A Cooling Anamaly of High-Mass White Dwarfs

We discovered anomaly in the cooling process of high-mass white dwarf stars. The "trafic jam" created by this anomaly can explain properties of the "Q branch", an unexpected branch-like feature revealed by the data of Gaia satellite. This anomaly is an extra delay of cooling in addition to the delay of crystallization. The settling of 22Ne in white dwarfs may account for the physical origin of this extra delay. Here is our paper and the catalog of selected white dwarfs:
and an explanation of the columns: column.txt

Observations from Gaia

The Q branch is revealed by the Hertzsprung--Russell diagram (a luminosity vs. color plot) of white dwarf stars, obtained from Gaia Data Release 2 (DR2). It is not only a pile-up or overdensity, but also a region with more fast-moving white dwarfs. We shall explain both of these features in one scenario, which requires an extra cooling delay produced by some mechanism around the branch. This poses a challenge to white dwarf models.

Two populations of white dwarfs

To explain both the number-density enhancement and the velocity excess on the Q branch, There must exist a special population of high-mass white dwarfs, which is subject to an extra cooling delay on the branch. This plot shows a simulated animation of this two-population cooling scenario. Our detailed analysis shows that the fraction of the extra-delayed population is about 7%, and the delay is 8 Gyr long.

Normal white dwarf cooling

White dwarfs cool down from the bright, blue end to the faint, red end of their cooling tracks. The cooling rate changes quite smoothly, so the number density of white dwarfs on this plot is also smooth.

Extra-delayed cooling

We find that for some white dwarfs, they must experience a very severe slowing-down of cooling rate, which makes them pile-up on the Q branch. This effect create both an over-dense region on the plot and an older age than the normal cooling (upon and after the pile-up).

Recovering the number-density enhancement

Recovering the age (and velocity) excess

The physics behind this cooling anomaly

With a simplified calculation, we found that the effective zone of 22Ne settling matches the Q branch very well on the H--R diagram. We look forward to numerical results from detailed white dwarf models.



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