Compact objects in the disk and globular clusters

Citation

Johnston, Helen Margaret (1992) Compact objects in the disk and globular clusters. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/828h-3x32. https://resolver.caltech.edu/CaltechETD:etd-09172008-152100

Abstract

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This thesis presents a multi-wavelength study of compact objects in globular clusters and in the disk. It is divided into two sections, and contains seven independent chapters, linked by the general theme of the study of the evolution and distribution of recycled pulsars and low mass X-ray binaries.

Part I deals with pulsars in globular clusters. Clusters appear to contain large numbers of pulsars, and this section is devoted to analyzing the pulsar content of the globular cluster system. The smearing of a pulsar signal in a binary orbit is analyzed numerically, and empirical relations between the observing time and the parameters of detectable binaries are presented. It appears that current searches should already have found a large fraction of cluster pulsars.

In Chapter 3, sensitive VLA images of four clusters are presented. No emission is detected from the dense cluster Liner 1, which suggests that the pulsar population of a cluster is not dependent only on its density. Seven unidentified sources are detected in or near the cores of three clusters. There appears to be no large population of pulsars in tight binaries.

In Chapter 4, the ultra-short period binary 411 1820-30 in NGC 6624 is detected at 1.4 GHz. The source is confirmed to be several arc seconds away from the cluster core, implying its negative period derivative cannot be due to gravitational acceleration by the cluster. It is suggested that identification of cluster X-ray sources via their radio emission is a potentially powerful tool.

In Chapter 5, a statistical analysis of all previous pulsar surveys, as well as new VLA and Westerbork observations, is used to derive the cluster pulsar luminosity function and the cluster density weighting function. The derived luminosity function is very similar to that of disk pulsars, and the density weighting function scales significantly less steeply than predicted by the tidal capture model. It is suggested that this may be due to the importance of single star-binary interactions.

Part II presents optical spectroscopy of a variety of compact binaries in the disk. Chapter 6 presents observations of the X-ray nova A0620-00. The mass of the compact object is confirmed to be above the maximum allowed mass for a neutron star, and therefore must be a black hole. More stringent limits are placed on the black hole mass by analysis of the emission lines from the disk.

Chapter 7 presents spectroscopy of the recurrent nova U Scorpii, which has been argued to be a very massive white dwarf, M ~ 1.4 M[...]. As a candidate for a white dwarf which may collapse to form a neutron star, this object is highly interesting for understanding the evolution of millisecond pulsars and LMXBs. From analysis of the absorption lines, an upper limit is placed on the mass of the white dwarf: M [...] 0.9 M[...](3σ). Hydrogen is also detected in absorption, suggesting that the observed high helium abundance in the emission lines is not due to the nature of the companion. Thus it appears there is no viable model for this interesting source.

In Chapter 8, radial velocities of nine disk LMXBs are presented. These are considered in the light of the direct formation model for LMXBs, where the neutron star is formed by the supernova explosion of a massive star in a binary system. With the inclusion of a random kick given to the neutron star at birth, the observations are found to be consistent with this model.

Thesis Files