68 Herculis

68 Herculis is a triple star[8] system located around 990[1] light years away from the Sun in the northern constellation of Hercules. In the astronomical community it is more commonly 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]

This article is about u Herculis. It is not to be confused with υ (upsilon) Herculis or U Herculis.
68 Herculis
Observation data
Epoch J2000      Equinox J2000
Constellation Hercules
Right ascension  17h 17m 19.56787s[1]
Declination +33° 06′ 00.3645″[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: −4.360[1] mas/yr
Dec.: −5.370[1] mas/yr
Parallax (π)3.3032 ± 0.1853[1] mas
Distance990 ± 60 ly
(300 ± 20 pc)
Absolute magnitude (MV)−2.66±0.30 (−2.35 + −1.15)[3]
Orbit[5]
Primary68 Her A
Companion68 Her B
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 A
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 B
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+3306[7]
Database references
SIMBADdata

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 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 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 C, lies at an angular separation of 10.2″ from the inner pair with a visual magnitude of 10.2.[8]

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. 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.
  3. Hilditch, R. W. (April 2005), "Astrophysical parameters for the eclipsing binary u Herculis", The Observatory, 125: 72–81, Bibcode:2005Obs...125...72H
  4. 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.
  5. 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. 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. 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.
  • Kaler, James B. (July 20, 2012), "68 Herculis", STARS, University of Illinois, retrieved 2019-06-15.
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