White Dwarfs
When a star like our Sun reaches the end of its life and runs out of fuel in its core, it sheds out its outer layers into space in a beautiful event called a planetary nebula.
At the center of the planetary nebula, what remains of the star, an extremely hot and dense core, is a newly born white dwarf.
Polluted white dwarfs
White dwarfs are extremely dense: their masses range between 0.1 and 1.4 times the mass of the Sun, but their radii are comparable with planetary ones. For this reason, the gravity on their surface is very strong and heavy elements (anything heavier than hydrogen and helium) sink very quickly inside the star and they should not visible in the atmosphere. However, in many white dwarfs, heavy elements have been observed as absorption lines in the atmosphere. The fact that we see these elements in the atmosphere, together with the short sinking time-scales, means that these white dwarfs are being polluted right now: objects with a heavy elements-rich composition are falling on them. Some white dwarfs even show evidence for a debris disk accreting onto the star.
In my work, I presented a new method to account for the observed debris discs around young white dwarfs and the presence of metal lines in their spectra: if the star system has a rich comet population (like our Solar System), the pollution can be accounted for by the interaction between comets (which are far away enough that their orbit can be perturbed by the mass-loss at the end if the star's life) and a possible planet close to the comet region (like the hypothetical Planet 9 in our Solar System).
Read more:
Polluting white dwarfs with perturbed exo-comets
Magnetic white dwarfs
White dwarfs are all born in the same manner, as the compact remnants of low and intermediate-mass stars up to about eight times the mass of our Sun. However, some white dwarfs are peculiar, as their surfaces are threaded with magnetic fields that range from a few thousand to a billion Gauss. We still do not know why some white dwarfs are born with magnetic fields thousands to millions of times stronger than the rest. Is magnetism the product of binary interactions with other stars, the cataclysmic merging of white dwarfs or some other process? Magnetic white dwarfs are on average more massive than non-magnetic ones, and this could indicate that the mass of the progenitor star is somehow related to the genesis of the magnetic field or that magnetic white dwarfs are the products of white dwarf mergers.
Read more:
A Massive Magnetic Helium Atmosphere White Dwarf Binary in a Young Star Cluster
Intermediate-mass stars become magnetic white dwarfs
Massive white dwarfs
The relationship between the main sequence mass of a star and its white dwarf remnant mass, known as the Initial-Final Mass Relation (IFMR), has become the subject of significant interest over the past few decades due to the insight it provides into the final stages of stellar evolution. Unstable mass loss, fluctuations in internal nuclear reactions and convective processes make these later stages challenging to model. Analysis of the IFMR reveals tension in the linearity of the relation, a property difficult to constrain due to the sparsity and significant scatter of current data.
One of the largest gaps in the IFMR remains at the high-mass end, where white dwarfs are both rare and faint. Ultramassive white dwarfs from a single progenitor set a lower limit on type Ia supernova production. These supernovae are used as standard candles to measure distances within the universe, and their production rate governs nucleosynthesis and the chemical evolution of galaxies. White dwarfs have been found with masses approaching the Chandrasekhar limit (1.38Msun), but formed by gaining mass from accretion or merger events rather than a single progenitor. Until recently, there were only two objects defining the upper limit of the IFMR. One is the current record holder GD 50, with a mass of 1.26 Msun and progenitor mass as high as 7.8. However, this object has some uncertainty in its parent cluster origin, while the other, a member of the Pleiades, is much less massive with a precursor mass ~6 Msun.
With my group at UBC, using Gaia Data Release 2, we search nearby open star clusters to look for young ultramassive white dwarfs.
Read more:
A Massive Magnetic Helium Atmosphere White Dwarf Binary in a Young Star Cluster
Massive White Dwarfs in Young Star Clusters (in preparation)