Kavli Affiliate: Andrew Vanderburg
| First 5 Authors: Daniel M. Krolikowski, Adam L. Kraus, Benjamin M. Tofflemire, Caroline V. Morley, Andrew W. Mann
| Summary:
Young exoplanets trace planetary evolution, particularly the atmospheric mass
loss that is most dynamic in youth. However, the high activity level of young
stars can mask or mimic the spectroscopic signals of atmospheric mass loss.
This includes the activity-sensitive He 10830 AA triplet, which is an
increasingly important exospheric probe. To characterize the He-10830 triplet
at young ages, we present time-series NIR spectra for young transiting planet
hosts taken with the Habitable-zone Planet Finder. The He-10830 absorption
strength is similar across our sample, except at the fastest and slowest
rotation, indicating that young chromospheres are dense and populate metastable
helium via collisions. Photoionization and recombination by coronal radiation
only dominates metastable helium population at the active and inactive
extremes. Volatile stellar activity, such as flares and changing surface
features, drives variability in the He-10830 triplet. Variability is largest at
the youngest ages before decreasing to $lesssim5-10$ mAA (or 3%) at ages
above 300 Myr, with 6 of 8 stars in this age range agreeing with no intrinsic
variability. He-10830 triplet variability is smallest and age-independent at
the shortest timescales. Intrinsic stellar variability should not preclude
detection of young exospheres, except at the youngest ages. We recommend
out-of-transit comparison observations taken directly surrounding transit and
observation of multiple transits to minimize activity’s effect. Regardless,
caution is necessary when interpreting transit observations in the context of
stellar activity, as many scenarios can lead to enhanced stellar variability
even on timescales of an hour.
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