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Published April 2017 | Submitted + Published
Journal Article Open

Four Sub-Saturns with Dissimilar Densities: Windows into Planetary Cores and Envelopes

Abstract

We present results from a Keck/HIRES radial velocity campaign to study four sub-Saturn-sized planets, K2-27b, K2-32b, K2-39b, and K2-108b, with the goal of understanding their masses, orbits, and heavy-element enrichment. The planets have similar sizes (R_P = 4.5–5.5 R⊕), but have dissimilar masses (M_P = 16–60 M⊕), implying a diversity in their core and envelope masses. K2-32b is the least massive (M_P = 16.5 ± 2.7 M⊕) and orbits in close proximity to two sub-Neptunes near a 3:2:1 period commensurability. K2-27b and K2-39b are significantly more massive at M_P = 30.9 ± 4.6 M⊕ and M_P = 39.8 ± 4.4 M⊕, respectively, and show no signs of additional planets. K2-108b is the most massive at M_P = 59.4 ± 4.4 M⊕, implying a large reservoir of heavy elements of about ≈50 M⊕. Sub-Saturns as a population have a large diversity in planet mass at a given size. They exhibit remarkably little correlation between mass and size; sub-Saturns range from ≈6–60 M⊕, regardless of size. We find a strong correlation between planet mass and host star metallicity, suggesting that metal-rich disks form more massive planet cores. The most massive sub-Saturns tend to lack detected companions and have moderately eccentric orbits, perhaps as a result of a previous epoch of dynamical instability. Finally, we observe only a weak correlation between the planet envelope fraction and present-day equilibrium temperature, suggesting that photo-evaporation does not play a dominant role in determining the amount of gas sub-Saturns accrete from their protoplanetary disks.

Additional Information

© 2017 The American Astronomical Society. Received 2016 December 1; revised 2017 January 30; accepted 2017 January 30; published 2017 March 7. We thank Konstantin Batygin and John Livingston for helpful discussions. E.A.P. acknowledges support from Hubble Fellowship grant HST-HF2-51365.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. for NASA under contract NAS 5-26555. This work has made use of data from the European Space Agency (ESA) mission Gaia, processed by Gaia Data Processing and Analysis Consortium (DPAC). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. Some of the data presented herein were obtained at the W M Keck Observatory (which is operated as a scientific partnership among Caltech, UC, and NASA). We thank the Caltech and NASA Keck Time Allocation Committees for providing HIRES time. This work included observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina), and Ministério da Ciência, Tecnologia e Inovação (Brazil). The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Software: Numpy/Scipy (Van Der Walt et al. 2011), Matplotlib (Hunter 2007), Pandas (McKinney 2010), Astropy (Astropy Collaboration et al. 2013), emcee (Goodman & Weare 2010; Foreman-Mackey et al. 2013), SME (Brewer et al. 2015), isochrones (Morton 2015), k2sc (Aigrain et al. 2015), batman (Kreidberg 2015), radvel (https://github.com/California-Planet-Search/radvel), k2phot (https://github.com/petigura/k2phot).

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Published - Petigura_2017_AJ_153_142.pdf

Submitted - 1702.00013.pdf

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Additional details

Created:
August 19, 2023
Modified:
October 24, 2023