Theta Eridani
  | |
| Observation data Epoch J2000 Equinox ICRS | |
|---|---|
| Constellation | Eridanus |
| θ1 Eri | |
| Right ascension | 02h 58m 15.6764s |
| Declination | −40° 18′ 16.839″ |
| Apparent magnitude (V) | 3.18 |
| θ2 Eri | |
| Right ascension | 02h 58m 16.4037s |
| Declination | −40° 18′ 16.906″ |
| Apparent magnitude (V) | 4.11 |
| Characteristics | |
| θ1 Eri | |
| Evolutionary stage | Subgiant + main sequence |
| Spectral type | A3IV-V |
| U−B color index | +0.14 |
| B−V color index | +0.128±0.012 |
| Variable type | Rotating ellipsoidal variable |
| θ2 Eri | |
| Evolutionary stage | Main sequence |
| Spectral type | A1V |
| B−V color index | +0.08 |
| Variable type | suspected |
| Astrometry | |
| θ1 Eri | |
| Radial velocity (Rv) | +10.6±0.7 km/s |
| Proper motion (μ) | RA: −52.273 mas/yr Dec.: +23.058 mas/yr |
| Parallax (π) | 19.9047±0.2062 mas |
| Distance | 167±1 ly (51.2±0.3 pc) |
| θ2 Eri | |
| Radial velocity (Rv) | +7.57±0.33 km/s |
| Proper motion (μ) | RA: −50.502 mas/yr Dec.: +16.613 mas/yr |
| Parallax (π) | 19.5270±0.1372 mas |
| Distance | 167±1 ly (51.2±0.3 pc) |
| Absolute magnitude (MV) | −0.59 (combined) |
| Orbit | |
| Primary | θ1 Eri A |
| Name | θ1 Eri B |
| Period (P) | 4.107704±0.000008 days |
| Semi-major axis (a) | (1.62±0.03)×10−3" (0.083±0.002 au) |
| Eccentricity (e) | 0.105±0.010 |
| Inclination (i) | 43.2±2.3° |
| Longitude of the node (Ω) | 30.7±1.4° |
| Periastron epoch (T) | 2,457,949.89±0.06 JD |
| Argument of periastron (ω) (secondary) | 332.2±5.4° |
| Semi-amplitude (K1) (primary) | 77.0±1.4 km/s |
| Semi-amplitude (K2) (secondary) | 81.9±0.4 km/s |
| Details | |
| θ1 Eri A | |
| Mass | 2.33±0.13 M☉ |
| Radius | 4.3±0.1 R☉ |
| Luminosity | 56 L☉ |
| Surface gravity (log g) | 3.54 cgs |
| Temperature | 7,600 K |
| Rotational velocity (v sin i) | 70 km/s |
| Age | 725 Myr |
| θ1 Eri B | |
| Mass | 2.19±0.13 M☉ |
| Radius | 3.95±0.10 R☉ |
| Luminosity | 52 L☉ |
| Surface gravity (log g) | 3.57 cgs |
| Temperature | 7,800 K |
| Rotational velocity (v sin i) | 60 km/s |
| Age | 725 Myr |
| θ2 Eri | |
| Mass | 2.30 M☉ |
| Radius | 3.2 R☉ |
| Luminosity | 44 L☉ |
| Surface gravity (log g) | 4.0 cgs |
| Temperature | 8,300 K |
| Metallicity [Fe/H] | −0.197 dex |
| Rotational velocity (v sin i) | 102.5 km/s |
| Age | 630 Myr |
| Other designations | |
| Theta Eri, θ Eri, CD−40°771, HIP 13847, SAO 216113, CCDM J02583-4018, WDS 02583-4018 | |
| θ1 Eri: Acamar, HD 18622, HR 897 | |
| θ2 Eri: HD 18623, HR 898 | |
| Database references | |
| SIMBAD | The system |
| θ1 Eri | |
| θ2 Eri | |
Theta Eridani, Latinized from θ Eridani, is a triple star system in the constellation of Eridanus, with a combined apparent magnitude of 2.88. The primary component has the proper name Acamar /ˈækəmɑːr/, the traditional name of the system. The system's distance based on parallax measurements is 51.2 ± 0.3 parsecs (166.99 ± 0.98 light-years).