NASAのミッションが、太陽系外惑星が縮小する理由の可能性を明らかにする(NASA Mission Reveals Possible Reason Behind Shrinking Exoplanets)

2023-11-17 カリフォルニア工科大学(Caltech)

＜関連情報＞

• https://www.caltech.edu/about/news/nasa-mission-reveals-possible-reason-behind-shrinking-exoplanets
• https://iopscience.iop.org/article/10.3847/1538-3881/acf9f9
• https://iopscience.iop.org/article/10.3847/1538-3881/ac3fa7
• https://iopscience.iop.org/article/10.3847/1538-3881/ac3f3b

K2のスケーリング VII. 中年齢における高温のサブ海王星の高い出現率の証拠 Scaling K2. VII. Evidence For a High Occurrence Rate of Hot Sub-Neptunes at Intermediate Ages

Jessie L. Christiansen, Jon K. Zink, Kevin K. Hardegree-Ullman, Rachel B. Fernandes, Philip F. Hopkins, Luisa M. Rebull, Kiersten M. Boley, Galen J. Bergsten, and Sakhee Bhure
The Astronomical Journal  Published 2023 November 15
DOI:10.3847/1538-3881/acf9f9

Abstract

The NASA K2 mission obtained high-precision time-series photometry for four young clusters, including the near-twin 600–800 Myr old Praesepe and Hyades clusters. Hot sub-Neptunes are highly prone to mass-loss mechanisms, given their proximity to the host star and the weakly bound gaseous envelopes, and analyzing this population at young ages can provide strong constraints on planetary evolution models. Using our automated transit detection pipeline, we recover 15 planet candidates across the two clusters, including 10 previously confirmed planets. We find a hot sub-Neptune occurrence rate of 79%–107% for GKM stars in the Praesepe cluster. This is 2.5–3.5σ higher than the occurrence rate of 16.54^+1.00_−0.98% for the same planets orbiting the ∼3–9 Gyr old GKM field stars observed by K2, even after accounting for the slightly supersolar metallicity ([Fe/H] ∼ 0.2 dex) of the Praesepe cluster. We examine the effect of adding ∼100 targets from the Hyades cluster and extending the planet parameter space under examination, and we find similarly high occurrence rates in both cases. The high occurrence rate of young, hot sub-Neptunes could indicate either that these planets are undergoing atmospheric evolution as they age, or that planetary systems that formed when the Galaxy was much younger are substantially different than from today. Under the assumption of the atmospheric mass-loss scenario, a significantly higher occurrence rate of these planets at the intermediate ages of Praesepe and Hyades appears more consistent with the core-powered mass-loss scenario for the origin of the planet radius valley, compared to the photoevaporation scenario.

若いミニ海王星TOI 560.01からヘリウムが放出される Escaping Helium from TOI 560.01, a Young Mini-Neptune

Michael Zhang, Heather A. Knutson, Lile Wang, Fei Dai, and Oscar Barragán
The Astronomical Journal  Published 2022 January 17
DOI:10.3847/1538-3881/ac3fa7

Abstract

We report helium absorption from the escaping atmosphere of TOI 560.01 (HD 73583b), an R = 2.8R_⊕, P = 6.4 day mini-Neptune orbiting a young (∼600 Myr) K dwarf. Using Keck/NIRSPEC, we detect a signal with an average depth of 0.68% ± 0.08% in the line core. The absorption signal repeats during a partial transit obtained a month later, but is marginally stronger and bluer, perhaps reflecting changes in the stellar wind environment. Ingress occurs on time, and egress occurs within 12 minutes of the white light egress, although absorption rises more gradually than it declines. This suggests that the outflow is slightly asymmetric and confined to regions close to the planet. The absorption signal also exhibits a slight 4 km s⁻¹ redshift rather than the expected blueshift; this might be explained if the planet has a modest orbital eccentricity, although the radial velocity data disfavors such an explanation. We use XMM-Newton observations to reconstruct the high-energy stellar spectrum and model the planet’s outflow with 1D and 3D hydrodynamic simulations. We find that our models generally overpredict the measured magnitude of the absorption during transit, the size of the blueshift, or both. Increasing the metallicity to 100× solar suppresses the signal, but the dependence of the predicted signal strength on metallicity is non-monotonic. Decreasing the assumed stellar EUV flux by a factor of three likewise suppresses the signal substantially.

若いミニ海王星HD 63433cからの進行中の質量放出の検出 Detection of Ongoing Mass Loss from HD 63433c, a Young Mini-Neptune

Michael Zhang, Heather A. Knutson, Lile Wang, Fei Dai, Leonardo A. dos Santos, Luca Fossati, Gregory W. Henry, David Ehrenreich, Yann Alibert, Sergio Hoyer, Thomas G. Wilson, and Andrea Bonfanti
The Astronomical Journal  Published 2022 January 17
DOI:10.3847/1538-3881/ac3f3b

Abstract

We detect Lyα absorption from the escaping atmosphere of HD 63433c, a R = 2.67R_⊕, P = 20.5 day mini-Neptune orbiting a young (440 Myr) solar analog in the Ursa Major Moving Group. Using Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph, we measure a transit depth of 11.1 ± 1.5% in the blue wing and 8 ± 3% in the red. This signal is unlikely to be due to stellar variability, but should be confirmed by an upcoming second transit observation with HST. We do not detect Lyα absorption from the inner planet, a smaller R = 2.15R_⊕ mini-Neptune on a 7.1 day orbit. We use Keck/NIRSPEC to place an upper limit of 0.5% on helium absorption for both planets. We measure the host star’s X-ray spectrum and mid-ultraviolet flux with XMM-Newton, and model the outflow from both planets using a 3D hydrodynamic code. This model provides a reasonable match to the light curve in the blue wing of the Lyα line and the helium nondetection for planet c, although it does not explain the tentative red wing absorption or reproduce the excess absorption spectrum in detail. Its predictions of strong Lyα and helium absorption from b are ruled out by the observations. This model predicts a much shorter mass-loss timescale for planet b, suggesting that b and c are fundamentally different: while the latter still retains its hydrogen/helium envelope, the former has likely lost its primordial atmosphere.