68 Herculis
Location of 68 Herculis (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Hercules
Right ascension 17h 17m 19.56781s[1]
Declination +33° 06′ 00.3684″[1]
Apparent magnitude (V) 4.80[2]
Characteristics
Spectral type B2 V + B8-9[3]
B−V color index −0.166±0.011[2]
Variable type Algol/semi-detached[4]
Astrometry
Radial velocity (Rv)−17.1±2.8[2] km/s
Proper motion (μ) RA: −4.298[1] mas/yr
Dec.: −5.621[1] mas/yr
Parallax (π)3.4346 ± 0.1164 mas[1]
Distance950 ± 30 ly
(291 ± 10 pc)
Absolute magnitude (MV)−2.66±0.30 (−2.35 + −1.15)[3]
Orbit[5]
Primary68 Her Aa
Companion68 Her Ab
Period (P)2.05102685 days
Semi-major axis (a)14.95±0.17 R☉
Eccentricity (e)0.0[6] (fixed)
Inclination (i)78.9±0.4°
Periastron epoch (T)50,344.99±0.009[6]
Argument of periastron (ω)
(secondary)		0.0[6] (fixed)°
Semi-amplitude (K1)
(primary)		101±1[6] km/s
Semi-amplitude (K2)
(secondary)		252±1.8[6] km/s
Details[5]
68 Her Aa
Mass7.88±0.26 M☉
Radius4.93±0.15 R☉
Luminosity4,786+343
−319 L☉
Surface gravity (log g)3.948±0.024 cgs
Temperature21,600±220 K
Rotational velocity (v sin i)145±5[6] km/s
68 Her Ab
Mass2.79±0.12 M☉
Radius4.26±0.06 R☉
Luminosity426.5+86.4
−71.7 L☉
Surface gravity (log g)3.625±0.013 cgs
Temperature12,600±550 K
Rotational velocity (v sin i)105±5[6] km/s
Other designations
u Her, 68 Her, BD+33°2864, HD 156633, HIP 84573, HR 6431, SAO 65913, WDS J17173+3306A[7]
Database references
SIMBADdata

68 Herculis is a triple star[8] system located around 950 light-years away from the Sun in the northern constellation of Hercules. In the astronomical community it is often referred to by its Bayer designation of u Herculis,[3] while 68 Herculis is the Flamsteed designation. The system is visible to the naked eye as a faint, blue-white-hued point of light with a peak apparent visual magnitude of 4.80.[2] It is approaching the Earth with a heliocentric radial velocity of −17 km/s.[2]

A light curve for 68 Herculis, plotted from TESS data[9]

The inner pair of this system form a well-studied[3] semidetached binary with the orbital plane oriented near the line of sight to the Earth, making it an Algol-type eclipsing binary. They have an orbital period of just over two days and a semimajor axis of 15 times the radius of the Sun, with the secondary component transferring mass to the hotter primary star.[5] The main eclipse reduces the magnitude of the system to 5.37, while the second eclipse lowers the brightness to magnitude 4.93.[4] Theoretical calculations suggest the donor star began with 7.2 times the mass of the Sun, the current primary at 3.6 solar masses, and their initial orbital period was around 1.35 days.[5]

The primary, designated component Aa or sometimes just A, displays Beta Cephei-like pulsational behavior. It appears to be a B-type main-sequence star with a stellar classification of B2 V.[3] The star has a high rate of spin, with a projected rotational velocity of 145 km/s.[6] It has nearly eight times the mass of the Sun and five times the Sun's radius. The star is radiating 4,786 times the Sun's luminosity from its photosphere at an effective temperature of 21,600 K.[5]

The secondary, component Ab or occasionally just B, has proven difficult to classify, but appears as a B-type star of type B8-9.[3] It is close to triple the mass of the Sun with 4.3 times the Sun's radius. The star is spinning with a projected rotational velocity of 105 km/s. It is radiating 426.5 times the Sun's luminosity from its photosphere at an effective temperature of 12,600 K.[5]

The third member of this system, component B, lies at an angular separation of 4.4″ from the inner pair with a visual magnitude of 10.2.[8] It shares a common proper motion and similar parallax to the eclipsing pair, and is modelled to be a main sequence star somewhat more massive, hotter, and more luminous than the Sun.[10] Any orbit would require thousands of years.[11]

References

  1. 1 2 3 4 5 Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
  2. 1 2 3 4 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.
  3. 1 2 3 4 5 6 Hilditch, R. W. (April 2005), "Astrophysical parameters for the eclipsing binary u Herculis", The Observatory, 125: 72–81, Bibcode:2005Obs...125...72H
  4. 1 2 Samus, N. N.; et al. (2017), "General Catalogue of Variable Stars", Astronomy Reports, 5.1, 61 (1): 80–88, Bibcode:2017ARep...61...80S, doi:10.1134/S1063772917010085, S2CID 125853869.
  5. 1 2 3 4 5 6 Kolbas, V.; et al. (November 2014), "Tracing CNO exposed layers in the Algol-type binary system u Her", Monthly Notices of the Royal Astronomical Society, 444 (4): 3118–3129, arXiv:1408.2681, Bibcode:2014MNRAS.444.3118K, doi:10.1093/mnras/stu1652.
  6. 1 2 3 4 5 6 7 8 Saad, Somaya; Nouh, Mohamed (June 2011), "A study of the B+B double-lined eclipsing binary u Her", Bulletin of the Astromical Society of India, 39: 277–287, Bibcode:2011BASI...39..277S.
  7. ↑ "68 Her", SIMBAD, Centre de données astronomiques de Strasbourg, retrieved 2019-06-14.
  8. 1 2 Eggleton, P. P.; Tokovinin, A. A. (September 2008), "A catalogue of multiplicity among bright stellar systems", Monthly Notices of the Royal Astronomical Society, 389 (2): 869–879, arXiv:0806.2878, Bibcode:2008MNRAS.389..869E, doi:10.1111/j.1365-2966.2008.13596.x, S2CID 14878976.
  9. ↑ MAST: Barbara A. Mikulski Archive for Space Telescopes, Space Telescope Science Institute, retrieved 4 January 2023.
  10. ↑ Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
  11. ↑ Tokovinin, Andrei (23 February 2018), "The Updated Multiple Star Catalog", The Astrophysical Journal Supplement Series, 235 (1): 6, arXiv:1712.04750, Bibcode:2018ApJS..235....6T, doi:10.3847/1538-4365/aaa1a5, eISSN 1538-4365.
  • Kaler, James B. (July 20, 2012), "68 Herculis", STARS, University of Illinois, retrieved 2019-06-15.
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