Tau Ursae Majoris

Tau Ursae Majoris (τ UMa) is the Bayer designation for a binary star in the northern circumpolar constellation of Ursa Major. It is visible to the naked eye, having an apparent visual magnitude of 4.66.^[2] With an annual parallax shift of 25.82 mas,^[1] it is located about 126 light years from the Sun. At that distance, the visual magnitude is diminished by an extinction factor of 0.19 due to interstellar dust.^[9]

Tau Ursae Majoris

Location of τ Ursae Majoris (circled)
Observation data Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation	Ursa Major
Right ascension	09h 10m 55.06553s[1]
Declination	+63° 30′ 49.0553″[1]
Apparent magnitude (V)	4.66[2]
Characteristics
Spectral type	kA5hF0mF5 II[3]
U−B color index	+0.14[2]
B−V color index	+0.35[2]
Astrometry
Radial velocity (Rv)	−9.80±0.30[4] km/s
Proper motion (μ)	RA: +102.37[1] mas/yr Dec.: −63.55[1] mas/yr
Parallax (π)	25.82±0.54 mas[1]
Distance	126 ± 3 ly (38.7 ± 0.8 pc)
Absolute magnitude (MV)	+1.73[5]
Orbit[6]
Period (P)	1,062.4 d
Eccentricity (e)	0.48
Periastron epoch (T)	2425721.6 JD
Argument of periastron (ω) (secondary)	349.4°
Semi-amplitude (K1) (primary)	3.9 km/s
Details
τ UMa A
Mass	1.8±0.1[7] M☉
Luminosity	16[8] L☉
Surface gravity (log g)	3.86±0.43[9] cgs
Temperature	7,343±100[9] K
Metallicity [Fe/H]	+0.57±0.15[9] dex
Rotational velocity (v sin i)	21[10] km/s
Other designations
τ UMa, 14 Ursae Majoris, BD+64°723, FK5 2727, HD 78362, HIP 45075, HR 3624, SAO 14796, WDS J09109+6331A[11]
Database references
SIMBAD	data

This is a single-lined spectroscopic binary star system with an orbital period of 2.9 years and an eccentricity of 0.48.^[6] The primary member, component A, is an evolved bright giant with a stellar classification of kA5hF0mF5 II.^[3] This notation indicates the star's spectrum shows the calcium K lines of an A5 star, the hydrogen lines of an F0 star, and the metallic lines of an F5 star.^[12] It is an evolved Am star of the ρ Puppis type, a class of evolved stars showing the Am chemical peculiarities.^[13] It is located in the instability strip of the Hertzsprung–Russell diagram but is not thought to be variable.^[7]

Naming

With φ, h, υ, θ, e, and f, it composed the Arabic asterism Sarīr Banāt al-Na'sh, the Throne of the daughters of Na'sh, and Al-Haud, the Pond.^[14] According to the catalogue of stars in the Technical Memorandum 33-507 - A Reduced Star Catalog Containing 537 Named Stars, Al-Haud were the title for seven stars : f as Alhaud I, this star (τ) as Alhaud II, e as Alhaud III, h as Alhaud IV, θ as Alhaud V, υ as Alhaud VI and φ as Alhaud VII .^[15]