AM Canum Venaticorum 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 cannibalises 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 larger 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.
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 Coma Berenices is the best-known star of this type. Spectra show mainly emission and the state is similar to a permanent minimum of the outbursting stars.
AM CVn variables' ultra-short orbital periods of 10–65 minutes indicate that both the donor star and accretor star are degenerate or semi-degenerate objects. While the accretor is always a white dwarf, the donor star can potentially be either a (helium or hybrid) white dwarf, a low-mass helium star or an evolved main-sequence star.
The observed states have been related to four binary system states:
- Ultrashort orbital periods less than 12 minutes have no accretion disk and show direct impact of the accreting material onto the white dwarf, or possibly have a very small accretion disk.
- Systems with periods between 12 and 20 minutes form a large stable accretion disk and appear permanently in outburst, comparable to hydrogen-free dwarf nova.
- Systems with periods of 20–40 minutes form variable disks which show occasional outbursts, comparable to hydrogen-free SU Ursae Majoris variables.
- Systems with orbital periods longer than 40 minutes form small stable accretion disks, comparable to a quiescent dwarf nova.
There are three possible types of donor stars in an AM CVn variable binary, although the accretor is always a white dwarf. Each binary type forms through a different evolutionary path, although all involve initially close main sequence binaries passing through one or more common envelope phases as the stars evolve away from the main sequence.
AM CVn stars with a white-dwarf donor can be formed when a binary consisting of a white dwarf and a low-mass giant evolve through a common-envelope (CE) phase. The outcome of the CE will be a double white-dwarf binary. Through the emission of gravitational radiation, the binary loses angular momentum, which causes the binary orbit to shrink. When the orbital period has shrunk to about 5 minutes, the least-massive (and the largest) of the two white dwarfs will fill its Roche lobe and start mass transfer to its companion. Soon after the onset of mass transfer, the orbital evolution will reverse and the binary orbit will expand. It is in this phase, after the period minimum, that the binary is most likely to be observed.
AM CVn stars with a helium-star donor are formed in a similar way, but in this case the giant that causes the common envelope is more massive and produces a helium star rather than a second white dwarf. A helium star is more expanded than a white dwarf, and when gravitational radiation brings the two stars into contact, it is the helium star which will fill its Roche lobe and start mass transfer, at an orbital period of roughly 10 minutes. As in the case of a white-dwarf donor, the binary orbit is expected to 'bounce' and start expanding soon after mass transfer is started, and we should typically observe the binary after the period minimum.
The third type of potential donor in an AM CVn system is the evolved main-sequence star. In this case, the secondary star does not cause a common envelope, but fills its Roche lobe near the end of the main sequence (terminal-age main sequence or TAMS). An important ingredient for this scenario is magnetic braking, which allows efficient angular-momentum loss from the orbit and hence a strong shrinkage of the orbit to ultra-short periods. The scenario is rather sensitive to the initial orbital period; if the donor star fills its Roche lobe too long before the TAMS the orbit will converge, but bounce at periods of 70–80 minutes, like ordinary CVs. If the donor starts mass transfer too long after the TAMS, the mass-transfer rate will be high and the orbit will diverge. Only a narrow range of initial periods, around this bifurcation period will lead to the ultra-short periods that are observed in AM CVn stars. The process of bringing the two stars into a close orbit under the influence of magnetic braking is called magnetic capture. AM CVn stars formed this way may be observed either before or after the period minimum (which can lie anywhere between 5 and 70 minutes, depending on exactly when the donor star filled its Roche lobe) and are assumed to have some hydrogen on their surface.
Before settling into an AM CVn state, binary systems may undergo several Helium nova explosions, of which V445 Puppis is a possible example. AM CVn systems are expected to transfer mass until one component becomes a dark sub-stellar object, but it is possible that they could result in a type Ia supernova, probably a sub-luminous form known as a type .Ia or Iax.
- Solheim, J.-E. (2010). "AM CVn Stars: Status and Challenges". Publications of the Astronomical Society of the Pacific 122 (896): 1133. Bibcode:2010PASP..122.1133S. doi:10.1086/656680.
- Nelemans, G. (August 2005). "AM CVn stars". In Hameury, J.-M.; Lasota, J.-P. The Astrophysics of Cataclysmic Variables and Related Objects, Proceedings of ASP Conference 330. San Francisco: Astronomical Society of the Pacific. p. 27. arXiv:astro-ph/0409676. Bibcode:2005ASPC..330...27N. ISBN 1-58381-193-1.
- Kotko, I.; Lasota, J.-P.; Dubus, G.; Hameury, J.-M. (2012). "Models of AM Canum Venaticorum star outbursts". Astronomy & Astrophysics 544: A13. Bibcode:2012A&A...544A..13K. doi:10.1051/0004-6361/201219156.