Spectroscopic Orbit

Definition

When two stars orbit each other and the orbital motion is detected spectroscopically from the changes in the Doppler shifts (also known as Doppler velocimetry), the system is traditionally called a spectroscopic binary. Accordingly, the orbital solution based on the observed changes in radial velocities of the two stars is called a spectroscopic orbit. When the companion is a planet, only the spectrum of the host star is detected. In this case, the planet and star revolve around their center of mass. Although the orbit of the primary star around the center of mass is smaller than that of the planet, it is still called a spectroscopic orbit. The five parameters that define a single-lined spectroscopic orbit are the orbital period P, the eccentricity of the elliptical orbit e, the velocity of the center of mass γ, the amplitude of the orbital velocity projected along the line of sight K, the orientation on the sky of the long axis of the ellipse ω, and the time of periastron...