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Comparing chemical abundances of a planet and the host star reveals theorigin and formation path. Stellar abundance is measured with high-resolutionspectroscopy. Planet abundance, on the other hand, is usually inferred fromlow-resolution data. For directly imaged exoplanets, the data are availablefrom a slew of high-contrast imaging/spectroscopy instruments. Here, we studythe chemical abundance of HR 8799 and its planet c. We measure stellarabundance using LBT/PEPSI (R=120,000) and archival HARPS data: stellar [C/H],[O/H], and C/O are 0.11$\pm$0.12, 0.12$\pm$0.14, and 0.54$^{+0.12}_{-0.09}$,all consistent with solar values. We conduct atmospheric retrieval using newlyobtained Subaru/CHARIS data together with archival Gemini/GPI and Keck/OSIRISdata. We model the planet spectrum with petitRADTRANS and conduct retrievalusing PyMultiNest. Retrieved planetary abundance can vary by $\sim$0.5 dex,from sub-stellar to stellar C and O abundances. The variation depends onwhether strong priors are chosen to ensure a reasonable planet mass. Moreover,comparison with previous works also reveals inconsistency in abundancemeasurements. We discuss potential issues that can cause the inconsistency,e.g., systematics in individual data sets and different assumptions in thephysics and chemistry in retrieval. We conclude that no robust retrieval can beobtained unless the issues are fully resolved.

On the Chemical Abundance of HR 8799 and the Planet c