Recurrent Nova RS Ophiuchi and type Ia supernovae

RS Ophiuchi is a famous recurrent nova.
Recently it had an outburst, brightening from magnitude 11 (hundred times fainter than faintest star visible to the naked eye) to fifth magnitude - faint but visible to the naked eye.

Recent analysis of the outburst (and here) suggest that RS Ophi is high mass and the mass grows with each outburst. If so, it will soon explode as a supernova.

Novae have been known for centuries. They are a "new stars", appearing in the sky where no star was before. We now know that they are sudden brightening of a faint pre-existing star. Brightness increase is typically about a factor of 10,000.

Novae occur when gas accreting onto a white dwarf builds up on the surface of the white dwarf until it detonates, in a sharp onset thermonuclear runaway reaction. A white dwarf, you will remember, is the compact (few thousand kilometer radius - or about Earth size) degenerate remnant core of a main sequence star which has exhausted the fuel available for fusion in its core.
The white dwarf is held up by the pressure of relativistic electrons in the highly dense core - crudely speaking, the electrons repel when pushed together providing a pressure which hold the white dwarf up against gravity.

Famously, there is a maximim mass for white dwarfs, the Chandrasekhar limit of about 1.4 times the mass of the Sun.
The material detonating in novae is on the surface, it is transferred to the white dwarf from a companion star, either a close main sequence companion, or an more evolved red giant star.

The typical white dwarf has a mass of about half the mass of the Sun. Some have lower masses, and there is a spread of masses up to almost, but not quite, 1.4 times the mass of the Sun.
If a white dwarf reaches the Chandrasekhar mass, then something has to give. The internal pressure is not enough to support the stars own weight. Either the star collapses - and the collapse either stops when a new source of pressure halts it, such as neutron degeneracy; or it doesn't halt, and a black hole forms; or the star blows up leaving now remnant.

The mass accreting onto a white dwarf add mass to the white dwarf. It comes down in a steady trickle.
Novae occur when the fresh material has built up to thick enough a layer that the combined pressure and temperature at the bottom of the layer can trigger a thermonuclear reaction which then runs away over the whole surface of the white dwarf. The energy released in the detonation is higher than the binding energy of the matter on the surface of the white dwarf, so matter is blown off into space.
Big question is, how efficiently does the explosion couple to the mass - is more mass blown off than accreted? ie is there net accumulation of mass, or net loss of mass during novae.

Now, there are two classes of novae - classical novae, and recurrent novae. The latter repeat on decadal time scales. They are rare. Now, realistically classical novae probably are mostly also recurrent, it is just that the recurrence interval is probably longer than the time we have been reliably observing the sky.

RS Oph is one of the more reliably recurrent novae, having erupted 6 times in the last century or so.
Recent papers in Nature here and here make a strong case that the recent outburst in RS Oph was asymmetric, and that the models closely constrain the white dwarf mass, the accretion rate of mass between outbursts, and the mass ejected.
These are interconnected, since we measure the luminosity of the material accreting on the white dwarf, which depends both on the mass accretion rate and the white dwarf mass.

The somewhat surprising result is that the RS Oph primary mass is high, in fact just about 1.4 times the mass of Sun. Further, the mass loss during outbursts is less than the estimated accreted mass between outbursts, so the primary is gaining mass. Quite quickly in fact. It should hit the Chandrasekhar limit in order million years, maybe less, maybe a little bit longer.

So recurrent novae now have a strong case for being massive systems, accreting net mass quite rapidly and heading for the Chandrasekhar limit. So what.

Well, the original stella nova - Tycho's Nova of 1572, was a supernova.
There are two types of supernovae, type I and II, each divided into subtypes.
As it happens, types Ib and Ic are actually of type II, but type Ia supernovae are of a genuinely different type.

Type II supernovae are the deaths of massive stars; type Ia supernovae are the detonation of a Chandrasekhar limit mass white dwarf.
They are relatively rare, maybe 1 in 5 supernova is type Ia, for about one per 300 years in the Milky Way. They are also incredibly important in modern cosmology. Type Ia supernovae are very luminous and can be detected from a very long way away, from billions of light years away. They also appear to be very uniform in luminosity and therefore are good standard candles. As such, they have been used to measure the geometry of the universe to large distances, and to show the universe is accelerating with time, implying the existence, and increasing dominance of dark energy.

There are several possible ways to make a type Ia.
When I were a lad, the favoured channel was the merger of two massive white dwarfs in close orbit through gravitational radiation. This will probably work, but we have never seen two white dwarfs which were both massive enough and close enough. If type Ia occur every 300 years, there have been about 50 million in the Milky Way to date, give or take. For obscure reasons we think type Ia supernovae take time to develop, their progenitors hang around for a while, so there ought to be millions of type Ia progenitors in the galaxy. So lots, but not too many...
Merging white dwarfs were problematic as type Ia progenitors, since their combined mass is unlikely to be just exactly the Chandrasekhar limit, if it is over, it is likely to be way over maybe 10-20%. Which would make type Ia poor standard candles, some would be overluminous.

Recurrent novae were another possible type Ia progenitor, they're good, they gain mass slowly and creep up on the Chandrasekhar limit, making good standard candles.
A problem though was whether recurrent novae were actually gaining mass.
Now it appears we know that they are.

However, Della Valle and Livio in 1996 looked at know recurrent novae, and give ~ million year lifetimes and net mass gain, you find the recurrent novae in the Milky Way reach the Chandrasekhar mass at the rate of about 5 per hundred thousand years. Which is much lower than the 3 per thousand years we estimate the type Ia rate to be!

So recurrent novae are probably progenitors of type Ia supernovae, but look like they may be the minority formation channel, they are too rare. Some other channel ought to dominate the type Ia formation.

Current suspicion falls on Supersoft x-ray sources - these are bright, "soft" x-ray sources. They are hot, but not very hot, and quite luminous. They are similar to recurrent novae, we think, white dwarf primaries accreting from secondary stars, but the difference is that we think the accretion rate is such that there is steady fusion on the surface. So no outbursts, no novae.

So we have good new evidence that the classical candidate to be the progenitor of the very important type Ia supernova, is in fact a type Ia progenitor. And, simultaneously, we infer that this formation mechanism is the minority contributor to type Ia formation. Most of them likely come from another source.

There you go.