# Nu2 Lupi

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Observation data Characteristics Epoch J2000      Equinox J2000 A star chart of the constellation of Lupus showing the position of ν2 Constellation Lupus Right ascension 15h 21m 48.15s ± 3.25[1] Declination −48° 19′ 03.46″ ± 3.16[1] Apparent magnitude (V) 5.7821 ± 0.0006[1] Spectral type G2V B−V color index 0.639 ± 0.003[1] Radial velocity (Rv) 68.7120 ± 0.0005[2] km/s Proper motion (μ) RA: -1622.61 ± 0.37[1] mas/yr Dec.: -275.62 ± 0.36[1] mas/yr Parallax (π) 67.51 ± 0.39[1] mas Distance 48.3 ± 0.3 ly (14.81 ± 0.09 pc) Absolute magnitude (MV) 4.9289 ± 0.0131[3] Mass 0.91 ± 0.03[4] M☉ Radius 1.04 ± 0.04[4] R☉ Luminosity 0.99 ± 0.01[5] L☉ Surface gravity (log g) 4.39 ± 0.02[5] cgs Temperature 5664 ± 14[5] K Metallicity [Fe/H] -0.34 ± 0.01[5] dex Rotation 25.0 ± 3.1 days[6] Rotational velocity (v sin i) 2.0 ± 0.5[7] km/s Age 10.36 ± 2.30[4] Gyr HD 136352, HIP 75181, Gliese 582, HR 5699 SIMBAD data Extrasolar Planets Encyclopaedia data

Nu2 Lupi (ν2 Lup) is a 6th magnitude G-type main-sequence star located approximately 48 light-years away in the constellation of Lupus. The physical properties of the star are similar to those of the Sun, though Nu2 Lupi is significantly older.

## Properties

Nu2 Lupi is a bright star, barely observable with the naked eye in good observing conditions, that lies towards the bottom Lupus near to the border with Norma and close to the galactic plane.

At over 1.6 arcseconds per year, Nu2 Lupi has a particularly large proper motion. This indicates that the star is nearby, which was confirmed by Earth-based parallax measurements during the last century such as that of the Gliese Catalogue of Nearby Stars, measuring 63.1 ± 7.8 milli-arcseconds. The much more accurate space-based Hipparcos parallax of 67.51 ± 0.39 milli-arcseconds gives a distance of 48.3 ± 0.3 light-years, making Nu2 Lupi one of the closest G-type main-sequence stars to the Sun.

Somewhat surprisingly, Nu2 Lupi also has a large radial velocity of 68.7 km/s. When combined with its large proper motion, it becomes apparent that the star is moving much faster through the galaxy than the Sun. This indicates that it the star is a member of an older, higher-motion stellar population, which is confirmed by the star's position on the Toomre diagram with Nu2 Lupi showing kinematics of a thick disk star.[8] This means that Nu2 Lupi must be considerably older than the Sun, which is supported by its spectroscopic parameters: the depth of the star's iron spectral lines implies an iron abundance of -0.34 ± 0.01 dex, equalling 46 ± 1% of the solar iron abundance - a typical value for a thick disk star. Similarly, the star's surface gravity of log 4.39 ± 0.02 g is somewhat lower than is typical for a main-sequence G-type star and indicates modest evolution, which when combined with a spectroscopically derived mass of 0.91 ± 0.03 M implies an age of 10.36 ± 2.30 billion years, over twice the solar age. Nu2 Lupi is therefore probably one of the oldest stars in the solar neighbourhood.

## Planetary system

On September 12, 2011 three low-mass planets were announced using data from the HARPS spectrograph.[2] These three planets are among about seven dozen planets discovered in September 2011, the most of any month during the exoplanet era that begun in early 1990s.

With a minimum mass of about 5 Earth masses, the innermost planet falls into the regime of Super-Earths. The two outer planets straddle the commonly accepted upper limit between Super-Earths and Neptunes at 10 M, so they could be either predominantly rocky or gaseous. All three planets orbit within 0.5 AU and are likely too hot to maintain liquid water.

The most recent published observation of this system for debris disks was in 2006 by the Spitzer telescope, searching for an excess of infra-red light that would indicate scattering of starlight by dust or planetesimals; no infra-red excess was detected.[9]

The Nu2 Lupi planetary system[2]
Companion
(in order from star)
Mass Semimajor axis
(AU)
Orbital period
(days)
b ≥5.28 ± 0.62 M 0.0933 ± 0.0015 11.577 ± 0.0056 0.18 ± 0.14
c ≥11.38 ± 0.10 M 0.1665 ± 0.0028 27.582 ± 0.0225 0.16 ± 0.07
d ≥9.59 ± 1.86 M 0.411 ± 0.0073 106.72 ± 1.0284 0.43 ± 0.24

3. ^ a b The relevant calculation for absolute magnitude is $M = m - 5 ((\log_{10}{D_L}) - 1)\!\,$, where $m\!\,$ is the apparent magnitude and $D_L\!\,$ is the distance in light-years.