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A Rare Brown Dwarf Eclipsing Binary

The SPECULOOS-South Observatory at Cerro Paranal, Chile houses four 1-meter robotic telescopes, now deployed in the service of identifying rocky planets transiting low-mass stars and brown dwarfs. Acronym untanglement: SPECULOOS stands for Search for habitable Planets EClipsing ULtra-cOOl Stars. An early success here is the just reported discovery of a triple brown dwarf system, with an inner binary that is eclipsing and a widely separated brown dwarf companion. The inner binary is what is known as a double-lined system, meaning that spectral lines from both eclipsing stars are visible in the data.

Data from the W. M. Keck Observatoy (Maunakea) and the 8-meter Very Large Telescope (VLT), each equipped with sensitive spectrometers, were used to confirm the discovery. Yesterday we saw how the analysis of a young exoplanet, DS Tuc Ab, could offer insights into how ‘hot Neptunes’ form. In a similar way, the brown dwarf triple system 2M1510A fills a needed gap in our data. A member of a 45 million year old moving group, the age of the system matches up with some currently imaged exoplanets, so we have another look at evolutionary models, this time of brown dwarfs, that may also be useful in the study of young planets.

This is also a rare find, as there is only one double-line eclipsing brown dwarf binary known — I’ll mercifully shorten its name to 2M0535 — but it’s so young (part of the 1 million year old Orion Nebular Cluster) that it’s difficult to work into current evolutionary models. The new triple system 2M1510, in which we can identify the mass and radius of the inner binary stars, will be a useful tool. As the authors of the paper on this work note: “The system’s age matches those of currently known directly imaged exoplanets so 2M1510A provides an opportunity to benchmark evolutionary models of brown dwarfs and young planets.”

Amaury Triaud (University of Birmingham, UK) is lead author of the study, which has just appeared in Nature Astronomy:

“Collecting a combination of mass, radius, and age is really rare for a star, let alone for a brown dwarf. Usually one or more of these measurements is missing. By drawing all these elements together, we were able to verify theoretical models for how brown dwarfs cool, models which are over 30 years old. We found the models match remarkably well with the observations, a testament to human ingenuity.”

Image: This illustration shows the average brown dwarf is much smaller than our sun and low mass stars and only slightly larger than the planet Jupiter. Credit: NASA Goddard Space Flight Center.

I was startled to find that six other brown dwarf triple systems have been identified, but 2M1510 is the only one with three components that are of approximately the same mass, Moreover, the separation ratio between the inner and outer orbits is the smallest of all the young triple brown dwarf systems yet found. As so-called ‘failed stars,’ brown dwarfs are in the murky region between star and giant planet, unable to sustain hydrogen fusion, but as far as we know forming like stars. We can consider them a key link between star and planet formation.

Brown dwarfs cool as they age, which reduces their radius as well as their luminosity. Young exoplanets do the same, making the two evolutionary models interesting to compare. That makes systems where we can get accurate mass, radius and age determinations crucial, which is why this is a helpful discovery. The authors explain what 2M1510 has revealed:

We find that widely used evolutionary models do reproduce the mass, radius and age of the binary components remarkably well, but overestimate their luminosity by up to 0.65 magnitudes, which could result in underestimations of 20% to 35% of photometric masses for directly imaged exoplanets and young-field brown dwarfs.

I suppose nothing elevated interest in planets around low-mass stars more than the seven small worlds around the TRAPPIST-1 system, in which the primary is an ultra-cool red dwarf star. Close-in rocky worlds around brown dwarfs are likewise a possibility, given that we’ve already identified planets at the brown dwarfs 2M1207, 2MASS J044144, and MOA-2007-BLG-192L, with the latter being roughly 3.3 Earth masses (this one was detected by gravitational microlensing, and is the smallest exoplanet I know of around a brown dwarf). But as we’re seeing, the usefulness of brown dwarfs in planet formation models is another reason that interest in these dim objects is intensifying.

The paper is Triaud et al., “An eclipsing substellar binary in a young triple system discovered by SPECULOOS,” Nature Astronomy 9 March 2020 (abstract).

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{ 7 comments… add one }
  • Geoffrey Hillend March 11, 2020, 16:37

    Those brown dwarfs are invisible to the visible spectrum; their spectra are black body thermal infrared where the luminosity determines the mass and also their spectral energy distribution determines their mass. Brown dwarf, Wikipedia.

  • George King March 11, 2020, 23:05

    So, with these cool brown dwarfs never being able to ignite, they never would produce (a) a stellar wind to blow away rotating gas and other disc material from a protoplanetary disc, and (b) at least stellar-level heat. It thus would seem that (1) any planets forming from the same disc as the brown dwarf would tend to have a relatively longer formation/accretion time (than if they had formed around a protostar that did ignite and enter the main sequence), (2) any frost line would be much, much further inward such that purely terrestrial worlds would be less common as opposed to maybe instead planets consisting of cores of metals, silicates and ices wrapped in much larger amounts of hydrogen and helium, all leading to (3) perhaps planets (in multi-planet systems) with far more homogeneity in composition, weighted heavily toward hydrogen and helium — and consistency in composition with the primordial disc — than in our solar system.

    Certainly nothing novel there for those who study these systems, but I never had given much thought to what sort of planets would tend to form around brown dwarfs (and excluding possibly captured rogue planets).

    Interesting little backwater systems initially formed by the same initial conditions as a main sequence stellar system — such as by a supernova shockwave causing a gas cloud to collapse in on itself — but with a different outcome — including likely also as to the types of planets formed — due to the insufficient mass in the wisp of a gas cloud that collapsed to form the system.

    Also mildly interesting to me (when I was reviewing related points on Wikipedia) that the German philosopher Immanuel Kant was involved in the formation of the nebular hypothesis of solar system formation. Studied Kant when I initially was a philosophy major, and I never knew that his intellect took him also in this direction. “Small world” kind of thing, across those oceans of time.

    • Antonio March 12, 2020, 16:59

      Brown dwarfs do ignite, but a different fusion reaction than normal stars, that fades away much earlier.

  • Michael Fidler March 12, 2020, 22:30

    What about Dyson spheres around brown dwarfs?

    In 2012 they could not find the number of brown dwarfs expected.

    Brown Dwarfs, Runts of Stellar Litter, Rarer than Thought.
    By Ian O’Neill June 12, 2012
    https://www.space.com/16112-brown-dwarf-stars-sun-rare.html

    In 2017 they found brown dwarfs forming by the dozen in star-forming region RCW 38.

    100 Billion Brown Dwarfs in the Milky Way?
    BY: JOHN BOCHANSKI JULY 18, 2017
    https://skyandtelescope.org/astronomy-news/100-billion-brown-dwarfs-milky-way/

    So where did they all go???

  • wdk March 13, 2020, 21:52

    Missed the period and separation for these bodies so far, but this system does stir the imagination – or conjectures.

    Speaking in terms of their satellites or planets ( whichever is appropriate), there would be some kinship with a system like Jupiter and its Galilean moons. Though brown dwarfs would experience a period of deuterium nuclear fusion vs. the hydrogen fusion, this episode is a lot shorter – and less energy is emitted overall. But these objects are also very young – 45 million years. Let’s say that the deuterium fusion period lasts less than a million years. Though that’s nothing compared to solar system age, that still would be a lot of heat to remove from the core based on the age cited. These objects might have some visible spillover from their infra red peaks.

    Another thing: despite not having significant solar winds or internal heat sources, like Jupiter, these bodies could very well have significant magnetospheres. And, correct me if I am wrong. If these objects are tightly packed and their poles are not aligned with their rotational axes, there might be a whole lot of charged particles associated with these systems and the magnetospheres might get tangled periodically. To lower the activity, I guess a planet would want somehow to rotate synchronously with its primary.

    Also, with a multi-body system such as described, there could be issues of long term stability if there are forms of energy dissipation. I would
    not be surprised if some of these brown dwarf systems get opportunities to merge into bigger bodies. Until they do get up to about 0.08 solar mass, they are not going to attract much attention.

  • Mike Serfas March 15, 2020, 21:48

    This makes me curious about polar n-gons on brown dwarfs. Looking at https://arxiv.org/pdf/0704.3106.pdf it appears that in one case the rotation is known to be 1/3 of a very precisely determined period of revolution … does that mean that you could pick out the periodic spectrographic signal of an n-gon from the noise as the star rotates? Or in stars so close together, could precession be fast enough to observe on a human timescale? Why do Jupiter and Saturn have hexagons (or a pentagon, weather permitting) but the Sun doesn’t anyway? How would an n-gon affect the magnetic phenomena of a star if it could exist?

  • stephen March 29, 2020, 22:21

    There was a speculation that in brown dwarfs, fusion would turn on and off repeatedly as it expands, cools, contracts and heats up again. somebody did the math and found that wouldn’t happen, but I wonder if everybody agrees with that latter finding?

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