Although we’re beginning to realize that brown dwarfs are widespread in the galaxy, we know surprisingly little about how they form. The question has an obvious impact on planetary formation models as well, but we won’t get a good read on the answer until we’ve been able to study brown dwarfs and other very low-mass stars (VLMS) in multiple systems. Right now, relying largely on the Hubble Space Telescope and direct imaging via adaptive optics, we’re unable to detect close binaries in such systems. That leaves radial velocity techniques to do the job.
And indeed, a brown dwarf binary designated PPl 15 was found in the Pleiades in the late 90’s with these methods. But the hope of landing a large number of close brown dwarf companions has faded. So far, despite ongoing work, we still have only three brown dwarf binaries confirmed through spectroscopy. And we’re still asking planet-sized questions: Can a brown dwarf support planets at just a few AU distance?
The assumption is yes, given our growing catalog of planets around low-mass M-dwarfs, but the only planet detection thus far is 2M1207b, which was discovered in so wide an orbit (55 AU) that researchers say it presumably formed quite differently from the Solar System and planets detected by radial velocity methods in other systems. In fact, the possibility that 2M1207b is actually a brown dwarf itself, forming star-like from the collapse of a gaseous nebula, cannot be discounted.
But now we have evidence for a low-mass companion orbiting the brown dwarf candidate Cha Hα 8, a member of a nearby star-forming region called Chamaeleon I. The companion object, say V. Joergens and A. Müller (Max-Planck-Institut für Astronomie, Königstuhl) in the discovery paper, is definitely low mass, orbiting in the range of 1 AU. Unfortunately, the mass of the primary is not yet well determined, leading to a range in the possible mass of the companion from 15.6 to 19.5 Jupiter masses.
Or perhaps less massive still, as the paper notes:
The discovery of the RV companion of Cha Hα 8, which has an RV semi-amplitude of only 1.6 km s−1 , is an important step towards RV planet detections of BD/VLMS. In fact, from the uncertainty in the orbit solution, it cannot be completely excluded that the companion of Cha Hα 8 has a mass in the planetary mass regime (~13 MJup). Follow-up RV measurements monitoring the next phase of periastron (April 2011) are necessary to investigate this further.
It’s an intriguing find. As the authors point out, binary stars are thought to form in such a way that the companions are of relatively equal mass, particularly when the separation is close. The exceptions that do exist only remind us how much we have to learn about the mechanics of these systems, especially brown dwarfs. Or are we dealing not with a star but a planet forming through disk instability models, one that may well have formed further out in the system and subsequently migrated inwards?
The paper is Joergens and Müller, “16–20 MJup RV companion orbiting the brown dwarf candidate Cha Hα 81,” in press at the Astrophysical Journal Letters and available as an online preprint.