Speaker
Description
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 |
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