Presentation #417.01 in the session Exoplanet Radial Velocities and Transits: Techniques.
The 2021 Extreme Precision Radial Velocity (EPRV) Working Group’s report, and the more recent 2022 SAG21 findings, emphasizes that stellar activity is a primary hurdle to confirming and characterizing Earth analogs (Earth mass planets orbiting in the habitable zones of Sun-like stars). As such, scientific interest has returned to stellar measurements, with a goal to characterize stellar activity and remove its signature(s) from RV data. The only star for which we can currently study such activity (and its impact on RVs) in great detail is the Sun, which represents only a small subset of local spectral types. For more distant stars, we can infer the presence of star spots and flares from time series photometry and sometimes produce crude images of the photospheres using methods such as interferometry and Doppler-tomography. Yet this is a far cry from understanding stellar activity in detail across a wide range of stellar types. While numerical tools (e.g. Gaussian processes) can be very helpful, their results can be misleading without a broad foundation of observed stellar variability to compare against.
We have proposed to NASA a SmallSat Explorer mission called STARSPOT (the STellar Activity Recorder and Spectro-Photometric ObservaTory) which could provide a missing link between Solar and stellar activity studies by collecting high-precision, spectrally and temporally resolved data for a representative set of F-G-K-M dwarf stars. These spectral time series would cover sequential rotation cycles and revisit targets yearly to sample long term magnetic cycles. The goal is to advance our understanding of stellar activity via characterization of specific active region phenomena, which in return could help to break the current 1m/s stellar activity RV barrier and aid in better modeling of the Transit Light Source Effect. In the STARSPOT concept, high stability spectroscopy with a passband from NUV to NIR is enabled by a low-noise space platform. A telescope based on free-form optical surfaces provides a large aperture and a large field of view, allowing for simultaneous observations of comparison stars with similar properties. This, and a constant solar reference, enables us to remove remaining systematics, which are already highly suppressed by design.