2003A&A...404..997I

Spectral evolution of the fast nova V1494 Aql was monitored soon after its discovery in December 1999 to September 2000. The first spectra showed prominent emission lines of H I and Fe II, while He I was seen in absorption. The radial velocities of the absorption components of H I, He I and N II rapidly increased (in the negative sense) during the early decline stage, while those of Fe II remained nearly constant. When a new spectrum was taken on February 6, 2000 after the seasonal interruption, this nova was in the transition stage. The spectra in the transition stage showed emission lines of H I, He I, He II, N II, N III, Si II, [N II], [O I], [O III], [Fe II], [Fe VI], [Ca V] etc., hence the emission lines of Fe II had disappeared. A quasi-periodic oscillation of luminosity with a time scale of about 16.5±1 days and a mean amplitude of about 1.2ag in V band was seen from February to the middle of April 2000. The emission lines of He II and [Ca V] disappeared around a light maximum of the oscillation, while the emission lines of N II and N III strengthened. At the same time high velocity (-2900 and +2830km/s) broad emission wings of H I lines appeared, which suggest an ejection of high velocity jets. The excitation state increased throughout the nebular stage. The last spectra taken in September 2000 showed highly excited emission lines up to [Fe VII] and [Fe X] λ6374.5. The interstellar extinction is estimated as E(B-V)=0.6±0.1 from the equivalent widths of the interstellar absorption components of Na I D1 and D2. Using this result, the distance to the nova is estimated as 1.6±0.2kpc. The mass and the helium abundance of the ejecta are estimated as 6.2±1.4x10^–5M_☉ and N(He)/N(H)=0.13±0.01, respectively. The electron density of the ejecta decreased as N_e∝t^–0.8 during the nebular stage, where t is time from light maximum. This low decline rate suggests that the ejecta had a ring like shape as well a large mass loss which may have continued throughout the nebular stage.