Browsing by Subject "star adaptive optics"

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The Mass-Luminosity Relation In The L/T Transition: Individual Dynamical Masses For The New J-Band Flux Reversal Binary SDSS J105213.51+442255.7Ab
(2015-05) Dupuy, Trent J.; Liu, Michael C.; Leggett, S. K.; Ireland, Michael J.; Chiu, Kuenley; Golimowski, David A.; Dupuy, Trent J.
We have discovered that SDSS J105213.51+442255.7 (T0.5 +/- 1.0) is a binary in Keck laser guide star adaptive optics imaging, displaying a large J- to K-band flux reversal (Delta J = -0.45 +/- 0.09 mag, Delta K= 0.52 +/- 0.05 mag). We determine a total dynamical mass from Keck orbital monitoring (88 +/- 5 M-Jup) and a mass ratio by measuring the photocenter orbit from CFHT/WIRCam absolute astrometry (M-B/M-A= 0.78 +/- 0.07). Combining these provides the first individual dynamical masses for any field L or T dwarfs, 49 +/- 3 M-Jup for the L6.5 +/- 1.5 primary and 39 +/- 3 M-Jup for the T1.5 +/- 1.0 secondary. Such a low mass ratio for a nearly equal luminosity binary implies a shallow mass-luminosity relation over the L/T transition (Delta log L-bol/Delta log M = 0.6(-0.8)(+0.6)). This provides the first observational support that cloud dispersal plays a significant role in the luminosity evolution of substellar objects. Fully cloudy models fail our coevality test for this binary, giving ages for the two components that disagree by 0.2 dex (2.0 sigma). In contrast, our observed masses and luminosities can be reproduced at a single age by "hybrid" evolutionary tracks where a smooth change from a cloudy to cloudless photosphere around 1300 K causes slowing of luminosity evolution. Remarkably, such models also match our observed JHK flux ratios and colors well. Overall, it seems that the distinguishing features SDSS J1052+ 4422AB, like a J-band flux reversal and high-amplitude variability, are normal for a field L/T binary caught during the process of cloud dispersal, given that the age (1.11(-0.20)(+0.17) Gyr) and surface gravity (log g = 5.0-5.2) of SDSS J1052+ 4422AB are typical for field ultracool dwarfs.
New Evidence For A Substellar Luminosity Problem: Dynamical Mass For The Brown Dwarf Binary Gl 417BC
(2014-08) Dupuy, Trent J.; Liu, Michael C.; Ireland, Michael J.; Dupuy, Trent J.
We present new evidence for a problem with cooling rates predicted by substellar evolutionary models that implies that model-derived masses in the literature for brown dwarfs and directly imaged planets may be too high. Based on our dynamical mass for Gl 417BC (L4.5+L6) and a gyrochronology system age from its young, solar-type host star, commonly used models predict luminosities 0.2-0.4 dex lower than we observe. This corroborates a similar luminosity-age discrepancy identified in our previous work on the L4+L4 binary HD 130948BC, which coincidentally has nearly identical component masses (approximate to 50-55 M-Jup) and age (approximate to 800 Myr) as Gl 417BC. Such a luminosity offset would cause systematic errors of 15%-25% in model-derived masses at this age. After comparing different models, including cloudless models that should not be appropriate for mid-L dwarfs like Gl 417BC and HD 130948BC but actually match their luminosities better, we speculate the observed overluminosity could be caused by opacity holes (i.e., patchy clouds) in these objects. Moreover, from hybrid substellar evolutionary models that account for cloud disappearance, we infer the corresponding phase of overluminosity may extend from a few hundred million years up to a few gigayears and cause masses to be overestimated by up to 25%, even well after clouds disappear from view entirely. Thus, the range of ages and spectral types affected by this potential systematic shift in luminosity evolution would encompass most known directly imaged gas-giants and field brown dwarfs.