AM CVn Stars: Status and Challenges

Abstract

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.

1000 cataclysmic variables from the Catalina Real-time Transient Survey

“…Currently, 39 AM CVn binaries are known (see e.g. Solheim 2010 for an overview, and Carter et al 2013a, 2014and Kupfer et al 2013 for recent discoveries).…”

Accretion Processes in CV AM CVn SDSS J090221.35+381941.9

“…As Figure 1 and Table 2 suggest that either the CV star or helium star channel is possible, the deciding factor may be the chemistry. As discussed in Solheim (2010), a H/He ratio of 10 −3 could indicate the hydrogen star channel whereas a H/He…”

“…Thus, both channels remain contenders for evolution of J0902. Let's look at each channel more closely, starting with the helium star channel: As discussed in Solheim (2010), once the donor's mass reduces to ∼ 0.01 − 0.03M ⊙ and the orbital period has grown to P orb 35 − 40 minutes, the donor may develop into a fully degenerate cooling white dwarf. The helium star channel and the white dwarf channel merge, as discussed in §1 and as shown in Figure 1, and the donor evolves into a hybrid white dwarf as it migrates from the helium star channel to the white dwarf channel.…”

“…This channel is known as the hydrogen star (a.k.a, CV) channel. Although not discussed in Solheim (2010), an end result that cannot be ruled out for this track is a supernova type Ia (SN Ia).…”

Massive double white dwarfs and the AM CVn birthrate

“…Despite large scale efforts to find AM CVn (Carter et al 2013), there are only 52 systems currently known. Hence, AM CVn represent the end product of a rare and fine-tuned evolution in binary systems (Solheim 2010).…”