The emission nebula associated with V1974 Cygni: a unique object?

Abstract

Through a program of narrow band imaging, we have observed the changing structure of the H alpha emission line around Nova Cyg 1992 (V1974 Cyg) at regular intervals from 1996 to 1999. Between 1994 and 1996, the nebular boundary advanced to the southwest at nearly the speed of light, implying that the nebula was created by an expanding wave of radiation originating in the explosion interacting with surrounding material. The expansion speed dropped to 0.35c during 1996-1999. We have taken spectra of the nebula in 1998 and 1999. Only Balmer lines are detected, no He I, [O III], [O II], [N II], or [S II]. There is also no trace of the high excitation nova lines (He II, NeV, etc). The Balmer lines are unresolved in velocity (FWHM less than or equal to 100 km s(-1)). These spectra show that the nebula is nor a reflection nebula, a conventional H II region, or a shock involving motions of the gas. The integrated H alpha luminosity of the nebula between 1996 and 1999 is in the range similar or equal to 1.3-2.2 x 10(35) erg s(-1)