Optical and Infrared Observations of the Bipolar Proto-Planetary Nebula Henize 401

Based on optical and near-infrared ground-based spectroscopy and optical and near-infrared Hubble Space Telescope (HST) images, we confirm the classification of Hen 401 as a bipolar proto-planetary nebula. HST images reveal a highly elongated morphology, consisting of two main bipolar lobes of unequal surface brightness separated by a dark lane of dust with a total extension of 28'' x 3'', superimposed on a much fainter background emission of elliptical shape extending over ~10'' x 5''. The simultaneous detection of spectral lines formed under very different density conditions, together with the strong obscuration observed along the equatorial plane, is interpreted as the signature of a circumstellar disk, which would be responsible for the strong collimation observed. We conclude that the emission observed is mainly due to scattering of the light originated in the fluorescence-excited inner region of the circumstellar disk. In addition, we also report the detection of strong molecular H2 emission in the near infrared, confirming that the onset of the H2 emission occurs in proto-PNs after the bipolar structure has developed, but before photoionization takes place.