Amher Am Herculis stars

- Usually considered a subdivision of the novalike cataclysmic variables, AM Her stars are also known as "polars." These binary systems contain a synchronously rotating, magnetized white dwarf and a cool companion that is near the main sequence. The accretion occurs towards the magnetic poles and is the reason for the name polar. The stars show polarized optical radiation, strong X-ray radiation, short period modulation and long-term bright and low states with orbital periods less than 3.5 hours, not recognized within the GCVS

Observation Key

Faint stars fsq Small amplitudes f\ Short periods <®> CCD or PEP

The exotic star AM Herculis is the namesake of the group of cataclysmic variables known as "polars," a

Tobte Cataclysmic Variable

^CanisVenaticorum

AM Herculis DO Herculis variable stars arranged in alphabetical order by designation Designation (and subclasses)

Novae

Novalike

SW Sextantis TOADS VY Sculptoris Supernovae

U Geminorum (dwarf novae)

WZ Sagittae V Sagittae Z Andromedae

AMCVN helium-rich, ultra-short-period CVs

AMHER CVs with strong magnetic field, known as "polars"

DQHER magnetic, fast spinning CVs known as

"intermediate polars" N (four subclasses indicated below) NA fast novae NB slow novae NC very slow novae

NR recurrent novae (three subclasses indicated below)

T Pyx stars U Sco stars T CrB stars

RW Tri UX UMa SW Sex TOADS

VYSCL anti-dwarf nova stars SN (two subclasses indicated below) SNI type I Supernovae SNII type II Supernovae UG (three subclasses indicated below] UGSS SS Cyg stars UGSU SU UMa stars UGZ Z Cam stars

WZSGE

VSGE V Sagittae stars ZAND Symbiotic systems

* Indicates a designation found within the literature but not recognized within the GCVS.

class of cataclysmic variables in which the magnetic field of the primary star (white dwarf) completely dominates the accretion flow of the system. AM Her was discovered in 1923 by M. Wolf in Heidelberg, Germany during a routine search for variable stars. The star was listed in the GCVS as an irregular variable with a range of brightness from OTO to 14T0. AM Her remained listed as an irregular until 1976, when the true nature of the star was finally understood.

The discovery of AM Herculis introduced a new class of highly magnetic stars to the group of cataclysmic variables. AM Her stars display magnetic field strengths so powerful that it prevents the formation of an accretion disk around the white dwarf (Figure 5.2) and locks the two stars of the binary system together so

Figure 5

conception CV showing dwarf's mi capturing material. A Good.

they always present the same face to each other. The white dwarf star spins at the same rate as the two stars orbit each other, a synchronous rotation that is the defining characteristic of an AM Her star.

The light curve of AM Her (Figure 5.3) appears to have the temperament of a super-violent tornado itself. There is apparently more than one source of radiation wreaking havoc on the star. The variations in AM Her may be thought to belong to two groups, the long-term changes and the short-term changes; characterized by the existence of two different states, one the active or "on" state, in which the luminosity fluctuates around 13™0, and the other inactive or "off' state, where the brightness remains at about 15™0. These two states are thought to be the result of active and inactive mass-transfer rates from the secondary to the primary star.

Figure 5

conception CV showing dwarf's mi capturing material. A Good.

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