The Solar Neighborhood. LV. M Dwarf Twin Binaries -- One in Five Twin Sibling Pairs Are Mismatched in Activity and/or Rotation

Abstract

We report on a study of 36 pairs of `twin' M dwarfs in wide binaries and assess how similarly the stars behave. Stars in each twin pair have BP, RP, $J$, $H$, and $K_s$ differing by $<$0.10 mag, mass estimates matching within $<$3%, and presumably the same age and composition. We utilize short- and long-term photometry, multi-epoch spectroscopy, and archival data to measure rotation periods, photometric activity levels, and H$\alpha$ equivalent widths for many systems. Speckle imaging, radial velocities, and long-term astrometry are used to identify unresolved companions, yielding three systems with unseen components. Among the 33 remaining twin systems, numerous remarkable pairs show nearly identical rotation rates and activity levels between their twin components, including cases throughout the lower main sequence and across a broad range of rotation-activity parameter space. In contrast, mismatches with $>$25% differences exist in rotation period for $21\%_{-7\%}^{+14\%}$ of twin pairs, in rotation amplitude for $67\%_{-15\%}^{+10\%}$ of pairs, in multi-year photometric variability for $33\%_{-9\%}^{+12\%}$ of pairs, and in H$\alpha$ activity for $21\%_{-6\%}^{+9\%}$ of pairs, with fully convective systems generally mismatched more often. Thus, roughly one out of five M dwarf twin sets does not match in rotation and/or activity despite otherwise identical fundamental parameters. Furthermore, we compile three key systems showing larger relative active/inactive H$\alpha$ mismatches. We propose the various mismatches likely stem from factors such as dynamo stochasticity, activity cycles, formative disk aspects, and/or star-planet interactions, depending on the system. These well-vetted twins offer ripe targets for many future investigations.