  | |
| Observation data Epoch J2000.0 Equinox J2000.0 | |
|---|---|
| Constellation | Aquila[1] |
| Right ascension | 19h 54m 14.8815s[2] |
| Declination | +08° 27′ 41.235″[2] |
| Apparent magnitude (V) | 4.722[3] |
| Characteristics | |
| Evolutionary stage | red clump[4] |
| Spectral type | G9.5 IIIb[5] |
| U−Bcolor index | +0.89[6] |
| B−Vcolor index | +1.049[7] |
| R−Icolor index | 0.57 |
| Astrometry | |
| Radial velocity (Rv) | −42.10 ± 0.14[7] km/s |
| Proper motion (μ) | RA: 101.694(107)mas/yr[2] Dec.: −81.141(77)mas/yr[2] |
| Parallax (π) | 17.5183±0.1006 mas[2] |
| Distance | 186 ± 1 ly (57.1 ± 0.3 pc) |
| Absolute magnitude (MV) | +0.96[1] |
| Details | |
| Mass | 1.74+0.21 −0.28[8] M☉ |
| Radius | 10.17±0.16[9] R☉ |
| Luminosity | 58.54+6.02 −12.37[8] L☉ |
| Surface gravity (log g) | 2.61±0.08[8] cgs |
| Temperature | 4,841[8] K |
| Metallicity[Fe/H] | −0.11+0.06 −0.1[8] dex |
| Rotational velocity (v sin i) | 5.2[7] km/s |
| Age | 980±180[10] Myr |
| Other designations | |
| Libertas,ξ Aquilae, Xi Aql, ξ Aql,59 Aquilae,BD+08 4261,GC 27558,HD 188310,HIP 97938,HR 7595,SAO 125210,PPM 168913,WDS J19542+0828AB[11] | |
| Database references | |
| SIMBAD | data |
Xi Aquilae is astar in theequatorialconstellation ofAquila. Its name is aBayer designation that isLatinized fromξ Aquilae, and abbreviatedXi Aql orξ Aql. The star has the official nameLibertas, pronounced/ˈlɪbərtæs/, aLatin word for liberty.[12] Based onparallax measurements, it is located at a distance of 186light-years (57parsecs) from theSun. This is an aginggiant star that is close to a billion years old.
A companion star has been detected usingspeckle interferometry0.09″ away.[13]
As of 2008, anextrasolar planet has been confirmed in orbit around the star. Designated Xi Aquilae b, this massive world was later named Fortitudo.[14]
ξ Aquilae (Latinised toXi Aquilae) is the star'sBayer designation. Following its discovery the planet was designated Xi Aquilae b.
In July 2014 theInternational Astronomical Union launchedNameExoWorlds, a process for giving proper names to certain exoplanets and their host stars.[15] The process involved public nomination and voting for the new names.[16] In December 2015, the IAU announced the winning names were Libertas for this star and Fortitudo for its planet.[17]
The winning names were those submitted by Libertyer, a student club atHosei University ofTokyo,Japan. The names which were originally proposed were in English and were 'Liberty' and 'Fortitude', but to comply with the IAU's rules they were modified to beLatin versions of the same words, and so the final names became 'Libertas' and 'Fortitudo' respectively. 'Aquila' is Latin for 'eagle', a popular symbol of liberty and embodiment of fortitude—emotional and mental strength in the face of adversity.[18]
In 2016, the IAU organized aWorking Group on Star Names (WGSN)[19] to catalog and standardize proper names for stars. In its first bulletin of July 2016,[20] the WGSN explicitly recognized the names of exoplanets and their host stars approved by the Executive Committee Working Group Public Naming of Planets and Planetary Satellites, including the names of stars adopted during the 2015 NameExoWorlds campaign. This star is now so entered in the IAU Catalog of Star Names.[12]
This star has anapparent visual magnitude of 4.722,[3] which, according to theBortle Dark-Sky scale, is bright enough to be viewed with the naked eye from dark suburban skies. The orbital motion of the Earth causes this star to undergo an annualparallax shift of 17.51 milliarcseconds. From this measurement, the distance to this star can be determined, yielding an estimate of approximately 186 light-years with an error of 1 light year.[2] The magnitude of the star is diminished by 0.09 from theextinction caused by interstellar gas and dust.[21]
Thespectrum of this star is considered a standard example of thestellar classification G9.5 IIIb,[5] where the G9.5 means that it belongs to the category ofG-type stars while theluminosity class of IIIb indicates that, at an estimated age of nearly one billion year,[10] is anevolved star that has reached thegiant stage. It is in thered clump,[4] meaning it is generating energy through the fusion of helium into carbon at its core.
Xi Aquilae has an estimated 174% of theSun's mass.[8] Its size has been measured usinginterferometry at theNavy Precision Optical Interferometer, which yields aradius 10.17 timesthat of the Sun.[9] It is radiating 58.5 times theSun's luminosity at aneffective temperature of4,841 K,[8] giving it the golden-hued glow of a G-type star.[22] The possibility of a binary stellar companion can be ruled out based upon observations with theCHARA array.[23]
In 2008, the presence of a planetary companion was announced, based uponDoppler spectroscopy results from theOkayama Astrophysical Observatory. This object, designated asXi Aquilae b, has at least 2.8Jupiter masses and is orbiting at an estimated 0.68astronomical unit from the star with a period of 136.75 days.[14] Any planets that once orbited to the interior of this object may have been consumed as the star entered thered giant stage and expanded in radius.[24] Later in 2024,astrometric measurements place an upper limit in the mass of37.1 MJ based onGaiaastrometry.[25]
| Companion (in order from star) | Mass | Semimajor axis (AU) | Orbital period (days) | Eccentricity | Inclination | Radius |
|---|---|---|---|---|---|---|
| b (Fortitudo) | ≥2.8[14] and <37.1[25] MJ | 0.68 | 136.75±0.25 | 0 (fixed) | — | — |
