OBSERVED VARIABILITY AT 1 and 4 μm IN THE Y0 BROWN DWARF WISEP J173835.52+273258.9

Abstract

We have monitored photometrically the Y0 brown dwarf WISEP J173835.52+273258.9 (W1738) at both nearand mid-infrared wavelengths. This 1 Gyr old 400 K dwarf is at a distance of 8pc and has a mass around 5 M Jupiter . We observed W1738 using two near-infrared filters at λ≈1 μm, Y and J, on Gemini Observatory and two mid-infrared filters at λ≈4 μm, [3.6] and [4.5], on the Spitzer observatory. Twenty-four hours were spent on the source by Spitzer on each of 2013 June 30 and October 30 UT. Between these observations, around 5 hr were spent on the source by Gemini on each of 2013 July 17 and August 23 UT. The mid-infrared light curves show significant evolution between the two observations separated by 4 months. We find that a double sinusoid can be fit to the [4.5] data, where one sinusoid has a period of 6.0±0.1 hr and the other a period of 3.0±0.1 hr. The nearinfrared observations suggest variability with a ∼3.0 hr period, although only at a 2σ confidence level. We interpret our results as showing that the Y dwarf has a 6.0±0.1 hr rotation period, with one or more large-scale surface features being the source of variability. The peak-to-peak amplitude of the light curve at [4.5] is 3%. The amplitude of the near-infrared variability, if real, may be as high as 5%-30%. Intriguingly, this size of variability and the wavelength dependence can be reproduced by atmospheric models that include patchy KCl and Na 2 S clouds and associated small changes in surface temperature. The small number of large features, as well as the timescale for evolution of the features, is very similar to what is seen in the atmospheres of the solar system gas giants.

Variability of Brown Dwarfs

“…Only Spitzer is sufficiently sensitive beyond 3 µm to allow Y dwarf studies at high photometric accuracy. Cushing et al (2016) and Leggett et al (2016) reported preliminary results of a Y dwarf variability survey, with the detection of similar variability amplitudes (∼3% at 4.5 µm) and periods (6−8.5 h) in WISEP J140518.40 + 553421.4(W 1405) and WISEP J173835.53 + 273258.9. The very red 3.6 µm to 4.5 µm color of Y dwarfs makes variability detection at 3.6 µm challenging, and only one epoch of the W1405 observations shows an unambiguous detection at 3.6 µm, leading to a 3.6 to 4.5 µm amplitude ratio close to unity.…”

“…Early effort concentrated on late-M to mid-Ls and did not include latertype objects because no relatively bright T dwarfs were known. This has changed due in large part to the SDSS and 2MASS surveys (e.g., Burgasser et al 1999;Leggett et al 2000), allowing and variability searches to be extended to cooler objects. Enoch et al (2003) performed a K s survey of L and T dwarfs with a sparse sampling of a few visits distributed over about a month.…”

Spitzer Light Curves of the Young, Planetary-mass TW Hya Members 2MASS J11193254–1137466AB and WISEA J114724.10–204021.3

“…To investigate the rotation periods of field-age brown dwarfs, we combine the rotation periods from Metchev et al (2015) with the compilations of rotation periods in Crossfield (2014) and Vos et al (2017), the rotation period of Luhman 16A (4.5-5.5 hr; Buenzli et al 2015) and Luhman 16B (5.05±0.10 hr; Burgasser et al 2014, 4.87±0.01 hr; Gillon et al 2013), and the two known rotation periods for the Y-type brown dwarfs WISE J140518.39+553421.3 (8.54±0.08 hr; Cushing et al 2016) and WISEP J173835.52+273258.9 (6.0±0.1 hr; Leggett et al 2016). To ensure we do not include low-mass stellar sources in this comparison, we limit our field-age sample to those objects with spectral types later than L2 (Dieterich et al 2014;.…”

3.8 μm Imaging of 400–600 K Brown Dwarfs and Orbital Constraints for WISEP J045853.90+643452.6AB

“…Interestingly, measurements of the atmospheres of the solar system giant planets show the upper layers to be warmer than expected; heat sources such as breaking gravity waves have been invoked (Matcheva & Strobel 1999;O'Donoghue et al 2016). The same effect may be present in cold brown dwarfs, which have similar radii and rotation periods Leggett et al 2016;Manjavacas et al 2019), and highly dynamic atmospheres (Apai et al 2017;Showman & Kaspi 2013).…”