HD 128429

HD 128429 is a binary star[3] system located at a distance of 88[1] light years from the Sun in the southern zodiac constellation of Libra. It has a yellow-white hue and is just barely visible to the naked eye with an apparent visual magnitude of 6.20.[5] The system is drifting closer to the Sun with a radial velocity of −66 km/s[5] and has a high proper motion, traversing the celestial sphere at the rate of 0.945″ per year.[9] It is a well-known high velocity star[10] system with a net heliocentric velocity of 158.8 km/s. The system is orbiting the through the galaxy with a high eccentricity of 0.62, which carries it from as close as 4.1 out to 17.5 kpc away from the Galactic center.[5]

This star was found to be a binary system based on variations in radial velocity data collected from the Hipparcos satellite.[11] The pair have an orbital period of 2.97 ± 0.21 years with photometric data yielding an angular separation of 21.28±2.88 mas.[3] Observations from the Gaia DR2 provide an estimated linear semimajor axis of 1.589 AU.[6] The eccentricity of the orbit is unknown, but has been assumed to be near zero.[3]

The visible member of this system, designated component Aa, has a stellar classification of F6V.[4] Superficially, it resembles 2–3[7] billion year old F-type main-sequence star that is generating energy through core hydrogen fusion. However, the star displays anomalies that are a challenge to explain through the normal star formation process. The first is the high velocity orbit of the star through the Milky Way, which would be very difficult for a young population I star to accomplish. The second is an abnormally low iron-to-magnesium [Fe/Mg] abundance ratio. This strongly suggests it is an ancient population II star that was formed during the early starburst phase of the galaxy about 12 billion years ago – a period when high levels of magnesium was released during supernovae explosions of massive stars. Both anomalies can be explained by a mass transfer that converted a much older star into a blue straggler.[3]

Evidence suggests that the companion, Ab, is a white dwarf star that evolved from an F- or G-type main-sequence star with a similar mass to the current primary. As component Ab became a red giant, it overflowed its Roche lobe and mass transfer took place. The white dwarf now has less than half the mass of the Sun, having transferred a substantial fraction of its mass to the current primary. The interaction would have circularized the orbit of the pair.[3]

The current primary has 1.32[3] times the mass of the Sun and 1.39 times the Sun's radius.[3] It has a low metallicity and is completely lacking in lithium.[3] The star is spinning with a projected rotational velocity of 16.2 km/s.[2] It is radiating 2.75[7] times the luminosity of the Sun from its photosphere at an effective temperature of 6,341 K.[2] The system is a source for X-ray emission.[12]