Theta Eridani, Latinized from θ Eridani, is a binary system in the constellation of Eridanus with a combined apparent magnitude of 2.88.[6] Its two components are designated θ1 Eridani, formally named Acamar /ˈækəmɑːr/ (the traditional name of the system),[13][14] and θ2 Eridani. The system's distance from the Sun based on parallax measurements is approximately 165 light-years.
  | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Eridanus |
| θ1 Eri | |
| Right ascension | 02h 58m 15.6764s[1] |
| Declination | −40° 18′ 16.839″[1] |
| Apparent magnitude (V) | 3.18[2] |
| θ2 Eri | |
| Right ascension | 02h 58m 16.4037s[3] |
| Declination | −40° 18′ 16.906″[3] |
| Apparent magnitude (V) | 4.11[2] |
| Characteristics | |
| θ1 Eri | |
| Spectral type | A3IV-V[4] |
| U−B color index | +0.14[5] |
| B−V color index | +0.128±0.012[6] |
| θ2 Eri | |
| Spectral type | A1V[4] |
| B−V color index | +0.08[5] |
| Variable type | suspected[7] |
| Astrometry | |
| θ1 Eri | |
| Radial velocity (Rv) | +11.9±2.6[6] km/s |
| Proper motion (μ) | RA: −57.132[1] mas/yr Dec.: +23.058[1] mas/yr |
| Parallax (π) | 19.9047 ± 0.2062 mas[1] |
| Distance | 164 ± 2 ly (50.2 ± 0.5 pc) |
| θ2 Eri | |
| Proper motion (μ) | RA: −50.502[3] mas/yr Dec.: +16.613[3] mas/yr |
| Parallax (π) | 19.5270 ± 0.1372 mas[3] |
| Distance | 167 ± 1 ly (51.2 ± 0.4 pc) |
| Absolute magnitude (MV) | −0.59 (combined)[6] |
| Details | |
| θ1 Eri | |
| Mass | 2.6[8] M☉ |
| Radius | 4.85[a] R☉ |
| Luminosity | 145[6] L☉ |
| Temperature | 8,200[8] K |
| Rotational velocity (v sin i) | 65.1[9] km/s |
| θ2 Eri | |
| Mass | 1.95[10] M☉ |
| Radius | 2.7[11] R☉ |
| Luminosity | 37[11] L☉ |
| Surface gravity (log g) | 3.83[10] cgs |
| Temperature | 8,293[10] K |
| Metallicity [Fe/H] | −0.197[10] dex |
| Rotational velocity (v sin i) | 102.5[12] km/s |
| Other designations | |
| θ1 Eri: Acamar, HD 18622, HR 897 | |
| θ2 Eri: HD 18623, HR 898 | |
| Database references | |
| SIMBAD | θ1 Eri |
| θ2 Eri | |
Nomenclature
Theta Eridani is the system's Bayer designation; θ1 and θ2 Eridani those of its two components.
The system bore the traditional name Acamar, derived from the Arabic آخِر النَّهْر [citation needed] Ākhir an-nahr, which means "the end of the river", via a Roman-alphabet handwriting misread "rn" to "m". In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[15] to catalog and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.[16] It approved the name "Acamar" for θ1 Eridani on 20 July 2016 and it is now so entered in the IAU Catalog of Star Names.[14]
The term "Ākhir an-nahr", or "Achr al Nahr", appeared in the catalogue of stars in the Calendarium of Al Achsasi Al Mouakket, which was translated into Latin as Postrema Fluminis.[17]
Historically, Acamar represented the end of the constellation Eridanus.[8][18] Now that distinction is held by the star Achernar, which shares the same Arabic etymology. Achernar is not visible from the Greek isles (latitudes > 33° North),[19] hence the choice of Acamar as the river's end during the time of Hipparchus and, later, Ptolemy.
In Chinese, 天園 (Tiān Yuán), meaning Celestial Orchard, refers to an asterism consisting of Theta Eridani, Chi Eridani, Phi Eridani, Kappa Eridani, HD 16754, HD 23319, HD 24072, HD 24160, Upsilon4 Eridani, Upsilon3 Eridani, Upsilon2 Eridani and Upsilon1 Eridani.[20] Consequently, the Chinese name for Theta Eridani itself is 天園六 (Tiān Yuán liù, English: the Sixth Star of Celestial Orchard).[21]
Stellar system
Theta Eridani is a binary system with some evidence suggesting it is part of a multiple star system.[7][22] The main star, θ1 Eridani, is of spectral class A4 with a +3.2 apparent magnitude. Its companion star, θ2 Eridani, is of spectral class A1 with an apparent magnitude of +4.1. The angular separation of the two stars is equal to 8.3 arcseconds.[8]