# Black dwarf binary systems

I know black dwarf stars do not exist yet, and that they are what white dwarfs end up after a long time. The question is really simple: would it be possible to have a binary system in which one of the two is a black dwarf, and the other companion star is dumping mass on the black dwarf, causing the black dwarf to supernova? I guess based on the timescales, the maximal initial mass of the second object could be calculated to see if it is possible that one object is at the black dwarf stage and the other still a giant. Or is the timescale on which the star goes from white dwarf to black dwarf big enough that it will never reach the black dwarf stage while the other one is still an AGB star?

Second part: If not possible that one is black dwarf and the other AGB star, is it possible if the companion is a brown dwarf, that they are close binary, and that a brown dwarf would dump mass on a black dwarf? This would need the binary system to be a very close binary system at this stage of the system.

Third part of the question: would the spectrum of the supernova be any different from the normal one with a white dwarf and a companion AGB star? I guess the spectral lines would be different, since the companion would have to be a very low-mass AGB star, since that companion would be about 20billion years or so, when the black dwarf star becomes a black dwarf.

• Technically, we don't know that black dwarf stars don't exist yet; we just know they shouldn't exist yet based on our current theories. Of course, for anything theoretically possible, it is impossible to know that it doesn't exist for sure--you can just gain better confidence that it probably doesn't exist. – called2voyage Mar 14 '14 at 12:27
• @called2voyage "Technically, we theorize that such objects might one day exist", is the phrase you are searching for? – usethedeathstar Mar 14 '14 at 12:42
• Succinctly, yes, but that phrasing misses the nuance that there is a possibility that we are incorrect and such objects already exist--we just haven't observed them yet. – called2voyage Mar 14 '14 at 13:44
• @called2voyage you need to define what a black dwarf is before confidently stating that there aren't any. See arxiv.org/abs/1406.0488 – Rob Jeffries Dec 28 '18 at 17:46

## 2 Answers

I see two real questions here. First, whether it's possible to have a black dwarf with a companion object. For a given black dwarf, this is unlikely, since the orbits would likely be unstable at the time scale required to produce a black dwarf. Given the size of the universe, however, it's not out of the question. A black dwarf could even capture a companion star after becoming black, although this would be extremely rare.

Assuming it's possible, the next question would be whether the supernova produced would be the same as a normal type 1A supernova. The supernova is generally triggered based on the critical (Chandrasekhar) mass which triggers carbon fusion. Since it's dependent on mass, it would likely be a "normal" type 1A supernova. Keep in mind not all 1A supernovae are identical to begin with, although they can still be "normalized" to function as a fairly reliable standard candle.

One thing to note is, the acretion process would cause the black dwarf to heat up, meaning it would not remain black.

• Downvoted because the cooling timescale for massive white dwarfs (the ones most likely to accrete and become type Ia supernovae) is short enough that cold white dwarfs (<3000 K) should exist and indeed have been found arxiv.org/abs/1406.0488 – Rob Jeffries Dec 28 '18 at 17:48

It is possible that accretion onto a white dwarf companion causes type Ia supernovae. This is most likely to occur if the white dwarf is massive to begin with.

The timescale for massive white dwarfs ($$>1 M_{\odot}$$) to cool below 3000 K and emit almost no radiation at visible wavelengths (which you might call a "black dwarf, though the term is not used extensively in the scientific literature AFAIK) is considerably shorter than for more typical white dwarfs of mass $$0.6 M_{\odot}$$ and can be of order 10 billion years. Such white dwarfs have been identified (Kaplan et al. 2014). The plot below shows the cooling curves of massive white dwarfs in the form of either surface temperature or absolute red magnitude vs time. Clearly they can cool (according to these widely used models) on timescales considerably less than the age of the universe.

Since massive white dwarfs evolve from massive progenitors with short lives, it would seem eminently possible for a lower mass companion to have a much longer evolutionary timescale and for mass transfer to occur later on that will trigger a supernova explosion, and within the current lifetime of our Galaxy. I see no reason for such a supernova to look any different to other type Ia supernovae.

However, the cooling time to a "black dwarf" would still be of order 10 billion years, so these events would not be occurring in high redshift galaxies that are considerably younger than the Milky Way.