AM CVn Stars: Status and Challenges

AM CVn stars are the outcome of a fine-tuned binary star evolution pathway. They are helium-rich and their binary orbital periods are less than 65 minutes. They evolve through one or two common envelope (CE) events, which are difficult to model. Observations of AM CVn stars are important to understand the CE phase. Thanks to intensive observing campaigns, the number of AM CVn stars has increased from 5 to 25 during the last 15 yr. We have witnessed long photometric campaigns, time-resolved spectroscopy, UV and X-ray observations, and progress in modeling of the internal structure of donor and accretor stars, disk structure, disk atmosphere, and their evolution. Two possible new members of the AM CVn family have orbital periods of less than 10 minutes. For these, four different models have been proposed, including one without mass transfer, driven by electricity generated by the secondary star moving in the magnetic field of the primary. Short-period AM CVn stars are among the first possible detectable sources of low-frequency gravitational wave (GW) radiation. They are also possible progenitors of a Type Ia supernova (SN Ia) and subluminous explosions, and they can produce helium novae during their evolution. From systematic searches in the Sloan Digital Sky Survey, it has been possible to estimate population densities that can be tested against population synthesis models. One important question to investigate is the relative importance of the three proposed birth channels: a low-mass white dwarf donor, a helium-star donor, or a highly evolved cataclysmic variable (CV) as a donor. A review of the research on AM CVn stars covering the last 15 yr is given, and the outlook for future research is discussed.