Spectral evolution of the nova V2468 Cygni: asymmetric and variable profiles of its emission lines

Abstract

The spectral evolution of the classical nova V2468 Cygni (Nova Cyg 2008) was monitored at Asiago Astrophysical Observatory in Italy and at Nishi-Harima Astronomical Observatory in Hyogo Japan from soon after the discovery in March 2008 to the end of 2009. The notable peculiarities in the spectra of this nova were the asymmetric and variable profiles of its emission lines. The emission-line profiles of H I varied in the early decline stage in 2008, while their profiles were nearly stable in 2009. On the other hand, large variations in the profiles of the emission lines of [O III], [Fe VII], and He II were observed in 2009. In the spectrum obtained on 2009 October 15, the red parts of the emission lines of [O III] strengthened with respect to the profiles in September, while the red parts of the lines of He II and [Fe VII] faded. The emission lines of [Fe VII] 5721 and [Fe VII] + [Ca V] 6087 exhibited convex profiles on 2009 September 27 and 28 in a similar way to the other lines, while only these lines had concave profiles on October 26 and 29. It seems to be difficult to explain the variations in the emission-line profiles with any known model of nova shells. The absolute magnitude at light maximum and the distance to the nova are estimated to be M V = −8.8 ± 0.3 mag and d = 5.5 ± 0.8 kpc. Using the intensities of H I, He I, and He II lines observed in September 2009, we estimated the helium abundance in number to be N (He) = 0.20 ± 0.01, and the mass of the ejecta to be 1.7 ± 1 × 10-5 M ⊙ . A very broad emission line from 3790 A to 3850 A was detected in the spectrum obtained on 2009 July 21. We propose that the multiple lines of [Fe V] 3F, 3839.3, 3794.9, 3851.3, 3819.9 etc. suddenly strengthened at that time, but the cause of this phenomenon is not known.