Tau Scorpii

Tau Scorpii (τ Sco, τ Scorpii) is a star in the southern zodiac constellation of Scorpius. It has the traditional name Alniyat or Al Niyat, which it shares with σ Scorpii. The name derives from the Arabic النياط an-niyāţ meaning "the arteries". The apparent visual magnitude of Tau Scorpii is +2.8, while parallax measurements yield a distance estimate of roughly 470 light-years (150 parsecs) from Earth.

Compared to the Sun, Tau Scorpii is a massive OB star with 15 times the Sun's mass and more than six times the radius of the Sun. It is radiating about 18,000 times the Sun's luminosity from its outer envelope at an effective temperature of 31,440 K. This gives it the blue-white hue characteristic of B-type stars. As yet there is no evidence of a companion in orbit around τ Sco. It is a magnetic star whose surface magnetic field was mapped by means of Zeeman–Doppler imaging. Tau Scorpii is rotating relatively slowly with a period of 41 days.

The spectrum of this star shows triply ionized oxygen (O IV) that is being generated by X-rays and the Auger ionization effect. Observations with the ROSAT space telescope showed it has a higher energy (harder) X-ray spectrum than is usual for B0 V stars. Over the energy range 0.8–1.2 keV, its X-ray luminosity is L_x = 1.8 × 10³¹ erg s⁻¹ with a large L_x to L_bol of log L_x/L_bol = –6.53 from ASCA measurements. ROSAT measurements showed a log L_x/L_bol ≃ –5.93 for the range 0.1-2.4 keV. The hard component of the X-ray spectrum from τ Sco as studied with XMM-Newton supports the presence of in-falling clumps of plasma in τ Sco.

This star is a proper motion member of the Upper Scorpius subgroup of the Scorpius-Centaurus OB association, the nearest such co-moving association of massive stars to the Sun. The Upper Scorpius subgroup contains thousands of young stars with mean age 11 million years at average distance of 470 light years (145 parsecs) A more recent analysis of the HR diagram position for Tau Scorpii estimates its effective temperature to be 29,850 Kelvin with a luminosity of 20,400 Suns, consistent with an isochronal age of 5 million years and an estimated mass of 14.5-14.7 solar masses.