The first part of the project has led to the identification of the oldest star in our galaxy that is accreting debris from orbiting planetesimals, making it one of the oldest rocky and icy planetary systems discovered in the Milky Way. The findings were published in the Monthly Notices of the Royal Astronomical Society by PhD student Abbigail Elms and received a press release. The research concludes that a faint white dwarf located 90 light years from Earth, as well as the remains of its orbiting planetary system, are over ten billion years old. The very red white dwarf WDJ2147-4035, detected by the space observatory Gaia of the European Space Agency, is polluted by planetary debris and was found to be around 10.7 billion years old, of which 10.2 billion years has been spent cooling as a white dwarf. By analysing the spectrum from WDJ2147-4035, the metals sodium, lithium, potassium and carbon were found to have accreted onto the star – making this the oldest metal-polluted white dwarf discovered so far. The debris found in the otherwise nearly pure-helium and high-gravity atmosphere of the red star WDJ2147-4035 are from an old planetary system that survived the evolution of the star into a white dwarf. By predictions from how quickly those metals are sinking into the star’s core, it is possible to look back in time and determine how abundant each of those metals were in the original planetary body. By comparing those abundances to astronomical bodies and planetary material found in our own solar system, we can guess at what those planet debris would have been like before the star died and became a white dwarf – but in the case of WDJ2147-4035, that has proven challenging. The red star WDJ2147-4035 is a mystery as the accreted planetary debris are very lithium and potassium rich and unlike anything known in our own solar system.
Some of the brightest and closest white dwarfs to the Sun were identified and observed spectroscopically as long ago as the 1910s. However, the white dwarf luminosity function peaks at faint magnitudes, and therefore most white dwarfs are cool and faint. The lack of precise parallax measurements for these faint white dwarfs meant that identifying nearby white dwarfs has been historically challenging. Following the launch of the space observatory Gaia of the European Space Agency in 2013, the first all-sky Gaia catalogues of white dwarf candidates with precise parallaxes were released. Using dedicated spectroscopic observations, PhD student Mairi O'Brien used the Gaia 3rd data release to confirm 203 new white dwarfs within a volume of 40 parsecs in a paper published in the Monthly Notices of the Royal Astronomical Society. The nature of 1079 Gaia white dwarf candidates out of the 1083 identified by Gaia within 40 parsecs of the Sun have now been spectroscopically confirmed in a follow-up MNRAS paper. This census of local white dwarfs has now reached >99% volume completeness, a major step forward compared to earlier studies. White dwarf volume samples have been found to have several practical advantages for deriving astrophysical relations. In particular, white dwarfs with cooling ages larger than 5 billion years are too dim to be seen at distances larger than 40-100 pc, resulting in increasingly age- and mass-biased samples. Older and heavier white dwarfs that have long cooling ages and short main-sequence lifetimes are intrinsically faint and only seen in the local volume, yet those provide a robust test of old planetary systems and stellar evolution models, e.g. using wide binaries.
We have used the 40 pc sample to derive the initial-to-final mass relation between Sun-like stars and their white dwarf remnants in a paper in collaboration with Dr Tim Cunningham, helping to better understand the local population of white dwarfs and the stellar mass loss on the giant phase after the main-sequence. The 40 pc sample was finally used to compare different techniques to extract the ages of stars (PhD student Emily Roberts, in progress). We find that the local star formation rate is close to constant in the last 10 billion years.
