Hubble Space Telescope imaging and ground‐based spectroscopy of old nova shells – I. FH Ser, V533 Her, BT Mon, DK Lac and V476 Cyg

In this paper we report on the first five out of eleven observations in our programme of Hubble Space Telescope ( HST ) imaging of old nova shells. We present new WFPC2 images of the shells around FH Ser and V533 Her taken in the F656N (H α +[N  ii ]) filter. We also show long‐slit spectra taken using the William Herschel Telescope on La Palma for these objects in the same spectral range. The shell around FH Ser is found to be a prolate ellipsoid of ellipticity 1.3±0.1 inclined at 62°±4° to the line of sight. The shell has an equatorial ring which is found to be due to increased emission in the two [N  ii ] lines rather than H α . The expansion velocity is best modelled by a true equatorial expansion rate of 490±20 km s −1 . The best‐fitting systemic velocity is −45 km s −1 . A synthetic image and synthetic spectra are also presented for this model for comparison with our observations. We derive a distance to FH Ser of 950±50 pc. The origin of the [N  ii ] equatorial ring is discussed in the context of a photoionization feature resulting from aspherical illumination by the central source rather than a simple density enhancement. It is possible, however, that the ring is also in part due to an extremely localized increase in the nitrogen abundance. The brightest part of the shell is found to have a surface brightness of 9.1×10 −15  erg cm −2  s −1  arcsec −2 . Similar imaging and spectroscopy of the nova V533 Her reveal a shell of radius 5±0.7 arcsec with an axial ratio of 1.2±0.2 and peak surface brightness of 1.3×10 −15  erg cm −2  s −1  arcsec −2 . The expansion velocity of this shell is 850±150 km s −1 and the distance is estimated to be 1250±300 pc. The shells around BT Mon, DK Lac and V476 Cyg were not detected with HST , implying 3 σ upper limits to the surface brightness in H α +[N  ii ] of 5.3–6.3×10 −16  erg cm −2 s −1  arcsec −2 .