FAST, GUSTY WINDS BLOWING FROM THE CORE OF THE PRE-PLANETARY NEBULA M 2-56
- Departamento de Astrofisica, Centro de AstrobiologIa, CSIC-INTA, Ctra de Torrejon a Ajalvir, km 4, 28850 Torrejon de Ardoz, Madrid (Spain)
- Instituto de Astrofisica de AndalucIa-CSIC, C/ Glorieta de la Astronomia s/n E-18008 Granada (Spain)
- Institut de Radioastronomie Millimetrique, 300 Rue de la Piscine, F-38406 Saint Martin d'Heres (France)
- Observatorio Astronomico Nacional, Apartado 112, Alcala de Henares, E-28803 Madrid (Spain)
We report optical long-slit spectra and direct imaging (ground-based and with the Hubble Space Telescope (HST)) of the pre-planetary nebula M 2-56 obtained at different epochs. The optical nebula is composed of 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 {approx}1.5'' x 1'' each and display closed, bow-shaped ends. The extended, faint outer lobes (OLs), which enclose the inner ones, have an angular size of {approx}13'' x 10''. 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 {approx}350 km s{sup -1} at the tips), we have measured the expansive proper motions of the knots ({Delta}{theta}{sub t} {approx} 0.03'' yr{sup -1}) by comparing HST images taken in 1998 and 2002. Moreover, we have discovered remarkable changes with time in the continuum and line emission spectrum of M 2-56. In 1998, we detected a burst of H{alpha} emission from the nebula nucleus that is interpreted as an indication of a dense, fast ({approx}350-500 km s{sup -1}) 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 cocoon (or shell-like structure) and an H II region around the central star that result from further post-asymptotic giant branch (post-AGB) mass loss after the F1-wind. Recent brightening of the scattered stellar continuum as well as an increase of scattered H{alpha} 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, leading to (1) acceleration of the molecular envelope that surrounds the optical nebula (kinematical age t{sub k} {approx} 1400 yr; Castro-Carrizo et al.), (2) the OLs (t{sub k} {approx} 350-400 yr), (3) the ILs (t{sub k} {approx} 40 yr), (4) the F1-wind (t{sub k} < 10 yr), and (5) the nuclear cocoon and H II region (t{sub k} {approx_lt} 2 yr?). The successive multiple post-AGB winds in M 2-56 are characterized by ejection speeds increasing with time. In contrast, the mass-loss rate and linear momentum show a time-decreasing trend.
- OSTI ID:
- 21448767
- Journal Information:
- Astrophysical Journal, Vol. 715, Issue 1; Other Information: DOI: 10.1088/0004-637X/715/1/143; ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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