5309 / H I 6563.00) = 0.447; ([Ne IV] 4720.77 / H I 6563.00) = 0.493. This indicates that the spectrum of V1405 Cas still shows a clear presence of coronal lines, even though recombination lines like He II 4685.7 Angstroms started dominating. Therefore, the nova V1405 Cas is still in the Coronal phase, ensuring that it is the slowest neon nova ( Atel #15796, ATel #15934).

The table below lists the integrated absolute intensity (in erg/cm²/s/Ang) of the most representative emission lines present on the 2023 November 23.75 UT spectrum, which is not corrected for interstellar reddening.

Line ID	Wavelength	Intensity
H I	4340.22	2.270E-14
He II	4685.48	3.714E-14
[Ne IV]	4720.77	1.393E-14
H I	4861.89	2.414E-14
[Fe VI]	4969.00	1.431E-14
[Ca V]	5309.22	1.264E-14
[Fe VI]	5714.99	1.535E-14
[Fe VII]	6086.29	1.949E-14
H I	6563.00	2.825E-14