55 Ursae Majoris

55 Ursae Majoris (55 UMa) is a triple star system in the constellation Ursa Major. Its apparent magnitude is 4.80. Two stars form a close spectroscopic binary with an orbital period of 2.55 days. The third star orbits the central pair every 1873 days.^[4] All three stars are A-type main-sequence stars.^[4]

55 Ursae Majoris

Observation data Epoch J2000      Equinox J2000
Constellation	Ursa Major
Right ascension	11h 19m 07.89965s[1]
Declination	+38° 11′ 08.0285″[1]
Apparent magnitude (V)	4.80[2] (4.78 / 5.3)[3]
Characteristics
Spectral type	A2V[2] (A1V + A2V + A1V)[4]
U−B color index	+0.04[5]
B−V color index	+0.09[5]
Astrometry
Radial velocity (Rv)	-3.0 ± 2[6] km/s
Proper motion (μ)	RA: -58.80[1] mas/yr Dec.: -65.33[1] mas/yr
Parallax (π)	17.00±0.34 mas[1]
Distance	192 ± 4 ly (59 ± 1 pc)
Absolute magnitude (MV)	-0.09 ± 0.20[4]
Orbit[4]
Primary	55 UMa Aa
Companion	55 UMa Ab
Period (P)	2.5537985 d
Eccentricity (e)	0.323
Periastron epoch (T)	2449602.368
Argument of periastron (ω) (secondary)	116.8°
Semi-amplitude (K1) (primary)	79.1 km/s
Semi-amplitude (K2) (secondary)	89.1 km/s
Orbit[4]
Primary	55 UMa A
Companion	55 UMa B
Period (P)	1872.7 ± 7.4 d
Semi-major axis (a)	0.0913 ± 0.0009″
Eccentricity (e)	0.126 ± 0.008
Inclination (i)	64.8 ± 0.8°
Longitude of the node (Ω)	130.0 ± 0.8°
Periastron epoch (T)	2448805 ± 18
Argument of periastron (ω) (secondary)	223.9 ± 3.7°
Semi-amplitude (K1) (primary)	8.4 ± 1.3 km/s
Semi-amplitude (K2) (secondary)	20.2 ± 6.9 km/s
Details[4]
55 UMa Aa
Mass	2.0 M☉
Surface gravity (log g)	4.25 cgs
Temperature	9230 ± 230 K
Rotational velocity (v sin i)	30 ± 4 km/s
55 UMa Ab
Mass	1.8 M☉
Surface gravity (log g)	4.25 cgs
Temperature	8810 ± 250 K
Rotational velocity (v sin i)	45 ± 5 km/s
55 UMa B
Mass	2.1 M☉
Surface gravity (log g)	4.25 cgs
Temperature	9290 ± 190 K
Rotational velocity (v sin i)	55 ± 5 km/s
Other designations
BD+38 2225, FK5 1293, HD 98353, HIP 55266, HR 4380, SAO 62491
Database references
SIMBAD	data