The Viewing Geometry of Brown Dwarfs Influences Their Observed Colors and Variability Amplitudes

Abstract

In this paper we study the full sample of known Spitzer [3.6 µm] 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• , while all objects in the sample displaying J-band variability have an inclination angle > 35• . J-band variability appears to be more affected by inclination than Spitzer [3.6 µm] 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 spacebased searches. However, in both bands we find that variability amplitude may reach a maximum at ∼ 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.

Detection of new strongly variable brown dwarfs in the L/T transition

“…These possible correlations were previously suggested by both Kirkpatrick et al (2010) and Metchev et al (2015), but investigated in depth by Vos et al (2017) for the majority of the known variable BDs. By determining the v sin i and calculating the inclinations of 19 targets, they found clear correlations with variability amplitude for both inclination and J − K S colour anomaly (i.e.…”

“…The resulting Fig. 8 includes 203 L8-T5 BDs from the database, all our targets and J2139+0220 for reference as it was shown by Vos et al (2017) to be maximally inclined. Even considering uncertainties in SpT of up to ±1.5, J2139 (∆ J−K S = −0.61) would be significantly redder than a typical BD of the same type.…”

“…As the collective knowledge in this field has grown substantially in the past few years, the need for a comprehensive variability catalogue arises. Vos et al (2017) brown dwarfs in the L/T transition gathered by criteria from the SIM-BAD database for various SpT, with the median colour shown as a grey star for a given SpT (size proportional to the number of BDs in that bin). J2139+0220 is included for reference as it is very strongly variable and has been shown to inclined at 90 • .…”

Spitzer Light Curves of the Young, Planetary-mass TW Hya Members 2MASS J11193254–1137466AB and WISEA J114724.10–204021.3

“…Vos et al (2017) investigated the relationship between inclination angle and variability amplitude for a sample of 19 brown dwarfs with measured variability and found a clear trend of increasing of J-band variability amplitudes with larger inclination angles. For Spitzer wavelengths, however, the differences between variability amplitudes of objects viewed close to equator-on (i≈90°) was marginal compared to objects with inclinations as low as ∼20°.…”

“…To investigate the rotation periods of field-age brown dwarfs, we combine the rotation periods from Metchev et al (2015) with the compilations of rotation periods in Crossfield (2014) and Vos et al (2017), the rotation period of Luhman 16A (4.5-5.5 hr; Buenzli et al 2015) and Luhman 16B (5.05±0.10 hr; Burgasser et al 2014, 4.87±0.01 hr; Gillon et al 2013), and the two known rotation periods for the Y-type brown dwarfs WISE J140518.39+553421.3 (8.54±0.08 hr; Cushing et al 2016) and WISEP J173835.52+273258.9 (6.0±0.1 hr; Leggett et al 2016). To ensure we do not include low-mass stellar sources in this comparison, we limit our field-age sample to those objects with spectral types later than L2 (Dieterich et al 2014;.…”