By Jacqueline Radigan, Giacconi Fellow at STScI
Formed like stars, but not massive enough to fuse hydrogen, brown dwarfs straddle the boundary between low mass stars and giant planets. Without a reservoir of nuclear fuel they spend their lives cooling. Consequently, the atmospheres of old and cold field brown dwarfs are higher surface gravity analogs to those of extrasolar planets. However, observations of free-floating brown dwarfs carry a key advantage over those of planets: they can be directly imaged without being washed out by the light of a parent star. As a result, the spectra obtained of nearby field brown dwarfs in the solar neighborhood are more numerous and of higher quality than their exoplanet counterparts. In fact the coolest brown dwarf yet discovered (Luhman 2014) has a temperature of only ~250 K (below the freezing point for water!), making it the coolest directly imaged object outside of our own Solar System.
At temperatures below ~2500 K (L spectral types), refractory species in brown dwarf atmospheres begin to condense to form exotic “dust’’ clouds (Burrows & Sharp 1999; Lodders 1999; Burrows et al. 2006). These first condensates are made of metal-oxides, silicate grains, and liquid iron. Further cooling leads to grain growth and the eventual settling of cloud layers below the visible photosphere. Thus at temperatures below ~1200 K, dust clouds have largely disappeared from view (T spectral types).
Figure 1. Reproduced from Radigan 2014. Color magnitude diagram of M, L and T dwarfs with known parallaxes (small circles) from the database of T. J. Dupuy. The points are divided by color into the spectral type bins used by Radigan et al. 2014: L-dwarfs (≤L8.5) in red, T-dwarfs (≥T4) in purple, and L/T transition dwarfs (L9–T3.5) in dark and light blue. Within the L/T transition bin T0–T2.5 dwarfs are shown in dark blue, while objects with L9–L9.5 and T3–T3.5 spectral types (the endpoints) are plotted in a lighter color to illustrate the degree of overlap with the L-dwarf and T-dwarf bins. Two L/T transition objects which are over-luminous for their spectral types (L9 and T0) are noted as possible binaries.
The brown dwarf spectral sequence, especially the transition from cloudy L- to clear T- spectral types is challenging to understand owing to the presence of clouds, complex chemistry, and weather. These complexities are highlighted by perplexing behavior observed at the L/T transition: at wavelengths around ~1 micron (near the peak of the spectral energy distribution) brown dwarfs briefly become brighter as they cool (Figure 1; Dahn et al. 2002; Tinney et al. 2003; Vrba et al. 2004; Dupuy & Liu 2012; Faherty et al. 2012). This phenomenon, which has proved notoriously difficult for modelers to reproduce (e.g. Marley et al. 2002, Allard et al. 2003, Burrows et al. 2006), is likely related to the mechanism of cloud dispersal at the L/T transition, and illustrates the limiting role clouds play in our understanding of cool atmospheres.
Dust cloud “weather” has long provided the most promising explanation for the counter-intuitive L/T transition brightening (Ackerman & Marley 2001, Burgasser et al. 2002) . The idea is that as dust clouds settle deeper in the atmosphere they begin to intersect with the dynamic troposphere, leading to the development of “holes”, or lower opacity sight-lines into the deep photosphere. Consequently, flux from warmer atmospheric layers is able to escape, and would explain the abrupt brightening seen at 1 micron wavelengths, where clouds are a dominant opacity source. This hypothesis makes a straightforward prediction: L/T transition brown dwarfs will have patchy atmospheres and should therefore exhibit variability on rotational timescales.
We tested this hypothesis by observing 56 brown dwarfs with mid-L to T spectral types over 60 nights using the Wide Field Infrared Camera (WIRC) on the Du Pont 2.5 m telescope at the Las Campanas Observatory (Radigan et al. 2014). Each target was observed continuously in the J-band (wavelengths of ~1.2 microns) for ~3-5 hours (a large fraction of a rotation period), along with several reference stars that happen to fall in the camera’s field of view. Differential photometry for the target and reference stars were computed to produce light curves. Using data for nearly 800 (presumably non-variable) reference stars we determined an empirical criterion for significant variability (with an expected 1% rate of false positives). Using our empirically calibrated criterion we found 9 of our 56 targets to be significantly variable (figure 2).
Figure 2. Reproduced from Radigan et al. 2014. Light curves of significantly variable (p > 99%) targets in our sample. Light curves are shown in the upper panels, and the light curve of a similar-brightness comparison star observed simultaneously is shown in the lower panels. All data shown were obtained using WIRC on the Du Pont 2.5 m telescope except for those of 2M0758+32, which were obtained using WIRCam on the Canada-France-Hawaii Telescope. All light curves have been binned down from their original cadence by factors of 3–7.
The spectral types, colors, and amplitudes of our variable targets are shown in figure 3. The high-amplitude variables are clustered directly at the L/T transition, providing the first direct evidence in support of cloud break-up. However, since not all L/T transition objects are highly variable, there may be more to the story. While large contrast between clouds and clearings produce large variability for some L/T transition brown dwarfs, how do we explain 1-micron brightening for the larger non-variable population? Some objects with patchy clouds may not be variable due to azimuthal symmetry, small scale features, or long rotation periods. Other supposed L/T transition objects may in fact be unresolved binaries with components outside the transition. Are these factors sufficient to explain all the non-variables? In the published paper we concluded: maybe. So while it is clear that large variability is more frequent at the L/T transition, more data are required to pin down a causal relationship between cloud break-up and 1 micron brightening.
Figure 3. Reproduced from Radigan et al. 2014. NIR spectral type vs. 2MASS J − Ks color for all targets observed in our program. Gray points show the population of known field L and T dwarfs with J <16.5. Purple circles show detections, with the linear symbol size is proportional to the peak-to-peak amplitude of variability detected (ranging from 0.9%–9% for the smallest to largest symbols). A gray dashed line encircles the objects considered part of the L/T transition sample. Our average sensitivity or completeness to sinusoidal signals of a given peak-to-peak amplitude is approximately equal in all spectral type bins.
Variable brown dwarfs, and notably the new population of high-amplitude variables at the substellar L/T transition, provide novel opportunities to constrain cloud properties and dynamics in cool atmospheres. Surface variations in cloud thickness produce a chromatic variability signature, providing an unprecedented opportunity to probe cloud structure via multi-wavelength monitoring as in e.g. Radigan et al. (2012) , Buenzli et al. (2012), Apai et al. (2013). Finally, mapping the evolution of features over multiple rotations will provide a way to study atmospheric dynamics in the high-gravity, non-irradiated regime.
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