  Context. Extreme precision radial velocity (RV) surveys seeking to detect
planets at RV semi-amplitudes of 10 cm/s are facing numerous challenges. One of
those challenges is convective blueshift caused by stellar granulation and its
suppression through magnetic activity which plays a significant role in hiding
planetary signals in stellar jitter. Aims. Previously we found that for main
sequence stars, convective blueshift as an observational proxy for the strength
of convection near the stellar surface strongly depends on effective
temperature. In this work we investigate 242 post main sequence stars, covering
the subgiant, red giant, and asymptotic giant phases and empirically determine
the changes in convective blueshift with advancing stellar evolution. Methods.
We used the third signature scaling approach to fit a solar model for the
convective blueshift to absorption-line shift measurements from a sample of
coadded HARPS spectra, ranging in temperature from 3750 K to 6150 K. We compare
the results to main sequence stars of comparable temperatures but with a higher
surface gravity. Results. We show that convective blueshift becomes
significantly stronger for evolved stars compared to main sequence stars of a
similar temperature. The difference increases as the star becomes more evolved,
reaching a 5x increase below 4300 K for the most evolved stars. The large
number of stars in the sample, for the first time, allowed for us to
empirically show that convective blueshift remains almost constant among the
entire evolved star sample at roughly solar convection strength with a slight
increase from the red giant phase onward. We discover that the convective
blueshift shows a local minimum for subgiant stars, presenting a sweet spot for
exoplanet searches around higher mass stars, by taking advantage of their
spin-down during the subgiant transition.
