Atmospheric escape from close-in exoplanets is thought to be crucial in shaping observed planetary populations. Recently, significant progress has been made in observing this process in action through excess absorption in-transit spectra and narrowband light curves. We model the escape of initially homogeneous planetary winds interacting with a stellar wind. The ram pressure balance of the two winds governs this interaction. When the impingement of the stellar wind on the planetary outflow is mild or moderate, the planetary outflow expands nearly spherically through its sonic surface before forming a shocked boundary layer. When the confinement is strong, the planetary outflow is redirected into a cometary tail before it expands to its sonic radius. The resultant transmission spectra at the He 1083 nm line are accurately represented by a 1D spherical wind solution in cases of mild to moderate stellar wind interaction. In cases of strong stellar wind interaction, the degree of absorption is enhanced and the cometary tail leads to an extended egress from transit. The crucial features of the wind–wind interaction are, therefore, encapsulated in the light curve of He 1083 nm equivalent width as a function of time. The possibility of extended He 1083 nm absorption well beyond the optical transit carries important implications for planning out-of-transit observations that serve as a baseline for in-transit data.
We report a search for excess absorption in the 1083.2 nm line of ortho (triplet) helium during transits of TOI-1807b and TOI-2076b, 1.25 and 2.5-R⊕ planets on 0.55- and 10.4-d orbits around nearby ∼200 Myr-old K dwarf stars. We limit the equivalent width of any transit-associated absorption to <4 and <8 mÅ, respectively. We limit the escape of solar-composition atmospheres from TOI-1807b and TOI-2076b to ≲1 and ≲0.1M⊕Gyr−1, respectively, depending on wind temperature. The absence of a H/He signature for TOI-1807b is consistent with a measurement of mass indicating a rocky body and the prediction by a hydrodynamic model that any H-dominated atmosphere would be unstable and already have been lost. Differential spectra obtained during the transit of TOI-2076b contain a He i-like feature, but this closely resembles the stellar line and extends beyond the transit interval. Until additional transits are observed, we suspect this to be the result of variation in the stellar He i line produced by rotation of active regions and/or flaring on the young, active host star. Non-detection of escape could mean that TOI-2076b is more massive than expected, the star is less EUV luminous, the models overestimate escape, or the planet has a H/He-poor atmosphere that is primarily molecules such as H2O. Photochemical models of planetary winds predict a semimajor axis at which triplet He i observations are most sensitive to mass-loss: TOI-2076b orbits near this optimum. Future surveys could use a distance criterion to increase the yield of detections.
more » « less- PAR ID:
- 10383470
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 518
- Issue:
- 3
- ISSN:
- 0035-8711
- Format(s):
- Medium: X Size: p. 3777-3783
- Size(s):
- p. 3777-3783
- Sponsoring Org:
- National Science Foundation
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