A dynamical model for the dwarf nova AH Herculis

We develop a dynamical model of the dwarf nova binary AH Herculis based upon high-resolution phase-resolved spectroscopy from the 2.5-m Hooker reflector at Mt Wilson. The distorted double-peaked Balmer emission lines from the accretion disc surrounding the white dwarf primary star have a peak-to-peak separation of 600 km s⁻¹. From radial velocity variations of the emission lines over a one-year baseline we determine the binary period P=0.258116±0.000004 day. We detect the weak absorption spectrum of the cool mass-losing companion star AH Her B, and use a cross-correlation method to measure its orbit radial velocity curve. The spectral type of AH Her B is early-to-middle K, but the Mg b lines are much weaker than expected relative to Fe I. The radial velocity semi-amplitudes $$K_\text {ems}=126\pm4\enspace \text {km}\enspace \text s^{-1}$$ for the emission line wings and $$K_\text {abs}=158\pm8\enspace \text {km}\enspace \text s^{-1}$$ for the absorption lines imply mass functions $$M_W\text {sin}^3\enspace i=0.34\pm0.04\enspace M_\odot,\enspace M_R\text {sin}^3\enspace i=0.27\pm0.03\enspace M_\odot,\enspace \text {and}\enspace a\text {sin}\enspace i=14.5\pm0.05\enspace R_\odot$$. If we impose an empirical ZAMS mass-radius relationship on the companion star, then $$i=46^\circ\pm3^\circ,\enspace M_W=0.95\pm0.10\enspace M_\odot\enspace \text {and}\enspace M_R=0.76\pm0.08\enspace M_\odot$$.