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Mu Arae

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   CAPTION: Mu Arae

   Mu Arae (NASA Nstars database)
   Mu Arae
                             Observation data
   Epoch J2000.0
   Constellation             Ara
   Right ascension           17^h 44^m 08.7^s
   Declination               −51° 50' 03"
   Apparent magnitude (V)    +5.12
                              Characteristics
   Spectral type             G3IV–V
   B-V colour index          0.694
   U-B colour index           ?
   Variable type             none
                                Astrometry
   Radial velocity (R[v])    −9.0 km/s
   Proper motion (μ)         RA: −15.06 mas/ yr
                             Dec.: −191.17 mas/ yr
   Parallax (π)              65.46 ± 0.80 mas
   Distance                  49.8 ± 0.6 ly
                             (15.3 ± 0.2 pc)
   Absolute magnitude (M[V]) +4.28
                                  Details
   Mass                      1.10 ± 0.05 M[☉]
   Radius                    1.3150 ± 0.0190 R[☉]
   Luminosity                1.75 ± 0.10 L[☉]
   Temperature               5813 ± 40 K
   Metallicity               210 ± 20%
   Rotation                  31 days
   Age                       6410 million years
                            Other designations
   15 Arae, HR 6585, HD 160691, Gl 691, CD-51°11094, FK5 662, HIP 86796,
   SAO 244981, GC 24024

   Mu Arae (μ Ara / μ Arae) is a Sunlike yellow-orange star located around
   50 light years away in the constellation Ara. The star has a planetary
   system with four known planets. The system's innermost planet was the
   first "hot Neptune" to be discovered.

Distance and visibility

   According to measurements made by the Hipparcos astrometric satellite,
   Mu Arae exhibits a parallax of 65.46 milliarcseconds as the Earth moves
   around the Sun. When combined with the known distance from the Earth to
   the Sun, this means the star is located at a distance of 49.8 light
   years (15.3 parsecs). Seen from Earth it has an apparent magnitude of
   +5.12 and is visible to the naked eye.

Stellar characteristics

   Mu Arae is estimated to be slightly more massive than our Sun at around
   1.10 solar masses. Based on the abundance of iron, it is around twice
   as enriched in heavy elements as our Sun, and is therefore described as
   metal-rich. Its surface temperature of around 5800 K is similar to our
   Sun. The star has a radius estimated to be 31.5% greater than our Sun
   and is 75% more luminous.

   As a star ages, the level of activity in its chromosphere is expected
   to decline. Based on observed activity levels, the age of Mu Arae is
   estimated to be around 6,410 million years or 1,450 million years,
   depending on the theoretical model used. Another way to estimate the
   star's age is to match its properties with models of stellar evolution.
   This method suggests an age of around 4,400 million years.

   Mu Arae has a listed spectral type of G3IV–V. The G3 part means the
   star is classified as yellow in colour, similar to our Sun (a G2V
   star). The star may be entering the subgiant stage of its evolution as
   it starts to run out of hydrogen in its core. This is reflected in its
   uncertain luminosity class, between IV (the subgiants) and V ( main
   sequence dwarf stars like the Sun).

Planetary system

   The orbits of the outer three planets in the Mu Arae system compared to
   those in our solar system. Central star is not to scale. At the scale
   of this picture, the innermost planet would be located at the edge of
   the disc representing the central star.
   Enlarge
   The orbits of the outer three planets in the Mu Arae system compared to
   those in our solar system. Central star is not to scale. At the scale
   of this picture, the innermost planet would be located at the edge of
   the disc representing the central star.

   As of August 2006, four extrasolar planets have been detected in orbit
   around Mu Arae. Three have high masses and are likely to be gas giants.
   The innermost planet has a mass comparable to that of Uranus and may
   either be a small gas giant or a large terrestrial planet.

Discovery

   In 2001, an extrasolar planet was announced by the Anglo-Australian
   Planet Search team, together with the planet orbiting Epsilon Reticuli.
   The planet, designated Mu Arae b, was thought to be in a highly
   eccentric orbit of around 743 days. The discovery was made by analysing
   variations in the star's radial velocity (measured by observing the
   Doppler shift of the star's spectral lines) as a result of being pulled
   around by the planet's gravity.

   Further observations revealed the presence of a second object in the
   system ( Mu Arae c), which was published in 2004. At the time, the
   parameters of this planet were poorly constrained and it was thought to
   be in an orbit of around 8.2 years with a high eccentricity.

   Later in 2004, a small inner planet designated Mu Arae d was announced
   with a mass comparable to that of Uranus in a 9-day orbit. This was the
   first of the class of planets known as "hot Neptunes" to be discovered.
   The discovery was made by making high-precision radial velocity
   measurements with the High Accuracy Radial Velocity Planet Searcher
   (HARPS) spectrograph.

   In 2006, two teams, one led by Krzysztof Goździewski and the other by
   Francesco Pepe independently announced four-planet models for the
   radial velocity measurements of the star, with a new planet ( Mu Arae
   e) in a near-circular orbit lasting approximately 311 days. The new
   model gives revised parameters for the previously known planets, with
   lower eccentricity orbits than in the previous model. The discovery of
   the fourth planet made Mu Arae the second known four-planet extrasolar
   system, after 55 Cancri.

System structure

   The Mu Arae system consists of an inner Uranus-mass planet in a tight
   9-day orbit and three massive planets, probably gas giants, on wide,
   near-circular orbits, which contrasts with the high-eccentricity orbits
   typically observed for long-period extrasolar planets. The Uranus-mass
   planet may be a chthonian planet, the core of a gas giant which has had
   its outer layers stripped away by stellar radiation. Alternatively it
   may have formed in the inner regions of the Mu Arae system as a rocky
   "super-Earth".

   The inner gas giants "e" and "b" are located close to the 2:1 orbital
   resonance which causes them to undergo strong interactions. The
   best-fit solution to the system is actually unstable: simulations
   suggest the system is destroyed after 78 million years, which is
   significantly shorter than the estimated age of the star system. More
   stable solutions, including ones in which the two planets are actually
   in the resonance (similar to the situation in the Gliese 876 system)
   can be found which give only a slightly worse fit to the data.

   Searches for circumstellar discs show no evidence for a debris disc
   similar to the Kuiper belt around Mu Arae. If Mu Arae does have a
   Kuiper belt, it is too faint to be detected with current instruments.

        Companion
   (In order from star)   Mass
                        ( M[J])  Orbital period
                                     (days)      Semimajor axis
                                                     ( AU)      Eccentricity
            d           >0.03321 9.6386 ± 0.0015    0.09094     0.172 ± 0.04
            e           >0.5219   310.55 ± 0.83      0.921     0.0666 ± 0.0122
            b            >1.676   643.25 ± 0.90      1.497      0.128 ± 0.017
            c            >1.814  4205.8 ± 758.9      5.235     0.0985 ± 0.0627

Habitability

   The gas giant planet "b" is located in the liquid water habitable zone
   of Mu Arae. This would prevent an Earthlike planet from forming in the
   habitable zone, however large moons of the gas giant could potentially
   support liquid water. On the other hand it is unclear whether such
   massive moons could actually form around a gas giant planet, thanks to
   an apparent scaling law between the mass of the planet and its
   satellite system. In addition, measurements of the star's ultraviolet
   flux suggest that any potentially habitable planets or moons may not
   receive enough ultraviolet to trigger the formation of biomolecules.

   Planet "e" would receive a similar amount of ultraviolet to the Earth
   and thus lies in the ultraviolet habitable zone, however, it would be
   too hot for any moons to support surface liquid water.

Planet naming conventions

   The established convention for extrasolar planets is that the planets
   receive lower-case Roman letters starting from "b", in order of
   discovery. This system is used by the team led by Goździewski, and has
   been adopted in this article for compatibility with the designations
   used previously for the 3-planet model.

   On the other hand, the team led by Pepe have proposed a modification of
   the designation system, where the planets are designated in order of
   characterisation. Since the parameters of the outermost planet were
   poorly constrained before the introduction of the 4-planet model of the
   system, this results in a different order of designations for the
   planets in the Mu Arae system. Both systems agree on the designation of
   the 670-day planet as "b". The Pepe system designates the 9-day planet
   as "c", the 310-day planet as "d" and the outer planet as "e".

   Since the International Astronomical Union has not defined an official
   system for designations of extrasolar planets, the issue of which
   convention is "correct" remains open.
   Retrieved from " http://en.wikipedia.org/wiki/Mu_Arae"
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