Tau Geminorum

Tau Geminorum, Latinized from τ Geminorum, is a star in the northern zodiac constellation of Gemini. It has the apparent visual magnitude of +4.42,^[2] making it visible to the naked eye under suitably good seeing conditions. This star is close enough to the Earth that its distance can be measured using the parallax technique, which yields a value of roughly 392 light-years (120 parsecs).^[1]

Artist's illustration of the giant star Tau Geminorum (left) and its brown dwarf companion—the dark disk at right.

Tau Geminorum

Location of τ Geminorum (circled)
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Gemini
Right ascension	07h 11m 08.3703s[1]
Declination	+30° 14′ 42.590″[1]
Apparent magnitude (V)	+4.42[2]
Characteristics
Spectral type	K2 III[3]
U−B color index	+1.41[2]
B−V color index	+1.261[3]
Astrometry
Radial velocity (Rv)	+22.02±0.07[3] km/s
Proper motion (μ)	RA: −30.725(167) mas/yr[1] Dec.: −48.515(166) mas/yr[1]
Parallax (π)	8.3261 ± 0.1591 mas[1]
Distance	392 ± 7 ly (120 ± 2 pc)
Absolute magnitude (MV)	−0.56±0.05[3]
Details
Mass	2.3±0.3[3] M☉
Radius	30.27+1.08 −1.09[4] R☉
Luminosity	364±14[4] L☉
Surface gravity (log g)	1.96±0.08[3] cgs
Temperature	4,583±70[4] K
Metallicity [Fe/H]	0.14±0.10[3] dex
Rotational velocity (v sin i)	5.8[5] km/s
Age	1.22±0.76[3] Gyr
Other designations
τ Gem, 46 Gem, BD+30 1439, HD 54719, HIP 34693, HR 2697, SAO 59858
Database references
SIMBAD	data

It is an evolved giant star of the spectral type K2 III. It has double^[6] the mass of the Sun and has expanded to 30 times the Sun's radius. Tau Geminorum is radiating 364^[4] times as much radiation as the Sun from its expanded outer atmosphere at an effective temperature of 4,583 K,^[4] giving it the characteristic orange-hued glow of a K-type star. It appears to be rotating slowly with a projected rotational velocity of 5.8 km/s.^[5]

Planetary system

This star has a brown dwarf or Super-Jupiter companion designated Tau Geminorum b, whose mass is at least 20.6 Jupiter masses.^[3] It was discovered in 2004 by Mitchell and colleagues, who also discovered Nu Ophiuchi b at the same time.^[7] This brown dwarf takes 305 days (0.84 years) to revolve around Tau Gem. It may also have a stellar companion; a magnitude 11, K0 dwarf at a projected separation of about 187 AU.^[3]

The Tau Geminorum planetary system^[3]

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
b	≥20.6 MJ	1.17	305.5 ± 0.1	0.031 ± 0.009	—	—

References

1. Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
2. Johnson, H. L.; et al. (1966), "UBVRIJKL photometry of the bright stars", Communications of the Lunar and Planetary Laboratory, 4 (99): 99, Bibcode:1966CoLPL...4...99J
3. Mitchell, David S.; et al. (July 2013), "Precise radial velocities of giant stars. V. A brown dwarf and a planet orbiting the K giant stars τ Geminorum and 91 Aquarii", Astronomy & Astrophysics, 555: 10, arXiv:1305.5107, Bibcode:2013A&A...555A..87M, doi:10.1051/0004-6361/201321714, S2CID 32245543, A87.
4. Baines, Ellyn K.; Clark, James H., III; Schmitt, Henrique R.; Stone, Jordan M.; von Braun, Kaspar (2023-12-01). "33 New Stellar Angular Diameters from the NPOI, and Nearly 180 NPOI Diameters as an Ensemble". The Astronomical Journal. 166 (6): 268. Bibcode:2023AJ....166..268B. doi:10.3847/1538-3881/ad08be. ISSN 0004-6256.{{cite journal}}: CS1 maint: multiple names: authors list (link)
5. Massarotti, Alessandro; et al. (January 2008), "Rotational and Radial Velocities for a Sample of 761 HIPPARCOS Giants and the Role of Binarity", The Astronomical Journal, 135 (1): 209–231, Bibcode:2008AJ....135..209M, doi:10.1088/0004-6256/135/1/209, S2CID 121883397
6. Luck, R. Earle; Heiter, Ulrike (June 2007), "Giants in the Local Region", The Astronomical Journal, 133 (6): 2464–2486, Bibcode:2007AJ....133.2464L, doi:10.1086/513194
7. Mitchell, D. S.; et al. (2004), "Four Substellar Companions Found Around K Giant Stars", Bulletin of the American Astronomical Society, 35: 1234, Bibcode:2003AAS...203.1703M