Research Interests

My area of research is stellar astronomy and I am interested in what resolved systems of stars can tell us about planet formation, dark matter, the age of the Universe, the dynamical evolution of stellar systems and the formation of galaxies. To investigate these diverse subjects I observe a wide range of objects including nearby stars, open and globular star clusters and the resolved components of our neighbouring galaxies. To accomplish my research goals I use a variety of telescopes particularly the twin Gemini Telescopes, the Canada France Hawaii Telescope and the Hubble Space Telescope.

HST images of the core of 47 Tucanae: optical on the left and UV on the right.

Stars up to about 8 times the mass of the Sun end their lives as white dwarfs (WDs). The core of such an object provides a physical laboratory with conditions that cannot be reproduced on Earth; temperature about 100 million degrees and density of order of 1 million times that of water. With this almost unique laboratory, it is possible to carry out exotic physical experiments. In the early stages after forming the WD, the central temperature is so hot that conditions favour the production of neutrinos in a reaction termed the plasmon neutrino process. For the first 20 million years of life, the WD cools mainly by the emission of neutrinos. These neutrinos are actually of very low energy, so their "detection" via the observed cooling rate of the WDs is a new window on the exotic physics of neutrino production. In addition to neutrinos, these very hot objects may also be producing other particles that could contribute to the cooling of these stars. An important additional cooling contributor could be axions, manufactured in the WD interior in processes similar to that of neutrinos. Axions are still hypothetical particles, invented by particle theorists to solve a problem in quantum chromodynamics. They could be of interest as a component of cold dark matter which is thought to dominate the Universe's mass budget.

To find the very hottest WDs in sizeable numbers, we need to observe in the core of populous clusters where many WDs were recently produced. Ancient globular star clusters are ideal for this purpose. In the core of a cluster we recently sampled, over 3000 such objects were discovered. Detailed analysis of the observed rate of cooling of the WDs will yield neutrino and possibly axion production rates that can be compared with theory.

Since WDs cool at quite a predictable rate, they are also quite wonderful clocks. The very coolest WDs are also the oldest. We have been obtaining extremely deep exposures with Hubble in order to located the coolest and oldest WDs in globular star clusters. This provides a new and much more precise method of dating these ancient stellar systems that were formed relatively soon after the Big Bang. Their ages coupled with their location in the Galaxy allows us to arrive at the chronology of formation of our Galaxy.

Before becoming a WD, its precursor is the most massive visible object in a star cluster. Because of this, the WD precursor sinks to the cluster center under the effects of gravity. Then, in a short period of time, the star loses about half its mass. Now it is too low in mass to be so centrally located, so the WD will begin diffusing slowly outward. Since each WD has a small clock on its back (its cooling age) and we can measure its distance from the cluster centre, we can estimate the diffusion constant in the cluster. We have applied this in one system thus far and have Hubble time in 2016 for another cluster with quite different physical parameters.

Left: Colour-magnitude diagram in the UV of the core of 47 Tucanae. Right: Radial distributions of young (light green) and older white dwarfs (dark green). Note the more extended radial distribution in the older stars.

In late 2015 we will be observing with Hubble on a unique project. One of the most difficult and important inputs into galaxy evolution models is the maximum mass of a star that can still produce a WD. Above this mass (6 to 8 times that of the Sun) the star produces a supernova. The chemical evolution of a galaxy is largely driven by the rate of supernovae explosions, so knowing which masses produce them is critical in understanding how galaxies evolve. UV imaging in globular clusters made us realize that young WDs were amongst the brightest stars in globular clusters; so bright that we could detect them in a nearby companion galaxy. We selected a sample of clusters in this galaxy whose stars just completing their nuclear evolution were between 4 and 9 times that of the Sun. We'll be looking to see which ones have been able to formed WDs.

The James Webb Space Telescope (JWST) will become operational during the next few years. In preparation for this, we have been carrying out pilot programs with Hubble to detect planetary remnants around WDs in ancient star clusters. I expect to use the infrared-optimized JWST to search for evidence of such debris around ancient WDs. A positive detection suggests the possibility of an early rise to life in the Universe.