Fast-evolving weather for the coolest of our two new substellar neighbours

Abstract

We present the results of intense photometric monitoring in the near-infrared (∼0.9 µm) with the TRAPPIST robotic telescope of the newly discovered binary brown dwarf WISE J104915.57-531906.1, the third closest system to the Sun at a distance of only 2 pc. Our twelve nights of time-series photometry reveal a quasi-periodic (P = 4.87 ± 0.01h) variability with a maximum peak-peak amplitude of ∼11% and strong night-to-night evolution. We attribute this variability to the rotational modulation of fast-evolving weather patterns in the atmosphere of the coolest component (∼T1-type) of the binary. No periodic signal is detected for the hottest component (∼L8-type). For both brown dwarfs, our data allow us to firmly discard any unique transit during our observations for planets ≥2 R ⊕ . For orbital periods smaller than ∼9.5 h, transiting planets are excluded down to an Earth-size.

WISE J072003.20-084651.2: AN OLD AND ACTIVE M9.5 + T5 SPECTRAL BINARY 6 pc FROM THE SUN

“…Data were reduced as described in Gillon et al (2013). After a standard pre-reduction (bias, dark, flatfield correction), aperture photometry was performed using IRAF/DAOPHOT2 (Stetson 1987).…”

A global cloud map of the nearest known brown dwarf

“…12) revealed two brown dwarfs only 2 parsecs away, making these the closest objects to the Solar system after the alpha Centauri system and Barnard's star. Both of these newly-discovered brown dwarfs are near the dust clearing temperature 13,14 , and one (Luhman 16B) exhibits strong temporal variability of its thermal radiation consistent with a rotation period of 4.9 hr 15 . Luhman 16AB's proximity to Earth makes these the first substellar objects bright enough to be studied at high precision and high spectral resolution on short timescales, so we observed both of these brown dwarfs for five hours (one rotation period of Luhman 16B) using the CRIRES spectrograph 16 at ESO's Very Large Telescope to search for spectroscopic variability.…”

“…The two objects have similar spectra but the absorption lines are broader for the B component: it exhibits a projected equatorial rotational velocity of 26.1 +/-0.2 km/s, vs. 17.6 +/-0.1 km/s for Luhman 16A. Taking Luhman 16B's rotation period 15 and considering that evolutionary models predict these objects to be 1.0+/-0.2 times the radius of Jupiter 17 , Luhman 16B's rotation axis must be inclined ≲30 deg from the plane of the sky; i.e., we are viewing this brown dwarf nearly equator-on. If the two brown dwarfs' axes are closely aligned (like those of the planets in our Solar system) then Luhman 16A rotates more slowly than Luhman 16B and the objects either formed with different initial angular momentum or experienced different accretion or spin-braking histories.…”

Cloud Atlas: Discovery of Rotational Spectral Modulations in a Low-mass, L-type Brown Dwarf Companion to a Star

“…To date, published light curves of various ultracool dwarfs show single or double peaks, whose shapes vary with time, sometimes even within a single rotation (Artigau et al 2009;Radigan et al 2012;Apai et al 2013;Gillon et al 2013;Buenzli et al 2015;Karalidi et al 2015;Metchev et al 2015;Lew et al 2016;Yang et al 2016). These observations are interpreted as single or multiple cloud structures in the atmosphere of brown dwarfs that can evolve at short timescales.…”