Strong brightness variations signal cloudy-to-clear transition of brown dwarfs
- Space Telescope Science Institute, 3700 San Martin Drive Baltimore, MD 21218 (United States)
- Département de Physique, Université de Montréal, C.P. 6128 Succ. Centre-Ville, Montréal, QC H3C 3J7 (Canada)
- Department of Astronomy and Astrophysics, University of Toronto, Toronto, ON M5S 3H4 (Canada)
We report the results of a J-band search for cloud-related variability in the atmospheres of 62 L4-T9 dwarfs using the Du Pont 2.5 m telescope at Las Campanas Observatory and the Canada-France-Hawaii Telescope on Mauna Kea. We find 9 of 57 objects included in our final analysis to be significantly variable with >99% confidence, 5 of which are new discoveries. In our study, strong signals (peak-to-peak amplitudes >2%) are confined to the L/T transition (4/16 objects with L9-T3.5 spectral types and 0/41 objects for all other spectral types). The probability that the observed occurrence rates for strong variability inside and outside the L/T transition originate from the same underlying true occurrence rate is excluded at >99.7% confidence. Based on a careful assessment of our sensitivity to astrophysical signals, we infer that 39{sub −14}{sup +16}% of L9-T3.5 dwarfs are strong variables on rotational timescales. If we consider only L9-T3.5 dwarfs with 0.8 < J – K {sub s} < 1.5, and assume an isotropic distribution of spin axes for our targets, we find that 80{sub −19}{sup +18}% would be strong variables if viewed edge-on; azimuthal symmetry and/or binarity may account for non-variable objects in this group. These observations suggest that the settling of condensate clouds below the photosphere in brown dwarf (BD) atmospheres does not occur in a spatially uniform manner. Rather, the formation and sedimentation of dust grains at the L/T transition is coupled to atmospheric dynamics, resulting in highly contrasting regions of thick and thin clouds and/or clearings. Outside the L/T transition we identify five weak variables (peak-to-peak amplitudes of 0.6%-1.6%). Excluding L9-T3.5 spectral types, we infer that 60{sub −18}{sup +22}% of targets vary with amplitudes of 0.5%-1.6%, suggesting that surface heterogeneities are common among L and T dwarfs. Our survey establishes a significant link between strong variability and L/T transition spectral types, providing evidence in support of the hypothesis that cloud holes contribute to the abrupt decline in condensate opacity and 1 μm brightening observed in this regime. More generally, fractional cloud coverage is an important model parameter for BDs and giant planets, especially those with L/T transition spectral types and colors.
- OSTI ID:
- 22370586
- Journal Information:
- Astrophysical Journal, Vol. 793, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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