This project addresses an important unsolved problem in the evolution of ordinary (1 to 8 solar mass) stars in their transition from the asymptotic giant branch to the planetary nebulae phase: the nature of the final mass loss and the origin of their jets. Enhanced equatorial mass loss and high-velocity jets dominate the early development of planetary nebulae and appear to be nearly ubiquitous, but their origins are unknown. Here, observations of the neutral gas and dust ejected by the stars will be used to address key aspects of this evolutionary phase.
1) The Geometry of AGB Envelopes: Deep imaging will be carried out at optical wavelengths to determine the structure of circumstellar envelopes at the tip of the asymptotic giant branch. The results will be used to determine the long-term evolution of the mass loss geometry leading up to the transition, which provides key constraints on possible jet triggering mechanisms.
2) Jets and Equatorial Tori: Observations will also be made of newly formed jets and tori in proto/young planetary nebulae using ultra high resolution (roughly 0.3 arc sec) millimeter interferometry. Spectro-imaging of the molecular gas will be carried out in lines of 12CO, 13CO, and other molecular species to determine the geometry, mass, and dynamics of the jets and tori, and their inter-relations.
The observations will be used to provide general constraints and to test specific scenarios of the final mass loss and jet formation. The results are expected to substantially improve our understanding of this key phase of stellar evolution.
This project will impact other fields of astrophysics as planetary nebulae constitute a new class of objects with jets, whose distinct characteristics may offer novel insights on how jets can form. This project also involves the education and research training of graduate and undergraduate students at New York University. It further fosters international scientific co-operation, and forms the basis for Professor Huggins' on-going outreach efforts which are broadly centered on stellar evolution. This includes "Stars in the Curriculum," a collaborative program with an education specialist and practicing teachers, designed to enhance the science curriculum in public schools in New York City.
In this project we investigated the late stages of evolution of ordinary stars (like the Sun) when they make the rapid transition from red giants to extended, diffuse, planetary nebulae. It has long been known that many planetary nebulae show complex spatial patterns, and it has been hypothesized that these are induced by companion stars or planets orbiting the giants and interacting with them, e.g., by tidal effects or engulfment. Our research explores aspects of this scenario. One study we carried out was to make deep optical images of circumstellar material around the precursor giants in order to investigate whether there are detectable symmetries in the mass-loss of these stars before the nebulae form. If companions are present, effects on the mass-loss geometry could be expected, even before the nebula-forming interaction takes place. Our imaging is able to detect the circumstellar material, and the results show that approximately 20% of the giants are surrounded by highly elliptical envelopes and up to 50% show more subtle symmetries. These results provide strong evidence that pre-shaping of the circumstellar material takes place, and strengthens the evidence for the scenario in which orbiting companions play a role in nebular formation. A second study we carried out was to investigate the energetics of the outflows of material during the actual formation of planetary nebulae in order to understand the basic ejection mechanism. Our approach compares theoretical models with direct observations of the ejected material made with telescopes operating at short radio (millimeter) wavelengths. The data provide information on the speed, energy, and momentum of the highly focused outflows - astrophysical "jets" - and the accompanying equatorial rings of material that are typically seen in newly forming planetary nebulae. Our analysis demonstrates that the jets result from material that is accelerated to high speeds and bursts out through the ejected stellar envelope. The energetics place strong constraints on the driving mechanism, and are found to be most consistent with models that power the jets by accreting part of the stellar material onto a companion. These and related studies contribute to more fully understanding the evolution of ordinary stars by refining the scenario in which companions play a significant role in the formation of planetary nebula. This project involved a team of undergraduate students, and the study of envelope shaping was done in collaboration with Dr. N. Mauron of the University of Montpellier.
