IUE and ISO observations of the bipolar proto-planetary nebula Hen 401 (IRAS 10178-5958)

We present ultraviolet (IUE) and infrared (ISO) observations of the bipolar proto-planetary nebula Hen 401 which, combined with previously available o ptical and near infrared data, are used to reconstruct the overall spectral energy distribution from 1150 Angstrom to 100 mum. The ISO spectrum is dom inated by strong PAH emission superimposed on a very cold continuum which i s interpreted as thermal emission originating in the C-rich cool dust (simi lar to 106 K) present in the circumstellar envelope, the remnant of the pre vious AGB phase. In addition, a second, hotter component detected in the ne ar infrared is attributed to thermal emission from hot dust (similar to 640 K), suggesting that mass loss and dust grain formation is still on-going d uring the current post-AGB phase. The ultraviolet (IUE) spectrum shows a st ellar continuum in the wavelength interval 2400 Angstrom to 3200 Angstrom w hich corresponds to a moderately reddened B8-type central star. Unexpectedl y, the UV flux in the wavelength interval 1150 Angstrom to 1900 Angstrom is very weak or absent with no evidence of a hotter binary companion which co uld explain the detection of the nebular emission lines observed in the ava ilable ground-based optical spectra of Hen 401. HST WFPC2 high resolution i mages also show no indication of a hot companion to the B8-type central sta r observed both in the optical and in the UV. The evolutionary implications of a possible single nature for the central star of Hen 401 are discussed.