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With the T Coronae Borealis nova likely to erupt soon, it got me thinking if novas could occur outside of a binary star system.

On the wiki page titled Nova:

All observed novae involve white dwarfs in close binary systems.

Classical nova eruptions are the most common type. They are likely created in a close binary star system consisting of a white dwarf and either a main sequence, subgiant, or red giant star.

Would it in theory be possible for such events to occur in other situations, say in a ternary system or from an influence of a black hole?

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To form a classic nova you need a white dwarf to be accreting matter, specfically hydrogen, onto its surface. The "obvious" source of this matter is another star that is very close to the white dwarf, such as a red giant. It is quite possible for a third star to be in the system (the wikipedia comment about "all in binary systems" is probably to indicate that no nova can occur in a solitary star system, and because any third star would play no part in the nova)

The jet of a black hole doesn't contain enough dense and slow moving hydrogen to accrete onto a white dwarf, so the association between novae and the jet in M87 is a not fully explained. Presumably the jet catalyses the nova in a binary system in some way.

There are other situations in which a star can suddenly or explosively brighten, such as type 2 supernovae. These don't involve accretion onto a white dwarf. These are not "classical novae".

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    $\begingroup$ Thanks for the answer. I'm specifically asking in context of "classical novae". May I ask why a potential third star (or any further stars in a multiple star system) would not play a part in the accretion leading to the nova? Would it not orbitally possible be for 3 (or more) stars to be in a close system with one another? $\endgroup$
    – Shawn Lim
    Commented Mar 28 at 8:37
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    $\begingroup$ Possibly a silly question on the side. In regards to "influence of a black hole", I wonder if the accretion disk of a black hole could contribute to a white dwarf accumulating enough matter for a nova to occur $\endgroup$
    – Shawn Lim
    Commented Mar 28 at 8:45
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    $\begingroup$ From the original paper "The enhanced rate of novae along M87’s jet is now firmly established, and unexplained." - arxiv.org/abs/2309.16856. But the accretion disk of a black hole is not the jets that your paper refer to. I'm not quite sure what you have in mind, I suppose a white dwarf could be orbiting in the (outer parts of a black hole accretion disk, and have it's own accretion, much as a proto planet can orbit in the accretion disc of a protostar. I don't (offhand) know of any examples of this $\endgroup$
    – James K
    Commented Mar 28 at 9:07
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    $\begingroup$ It's not possible for 3 stars to be in a close system, as that makes an unstable three-body-problem. The only stable three body orbits are hierarchical, That is the the white-dwarf-red-giant binary must either be alone, or be orbiting a third body, or be orbited by a third body. And given the likely distribution of mass, any three body system is likely to be a circumbinary, with a third star orbiting the binary at some distance. Otherwise it's just not stabel $\endgroup$
    – James K
    Commented Mar 28 at 9:11
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    $\begingroup$ Thanks for the clear explanations. I suppose I ought to clarify on the black hole part. In the paper and article referenced, M87's jet seems to have increased Nova rates of binary systems along its path, possibly due to the hydrogen-rich plasma it irradiates. This made me curious as to whether a black hole could in a binary system with a white dwarf, be directly responsible for a nova occuring. $\endgroup$
    – Shawn Lim
    Commented Mar 28 at 10:11

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