Stellar Radial Velocities: The Continuing Pursuit For Decimeter Precision
Mark H. Slovak

The recent detection of an exoplanet (Proxima b) around Proxima Centauri highlighted the increasing precision of extrinsic stellar radial velocity (vr) measurements. A thousand fold improvement (from ±1 km/s to ±1 m/s) has been achieved over the last few decades. While intrinsic stellar variability may set a "background" limit of ±1 m/s for cool stellar systems, the ultimate precision of a few decimeters/s (±1 - 2 dm/s) is the current goal of astronomical spectroscopists.

Increased precision also permitted a fundamental improvement to a crucial "rung" of the cosmic distance "ladder": high precision measurements of intrinsic (pulsation) radial velocities of galactic Cepheid variables (along with contemporaneous BVR photometry). The unique Cepheid period-luminosity-color (P-L-C) relation provides the crucial bridge in the distance scale from direct parallax to "standard candles" (Type Ia supernovae). The development of a second generation coudé radial velocity spectrometer (RVM), which provided the basis for an extensive Cepheid observing program, is described as well several unexpected discoveries, including the triple Cepheid system S Sagittae.