Complex Brown Dwarf Systems ‘Baffle’ Astronomers

A couple of brown dwarfs have been discovered in a close binary system some 240 light years away, whose two stars circle each other at a distance of about 19AU, similar to that of Uranus around the Sun.   The two new exoplanets orbit close to the primary Sun-like star HD 87646 (1).  These two sub-stellar companions are HD 87646b, which is a minimum 12MJupiter sub-brown dwarf (a ‘hot Jupiter’-type exoplanet) orbiting every 13 days just 0.117AU from the star (2); and  HD 87646c, which is a 57MJupiter brown dwarf circling the star every 673 days (1).  The orbital eccentricity of the brown dwarf is greater than that of the inner sub-brown dwarf, which is in keeping with other observations of brown dwarfs orbiting stars.

brown_dwarfs

Image Credit: Janella Williams, Penn State University

The international team that discovered this remarkable system is perplexed as to how it might have come about:

“Given the fact that HD 87646 is the first known system to have two massive substellar objects orbiting a star in a close binary and the masses of the two objects are close to the minimum masses for burning deuterium and hydrogen, these peculiarities raise questions about the system’s formation and evolution.

“”The large masses of these two substellar objects suggest that they could be formed as stars with their binary hosts: a large molecular cloud collapsed and fragmented into four pieces; the larger two successfully became stars and formed the HD 87646 binary, and the other smaller ones failed to form stars and became the substellar objects in this system. This scenario might be relevant for the binary stars but seems problematic for the two substellar objects on orbits within one AU because it is unclear whether fragmentation on such a small scale can occur,” the paper reads (1)

“Other hypothesis offered by the scientists is that the two newly discovered giant objects were formed like giant planet in a protoplanetary disk around HD 87646A. However, they added that such massive disks are rare in close binaries, and further investigation is needed to confirm this explanation.” (3)

Given the large number of substantial exoplanets and brown dwarfs discovered over the last couple of decades, finding a couple more in a close binary system may not seem so surprising.  We’ve got used to the fact that the classical planet-forming model, which described how the solar system came about so well, simply does not apply to the diversity of planetary systems emerging from the ballooning exoplanet data (now >500).

binary_planets

Image credit: NASA

According to the core accretion model, the nature of the close binary system should create problems for giant planet formation, because gravitational perturbations inhibit the growth of planetessimals in the protoplanetary disk, meaning that the formation of a planetary core is slowed down.  The dust disk will essentially dissipate before the planetary core can amass gaseous material around it to form a gas giant of brown dwarf.  Furthermore, given the sheer size of these brown dwarf companions, the initial disk would have to be massive to have formed them.  This isn’t generally seen in young close binary systems, meaning that the disk should have dissipated way faster than the time required for giant planet formation (1).  This is a similar issue to that recently raised for rocky planet formation (4).

Another theory of planet formation relies on disk instability, which doesn’t suffer the same problems in this case as the core accretion model.  The authors are, however, cautious of advocating this mechanism for the complex HD 87646 case as it’s not clear whether disk instability can be triggered in the presence of a close stellar companion (1).  The planet-forming mechanism in this case remains a mystery, although the authors do offer ideas about the migration and scattering of these companion objects post-formation.

An alternative which would not have been considered by mainstream astrophysicists is the radical model of planet formation proposed by the French engineer François Berguerand, whom advocates the action of a passing star to draw out a spindle of stellar matter from a young star.  This spindle of dense matter then separates out into gaseous planets (5).  This close binary system would seem to provide the perfect opportunity for the drawing out of such matter to create the substantial brown dwarf companions for the primary star.  I’ve written to François for comment.

 

Brown Dwarf Discovered Orbiting Star HR 2562 

Perhaps also of some relevance to the case of HD 87646 is the recent announcement of a 30MJupiter brown dwarf discovered lurking within the inner hole of a dust disk encircling the star HR 2562 (6).  At 1.3 M

Sun, the masses of these primary stars are essentially the same.   Although the presence of the circumstellar disk would seem to imply a youthful star, the age of HR 2562 remains a subject of some debate.  It’s thought to be about 600 million years old, based upon evolutionary models.

dust_disk_planet

Image Credit: Karen L. Teramura, UH IfA

The disk spans from 38 to 75 AU away from the host star, with the brown dwarf, designated HR 2562B, located some 20AU from the star (7).  This is a similar distance as the secondary star in the HD 87646 binary.

Most star systems have lost their dust disks by 300-400 million years (8).  Rare circumstellar dust disks around older stars are then thought to arise as a result of collisional cascades between rocky objects, like asteroids and comets, in planetary systems (9) in what are otherwise dynamically calm environments.  As a result of this hypothesis, older star systems which exhibit dust disks are considered good candidates for the presence of terrestrial planets (10).  The solar system should perhaps also exhibit this kind of dust disk, given its perceived general stability.  the fact that it doesn’t is thought to be due to the decimation of the Kuiper Belt which took place around 3.9 billion years ago:

“Intriguingly, our solar system looks unusual… because it has rocky planets as well giant planets that are relatively calm in terms of their orbital dynamics, but no debris disk.

““So, why don’t we have this dust?” [Sean] Raymond [of the Observatory of Bordeaux, France] said. “Well, it turns out that for more than half a billion years after its formation, the solar system did have a bright debris disk!  As is the case for other stars, the dust was produced by icy planetesimals occasionally colliding and grinding themselves away.  This population of planetesimals was the primordial Kuiper belt, and we think it contained about 50 Earth masses of material, at least 100 times more mass than it does today.”

“However, the Kuiper belt was almost completely cleared out during a period instability about 3.8 billion years ago that is linked with the “Late Heavy Bombardment,” when asteroids were slung around the solar system, leaving planets with scars often still visible today.” (11)

I consider this event to be the result of the infiltration into the planetary zone of a rogue sub-brown dwarf object which, I believe, still remains undiscovered in the outer reaches of the solar system.  Its presence has indeed eliminated much of the Kuiper Belt, and created the ‘Kuiper Cliff’ at about 50AU.

By contrast, what we have in the case of HR 2562 is an inner brown dwarf, surrounded by an outer circumstellar disk.  This would then imply that the bulk of the system’s rocky material – asteroids, terrestrial planets, planetessimals – lies beyond the the reach of a substantial inner sub-stellar companion.

Or is it actually an example of the way a sub-stellar companion can ‘sweep out’ a region of space?  In the case of our solar system, this is what I believe has occurred between the Kuiper Belt and the inner Oort Cloud (i.e. between 50 and 2000AU).

 

Written by Andy Lloyd,

29th August 2016

References:

1)  Bo Ma et al “Very Low-Mass Stellar and Substellar Companions to Solar-like Stars From MARVELS VI: A Giant Planet and a Brown Dwarf Candidate in a Close Binary System HD 87646” 11th Aug 2016, http://arxiv.org/abs/1608.03597

2)  New World Atlas “HD 87646 b” https://exoplanets.nasa.gov/newworldsatlas/3376/

3)  Tomasz Nowakowski “Giant planet and brown dwarf discovered in a close binary system HD 87646” 16th August 2016 http://phys.org/news/2016-08-giant-planet-brown-dwarf-binary.html with thanks to Lee

4)  Scott J. Kenyon et al “Rocky Planet Formation: Quick and Neat” 18th August 2016, http://arxiv.org/abs/1608.05410

5)  Andy Lloyd “Our Solar System: An Alternative Birth” 25th September 2015 http://www.andylloyd.org/darkstarblog30.htm

6)  Tomasz Nowakowski “Astronomers find a brown dwarf companion to a nearby debris disk host star” 26th August 2016 http://phys.org/news/2016-08-astronomers-brown-dwarf-companion-nearby.html with thanks to Mark

7)  Quinn M. Konopacky et al “Discovery of a Substellar Companion to the Nearby Debris Disk Host HR 2562” 23rd August 2016, arxiv.org/abs/1608.06660

8)  H. J. Habing et al “Disappearance of stellar debris disks around main-sequence stars after 400 million years” Nature 401, 456-458, 30 September 1999, http://www.nature.com/nature/journal/v401/n6752/full/401456a0.html

9)  Rahul Patel “Debris Disks: Searching for Dust to Find Planets” 4th March 2015 https://sciencesprings.wordpress.com/2015/03/07/from-gpi-debris-disks-searching-for-dust-to-find-planets/

10)   S. N. Raymond et al “Debris disks as signposts of terrestrial planet formation” June 2011, Astronomy & Astrophysics, Volume 530, id.A62, 23 pp, http://adsabs.harvard.edu/abs/2011arXiv1104.0007R

11)  Astrobiology Magazine “Stars with Dusty Disks Should Harbor Earth-like Worlds” 8th March 2012 http://www.astrobio.net/news-exclusive/stars-with-dusty-disks-should-harbor-earth-like-worlds/

 

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