# How do isolated binary compact systems survive their formational supernovae?

In a broad sense I'm trying to understand how a binary black hole or neutron star systems can form from isolated binary massive star systems (i.e., other than from migration within rich BH/NS clusters), and especially, how these can form within the constraints that allow for observed merger rates. I was hoping that this question would answer me, but it doesn't, because I realised I've never seen an explanation of how a binary system could survive a SN of one of its stars in the first place.

As far as I understand, we think one route to observed BH/NS mergers is a binary system of massive stars. This gives a scenario of 2 massive stars in an isolated binary system, one of which undergoes core collapse and type 2 SN. The two stars must be quite close (or later perturbed) and not greatly "kicked" on formation of either compact object, otherwise they couldn't later merge in the observed time. They probably also have similar ages (formed in the same cloud).

But in this scenario

• How does the second-to-collapse star, survive being ripped apart by a nearby SN at such short distance, and remain in a small enough orbit, which we know happens since we later observe the merger of two compact objects? How does it survive structurally and regain stability in a form capable of future SN, given the intense flood of neutrinos and other energy/particles, some of which can penetrate to its core? Or does the first SN directly trigger the second SN?

• Even assuming the second star survives with enough mass and undisrupted structure for SN (or is triggered directly into SN), wouldn't the transferred momentum from the first SN ejecta "on its doorstep" be sufficient to simultaneously unbind the binary system, or the momentum transferred from the second SN be enough to eject the first NS/BH, so that the two resulting compact objects no longer form a binary system?

• What makes you think that a supernovae deploys enough energy to propel a star into outer space or blows it away completely? It is a lot of energy, but not that lot. – J. Chomel Jun 25 '18 at 8:19
• The first SN not only deploys a large amount of energy, which is capable of transferring momentum (although only about <10% of a solid angle is transferred to the other star depending on the system), but also, it loses a significant fraction of its mass, which means post-SN, the two stars are moving too fast to maintain their orbit and will tend to separate anyway, and the energy needed to unbind will also be considerably lower. I'm not sufficiently sure of any calculations I do, to try and work out whether these unbind most or just some, binary massive systems, or the impact on the 2nd star. – Stilez Jun 25 '18 at 8:31
• @J.Chomel Supernova explosions can be quite asymmetrical, and some neutron stars get a significant kick from this one well-known example being the pulsar B1508+55. – PM 2Ring Jun 25 '18 at 10:18
• @J.Chomel As PM 2Ring said, pulsar kicks have been observed in several neutron stars. The same phenomenon occurs in some black holes. – HDE 226868 Jun 25 '18 at 13:46
• Also the SN doesn't have to blow the star away or unbind it. If the energies (which will be asymmetrically applied) are enough to destabilise the star generally, or its core, or perhaps even its envelope or fusion processes (SN output includes a proportion which will penetrate and may interact within the core of its companion), that might be enough and have a lower threshold. – Stilez Jun 25 '18 at 21:00

OK, here's what I've been able to piece together with a little research (not really my field, so I may get a few things wrong).

The short answer is a combination of: A) It's pretty hard to actually disrupt a massive star via a neighboring supernova, and the two stars aren't intially that close to each other; and B) Lots of complicated effects due to the binary interaction.

I'll focus on what seems to be the best current model for forming a compact double neutron-star (DNS) system -- one with a small enough separation in the resulting neutron stars so that gravitational-wave emission can cause them to merge within a few billion years -- which is discussed in this 2017 article by Tauris et al.. It looks a bit like this:

1. Massive binary star system forms, with moderate separation (not as close as the final DNS binary will be).

2. Primary star (initially more massive) evolves faster, to the point where it expands and fill its Roche lobe, leading to mass transfer to the secondary.

3. This means the primary loses mass while the secondary gains mass. So when the primary reaches the supernova stage, it has less mass in its envelope to eject -- and the companion star may now be the more massive of the two.

4. Primary's core collapses, producing first SN. This probably won't destroy the companion; it might disrupt the system -- not so much because the companion is kicked a lot but because the NS remnant is lower mass than the pre-SN primary star, and it probably gets a kick from explosion asymmetries. But recall that the companion star may now be more massive, so the gravitational disruption is less.

5. Companion star evolves, expanding to overfill its Roche lobe, leading to a "common envelope" phase where the NS is actually inside the companion star's outer envelope.

6. Common envelope phase leads to

A. Significant shrinking of the orbit as the NS experiences dynamical friction;

B. Ejection of much of the companion star's outer envelope.

7. System is now much more compact, with a NS and a stripped helium-star companion.

8. Possible further mass transfer from companion onto neutron star, leading to even less of an envelope around the companion and an even lower mass.

9. Companion's core collapses, causing second SN. This has little effect on the first NS (i.e., the former primary star), since the NS is a very small target with extreme density and surface gravity: very hard to disrupt. In addition, the companion has very little envelope left to eject at this point.

Some key points:

• The binary starts off with a significantly wider separation than the final DNS will have, so the effects of the first SN on the companion will be less extreme than you might imagine;

• Mass transfer (and ejection) during the Roche overflow and common envelope stages removes mass from the stars, which means there's less envelope to eject when a SN kicks off (so it does less damage to the other star/NS) and also that the SN ejection of a star's envelope doesn't have a strong effect on the gravitational binding energy, since there's less of an envelope to eject.

This paper by Liu et al. (2015) presents some modern simulations of the effect of a SN blast wave on a nearby companion star. It turns out to be pretty hard to disrupt the companion: even for relatively small separations, the companion only loses about 10% of its mass, and this goes down to a few percent or less as the separation increases. (Remember that the companion is itself a massive star most likely still on the main sequence, and so relatively compact, with high surface gravity.)

Can the explosion push the companion star away? Probably not very much: again, the companion star is itself quite massive -- maybe more massive than the original primary because of the mass transfer in stage 2 (above) -- so it's hard to push it around.

• Sounds plausible. I guess if the 1st star to collapse gets a small kick, it will drag the companion with it, unless the kick speed is greater than the escape velocity. – PM 2Ring Jun 27 '18 at 2:47
• Reading the two cited papers which are relevant and interesting. I guess part of the question is that there should be an energy/effect that's far less than that needed for disruption/binary unbinding, which alters the orbit enough to prevent merger in the time seen, or to disrupt the partner's fusion processes (does the flood of neutrinos have an effect when unlike EM, it reaches the partner's core?). So its not as simple as "not enough energy to unbind the binary or destroy the other star" - a lot lesser effect would suffice to prevent a double compact system later forming in the time seen. – Stilez Jun 28 '18 at 17:43
• @Stilez -- Very roughly, the energy contribution from neutrinos inside the SN occurs at radii out to $\sim 100$ km. But the companion star is at a distance of, say, $\sim 0.1$ AU. This means the neutrino flux will be diluted (by the inverse-square law) by a factor of $\sim 10^{10}$ by the time it reaches the companion star. So the neutrinos won't have nearly the effect on the companion that they do inside their parent supernova. – Peter Erwin Jun 28 '18 at 18:51