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We obtained an LBT/PEPSI spectrum with very high resolution and highsignal-to-noise ratio (S/N) of the K0V host Kepler-444, which is known to host5 sub-Earth size rocky planets. The spectrum has a resolution of R=250,000, acontinuous wavelength coverage from 4230 to 9120A, and S/N between 150 and550:1 (blue to red). We performed a detailed chemical analysis to determine thephotospheric abundances of 18 chemical elements, in order to use the abundancesto place constraints on the bulk composition of the five rocky planets. Ourspectral analysis employs the equivalent width method for most of our spectrallines, but we used spectral synthesis to fit a small number of lines thatrequire special care. In both cases, we derived our abundances using the MOOGspectral analysis package and Kurucz model atmospheres. We find no correlationbetween elemental abundance and condensation temperature among the refractoryelements. In addition, using our spectroscopic stellar parameters and isochronefitting, we find an age of 10+/-1.5 Gyr, which is consistent with theasteroseismic age of 11+/-1 Gyr. Finally, from the photospheric abundances ofMg, Si, and Fe, we estimate that the typical Fe-core mass fraction for therocky planets in the Kepler-444 system is approximately 24 per cent. If ourestimate of the Fe-core mass fraction is confirmed by more detailed modeling ofthe disk chemistry and simulations of planet formation and evolution in theKepler-444 system, then this would suggest that rocky planets in moremetal-poor and alpha-enhanced systems may tend to be less dense than theircounterparts of comparable size in more metal-rich systems.

PEPSI deep spectra. III. A chemical analysis of the ancient planet-host star Kepler-444