WHY WE SHOULD KEEP POKING THE BEEHIVE: SIMULTANEOUS K2 AND SPECTROSCOPIC OBSERVATIONS OF PRAESEPE

FORTY-FIVE YEARS AGO ANDREW SKUMANICH PROPOSED THAT ROTATION AND CHROMOSPHERIC ACTIVITY A PROXY FOR MAGNETIC FIELD STRENGTH DECREASE AT ROUGHLY THE SAME RATE FOR SOLAR-TYPE STARS BETWEEN THE AGES OF 100 AND 5 GYR. THIS IS THOUGHT TO RESULT FROM A FEEDBACK LOOP IN WHICH WINDS CARRY ANGULAR MOMENTUM AWAY FROM THE STAR BRAKING ITS ROTATION AND DIMINISHING THE SHEAR BETWEEN ITS RADIATIVE AND CONVECTIVE ZONES WHICH IS RESPONSIBLE FOR GENERATING THE MAGNETIC FIELD. THE RESULTING WEAKER MAGNETIC FIELD THEN PRODUCES WEAKER WINDS; THESE CONTINUE TO SPIN DOWN THE STAR AND FURTHER WEAKEN ITS MAGNETIC FIELD BUT AT A DIMINISHED RATE. WHILE WORK IN THE ENSUING DECADES HAS CONFIRMED THE GENERAL PICTURE DESCRIBED BY SKUMANICH THE DETAILS STILL ELUDE US. BUT WE CONTINUE TO TRY TO CONSTRAIN THE DEPENDENCE OF ROTATION AND ACTIVITY ON AGE BECAUSE A WELL-CALIBRATED AGE-ROTATION-ACTIVITY RELATION (ARAR) WOULD BE VERY VALUABLE PARTICULARLY FOR LOWER-MAS STARS. EARTH-LIKE PLANETS ARE MOST LIKELY TO BE DISCOVERED IN THE HABITABLE ZONES OF NEARBY OLD M-TYPE FIELD STARS (CF. THE 1.3 EARTH-MASS PLANET AROUND PROXIMA CENTAURI). UNDERSTANDING THESE PLANETS' RADIATION ENVIRONMENTS AND POTENTIAL HABITABILITY DEMANDS A ROBUST ARAR THAT COULD BE APPLIED TO THEIR PARENT STARS. IF WE KNEW HOW ROTATION OR MAGNETIC ACTIVITY TRACERS SUCH AS X-RAY OR H-ALPHA EMISSION DEPEND ON AGE A MEASUREMENT OF ANY ONE OF THESE QUANTITIES COULD BE USED TO DETERMINE AN ACCURATE AGE FOR ANY ISOLATED FIELD STAR. CURRENTLY HOWEVER WE INVERT THE PROCESS BY E.G. ADOPTING CANONICAL AGES FOR FIELD STARS TO CONSTRAIN THE BEHAVIOR OF THE ARAR AT THE OLDEST AGES. FEW OPEN CLUSTERS HAVE BEEN SYSTEMATICALLY SURVEYED FOR ROTATION AND STELLAR ACTIVITY AND THERE IS A SCARCITY OF ACCESSIBLE CLUSTERS OLDER THAN ~150 MYR. THE HYADES AND PRAESEPE (BOTH ~650 MYR) SERVE AS THE ONLY ANCHORS FOR OUR UNDERSTANDING OF STELLAR ACTIVITY ACROSS A WIDE RANGE OF MASSES BETWEEN A FEW 100 MYR AND THE AGE OF FIELD STARS (>2 GYR). WE HAVE ALREADY EXTENSIVELY SURVEYED BOTH CLUSTERS FOR ROTATION INCLUDING WITH K2 AND FOR X-RAY AND H-ALPHA EMISSION (AG EROS ET AL. 2011 DOUGLAS ET AL. 2014 2016 AND IN PREP). HERE WE PROPOSE TO COMPLETE THE ROTATIONAL CENSUS FOR PRAESEPE BY OBSERVING ~500 MEMBERS IN K2 CAMPAIGN 16. THIS SHOULD YIELD ~150 NEW PERIODS FOR THE CLUSTER; MORE IMPORTANTLY COMBINING DATA FROM CAMPAIGNS 5 AND 16 WITH PREVIOUS GROUND-BASED PERIOD MEASUREMENTS IS A UNIQUE OPPORTUNITY TO EXAMINE ACTIVITY CYCLES IN AN ADDITIONAL ~350 STARS. TO OUR KNOWLEDGE THERE HAVE BEEN NO LARGE-SCALE ATTEMPTS TO REVISIT PERIOD MEASUREMENTS FOR CLUSTER STARS AND THESE OBSERVATIONS WILL PROVIDE US WITH VALUABLE INSIGHT ON THE STABILITY OF THE AGE-ROTATION RELATION OVER A BASELINE OF AT LEAST ~3 YEARS (AND ~A DECADE FOR SOME STARS). IN ADDITION WE PLAN TO CONDUCT SIMULTANEOUS MULTI-EPOCH SPECTROSCOPY OF ~200 PRAESEPE M DWARFS LIKELY WITH THE HYDRA MULTI-OBJECT SPECTROGRAPH ON WIYN. THIS WILL ALLOW US TO MEASURE H-ALPHA EMISSION THE MAIN CHROMOSPHERIC ACTIVITY INDICATOR FOR LOW-MASS STARS AS A FUNCTION OF ROTATIONAL PHASE. THESE DATA WILL TEST WHETHER THE OBSERVED H-ALPHA VARIABILITY IS DUE TO ACTIVE REGIONS ROTATING IN AND OUT OF VIEW. ALTERNATELY IF STARS WITH STRONG H-ALPHA VARIABILITY ALSO SHOW STRONGLY VARIABLE SPOTS THAT WOULD IMPLY THAT THE MAGNETIC FIELD IS RAPIDLY REARRANGING ITSELF. C16 PROVIDES A UNIQUE OPPORTUNITY TO USE SIMULTANEOUS HIGH-PRECISION PHOTOMETRY AND MULTI-EPOCH SPECTROSCOPY TO ILLUMINATE THE CAUSE AND EXTENT OF H-ALPHA VARIABILITY. OUR WORK WILL LEVERAGE EXISTING DATA OUR ACCESS TO MDM OF WHICH COLUMBIA UNIVERSITY OWNS 25% THE NN-EXPLORE PROGRAM AT WIYN AND K2'S CAPABILITIES TO INVESTIGATE THE STABILITY OF STARSPOT DISTRIBUTIONS AND H-ALPHA MEASUREMENTS. PRAESEPE IS A BENCHMARK CLUSTER FOR THE ARAR WHICH CAN BE USED TO MEASURE THE AGES OF EXOPLANET HOST STARS. THIS PROPOSAL THEREFORE SUPPORTS NASA'S GOAL OF UNDERSTANDING EXOPLANETARY SYSTEMS.