Brown Dwarfs Come of Age


A few months ago I attended my first conference on brown dwarfs, in Fuerteventura,  one of the Canary Islands off the west coast of Africa.

Coming from a background in exoplanets I was positively surprised at the many crossovers between brown dwarfs and exoplanets. One such crossover was the formation mechanisms which govern brown dwarfs. They mostly form the way stars form. However, as we enter the mass regime of the largest planets, there is a changeover where brown dwarfs are more likely to form like giant exoplanets do, via disk fragmentation. At the conference it was shown that this changeover seems to happen at 42 MJup (I think Douglas Adams was onto something here):

Less than 42 MJup and you form brown dwarfs via disk instability and gravitational fragmentation, much like giant planets. This formation process is limited, however, by the radiative feedback from protostars which largely suppress disk fragmentation.

More massive than 42 MJup and you get a scaled-down version of low-mass star formation with cloud contraction.

This is not to say that hybrid scenarios can’t exist, such as fragments being ejected from protostellar disks followed by cooling and contraction to stellar densities. Also, wide companion brown dwarfs with masses less than 42 MJup are a bit of a puzzle since the disk formation scenario is unlikely due to their wide separations.

We are currently able to resolve and see disks around brown dwarfs with both ALMA and Herschel. In fact some seem to have disks with enough material to form Earth and Neptune-mass planets. New disk fragmentation models demonstrate this and are able to produce ejected proto-brown-dwarfs which themselves form disks. Binaries are not as easily made.

On the observational front, the conference attendees were presented with a beautiful rotation curve  of a sub mm grain disk around the Taurus brown dwarf 2M0444. At the moment the S/N of the observations are too low to derive a dynamical mass, despite this, I was impressed how it was so well resolved.

The Initial Mass Function (IMF) of brown dwarfs in clusters were discussed. The lack of mass-luminosity relationship (brown dwarfs cool steadily with time) makes obtaining an accurate IMF difficult. What can be said though is that brown dwarfs have their own IMF. Ingo Thies showed how it was still possible to have a smoothly declining binary fraction even with a disjointed IMF between stars and brown dwarfs.

Moving onto the coldest and lowest mass objects, Trent Dupuy presented his new parallax measurements done with Spitzer. These sort of parallax measurements are essential for calculating the effective temperature of brown dwarfs (via Stephan-Boltzmann law), since without accurate distances, there are a lot of degeneracies present. Currently Spitzer is is the only telescope that can measure both the flux and distances for Y-dwarfs. One of the take-home messages were that Y-dwarfs are warmer (400+ K not 300K) than expected compared to model atmospheres and that we may not know how to get the spectral type of Y-dwarfs yet. A quote still ringing in my ears comes to mind:

“Y dwarfs: New spectral type? OR spectral hype”

- Anonomyous

Knowing the age of brown dwarfs are important for a whole host of reasons. For instance knowing the age and effective temperature of a brown dwarf we can infer its mass. Also, as Jacqueline Faherty argued, young brown dwarfs open up a gateway to understanding directly imaged exoplanets. This is because they are in general easier to observe and also share a lot of the same spectral, photometric and luminosity properties. Age also allows us to study gravity-sensitive observables and disentangle secondary parameters such as dust properties (proceeding).

Binary brown dwarfs are an excellent resource for determining fundamental parameters such as metallicity, age, mass, temperature and stellar rotation. Adam Burgasser presented us with Luhman 16AB, a well-separated binary at the L-T transition, which just so happens to be the third closest system to the sun a mere 2 pc away (proceeding). If this wasn’t enough, the object is highly variable (with the variability likely coming from the T-type companion). It was clear from the talks at the conference that the binary fraction of brown dwarfs (which is about 20%) is still not very well known, with most of the statistics coming from directly-imaged systems.

We heard about the discovery of halo T dwarfs and many new L subdwarfs. Katelyn Allers presented us with an index-based method for classifying the gravities of M6-L5 dwarfs (consistent with gravity classifications using optical spectroscopy), which has the advantage of distinguishing between young and dusty objects (paper).

For the latest spectral class, the Y-dwarfs, Caroline Morley showed us her work on a new grid of model spectra which include the effects of water clouds in the atmospheres of these cold brown dwarfs. The largest effect by water clouds is in the mid-infrared where water ice is a strong absorber for objects colder than ~350K.

Brown dwarfs have strong magnetic fields and are known to show brief brightness eruptions (similar to flares that occur on the Sun). There are various ways one can infer magnetism from (Chromospheric Hα, CaII and H&K emission, optical variability etc..), but as Janella Williams explained, “In Brown Dwarf Land, radio is king”, highlighting that radio emission is the best tracer for magnetic activity after ~M8 spectral type. Radio observations of magnetic activity is currently ongoing with the VLA looking at 18 L/T dwarfs close by (d<13pc).

Rapidly rotating brown dwarfs with dust present in their atmospheres, emit linear polarised light at optical and near-IR wavelengths. Paulo Miles-Páez showed us observations of rapidly rotating ultracool dwarfs which showed that polarised objects observed during consecutive days were constant within the uncertainties, however, with observations taken months apart, significant flux variations were observed suggesting the possibility of weather patterns. He suggested that simultaneous linear polarimetric measurements in different filters could help constrain the grain size of the dust in the atmospheres. As Adam Burgasser pointed out, this could be an interesting way to study the structure of clouds in sources which are not variable.

Jacqueline Radigan presented us with her results on monitoring L/T transition brown dwarfs with Spitzer. There seems to be no trends between spectral type, rotation period, activity or amplitude. Differential rotation leading to winds appear to be present in brown dwarfs. Repeat visits at multiple wavelengths will be essential to know what is going on.

Finally I thought I would end with an interesting poll held at the conference:


Should we continue using the deuterium burning mass limit in the definition of brown dwarfs?

Y: 46

N: 16


Should another mass limit be involved in the definition (e.g 42 MJup) (perhaps in addition)?

Y: 6

N: 55


Should we insist that a planet orbit a star or a stellar remnant?

Y: 32

N: 30


Free-floaters in clusters/field -called sub-BDs (or another different name)? Should we call them something else?

Y: 7

N: 55


About Author

Editor of Observational exoplanet and brown dwarf astronomer studying the atmospheres of exoplanets. Interested in public outreach and conveying my interest in astronomy to others. Follow me on Twitter or Google+. (More)