The Viewing Geometry of Brown Dwarfs Influences Their Observed Colours and Variability Properties

Abstract

In this paper we study the full sample of known Spitzer [$3.6\mu$m] and $J$-band variable brown dwarfs. We calculate the rotational velocities, $v\sin i$, of 16 variable brown dwarfs using archival Keck NIRSPEC data and compute the inclination angles of 19 variable brown dwarfs. The results obtained show that all objects in the sample with mid-IR variability detections are inclined at an angle $>20^{\circ }$, while all objects in the sample displaying $J$-band variability have an inclination angle $>35^{\circ }$. $J$-band variability appears to be more affected by inclination than \textit{Spitzer} [$3.6\mu$m] variability, and is strongly attenuated at lower inclinations. Since $J$-band observations probe deeper into the atmosphere than mid-IR observations, this effect may be due to the increased atmospheric path length of $J$-band flux at lower inclinations. We find a statistically significant correlation between the colour anomaly and inclination of our sample, where field objects viewed equator-on appear redder than objects viewed at lower inclinations. Considering the full sample of known variable L, T and Y spectral type objects in the literature, we find that the variability properties of the two bands display notably different trends, due to both intrinsic differences between bands and the sensitivity of ground-based versus space-based searches. However, in both bands we find that variability amplitude may reach a maximum at $\sim 7-9$hr periods. Finally, we find a strong correlation between colour anomaly and variability amplitude for both the $J$-band and mid-IR variability detections, where redder objects display higher variability amplitudes.

14 pages, 11 figures, accepted for publication in ApJ

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