Rotation periods and photometric variability of rapidly rotating ultra-cool dwarfs

TL;DR

This study measured rotation periods and photometric variability in ultra-cool dwarfs, revealing a high variability rate and a correlation between rapid rotation and linear polarization, enhancing understanding of their atmospheric properties.

Contribution

It provides new rotation period measurements for 18 ultra-cool dwarfs and links rotation speed with photometric variability and polarization, offering insights into their atmospheric dynamics.

Findings

50% of the sample showed photometric variability at 2σ level.

65% of fast-rotating dwarfs exhibit variability with amplitudes ≤20 mmag.

Shorter rotation periods correlate with higher linear polarization.

Abstract

We used the optical and near-infrared imagers located on the Liverpool, the IAC80, and the William Herschel telescopes to monitor 18 M7$-$L9.5 dwarfs with the objective of measuring their rotation periods. We achieved accuracies typically in the range $\pm$1.5$-$28 mmag by means of differential photometry, which allowed us to detect photometric variability at the 2$\sigma$ level in the 50\% of the sample. We also detected periodic modulation with periods in the interval 1.5$-$4.4 h in 9 out of 18 dwarfs that we attribute to rotation. Our variability detections were combined with data from the literature; we found that 65$\,\pm\,$18 $\%$ of M7$-$L3.5 dwarfs with $v\,$sin$\,i\ge30$ km s$^{-1}$ exhibit photometric variability with typical amplitudes $\le$20 mmag in the $I$-band. For those targets and field ultra-cool dwarfs with measurements of $v\,$sin$\,i$ and rotation period we derived the expected inclination angle of their rotation axis, and found that those with $v\,$sin$\,i\ge30$ km s$^{-1}$ are more likely to have inclinations $\gtrsim40$ deg. In addition, we used these rotation periods and others from the literature to study the likely relationship between rotation and linear polarization in dusty ultra-cool dwarfs. We found a correlation between short rotation periods and large values of linear polarization at optical and near-infrared wavelengths.