White dwarf stars are glowing embers, the remains of small stars that have run out of fuel to burn Most white dwarfs are hot lumps of charcoal, gradually radiating their heat into space. But a few are made of helium and it is these that we look at today.
Because they generate no heat of their own, white dwarfs will eventually become so cold that they should stop emitting significant amounts of heat and light. But this cooling process takes ages, far longer even than the age of the universe, so no so-called black dwarfs are yet thought to exist.
The precise rate at which white dwarfs cool depends on their internal structure. That’s fairly well understood for plain vanilla white dwarfs. But the helium flavour holds some surprises.
The conventional view is that helium under high pressure forms a plasma of nuclei in a sea of electrons. When the pressure increases, the nuclei become ordered forming a crystal. The properties of this helium crystal determine how quickly the star cools.
Recently, however, astrophysicists have pointed that the helium can also form a Bose-Einstein condensate. The question that Paulo Bedaque at the University of Maryland in College Park and a few pals investigate is how the presence of such a condensate might affect the properties of the star.
It turns out that helium condensates have an extraordinary rich behaviour in which various kinds of quasiparticles can form. These quasiparticles are essentially quantised excitations in the condensate and have been well studied for ordinary condensates.
Because these quasiparticles transport energy through and out of a condensate, they reduce its specific heat.
What Bedaque and co have found is an entirely new quasiparticle that emerges in helium white dwarfs because of the extra constraints on the behaviour of the condensate at the centre of such a dense object.
This quasiparticle, they say, reduces the specific heat of the white dwarf core by two orders of magnitude compared to a crystalline core.
The consequences aren’t difficult to fathom. The lower specific heat means that helium white dwarfs ought to cool down significantly faster than previously thought. In fact, Bedaque and co suggest this more rapid cooling ought to be detectable.
As it turns out, there is a known anomaly in the temperatures of helium white dwarfs. A couple of years ago, astronomers found a group of helium white dwarfs in a globular cluster a few thousand light years from here.
Plot the temperature and magnitude of white dwarfs and astronomers usually find that the sequence of stars becomes cooler and less bright until they are no longer visible to whatever telescope they are using.
But with these helium white dwarfs, astronomers saw something else: the sequence ended well above the magnitude limit of the observations (which were made with Hubble). For some reason, the dimmest, coolest, oldest stars aren’t where they are supposed to be
Nobody knows why but the new quasiparticle and the cooling it causes, raises a possibility. Perhaps helium white dwarfs go through some kind of internal transition as they get older that causes them to suddenly cool much faster than expected. That’s why the oldest coolest stars don’t follow the usual pattern of cooling.
For the moment, that’s just a guess. Bedaque and co say that a great deal more modelling is necessary to fully understand how all this might work out. More data on real helium white dwarfs would help too.
Looks like a few interesting months ahead for these guys.
Ref: arxiv.org/abs/1111.1343: Nuclear Condensate And Helium White Dwarfs