Giants around giants: 13 year observations of the EXPRESS RV program

Evolved stars are excellent targets for precision radial velocity studies for three main reasons: i) they are cooler and rotate slower than their former main-sequence progenitor, which allows us to achieve a radial velocity precision at the m/s level for intermediate-mass stars, ii) we can study the dynamical evolution of planetary orbits due to the interaction with the expanding stellar envelope, and iii) they are significantly brighter than main sequence stars at a similar distance, allowing high SNRs to be acquired in an efficient manner.

Since 2009, we have been conducting a radial velocity survey called EXPRESS (EXoPlanets aRound Evolved StarS) aimed at detecting giant planets and studying the correlation between their orbital parameters and the host star properties (mass, metallicity, age) and to investigate whether planets can survive the stellar evolution after the main-sequence. For this, we have obtained thousands of individual radial velocities epochs over the course of 13 years, for a sample of 166 bright giant stars, resulting in the detection of ~30 planetary systems, including multi-planet systems, brown dwarfs in the desert, a gas giant in a compact binary system, among others. Figure 1 shows the orbital period versus host star mass, for planets detected among the EXPRESS sample. In addition, we have detected more than 20 stellar binary companions, resolved with RVs, astrometry and high-contrast direct imaging observations. Moreover, we have recently combined our data with those obtained by the PPPS and the Lick Surveys. Among our results we highlight:

1) Giant planets are more frequent around metal-rich giant stars.
2) The fraction of giant planets increases with the stellar mass, up to a maximum at ~1.7 Msun. Beyond ~3.0 Msun, no planets are found.
3) A surprisingly high fraction of giant planets around low-luminosity RGB stars of 39.4 +/- 8.0%.
4) An overall higher fraction of giant planet around RGB stars (f~14%) compared to post-RGB Horizontal-branch stars (f ~ 7%).

In this talk I will describe our project, including observations, data reduction, planet detections and the results in collaboration with the PPPS and the Lick Survey.
Finally, I will discuss our findings in the context of planetary formation and evolution, particularly after the main-sequence.

Figure 1: Orbital period versus stellar mass for EXPRESS planets. The symbol size is proportional to the minimum planet mass (Mp sini). Planets in multiple systems are connected by a dashed line. The red and blue dots correspond to RGB and HB host stars, respectively.