2019MNRAS.483..480V

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 per cent. 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 per cent. We determine a probability of 98 per cent 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 light curves to measure a rotational period of 8.45 ± 0.05 h for PSO 318.5-22.