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We present a photometric variability survey of young planetary-mass objects using the New Technology Telescope in the JS and KS bands. Surface gravity plays an important role in the atmospheric structure of brown dwarfs, as young low-gravity L dwarfs have a higher variability rate than field L dwarfs. In this study, we extend variability studies to young T-type planetary-mass objects and investigate the effects of surface gravity on the variability of L and T dwarfs across a large sample. We conduct continuous monitoring for 18 objects with spectral types from L5 to T8 and detect four new variables and two variable candidates. Combining with previous variability surveys of field and young L and T objects, we find that young objects tend to be more variable than field objects within peak-to-peak variability amplitude ranges of 0.5–10 per cent and period ranges of 1.5–20 h. For the first time, we constrain the variability rate of young T dwarfs to be  |$56_{-18}^{+20}$|  per cent compared to  |$25_{-7}^{+8}$|  per cent for field T dwarfs. Both field and young samples have higher variability rates at the L/T transition than outside the L/T transition. The differences in the variability rates between field and young samples are about 1σ and therefore larger sample sizes are needed to confirm and refine the results. Besides the L/T transition, young L dwarfs with strong variability tend to assemble in a narrow spectral type range of L6–L7.5. This work supports the critical role of surface gravity on the atmospheric structure from L to T spectral types. [ABSTRACT FROM AUTHOR]

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 |$30^{+16}_{-8}{{\ \rm per\ cent}}$|⁠. 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 |$11^{+13}_{-4}{{\ \rm per\ cent}}$|⁠. We determine a probability of |$98{{\ \rm per\ cent}}$| 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 JS and KS 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 JS and KS bands and find higher JS amplitudes. We use the JS light curves to measure a rotational period of 8.45 ± 0.05 h for PSO 318.5–22. [ABSTRACT FROM AUTHOR]

We present the detection of [3.6 μm] photometric variability in two young, L/T transition brown dwarfs, WISE J004701.06+680352.1 (W0047) and 2MASS J2244316+204343 (2M2244) using the Spitzer Space Telescope. We find a period of 16.4 ±0.2 h and a peak-to-peak amplitude of 1.07 ± 0.04 per cent forW0047, and a period of 11±2 h and amplitude of 0.8 ± 0.2 per cent for 2M2244. This period is significantly longer than that measured previously during a shorter observation. We additionally detect significant J-band variability in 2M2244 using the Wide-Field Camera on UKIRT. We determine the radial and rotational velocities of both objects using Keck NIRSPEC data. We find a radial velocity of -16.0+0.8 -0.9 km s-1 for 2M2244, and confirm it as a bona fide member of the AB Doradus moving group. We find rotational velocities of v sin i = 9.8 ± 0.3 and 14.3 -1.5+1.4 km s-1 for W0047 and 2M2244, respectively. With inclination angles of 85-9+5◦ and 76+-2014◦, W0047 and 2M2244 are viewed roughly equator-on. Their remarkably similar colours, spectra and inclinations are consistent with the possibility that viewing angle may influence atmospheric appearance. We additionally present Spitzer [4.5 μm] monitoring of the young, T5.5 object SDSS111010+011613 (SDSS1110) where we detect no variability. For periods <18 h, we place an upper limit of 1.25 per cent on the peak-to-peak variability amplitude of SDSS1110. [ABSTRACT FROM AUTHOR]