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 similar to 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 H-2 emission in the near infrared, confirming that the onset of the H-2 emission occurs in proto-PNs after the bipolar structure has developed, but before photoionization takes place.