Michael Gully-Santiago The University of Texas at Austin DPS- EPSC 2023 Division for Planetary Sciences the American Astronomical Society October 3, 2023 San Antonio, TX A Large and Variable Leading Tail of Helium in HAT-P-67b, a Sub-Saturn Undergoing Runaway Inflation Caroline V. Morley, Jessica Luna, Morgan MacLeod, Antonija Oklopčić, Aishwarya Ganesh, Quang H. Tran, Zhoujian Zhang, Brendan P. Bowler, William D. Cochran, Daniel M. Krolikowski, Suvrath Mahadevan, Joe P. Ninan, Guðmundur Stefánsson, Andrew Vanderburg, Joseph A. Zalesky, Gregory R. Zeimann Gully-Santiago et al. arXiv 2307.08959

The observed mass-radius diagram for Solar System planets

The observed mass-radius diagram for exoplanets

Why so few inflated sub- Saturns? The Inflated Sub-Saturn Cliff

Scenario 1: Nature does not make them. Scenario 2: Nature makes them, but they are unstable. Why so few inflated sub- Saturns?

Scenario 1: Migration prevents sub-Saturns from reaching high Teq . Scenario 2: They reach high Teq , but quickly undergo Runaway Inflation. Thorngren & Fortney 2018

Scenario 1: Migration prevents sub-Saturns from reaching high Teq . Scenario 2: They reach high Teq , but quickly undergo Runaway Inflation. Thorngren & Fortney 2018 What does planetary theory expect?

The steady state Radius-Mass slope steepens with increasing Teq . Thorngren & Fortney 2018

Thorngren & Fortney 2018 The steady state Radius-Mass slope steepens with increasing Teq .

Thorngren & Fortney 2018 The steady state Radius-Mass slope steepens with increasing Teq .

Thorngren & Fortney 2018 A positive feedback loop ensues.

Thorngren & Fortney 2018 A positive feedback loop ensues.

Thorngren & Fortney 2018 A positive feedback loop ensues. Losing mass makes you larger, which makes you lose mass faster.

Inflated sub-Saturns should exhibit profound mass loss rates. Thorngren & Fortney 2018

Inflated sub-Saturns are rare. Thorngren & Fortney 2018

This talk: Helium 10833 Å observations of HAT-P-67 b for HAT-P-32 b, see-- Zhang, Morley, Gully-Santiago et al. 2023 DOI: (10.1126/sciadv.adf8736)

HAT-P-67 an F subgiant Zhou et al. 2017

HAT-P-67 b a very low density, inflated hot Saturn Zhou et al. 2017

Habitable Zone Planet Finder (HPF) Helium Exospheres Survey λ = 8100 – 12,800 Å R = 55,000 Hobby Eberly Telescope (HET), Texas, USA We get abundant orbital phase coverage: Large orbital phase coverage Visits In–Transit Out-of-Transit HAT-P-67 b 7 35

HAT-P-67 b with HPF 39 nights over 3 years 13.8 hours of on-sky integration time 152 individual exposures

HAT-P-67 b shows conspicuous variability in He I 10833 Å.

Up to 10% transit depths.

HAT-P-67 b also has an extended ingress.

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Leading tail resides in the stellar rest frame.

Weak trailing tail blueshifts indicating acceleration away from the planet.

The leading tail is direct evidence for preferential dayside mass loss.

̇ 𝑴 ~ 2 ×1013 g/s (105 M ⨁ / Gyr ) with 1D Parker Winds models † (p-winds) †Significant uncertainty: - XUV radiation - T0 - 3D effects (streams) - self-shielding Dos Santos et al. 2022 with Mp < 100 M ⨁ implies inflationary timescale 𝜏infl < 1 Gyr

Thorngren, Lee & Lopez 2023 XUV irradiation removes hot Saturns from the mass-radius plane. Mass loss is a positive feedback loop near the 0.1 g/cm3 threshold.

Ohmic Dissipation and XUV irradiation make different quantitative predictions for inflation timescales. HAT-P-67 b Theory: 𝜏infl ~ 5-50 Myr Observed: 𝜏infl < 1000 Myr

XUV irradiation better matches the population of hot Saturns 0.1 g cm-3 threshold divides observed planet sample from sub-Saturn cliff.

Conclusions We have detected up to 10% transit depth of He I 10833 Å from HPF spectra of HAT-P-67 b. The excess absorption preceeds the transit by up to 130 planetary radii in a large leading tail. The prominence of this leading tail is direct evidence for preferential dayside mass loss. We estimate a mass loss rate of 2 x 1013 g/s, and lifetime less than a Gyr. This pattern broadly agrees with theoretical predictions and explains the lack of inflated sub-Saturns. Gully-Santiago et al. arXiv 2307.08959