Tau9 Eridani

Tau9 Eridani (τ9 Eri) is a binary star in the constellation Eridanus. It is visible to the naked eye with an apparent visual magnitude of 4.63. The distance to this system can be estimated using the parallax method, which yields a value of roughly 327 light years.

τ9 Eridani

A light curve for Tau9 Eridani plotted from TESS data
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Eridanus
Right ascension 03h 59m 55.48381s
Declination −24° 00 58.3798
Apparent magnitude (V) 4.63
Characteristics
Spectral type B9.5V Si
U−B color index −0.40
B−V color index −0.13
Variable type α2 CVn
Astrometry
Radial velocity (Rv)25.5±0.5 km/s
Proper motion (μ) RA: +12.12 mas/yr
Dec.: +16.48 mas/yr
Parallax (π)9.96 ± 0.22 mas
Distance327 ± 7 ly
(100 ± 2 pc)
Absolute magnitude (MV)−0.44
Orbit
Period (P)5.95382 days
Eccentricity (e)0.129
Periastron epoch (T)246991.65 JD
Argument of periastron (ω)
(secondary)
183.2°
Semi-amplitude (K1)
(primary)
40.0 km/s
Semi-amplitude (K2)
(secondary)
89.9 km/s
Details
τ9 Eri A
Mass3.6 M
Radius3.06 R
Luminosity209 L
Surface gravity (log g)4.11±0.11 cgs
Temperature12,580 K
Rotational velocity (v sin i)26.8 km/s
Age140 Myr
τ9 Eri B
Mass1.6 M
Radius1.5 R
Luminosity6.3 L
Temperature7530 K
Rotational velocity (v sin i)15 km/s
Other designations
τ9 Eridani, τ9 Eri, 36 Eridani, CD−24° 2022, FK5 2287, HD 25267, HIP 18673, HR 1240, SAO 169017.
Database references
SIMBADdata

This is a double-lined spectroscopic binary system with an orbital period of 5.95382 days and an eccentricity of 0.12. The primary component, τ9 Eri A, is a magnetic chemically peculiar star with a stellar classification of B9.5V Si, indicating that it is a B-type main sequence star that shows abundance anomalies in its silicon absorption lines. It is an Alpha² Canum Venaticorum variable with a rotational periodicity of 5.954 days. The averaged strength of the stellar effective magnetic field is 240.6±91.0 G.

The primary component Tau9 Eridani A has an estimated 326% the mass of the Sun and 3.1 times the Sun's radius. It shines with 166 times the solar luminosity from an outer atmosphere at an effective temperature of 10,866 K. The star is spinning with a projected rotational velocity of 30 km/s. The secondary star, designated Tau9 Eridani B, is a late A-type or early F-type star with a mass of 1.6 solar masses and an effective temperature of 7530 K.

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