Title: EPIC 210894022b - A short period super-Earth transiting a metal poor, evolved old star,
Abstract: The star EPIC 210894022 has been identified from a light curve acquired
through the K2 space mission as possibly orbited by a transiting planet. Our
aim is to confirm the planetary nature of the object and derive its fundamental
parameters. We combine the K2 photometry with reconnaissance spectroscopy and
radial velocity (RV) measurements obtained using three separate telescope and
spectrograph combinations. The spectroscopic synthesis package SME has been
used to derive the stellar photospheric parameters that were used as input to
various stellar evolutionary tracks in order to derive the parameters of the
system. The planetary transit was also validated to occur on the assumed host
star through adaptive imaging and statistical analysis. The star is found to be
located in the background of the Hyades cluster at a distance at least 4 times
further away from Earth than the cluster itself. The spectrum and the space
velocities of EPIC 210894022 strongly suggest it to be a member of the thick
disk population. We find that the star is a metal poor ([Fe/H]=-0.53+/-0.05
dex) and alpha-rich somewhat evolved solar-like object of spectral type G3 with
Teff=5730+/-50 K, logg=4.15+/-0.1 (cgs), radius of 1.3+/-0.1 R_Sun, and mass of
0.88+/-0.02 M_Sun. The RV detection together with the imaging confirms with a
high level of significance that the transit signature is caused by a
super-Earth orbiting the star EPIC 210894022. We measure a mass of 8.6+/-3.9
M_Earth and a radius of 1.9+/-0.2 R_Earth. A second more massive object with a
period longer than about 120 days is indicated by a long term linear
acceleration. With an age of > 10 Gyrs this system is one of the oldest where
planets is hitherto detected. Further studies of this planetary system is
important since it contains information about the planetary formation process
during a very early epoch of the history of our Galaxy.