On the Chemical Abundance of HR 8799 and the Planet c

Ji Wang, Jason J. Wang, Bo Ma, Jeffrey Chilcote, Steve Ertel, Olivier Guyon, Ilya Ilyin, Nemanja Jovanovic, Paul Kalas, Julien Lozi, Bruce Macintosh, Klaus G. Strassmeier, Jordan Stone

Research output: Contribution to journalArticlepeer-review

27 Scopus citations


Comparing chemical abundances of a planet and the host star reveals the origin and formation pathway of the planet. Stellar abundance is measured with high-resolution spectroscopy. Planet abundance, on the other hand, is usually inferred from low-resolution data. For directly imaged exoplanets, the data are available from a slew of high-contrast imaging/spectroscopy instruments. Here, we study the chemical abundance of HR 8799 and its planet c. We measure stellar abundance using LBT/PEPSI (R = 120,000) and archival HARPS data: stellar [C/H], [O/H], and C/O are 0.11 ± 0.12, 0.12 ± 0.14, and 0.54-0.09+0.12, all consistent with solar values. We conduct atmospheric retrieval using newly obtained Subaru/CHARIS data together with archival Gemini/GPI and Keck/OSIRIS data. We model the planet spectrum with petitRADTRANS and conduct retrieval using PyMultiNest. Retrieved planetary abundance can vary by ∼0.5 dex, from sub-stellar to stellar C and O abundances. The variation depends on whether strong priors are chosen to ensure a reasonable planet mass. Moreover, comparison with previous works also reveals inconsistency in abundance measurements. We discuss potential issues that can cause the inconsistency, e.g., systematics in individual data sets and different assumptions in the physics and chemistry in retrieval. We conclude that no robust retrieval can be obtained unless the issues are fully resolved.

Original languageEnglish (US)
Article number150
JournalAstronomical Journal
Issue number3
StatePublished - Sep 2020

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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