4 Ursae Majoris

4 Ursae Majoris (sometimes abbreviated 4 Uma) is the Flamsteed designation of a star in the northern circumpolar constellation of Ursa Major. It also bears the Bayer designation of Pi2 Ursae Majoris (Pi2 UMa, π2 Ursae Majoris, π2 UMa) and is traditionally named Muscida. With an apparent visual magnitude of +4.6,[2] this star is visible from suburban or darker skies based upon the Bortle Dark-Sky Scale. From parallax measurements made during the Hipparcos mission, this star is at a distance of 256 light-years (78 parsecs) from Earth.[1] As of 2011, one extrasolar planet has been confirmed to be orbiting the star.

This article is about π2 Ursae Majoris. For other stars with this Bayer designation, see π Ursae Majoris.
Pi2 Ursae Majoris
Diagram showing star positions and boundaries of the Centaurus constellation and its surroundings
Diagram showing star positions and boundaries of the Centaurus constellation and its surroundings

Location of π2 Ursae Majoris (circled) near the center
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Ursa Major
Right ascension 08h 40m 12.81767s[1]
Declination +64° 19′ 40.5700″[1]
Apparent magnitude (V) +4.620[2]
Characteristics
Spectral type K2 III[3]
U−B color index +1.193[2]
B−V color index +1.159[2]
Astrometry
Radial velocity (Rv)+14.62[4] km/s
Proper motion (μ) RA: −60.05[1] mas/yr
Dec.: +26.40[1] mas/yr
Parallax (π)12.74 ± 0.26 mas[1]
Distance256 ± 5 ly
(78 ± 2 pc)
Absolute magnitude (MV)0.150[3]
Details
Mass1.234 ± 0.15[5] M☉
Radius18.79±0.38[6] R☉
Luminosity112.4±10.0[6] L☉
Surface gravity (log g)1.8 ± 0.15[5] cgs
Temperature4336±99[6] K
Metallicity [Fe/H]-0.25 ± 0.04[5] dex
Rotational velocity (v sin i)8[7] km/s
Age4.18 ± 1.95[3] Gyr
Other designations
π2 Ursae Majoris, π2 UMa, Pi2 UMa, 4 Ursae Majoris, BD+64°698, FK5 2677, GC 11850, HD 73108, HIP 42527, HR 3403, PPM 16713, SAO 14616
Database references
SIMBADdata

Properties

This star has a stellar classification of K2 III,[3] indicating that, at an estimated age of around four billion years,[3] it is an evolved star that has reached the giant stage. It has a mass about 1.2 times larger than the Sun, but has expanded to 18 times the Sun's girth.[5] The effective temperature of the star's outer atmosphere is 4,415 K.[5] This heat gives it the cool, orange-hued glow of a K-type star.[8]

Pi2 Ursae Majoris is a member of the Milky Way galaxy's thin disk population. It is following an orbit through the galaxy with an eccentricity of 0.10, which carries it as close to the Galactic Center as 27.7 kilolight-years (8.5 kiloparsecs) and as far as 34.1 kly (10.5 kpc). The inclination of this orbit lies close to the galactic plane, so it departs this plane by no more than 260 ly (80 pc).[3]

Planetary system

Based upon observed radial velocity changes in the star, in 2007 the presence of a planetary companion was announced. The planet, designated 4 Ursae Majoris b, is at least seven times more massive than Jupiter. Its orbit is eccentric, orbiting 4 Ursae Majoris at 87% the distance from Sun to Earth. Compared to the Sun, this star has a lower abundance of elements other than hydrogen and helium, what astronomers term the star's metallicity. This is curious, because most main-sequence stars with planets tend to have a higher abundance of metals.[5]

The 4 Ursae Majoris planetary system[5]
Companion
(in order from star)		 Mass Semimajor axis
(AU)		 Orbital period
(days)		 Eccentricity Inclination Radius
b >7.1 ± 1.6 MJ 0.87 ± 0.04 269.3 ± 1.96 0.432 ± 0.024 — —

Naming and etymology

With π1, σ1, σ2, ρ, A and d, it composed the Arabic asterism Al Ṭhibā᾽, the Gazelle.[9] According to the catalogue of stars in the Technical Memorandum 33-507 - A Reduced Star Catalog Containing 537 Named Stars, Al Ṭhibā were the title for seven stars: A as Althiba I, π1 as Althiba II, this star (π2) as Althiba III, ρ as Althiba IV, σ1 as Althiba V, σ2 as Althiba VI, and d as Althiba VII.[10]

See also

References
  1. van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357, S2CID 18759600
  2. Jennens, P. A.; Helfer, H. L. (September 1975), "A new photometric metal abundance and luminosity calibration for field G and K giants", Monthly Notices of the Royal Astronomical Society, 172 (3): 667–679, Bibcode:1975MNRAS.172..667J, doi:10.1093/mnras/172.3.667
  3. Soubiran, C.; et al. (2008), "Vertical distribution of Galactic disk stars. IV. AMR and AVR from clump giants", Astronomy and Astrophysics, 480 (1): 91–101, arXiv:0712.1370, Bibcode:2008A&A...480...91S, doi:10.1051/0004-6361:20078788, S2CID 16602121
  4. Famaey, B.; et al. (January 2005), "Local kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the concept of superclusters", Astronomy and Astrophysics, 430: 165–186, arXiv:astro-ph/0409579, Bibcode:2005A&A...430..165F, doi:10.1051/0004-6361:20041272, S2CID 17804304
  5. Döllinger, M. P.; et al. (2007), "Discovery of a planet around the K giant star 4 Ursae Majoris", Astronomy and Astrophysics (abstract), 472 (2): 649–652, arXiv:astro-ph/0703672, Bibcode:2007A&A...472..649D, doi:10.1051/0004-6361:20066987, S2CID 17662368
  6. Baines, Ellyn K.; et al. (2010). "Angular Diameters and Effective Temperatures of 25 K Giant Stars from the CHARA Array". Astrophysical Journal. 710 (2): 1365–1374. arXiv:0912.5491. Bibcode:2010ApJ...710.1365B. doi:10.1088/0004-637X/710/2/1365. S2CID 799107.
  7. Bernacca, P. L.; Perinotto, M. (1970), "A catalogue of stellar rotational velocities", Contributi Osservatorio Astronomico di Padova in Asiago, 239 (1): 1, Bibcode:1970CoAsi.239....1B
  8. "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, archived from the original on February 22, 2012, retrieved 2012-01-16
  9. Allen, Richard Hinckley (1899), Star-Names and Their Meanings, New York: G. E. Stechert, p. 444
  10. Rhoads, Jack W. (November 15, 1971), Technical Memorandum 33-507-A Reduced Star Catalog Containing 537 Named Stars (PDF), Jet Propulsion Laboratory, California Institute of Technology.
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