AM Canum Venaticorum

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AM Canum Venaticorum
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Canes Venatici
Right ascension 12h 34m 54.60s[1]
Declination +37° 37′ 44.1″[1]
Apparent magnitude (V) +14.18
Characteristics
Spectral type DBp
U−B color index -1.01[2]
B−V color index −0.23[2]
Variable type AM CVn
Astrometry
Distance 1,976+440
−300
ly
(606+135
−93
[3] pc)
Absolute magnitude (MV) 4.90+0.37
−0.45
[4]
Orbit[3]
Period (P) 1,028.7322 ± 0.0003 sec
Inclination (i) 43 ± 2°
Details
A
Mass 0.68 ± 0.06[3] M
B
Mass 0.125 ± 0.012[3] M
Other designations
EGGR 91, HZ 29, GSC 03018-02523, PG 1232+379, WD 1232+37, AAVSO 1229+38.[2]

AM Canum Venaticorum is a cataclysmic variable binary star in the constellation of Canes Venatici. It is the type star of its class of variables, the AM CVn stars. Based upon parallax measurements with the Hubble Space Telescope, this system is located at a distance of about 2,000 light-years (610 parsecs) from the Earth. It has a proper motion of 34.25 ± 0.88 mas yr–1 at a position angle of 67.0 ± 1.7.[4]

During 1939–40, a survey for faint white dwarf stars was carried out using an 18 inches (46 cm) Schmidt telescope at Palomar observatory. Part of the survey was made around the north galactic pole in order to exclude stars of stellar classifications O, B, and A, as these higher mass, shorter-lived stars tend to be concentrated along the plane of the Milky Way galaxy where new star formation occurs. Out of the stars observed, a list of faint blue stars was constructed by Milton L. Humason and Fritz Zwicky in 1947,[5] with their blue hue suggesting a relatively high effective temperature. The 29th star on their list, HZ 29, was found to have the most peculiar spectrum out of the set. It displayed an absence of hydrogen lines, but broad, diffuse lines of neutral (non-ionized) helium.[6] This was interpreted as a hydrogen-deficient white dwarf star. In 1962, this star was observed with a photoelectric detector and was found to vary in magnitude over a period of 18 minutes. The light curve of the variation displayed a double sinusoid pattern.[7] Later, a flickering behavior was observed, which suggested a mass transfer.[4]

The model developed to explain the observations was that AM Canum Venaticorum is a binary system consisting of a pair of white dwarfs in a close orbit. The primary is a more massive white dwarf composed of carbon/oxygen, while the secondary is a less massive white dwarf made of helium, with no hydrogen but traces of heavier elements.[4] (In some AM Canum Venaticorum-type variables, the secondary can be a semi-degenerate object such as Subdwarf B star instead of a helium white dwarf.) Gravitational wave radiation is causing a loss of angular momentum in the orbit, leading to the transfer of helium from the secondary to the primary as the two draw closer.[8] This transfer is occurring because the secondary is overflowing its Roche lobe—a tear drop shaped lobe created by the gravitational interaction between the two stars.[4]

The mass transfer rate between the white dwarfs is estimated as about 7 × 10−9 solar masses per year, which is creating an accretion disk around the companion white dwarf.[3] The energy output from the mass flow onto this accretion disk is actually the primary contributor to the visual luminosity of this system; outshining both of the white dwarfs. The temperature of this disk is about 30,000 K.[3]

High speed photometry of the system shows multiple periods of variation in the luminosity. The main period of 1028.73 seconds (17m 8.73s) is the orbital period of the pair.[8] A secondary period of 1,051 seconds (17m 31s) is believed to be caused by a superhump—an elevated outburst in the signal that occurs with a period slightly longer than the orbital period. The superhump may be the result of an elongation of the accretion disk in combination with precession. The elliptical disk precesses about the white dwarf over a time interval much longer than the orbital period, causing a slight change in the orientation of the disk over each orbit.[9]

Normally this star system only exhibits magnitude variations of 0.05. However, AM CVn star systems such as this are nova-like objects that are known to randomly generate intense flares in luminosity. AM Canum Venaticorum displayed just such flaring behavior twice during the period 1985–1987, with these flares showing rapid fluctuations in luminosity. A 1986 flare caused an increase in magnitude of up to Δm = 1.07 ± 0.03 and lasted for 212 seconds. The amount of energy released during this event is estimated as 2.7 × 1036 erg.[10] These flashes are caused by the brief thermonuclear fusion of helium being accumulated along an outer shell by the primary.[11]

References[edit]

  1. ^ a b Cutri, R. M. et al. (March 2003), "2MASS All-Sky Catalog of Point Sources", VizieR On-line Data Catalog: II/246, Bibcode:2003yCat.2246....0C 
  2. ^ a b c "alf Lyn -- Variable Star", SIMBAD (Centre de Données astronomiques de Strasbourg), retrieved 2011-12-12 
  3. ^ a b c d e f Roelofs, G. H. A. et al. (September 2006), "Kinematics of the ultracompact helium accretor AM Canum Venaticorum", Monthly Notices of the Royal Astronomical Society 371 (3): 1231–1242, arXiv:astro-ph/0606327, Bibcode:2006MNRAS.371.1231R, doi:10.1111/j.1365-2966.2006.10718.x 
  4. ^ a b c d e Roelofs, G. H. A. et al. (September 2007), "Hubble Space Telescope Parallaxes of AM CVn Stars and Astrophysical Consequences", The Astrophysical Journal 666 (2): 1174–1188, arXiv:0705.3855, Bibcode:2007ApJ...666.1174R, doi:10.1086/520491 
  5. ^ Humason, M. L.; Zwicky, F. (January 1947), "A Search for Faint Blue Stars", Astrophysical Journal 105: 85, Bibcode:1947ApJ...105...85H, doi:10.1086/144884 
  6. ^ Greenstein, Jesse L.; Matthews, Mildred S. (July 1957), "Studies of the White Dwarfs. I. Broad Features in White Dwarf Spectra", Astrophysical Journal 126: 14, Bibcode:1957ApJ...126...14G, doi:10.1086/146364 
  7. ^ Smak, J. (February 1967), "18-min. Light-Variations of HZ 29", Information Bulletin on Variable Stars 182: 1, Bibcode:1967IBVS..182....1S 
  8. ^ a b Nelemans, G.; Steeghs, D.; Groot, P. J. (September 2001), "Spectroscopic evidence for the binary nature of AM CVn", Monthly Notices of the Royal Astronomical Society 326 (2): 621–627, arXiv:astro-ph/0104220, Bibcode:2001MNRAS.326..621N, doi:10.1046/j.1365-8711.2001.04614.x 
  9. ^ Pearson, K. J. (July 2007), "Are superhumps good measures of the mass ratio for AM CVn systems?", Monthly Notices of the Royal Astronomical Society 379 (1): 183–189, arXiv:0705.0141, Bibcode:2007MNRAS.379..183P, doi:10.1111/j.1365-2966.2007.11932.x 
  10. ^ Marar, T. M. K. et al. (January 1988), "Flares on AM Canum Venaticorum", Astronomy and Astrophysics 189 (1–2): 119–123, Bibcode:1988A&A...189..119M 
  11. ^ Bildsten, Lars; Shen, Ken J.; Weinberg, Nevin N.; Nelemans, Gijs (June 2007), "Faint Thermonuclear Supernovae from AM Canum Venaticorum Binaries", The Astrophysical Journal 662 (2): L95–L98, arXiv:astro-ph/0703578, Bibcode:2007ApJ...662L..95B, doi:10.1086/519489 

Coordinates: Sky map 12h 34m 54.58s, +37° 37′ 43.4″