Cloud Atlas: High-contrast Time-resolved Observations of Planetary-mass Companions

Abstract

Directly-imaged planetary-mass companions offer unique opportunities in atmospheric studies of exoplanets. They share characteristics of both brown dwarfs and transiting exoplanets, therefore, are critical for connecting atmospheric characterizations for these objects. Rotational phase mapping is a powerful technique to constrain the condensate cloud properties in ultra-cool atmospheres. Applying this technique to directly-imaged planetarymass companions will be extremely valuable for constraining cloud models in low mass and surface gravity atmospheres and for determining the rotation rate and angular momentum of substellar companions. Here, we present Hubble Space Telescope Wide Field Camera 3 near-infrared time-resolved photometry for three planetary-mass companions, AB Pic B, 2M0122B, and 2M1207b. Using two-roll differential imaging and hybrid point spread function modeling, we achieve sub-percent photometric precision for all three observations. We find tentative modulations (<2 ) for AB Pic B and 2M0122B but cannot reach conclusive results on 2M1207b due to strong systematics. The relatively low significance of the modulation measurements cannot rule out the hypothesis that these planetary-mass companions have the same vertical cloud structures as brown dwarfs. Our rotation rate measurements, combined with archival period measurements of planetary-mass companions and brown dwarfs do not support a universal mass-rotation relation. The high precision of our observations and the high occurrence rates of variable low-surface gravity objects encourage high-contrast time-resolved observations with the James Webb Space Telescope.

Cloud Atlas: High-precision HST/WFC3/IR Time-resolved Observations of Directly Imaged Exoplanet HD 106906b

“…Additionally, the 1.4 m water absorption or the F139M band sometimes show reduced rotational modulation amplitude (e.g., Apai et al 2013), due to water vapor opacity elevating the photosphere at this wavelength. Therefore, rotational modulations only appearing in the band with the shortest wavelength of our observation is qualitatively consistent with model predictions and previous observations, particularly those for planetary-mass companions Zhou et al 2019). If we assume that the wavelength dependence of HD106906b's rotational modulations is the same as that measured in 2M1207b ) as 2M1207b (a mid-L-type planetary-mass companion) is HD106906b's close analog that also has modulation detected, we would expect the modulation amplitude in the F153M band to be 0.6%.…”

“…HD106906b's flux intensity is measured by PSF fitting to the primary-subtracted images. Details of the PSF-fitting procedures can be found in Zhou et al (2019). We construct 9× over-sampled PSFs using the TinyTim PSF modeling software (Krist 1995).…”

“…From the astrophysical perspective, we can qualitatively evaluate the likelihood for HD106906b, an early L-type planetary-mass companion to be rotationally modulated only in the F127M band but not in the other two bands. Multiwavelength and time-resolved observations of ultra-cool dwarfs have found that the rotational modulations for the majority brown dwarfs and planetary-mass companions are wavelength-dependent and have higher amplitudes at shorter wavelengths than longer wavelengths (e.g., Apai et al 2013;Yang et al 2015;Zhou et al 2016;Schlawin et al 2017;Zhou et al 2019). These findings are consistent with a model prediction based on Mie-scattering calculation (Hiranaka et al 2016;Lew et al 2016;Schlawin et al 2017).…”

A measurement of the wind speed on a brown dwarf

“…Gas giant exoplanets and brown dwarfs (objects with masses of 13-72 times that of Jupiter) rotate on timescales of hours to days (1)(2)(3)(4). If there are any inhomogeneous features at the top of their atmospheres, such as clouds, the rotational modulation causes quasi-periodic variability in their brightness.…”

Cloud Atlas: Weak Color Modulations Due to Rotation in the Planetary-mass Companion GU Psc b and 11 Other Brown Dwarfs

“…A particularly important probe of atmospheric properties is time-resolved high-precision (subpercent level) spectrophotometry, that can-through the rotation of the target-explore nonuniform brightness distribution in an atmosphere with fixed gravity and interior temperature. Such rotational mapping studies have been used successfully to constrain cloud properties in ultracool atmospheres, including those with planetary masses (e.g., Apai et al 2013;Buenzli et al 2014;Metchev et al 2015;Zhou et al 2016Zhou et al , 2019Apai et al 2017;Biller 2017;Manjavacas et al 2019;Miles-Páez et al 2019). The wavelength dependence of rotation modulations sheds light on the variations of cloud particle sizes, molecular abundances, and photospheric temperatures.…”