Trends in ultracool dwarf magnetism: Papers I and II

Well, the pair of papers that I’ve been working on for much of this year have finally hit, showing up as 1310.6757 and 1310.6758. I’m very happy with how they turned out, and it’s great to finally get them out the door!

These papers are about magnetism in very small stars and brown dwarfs, which we refer to as “ultracool dwarfs” or UCDs. Observations show that UCDs produce strong magnetic fields that can lead to large flares. However, the internal structure of these objects is very different than that of the Sun (no radiative core), in a way that makes it challenging to develop a theory of how UCDs produce their magnetic fields, and of what configuration those fields assume.

So we turn to observations for guidance. Our papers present new observations of seven UCDs made with the Chandra space telescope, detecting X-rays, and the recently-upgraded Very Large Array, detecting radio waves. Magnetic short circuits (“reconnection events”) are understood to lead to both X-ray and radio emission, and observations in these bands have turned out to provide very useful diagnostics of magnetism in both distant stars and the Sun.

When people such as my boss started studying UCD magnetism, they soon discovered that that the radio and X-ray emission of these small, cool objects has several surprising features when compared to Sun-like stars. We hope that by understanding these surprising observational features, we can develop a better theoretical understanding of what’s going on “under the hood.” This in turn will help us grapple with some challenging basic physics and also inform our understanding of what the magnetic fields of extrasolar planets might be like, which has large implications for their habitability (e.g.).

The first paper considers the ratio of radio to X-ray brightness. While this ratio is fairly steady across many stars, in some UCDs the radio emission is much too bright. The second paper considers X-ray brightness as a function of rotation rate. UCDs tend to rotate rapidly, and if they were Sun- like stars this would lead to them having fairly bright X-ray emission regardless of their precise rotation rate. But instead, they have depressed levels of X-ray emission, and the faster they rotate the fainter they seem to get.

Our papers make these effects clearer than ever, thanks to both the new data and to work we did to build up a database of relevant measurements from the literature. I’m really excited about the database since it’s not a one-off effort; it’s an evolving, flexible system inspired by the architecture of the Open Exoplanet Catalogue (technical paper here). It isn’t quite ready for prime time, but I believe the system to be quite powerful and I hope it can become a valuable, living resource for the community. More on it anon.

One of the things that the database helps us to see is that even if you look at two UCDs that are superficially similar, their properties that are influenced by magnetism (e.g., radio emission) may vary widely. This finding matches well with results from studies using an entirely unrelated technique called Zeeman-Doppler imaging (ZDI). The researchers using ZDI can measure certain aspects of the UCD magnetic fields directly, and they have concluded that these objects can generate magnetic fields in two modes that lead to very different field structures. These ideas are far from settled — ZDI is a complex, subtle technique — but we’ve found them intriguing and believe that the current observations match the paradigm well.

One of my favorite plots from the two papers is below. The two panels show measurements of two UCD properties: X-ray emission and magnetic field strength, with the latter being a representative value derived from ZDI. Each panel plots these numbers as a function of rotation (using a quantity called the Rossby number, abbreviated “Ro”). The shapes and colors further group the objects by mass (approximately; it’s hard to measure masses directly).

X-rays and magnetic field versus rotation. There’s scatter, but the general trends in the two parameters (derived from very different means) are surprisingly similar. From [1310.6758](
X-rays and magnetic field versus rotation. There’s scatter, but the general trends in the two parameters (derived from very different means) are surprisingly similar. From 1310.6758.

What we find striking is that even though the two panels show very different kinds of measurements, made with different techniques and looking at different sets of objects, they show similar trends: wide vertical scatter in the green (lowest-mass) objects; low scatter and high values in the purple (medium-mass) objects; and low scatter with a downward slope in the red (relatively high-mass) objects. This suggests to us that the different field structures hypothesized by the ZDI people result in tangible changes in standard observational quantities like X-ray emission.

In our papers we go further and sketch out a physical scenario that tries to explain the data holistically. The ZDI papers have argued that fast rotation is correlated with field structure; we argue that this can explain the decrease of X-rays with rotation, if the objects with low levels of X-rays have a field structure that produces only small “short circuits” that can’t heat gas to X-ray emitting temperatures. But if these short circuits manage to provide a constant supply of energized electrons, that could explain the overly bright radio emission. The other objects may produce fewer, larger flares that can cause X-ray heating but are too infrequent to sustain the radio-emitting electrons. (There are parallels of this idea in studies of the X-ray flaring properties of certain UCDs.)

Our papers only sketch out this model, but I think we provide a great jumping-off point for more detailed investigation. What I’d like to do for Paper III is do a better job of measuring rotation; right now, we use a method that has some degeneracies between actual rotational rate and the orientation of the object with regards to Earth. Some people have argued that orientation is in fact important, so using different rotation metrics could help test our model and the orientation ideas. And of course, it’s important to get more data; our “big” sample has only about 40 objects, and we need more to really solidly investigate the trends that we think we see.

One great part of this project is that I worked on it not only with my boss Edo Berger, but also with a fantastic summer REU student from Princeton, Ben Cook. Ben’s the lead author of Paper II and he did fantastic work on many aspects of the overall effort. It was a pleasure working with him and I suspect he’ll do quite well in the years to come.