2018A&A...612A..46M

Context. With the Large Binocular Telescope (LBT), we obtained a spectrum with PEPSI, its new optical high-resolution echelle spectrograph. The spectrum has very high resolution and a high signal-to-noise (S/N) and is of the K0V host Kepler-444, which is known to host five sub-Earth-sized rocky planets. The spectrum has a resolution of R~=250000, a continuous wavelength coverage from 4230Å to 9120Å, and an S/N between 150-550:1 (blue to red).
Aims. We performed a detailed chemical analysis to determine the photospheric abundances of 18 chemical elements. These were used to place constraints on the bulk composition of the five rocky planets.
Methods. Our spectral analysis employs the equivalent-width method for most of our spectral lines, but we used spectral synthesis to fit a small number of lines that required special care. In both cases, we derived our abundances using the MOOG spectral analysis package and Kurucz model atmospheres.
Results. We find no correlation between elemental abundance and condensation temperature among the refractory elements (T_C>950K). In addition, using our spectroscopic stellar parameters and isochrone fitting, we find an age of 10±1.5Gyr, which is consistent with the asteroseismic age of 11±1Gyr. Finally, from the photospheric abundances of Mg, Si, and Fe, we estimate that the typical Fe-core mass fraction for the rocky planets in the Kepler-444 system is approximately 24%.
Conclusions. If our estimate of the Fe-core mass fraction is confirmed by more detailed modeling of the disk chemistry and simulations of planet formation and evolution in the Kepler-444 system, then this would suggest that rocky planets in more metal-poor and α-enhanced systems may tend to be less dense than their counterparts of comparable size in more metal-rich systems.