Nebula Nuclei Research Articles

FAST, GUSTY WINDS BLOWING FROM THE CORE OF THE PRE-PLANETARY NEBULA M 2-56

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 ∼15 × 1'' each and display closed, bow-shaped ends. The extended, faint outer lobes (OLs), which enclose the inner ones, have an angular size of ∼13'' × 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 ∼350 km s−1 at the tips), we have measured the expansive proper motions of the knots (Δθt ∼ 003 yr−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 Hα emission from the nebula nucleus that is interpreted as an indication of a dense, fast (∼350–500 km s−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 Hα 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 tk ∼ 1400 yr; Castro-Carrizo et al.), (2) the OLs (tk ∼ 350–400 yr), (3) the ILs (tk ∼ 40 yr), (4) the F1-wind (tk < 10 yr), and (5) the nuclear cocoon and H ii region (tk ≲ 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.

The Relative Sizes of Planetary Nebulae

The sizes of planetary nebulae are poorly known, principally because the distances to these sources are not very well established. We point out in the following, however, that if one is careful in selecting nebulae having particular ranges of galactic latitude, and narrow ranges of surface brightness, then it should be possible to establish mean trends in nebular diameter. We have, in this way, been able to determine that where brightness temperatures are the same, then circular and elliptical sources, and the nuclei of bipolar nebulae (BPNe) all have closely similar mean dimensions. It is also likely that circular and elliptical sources have closely similar masses (the case of the BPNe is less clear). Finally, we note that halos are typically ∼3 times larger than circular and elliptical PNe, and the lobes of BPNe are ∼5 times as large, making the latter envelopes the largest of all nebular structures.

New Emission Line Planetary Nebulae Nuclei in the Direction of the Galactic Center

We have discovered new Wolf-Rayet type and weak emission-lines nuclei of the planetary nebulae in the Galactic center.

Energetic non-thermal process in galactic objects and ground-based gamma-ray astronomy

Very high energy γ-rays at TeV energies so far have been detected from pulsar nebula, supernova remnants and active galactic nuclei, and explained by the emission of the electron and positron progenitor. The copious production of the energetic, nonthermal electrons and positrons generally implies an intense radiation field in the source region and a considerable amount of optical depth for high energy γ-rays to affect their energy spectrum. The possibility of new classes of γ-ray sources at TeV energies are also argued in relevance to cosmic rays.

The dynamics of double-envelope planetary nebulae

A number of criteria are elaborated based on the careful analysis of nebulae images which confirm the version of the origin of double-envelope planetary nebulae by means of dynamical separation but not by multiple ejection. The importance of stellar winds in the origin of the gigantic halos around double envelope nebulae is outlined. The problem concerning the existence of two types of filamentary planetary nebulae (Figure 8) is raised: type A, filaments are the fragmentations of the “Rayleigh-Taylor instability”, the result of dynamical interaction of the envelope with the outer interstellar matter (NGC 6543), and type B, the origin of filaments is connected with the “Magnetic field fluctuations” (A 43, A 72).The possibility of the origin of three-envelope nebulae in the framework of the dynamic separation version is discussed (NGC 7293). Attention is paid to the impossibility of outbursts with low velocities (20–30 km s−1) from hot stars, i.e., the nuclei of nebulae.

The Possibility of Beta decay in the Atmospheres of Nuclei of Planetary Nebulae

At present there is some basis for questioning the usual presentation of the nuclei of planetary nebulae as being ordinary stars. If the quantity of mass released by a star during its formation or at some stage of its evolution serves as a criterion for its not being stationary, then the nuclei of planetary nebulae are hardly the most non-stationary objects. A strong deviation from Planck's radiation law may also serve as a criterion for non-stationarity when the deviation occurs in the far short-wave region of the spectrum in nuclei and similar objects (e.g. stars of Wolf-Rayet which are not nuclei of planetary nebulae). Finally, the existence of synchrotron radio radiation in some planetary nebulae must be explained by the exceptional activity of their nuclei. It would seem that the nuclei of these nebulae are suppliers if not of typical relativistic electrons then at least of electrons with high energies.