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Astrophysics with white dwarfs Kalirai, Jasonjot Singh

Abstract

White dwarfs are the end products of the entire stellar evolutionary process in all intermediate and low mass stars. Over 99% of all stars in our Galaxy will eventually end their lives as white dwarfs. Observationally, studying white dwarfs has proven to be very difficult, primarily due to the faintness of the objects. Bright white dwarfs with M[sub v] = 11 have a luminosity only l/300th of the Sun's intrinsic brightness, while the faintest white dwarfs are 100,000x fainter than the Sun. In this thesis, we describe three related projects aimed at better understanding white dwarfs themselves, as well as their role as inhabitants of our Galaxy. The data that we have acquired to study these faint stars are of unprecedented quality and depth, thereby making possible several scientific results that have eluded investigation in decades of previous effort. First, we provide new insight into one of the most important questions in astrophysics today, what is the nature of the dark matter? Specifically, we are able to marginally rule out the most likely candidates based on microlensing results, namely white dwarfs, as a strong contribution to the dark matter. This study represents the deepest ever look into the Galactic halo and uses Hubble Space Telescope (HST) data. Secondly, we present results from the continuing study of open star clusters in the Canada France Hawaii Telescope (CFHT) Open Star Cluster Survey. This work has improved the quality of the photometry of open star clusters by over an order of magnitude compared to what had been previously possible. We present our findings for two very young clusters, NGC 2168 (M35) and NGC 2323 (M50), including a study of their white dwarf populations. These two clusters, and the white dwarfs that we have found within them, will prove to be crucial in constraining one of the most fundamental relations in stellar evolution, the initial-final mass relationship. In the third project, we use the 8-metre Gemini North and 10-metre Keck telescopes to simultaneously obtain spectra for 22 white dwarfs in the rich cluster NGC 2099. This work represents a planned follow-up study of the white dwarfs in the richest clusters that we identified in the CFHT Open Star Cluster Survey, and has produced several interesting results. First, all white dwarfs in this cluster are hydrogen rich suggesting perhaps that the ratio of hydrogen to helium white dwarfs is different in clusters than in the field, or that all massive white dwarfs are hydrogen rich. Secondly, the NGC 2099 white dwarfs provide the first ever confirmation of a white dwarf cooling age for a star cluster. Thirdly, with just this one cluster, we are able to almost double the number of white dwarfs that exist on the initial-final mass plane, and provide very strong, tight constraints on a key part of the initial-final mass relationship. The previous constraints on this relationship, which show a large scatter, had taken over 30 years to establish. Our findings directly show that stars with masses between 2.8-3.5 M lose 75% of their mass through stellar evolution. Additionally, for the first time, we are beginning to see the effects of metallicity on the initial-final mass relationship. Finally, we are now in a position to obtain further spectroscopy of white dwarfs in the other rich clusters that we have imaged with CFHT, and, in the very near future, plan to put over 100 data points on the initial-final mass plane.

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