New radial velocity observations of AH Her: evidence for material outside the tidal radius

Abstract

Spectroscopic observations of AH Herculis during a deep quiescent state are put forward. We found the object in a rare long minima, allowing us to derive accurately the semi-amplitudes: $K_1=121\pm 4$ \kms\ and $K_2=152\pm 2$ \kms\ and its mass functions $M_W{\sin }^{3}i=0.30\pm 0.01$ M${}_{\odot }$ and $M_R{\sin }^{3}i=0.24\pm 0.02$ M${}_{\odot }$, while its binary separation is given by $a\sin i=1.39\pm 0.02$~R${}_{\odot }$. The orbital period $P_{orb}$~=~ 0.25812~$\pm 0.00032$~days was found from a power spectrum analysis of the radial velocities of the secondary star. These values are consistent with those determined by \citet{Horne:1986}. Our observations indicate that K5 is the most likely spectral type of the secondary. We discuss why we favour the assumption that the donor in AH Her is a slightly evolved star, in which case we find that the best solution for the inclination yields $i=48^{\circ }\pm 2^{\circ }$. Nonetheless, should the donor be a ZAMS star, we obtain that the inclination is between $i=43^{\circ }$ and $i=44^{\circ }$. We also present Doppler tomography of H$\alpha$ and H$\beta$, and found that the emission in both lines is concentrated in a large asymmetric region at low velocities, but at an opposite position to the secondary star, outside the tidal radius and therefore at an unstable position. We also analyse the H$\alpha$ and H$\beta$ line profiles, which show a single broad peak and compare it with the previous quiescent state study which shows a double-peaked profile, providing evidence for its transient nature.

10 pages, 8 figures, 2 tables, accepted for publication on MNRAS

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