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The Upper Edge of the Neptune Desert Is Stable Against Photoevaporation

Vissapragada, Shreyas and Knutson, Heather A. and Greklek-McKeon, Michael and Oklopčić, Antonija and Dai, Fei and dos Santos, Leonardo A. and Jovanovic, Nemanja and Mawet, Dimitri and Millar-Blanchaer, Maxwell A. and Paragas, Kimberly and Spake, Jessica J. and Vasisht, Gautam (2022) The Upper Edge of the Neptune Desert Is Stable Against Photoevaporation. . (Unpublished)

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Transit surveys indicate that there is a deficit of Neptune-sized planets on close-in orbits. If this "Neptune desert" is entirely cleared out by atmospheric mass loss, then planets at its upper edge should only be marginally stable against photoevaporation, exhibiting strong outflow signatures in tracers like the metastable helium triplet. We test this hypothesis by carrying out a 12-night photometric survey of the metastable helium feature with Palomar/WIRC, targeting seven gas-giant planets orbiting K-type host stars. Eight nights of data are analyzed here for the first time along with reanalyses of four previously-published datasets. We strongly detect helium absorption signals for WASP-69b, HAT-P-18b, and HAT-P-26b; tentatively detect signals for WASP-52b and NGTS-5b; and do not detect signals for WASP-177b and WASP-80b. We interpret these measured excess absorption signals using grids of Parker wind models to derive mass-loss rates, which are in good agreement with predictions from the hydrodynamical outflow code ATES for all planets except WASP-52b and WASP-80b, where our data suggest that the outflows are much smaller than predicted. Excluding these two planets, the outflows for the rest of the sample are consistent with a mean energy-limited outflow efficiency of ε = 0.41^(+0.16)_(−0.13). Even when we make the relatively conservative assumption that gas-giant planets experience energy-limited outflows at this efficiency for their entire lives, photoevaporation would still be too inefficient to carve the upper boundary of the Neptune desert. We conclude that this feature of the exoplanet population is a pristine tracer of giant planet formation and migration mechanisms.

Item Type:Report or Paper (Discussion Paper)
Related URLs:
URLURL TypeDescription Paper ItemJournal Article
Vissapragada, Shreyas0000-0003-2527-1475
Knutson, Heather A.0000-0002-0822-3095
Greklek-McKeon, Michael0000-0002-0371-1647
Oklopčić, Antonija0000-0002-9584-6476
Dai, Fei0000-0002-8958-0683
dos Santos, Leonardo A.0000-0002-2248-3838
Jovanovic, Nemanja0000-0001-5213-6207
Mawet, Dimitri0000-0002-8895-4735
Millar-Blanchaer, Maxwell A.0000-0001-6205-9233
Spake, Jessica J.0000-0002-5547-3775
Vasisht, Gautam0000-0002-1871-6264
Additional Information:We thank the Palomar Observatory telescope operators, support astronomers, and directorate for their support of this work, especially Tom Barlow, Andy Boden, Carolyn Heffner, Paul Nied, Joel Pearman, Kajse Peffer, and Kevin Rykoski. We also thank Yayaati Chachan, Philipp Eigm¨uller, Julie Inglis, Shubham Kanodia, Hannah Wakeford, and Nicole Wallack for insightful conversations. SV is supported by an NSF Graduate Research Fellowship. HAK acknowledges support from NSF CAREER grant 1555095. AO gratefully acknowledges support from the Dutch Research Council NWO Veni grant. Facilities: ADS, NASA Exoplanet Archive, Hale 200-inch, TESS. Software: numpy (Harris et al. 2020), scipy (Virtanen et al. 2020), astropy (Astropy Collaboration et al. 2013, 2018), matplotlib (Hunter 2007), p-winds (dos Santos et al. 2022), theano (The Theano Development Team et al. 2016), pymc3 (Salvatier et al. 2016), exoplanet (Foreman-Mackey et al. 2021a,b), ldtk (Husser et al. 2013; Parviainen & Aigrain 2015), corner (Foreman-Mackey 2016)
Group:Astronomy Department, Division of Geological and Planetary Sciences
Funding AgencyGrant Number
NSF Graduate Research FellowshipUNSPECIFIED
Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO)UNSPECIFIED
Record Number:CaltechAUTHORS:20220802-224623032
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:116046
Deposited By: George Porter
Deposited On:03 Aug 2022 15:06
Last Modified:28 Nov 2022 23:43

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