Extreme magnetic activity in NLTT 33370 AB

The paper that I’ve been working on for much of this year is finally submitted to The Astrophysical Journal and out on Arxiv! It’s the fourth in a series using simultaneous observations in multiple bands to better understand magnetic activity in the coolest stars and brown dwarfs. This particular paper targets a fascinating system called NLTT 33370 AB (aka 2MASS J13142039+1320011; it’s a little unfortunate that both names are nigh-unpronounceable).

NLTT 33370 came to our attention in 2011 when it was detected as a very bright radio emitter as part of a larger radio survey of the so-called “ultracool dwarfs”. In fact, it’s the brightest known radio emitter in this class. But it turns out that NLTT 33370 is doubly special because it is also a (relatively) short-period visual binary that will yield precise dynamical mass measurements of its components, which turn out to be nearly identical. It’s generally very difficult to measure masses directly in astronomy and so systems like NLTT 33370 provide important benchmarks for checking theories of stellar formation and evolution.

Even better, Josh Schlieder has shown that this system is fairly young, with an age of about 100 million years, which is extra-unusual for a mass benchmark. This is important since you want to check that your stellar evolution theories work throughout a star’s lifetime, not just when it’s old and settled.

There’s an interplay between these two unusual aspects of the system, however. The bright radio emission of NLTT 33370 indicates that it’s an extremely magnetically active system, and high levels of magnetic activity are in turn expected to have a strong impact on the structure and evolution of low-mass stars (see, e.g., MacDonald & Mullan 2009, Stassun et al 2012). If you want to test those evolution theories correctly, then, you have to understand how the magnetic phenomena might be affecting things.

On the flip side, the sheer brightness of this system makes it a great laboratory for studying how magnetic fields are generated and dissipated in the ultracool regime. This is something that we just don’t understand very well at the moment. However, any studies on this topic need to keep in mind this system’s unusual youth — as well as the simple fact that one is looking at a somewhat tight binary system, and not a single star.

Our NLTT 33370 project was designed to tackle the latter question of how the magnetic fields are generated and dissipated. We did this by observing the system in bands across the EM spectrum: radio, optical, the line, UV, and X-ray. These all give complementary handles on the different ways that different parts of the star are affected by its magnetism. While studies of this kind (including a recent one of ours) often rely on single brightness measurements taken at quasi-random times, one of the hallmarks of magnetic activity is erratic variability on many time scales, so it can be hard to interpret such measurements. It’s far better to monitor each of these bands for long times, so you understand how they vary, and to do this at the same time in every band, so you can have as complete a view as possible of what’s going on at any given time. Of course, it’s far more difficult to observe an object in this way, because you need a lot of telescope time and the logistics are hard to work out. But the whole point of research is that you’d better be pushing the boundaries of what’s possible.

The centerpiece observations in our study were long stares with the upgraded Very Large Array (radio) and the


observatory (optical) in 2012 and 2013. In 2013 we added simultaneous X-ray monitoring with the Chandra space telescope. I won’t show the complete data sets here since giving them a proper explanation would take up more space than I’d like, but rest assured that we saw a whole smorgasbord of magnetic phenomena. For a small sample, here are the radio-band light curves, showing both rapid flares and steadier, periodic variability at two frequencies. Each panel shows about ten hours of data:

The radio light curve of NLTT 33370 AB as obtained by the upgraded Very Large Array. From Williams et al. ([1409.4411](http://arxiv.org/abs/1409.4411)).
The radio light curve of NLTT 33370 AB as obtained by the upgraded Very Large Array. From Williams et al. (1409.4411).

Strikingly, there’s a dramatic change in the radio emission between 2012 and 2013. Besides the changes that you can see above, the timing of the radio modulations relative to the optical ones shifts by about 180° — a clue that we only obtained thanks to the simultaneous observations made with MEarth. This kind of evolution was totally unexpected. Even more strangely, while the total radio emission undergoes this striking change, the “circularly polarized” sub-component (not shown above) stays more or less the same. These facts led us to speculate that the 2013 (“Campaign 2”) data probably combine a steadier radio-emitting structure with emission from a single large release of magnetic energy some time before our observation. But with only a night’s worth of observations, we don’t know how frequently such flares might occur, or how much energy they might contain, or how long their radio emission might last.

This is just one example of why it’s so important to monitor these objects in multiple bands simultaneously, for multiple consecutive rotations. But longer-term monitoring is also incredibly valuable — as the data above show, several different kinds of magnetic flares happen in this system, and you need to build up a good-sized sample of them to truly understand their rates and sizes. Thanks to the goals of the MEarth observatory and the support of its team (most notably co-author Jonathan Irwin), we got this kind of longer-term monitoring in the optical band. Those data showed yet another surprise: the optical variations are composed of two modulations at periods that are slightly (2%) different. This figure shows all of the optical data spanning 3.2 years crammed together; the parts with smooth curves are nights of intensive observations, while the noisy-looking parts are months where we get just a few measurements every night.

Optical light curve of NLTT 33370 AB from MEarth. The gaps between observations are squashed together so that all of the data points can be seen. Different colors represent data from different years. From Williams et al. ([1409.4411](http://arxiv.org/abs/1409.4411)).
Optical light curve of NLTT 33370 AB from MEarth. The gaps between observations are squashed together so that all of the data points can be seen. Different colors represent data from different years. From Williams et al. (1409.4411).

The figure doesn’t make this apparent, but you can’t explain the data with a single periodicity, even if you let the modulation shape wiggle around. If you model the variation with multiple periodicities, however, you can do a very good job, even when you use a very simple sine curve to model the variations.

If these observations were of the Sun, you might suggest that the two periodic features correspond to two different sunspots. Like many astrophysical objects, the Sun has “differential rotation” where its atmosphere rotates at different rates depending on the latitude. Two sunspots at different latitudes would then imprint different rotation periods on the optical light curve. However, detailed studies of very cool stars such as the components of NLTT 33370 AB seem to show that they do not have differential rotation, even at levels much smaller than the 2% effect observed here. It’s of course possible that the two rotation rates correspond to each of the two stars in the binary, but their very similar values are, well, awfully suspicious. We’re still not sure what to make of this result.

There are a ton of other intriguing things about this system, including the fact that only one of the objects seems to emit radio waves, different kinds of X-ray flares that we observe, the possible phasing of periodic modulations in other bands such as the UV and Hα, and the frequency structure of the rapid radio flares. But this post is already quite lengthy! If you’d like to know more, you should dive into the full research paper.

We certainly hope to keep on studying NLTT 33370 AB in the future. We’d like to obtain a few key, specialized measurements that will give us some vital context for better understanding the data we have here; but also, continued observations of the kinds shown above will help us truly understand the different flaring events that happen in this system. Because NLTT 33370 AB is so unusually active and it can be a reference point for such precise physical measurements, I’m sure it will quite extensively studied going forward.