A study of planetary nebulae in and towards the Galactic Bulge

UoM administered thesis: Phd

  • Authors:
  • Bryan Rees

Abstract

The University of ManchesterFaculty of Engineering and Physical SciencesABSTRACT OF THESIS submitted by Bryan ReesFor the degree of Doctor of Philosophyand entitled "A study of planetary nebulae in and towards the Galactic Bulge"Date of submission 30/06/2011A planetary nebula (PN) consists of material, mainly gas, that has been ejected from a star on the asymptotic giant branch of its life cycle. This material emits electromagnetic radiation due to photoionization and recombination, collisional and radiative excitation or free-free radiation. The envelope of material moves outwards from the central star and may take one of a variety of shapes. These shapes are believed to be sculpted by the stellar wind, magnetic fields and interactions with a binary companion. However, within a time scale of as little as 10 000 years the nebula fades from view and merges with the interstellar medium.Similar variations in the shape of planetary nebulae (PNe) can be seen in both the Galactic Bulge and Disc and in the Magellanic Clouds. It is therefore reasonable to assume that the shaping process is universal. By classifying PNe by morphology and relating those shapes to other nebular properties we have attempted to derive information about that shaping process.We have used photometric narrowband observations of a sample of PNe listed in the Strasbourg-ESO Catalogue of Galactic Planetary Nebulae to investigate the relationship between PN morphology and the other PN characteristics. The high resolution images were made using ESO's New Technology Telescope and the Hubble Space Telescope. The information we could obtain directly from the observations was augmented by information in the literature in order to address that question. The observations were used to classify the morphologies of 154 PNe, to estimate the sizes of 138 of those nebulae that we considered to lie within the Galactic Bulge, to determine the orientations of 130 of those Bulge nebulae and to derive photometric fluxes for the 69 PNe which had observations of standard stars made during the same night. Information on central star binarity, nebular abundances and radial and expansion velocity was obtained from the literature.Our photometrically derived PNe line fluxes were used to verify 59 H-beta and 69 [OIII] catalogued values (which were obtained using spectroscopy). We found sufficient discrepancy between the values for 9 PNe to merit a further check taking place.We found no distinguishing relationship between PN morphology and any of PN size, radial velocity, or angular location within the Bulge. The abundances of He and O, and the N/O ratio, are generally lower in bipolar nebulae than in those nebulae with no apparent internal structure. We are unable to come to any conclusion as to a relationship between PN morphology and stellar metallicity.Given the short lifespan of PNe and the age of the Bulge it appears that almost all PNe in the Bulge must be associated with low mass stars. The high ratio of bipolar PNe we found in our Bulge sample suggests that, at least within the Bulge, bipolar nebulae are not necessarily associated with high mass stars. Our results show that unlike the orientations of other types of PNe the orientations of the bipolar nebulae in the Bulge are not randomly distributed. Measured to a line tip to tip along the lobes they peak and have their mean approximately along the Galactic Plane. This suggests that the bipolar PNe originate in a different environment from other morphological types, perhaps related to binary separation. However, we find that bipolarity does not imply common-envelope evolution. If the hypothesis that bipolar nebulae are formed in binary star systems is correct, binary systems in the Galactic Bulge have angular momentum vectors that are preferentially aligned along the Galactic Plane. As the orientation appears to be unrelated to lobe size and hence nebular age, the alignment implies that the non-random nature of the angular momentum vectors originated at the time the Bulge stellar population formed. We suggest that it is due to the direction and strength of the ambient magnetic fields.

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Original languageEnglish
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Award date31 Dec 2011