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]

HD 128429
Observation data
Epoch J2000      Equinox J2000
Constellation Libra
Right ascension 14h 36m 59.79592s[1]
Declination −12° 18′ 19.0687″[1]
Apparent magnitude (V) 6.197±0.005[2]
Characteristics
Evolutionary stage Blue straggler[3]
Spectral type F6V[4]
B−V color index 0.464±0.011[5]
Astrometry
Radial velocity (Rv)−66.1±0.5[5] km/s
Proper motion (μ) RA: −868.62[1] mas/yr
Dec.: −364.52[1] mas/yr
Parallax (π)37.1800 ± 0.4010 mas[1]
Distance87.7 ± 0.9 ly
(26.9 ± 0.3 pc)
Absolute magnitude (MV)3.66[5]
Orbit[3]
CompanionAb
Period (P)1,086±77 d
Semi-major axis (a)1.589 AU[6]
Eccentricity (e)≤ 0.15
Details[2]
Aa
Mass1.32 M☉
Radius1.39±0.06 R☉
Luminosity2.75[7] L☉
Surface gravity (log g)4.28±0.10 cgs
Temperature6,341±70 K
Metallicity [Fe/H]−0.10±0.06 dex
Rotational velocity (v sin i)16.2±0.5 km/s
Age1.7±0.6[3] Gyr
3.10[7] Gyr
Ab
Mass0.49±0.09 M☉[3]
0.130+0.036
−0.015[6] M☉
Other designations
BD−11°3770, FK5 1381, HD 128429, HIP 71469, HR 5455, SAO 158677, LAL 26630, LHS 2953[8]
Database references
SIMBADdata

Binary system

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]

Properties

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]

References
  1. Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
  2. Fuhrmann, Klaus; Chini, Rolf (January 2019), "On ancient solar-type stars", Monthly Notices of the Royal Astronomical Society, 482 (1): 471–489, Bibcode:2019MNRAS.482..471F, doi:10.1093/mnras/sty2660.
  3. Fuhrmann, Klaus; Chini, Rolf (October 2017), "On the ancient field blue straggler HR 5455", Monthly Notices of the Royal Astronomical Society, 471 (2): 1888–1891, Bibcode:2017MNRAS.471.1888F, doi:10.1093/mnras/stx1784.
  4. Gray, R. O.; et al. (July 2006), "Contributions to the Nearby Stars (NStars) Project: spectroscopy of stars earlier than M0 within 40 pc-The Southern Sample", The Astronomical Journal, 132 (1): 161–170, arXiv:astro-ph/0603770, Bibcode:2006AJ....132..161G, doi:10.1086/504637, S2CID 119476992.
  5. Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv:1108.4971, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015, S2CID 119257644.
  6. Kervella, Pierre; et al. (March 2019), "Stellar and substellar companions of nearby stars from Gaia DR2. Binarity from proper motion anomaly", Astronomy & Astrophysics, 623: 23, arXiv:1811.08902, Bibcode:2019A&A...623A..72K, doi:10.1051/0004-6361/201834371, S2CID 119491061, A72
  7. Luck, R. Earle (January 2017), "Abundances in the Local Region II: F, G, and K Dwarfs and Subgiants", The Astronomical Journal, 153 (1): 19, arXiv:1611.02897, Bibcode:2017AJ....153...21L, doi:10.3847/1538-3881/153/1/21, S2CID 119511744, 21.
  8. "HD 128429". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2019-11-28.
  9. Luyten, Willem J. (1979), LHS catalogue. A catalogue of stars with proper motions exceeding 0″.5 annually (2nd ed.), Minneapolis: University of Minnesota, Bibcode:1979lccs.book.....L.
  10. Heck, A. (June 1978), "Absolute luminosity calibration of F stars", Astronomy and Astrophysics, 66: 335–342, Bibcode:1978A&A....66..335H.
  11. Bartkevicius, A.; Gudas, A. (2001), "Kinematics of Hipparcos Visual Binaries. I. Stars with Orbital Solutions", Baltic Astronomy, 10 (4): 481–587, Bibcode:2001BaltA..10..481B, doi:10.1515/astro-2001-0402, S2CID 117700350.
  12. Haakonsen, Christian Bernt; Rutledge, Robert E. (September 2009), "XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources", The Astrophysical Journal Supplement, 184 (1): 138–151, arXiv:0910.3229, Bibcode:2009ApJS..184..138H, doi:10.1088/0067-0049/184/1/138, S2CID 119267456.
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