HomeScienceCosmic indigestion: Swallowing a neutron star can cause star to explode

Cosmic indigestion: Swallowing a neutron star can cause star to explode

Diagram of a shell of material with a bright point in the middle.
Enlarge / A model of the supernova exploding inside a torus of gas ejected years earlier.

One of the stranger features of our Universe is the existence of what you might call dual-core stars. Many stars exist as part of a multistar system, and in some cases their orbits are extremely close. Couple that with the fact that stars can expand as they age, and you enter a situation where the outer edges of one star may engulf a second. Friction can then draw their orbits closer, resulting in the core of both stars orbiting within a large shared envelope of plasma.

Things can get more complicated still when you consider that the stars won’t necessarily have life cycles that line up well—one of them could easily explode before the other, leaving behind a black hole or a neutron star. That can lead to some bizarre situations, like a star that replaces its core with a neutron star.

Now, researchers are saying they’ve probably found a more violent alternative to that merger. In this case, the neutron star didn’t settle neatly into the core of its companion star. Instead, the companion star lost its outer layers to space, then saw its core disrupted in a way that caused it to explode.

Something new

The object in question, technically named VT J121001+495647, was identified by a team that compared two different surveys of the sky done at radio frequencies. Basically, they looked for objects that were present when a survey was done during 2017-2018 and weren’t present back when the same area of sky was covered in a 1994-2005 survey. The brightest of these objects was what’s being called VT J121001+495647.

A check of a hydrogen emission line from the object indicated it was very broad instead of forming a sharp peak. This tells us that VT J121001+495647 has multiple components. Some of them are moving toward us, and so the light they produce is blue-shifted, broadening the peak in that direction. And another part is moving away from us and thus broadening the peak in the red direction. The simplest explanation for this is that VT J121001+495647 is the debris of an exploded star, expanding out in all directions.

More accurately, what we’re seeing is those debris slamming into material that had been ejected from the star earlier, prior to its explosion. But the researchers calculate that the total amount of mass that was ejected earlier has to be at least the same as the mass of the Sun, which is a large amount of material—too large to be accounted for by a solar-wind-like process. So it must have nt just been ejected, but ejected relatively recently for the supernova debris to be slamming into it already.

Confusing matters further, the team searched the archives for unusual events at the same location and came up with one: an X-ray burst that was picked up by an instrument on the International Space Station. The event was unusual in comparison to other X-ray bursts in that it was very luminous but brief, and it didn’t extend to higher-energy wavelengths. The burst dates to three years prior to the imaging of VT J121001+495647, so it could have been from the supernova itself. But the duration and energy of the burst didn’t match what we typically see from forming neutron stars or black holes.

Making sense of it all

Putting this all together, we have evidence that the star erupted a lot of material a few years before it exploded and that the timing of this eruption may be linked to an X-ray burst. There are a number of ways to explain this, but most don’t seem to match up with the data especially well. For example, instabilities in the fusion reactions at the core of the star can cause an eruption, but they’re only large enough to spit out a Sun’s worth of material in the last few years before the explosion. And if that were the case, the material would still be closer to where the star was than it actually is.

Interactions with a companion star could potentially produce a shell of material of the right density and distance. But the chances of them occurring so close to the star’s explosion are extremely small.

So, the researchers propose an alternative: the ejection of material did come via interactions with a companion object. And the interactions also are what triggered the supernova.

How does that work? The theoretical ideas behind it turn out to have been set forth in a series of papers in the middle of the last decade. If the companion star has already exploded, it would then be in the form of a neutron star or black hole. If that ended up inside the envelope of the normal star, it would form an accretion disk and jets. These would blast out a lot of the material in the outer layers of the star, forming the more distant material that’s creating the radio signal.

The timing of events. In the upper panels, a black hole or neutron star strips off the outer layers of a star, creating a disk of material around it. In the lower panels, it disrupts the star's core, setting off a supernova that sends debris into the gas ejected earlier.
Enlarge / The timing of events. In the upper panels, a black hole or neutron star strips off the outer layers of a star, creating a disk of material around it. In the lower panels, it disrupts the star’s core, setting off a supernova that sends debris into the gas ejected earlier.

If the compact companion object ended up close enough to the star’s core, however, it could disrupt the core via tidal interactions. And, from the star’s perspective, that’s a problem, because the orderly fusion of elements in the core provides the energy needed to keep gravity from collapsing the star. Mess with the core, and the whole thing can come crashing down, producing a supernova. During this process, the infalling companion could have briefly formed an accretion disk and jets, which would then account for the X-ray burst found in the archives.

The star would have had to have been massive enough to explode at some point anyway. But the disruption hastened the process along.

As with most cases like this, there are a couple of other oddball objects we’ve seen in the past that could also have been created by a similar mechanism. Unfortunately, they don’t have the same wealth of data that we have for VT J121001+495647, so the timing of events can’t be nailed down so precisely. But with a good model of what to look for, don’t be surprised if we’re now able to identify others.

Science, 2021. DOI: 10.1126/science.abg6037  (About DOIs).

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