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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 Tinyanont, Samaporn and Vasisht, Gautam (2022) The Upper Edge of the Neptune Desert Is Stable Against Photoevaporation. Astronomical Journal, 164 (6). Art. No. 234. ISSN 0004-6256. doi:10.3847/1538-3881/ac92f2.

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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 data sets. 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_(-0.13)^(+0.16). 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:Article
Related URLs:
URLURL TypeDescription ItemDiscussion Paper
Vissapragada, Shreyas0000-0003-2527-1475
Knutson, Heather A.0000-0002-5375-4725
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
Paragas, Kimberly0000-0003-0062-1168
Spake, Jessica J.0000-0002-5547-3775
Tinyanont, Samaporn0000-0002-1481-4676
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 Aaron Bello-Arufe, Yayaati Chachan, Philipp Eigmüller, Akash Gupta, Julie Inglis, Shubham Kanodia, Katja Poppenhaeger, Hannah Wakeford, and Nicole Wallack for insightful conversations. We acknowledge the referees for thorough reports that improved the quality of this paper. S.V. is supported by an NSF Graduate Research Fellowship. H.A.K. acknowledges support from NSF CAREER grant 1555095. A.O. gratefully acknowledges support from the Dutch Research Council NWO Veni grant.
Group:Astronomy Department, Division of Geological and Planetary Sciences
Funding AgencyGrant Number
NSF Graduate Research FellowshipUNSPECIFIED
Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO)UNSPECIFIED
Issue or Number:6
Record Number:CaltechAUTHORS:20221114-805047300.13
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:117859
Deposited By: Research Services Depository
Deposited On:29 Nov 2022 00:06
Last Modified:29 Nov 2022 17:27

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