Chemistry and morphology of evolved circumstellar envelopes

Abstract

Asymptotic giant branch stars, proto-planetary nebulae and planetary neb- ulae are evolved stars that are well known to be the source of molecular species in the interstellar medium. Synthesis of complex molecules occurs readily under the physical conditions found within the circumstellar envelopes of asymptotic giant branch stars. As evident from the rapidly changing chemical composi- tion at different stages of stellar evolution, carbon dredged-up by convectional instabilities facilitates rapid formation and reprocessing of molecules. These molecules are then distributed to the interstellar medium as the circumstellar envelope is gradually shed in the proto-planetary nebula and planetary nebula phases via a series of mass ejection by stellar winds. Physical conditions inthe envelope also do not remain constant as an object evolves, and different conditions will permit different chemical processes. Hence, it is important to track the chemical and physical changes in the circumstellar envelope as a star proceeds on its evolutionary track by conducting spectral line surveys on a large sample of objects at different stages of stellar evolution.

Presented in this thesis are the results from the unbiased systematic line surveys targeting the carbon-rich asymptotic giant branch stars IRC+10216 and CIT 6. These surveys complement a series of previous studies that covered objects at other stages of the evolutionary track as part of an on-going effort to paint a comprehensive chemical picture of late-stage stellar evolution. Con- ducted using the 12m radio antenna at the Arizona Radio Observatory, the surveys aim to expand wavelength coverage of the two objects. Since the ob- servations are in the λ3 mm window, they are predominately targeting features arising from rotational transitions of molecular species. While no new carri- ers have been identified, some of the features associated with transitions from H2CCCC, NaCl and SiC4 are novel detections. Spectroscopic analysis allowed the identification of the more probable chemical pathways taken by the forma- tion of polyyne and cyanopolyyne chains. Further examination of the spectral data also supported the hypothesis that CIT6 is a more evolved asymptotic giant branch specimen than the prototypical IRC+10216.

Besides chemistry, this thesis also looks into the morphology of evolved stel- lar objects. It is well-observed that the morphology of planetary nebulae can deviate significantly from the spherical symmetry typical in the asymptotic giant branch phase. Also presented in the present work is a series of three-dimensional models of planetary nebulae aiming to explore the intrinsic morphologies of these objects. Based on the high-resolution images from the Hubble Space Telescope and the Gemini North Telescope, these models revealed that many of the objects that were previously misclassified as elliptical or bipolar are in fact multipolar in nature. A new classification system, which differentiates objects depending on whether precessing collimated outflows are required to result in the observed geometry, is developed to more appropriately characterize the planetary neb- ulae. The models also highlighted the ability of interacting shell structures brought forth by cavities in a confining medium to form complex internal struc- tures. This suggests that morphologies of planetary nebulae are not as diverse as thought previously.

This thesis also presents the results from a near-infrared observational study that aim to examine the internal and extended structures of planetary nebu- lae. By employing the Canada-France-Hawaii Telescope to image a sample of planetary nebulae at unprecedented depth and wide field-of-view, the present study have yielded first detections in a number of small-scale structures, such as knots and filaments, in NGC650, NGC3587, PNSaWe3 and IPHASPN-1. A detailed model of Kn 26 is constructed base on these high-resolution images. The narrowband image in molecular hydrogen of NGC 1514 and PN SaWe 3 have also been presented for the first time by this work. Comparison between the acquired near-infrared and archived optical images revealed molecular hydro- gen can exist within ionized regions. From the distribution and morphology of the structures detected, it is inferred that hydrogen molecules in the central region of NGC 650 are primordial molecules that survived photodissociation by being shielded within dense knots. Molecular hydrogen in NGC3587, by con- trast, seems to be second-generation molecules that formed on surfaces of dust subsequent to the high luminosity phase.