One of my favorite types of scientific problems is when you see something that is impossible and you know about it.
White dwarfs are the cores of stars that die. After some time, the star swells up, blows off its outer layers, and leaves a hot, dense core exposed to space. The Sun will eventually die and be replaced by a white dwarf.
The white dwarf's mass depends on how big the star was. The bigger the star, the bigger the white dwarf. But this leads to a different conclusion. The lowest mass white dwarfs we see are less than the Sun's mass. The stars that form them have to be very low mass.
Do you know the problem? The universe is only 14 billion years old. It is not old enough to have made any ELMs.
We see them. How is this possible?
It is a good bet that the Universe knows what it is doing when there are problems like this in science. You missed something, so what is going on?
In this case, it is other stars. Specifically, the stars. Maybe the low mass star that made the ELM was close to another star. The systems behave differently than single stars do. If the low mass star is a companion to a previously made white dwarf, the primary can pull material off the companion star, stripping its outer layers away. If the timing is right, this can leave behind a white dwarf that would take far longer than the age of the Universe to make.
Okay, cool. You have a nice hypothesis. Is there any evidence for it?
The progenitors of ELMs have been revealed thanks to a clever use of archives and new observations.
The first piece of the puzzle is that all known ELMs are in systems where the primary is a white dwarf. To make them you need a low mass star in a close vicinity of a white dwarf, and those can be found through sky surveys.
The low mass star companion has to be near the end of its life, so it has just started making a white dwarf core. When the outer layers are stripped off, the white dwarf is much lower in mass. The stars losing their outer layers to the primary will be hotter and smaller than usual, making it possible to distinguish them from other stars.
The chain of logic is long and complex, dealing with things like how stars evolve, and one of the few reliable things we can do. The point is that the progenitor systems of ELMs can be found.
The astronomer first looked through the data. The European Space Agency's satellite, called Gaia, measures the brightness, positions, and motions of over a billion stars in the sky. A search like this would have been impossible before.
The numbers wouldn't be reliable so they cut stars too far. They had 660,000 stars. They looked for stars that were fainter than normal stars but brighter than white dwarfs. The number was reduced to 8,300. They looked for ones that had good observations of their brightness over time using the Zwicky Transient Facility and another survey done by a ground-based telescope. They looked at the remaining binaries and found a low mass star in close proximity of a white dwarf primary.
They had 51 candidate systems. The key was breaking the light up of the stars into individual wavelengths. This can give information on temperature, size, mass, and more of the stars. They were able to do this with 21 systems, all of which were low mass stars. Whoa. They are going to follow up on the remaining 31
This is very impressive sleuthing. Being able to remove the low-mass star/white dwarf binaries from the sky is a great feat.
The conclusion is that ELMs are not older than the Universe and are made in these kinds of binaries. How often they form is 60 per million years on average, and how many we should expect out there is roughly 2 for every cubic volume of space in the galaxy.
In some cases two white dwarfs spiral together in the progenitors of Type Ia supernovae, and in millisecond pulsars, where the two stars are very close together. These are used to measure the expansion rate of the universe.
Understanding how low mass white dwarfs are made is important, with implications on the scale of the Universe itself. It is nice to know that the Universe knows what it is doing.
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