Ultraviolet spectra of planetary nebulae - VI. NGC 7662

Abstract

A detailed study of NGC 7662 is based on UV results obtained from 15 IUE spectra and on observations of other workers at optical, IR and radio wavelengths. Improved techniques are used to extract IUE data for an extended source. Relative fluxes in the different apertures which have been used are obtained using the brightness contours of Coleman, Reay & Worswick. There is close agreement between the reddening deduced from the ratios He II (λ 1640)/(λ 4686) and (radio)/(Hβ) and the nebular continuum emission observed with the IUE large slots agrees closely with that predicted using absolute radio and Hβ fluxes. The fluxes in nebular emission lines observed with the small slots are smaller than expected from brightness distributions; it is concluded that, for an extended source, the small slots have aperture transmission factors of 0.85 for SWP and 0.46 for LWR. The central star is fainter than has been previously supposed (by more than two magnitudes). The blackbody He II Zanstra temperature of 113 000 K is consistent with the UV colour temperature. Previous work on colour temperatures of central stars is discussed critically. Two models are discussed. For Model I the star is assumed to radiate like a blackbody and absorption by internal dust is neglected. This gives results which are not in agreement with observations for the flux in the C III λ 2297 di-electronic recombination line, and for the thermal infrared (TIR) emission measured by Moseley. For Model II the stellar flux is interpolated from LTE model atmosphere results of Hummer & Mihalas. The He II Zanstra temperature is 120 000 K. The C3+ abundance is such as to give a C III λ 2297 flux in agreement with observations but an emission in the C IV λ 1549 resonance lines greater than that observed by a factor of 3. The C IV emission is assumed to be attenuated by dust. The required dust optical depth is τD = 0.1. The dust is assumed to consist of large graphite grains. The calculated TIR flux agrees with that observed. Model II gives fluxes for lines of H, He, C, N, 0, Ne, Mg, Si and S with an agreement with UV, optical and IR observations which is generally satisfactory. Abundances are deduced. The use of the model atmosphere flux gives a marked improvement for the C3+/C2+ and N3+/N2+ ratios. Model II is considered to be much more realistic than Model I. The O abundance is less than half of the solar value, C/O three times solar, N/O is 40 per cent higher than solar and Ne/O is solar, as is S/H. The gasphase Mg abundance is less than solar by a factor of at least 50, and that of Si by a factor of 7.