Delta Geminorum
Star in the constellation Gemini
From Wikipedia, the free encyclopedia
Delta Geminorum (δ Geminorum, abbreviated Delta Gem, δ Gem), formally named Wasat /ˈweɪsət/,[18][19] is a binary star system in the constellation of Gemini.
 | |
| Observation data Epoch J2000 Equinox ICRS | |
|---|---|
| Constellation | Gemini |
| δ Gem A | |
| Right ascension | 07h 20m 07.3746s[1] |
| Declination | +21° 58′ 56.295″[1] |
| Apparent magnitude (V) | +3.53[2] |
| δ Gem B | |
| Right ascension | 07h 20m 07.0768s[3] |
| Declination | +21° 58′ 52.183″[3] |
| Apparent magnitude (V) | +8.18[4] |
| Characteristics | |
| δ Gem A | |
| Evolutionary stage | main sequence[5] |
| Spectral type | F2VkF0mF0[6] |
| U−B color index | +0.04[2] |
| B−V color index | +0.34[2] |
| δ Gem B | |
| Spectral type | K6V[7] |
| Astrometry | |
| δ Gem A | |
| Radial velocity (Rv) | −15.3±1.5[8] km/s |
| Proper motion (μ) | RA: –27.722 mas/yr[1] Dec.: –14.702 mas/yr[1] |
| Parallax (π) | 53.7692±0.2403 mas[1] |
| Distance | 60.7 ± 0.3 ly (18.60 ± 0.08 pc) |
| Absolute magnitude (MV) | +1.95[9] |
| δ Gem B | |
| Proper motion (μ) | RA: −28.196 mas/yr[3] Dec.: +13.306 mas/yr[3] |
| Parallax (π) | 53.9507±0.0452 mas[3] |
| Distance | 60.45 ± 0.05 ly (18.54 ± 0.02 pc) |
| Orbit[10] | |
| Period (P) | 550±450 yr |
| Semi-major axis (a) | 13.8±1.6″ |
| Eccentricity (e) | 0.98±0.30 |
| Inclination (i) | 75±10° |
| Longitude of the node (Ω) | 128±51° |
| Periastron epoch (T) | 2136±57 |
| Argument of periastron (ω) (secondary) | 266±32° |
| Details | |
| δ Gem A | |
| Mass | 1.57[11] M☉ |
| Radius | 2.06±0.25[12] R☉ |
| Luminosity | 9.88[13] L☉ |
| Temperature | 6,900[14] K |
| Metallicity [Fe/H] | –0.26[14] dex |
| Rotational velocity (v sin i) | 129.7[15] km/s |
| Age | 1.6[14] Gyr |
| δ Gem B | |
| Mass | 0.810+0.111 −0.080[16] M☉ |
| Radius | 0.597+0.043 −0.039[16] R☉ |
| Luminosity | 0.189+0.006 −0.005[16] L☉ |
| Surface gravity (log g) | 4.795+0.115 −0.067[16] cgs |
| Temperature | 4,931+123 −138[16] K |
| Other designations | |
| Wasat, 55 Geminorum, BD+22°1645, FK5 279, Gl 271, HD 56986, HIP 35550, HR 2777, SAO 79294[17] | |
| Database references | |
| SIMBAD | δ Gem A |
| δ Gem B | |
Nomenclature
δ Geminorum (Latinised to Delta Geminorum) is the system's Bayer designation.
It bore the traditional name Wasat, which derives from the Arabic word for "middle".[20][21] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[22] to catalogue and standardize proper names for stars. The WGSN approved the name Wasat for Delta Geminorum A on 21 August 2016 and it is now so entered in the IAU Catalog of Star Names.[19]
In Chinese, 天樽 (Tiān Zūn), meaning Celestial Wine Cup, refers to an asterism consisting of Delta Geminorum, 57 Geminorum and Omega Geminorum.[23] Consequently, Delta Geminorum itself is known as 天樽二 (Tiān Zūn èr, English: the Second Star of Celestial Wine Cup.).[24] From this Chinese name, the name Ta Tsun has appeared.[25]
Properties
Delta Geminorum A is a main-sequence star with the stellar classification F2VkF0mF0.[6] This star has 1.57 times the mass of the Sun[11] and is rotating rapidly with a projected rotational velocity of 129.7 km s−1.[15] The estimated age is 1.6 billion years.[14]
It has an apparent visual magnitude of +3.53,[2] allowing it to be seen with the naked eye. It is 0.18 degree south of the ecliptic so it is occasionally occulted by the Sun, Moon and, rarely, by a planet; and is eclipsed by the sun from about 10-12 July.[27] Thus the star can be viewed the whole night, crossing the sky, in mid-January. The last occultation by a planet was by Saturn on June 30, 1857, and the next will be by Venus on August 12, 2420.[citation needed] In 1930, the dwarf planet Pluto was discovered about 0.5° to the east of this star by American astronomer Clyde Tombaugh.[28]
Delta Geminorum A has been reported to be a single-lined spectroscopic binary in 1964, with a preliminary orbital solution indicating an orbital period of 2,238.6 days, a minimum semi-major axis of 5.22 au and an eccentricity of 0.353,[29] although these orbital elements could not be confirmed with proper motion measurements[30] and the companion was never directly detected,[31][32] making it unlikely that Delta Geminorum A is a binary system.[31] The binary mass function, together with the mass of the visible star, would imply a secondary mass of at least 6 solar masses, making it a black hole.[32]
The companion is a cooler class K star,[7] not apparent to the naked eye, but clearly visible in a small telescope.[20] The orbital period is uncertain, 550±450 years, and the eccentricity is also uncertain, at 0.98±0.30. The angular semi-major axis is somewhat better-determined, at 13.8±1.6".[10] The star has 59% the radius of the Sun, 18.9% the luminosity and an effective temperature of 4,952 K.[33]
The system is now approaching the Solar System at a radial velocity of −15.3 km/s, and is expected to reach its closest distance in 1.1 million years, at about 6.7 ly (2.1 pc).[8]