Fast, gusty winds blowing from the core of the pre-planetary nebula M2-56

Abstract

We report optical long-slit spectra and direct imaging (ground-based and with HST) of the pre-planetary nebula (pPN) M2-56 obtained at different epochs. The optical nebula is composed by shock-excited material distributed in two pairs of nested lobes with different sizes and surface brightness. The compact, bright inner lobes (ILs) have an angular size of ~1.5"x1" each and display closed, bow-shaped ends. The extended, faint outer lobes (OLs), which enclose the inner ones, have an agular size of ~13"x10". Within the ILs and the OLs the velocity increases with the distance to the center, however, the ILs show expansion velocities larger than the OLs. Consistent with the large speeds reached by the ILs (of up to ~350 km/s at the tips), we have measured the expansive proper motions of the knots (~0.03 arcsec/yr) by comparing two-epoch HST images. Moreover, we have discovered remarkable changes with time in the continuum and line emission spectrum of M2-56. In 1998, we detected a burst of Halpha emission from the nebula nucleus that is interpreted as an indication of a dense, fast (~350-500 km/s) bipolar wind from the nebula's core (referred to as "F1-wind"). Such a wind has been recently ejected (after 1989) probably as a short-duration mass-loss event. Our data also reveal an optically thick, compact structure (cocoon?) and a HII region around the central star that result from further post-AGB mass-loss after the F1-wind. Recent brightening of the scattered stellar continuum as well as an increase of scattered Halpha emission along the lobes is reported, both results pointing to a decrease of the optical depth of the circumstellar material enshrouding the star. The data presented here unveil the complex post-AGB mass-loss history of this object, whose rapid evolution is driven by multiple episodes of mass outflow, not regularly spaced in time... (abridged).