“The Rotation Period and Magnetic Field of the T Dwarf 2MASSI J1047539+212423”
2015 August 5
Long time, no blog! I’ve been busy with a bunch of things this summer. These have mostly not been my latest paper (DOI, arxiv, ADS), which was submitted in February, but it’s finally out in ApJ so it feels like a good time to finally write about it.
The full title is “The Rotation Period and Magnetic Field of the T Dwarf 2MASSI J1047539+212423 Measured From Periodic Radio Bursts” and that pretty much sums it up. The work builds on an earlier paper describing VLA follow-up of this awesome, cold brown dwarf, a.k.a. 2M 1047+21, which was discovered to have radio emission by Route & Wolszczan in 2012. The main thing about this radio-emitting brown dwarf is that it’s much colder than the other published radio emitters; its effective temperature is just 900 K, while the next-coolest emitter at the time of discovery is at 1900 K. This is really encouraging since a big theme in the field is pushing to detect radio emission from cooler, and less massive objects, hopefully down into the (exo)planetary regime one day. The initial detections suggested that even at 1000 Kelvin cooler, the radio and magnetic activity levels held up pretty well.
Our follow-up paper confirms this. With a longer VLA observation, we both confirmed the first detection and discovered the periodic, highly- polarized radio bursts that are a hallmark of planet-style auroral current systems. This isn’t shocking, since both planets and higher-mass brown dwarfs show these bursts, but this is great evidence that the processes really do scale continuously between these regimes. And we also get to read the object’s rotation period off of the burst periodicity. The burst shapes are erratic so it’s a bit challenging to do this very precisely (see the paper if you’re curious), but the basic number is about 1.8 hours for a full rotation. This is about 150 times the Earth’s rotation speed, if you’re talking miles per hour, though it’s in line with similar objects. That being said, this is one of only a handful of rotation periods measured for mid/late T dwarfs, since they’re very faint and seem to have pretty uniformly-colored atmospheres (so that it’s hard to see clouds rotating in and out of view).
We also observed 2M 1047+21 at slightly higher frequencies and saw a single burst that seems to be due to the same process. The radio frequency tells you about the dwarf’s magnetic field strength, so we deduced that this dwarf’s magnetic field reaches strengths of about 3.5 kG. That’s as strong as you get in active stars, and about 250 times stronger than what we see in Jupiter, even though this object is only about 8 times warmer than Jupiter. This makes me wonder whether there’s some kind of big dropoff in magnetic field strengths somewhere between the T dwarfs and planets, or whether maybe some planets can have magnetic fields much stronger than we expect.
Since the discovery of radio emission from 2M 1047+21, Melodie Kao and Gregg Hallinan at Caltech have announced that they’ve detected radio emission from similarly cold objects, although the work isn’t published yet. Their target-selection strategy also yielded a much higher success rate than we typically have in this field. This is pretty encouraging and means that the race is on to see if we can get radio emission from even colder objects!
The submitted version of the paper is available not only on arxiv.org, but also on my personal website as an HTML document using the “Webtex” system I’ve been developing. My goal here is to make it so that scientific articles are better looking and more valuable on the computer screen than on paper; all of our writing tools are designed to make printed documents, leading to a bunch of practices that don’t make sense in the electronic context (footnotes!). I’ve found it hard to articulate why I think this is so important, and I don’t think the current prototype makes it self-evident, but it’s what I’ve got right now. (That being said, there’s a lot going on under the hood!)
There’s also a video of me giving a short talk about this work at the Geneva “BCool” workshop: