AM Canum Venaticorum star
Rare hydrogen-poor cataclysmic variables with ultra-short orbital periods.
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AM Canum Venaticorum stars, or AM CVn stars, are an uncommon kind of cataclysmic variable. They get their name from their prototype, AM Canum Venaticorum. In these hot, blue binary systems, a white dwarf pulls in hydrogen-poor material from a small, dense companion star.
The two stars orbit each other extremely quickly, with periods under about an hour. Their spectra are unusual, dominated by helium lines while hydrogen is either missing or very faint. These systems are expected to produce strong gravitational waves, powerful enough for the Laser Interferometer Space Antenna (LISA) to detect.
Quick Facts
- Orbital period range
- 10–65 minutes
- Accretor temperature range
- 10,000–20,000 K (up to over 100,000 K in some cases)
- Donor mass range
- 0.01–0.1 solar mass (when heavily stripped)
- Accretion disk brightness range
- absolute magnitude 5–8 in high state, 3–5 magnitudes fainter in low state
- Superhump period
- different from orbital period, larger amplitude
Facts from the source article.
Background
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 Boötis are stars that show outbursting behaviour. These stars occasionally show longer, and sometimes little brighter, superoutbursts. The interval between outbursts is longer on average for stars with longer periods.
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. In the low state, there is no brightness variation but the spectra vary with periods longer than 40 minutes up to around an hour. GP Comae Berenices is the best-known star of this type.
In addition, extreme short period (< 12 minutes) stars show only tiny very rapid brightness variations; ES Ceti and V407 Vulpeculae show this behaviour.
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Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: AM Canum Venaticorum star (CC BY-SA 4.0).
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