A High-resolution Non-detection of Escaping Helium in the Ultrahot Neptune LTT 9779b: Evidence for Weakened Evaporation
Abstract
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 ∼ 68,000) on the 6.5 m Clay/Magellan II Telescope, aiming to detect an extended upper atmosphere in the He 10830 Å triplet. We found no detectable planetary absorption: in a 0.75 Å passband centered on the triplet, we set a 2σ upper limit of 0.12% (δ R p /H < 14) and a 3σ upper limit of 0.20% (δ 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 $\dot{M}\lt {10}^{10.03}$ g s‑1 and $\dot{M}\lt {10}^{11.11}$ 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 Å.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- February 2024
- DOI:
- 10.3847/2041-8213/ad23cf
- arXiv:
- arXiv:2401.16474
- Bibcode:
- 2024ApJ...962L..19V
- Keywords:
-
- Exoplanet atmospheres;
- Exoplanet atmospheric evolution;
- 487;
- 2308;
- Astrophysics - Earth and Planetary Astrophysics
- E-Print:
- 11 pages, 3 figures, accepted for publication in ApJL