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

Abstract

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 thatHD1 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.