Starspots and Undetected Binary Stars Have Distinct Signatures in Young Stellar Associations

Young stars form in associations, meaning that young stellar associations provide an ideal environment to measure the age of a nominally coeval population. Isochrone fitting, which is the typical method for measuring the age of a coeval population, can be impacted by observational biases that obscure the physical properties of a population. One feature in isochrone fits of star-forming regions is an apparent mass-dependent age gradient, where lower-mass stars appear systematically younger than higher-mass stars. Starspots and stellar multiplicity are proposed mechanisms for producing the mass-dependent age gradient, but the relative importance of starspots versus multiplicity remains unclear. We performed a synthetic red optical low-resolution spectroscopic survey of a simulated analog to a 10 Myr stellar association including mass-dependent multiplicity statistics and age-dependent starspot coverage fractions. We found that undetected starspots alone do not produce an apparent mass-dependent age gradient, but instead uniformly reduce the average measured age of the population. We also found that binaries continue to produce an apparent mass-dependent age gradient and introduce more scatter in the age measurement than spots, but are easily removed from the population as long as there are good distance measurements to each target. We conclude that it is crucial to incorporate treatments of both starspots and undetected stellar multiplicity into isochrone fits of young stellar associations to attain reliable ages.