Title: Large Amplitude Variations of an L/T Transition Brown Dwarf: Multi-wavelength Observations of Patchy, High-Contrast Cloud Features
Authors: Jacqueline Radigan, Ray Jayawardhana, David Lafreniere, Etienne Artigau, Mark Marley, Didier Saumon
First Author’s Institution: Department of Astronomy, University of Toronto
Calling brown dwarfs “failed stars” always seems a little mean to me. They’re not really failing at anything! They just don’t have high enough central temperatures to fuse hydrogen into helium; however, since they are more massive than planets, they can fuse deuterium in their core. (See this astrobite for a great briefing on brown dwarf spectroscopy for the newly-discovered Y dwarfs.)
The coolest thing about brown dwarfs (in my moderately biased opinion as someone who researches exoplanets) is that they are a beautiful testbed for planetary atmosphere modeling. Exoplanet atmospheres are really hard to observe. We struggle to get a spectrum with more than a dozen points! But brown dwarfs—because they’re not orbiting a star—are easily spatially resolved, and we have observed many brown dwarfs with moderate to high resolution spectroscopy (hundreds or thousands of points per spectrum). Since cool brown dwarfs are the same temperature as planets at their surface, we can use them to test our models while we wait for exoplanet observations to catch up.
L/T Transitional Objects: The Awkward Teenage Years
“L dwarfs” are hot brown dwarfs, which range in effective temperature from ~1400 K to just over 2000 K. They have significant cloud decks—made of iron and rock!—which blanket the surface in a dusty layer. Brown dwarfs cool as they age, and eventually become “T dwarfs.” These T dwarfs are colder brown dwarfs (under ~1200 K). They seem to have clear atmospheres, without a significant cloud layer. (See the first picture for artist’s renditions of each, and how they compare in size to stars/planets).
Objects transition very quickly from being dusty L dwarfs to clear T dwarfs. While brown dwarfs at 1400 K are covered with thick clouds, brown dwarfs that are just 100-200 K cooler are cloud-free. But we don’t know how exactly this happens. Do the clouds just sink down below the visible atmosphere? Do they break up and become patchy?
The authors of this paper set out to observe transitional objects—ones with a temperature between L dwarfs and T dwarfs—to determine how this transition works.
Observations of a Variable Brown Dwarf
This brown dwarf (with the catchy name 2MASS J21392676+0220226—or 2M2139 for short) was observed in the near-infrared bands J (1.3 microns), H (1.4 microns), and K (2.1 microns) for several nights. The flux was measured in each of these filters and compared over the course of the night. It was observed to be incredibly variable! The flux in each band varied by several percent with a period of about 8 hours. The amount that the amplitude varied also seemed to change from night to night.
This seems to indicate that there are some kinds of spots or features within the brown dwarf atmosphere. As the brown dwarf rotates, different parts of the surface come into view. Two suggestions are put forth:
- There are patchy clouds. Some fraction of the atmosphere still has the iron/silicate clouds (like an L dwarf) but some sections are clear (like a T dwarf). This would tell us that the L-T transition happens by the breakup of clouds and gradual clearing of the atmosphere.
- There could be hotter spots and colder spots, induced magnetically in the atmosphere.
The authors create a model of the brown dwarf surface using a combination of different 1D models. Each model has a temperature and cloud thickness parameter; they test each different combination (varying the covering fraction of clouds) to find the combined model that fits both the near-infrared spectrum in its entirety and the variations in flux that they’ve measured photometrically.
Swirling, Patchy Clouds
They find that only a few combinations of parameters fit both the spectrum and the photometry. They need a combination of thick clouds and clearer areas (#1 from above) to fit the data. This means that, according the their results, there are big dusty storms that cover large fractions of 2M2139’s surface!
Of course, combining 1D models in this way might not actually give us physically reasonable atmospheres. This type of modeling is great first step, but to really understand the atmospheres of objects with variable cloud cover like 2M2139, we’ll need to move to 3D models of the whole substellar atmosphere. While 1D models give us a good understanding of the vertical pressure-temperature profile and vertical cloud structure in the atmosphere, by nature they do not tell us about horizontal structure. To really understand the kinds of flows, winds, and motion in the atmosphere that would give rise to cloudy and clear areas of sky, it would be helpful to model it as a 3-dimensional object.