The California-Kepler survey. X. The radius gap as a function of stellar mass, metallicity, and age
File(s)
Published version
Author(s)
Type
Journal Article
Abstract
In 2017, the California-Kepler Survey (CKS) published its first data release (DR1) of high-resolution optical spectra of 1305 planet hosts. Refined CKS planet radii revealed that small planets are bifurcated into two distinct populations, super-Earths (smaller than 1.5 R⊕) and sub-Neptunes (between 2.0 and 4.0 R⊕), with few planets in between (the "radius gap"). Several theoretical models of the radius gap predict variation with stellar mass, but testing these predictions is challenging with CKS DR1 due to its limited M⋆ range of 0.8–1.4 M⊙. Here we present CKS DR2 with 411 additional spectra and derived properties focusing on stars of 0.5–0.8 M⊙. We found that the radius gap follows Rp ∝ Pm with m = −0.10 ± 0.03, consistent with predictions of X-ray and ultraviolet- and core-powered mass-loss mechanisms. We found no evidence that m varies with M⋆. We observed a correlation between the average sub-Neptune size and M⋆. Over 0.5–1.4 M⊙, the average sub-Neptune grows from 2.1 to 2.6 R⊕, following ${R}_{p}\propto {M}_{\star }^{\alpha }$ with α = 0.25 ± 0.03. In contrast, there is no detectable change for super-Earths. These M⋆–Rp trends suggest that protoplanetary disks can efficiently produce cores up to a threshold mass of Mc, which grows linearly with stellar mass according to Mc ≈ 10 M⊕(M⋆/M⊙). There is no significant correlation between sub-Neptune size and stellar metallicity (over −0.5 to +0.5 dex), suggesting a weak relationship between planet envelope opacity and stellar metallicity. Finally, there is no significant variation in sub-Neptune size with stellar age (over 1–10 Gyr), which suggests that the majority of envelope contraction concludes after ∼1 Gyr.
Date Issued
2022-04-01
Date Acceptance
2022-01-23
Citation
The Astronomical Journal, 2022, 163 (4), pp.179-179
ISSN
0004-6256
Publisher
American Astronomical Society
Start Page
179
End Page
179
Journal / Book Title
The Astronomical Journal
Volume
163
Issue
4
Copyright Statement
© 2022. The Author(s). Published by the American Astronomical Society.
License URL
Sponsor
The Royal Society
The Royal Society
Commission of the European Communities
Identifier
https://iopscience.iop.org/article/10.3847/1538-3881/ac51e3
Grant Number
UF150412
RGF\R1\180046
853022
Subjects
Astronomy & Astrophysics
0201 Astronomical and Space Sciences
Publication Status
Published online
Date Publish Online
2022-03-18