Rapidly Rotating But Not So Variable: A Brown Dwarf As Seen From Earth

Title: Tracing the Top-of-the-atmosphere and Vertical Cloud Structure of a fast-rotating late T-dwarf

Authors: Elena Manjavacas, Theodora Karalidi, Xianyu Tan, Johanna M. Vos, Ben W. P. Lew, Beth A. Biller, Natalia Oliveros-Gomez

First Author’s Institutions: AURA for the European Space Agency, MD, USA; Johns Hopkins University, MD, USA

Status: Accepted to Astrophysical Journal

While they have a reputation as “failed stars”, brown dwarfs might have more in common with their gas giant planet cousins. With swirling, patchy clouds in their atmospheres, the light curves of brown dwarfs have been seen to vary in amplitude as they rotate and bring different faces with variable cloud coverage into view. By observing brown dwarfs over their rotation periods with spectrophotometry, astronomers can simultaneously measure how much the atmosphere is changing in multiple wavelength bands. This technique means a 3D map of a brown dwarf’s atmosphere can be built, since different wavelengths probe different levels of pressure within an atmosphere. Although most spectrophotometric observations of brown dwarfs have used the Hubble Space Telescope, the authors of today’s paper employed the ground-based Keck I telescope to study 2M0050–3322, a rapidly rotating T-type brown dwarf.

Seeing One Atmosphere Through Another

Since the levels of variability in brown dwarf atmospheres can be small, it is important to characterise any other non brown dwarf sources of noise in the data. For these ground-based observations, particular care has to be taken given that the Earth’s atmosphere will also be changing over the course of the observations. Using the MOSFIRE multi-object spectrograph, the authors observed 2M0050–3322 for two of its rotation periods (around 2.5 hours in total). They also observed several other nearby stars to help calibrate the impacts of things like the local humidity and temperature on the measurements. The light curves of all the objects were obtained at multiple different infra-red wavelengths, including in the J and H bands and in a region slightly redder than the H band which the authors denote as the CH4-H2O band. By dividing each 2M0050–3322’s light curves by the median light curve of the calibration stars, 2M0050–3322’s light curves could be corrected for any impacts of the Earth’s atmosphere to find the true variability of the brown dwarf, as shown in Figure 1.

Two plots of normalised counts (y axis) vs time (x axis), with one below the other. Both plots extend from a time of 0 to a time of 2.75 hours. On the top plot: J band photometry is plotted in blue from 0 - 0.5 hours, and is scattered around normalised counts of 1, labelled as 1.48 +- 0.76%. J band spectra is plotted in green from 0 .75-1.2 hours, and is scattered around normalised counts of 1, labelled as 0.62 +- 0.18%. H band spectra is plotted in purple from 1.25-2.6 hours, and is scatttered around normalised counts of 1, labelled as 1.26+-0.93%. On the bottom plot: the blue and green as the same as above. The purple points extending across the same time now refer to the CH4-H2O band and have a larger scatter, with the last data point 2 sigma away from normalised counts =1. The purple points are labelled as 5.33+-2.02%
Figure 1: Light curves of 2M0050–3322 in J, H and CH4-H2O bands. The CH4-H2O band shows the biggest fluctuations, but all are best fit by a flat line. Figure 7 from the paper.

Over the course of their observations, the authors found that 2M0050–3322 had a minimum to maximum fluctuations of ~1% in the J and H band, and a higher 5% amplitude in the redder CH4-H2O band. This seemingly low level of variation was also confirmed by fitting flat and sinusoidal models to the light curves, with a flat line proving to best the preferred fit for all the observations. 

Models to the Rescue?

With observations to hand, the authors then sought to compare their results to models of 2M0050–3322, to see if a similar lack of variation was present. General Circulation Models (GCMs) of the thermal flux of the atmosphere predict a slightly sinusoidal light curve with almost a 1% variation, matching the amplitude seen in the J and H band observations! Meanwhile, models of the structure of clouds in the atmosphere show that 2M0050–3322 has various types of clouds at different pressures, meaning that each of the observation bands could be probing different clouds.

A visual depiction of the atmosphere of the brown dwarf. A tall rectangle is shaded from pale blue at the top to dark blue at the bottom, with the top labelled "top of the atmosphere". On the left hand side, pressure increments are labelled from 0.5 bar at the top to 50 bar at the bottom. At 20 bar, roughly a third of the way up from the bottom, an orange cloud extends across the plot and is labelled MnS. At 2 bar, in the middle of the image two green clouds are shown with a small gap between them, both labelled Na2S. At 1 bar, roughly two thirds of the way up from the bottom two small grey clouds are labelled KCl. An arrow points upwards from 20 bar to just below 2 bar, overlapping the orange cloud, and is labelled J-band, indicating where J band observations probe. Another arrow points upwards from where the J band arrow stops to around 2 bar and is labelled H band, crossing the green cloud. Finally, a third arrow extends from 2 bar up to 1 bar, crossing the green cloud and almost touching the grey cloud, and is labelled CH4-H2O band.
Figure 2: Visual representation of the cloud structure of 2M0050–3322. Three types of clouds are seen to form in the atmosphere, each at different pressure levels. KCl and Na2S clouds are seen to form at similar pressure levels as those probed by the CH4-H2O band, possibly explaining why it shows more variation than the J and H bands. Figure 15 in the paper.

Figure 2 shows that the CH4-H2O band traces similar pressure levels as those where sodium sulphide (Na2S) and potassium chloride (KCl) clouds condense. This could explain why the CH4-H2O light curves show more variability than the other bands, which reach deeper into the atmosphere and therefore do not probe these clouds. While these modelling efforts begin to explain the observations of the brown dwarf, the authors caution that longer-term monitoring is likely needed to fully explain the mysteries of 2M0050–3322.

Astrobite edited by William Balmer

Featured image credit: NASA/JPL-Caltech 

About Lili Alderson

Lili Alderson is a second year PhD student at the University of Bristol studying exoplanet atmospheres with space-based telescopes. She spent her undergrad at the University of Southampton with a year in research at the Center for Astrophysics | Harvard-Smithsonian. When not thinking about exoplanets, Lili enjoys ballet, film and baking.

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