2000A&A...361..725C

Through a program of narrow band imaging, we have observed the changing structure of the Hα 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≤100km/s). These spectra show that the nebula is not a reflection nebula, a conventional H II region, or a shock involving motions of the gas. The integrated Hα luminosity of the nebula between 1996 and 1999 is in the range ≃1.3-2.2x10³⁵erg/s. The Balmer decrement is normal for recombinations of a lightly reddened plasma. The lack of forbidden emission lines can only be understood if the electron temperature is low. This condition results if the energies of the ejected photoelectrons are shared among electrons, protons, and neutrals in a partially ionized medium. The He I lines are suppressed if the flash ionizing spectrum is truncated at or below the He⁰ ionization edge. The ionized material is on the front face of neutral sheets. The density is poorly determined, but is probably very large (∼10⁴cm^–3) in order to explain the brightest region of the nebula. The dynamical timescale is about a year and the recombination timescale of the same order. Bright patches are observed to fade in these times. The energy required to ionize the nebula is the bolometric luminosity of the nova for 30 days, smaller than the time during which the temperature of the nova photosphere was in the right range to produce the ionizing photons. We have also undertaken sensitive surveys of Hα nebulae around recent novae but find no evidence of other such nebulae, so this type of object must be rare.