This paper explores the effects of post-asymptotic giant branch (AGB) winds driven solely by magnetic pressure from the stellar surface. It is found that winds can reach high speeds under this assumption and lead to the formation of highly collimated proto-planetary nebulae. Bipolar knotty jets with periodic features and constant velocity are well reproduced by the models. Several wind models with terminal velocities from a few tens of km s À1 up to 10 3 km s À1 are calculated, yielding outflows with linear momenta in the range 10 36 -10 40 g cm s À1 , and kinetic energies in the range 10 42 -10 47 ergs. These results are in accord with recent observations of proto-planetary nebulae that have pointed out serious energy and momentum deficits if radiation pressure is considered as the only driver for these outflows. Our models strengthen the notion that the large mass loss rates of post-AGB stars, together with the short transition times from the late AGB to the planetary nebula stage, could be directly linked with the generation of strong magnetic fields during this transition stage. Subject heading gs: hydrodynamics -ISM: individual ( He 2-90, He 3-401, M2-9, Mz 3, OH 231.8+4.2) -planetary nebulae: general -stars: AGB and post-AGB
After a hot white dwarf ceases its nuclear burning, its helium may briefly and explosively reignite. This causes the star to evolve back into a cool giant, whereupon it experiences renewed mass ejection before reheating. A reignition event of this kind was observed in 1996 in V4334 Sgr (Sakurai's object). Its temperature decrease was 100 times the predicted rate. To understand its unexpectedly fast evolution, we have developed a model in which convective mixing is strongly suppressed under the influence of flash burning. The model predicts equally rapid reheating of the star. Radio emission from freshly ionized matter now shows that this reheating has begun. Such events may be an important source of carbon and carbonaceous dust in the Galaxy.
CK Vul is classified as, amongst others, the slowest known nova, a hibernating nova or a very late thermal pulse object. Following its eruption in ad 1670, the star remained visible for 2 yr. A 15‐arcsec nebula was discovered in the 1980s, but the star itself has not been detected since the eruption. We here present radio images which reveal a 0.1‐arcsec radio source with a flux of 1.5 mJy at 5 GHz. Deep Hα images show a bipolar nebula with a longest extension of 70 arcsec, with the previously known compact nebula at its waist. The emission‐line ratios show that the gas is shock‐ionized, at velocities >100 km s−1. Dust emission yields an envelope mass of ∼5 × 10−2 M⊙. Echelle spectra indicate outflow velocities up to 360 km s−1. From a comparison of images obtained in 1991 and 2004 we find evidence for expansion of the nebula, consistent with an origin in the 1670 explosion; the measured expansion is centred on the radio source. No optical or infrared counterpart is found at the position of the radio source. The radio emission is interpreted as thermal free–free emission from gas with Te∼ 104 K. The radio source may be due to a remnant circumbinary disc, similar to those seen in some binary post‐AGB stars. We discuss possible classifications of this unique outburst, including that of a sub‐Chandrasekhar mass supernova, a nova eruption on a cool, low‐mass white dwarf or a thermal pulse induced by accretion from a circumbinary disc.
The outflowing proper motions of 15 knots in the dominant northwestern lobe of the high‐excitation poly‐polar planetary nebula NGC 6302 have been determined by comparing their positions relative to those of faint stars in an image taken at the San Pedro Martir Observatory in 2007 to those in a South African Astronomical Observatory archival plate obtained by Evans in 1956. The Hubble‐type expansion of this lobe is now directly confirmed in a model‐independent way from these measurements. Furthermore, an unambigous distance to NGC 6302 of 1.17 ± 0.14 kpc is now determined. Also, all the velocity vectors of the 15 knots (and two others) point back to the central source. An eruptive event from within the central torus ≈2200 yr previously must have created the high‐speed lobes of NGC 6302.
We present the first detailed kinematical analysis of the planetary nebula Abell 63, which is known to contain the eclipsing close‐binary nucleus UU Sge. Abell 63 provides an important test case in investigating the role of close‐binary central stars on the evolution of planetary nebulae. Longslit observations were obtained using the Manchester echelle spectrometer combined with the 2.1‐m San Pedro Martir Telescope. The spectra reveal that the central bright rim of Abell 63 has a tube‐like structure. A deep image shows collimated lobes extending from the nebula, which are shown to be high‐velocity outflows. The kinematic ages of the nebular rim and the extended lobes are calculated to be 8400 ± 500 and 12 900 ± 2800 yr, respectively, which suggests that the lobes were formed at an earlier stage than the nebular rim. This is consistent with expectations that disc‐generated jets form immediately after the common envelope phase. A morphological–kinematical model of the central nebula is presented and the best‐fitting model is found to have the same inclination as the orbital plane of the central binary system; this is the first proof that a close‐binary system directly affects the shaping of its nebula. A Hubble‐type flow is well‐established in the morphological–kinematical modelling of the observed line profiles and imagery. Two possible formation models for the elongated lobes of Abell 63 are considered, (i) a low‐density, pressure‐driven jet excavates a cavity in the remnant asymptotic giant branch (AGB) envelope; (ii) high‐density bullets form the lobes in a single ballistic ejection event.
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