TESS observations of southern ultra fast rotating low mass stars

TL;DR

This study systematically searches for ultra fast rotating low mass stars in the southern hemisphere using TESS data, revealing a decrease in flaring activity among the fastest rotators, contrary to expectations.

Contribution

It provides the first large-scale identification of ultra fast rotators among low mass stars using TESS and Gaia data, and investigates their flare activity.

Findings

Flares decrease significantly at periods <0.2 days.

Fewer flares are observed in the fastest rotators, contrary to expectations.

Potential reasons include binary membership or wavelength-dependent emission.

Abstract

In our previous study of low mass stars using TESS, we found a handful which show a periodic modulation on a period <1 d but also displayed no flaring activity. Here we present the results of a systematic search for Ultra Fast Rotators (UFRs) in the southern ecliptic hemisphere which were observed in 2 min cadence with TESS. Using data from Gaia DR2, we obtain a sample of over 13,000 stars close to the lower main sequence. Of these, we identify 609 stars which lie on the lower main sequence and have a periodic modulation <1 d. The fraction of stars which show flares appears to drop significantly at periods <0.2 d. If the periods are a signature of the rotation rate, this would be a surprise, since faster rotators would be expected to have a stronger magnetic field and, therefore, produce more flares. We explore possible reasons for our finding: the flare inactive stars are members of binaries, in which case the stars rotation rate could have increased as the binary orbital separation reduced due to angular momentum loss over time, or that enhanced emission occurs at blue wavelengths beyond the pass band of TESS. Follow-up spectroscopy and flare monitoring at blue/ultraviolet wavelengths of these flare inactive stars are required to resolve this question.