An analysis of the Ap binary HD 81009

We present a detailed investigation of the orbit, component characteristics and magnetic field of the single spectrum (SB1), visual Ap binary HD 81009 . By simultaneously modeling new and archival radial velocity measurements an d new and archival speckle interferometric measurements (obtained with the CHARA array) we obtain a unique model of the orbital geometry and constrain ts on the component masses of HD 81009. Additional constraints provided by the Hipparcos parallax and component magnitude difference and the optical s pectral energy distribution allow us to determine a self-consistent solutio n for the basic physical properties of the components. HD 81009 is a highly eccentric (e = 0.718), long-period (P-orb = 29.3 y) binary composed of two main sequence A-type stars. While its presence is required in order to exp lain the astrometric and photometric observations, the cooler secondary com ponent is never detected spectroscopically, and is therefore inferred to ro tate somewhat more rapidly. than the hotter component. The hotter primary component is identified as the slowly-rotating (P-rot = 33(d).984) magnetic Ap star. We have modeled the magnetic field geometry of this star using new and archival longitudinal magnetic field and mean magn etic field modulus observations. The rotational variations of the magnetic quantities are consistent with a decentered dipole surface magnetic field g eometry with small magnetic obliquity (beta < 20<degrees>). This is consist ent with the observation of Landstreet & Mathys (2000), who report that nea rly all magnetic Ap stars with periods longer than around 25 days exhibit b eta < 20<degrees>, implying that their magnetic fields are approximately al igned with their rotational axes.