99 Herculis

This article is about b Herculis. Not to be confused with β (Beta) Herculis.

99 Herculis is the Flamsteed designation for a binary star system in the northern constellation of Hercules. It has the Bayer designation b Herculis, while 99 Herculis is the Flamsteed designation. This system has an apparent visual magnitude of 5.1,^[2] which, according to the Bortle scale, makes it faintly visible to the naked eye from suburban skies. Measurements made with the Hipparcos spacecraft show an annual parallax shift of 0.064″,^[1] corresponding to a physical distance of about 51.0 ly (15.6 pc) from the Sun. The system is moving further from the Earth with a heliocentric radial velocity of +1.7 km/s.^[6]

99 Herculis

Observation data Epoch J2000      Equinox J2000
Constellation	Hercules
Right ascension	18h 07m 01.53971s[1]
Declination	+30° 33′ 43.6896″[1]
Apparent magnitude (V)	5.066[2] (A: 5.10; B: 8.45)[3]
Characteristics
Spectral type	F7 V[4] + K4 V[5]
U−B color index	–0.056[2]
B−V color index	+0.548[2]
Astrometry
Radial velocity (Rv)	+1.7[6] km/s
Proper motion (μ)	RA: –100.32[1] mas/yr Dec.: +110.08[1] mas/yr
Parallax (π)	63.93 ± 0.34 mas[1]
Distance	51.0 ± 0.3 ly (15.64 ± 0.08 pc)
Absolute magnitude (MV)	4.08[7]
Orbit[5]
Period (P)	56.3 ± 0.1 yr
Semi-major axis (a)	1.06 ± 0.02″
Eccentricity (e)	0.766 ± 0.004
Inclination (i)	39° ± 2°
Longitude of the node (Ω)	41° ± 2°
Periastron epoch (T)	1997.62 ± 0.05
Argument of periastron (ω) (secondary)	116° ± 2°
Details
99 Her A
Mass	0.94[5] M☉
Radius	1.1[8] R☉
Luminosity	1.96[5] L☉
Surface gravity (log g)	4.17[4] cgs
Temperature	5,938[4] K
Metallicity [Fe/H]	–0.60[4] dex
Rotational velocity (v sin i)	5[7] km/s
Age	9.37[4] Gyr
99 Her B
Mass	0.46[5] M☉
Radius	0.74[8] R☉
Luminosity	0.14[5] L☉
Other designations
b Her, 99 Her, BD+30°3128, GJ 704, HD 165908, HIP 88745, HR 6775, SAO 66648[9]
Database references
SIMBAD	99 Her AB
	99 Her A
	99 Her B

The binary nature of this star system was first discovered in 1859 by English astronomer W. R. Dawes.^[10] The two stellar components orbit around their common center of mass, or barycenter, with a period of 56.3 years and an eccentricity of 0.766. The semi-major axis of their orbit spans an angle of 1.06 arcseconds, which corresponds to a physical dimension of 16.5 AU. The plane of their orbit is inclined by an angle of about 39° to the line-of-sight from the Earth. Reports of a third component of this system now appear doubtful.^[5]

The primary component, 99 Herculis A, is an F-type main sequence star with a stellar classification of F7 V.^[4] It has 94%^[5] of the Sun's mass and nearly double the luminosity, but has an estimated 10% greater radius.^[8] The effective temperature of the star's outer atmosphere is 5,938 K,^[4] giving it a white-hued glow.^[11] This is a metal-poor star, showing overall abundances of elements—other than hydrogen or helium—equal to 60% of those in the Sun.^[12]

The secondary component, 99 Herculis B, is fainter by 3.35 magnitudes compared to the primary. It is a K-type main sequence star with a classification of K4 V.^[5] With 46%^[5] of the mass of the Sun, it has 74%^[8] of the Sun's radius but shines with just 14%^[5] of the Sun's luminosity.

Images from the Herschel Space Observatory show that a disk of dusty debris is orbiting the barycenter at an average radius of 120 AU. Oddly, the disk appears to be misaligned with the orbital plane of the binary system. This may be the result of an interaction within another star system some time in the past. Most of the disk emission appears to be caused by icy objects having a diameter of 10 cm or less, with a net mass of about ten times the mass of the Earth.^[5]

References

1. van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357, S2CID 18759600.
2. Rakos, K. D.; et al. (February 1982), "Photometric and astrometric observations of close visual binaries", Astronomy and Astrophysics Supplement Series, 47: 221–235, Bibcode:1982A&AS...47..221R.
3. Eggen, O. J. (February 1965), "Masses, luminosities, colors, and space motions of 228 visual binaries", Astronomical Journal, 70: 90, Bibcode:1965AJ.....70...19E, doi:10.1086/109676.
4. Maldonado, J.; et al. (May 2012). "Metallicity of solar-type stars with debris discs and planets". Astronomy & Astrophysics. 541: A40. arXiv:1202.5884. Bibcode:2012A&A...541A..40M. doi:10.1051/0004-6361/201218800. S2CID 46328823.
5. Kennedy, G. M.; et al. (April 2012), "99 Herculis: host to a circumbinary polar-ring debris disc", Monthly Notices of the Royal Astronomical Society, 421 (3): 2264–2276, arXiv:1201.1911, Bibcode:2012MNRAS.421.2264K, doi:10.1111/j.1365-2966.2012.20448.x, S2CID 53710762.
6. Nordström, B.; et al. (May 2004), "The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of ˜14,000 F and G dwarfs", Astronomy and Astrophysics, 418 (3): 989–1019, arXiv:astro-ph/0405198, Bibcode:2004A&A...418..989N, doi:10.1051/0004-6361:20035959, S2CID 11027621.
7. Takeda, Yoichi; et al. (February 2005), "High-Dispersion Spectra Collection of Nearby F--K Stars at Okayama Astrophysical Observatory: A Basis for Spectroscopic Abundance Standards", Publications of the Astronomical Society of Japan, 57 (1): 13–25, Bibcode:2005PASJ...57...13T, doi:10.1093/pasj/57.1.13.
8. Pasinetti Fracassini, L. E.; et al. (February 2001). "Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics". Astronomy and Astrophysics. 367 (2): 521–524. arXiv:astro-ph/0012289. Bibcode:2001A&A...367..521P. doi:10.1051/0004-6361:20000451. S2CID 425754. Note: Search the VizieR catalogue II/224 for HD 165908.
9. "b Her". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2013-10-10.
10. van Biesbroeck, G. (1947), "The orbit of 99 Herculis = Alvan Clark 15", Astronomical Journal, 53: 23–24, Bibcode:1947AJ.....53...23V, doi:10.1086/106040.
11. "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, archived from the original on March 18, 2012, retrieved 2012-01-16.
12. Adelman, Saul J.; et al. (August 2000), "Elemental abundance analyses with DAO spectrograms - XXIII. The superficially normal stars 28 And (A7III) and 99 Her (F7V)", Monthly Notices of the Royal Astronomical Society, 316 (3): 514–518, Bibcode:2000MNRAS.316..514A, doi:10.1046/j.1365-8711.2000.03577.x.