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

¹ National Astronomical Observatory, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
² Indian Institute of Astrophysics, Bangalore-560034, India
³ ISO Data Centre, Astrophysics Division, Space Science Department of ESA, VILSPA, 28080 Madrid, Spain
⁴ Osservatorio Astronomico di Capodimonte, Via Moiariello 16, 80131 Naples, Italy
⁵ Institute of Astronomy, School of Science, University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁶ Kiso Observatory, School of Science, University of Tokyo, Mitake, Kiso, Nagano 397-0101, Japan
⁷ Kapteyn Astronomical Institute, Postbus 800, 9700 AV Groningen, The Netherlands
⁸ Laboratorio de Astrofísica Espacial y Física Fundamental, INTA, Apartado de Correos 50727, 28080 Madrid, Spain

Corresponding author: M. Parthasarathy, partha@iiap.ernet.in

Received: 6 April 2001
Accepted: 29 June 2001

Abstract

We present ultraviolet (IUE) and infrared (ISO) observations of the bipolar proto-planetary nebula Hen 401 which, combined with previously available optical and near infrared data, are used to reconstruct the overall spectral energy distribution from 1150 Å to 100 μm. The ISO spectrum is dominated by strong PAH emission superimposed on a very cold continuum which is interpreted as thermal emission originating in the C-rich cool dust (~106 K) present in the circumstellar envelope, the remnant of the previous AGB phase. In addition, a second, hotter component detected in the near infrared is attributed to thermal emission from hot dust (~640 K), suggesting that mass loss and dust grain formation is still on-going during the current post-AGB phase. The ultraviolet (IUE) spectrum shows a stellar continuum in the wavelength interval 2400 Å to 3200 Å which corresponds to a moderately reddened B8-type central star. Unexpectedly, the UV flux in the wavelength interval 1150 Å to 1900 Å is very weak or absent with no evidence of a hotter binary companion which could explain the detection of the nebular emission lines observed in the available ground-based optical spectra of Hen 401. HST WFPC2 high resolution images also show no indication of a hot companion to the B8-type central star 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.