XY Ursae Majoris
Eclipsing binary star system in the constellation of Ursa Major
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XY Ursae Majoris is a short period binary star system in the northern circumpolar constellation of Ursa Major. It is an eclipsing binary with a baseline apparent visual magnitude of 9.50.[3] The system is located at a distance of 221.5 light years from the Sun based on parallax measurements,[2] but is drifting closer with a radial velocity of −10 km/s.[6] It has a relatively high proper motion, traversing the celestial sphere at the angular rate of 0.191″·yr−1.[10]
| Observation data Epoch J2000.0 Equinox ICRS | |
|---|---|
| Constellation | Ursa Major |
| Right ascension | 09h 09m 55.935s[2] |
| Declination | +54° 29′ 17.72″[2] |
| Apparent magnitude (V) | 9.50[3] |
| Characteristics | |
| Evolutionary stage | main sequence[2] |
| Spectral type | G2V + K5V[4] |
| B−V color index | 0.765±0.039[3] |
| Variable type | Detached eclipsing binary, RS CVn[5] |
| Astrometry | |
| Radial velocity (Rv) | −9.98±0.83[6] km/s |
| Proper motion (μ) | RA: −49.781 mas/yr[2] Dec.: −182.641 mas/yr[2] |
| Parallax (π) | 14.7223±0.0137 mas[2] |
| Distance | 221.5 ± 0.2 ly (67.92 ± 0.06 pc) |
| Absolute magnitude (MV) | 5.41[3] |
| Orbit[7] | |
| Period (P) | 0.4789961 d |
| Semi-major axis (a) | 3.05±0.01 R☉[8] |
| Eccentricity (e) | 0.0 |
| Inclination (i) | 79.81[8]° |
| Periastron epoch (T) | 2,453,821.6344±0.0002 |
| Semi-amplitude (K1) (primary) | 124.74±0.28 km/s |
| Details | |
| Primary | |
| Mass | 1.01±0.01[8] M☉ |
| Radius | 1.22±0.01[8] R☉ |
| Luminosity | 1.10±0.02[8] L☉ |
| Temperature | 5,780[4] K |
| Secondary | |
| Mass | 0.65±0.01[8] M☉ |
| Radius | 0.58±0.01[8] R☉ |
| Luminosity | 0.09±0.01[8] L☉ |
| Temperature | 3,850[4] K |
| Other designations | |
| XY UMa, AG+54 702, BD+55 1317, HD 237786, HIP 44998, SAO 27143, PPM 32024[9] | |
| Database references | |
| SIMBAD | data |
The variability of this system was discovered by W. Strohmeier and its period was determined by R. Kippenhahn, with the findings announced in 1955.[11] It was found to be a detached eclipsing binary system, and by 1963 a variable primary component had been noted.[12] E. H. Geyer made intermittent light curve studies of the system starting with its discovery up until 1975,[11] ascribing variability in the light curve to star spot activity on the primary component. The orbital period of the pair was determined to be 0.479 d,[13] with the orbital plane inclined at an angle of 79.84° to the line of sight from the Earth.[8]
By 1990, enough data had been collected to identify a long term variation of the period, and it was hypothesized this was caused by a third body in the system orbiting the close binary.[14] The orbital period of this component was estimated to be ~30 years in 2001,[15] then refined to 26.7 years by 2010. If the orbital plane of this component is the same as the inner pair, its mass would be 18% of the mass of the Sun. An alternative solution to the period change suggests magnetic activity causes shifts in the angular momentum of the system, but this is considered less likely.[8]
This is a double-lined spectroscopic binary star system with an orbital period of 11.5 h.[7] The primary component is a G-type main-sequence star with a stellar classification of G2V.[4] It is an RS Canum Venaticorum-type variable that is magnetically active[5] and a bright X-ray source.[16] Despite this, relatively few optical flares have been observed.[16] The cooler secondary is a K-type main-sequence star of class K5V that is smaller and less massive than the primary.[4]