Stellar Helium Variability and its Effect on the Detection of Young Planet Exospheres

The evolutionary pathway of small rocky planet atmospheres, which dictates habitability, is ill-constrained. Planets lose atmosphere over time, but the driver is unclear (e.g. photoevaporation, contraction heat). The cause of this atmospheric sculpting affects the type of atmosphere that is retained and the potential desiccation of the planetary surface from UV irradiation. Observing planetary atmospheres over time, especially in youth when mass loss is most dynamic, can constrain the evolutionary mechanisms. Such a study requires confirmation of evaporating atmospheres, or exospheres, by detecting the planetary mass loss via transit spectroscopy. However, stellar activity causes temporally coherent noise across the spectrum that can manifest as changes in the spectral line strength and prevent the detection of an exosphere. This includes the Helium 10830 A infrared triplet, which is an increasingly popular probe of atmospheric loss. The Helium feature is intrinsically variable in M-dwarfs, but its variability is not known in other types of stars, particularly at young ages when both stellar activity and planetary mass loss are greatest. We present years-long spectral time series from the Habitable-zone Planet Finder (HPF) of 10 young transiting planet hosts (ages of 20 Myr to 1 Gyr) to measure the amplitude and timescale of young stellar He variability. The long-term variability in the He triplet is large enough to impede exosphere detection at the youngest ages, but approaches the field level above roughly 500 Myr. We show that the He line variability could potentially be a tracer of RV jitter, and use the relationship between the typical He line profiles and stellar properties to extract information of the nature of the stellar He feature. These results show that accurate, contemporaneous models of the out-of-transit line profile are needed to precisely measure excess absorption at young ages.