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On the Chemical Abundance of HR 8799 and the Planet c
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.
Untangling the Sources of Abundance Dispersion in Low-metallicity Stars
We measure abundances of 12 elements (Na, Mg, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni) in a sample of 86 metal-poor (−2 ≲ [Fe/H] ≲ −1) subgiant stars in the solar neighborhood. Abundances are derived from high-resolution spectra taken with the Potsdam Echelle Polarimetric and Spectroscopic Instrument on the Large Binocular Telescope, modeled using iSpec and MOOG. By carefully quantifying the impact of photon-noise (<0.05 dex for all elements), we robustly measure the intrinsic scatter of abundance ratios. At fixed [Fe/H], the rms intrinsic scatter in [X/Fe] ranges from 0.04 (Cr) to 0.16 dex (Na), with a median of 0.08 dex. Scatter in [X/Mg] is similar, and accounting for [α/Fe] only reduces the overall scatter moderately. We consider several possible origins of the intrinsic scatter with particular attention to fluctuations in the relative enrichment by core-collapse supernovae (CCSN) and Type Ia supernovae and stochastic sampling of the CCSN progenitor mass distribution. The stochastic sampling scenario provides a good quantitative explanation of our data if the effective number of CCSN contributing to the enrichment of a typical sample star is N ∼ 50. At the median metallicity of our sample, this interpretation implies that the CCSN ejecta are mixed over a gas mass ∼6 × 104 M ⊙ before forming stars. The scatter of elemental abundance ratios is a powerful diagnostic test for simulations of star formation, feedback, and gas mixing in the early phases of the Galaxy.
On the lithium abundance of the visual binary components ξ Boo A (G8V) and ξ Boo B (K5V)
A spectroscopic investigation of the lithium resonance doublet in ξ Boo A and ξ Boo B in terms of both abundance and isotopic ratio is presented. We obtained new R = 130,000 spectra with a signal-to-noise ratio (S/N) per pixel of up to 3200 using the 11.8 m LBT and PEPSI. From fits with synthetic line profiles based on 1D-LTE MARCS model atmospheres and 3D-NLTE corrections, we determine the abundances of both isotopes. For ξ Boo A, we find A(Li) = 2.40 ± 0.03 dex and 6Li/7Li <1.5 ± 1.0% in 1D-LTE, which increases to ≈2.45 for the 3D-NLTE case. For ξ Boo B we obtain A(Li) = 0.37 ± 0.09 dex in 1D-LTE with an unspecified 6Li/7Li level. Therefore, no 6Li is seen on any of the two stars. We consider a spot model for the Li fit for ξ Boo B and find A(Li) = 0.45 ± 0.09 dex. The 7Li abundance is 23 times higher for ξ Boo A than the Sun's, but three times lower than the Sun's for ξ Boo B while both fit the trend of single stars in the similar-aged M35 open cluster. Effective temperatures are redetermined from the TiO band head strength. We note that the best-fit global metallicities are −0.13 ± 0.01 dex for ξ Boo A but +0.13 ± 0.02 dex for ξ Boo B. Lithium abundance for the K5V benchmark star 61 Cyg A was obtained to A(Li) ≈ 0.53 dex when including a spot model but to ≈0.15 dex without a spot model.
On the binary orbit of Henry Draper one (HD 1)
We present our final orbit for the late-type spectroscopic binary Henry Draper one (HD 1). area total of 553 spectra from 13 years of observations are used with our robotic STELLA facility and its high-resolution echelle spectrograph SES. Its long-term radial velocity stability is ≈50 m s−1. A single radial velocity of HD 1 reached an rms residual of 63 m s−1, close to the expected precision. Spectral lines of HD 1 are rotationally broadened with a v sin i of 9.1±0.1 km s−1. The overall spectrum appears single-lined and yielded an orbit with an eccentricity of 0.5056±0.0005 and a semiamplitude of 4.44 km s−1. We constrain and refine the orbital period based on the SES data alone to 2, 318.70±0.32 days, compared to 2, 317.8±1.1 days when including the older dataset published by DAO and Cambridge/Coravel. Owing to the higher precision of the SES data, we base the orbit calculation only on the STELLA/SES velocities so as to not degrade its solution. We redetermine astrophysical parameters for HD 1 from spectrum synthesis and, together with the new Gaia DR-2 parallax, suggest a higher luminosity than published previously.We conclude thatHD1 is a slightly metal-deficient K0 III-II giant 217 times more luminous than the Sun. The secondary remains invisible at optical wavelengths. We present evidence for the existence of a third component.