Stellar magnetic fields, and their tracers, are fundamentally variable on timescales ranging from minutes to decades to Gyr. Time domain data allows us to study stellar rotation, star spot lifetimes and configurations, stochastic eruptions like flares and CMEs, and shifts in stellar dynamos. In the current era of surveys, we can trace these phenomena on large numbers of cool stars, detailing the changes in magnetic activity with mass, rotation, and age.
For the last several decades, most magnetic activity work focused on spectroscopic or X-ray activity tracers, with CaII H&K cycles measured for a subset of nearby solar-type stars. The Kepler/K2 missions represented a giant step forward in studies of flares and starspot signatures: these missions' exquisitely detailed light curves facilitated studies of magnetism in tens of thousands of stars extending down into the M dwarfs (McQuillan et al. 2014, Douglas et al. 2016, 2019). TESS, Gaia, and new ground-based surveys will provide coverage of nearly the entire sky, while LSST will yield long-baseline, multi-wavelength coverage. We propose a session to highlight this ongoing time-domain work and discuss ideas and methods for studying magnetic activity with upcoming surveys.
From Rotation Periods to Starspot Maps
The Kepler/K2 mission provided a wealth of rotation periods for Sun-like and lower-mass stars, representing an explosive step forward for gyrochronology. K2 in particular targeted stars at both very young and very old ages, and yielded new insights into very low-mass stars in particular. Rotation analysis from Kepler/K2 is mostly complete, and magnetic analysis is expanding into new areas. In particular, the precise photometry has facilitated detailed maps of stellar surfaces and analyses of differential rotation and starspot evolution (Davenport et al. 2015, Rebull et al 2016, Roettenbacher & Vida 2018).
As we move into the future, new surveys offer the opportunity to survey stars across the entire sky with a variety of cadences and baselines. TESS will survey nearly the entire sky over its prime and extended missions, and near the ecliptic poles it will obtain year-long light curves for nearby M dwarfs. Gaia has also yielded long-baseline data for stars across the entire sky, and LSST will provide multi-band photometry over 10 years that will complement higher-cadence single-band surveys. We plan to solicit talks not only on current missions such as TESS and Gaia, but also to encourage speakers to discuss their plans for incorporating future mission data.
Future missions like PLATO and LSST will continue this innovative trend, with new techniques to produce more precise and longer-baseline data. LSST in particular will observe the whole sky in multiple bands, permitting unprecedented multiwavelength studies of starspots. Furthermore, combining surveys will yield longer baselines for more and more stars, allowing an expansion of the time domain studies to multi-year and decadal activity cycles. From these, we may detect more magnetic field cycling like the Sun's eleven year cycle, and learn to understand how common or uncommon that phenomenon might be across the main sequence. We will recruit speakers who plan to use these surveys to discuss challenges and paths forward in this new multi-band, long-baseline domain.
The Power of Flares in Surveys
Flare science is experiencing a renaissance. From Kepler's long-baseline to TESS's sky coverage, as well as ground-based contributions contributions from MEarth, NGTS and ASAS-SN, we now have a wealth of flare observations with which to characterize stellar magnetic fields (Ilin et al. 2019). Flares have long been known to exist on stars across the main sequence, on a variety of objects with different ages and magnetic field strengths, but we can now begin to classify how the presence and frequency of flares varies with mass, and rotation (Davenport et al. 2019). In particular, NGTS and TESS have observed flares on more types of stars than before, including later ultracool dwarfs and fast rotators that may have different dynamo mechanisms than Sun-like stars (e.g., Jackman et al. 2019).
Also, with LSST on the horizon, the community will need to determine how to robustly analyze sparse cadence flares. Current surveys such as ASAS-SN offer insights into analyzing flares where we have only one or a handful of data points during an extended event. How can flares be detected, characterized, and used to diagnose magnetic field strength and age with a minimum of observations? Our proposed splinter session will include discussions of these challenges and how to handle them observationally and statistically.
Relevant and Timely
This splinter is strongly linked to the central topics of Cool Stars 21. The Cool Stars conferences have a long tradition of including some of the most exciting results on Solar and stellar magnetic activity. Our splinter topic is focused on some of the newest results in this area, and will bridge several focus areas for Cool Stars 21. In particular, our focus on surveys will link magnetism with environments and stellar populations because the new era of all-sky surveys bridges many different dynamical environments. And since we will focus specifically on surveys and on prospects for future studies, our splinter session will not significantly duplicate the content of the plenary sessions. Furthermore, with our focus on the future, we plan to emphasize the participation of early-career astronomers both in the talks and in discussions.
Our topic is also timely. The TESS mission is now in its second year, and by the time of Cool Stars 21 it will have surveyed nearly the entire sky. Gaia will also provide light curves for over 105 stars, complementing TESS's shorter baseline. And in the next few years, LSST and PLATO will come online and provide additional windows into time-domain studies of stellar magnetism. As we take stock of the accomplishments of Kepler and other past surveys, it's also important to also look to the future and discuss as a community how we will best take advantage of upcoming missions and ongoing data releases.