Hot Jupiters orbiting extremely close to their host star may experience atmospheric escape due to the large amounts of high-energy radiation they receive. Understanding the conditions under which this occurs is critical, as atmospheric escape is believed to be a driving factor in sculpting planetary populations. In recent years, the near-infrared 10830 Å helium feature has been found to be a promising spectral signature of atmospheric escape, because it can be observed using ground-based telescopes, does not suffer from interstellar absorption (which is a challenge for Lyα observations), and is not as impacted by stellar activity as Hα.

We use transmission spectroscopy to search for excess helium absorption in the extended atmosphere of WASP-48b, a hot Jupiter orbiting a slightly evolved, rapidly-rotating F star. The data were collected using the Habitable Zone Planet Finder spectrograph on the Hobby-Eberly Telescope. Observations were taken over the course of seven nights, from which we obtain three transits. No detectable helium absorption is seen, as absorption depth is limited to -0.0030±0.0050 at 0.6 σ. This non-detection follows our current understanding of decreasing stellar activity (and thus high-energy radiation) with age. We use a 1D isothermal Parker wind model to compare with our observations and find there is still some degeneracy in the range of planetary thermosphere temperatures and mass-loss rates that could explain our findings.

We explore our results within the context of the full sample of helium detections and non-detections to date, which should assist in optimizing future observational parameters for helium searches. Surprisingly, comparing helium absorption with the stellar activity index log R'HK reveals a large spread in the correlation between these two factors, suggesting that there are additional parameters influencing the helium absorption strength.