15-20 March 2020
Europe/Berlin timezone

The X-ray activity-rotation-age relation of M dwarfs

Not scheduled




Enza Magaudda


The activity-rotation relation of M dwarfs provides observational evidence of the stellar dynamo, which are poorly understood for low-mass stars especially in the fully convective regime. Effects of rotational evolution are also encoded in the empirical rotation-activity relation, because stellar rotation slows down throughout a star's main-sequence life and, consequently, the dynamo efficiency also decreases over time causing a decrease of X-ray luminosity.
From past studies it is known that the X-ray activity-rotation relation splits into two regimes with different rotational dependence: the saturated regime for faster rotating stars and the unsaturated regime for slower rotating stars, with a transition between the two regimes at a rotation period $\rm P_{rot}\sim10$\,d.
I present our study of the behaviors in both regimes with new XMM-Newton and Chandra X-ray observations and new rotation periods from the Kepler Two-Wheel (K2) mission.
We have updated data sets from the previous literature. The combined database has allowed us to study the activity-rotation relation of M dwarfs in three different mass ranges.
Our new findings include:
1. a non-constant X-ray luminosity ($\rm L_{x}$) in the saturated regime,
2. a mass-dependence of the slope in the unsaturated regime
3. a remarkable gap with a paucity of objects at $\rm L_{x}/L_{bol} \sim 10^{-4}$, probably associated to a phase of stalled rotational evolution followed by an episode of rapid spin-down,
4. different slopes in the unsaturated regime for different parametrizations of the Rossby number ($\rm R_{O} = P_{rot}/\tau_{conv}$, where $\rm \tau_{conv}$ is the convective turnover time).
Combining our best fit parameters from the $\rm L_{x}-P_{rot}$ analysis with spin-down models we constructed the first $\rm L_{x}-$age relation for M dwarfs and we compared it to the activity of M stars with known age.
Finally I show how we use the eFEDS field of eROSITA combined with new rotation periods from TESS to obtain additional constraints on the activity-rotation relation of M dwarfs, especially in the critical transition between the saturated and the unsaturated regime.

Presenter status eROSITA consortium member

Primary author


Beate Stelzer (IAAT) Dr Stefanie Raetz (Institut für Astronomie und Astrophysik, Eberhard-Karls Universität Tübingen, Sand 1, D-72076 Tübingen, Germany) Prof. Kevin Covey (Department of Physics & Astronomy, Western Washington University, Bellingham WA 98225-3164, USA) Prof. Sean Matt (University of Exeter, Department of Physics & Astronomy, Physics Bldg., Stocker Road, Exeter EX4 4QL, UK) Prof. Alexander Scholz (SUPA, School of Physics & Astronomy, University of St Andrews, North Haugh, St Andrews, KY169SS, UK)

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