Escaping Helium from TOI 560.01, a Young Mini-Neptune

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.

The Upper Edge of the Neptune Desert Is Stable Against Photoevaporation

“…Our survey also revealed that 1D mass loss models overpredict the metastable helium signatures from WASP-52b and WASP-80b, in agreement with previous work by Fossati et al (2021). Although it cannot explain WASP-80b, our estimates suggest that a strong stellar wind might confine the outflow of WASP-52b, similar to what has been proposed for WASP-107b (Spake et al 2021;Wang & Dai 2021b) and TOI-560.01 (Zhang et al 2022). If the helium line for WASP-52b can be detected with more sensitive observations at higher resolving power, we would expect to see a characteristic blueshifted line profile and/or an extended tail of postegress absorption in this scenario.…”

“…If the outflow is confined into a comet-like shape by a strong stellar wind, it could reduce the magnitude of the helium ab-sorption (e.g., McCann et al 2019;Carolan et al 2020;MacLeod & Oklopčić 2022). This phenomenon has been directly detected for the hot Jupiter WASP-107b (Khodachenko et al 2021;Spake et al 2021;Wang & Dai 2021b), and may also explain the relatively weak helium signal measured for the mini-Neptune TOI-560.01 (Zhang et al 2022). Importantly, both WASP-52 and WASP-80 appear to be relatively young, suggesting that they may have correspondingly enhanced stellar winds.…”

“…From 2019-2021, we used our narrowband helium photometer to survey the metastable helium line in a seven-planet sample near the upper edge of the Neptune desert. There are currently ten published helium detections in the literature: WASP-107b (Spake et al 2018;Allart et al 2019;Kirk et al 2020;Spake et al 2021), HAT-P-11b (Allart et al 2018;Mansfield et al 2018), WASP-69b (Nortmann et al 2018;Vissapragada et al 2020b), HD 209458b (Alonso-Floriano et al 2019, HD 189733b (Salz et al 2018;Guilluy et al 2020), GJ 3470b (Palle et al 2020;Ninan et al 2020), HAT-P-18b (Paragas et al 2021), HAT-P-32b (Czesla et al 2021), TOI-560.01 (Zhang et al 2022), and GJ 1214b(Orell-Miquel et al 2022. These observations, along with many non-detections and tentative detections in the literature, span wide ranges in planetary mass and radius, stellar spectral type and activity level, system age, observing methodology, and data reduction technique.…”

A Mini-Neptune from TESS and CHEOPS Around the 120 Myr Old AB Dor member HIP 94235

“…Rockcliffe et al (2021) reported a non-detection of escaping hydrogen for the 650 Myr old K2-25b, with one possibility for the non-detection being factors related to the star's youth. Zhang et al (2022a) reported escaping He I for the 500 Myr old mini-Neptune HD 73583b, showing an excess 0.68% absorption for the He I 10830 Å line. Tentative detections of atmospheric escape have been reported for planets in the V1298 Tau system in Ca II, H-α (Feinstein et al 2021), and He I (Gaidos et al 2022) The K2 mission (Howell et al 2014) yielded some of the first young transiting planets (e.g.…”

“…Real-time measurements of photoevaporation for younger systems can help establish the timescale for this process and its influence on the radius evolution of planets. Zhang et al (2022b) detected the Lyman-α transit of the outer planet HD 63433 c in a 400 Myr old planetary system. They also reported the lack of a detectable escaping hydrogen atmosphere for the inner planet, suggesting that it may have already undergone run-away evaporation and lost its primordial atmosphere.…”