A Search for Variability in Exoplanet Analogues and Low-Gravity Brown Dwarfs

Abstract

We report the results of a J-band survey for photometric variability in a sample of young, low-gravity objects using the New Technology Telescope (NTT) and the United Kingdom InfraRed Telescope (UKIRT). Surface gravity is a key parameter in the atmospheric properties of brown dwarfs and this is the first large survey that aims to test the gravity dependence of variability properties. We do a full analysis of the spectral signatures of youth and assess the group membership probability of each target using membership tools from the literature. This results in a 30 object sample of young low-gravity brown dwarfs. Since we are lacking in objects with spectral types later than L9, we focus our statistical analysis on the L0-L8.5 objects. We find that the variability occurrence rate of L0-L8.5 low-gravity brown dwarfs in this survey is 30 +16 −8 %. We reanalyse the results of Radigan (2014) and find that the field dwarfs with spectral types L0-L8.5 have a variability occurrence rate of 11 +13 −4 %. We determine a probability of 98% that the samples are drawn from different distributions. This is the first quantitative indication that the low-gravity objects are more likely to be variable than the field dwarf population. Furthermore, we present follow-up J S and K S observations of the young, planetary-mass variable object PSO 318.5-22 over three consecutive nights. We find no evidence of phase shifts between the J S and K S bands and find higher J S amplitudes. We use the J S lightcurves to measure a rotational period of 8.45 ± 0.05 hr for PSO 318.5-22.

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

“…Recent studies of field planetary-mass objects (e.g., Biller et al 2015;Vos et al 2018;Vos et al 2019) and planetarymass companions (e.g., Zhou et al 2016;Manjavacas et al 2017) begun to explore this parameter space. Encouragingly, low surface gravity objects are found more likely to be variable (e.g, Metchev et al 2015;Vos et al 2019), thus are better targets for rotational modulation studies. Furthermore, two objects out of three from the sample of Apai et al (2017) that demonstrated planetary waves are classified as low-gravity or planetary-mass objects.…”

Atmospheric Dynamics of Hot Giant Planets and Brown Dwarfs

“…Fourth, many brown dwarfs exhibit IR variability over rotational timescales, which probably results from regional patches of differing temperature and cloud opacity-and therefore differing IR fluxes to space-moving in and out of view as the brown dwarf rotates (left panel of Fig. 23, Artigau et al 2009;Radigan et al 2012;Apai et al 2013;Wilson et al 2014;Buenzli et al 2015;Metchev et al 2015;Lew et al 2016;Vos et al 2019;; for reviews, see Biller 2017 and Artigau 2018). Interestingly, the shape of the lightcurves changes on short timescales, indicating that the spatial pattern of patchiness changes over time-presumably due to time-evolving organization of turbulence, vortices, or other atmospheric structures over the globe.…”

On the Correlation between L Dwarf Optical and Infrared Variability and Radio Aurorae

“…Metchev et al (2015) identified a rotation period of 2.7±0.1 hr and saw variability in the Spitzer Infrared Array Camera (IRAC) channels 1 (3.6 μm) and 2 (4.6 μm) with magnitude changes of 0.56±0.03% and 0.87±0.09%, respectively. Additionally, Enoch et al (2003) observed K band variability of 0.10±0.02 mag and no variability in the J band, the latter of which was confirmed by Vos et al (2019). Schmidt et al (2015) placed an upper limit on the Hα activity of 2M0103+19 of log(L Hα /L bol )<−5.96.…”

“…The authors also reported a rotation period of ∼24 hr. Vos et al (2019) observed no variability in the J band. There have been no Hα observations of 2M1615+49.…”

“…measured a rotation period of 3.7±0.8 hr based on J band variability with an amplitude of 0.6±0.1%. Vos et al (2019) similarly reported a J band variability amplitude of 0.7±0.3. Schmidt et al (2015) placed an upper limit on the Hα activity of 2M1425-36 of log(L Hα /L bol )<−5.03.…”