On the binary orbit of Henry Draper one ( HD 1)

We present our final orbit for the late type spectroscopic binary Henry Draper one (HD 1). area total of 553 spectra from 13 years of observations are used with our robotic STELLA facility and its high resolution echelle spectrograph SES. Its long term radial velocity stability is ≈50 m s^-1. A single radial velocity of HD 1 reached an rms residual of 63 m s^-1, close to the expected precision. Spectral lines of HD 1 are rotationally broadened with a v sin i of 9.1 ± 0.1 km s^-1. The overall spectrum appears single lined and yielded an orbit with an eccentricity of 0.5056 ± 0.0005 and a semiamplitude of 4.44 km s^-1. We constrain and refine the orbital period based on the SES data alone to 2, 318.70 ± 0.32 days, compared to 2, 317.8 ± 1.1 days when including the older dataset published by DAO and Cambridge/Coravel. Owing to the higher precision of the SES data, we base the orbit calculation only on the STELLA/SES velocities so as to not degrade its solution. We redetermine astrophysical parameters for HD 1 from spectrum synthesis and, together with the new Gaia DR 2 parallax, suggest a higher luminosity than published previously. We conclude that HD 1 is a slightly metal deficient K0 III II giant 217 times more luminous than the Sun. The secondary remains invisible at optical wavelengths. We present evidence for the existence of a third component.