AM CVn star

An AM CVn star, or AM Canum Venaticorum star, is a rare type of cataclysmic variable star named after their type star, AM Canum Venaticorum. In these hot blue binary variables, a white dwarf accretes hydrogen-poor matter from a compact companion star.

These binaries have extremely short orbital periods (shorter than about one hour) and have unusual spectra dominated by helium with hydrogen absent or extremely weak. They are predicted to be strong sources of gravitational radiation, strong enough to be detected with the Laser Interferometer Space Antenna.

AM CVn stars differ from most other cataclysmic variables (CVs) in the lack of hydrogen lines from their spectra. They show a broad continuum corresponding to hot stars with complex absorption or emission lines. Some stars show absorption lines and emission lines at different times. AM CVn stars have long been known to exhibit three types of behaviour: an outbursting state; a high state; and a low state.

In the outbursting state, stars show strong variability with periods of 20–40 minutes. The stars V803 Centauri and CR Bootes are stars that show outbursting behaviour. These stars occasionally show longer, and sometimes little brighter, super-outbursts. The interval between outbursts is longer on average for stars with longer periods. The spectra show strong helium absorption lines during the outbursts, with many weaker emission lines of helium and iron near minimum. The spectral lines are typically doubled, producing broad flat-bottom absorption lines and sharp double-peaked emission lines. This is the most common type of AM CVn variable, possibly because they are most easily detected.

In the high state, stars show brightness variations of a few tenths of a magnitude with multiple short periods, less than or around 20 minutes. AM CVn itself shows this state, along with the other bright example HP Librae. Variations often occur most strongly with one or two periods, and the beat period between them. The spectra show absorption lines mainly of helium, and the high state is so named as it is similar to a permanent outburst.

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