Podcast: Inevitable Supernova
Written by Fraser Cain
Consider the dramatic binary system of RS Ophiuchi. A tiny white dwarf star, about the size of our Earth, is locked in orbit with a red giant star. A stream of material is flowing from the red giant to the white dwarf. Every 20 years or so, the accumulated material erupts as a nova explosion, brightening the star temporarily. But this is just a precursor to the inevitable cataclysm – when the white dwarf collapses under this stolen mass, and then explodes as a supernova. Dr. Jennifer Sokoloski has been studying RS Ophiuchi since it flared up earlier this year; she discusses what they've learned so far, and what's to come.
Listen to the interview: Inevitable Supernova (5.5 MB)
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Fraser Cain: What did you see at RS Ophiuchi?
Dr. Jennifer Sokoloski: Well, we were looking at this binary system that had a nova explosion. Looking in the X-rays, we something that was related to the fact that this binary is actually an extremely unusual system for a nova. In most novae, you have a binary, so two stars, which are gravitationally bound and orbiting each other, and one of those is a white dwarf. Material on the surface of the white dwarf piles up and piles up until it gets so dense, and under such high pressure and under such conditions of heat that it will undergo a thermonuclear explosion. On a normal nova-producing binary, it ejects material into relatively free space. In this one, what happened is that it ejected this material into a very dense nebula. Because it was in an unusual environment. When the material that was ejected from the explosion crashed through this nebula, it got shock heated and produced very strong X-ray emissions. That's what we were looking at. It allowed us to determine some properties of this stuff that was thrown out.
Fraser: So let's see if I understand correctly, you've got the white dwarf star, and it's going around another red giant star. And there's debris left over from the stuff these stars have given off in the past.
Dr. Sokoloski: Yeah, exactly, the red giant has a strong wind normally, unrelated to the nova. It produces a wind, and so before the nova occurred, you can think of this binary as being engulfed in this dense nebula, this dense wind from the red giant. And so when the nova exploded, this stuff has all this material to crash into, and that's what made it light up, and allowed us to see something that you don't normally see in a nova.
Fraser: About how often would this happen? It's dragging this material off and piles it up and then explodes. How often would that happen?
Dr. Sokoloski: That's a good question, because again that highlights why RS Oph is different than most novae. For most novae, it takes about 10,000 years for the material to pile up enough for it to ignite. In RS Oph, it only takes 20 years. It's one of the shortest times between nova explosions on the same star. The reason for that is that the white dwarf is very massive. When you have a white dwarf that's very massive, the gravitational field at the surface is very very strong. So then as the material piles up, the wind from the red giant hits the white dwarf and starts piling and piling. It's in such a strong gravitational field that the field does some of the crushing. So it crushes it down and allows it to ignite with much less material than in a more standard way with a white dwarf.
Fraser: Now let's say that we were in the environment of this system, what would it look like?
Dr. Sokoloski: You have a very large red giant, and lots of wind blowing off this red giant. And the wind is actually glowing. It actually, itself, is glowing radiation. The white dwarf, which is nearby, is tiny. It's the size of the Earth, and the red giant is much much larger – say, 40 times the size of the Sun. The white dwarf probably has a disk around it, because the system has angular momentum as these two objects orbit each other. The material forms a disk around the white dwarf, and so you have the red giant, the small white dwarf with the accretion disk. Before the nova happens, it's sort of happily in that configuration. Then once the nova occurs, things change dramatically. The explosion ejects all this material from the surface of the white dwarf and obliterates the disk. The disk is wiped away. It produces a shockwave that moves outward very quickly. Within a day or two, the shockwave is larger than the binary system, and then moves outward and outward. We observed this, basically within the first three weeks. And so by that time, by day 2 all the way throughout the first 3 weeks, we're looking at emission related to this shockwave that's moving outward is now much larger than the size of the binary.
Fraser: And you're saying that this movement through this material tells you a bit about what's going on. What kinds of information have you been able to glean from this?
Dr. Sokoloski: There are two main things. If you look at the velocity of the shockwave, that tells you something about the amount of material that's really pushing the shock. In particular, when the material begins to slow down. For example, if you had the material on the white dwarf – a massive pile of the fuel – and that ignites and gets ejected, if it's very massive, it would move out at a constant velocity for quite a long time, sort of impervious to the nebula. It would be moving outwards until the nebula begins to have an impact to slow it down. We saw something that was the opposite of that. The shockwave almost immediately started slowing down. So what that tells us is that the amount of material that's pushing the shockwave is small compared to the amount of material that's in the nebula. So, by looking at the dynamics of this shock, we can learn about the amount of material that's on the surface of the white dwarf, and that in turn tells us that the white dwarf is very massive, because, as I told you before, in order to get a nova explosion with very little mass, that tells us that the white dwarf has to be very heavy itself.
Fraser: And does a heavy white dwarf mean anything?
Dr. Sokoloski: Well, this is one of the most interesting implications. White dwarfs can only get so massive. If it gets too close to a special number, which is about 1.4 times the mass of the Sun, it'll explode in a supernova. It just can't hold up any more weight than that. And so what we found is that this white dwarf is, in fact, just at that limit. So by looking at this smaller explosion, this nova, what we find is that this white dwarf is very very close to exploding in a much larger event, a supernova. In fact, that kind of supernova is particularly interesting to a lot of people because that's what people use to study the expansion of the Universe.
Fraser: Right, this is a Type 1A supernova. What the implications of that be in the environment of this poor duo.
Dr. Sokoloski: Well, if that happens, all bets are off. I don't know what would happen actually to the red giant. But from our perspective, from the perspective of the Earth, if you weren't even at an unsafe distance near the binary. From here it would be a very dramatic thing. You would look up in the sky and it would be one of the brightest things in the sky. It wouldn't be quite as bright as the Moon, but it would be brighter than any planet. That's why people use them for cosmology, because these explosions are so bright, you can see them very very far away in the Universe. So one reason why it's interesting that we're seeing it before the star has gone supernova is because people are usually look at systems like this after they go supernova. And so now we have the opportunity to try and study it, and learn about these kinds of systems, before the supernova occurs, and hopefully that will help us understand some of the subtleties of how bright the supernova is, and how they're used in cosmology.
Fraser: And how much time do you think you've got before you lose your research subject?
Dr. Sokoloski: Well, that would keep me busy for the rest of my career, so I wouldn't lose anything. But, I don't know. It's hard to answer your question, because we know that it's on the cusp – it's very close to going supernova – but I can't tell you if it's going to be tomorrow or 1000, or 100,000 years from now unfortunately.
Fraser: Do you think within the 100,000 year range it's likely?
Dr. Sokoloski: So yes, in that sense, in the timescale of the Universe, in a cosmological timescale, it's going to happen very soon. Just from a human perspective, that it's hard to say; whether it's 10,000 or a 100,000 years soon.
Fraser: Well, let's say that it doesn't explode within the next couple of years and change your work's pursuit, what are you going to be looking for next?
Dr. Sokoloski: That reminds me of the other answer to your question where you asked, what do we learn from this. The other thing, while we were watching this blast move outward was that we saw that there are certain expectations for how the brightness would change if you had a perfectly spherical outward motion, with certain other properties that people associate with – that theorists working on these kinds of objects assume. We observed that those properties weren't obeyed, that the brightness decreased much more quickly. And so that tells us that it's possible this is not a nice neat spherical shell. Some radio observations have shown us that you might actually have a ring structure with jets. We know there are jets, we've seen them in the radio, and so now a lot of people are doing work to try to understand in systems like this, in RS Oph itself and other stellar explosions, what produces these structures that are not simple spherical outflows but jets that are a common phenomenon in stellar explosions and also in the Universe. From galaxies people see jets, it seems to be a very common structure. So, for RS Oph, we're trying to understand, is this something intrinsic to a nova explosion, that the explosion itself is asymmetric, and not on the same strength all over the surface of the star. Is everywhere the same or is it stronger or weaker at the poles, for example, or at the equator. Or is it possible that there's something in the environment? Because this is a binary star, it's a system with a preferred axis and plane of rotation that the ejecta interacts with. Material that might be in a disk around the binary, and that's what produces the structure we see. So I guess the next step for RS Oph is: why is it asymmetric, why do you get jets?
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