A High-resolution Non-detection of Escaping Helium in the Ultrahot Neptune LTT 9779b: Evidence for Weakened Evaporation

The recent discovery of "ultrahot" (P < 1 day) Neptunes has come as a surprise: some of these planets have managed to retain gaseous envelopes despite being close enough to their host stars to trigger strong photoevaporation and/or Roche lobe overflow. Here, we investigate atmospheric escape in LTT 9779b, an ultrahot Neptune with a volatile-rich envelope. We observed two transits of this planet using the newly commissioned WINERED spectrograph (R similar to 68,000) on the 6.5 m Clay/Magellan II Telescope, aiming to detect an extended upper atmosphere in the He 10830 angstrom triplet. We found no detectable planetary absorption: in a 0.75 angstrom passband centered on the triplet, we set a 2 sigma upper limit of 0.12% (delta R-p/H < 14) and a 3 sigma upper limit of 0.20% (delta R-p/H < 22). Using a H/He isothermal Parker wind model, we found corresponding 95% and 99.7% upper limits on the planetary mass-loss rate of M<10(10.03) g s(-1) and M<10(11.11) g s(-1), respectively, smaller than predicted by outflow models even considering the weak stellar X-ray and ultraviolet emission. The low evaporation rate is plausibly explained by a metal-rich envelope, which would decrease the atmospheric scale height and increase the cooling rate of the outflow. This hypothesis is imminently testable: if metals commonly weaken planetary outflows, then we expect that JWST will find high atmospheric metallicities for small planets that have evaded detection in He 10830 angstrom.