Can the Recurrent Novae RS Oph Become Type Ia Supernovae?

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The classical scenario for creating Type Ia supernovae is a white dwarf star accreting mass from a nearby star entering the red giant phase. The growing red giant fills its Roche lobe and matter falls onto the white dwarf, pushing it over the Chandrasekhar limit causing a supernova. However, this assumes that the white dwarf is already right at the tipping point. In many cases, the white dwarf is well below the Chandrasekhar limit and matter piles up on the surface. It then ignites as a smaller nova blowing off most (if not all) of the material it worked so hard to collect.

A new paper by a group of European astronomers considers how this cycle will affect the overall accumulation of mass on the white dwarfs which undergo recurrent novae. In a previous, more simplistic 1D study (Yaron et al. 2005) simulations revealed that a net mass gain is possible if the white dwarf accumulates an average of 10-8 times the mass of the Sun each year. However, at this rate, the study suggested that most of the mass would be lost again in the resulting novae, and even a minuscule gain of 0.05 solar masses would take on the order of millions of years. If this was the case, then building up the required mass to explode as a Type Ia supernova would be out of reach for many white dwarfs since, if it took too much longer, the companion’s red giant phase would end and the dwarf would be out of material to gobble.

For their new study, the European team simulated the case of RS Ophiuchi (RS Oph) in a 3D situation. The simulation did not only take into consideration the mass loss from the giant onto the dwarf, but also included the evolution of the orbits (which would also influence the accretion rates) and varied rates for the velocity of the matter being lost from the giant. Unsurprisingly, the team found that for slower mass loss rates from the giant, the dwarf was able to accumulate more. “The accretion rates change from
around 10%  [of the mass of the red giant] in the slow case to roughly 2% in the fast case.”

What was not immediately obvious is that the loss of angular momentum as the giant shed its layers resulted in a decrease in the separation of the stars. In turn, this meant the giant and dwarf grew closer together and the accretion rate increased further. Overall they determined the current accretion rate for RS Oph was already higher than the 10-8 solar masses per year necessary for a net gain and due to the decreasing orbital distance, it would only improve. Since RS Oph’s mass is precipitously close to the 1.4 solar mass Chandrasekhar limit, they suggest, “RS Oph is a good candidate for a progenitor of an SN Ia.”

Astronomers Find Type Ia Supernova Just Waiting to Happen

Type Ia supernovae are a mystery because no one can predict when or where one might occur. But astronomers are hedging their bets on V445 Puppis. A so-called “vampire white dwarf” that underwent a nova outburst after gulping down part of its companion’s matter in 2000, now, it appears this double star system is a prime candidate for exploding. “Whether V445 Puppis will eventually explode as a supernova, or if the current nova outburst has pre-empted that pathway by ejecting too much matter back into space is still unclear,” said Patrick Woudt, from the University of Cape Town and lead author of the paper reporting the results. “But we have here a pretty good suspect for a future Type Ia supernova!”

This is the first, and so far only nova showing no evidence at all for hydrogen, and provides the first evidence for an outburst on the surface of a white dwarf dominated by helium. “This is critical, as we know that Type Ia supernovae lack hydrogen,” said Danny Steeghs, from the University of Warwick, UK, “and the companion star in V445 Pup fits this nicely by also lacking hydrogen, instead dumping mainly helium gas onto the white dwarf.”

Click here to watch a movie of the expanding shell of V445 Puppis.

The astronomers have determined the system is about 25,000 light-years from the Sun, and it has an intrinsic brightness of over 10,000 times our Sun. This implies that the vampire white dwarf in this system has a high mass that is near its fatal limit and is still simultaneously being fed by its companion at a high rate.

“One of the major problems in modern astrophysics is the fact that we still do not know exactly what kinds of stellar system explode as a Type Ia supernova,” said Woudt, “As these supernovae play a crucial role in showing that the Universe’s expansion is currently accelerating, pushed by a mysterious dark energy, it is rather embarrassing.”

Shell around V445 Puppis  (March 2005). Credit: ESO
Shell around V445 Puppis (March 2005). Credit: ESO

Woudt and his team used the ESO’s Very Large Telescope (VLT) to obtain very sharp images of V445 Puppis over a time span of two years. The images show a bipolar shell, initially with a very narrow waist, with lobes on each side. Two knots are also seen at both the extreme ends of the shell, which appear to move at about 30 million kilometers per hour. The shell — unlike any previously observed for a nova — is itself moving at about 24 million kilometers per hour. A thick disc of dust, which must have been produced during the last outburst, obscures the two central stars.

As Steeghs said, one defining characteristic of Type Ia supernovae is the lack of hydrogen in their spectrum. Yet hydrogen is the most common chemical element in the Universe. Such supernovae most likely arise in systems composed of two stars, one of them being the end product of the life of sun-like stars, or white dwarfs. When such white dwarfs, acting as stellar vampires that suck matter from their companion, become heavier than a given limit, they become unstable and explode.

The build-up is not a simple process. As the white dwarf cannibalizes its prey, matter accumulates on its surface. If this layer becomes too dense, it becomes unstable and erupts as a nova. These controlled, mini-explosions eject part of the accumulated matter back into space. The crucial question is thus to know whether the white dwarf can manage to gain weight despite the outburst, that is, if some of the matter taken from the companion stays on the white dwarf, so that it will eventually become heavy enough to explode as a supernova.

Read the team’s paper.

Source: ESO

Chandrasekhar Limit

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When a human puts on too much weight, there is an increased risk of heart attack; when a white dwarf star puts on too much weight (i.e. adds mass), there is the mother of all fatal heart attacks, a supernova explosion. The greatest mass a white dwarf star can have before it goes supernova is called the Chandrasekhar limit, after astrophysicist Subrahmanyan Chandrasekhar, who worked it out in the 1930s. Its value is approx 1.4 sols, or 1.4 times the mass of our Sun (the exact value depends somewhat on the white dwarf’s composition how fast it’s spinning, etc).

White dwarfs are the end of the road for most stars; once they have used up all their available hydrogen ‘fuel’, low mass stars shed their outermost shells to form planetary nebulae, leaving a high density core of carbon, oxygen, and nitrogen (that’s a summary, it’s actually a bit more complicated). The star can’t collapse further because of electron degeneracy pressure, a quantum effect that comes from the fact that electrons are fermions (technically, only two fermions can occupy a given energy state, one spin up and one spin down).

So what happens in the core of a massive star, one whose core weighs in at more than 1.4 sols? As long as the star is still ‘burning’ nuclear fuel – helium, then carbon etc, then neon, then … – the core will not collapse because it is very hot (electron degeneracy pressure won’t hold it up ’cause it’s too massive). But once the core gets to iron, no more burning is possible, and the core will collapse, spectacularly, producing a core collapse supernova.

There is a way a white dwarf can go out with a bang rather than a whimper; by getting a little help from a friend. If the white dwarf has a close binary companion, and if that companion is a giant star, some of the hydrogen in its outer shell may end up on the white dwarf’s surface (there are several ways this can happen). The white dwarf thus adds mass, and every so often the thin hydrogen envelope blows up, and we see a nova. One day, though, the extra mass may put it over the limit, the Chandrasekhar limit … the temperature in its center gets high enough that the carbon ‘ignites’, the ‘flame’ spreads throughout the star, and it becomes a special kind of supernova, a Ia supernova.

For more technical details of the Chandrasekhar limit, Richard Fitzpatrick of the University of Texas at Austin has an online Thermodynamics & Statistical Mechanics course, which includes a page on the Chandrasekhar limit.

Supernovae are very important to astronomy, so you won’t be surprised to learn that there are lots of Universe Today stories on the Chandrasekhar limit! Some examples: White Dwarf Theories Get More Proof, White Dwarf “Close” to Exploding as Supernova, and Colliding White Dwarfs Caused a Powerful Supernova.

Astronomy Cast Episode 90 (The Scientific Method) includes a look at how Chandrasekhar worked out the limit that now bears his name, and Where Do Stars Go When They Die? also covers this topic.

References:
Wikipedia
http://www.bluffton.edu/~bergerd/NSC_111/stars.html