Mira B
Star in the constellation Cetus
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Mira B, also known as VZ Ceti, is the companion star to the variable star Mira, probably a white dwarf. The two are separated by around 100 AU. Suspected as early as 1918, it was visually confirmed in 1923 by Robert Grant Aitken, and has been observed more or less continually since then, most recently by the Chandra X-Ray Observatory.[7]
The red giant star Mira (right), and its companion Mira B on the left. Taken on December 11, 1995, by the Hubble Space Telescope using the Faint Object Camera. | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Cetus |
| Right ascension | 02h 19m 20.80s[1] |
| Declination | −02° 58′ 40.0″[1] |
| Apparent magnitude (V) | 9.5–12.0[2] |
| Characteristics | |
| Evolutionary stage | white dwarf[3] |
| Spectral type | DA[4] |
| Variable type | unique[2] |
| Astrometry | |
| Distance | approx. 300[5] ly (approx. 90 pc) |
| Details | |
| Mass | 0.24±0.04 or 0.42±0.04[3]: 6 M☉ |
| Radius | 0.015 or 0.020[3]: 3 R☉ |
| Temperature | 14,000[6] K |
| Other designations | |
| VZ Cet, ο Cet B, WDS J02193-0259Ab, CCDM J02194-0258P, WD 0216-032 | |
| Database references | |
| SIMBAD | data |
Long known to be erratically variable itself, its fluctuations seem to be related to its accretion of matter from Mira's stellar wind, which makes it a symbiotic star.[8]
Orbit
Current research
In January 2007, astronomers at the Keck Observatory announced the discovery of a protoplanetary disk around Mira B. Discovered via infrared data, the disk is apparently derived from captured material from Mira itself; Mira B accretes as much as one percent of the matter lost by its primary. Though planetary formation is perhaps unlikely as long as the disk is in active accretion, it may proceed apace once Mira A completes its red giant phase and becomes a white dwarf remnant.[12]
Several factors, such as low x-ray luminosity, suggest that Mira B is actually a normal main-sequence star of spectral type K and roughly 0.7 solar mass, rather than a white dwarf as first envisioned. However, a 2010 analysis of rapid optical brightness variations has indicated that Mira B is, in fact, a white dwarf.[6] Although most white dwarfs have masses close to 0.6 M☉ and Mira B is generally assumed to have it too, measurements based on the accretion suggest a mass value of 0.24±0.04 M☉ in the case of accretion through overflow of the red giant's Roche lobe by its stellar wind, or 0.42±0.04 M☉ in the case of Bondi–Hoyle–Lyttleton accretion. Mass-radius relations give radii of 0.020 or 0.015 R☉, respectively. The case of wind roche lobe overflow is more probable, so Mira B is likely an extremely low-mass white dwarf.[3]