Slowly Cooling White Dwarfs.
White dwarfs are the remnants of a Sun-like star. Any stars with a mass in the range of 0.8–10 Mₛᵤₙ are destined to end their lives as white dwarfs. They are so dense that they are supported against gravity by electron degeneracy pressure. This pressure arises when electrons are packed very closely, as allowed by the laws of quantum mechanics. Any further push by gravity is resisted by the degeneracy pressure, thus allowing white dwarfs to be stable.
With time, white dwarfs evolve towards cooler temperatures and lower luminosities because they cannot generate energy and spend their lives radiating away the thermal energies of their constituent ions.
The evolution of white dwarfs is generally described as a simple cooling process.
This is because there are not enough constituents available in white dwarfs to allow them to generate energy. Thus they radiate energy and move towards cooler temperatures. Because of this simple cooling nature shown by them, white dwarfs are also used as cosmic chronometers.
How were these slowly cooling white dwarfs discovered?
While studying the M13 galaxy, scientists observe that the white dwarfs in M13 are more luminous than in the M3 galaxy. Both M3 and M13 are globular clusters and the stellar mass distribution along the horizontal branch of these two systems is almost identical thus, the white dwarfs' luminosity function in them should be similar if white dwarfs evolve in a simple cooling process.
The search for the explanation of the anomaly in M13 led scientists to an alternative cooling mechanism in white dwarfs.
One of the possible explanations is that the white dwarfs in the M13 galaxy cool more slowly than their counterparts in the M3 galaxy and this leads to the difference in the observed luminosity functions in both galaxies.
How does the cooling slow down?
The possible explanation is that the thermonuclear burning in their envelope leads to the slowing down of the cooling process of white dwarfs.
Even a small amount of hydrogen left out from the previous evolutionary stage is enough for slow thermal burning.
Another interesting fact is that this process is more favorable in the low metallicity low luminosity regime which is a characteristic of globular clusters. This increase in the cooling time means that there are more white dwarfs for any given luminosity.
This leads to an increase in the luminosity function which in turn can explain the observed difference between the luminosity functions of M3 and M13. Slowly cooling white dwarfs are expected in clusters with moderate metallicity.
Standard white dwarfs like those in M3 have a hydrogen content below the threshold for hydrogen burning and thus progressively cool in time with no sources of active energy production.
M13 has a blue tail in the horizontal branch of the stellar evolution of the galaxy whereas it is absent in M3. Thus, another characteristic of slow-cooling white dwarfs is that they are found in galaxies with blue tails in the horizontal branch.
This slow cooling white dwarf mechanism explains the observed luminosity difference in the M13 and M3 galaxy and as it only occurs in the M13 galaxy and not in the M3, this process is also linked with the blue tail in the horizontal branch of M13 which is absent in the M3 galaxy.
