Techniques for Finding Close-in, Low-mass Planets around Subgiants

Jupiter-mass planets with large semi-major axes ($a > 1.0$ AU) occur at a higher rate around evolved intermediate mass stars. There is a pronounced paucity of close-in ($a < 0.6$ AU), intermediate period ($5 < P < 100$ days), low-mass ($M_{rm planet} < 0.7M_{rm Jup} $) planets, known as the `Planet Desert. Current radial velocity methods have yet to yield close-in, low-mass planets around these stars because the planetary signals could be hidden by the (5-10) m s$^{-1}$ radial velocity variations caused by acoustic oscillations. We find that by implementing an observing strategy of taking three observations per night separated by an optimal $Delta t$, which is a function of the oscillation periods and amplitudes, we can average over the stellar jitter and improve our sensitivity to low-mass planets. We find $Delta t$ can be approximated using the stellar mass and radius given by the relationship $Delta t = $1.79 $(M/M_{odot})^{-0.82} ~(R/R_{odot})^{1.92}$. We test our proposed method by injecting planets into very well sampled data of a subgiant star, $gamma$ Cep. We compare the fraction of planets recovered by our method to the fraction of planets recovered using current radial velocity observational strategies. We find that our method decreases the RMS of the stellar jitter due to acoustic oscillations by a factor of three over current single epoch observing strategies used for subgiant stars. Our observing strategy provides a means to test whether the Planet Desert extends to lower mass planets.