Wolf 503 b: Characterization of a Sub-Neptune Orbiting a Metal-poor K Dwarf

Polanski, Alex S.; Crossfield, Ian J. M.; Burt, Jennifer A.; Nowak, Grzegorz; López-Morales, Mercedes; Mortier, Annelies; Poretti, Ennio; Behmard, Aida; Benneke, Björn; Blunt, Sarah; Bonomo, Aldo S.; Butler, R. Paul; Chontos, Ashley; Cosentino, Rosario; Crane, Jeffrey D.; Dumusque, Xavier; Fulton, Benjamin J.; Ghedina, Adriano; Gorjian, Varoujan; Grunblatt, Samuel K.; Harutyunyan, Avet; Howard, Andrew W.; Isaacson, Howard; Kosiarek, Molly R.; Latham, David W.; Luque, Rafael; Martinez Fiorenzano, Aldo F.; Mayor, Michel; Mills, Sean M.; Molinari, Emilio; Nagel, Evangelos; Pallé, Enric; Petigura, Erik A.; Shectman, Stephen A.; Sozzetti, Alessandro; Teske, Johanna K.; Wang, Sharon Xuesong; Weiss, Lauren M.
Bibliographical reference

The Astronomical Journal

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Using radial-velocity measurements from four instruments, we report the mass and density of a 2.043 ±0.069 R⊕ sub-Neptune orbiting the quiet K-dwarf Wolf 503 (HIP 67285). In addition, we present improved orbital and transit parameters by analyzing previously unused short-cadence K2 campaign 17 photometry and conduct a joint radial-velocity-transit fit to constrain the eccentricity at 0.41 ± 0.05. The addition of a transit observation by Spitzer also allows us to refine the orbital ephemeris in anticipation of further follow-up. Our mass determination, 6.26 ${}_{-0.70}^{+0.69}$ M⊕ , in combination with the updated radius measurements, gives Wolf 503 b a bulk density of $\rho ={2.92}_{-0.44}^{+0.50}$ g cm-3. Using interior composition models, we find this density is consistent with an Earth-like core with either a substantial H2O mass fraction (45 ${}_{-16}^{+19}$ %) or a modest H/He envelope (0.5% ± 0.3%). The low H/He mass fraction, along with the old age of Wolf 503 (11 ± 2 Gyr), makes this sub-Neptune an opportune subject for testing theories of XUV-driven mass loss while the brightness of its host (J = 8.3 mag) makes it an attractive target for transmission spectroscopy.
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