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).