Laura Rogers
Once a star, similar to our Sun, ceases fusing hydrogen, it will undergo a violent stage of stellar evolution resulting in the production of a small, hot and dense stellar core. This is a white dwarf. It has been demonstrated that although the inner planetary system is engulfed during the red giant stage, the outer planetary system can survive. Evidence for the survival of outer planetary systems to the white dwarf phase comes from observations of planetary material ‘polluting' the atmosphere of white dwarfs. These observations are unique in providing the composition of exo-planetary material, but how does the material ultimately end up there? Infrared observations of dust very close to white dwarfs reveal how planetary material arrives in the atmosphere of white dwarfs. We expect the scattering of planetary bodies that leads to pollution to be a stochastic process, where objects get scattered in randomly, with the potential for variability on human timescales. Such variability has been found for the white dwarf WDJ0959-0200 among others, these discoveries are often serendipitous. I present the results from a large scale near-infrared monitoring campaign of 34 white dwarfs with dust using the United Kingdom Infra-Red Telescope over a baseline of 3 years. I address the following questions: How often does the dust vary in the near-infrared? What does this tell us about how the material is accreting onto the white dwarf? Can we use these observations to learn about the number of planetary bodies accreting- is it one body, or many bodies?
Once a star, similar to our Sun, ceases fusing hydrogen, it will undergo a violent stage of stellar evolution resulting in the production of a small, hot and dense stellar core. This is a white dwarf. It has been demonstrated that although the inner planetary system is engulfed during the red giant stage, the outer planetary system can survive. Observations have shown that between 25 and 50 percent of white dwarfs show evidence for exoplanetary material ‘polluting’ their atmosphere. It is hypothesised that chunks of planetary material from the outer planetary system get perturbed towards the white dwarf. Due to the strong gravitational field of white dwarfs, the material gets torn apart once it is close and forms a circumstellar dust reservoir – the white dwarfs with this close in dust are called dusty white dwarfs. This planetary material then accretes onto the atmosphere of the white dwarf. The only way to directly study the bulk chemical composition of an exoplanetary body is to observe these white dwarfs which have accreted planetary material.
In order to learn about the composition of exoplanetary material, we need to understand how this planetary material arrives in the atmosphere of the white dwarfs. We specifically want to understand how the material gets from the dust reservoir to the atmosphere of the white dwarf. Does the material accrete steadily, or variably? What processes contribute to this accretion? We also want to understand the number of bodies involved. Is it one body letting out a steady stream of material? Or is it many bodies? If it is the latter, is it a steady disc or stochastic (random), with highly variable accretion from the scattering and disruption of many bodies? To investigate these questions, we study dusty white dwarfs, white dwarfs which both have a dust reservoir close in to them and exoplanetary material ‘polluting’ their atmosphere. We observe these systems to search for variability in the infrared dust flux. Xu & Jura (2014) discovered the first dusty white dwarf which showed large amounts of dust variability. The infrared dust flux dropped by 30% within a year.
Often dust variability is discovered serendipitously. We systematically monitor 34 dusty white dwarfs for 3 years using the United Kingdom Infrared Telescope. We observe the dusty white dwarfs in the J, H and K near-infrared photometric bands to obtain a number of brightness measurements over the 3 year period. We take all the brightness measurements over the 3 years and for each object find the best fitting median magnitude (how bright the object is) and standard deviation, which we take as a proxy for the variability. We compare the white dwarfs' variability to what is expected for non-variable field stars. None of the values of the white dwarfs’ variability lie significantly above that expected from non-variable field stars. Therefore, for all 34 white dwarfs, over the length of the survey the near-infrared dust flux appears stable within the errors of the survey. This means that the accretion, which leads to exoplanetary material being in the atmosphere of white dwarfs, appears stable, and the disruption events which lead to large dust variabilities are rare and occur on timescales longer than 3 years.