“The First Millimeter Detection of a Non-Accreting Ultracool Dwarf”

Image credit: NRAO/AUI/NSF; Dana Berry / SkyWorks

Our latest paper is finally out! It’s called “The First Millimeter Detection of a Non-Accreting Ultracool Dwarf” and is accepted to The Astrophysical Journal. The Center for Astrophysics and the NRAO are putting out press releases about it including a cool animation.

This is a project that Edo and I did in collaboration with a group of brown dwarf experts in the UK: Sarah Casewell, Craig Stark, Stu Littlefair, and Christiane Helling. Craig is an expert in exotic plasma processes and came up with an idea about how some of them might operate in the upper atmospheres of very cool stars and brown dwarfs — in line with an overall physical model that Christiane has been developing for quite a while. If so, these processes would cause these cool stars to become quite bright at millimeter wavelengths (or, equivalently, frequencies around 100 GHz; these are the sorts of wavelengths used in new full-body airport scanners). The ALMA telescope would be the perfect tool to try to detect this emission, and just about two years ago Sarah got in touch with me about collaborating on this project since I have experience working with the long-wavelength interferometric data that ALMA produces.

ALMA has annual proposal deadlines and generally does things at a … stately … pace. Sarah led up the proposal, which was eventually accepted, and ALMA finally delivered our data in late April of this year.

Craig’s plasma processes are, well, quite exotic, so frankly we weren’t sure if we were going to see anything. So I was quite excited when I took a first look at the data and saw a strong detection of our target, a famous low-mass object called TVLM 513–46546! (Well, it’s famous among people who care about exotic phenomena in ultracool dwarfs.) Stuart promptly went out and obtained a night’s worth of visible-light monitoring of our target, since it’s known to vary periodically at visible wavelengths — if there were any variability in the ALMA data, it would be interesting to see how it compared to the optical, and you want contemporaneous data in order to be able to compare effectively.

Once we got to sit down and look at the data carefully, it didn’t seem as if the emission matched Craig’s exotic plasma processes. Instead, it comes from a different surprising source! It seems to come from the same general kind of synchrotron process that is likely responsible for TVLM 513’s emission at centimeter radio wavelengths (at least its non-bursting emission, but that’s a story for another time), just operating at much high energies. The physics of the synchrotron process tell us that emission at ALMA wavelengths must be coming from very energetic electrons — ones moving close to the speed of light — gyrating in fairly strong magnetic fields. This is really awesome! It’s kind of incredible to me that this big, relatively cold ball of gas can both self-generate strong magnetic fields (in some places thousands of times stronger than the Earth’s) and somehow channel its energy into accelerating electrons to at least 99% the speed of light.

Even though you might think that TVLM 513 would be a fairly dinky little object, our data help paint a picture in which it’s surrounded by an intense radiation environment — much stronger than the Sun’s, in fact. This is important since astronomers these days are very interested in finding planets around low-mass red dwarfs. In order to be warm enough to have liquid water, such planets would need to orbit fairly close to their host stars, but this means that they’d also be that much closer to the kind of radiation sources we observed with ALMA. It’s an active area of research to try to understand how harmful this radiation, and related effects, would be for any life on such planets.

Our finding is also very interesting physically since the electrons that produce the ALMA emission probably lose all their energy in a matter of a few hours. So, if these electrons are accelerated to such high energies only in big bursty events, we got very lucky during our observation. On the other hand, if the emission we observe turns out to be very steady, that implies that the acceleration process is probably operating continuously at a low level. The mechanism of this acceleration process is precisely one of the big mysteries in this field, so I’m excited about the possibility of using the time-dependence of the ALMA emission to learn more.

In fact, there’s a hint in our data that the ALMA emission had a bit of a burst in synch with the time when the optical modulation would have been at its maximum. (We’re glad that Stu got his data!) My best estimate is that there’s about a 7% chance that the observed data are just a noise blip, which isn’t too high, but is higher than you generally want when trying to make a scientific claim. To be honest, I’m not quite sure what the explanation would be if these pieces of emission turn out to be synchronized, but you can bet that I want to find out if they are.

Unfortunately (or is it ironically?) we got these data the day before the 2015 ALMA proposal deadline, so we weren’t able to submit a plan for follow-up observations. We’ll certainly submit one in 2016, which means that hopefully we’ll get more data in 2017. I’d like to get data sooner, but ALMA is the only telescope in the world that’s powerful enough to get the data we need, so it’s hard to complain too much!

This result opens the door to a lot of new investigations. While there are lots of details that we’d like to fill in about TVLM 513, I’d really like to observe a larger sample of similar objects and see how many can sustain this high energy acceleration process. Properties of the emission such as its time-dependence and polarization will fill in the physics and help us understand what’s driving these processes, as well as painting a clearer picture of how this radiation might affect habitability.

To be honest, there’s probably a small silver lining in the fact that we’ll have to wait a bit to try to get more ALMA data. The way the ALMA schedules observations right now is very rigid, in the sense that it’s hard to make special requests: right now you can’t even request a single observation that lasts longer than two hours. (I had a proposal for a four-hour observation rejected as being “technically impossible”; not that I’m bitter.) The kinds of observations that we’d really like to do — variability monitoring; simultaneous observations with the Very Large Array — do not fit well into that scheme at all. There are not-crazy reasons for the current way of doing things, but I think it does preclude a lot of exciting science. We will advocate for more astronomer-friendly rules and with luck we’ll have more flexibility the next time we apply.