Top-of-the-atmosphere and Vertical Cloud Structure of a Fast-rotating Late T Dwarf

Abstract

Only a handful of late T brown dwarfs have been monitored for spectrophotometric variability, leaving incomplete the study of the atmospheric cloud structures of the coldest brown dwarfs, which share temperatures with some cold, directly imaged exoplanets. 2MASS J00501994–332240 is a T7.0 rapidly rotating, field brown dwarf that showed low-level photometric variability in data obtained with the Spitzer Space Telescope. We monitored 2MASS J00501994–332240 during ∼2.6 hr with MOSFIRE, installed at the Keck I telescope, with the aim of constraining its near-infrared spectrophotometric variability. We measured fluctuations with a peak-to-peak amplitude of 1.48% ± 0.75% in the J-band photometric light curve, an amplitude of 0.62% ± 0.18% in the J-band spectrophotometric light curve, an amplitude of 1.26% ± 0.93% in the H-band light curve, and an amplitude of 5.33% ± 2.02% in the CH4 − H2O band light curve. Nevertheless, the Bayesian information criterion does not detect significant variability in any of the light curves. Thus, given the detection limitations due to the MOSFIRE sensitivity, we can only claim tentative low-level variability for 2M0050–3322 in the best-case scenario. The amplitudes of the peak-to-peak fluctuations measured for 2MASS J00501994–332240 agree with the variability amplitude predictions of general circulation models for a T7.0 brown dwarf for an edge-on object. Radiative transfer models predict that the Na2S and KCl clouds condense at pressures lower than that traced by the CH4–H2O band, which might explain the higher peak-to-peak fluctuations measured for this light curve. Finally, we provide a visual recreation of the map provided by general circulation models and the vertical structure of 2MASS J00501994–332240 provided by radiative transfer models.

Informed Systematic Method to Identify Variable Mid- and Late-T Dwarfs

“…GCMs provide instantaneous model maps for the top-of-the atmosphere thermal flux, and predict the most likely light curve for such an object. The variability amplitude predicted for a T dwarf of a similar spectral type to 2M2228 (2M0050-3322, Manjavacas et al 2022) is about ∼1% for an edge-on object. The predicted variability amplitude decreases to about 0.2% for a pole-on object.…”

“…Similarly, Buenzli et Given that most of the cloud deck is expected to be at pressures above ∼1 bar (Yang et al 2016), in principle, we do not expect a high variability in the Spitzer channels. A visual representation of the atmospheric structure of an object of similar spectral type, 2M0050-3322, can be found in Manjavacas et al (2022), their Figure 19. Nevertheless, the sensitivity of the Spitzer Space Telescope is higher than that of ground-based observatories.…”

Measuring the variability of directly imaged exoplanets using vector Apodizing Phase Plates combined with ground-based differential spectrophotometry

“…However, this is also inherently the case for any study that explores the variability of isolated brown dwarfs and planetary-mass objects, stellar variability due to star spots or other sources, or transiting exoplanet studies where exoplanets transit variable stars (e.g. Gelino et al 2002;Rockenfeller et al 2006;Biller et al 2013Biller et al , 2015Biller et al , 2021Girardin et al 2013;Radigan et al 2014;Wilson et al 2014;Naud et al 2017;Eriksson et al 2019;Vos et al 2019;Manjavacas et al 2021Manjavacas et al , 2022. A subsequent study is forthcoming in which we further investigate the precision that can be reached with this technique, and use injection-recovery tests to assess the extent to which known, simulated variability signals can be recovered (Sutlieff et al, in preparation).…”

“…For non-coronagraphic, ground-based observations of isolated brown dwarfs and planetary-mass objects, nonvariable stars present in the field of view have often been used as photometric references to enable many successful measurements of variability (e.g. Gelino et al 2002;Girardin et al 2013;Biller et al 2013Biller et al , 2015Radigan et al 2014;Wilson et al 2014;Naud et al 2017;Eriksson et al 2019;Vos et al 2019;Manjavacas et al 2021Manjavacas et al , 2022. However, the typically narrow field of view of ground-based coronagraphic imagers generally precludes the use of field stars as photometric references for observations of companions, and widely used focal-plane coronagraphs block the host star to enable the detection of the companion (Soummer 2005;Mawet et al 2012;Ruane et al 2018).…”