A High-resolution Non-detection of Escaping Helium in the Ultrahot Neptune LTT 9779b: Evidence for Weakened Evaporation
| Autor: | Shreyas Vissapragada, Patrick McCreery, Leonardo A. Dos Santos, Néstor Espinoza, Andrew McWilliam, Noriyuki Matsunaga, Jéa Adams Redai, Patrick Behr, Kevin France, Satoshi Hamano, Charlie Hull, Yuji Ikeda, Haruki Katoh, Hideyo Kawakita, Mercedes López-Morales, Kevin N. Ortiz Ceballos, Shogo Otsubo, Yuki Sarugaku, Tomomi Takeuchi |
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| Jazyk: | angličtina |
| Rok vydání: | 2024 |
| Předmět: | |
| Zdroj: | The Astrophysical Journal Letters, Vol 962, Iss 1, p L19 (2024) |
| Druh dokumentu: | article |
| ISSN: | 2041-8213 2041-8205 |
| DOI: | 10.3847/2041-8213/ad23cf |
| Popis: | 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 2 σ upper limit of 0.12% ( δ R _p / H < 14) and a 3 σ 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. |
| Databáze: | Directory of Open Access Journals |
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