Artwork depicting a binary brown dwarf system. Credit: Janella Williams, Penn State University
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Artwork depicting a binary brown dwarf system. Credit: Janella Williams, Penn State University

A rare find: An eclipsing binary brown dwarf in a triple brown dwarf system

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Mar 10, 2020, 9:00 AM EDT (Updated)

One of the more difficult tasks in astronomy is getting accurate physical measurements of stars. The most fundamental properties of a star are its mass, radius, and age — these tell you a huge amount about their behavior and how they work. We can determine some of these by using other properties; for example, the temperature of a star is generally directly measurable by taking a spectrum of it, and for stars like the Sun the temperature depends on the mass (more massive stars are hotter). So it's possible to figure some of these out.

But we rely on models a lot to do this. We understand the physics of how stars work, and can use that to figure out the age by looking at different properties. The problem is we can't always know we have the physics right! So the best way to get these three basic properties is to observe them directly.

But that's tough! Getting the mass of a single star sitting out in space by itself is a difficult thing to do, if not impossible. Same for its radius.

Still, sometimes nature is generous. If two stars are orbiting each other then their orbit depends on their gravity in a very simple way, and we can determine their combined masses that way. And if we see that orbit edge-on, then every half an orbit one star passes directly in front of the other from our viewpoint, generating eclipses, and from that the radii of the stars can be found.

All of this is even harder for brown dwarfs, objects that are more massive than planets but less than stars. They don't have enough mass to ignite nuclear fusion in their cores, making them different than actual stars like the Sun. These gray-area objects have weird properties that make them more difficult to study; for example they're much fainter than stars, so getting good data on them is hard. Even when we find a pair orbiting each other it can be difficult to figure out much about them.

And this is why the newly discovered system 2M1510 is so important: Not only does it have a pair of brown dwarfs orbiting each other, but there's also an eclipse every orbit! Once astronomers knew what they had they pounced on it.

2M1510 (arrowed) is a triple-brown-dwarf system, faint even in this infrared survey image. Credit: ALADIN / 2MASS

2M1510 was first found in an infrared survey of the sky called 2MASS (the 2-Micron All Sky Survey), where it was seen to be two separate faint objects of slightly unequal brightness. The European Gaia space-based survey found 2M1510 to be about 120 light years from Earth, which meant the two components are separated by 37 billion km; a fair distance but still close enough together to be a binary.

It was first thought they were very low mass stars, like the Sun but a lot dimmer and cooler. But their motion through space indicated they were part of a loose cluster of stars called the Argus Association. This was a critical discovery, because these stars in the association are very young, about 45 million years old. When a star is young it's very hot, and that makes it look like a more massive star than it really is. When astronomers accounted for the ages of the stars, they realized that 2M1510 must be made up of two brown dwarfs, not actual stars.

But this gets better. A team of astronomers used an observatory called SPECULOOS (a terrible acronym for Search for habitable Planets EClipsing ULtra-cOOl Stars), a collection of four 1-meter telescopes in Chile, to observe the system. What they found is that the slightly brighter component of the system, 2M1510A, was itself a binary consisting of two brown dwarfs. They were able to confirm this using the Keck telescope in Hawaii, too, where spectra showed the motion of the two stars around each other.

This made 2M1510 a triple brown dwarf system, with two orbiting each other in a binary, and the third, 2M1510B, orbiting farther out. This is called a hierarchical triple system. Coooool.

Left: During the eclipse, the primary brown dwarf blocks about 4% of the secondary brown dwarf’s light. Right: As the two revolve around each other, they appear to move toward us and away from us at a maximum of about 18 km/sec, revealing their masses and the shape of their orbit. Credit: Triaud et al.

And this gets better yet. The two brown dwarfs in the binary are in a very tight orbit, taking just 21 days to go around each other. The orbit is mildly elliptical, and is about 19 million kilometers along its long axis, so the two are very close together (the Earth orbits the Sun 150 million km out from the Sun, by comparison). By taking spectra and observing them carefully, the masses of the two objects can be found, and they are indeed wee: One is 0.038 times the mass of the Sun (or, if you prefer, 40 times the mass of Jupiter) and the other is 0.0375 times the Sun's mass (39.3 Jupiters). This makes them very close in mass.

Once per orbit the slightly more massive one (called the primary, in this case specifically 2M1510Aa) passes between us and the slightly less massive one (called the secondary, 2M1510Ab). It's a grazing eclipse — like a partial solar eclipse — where it blocks 4% of the star for 90 minutes. That's not ideal, but it's enough to get the combined radii of the two brown dwarfs: 0.315 times the Sun's radius, or 3.15 times Jupiter's. Since they are almost exactly the same mass it's fair to assume they have the same radius, which makes them each about 1.57 times the size of Jupiter.

That sounds about right. Brown dwarfs are weird; once you get an object about the mass of Jupiter, adding mass to it increases its density, so it actually gets smaller, not bigger. In this case these objects are still very hot from their formation, and that leftover heat puffs them up, so they can be much more massive than Jupiter but only a little bit bigger. Weirdly, this makes them extremely dense, almost twice as dense as iron!

Like I said, brown dwarfs are weird.

Artwork depicting a binary brown dwarf system. Credit: Janella Williams, Penn State University

But the beauty of all this is that it will really help us understand these odd objects. Only one other eclipsing binary brown system is known, but it's much younger (roughly a million years) so it's still affected by the vagaries of youth (strong magnetic fields, material still falling onto the objects, and so on). 2M1510 is older and better established, less likely to have outside effects screwing things up. It will become a new benchmark for brown dwarfs, allowing astronomers to test their models of how they form and evolve — and in fact the paper goes into that a bit, showing how the models do well for some things, but, for example, overestimate the brightness of brown dwarfs by about 50%. That in turn can lead to the mass estimates being off — and that's critical when a brown dwarf is near the mass limit to becoming a star. If you overestimate the mass you might think it should be a star (massive enough to fuse hydrogen into helium in its core) when it isn't.

Bear in mind the first brown dwarf was only found in the 1990s! So we've been studying them for a while and have learned a lot, but in some ways we're still in the early days of this. It's not often astronomers find a new object that becomes the standard for a field, but that's what we have here… at least until an even better system is found, one perhaps where both stars eclipse each other every half orbit. Until then, though, 2M1510 is the system to study.

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