The system of cometary knots in the Helix planetary nebula (NGC 7293) has been systematically observed using ground‐based images and long‐slit, high‐resolution spectroscopy. CCD images in the [N ii] 6584‐Å line, taken with the ESO NTT, are used to determine the spatial distribution of the knots; images in the [O iii] 5007‐Å line distinguish their position within the nebula on the basis of the absorption of the central [O iii] 5007‐Å‐emitting zone of the Helix nebula. The kinematics of 50 of the brighter knots and their tails were studied primarily with the Manchester Echelle Spectrograph (MES) as well as the ESO NTT using the EMMI spectrograph. Three regions were covered by multiple exposures of a 10‐element multislit with MES to obtain [N ii] 6584‐Å line profiles along 300 individual slit positions. In addition, many long‐slit MES spectra, placed diametrically across the nebula, were obtained to compare the kinematics of the knots with the large‐scale kinematical structure of the nebula.
The global expansion of the system of knots is around 14 km s−1, some 17 km s−1 less than for the overrunning gas, and they are distributed in a thick central disc. The velocity field of the system of knots is similar to that of molecular (CO) emission but with a lower expansion velocity. Most knots are external to the central [O iii] 5007‐Å‐emitting region, but some still show localized [O iii] 5007‐Å emission from their heads.
The kinematical structure of two of the latter knots as revealed by profiles of the [O i] 6300‐Å, [N ii] 6584‐Å, [O iii] 5007‐Å and Hα emission lines is considered in detail and compared with their CO emission‐line profiles. The interstellar absorption by the core of a further knot is derived from an [O iii] 5007‐ÅHST image combined with MES [O iii] 5007‐Å profiles. Accurate masses, densities and ages are determined as a consequence. Furthermore, a kinematical model shows that the [N ii] 6584‐Å‐emitting flow around a dusty, molecular globule (the core of an ionized knot) arises mainly from the expanding ionized gas of the Helix nebula engulfing the knot. The flow, parallel to the globule's photoionized surface, is mildly supersonic. The relationship between the system of knots and the large‐scale structure is discussed.
In this paper we present our experiences from a decade of plug-in development in the jABC framework, that is characterized by rigorous application of simplicity principles in two dimensions. First, the scope of the plug-in development is clearly defined: The jABC readily provides a sophisticated graphical user interface, which has been tailored to working with all kinds of directed graphs. Within this scope, plug-in development can deliberately focus on the actual functionality, like providing semantics to graphs, without having to deal with tedious but semantically irrelevant issues like user interfaces. Second, plug-in functionality can be itself conveniently modeled as a workflow within the jABC. We illustrate our approach by means of two mature plug-ins: Genesys, a plug-in that adds arbitrary code generator functionality to the jABC, and PROPHETS, a plug-in that eases user-level definition of workflows by completing model sketches by means of synthesis capabilities, so that they become complete and executable. We summarize our experience so far and derive general design principles for "lightweight plug-in development", that we are going to realize in the next generation of the jABC, which will be implemented itself as a collection of Eclipse plug-ins.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.