Tannock and colleagues presented the discovery of rapid photometric variability in three ultra-cool dwarfs from long-duration monitoring with the Spitzer Space Telescope. The T7, L3.5, and L8 dwarfs have the shortest photometric periods known to date: 1.080 h, 1.14 h, and 1.23 h, respectively. Their study recently appeared in Journal Zenodo.
They compared the near-infrared spectra to photospheric models to determine the objects’ fundamental parameters and radial velocities (RVs) (which is shown in Table 1 below).
All three objects are likely substellar. At a spectral type of L3.5, 2MASS J0407+1546 is the warmest, have fairly cloudy atmosphere and potentially most massive among their three L and T dwarfs. While, based on T7 spectral type, researchers expect that 2MASS J0348+6022 have relatively clear and cloudless atmosphere and that of 2MASS J1219+3128 have dusty atmosphere. Their rapid rotations and Hα emissions may well indicate the presence of an aurora. Based on radio detections of three L and T dwarfs with short (1.5 h–2.2 h) rotation periods, rapid rotation is key to powering auroral emissions via the electron synchrotron maser instability.
“They are excellent candidates for seeking auroral radio emission, which has been linked to rapid rotation in ultra-cool dwarfs.“— told tannock, lead author of the study.
They confirmed the rapid rotation through moderate resolution infrared spectroscopy that reveals projected rotational velocities of 103.5 km s¯1 for 2MASS J0348−6022 (T7), 79.0 km s¯1 for 2MASS J1219+3128 (L8) and 82.6 km s¯1 for 2MASS J0407+1546 (L3.5), these are among the most rapidly spinning ultra-cool dwarfs known to date. They also found that the objects have oblateness factors of between 5% and 8%, which also makes them the most oblate field ultra-cool dwarfs known to date and ranks them among the best targets for seeking net optical or infrared polarization.
They also considered the role of the centrifugal acceleration on surface gravity, and found that while the effect can be significant, at .0.1 dex in surface gravity it can be difficult to discern with current photospheric models.
Including this paper, 78 L-, T-, and Y-dwarf rotation periods have now been measured. The clustering of the shortest rotation periods near 1 h suggests that brown dwarfs are unlikely to spin much faster.
Featured image: Pixel phase-corrected flux at 3.6 and 4.5 as a function of centroid position in both the x- and y-directions. The centroids are measured relative to the average centroids across all exposures. The Pearson correlation coefficients (r) are given in each panel and they found that there is no correlation between the flux and centroid positions on the detector. They concluded that there is no residual periodic effect on the photometry after correcting for Spitzer’s pointing wobble. © Tannock et al.
Reference: Tannock, Megan E., Metchev, Stanimir, Heinze, Aren, Miles-Páez, Paulo, Gagné, Jonathan, Burgasser, Adam, … Plavchan, Peter. (2021). Weather on Other Worlds. V. The Three Most Rapidly Rotating Ultra-Cool Dwarfs. Presented at the The 20.5th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun (CS20.5), virtually anywhere: Zenodo. https://zenodo.org/record/4560684 http://doi.org/10.5281/zenodo.4560684
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