| Abstract
| - Context. AM Canum Venaticorum stars are close, interacting binary systems with a white dwarf primary surrounded by a helium-dominated accretion disk. Because of the short orbital periods, the unseen secondary must be compact, too. Different evolutionary scenarios for the formation of these systems resulting in different kinds of secondaries are possible. Aims. Quantitative spectral analyses of accretion-disk spectra can reveal the nature of the secondary. We investigate a particular AM CVn system, discovered recently by the Palomar Transient Factory, PTF 09hpk. It is the only known AM CVn star exhibiting unambiguously both outbursts and superoutbursts. We characterize the accretion disk in quiescence and determine abundances of trace elements. Methods. We calculated line-blanketed nonlocal thermodynamic equilibrium (non-LTE) models and computed accretion-disk spectra by self-consistent solution of radiative transfer and vertical structure equations. These are compared to optical spectra taken during quiescence. Results. In quiescence, the disk has a uniform effective temperature of 6000 K. An optically thin boundary layer causes the observed He iiλ 4686 Å emission feature. The disk around the massive white dwarf primary ( 1.2 M⊙) is dominated by helium and is strongly hydrogen-deficient ( <−5 dex relative to the solar value). For heavy elements (C, N, O, Mg, Si, Fe), we find moderate to strong subsolar abundances, with depletions down to −2.5 dex for iron. The N/O ratio is >10. Conclusions. According to the high N/O ratio, the secondary of PTF 09hpk is either a helium white dwarf or a helium star. The low metallicity points to a halo origin of the system or it might be the consequence of heavy-metal sedimentation if the secondary is an old, low-mass helium white dwarf.
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