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.
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.
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.
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).