THE PLANETARY-NEBULA ABANDONED BY ITS CENTRAL STAR

Narrowband images have been obtained of the nebula Sh 2-174, to follow up the recent claim that the white dwarf situated on one edge, GD 561 , is its displaced central star. The Ha and [N II] emission originates from the cleft-hoof morphology apparent on the POSS, but the [O III] is centred on the white dwarf. This confirms that the star and the neb ula are physically related. With the severely distorted morphology app arent in Ha and [N II] combined with the location of the former centra l star, Sh 2-174 represents the most advanced case so far of the PN-IS M interaction. Spectra taken of GD 561 suggest a surface gravity log g approximate to 6.8 and effective temperature T(eff)approximate to 65 000 K. Assuming a standard single star evolution, this would lead to a n exceptionally low mass and a post-AGB age of similar to ten million years, which is incompatible with the several hundred thousand years w hich is generally supposed to be a typical PN lifetime. We suggest ins tead that GD 561 is part of a close binary system, having lost its out er envelope after the termination of core H burning, and is now a smal l similar to 0.3.M circle dot degenerate He dwarf. However, this scena rio requires the existence of a faint companion.