New Features of the Symbiotic Recurrent Nova V407 Cygni Found in the Outburst in 2010

The spectral evolution of V407 Cyg during the outburst in 2010 is reported. The coronal emission lines [Fe x] 6374, [Ar x] 5535, and [Ar xi] 6919 strengthened rapidly around day 20 of the outburst, which coincided with the growth of soft X-ray flux. On the other hand, the non-coronal forbidden lines [O i], [N ii], [O iii], etc. strengthened between days 40 and 60, which coincided with the fading of the soft X-rays. The luminosities in the UV and optical regions also decreased in the same period, and were seen as bends on the light curves. Our observations suggest that the fading of the soft X-rays and the bends on the light curves were likely related to a rapid decreasing of the emission measure of the nebulosity, and may not have been due to the cessation of the hydrogen burning in the system. It seems that the coronal emission lines were emitted by the collision of the ejecta with the circumstellar envelope as well as the soft X-rays. Intensities of the emission lines of He i relative to Hβ probably increased in the first 3 days of the outburst and decreased in the successive 10 days and then recovered, while no other line showed a similar variation. The variations in intensity of the He i emission lines coincided with that of the γ-ray flux in the first two weeks of the outburst. The helium abundance derived from the intensities of the He i and He ii emission lines relative to Hβ appeared to have decreased from N(He) = 0.17 to 0.076 according to the fading of the He i lines, and then increased to 0.20 when the intensities recovered in the later stage of the outburst. It is unlikely that the helium abundance really changed so much during the outburst. There might have been an unknown mechanism that weakened the He i emission lines. The distance to V407 Cyg is estimated to be 5.3+/- 1 kpc. The absolute V mag at maximum light of the outburst was probably fainter by about 0.6 mag than that expected from the time for a 2 mag decline.