AC Herculis

AC Herculis, is an RV Tauri variable and spectroscopic binary star in the constellation of Hercules. It varies in brightness between apparent magnitudes 6.85 and 9.0.

AC Herculis
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Hercules
Right ascension 18h 30m 16.23850s[1]
Declination +21° 52′ 00.6080″[1]
Apparent magnitude (V) 6.85 - 9.00[2]
Characteristics
Evolutionary stage Post-AGB star[3]
Spectral type F2pIb-K4e(C0,0)[2]
U−B color index +0.17 - +0.94[4]
B−V color index +0.52 - +1.09[4]
Variable type RVa[2]
Astrometry
Radial velocity (Rv)−30.00[5] km/s
Proper motion (μ) RA: -2.82 ± 1.12[1] mas/yr
Dec.: -0.18 ± 1.16[1] mas/yr
Parallax (π)0.7843 ± 0.0289 mas[6]
Distance4,200 ± 200 ly
(1,280 ± 50 pc)
Orbit[7]
Period (P)1,194 days
Semi-major axis (a)2.7 AU
Eccentricity (e)0.12
Inclination (i)50°
Details[7]
A
Mass0.6 M☉
Radius47.1+4.7
−4.1[8] R☉
Luminosity2475+183
−209[8] L☉
Surface gravity (log g)0.65 cgs
Temperature5900[8] K
Metallicity [Fe/H]−1.5 dex
B
Mass1.2 M☉
Other designations
AC Her, GSC 01581-01726, HD 170756, HIP 90697, BD+21°3459, 2MASS J18301623+2152007, IRAS 18281+2149, AAVSO 1826+21
Database references
SIMBADdata

AC Her is an RVa star, meaning it is an RV Tauri variable whose maximum and minimum magnitudes do not slowly vary over hundreds of days. It also is a very clear example of a common type of RV Tauri light curve where the maximum following a deep minimum is brighter than the maximum following a shallow minimum. In each period of 75.46 days[8] it has two maxima and two minima.[9]

AC Her is also a binary star, its companion was detected with spectroscopy and long baseline interferometry. Interferometric observations from the CHARA Array enabled its three dimensional orbit with a semimajor axis of 2.01 ± 0.01 mas, equivalent to 2.83 ± 0.08 au.[10] The orbit has an inclination of 142.9 ± 1.1 degrees and a longitude of the ascending node of 155.1 ± 1.8 degrees. The invisible secondary is more massive (1.40 ± 0.12 M☉) than the supergiant primary ( 0.73 ± 0.13 M☉). The orbital period is 1187.7 days. The two stars are also surrounded by a dusty disc filling the region between 34 and 200 astronomical units (AU).[7]

Little is known of the secondary star except that its mass is around 1.2 M☉, deduced from the mass ratio of the binary system and the modelled mass of the primary star. The primary itself is calculated to have a mass of 0.6 M☉, but a luminosity of 2,500 L☉. It is slightly cooler than the sun, although this varies by over a thousand K as the star pulsates.[4]

The total system mass can be estimated from the dynamics of the disc, and this gives a value of 1.5 M☉, slightly lower than from other methods.[3]

References
  1. van Leeuwen, F. (2007). "Validation of the New Hipparcos Reduction". Astronomy and Astrophysics. 474 (2): 653–64. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. S2CID 18759600.
  2. Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S. 1. Bibcode:2009yCat....102025S.
  3. Bujarrabal, V.; Castro-Carrizo, A.; Alcolea, J.; Van Winckel, H. (2015). "Detection of Keplerian dynamics in a disk around the post-AGB star AC Herculis". Astronomy & Astrophysics. 575: 4. arXiv:1502.01607. Bibcode:2015A&A...575L...7B. doi:10.1051/0004-6361/201525742. S2CID 118426089.
  4. Dawson, D. W. (1979). "A photometric investigation of RV Tauri and yellow semiregular variables". Astrophysical Journal Supplement Series. 41: 97. Bibcode:1979ApJS...41...97D. doi:10.1086/190610.
  5. Gontcharov, G. A. (2006). "Pulkovo Compilation of Radial Velocities for 35 495 Hipparcos stars in a common system". Astronomy Letters. 32 (11): 759–771. arXiv:1606.08053. Bibcode:2006AstL...32..759G. doi:10.1134/S1063773706110065. S2CID 119231169.
  6. 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.
  7. Hillen, M.; De Vries, B. L.; Menu, J.; Van Winckel, H.; Min, M.; Mulders, G. D. (2015). "The evolved circumbinary disk of AC Herculis: A radiative transfer, interferometric, and mineralogical study" (PDF). Astronomy & Astrophysics. 578: A40. arXiv:1503.03984. Bibcode:2015A&A...578A..40H. doi:10.1051/0004-6361/201425372. S2CID 53554889.
  8. Bódi, A.; Kiss, L. L. (2019), "Physical Properties of Galactic RV Tauri Stars from Gaia DR2 Data", The Astrophysical Journal, 872 (1): 60, arXiv:1901.01409, Bibcode:2019ApJ...872...60B, doi:10.3847/1538-4357/aafc24, S2CID 119099605
  9. Preston, G. W.; Krzeminski, W.; Smak, J.; Williams, J. A. (1963). "A Spectroscopic and Photoelectric Survey of the RV Tauri Stars". Astrophysical Journal. 137: 401. Bibcode:1963ApJ...137..401P. doi:10.1086/147520.
  10. Anugu, N.; Kluska, K.; Gardner, T.; Monnier, J. D. (2013). "Three-dimensional Orbit of AC Her Determined: Binary-induced Truncation Cannot Explain the Large Cavity in This Post-AGB Transition Disk". Astrophysical Journal. 950 (2): 149. arXiv:2305.02408. Bibcode:2023ApJ...950..149A. doi:10.3847/1538-4357/acd1e6.
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