EZ Canis Majoris
Epoch J2000.0 Equinox J2000.0 (ICRS)
|Right ascension||06h 54m 13.04410s|
|Declination||−23° 55′ 42.0150″|
|Apparent magnitude (V)||6.91 6.71 to 6.95|
|U−B color index||−0.89|
|B−V color index||−0.28|
|Proper motion (μ)|| RA: −4.431±0.057 mas/yr |
Dec.: 2.884±0.092 mas/yr
|Parallax (π)||0.4119 ± 0.0503 mas|
|Distance||4,900 ly |
|Absolute magnitude (MV)||−5.33|
|Period (P)||3.63 d|
|Semi-major axis (a)||0.13 au|
|Luminosity (bolometric)||620,000 L☉|
EZ Canis Majoris (abbreviated to EZ CMa, also designated as WR 6) is binary system in the constellation of Canis Major. The primary is a Wolf-Rayet star and it is one of the ten brightest Wolf-Rayet stars, brighter than apparent magnitude 7.
EZ CMa has an apparent visual magnitude which varies between 6.71 and 6.95 over a period of 3.766 days, along with changes in the spectrum. It has been proposed that it could be a binary star, with a neutron star as companion that would complete an orbit around the Wolf-Rayet with that period, being it the cause of those variations. The General Catalogue of Variable Stars lists it as a possible cataclysmic variable on this basis. Its has been argued that the companion does not exist and spectral variations are caused by activity on the star's surface.
Observations of the light variations over a four month period from late 2015 to early 2016 confirmed the clear 3.76 d variations. This was interpreted as a 3.66 d orbital period with rapid apsidal precession completing a full rotation in about 100 days. The orbit is inclined at around 60-74 degrees and there are two eclipses during each orbit.
Wolf Rayet star and nebula
The spectral type of WN4 indicates an extremely hot star, and this leads to a very high luminosity, mostly emitted as ultraviolet radiation. The spectrum shows a star entirely devoid of hydrogen at the surface.
EZ CMa is surrounded by a faint bubble nebula, a small HII region blown by stellar winds up to 1,700 km/s and ionised by the intense UV radiation. This is catalogued as Sharpless 308 or just S308. It is likely to be a member of the very scattered open cluster Collinder 121, found around the orange supergiant ο1 CMa.
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