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TZ Fornacis
	; A blue band light curve for TZ Fornacis, adapted from Andersen et al. (1991)
Observation data; Epoch J2000.0 Equinox J2000.0
Constellation	Fornax
Right ascension	03h 14m 40.093s
Declination	−35° 33′ 27.59″
Apparent magnitude (V)	6.88 (7.51 + 7.76)
Characteristics
Evolutionary stage	Giant + subgiant
Spectral type	G8 III + F7 III
B−V color index	0.740±0.006
Variable type	Algol
Astrometry
Radial velocity (Rv)	18.0±2.4 km/s
Proper motion (μ)	RA: 34.598 mas/yr ; Dec.: 17.389 mas/yr
Parallax (π)	5.4708 ± 0.0193 mas
Distance	596 ± 2 ly ; (182.8 ± 0.6 pc)
Absolute magnitude (MV)	0.70
Orbit
Period (P)	75.66647±0.00006 d
Semi-major axis (a)	2.993±0.030 mas; 0.5564±0.0001 AU
Eccentricity (e)	0.00002±0.00003
Inclination (i)	85.68±0.05°
Longitude of the node (Ω)	65.99±0.03°
Argument of periastron (ω); (secondary)	269.93±0.04°
Semi-amplitude (K1); (primary)	38.90±0.01 km/s
Semi-amplitude (K2); (secondary)	40.87±0.02 km/s
Details
TZ For 1
Mass	2.057±0.001 M☉
Radius	8.28±0.22 R☉
Luminosity	37.2+1.8; −1.7 L☉
Surface gravity (log g)	2.91±0.02 cgs
Temperature	4,930±30 K
Metallicity [Fe/H]	0.02±0.05 dex
Rotational velocity (v sin i)	6.1±0.3 km/s
Age	1.20±0.10 Gyr
TZ For 2
Mass	1.958±0.001 M☉
Radius	3.94±0.17 R☉
Luminosity	22.9+1.5; −1.6 L☉
Surface gravity (log g)	3.35±0.02 cgs
Temperature	6,650±200 K
Metallicity [Fe/H]	−0.05±0.10 dex
Rotational velocity (v sin i)	45.7±1.0 km/s
Other designations
	CD−36° 1218, FK5 1090, GC 3886, HD 20301, HIP 15092, SAO 194176, PPM 278845
Database references
SIMBAD	data

TZ Fornacis is an eclipsing binary star system in the southern constellation of Fornax. It has the designation HD 20301 from the Henry Draper Catalogue; TZ Fornacis is the variable star designation, abbreviated TZ For. This target is a challenge to view with the naked eye, having a peak apparent visual magnitude of 6.88.^[2] During an eclipse, the magnitude drops to 7.05.^[5] This system is located at a distance of approximately 596 light years from the Sun based on parallax measurements,^[3] and is drifting further away with a radial velocity of roughly 18 km/s.^[6]

In 1977, J. Andersen and B. Nordström identified HD 20301 as a double-lined spectroscopic binary star system.^[8] The same year, E. H. Olsen determined this to be an eclipsing binary system that included an aging giant star. This made it of interest to astronomers because the orbital elements could be used to more precisely determine the mass and radius of an evolved star.^[9] The two stars are of similar mass and both have evolved away from the main sequence, giving them an enlarged radius.^[1]

The orbital plane for this system is nearly aligned with the line of sight to the Earth, so with each orbit the components are seen to eclipse each other over an orbital period of 75.67 days. However, these eclipses are shallow so there is not a complete occultation. The orbit has been circularized by tidal forces between the stars, but only the more massive component has had its rotation tidally synchronized with the orbit.^[1] The system is fully detached with neither component overrunning its Roche lobe.^[4]

The primary component, designated TZ Fornacis 1, has a spectrum that matches an aging G-type giant star with a stellar classification of G8 III. It is estimated to be 1.2 billion years old and is spinning in synchronicity with the orbital rotation. Based on the abundance of iron, the metallicity of this star is essentially the same as in the Sun. It has double the mass of the Sun and has expanded to over 8 times the Sun's radius. The star is radiating 37 times the luminosity of the Sun from its enlarged photosphere at an effective temperature of 4,930 K.^[4]

The secondary star, designated TZ Fornacis 2, is an F-type subgiant star with a class of F7 III. Models suggest it has just left the main sequence.^[4] The star is still small enough that its rotation rate hasn't been significantly impacted by tidal interaction. It has a relatively high projected rotational velocity of 46 km/s.^[10] The star has nearly double the mass of the Sun and four times the Sun's radius. It is radiating 23 times the luminosity of the Sun at an effective temperature of 6,650 K, making it the hotter star in this system.^[4]

References

^ ^a ^b ^c Andersen, J.; et al. (June 1991), "TZ Fornacis : stellar and tidal evolution in a binary with a fully-fledged red giant (absolute dimensions of eclipsing binaries. XVII).", Astronomy and Astrophysics, 246: 99, Bibcode:1991A&A...246...99A.
^ ^a ^b ^c ^d ^e 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.
^ ^a ^b ^c ^d ^e 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.
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ Gallenne, A.; et al. (February 2016), "The Araucaria Project: High-precision orbital parallax and masses of the eclipsing binary TZ Fornacis", Astronomy & Astrophysics, 586, id. A35, arXiv:1511.07971, Bibcode:2016A&A...586A..35G, doi:10.1051/0004-6361/201526764.
^ ^a ^b Samus, N. N.; et al. (2017), "General Catalogue of Variable Stars", Astronomy Reports, GCVS 5.1, 61 (1): 80–88, Bibcode:2017ARep...61...80S.
^ ^a ^b Gontcharov, G. A. (2006), "Pulkovo compilation of radial velocities for 35495 stars in a common system", Astronomy Letters, 32 (11): 759–771, arXiv:1606.08053, Bibcode:2006AstL...32..759G, doi:10.1134/S1063773706110065, S2CID 119231169.
^ "TZ For", SIMBAD, Centre de données astronomiques de Strasbourg, retrieved 2024-01-18.
^ Andersen, J.; Nordström, B. (September 1977), "Bright southern stars of astrophysical interest", Astronomy and Astrophysics, Supplement Series, 29: 309–312, Bibcode:1977A&AS...29..309A.
^ Olsen, E. H. (August 1977), "HD 20301: an Eclipsing, Double-Lined Early G Giant", Information Bulletin on Variable Stars, 1317: 1, Bibcode:1977IBVS.1317....1O.
^ Higl, J.; et al. (September 2018), "An analysis of the TZ Fornacis binary system", Astronomy & Astrophysics, 617, id. A36, Bibcode:2018A&A...617A..36H, doi:10.1051/0004-6361/201833112.

Claret, A. (August 2022), "Exploratory scenarios for the differential nuclear and tidal evolution of TZ Fornacis", Astronomy & Astrophysics, 664, id. A101, arXiv:2206.11069, Bibcode:2022A&A...664A.101C, doi:10.1051/0004-6361/202243573.
Valle, G.; et al. (April 2017), "Statistical errors and systematic biases in the calibration of the convective core overshooting with eclipsing binaries. A case study: TZ Fornacis", Astronomy & Astrophysics, 600, id. A41, arXiv:1612.07066, Bibcode:2017A&A...600A..41V, doi:10.1051/0004-6361/201628240.
Claret, A. (February 2011), "Peculiar rotation in evolved binary systems: stellar and tidal evolution of TZ Fornacis", Astronomy and Astrophysics, 526, id. A157, Bibcode:2011A&A...526A.157C, doi:10.1051/0004-6361/201015128, hdl:10261/45203.
Claret, A.; Gimenez, A. (April 1995), "Stellar and tidal evolution of TZ Fornacis: a case of asynchronism", Astronomy and Astrophysics, 296: 180, Bibcode:1995A&A...296..180C.
Clausen, J. V.; et al. (June 1991), "Four-colour photometry of eclipsing binaries. XXXIII. Light curves of TZ Fornacis", Astronomy and Astrophysics, Supplement Series, 88: 535, Bibcode:1991A&AS...88..535C.
Andersen, J.; et al. (1984), Maeder, Andre; et al. (eds.), "A New Test Case for Normal Giant Evolution - TZ-Fornacis", Observational Tests of the Stellar Evolution Theory. International Astronomical Union Symposium No. 105, held in Geneva, Switzerland, September 12–16, 1983, Dordrecht, The Netherlands, Boston, MA, Hingham, MA.: D. Reidel Publishing Company, p. 397, Bibcode:1984IAUS..105..397A.