2 5 M ay 2 00 9 Near - Infrared Light Curves of the Brown Dwarf Eclipsing Binary 2 MASS J 05352184 – 0546085 : Can Spots Explain the Temperature Reversal ?

We present near-infrared JHKS light curves for the double-lined eclipsing binary system 2MASS J05352184-0546085, in which both components have been shown to be brown dwarfs with an age of ∼ 1 Myr. We analyze these light curves together with the previously published IC -band light curve and radial velocities to provide refined measurements of the system’s physical parameters. The component masses and radii are here determined with an accuracy of ∼ 6.5% and ∼ 1.5%, respectively. In addition, we confirm the previous surprising finding that the primary brown dwarf has a cooler effective temperature than its lower-mass companion. Next, we perform a detailed study of the residual variations in the out-of-eclipse phases of the light curves to ascertain the properties of any inhomogeneities (e.g. spots) on the surfaces of the brown dwarfs. Our analysis reveals two low-amplitude (∼ 0.02 mag) periodic signals, one attributable to the rotation of the primary with a period of 3.293 ± 0.001 d and the other to the rotation of the secondary with a period of 14.05± 0.05 d. Both periods are consistent with the measured v sin i and radii. Finally, we explore the effects on the derived physical parameters of the system when spots are included in the modeling of the light curves. The observed low-amplitude rotational modulations are well fit by cool spots covering a small fraction (. 10%) of the brown dwarfs’ surfaces. Such small spots negligibly affect the physical properties of the brown dwarfs, and thus by themselves cannot explain the primary’s unexpectedly low surface temperature. To mimic the observed ∼ 200 K suppression of the primary’s temperature, our model requires that the primary possess a very large spot coverage fraction of ∼ 65%. These spots must in addition be symmetrically distributed on the primary’s surface so as to not produce photometric variations larger than observed. Altogether, a spot configuration in which the primary is heavily spotted while the secondary is lightly spotted—consistent with the idea that the primary’s magnetic field is much stronger than the secondary’s—can explain the apparent temperature reversal and can bring the temperatures of the brown dwarfs into agreement with the predictions of theoretical models.