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TW Horologii
	; A visual band light curve for TW Horologii, plotted from data published by Tabur et al. (2009)
Observation data; Epoch J2000 Equinox J2000
Constellation	Horologium
Right ascension	03h 12m 33.16062s
Declination	−57° 19′ 17.5710″
Apparent magnitude (V)	5.71 (5.52 to 5.95)
Characteristics
Evolutionary stage	Asymptotic giant branch
Spectral type	C-N4IIIb: (C23.5)
U−B color index	+2.93
B−V color index	+2.419±0.014
Variable type	SRb
Astrometry
Radial velocity (Rv)	+14.3±2.9 km/s
Proper motion (μ)	RA: +18.492 mas/yr ; Dec.: +13.298 mas/yr
Parallax (π)	2.3755 ± 0.1295 mas
Distance	1,370 ± 70 ly ; (420 ± 20 pc)
Absolute magnitude (MV)	−1.79
Absolute bolometric; magnitude (Mbol)	−4.62
Details
Mass	3.29±0.65 M☉
Radius	303 R☉
Luminosity	6,700 L☉
Surface gravity (log g)	0.049 cgs
Temperature	3,000 K
Other designations
	TW Hor, CD−57°626, FK5 118, HD 20234, HIP 14930, HR 977, SAO 233037
Database references
SIMBAD	data

TW Horologii is a carbon star and semiregular variable in the southern constellation of Horologium,^[12] near the eastern constellation border with Reticulum. It has a ruddy hue and, with an apparent visual magnitude that ranges from 5.52 down to 5.95,^[4] is visible to the naked eye and one of the brightest carbon stars.^[12] Based on parallax measurements, it is located at a distance of approximately 1,370 light years from the Sun.^[2] It is drifting further away with a radial velocity of +14 km/s.^[3] In the past this star has been considered a member of the open cluster NGC 1252, but this now seems unlikely.^[13]

This is an aging red giant star on the asymptotic giant branch^[7] with a stellar classification of C-N4IIIb: and a C₂ index of C₂3.5. It has been listed as a standard star for that MK spectral class.^[5] The star is classified as a semiregular variable of type SRb and has a periodicity of 158 days.^[4] It has expanded to 303 times the radius of the Sun and, on average, is radiating 6,700 times the Sun's luminosity from its swollen photosphere at an effective temperature of 3,000 K.^[9] If it replaced the Solar System, the perihelion of Mars would be inside the star. The short-lived element technetium has been observed in the spectrum, an indicator of thermal pulses during helium shell burning.^[14]

Based on the detection of excess ultraviolet excess, it is most likely a binary star system.^[15] An analysis of the motion of TW Horologii suggests a low-mass companion, although the UV excess suggests it is hot.^[8]

In 2013 the luminosity of Mira variables, based on Hipparcos parallaxes, was used to calibrate a Period-luminosity relationship for carbon stars. The absolute magnitude of TW Horologii was calculated to be −1.79.^[3]

Notes

^ Applying the Stefan–Boltzmann law with a nominal solar effective temperature of 5,772 K:
${\sqrt {{\biggl (}{\frac {5,772}{3,000}}{\biggr )}^{4}\cdot 6,700}}=303\ R_{\odot }$ .

References

^ Tabur, V.; Bedding, T. R.; Kiss, L. L.; Moon, T. T.; Szeidl, B.; Kjeldsen, H. (December 2009). "Long-term photometry and periods for 261 nearby pulsating M giants". Monthly Notices of the Royal Astronomical Society. 400 (4): 1945–1961. arXiv:0908.3228. Bibcode:2009MNRAS.400.1945T. doi:10.1111/j.1365-2966.2009.15588.x.
^ ^a ^b ^c ^d ^e ^f 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.
^ ^a ^b ^c ^d ^e ^f 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 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.
^ ^a ^b Barnbaum, Cecilia; et al. (1996). "A Moderate-Resolution Spectral Atlas of Carbon Stars: R, J, N, CH, and Barium Stars". Astrophysical Journal Supplement. 105: 419. Bibcode:1996ApJS..105..419B. doi:10.1086/192323.
^ Nicolet, B. (1978). "Catalogue of homogeneous data in the UBV photoelectric photometric system". Astronomy and Astrophysics Supplement Series. 34: 1–49. Bibcode:1978A&AS...34....1N.
^ ^a ^b Guandalini, R.; Cristallo, S. (2013). "Luminosities of carbon-rich asymptotic giant branch stars in the Milky Way". Astronomy & Astrophysics. 555: 7. arXiv:1305.4203. Bibcode:2013A&A...555A.120G. doi:10.1051/0004-6361/201321225. S2CID 54918450. A120.
^ ^a ^b Kervella, Pierre; Arenou, Frédéric; Mignard, François; Thévenin, Frédéric (2019). "Stellar and substellar companions of nearby stars from Gaia DR2. Binarity from proper motion anomaly". Astronomy and Astrophysics. 623: A72. arXiv:1811.08902. Bibcode:2019A&A...623A..72K. doi:10.1051/0004-6361/201834371. S2CID 119491061.
^ ^a ^b ^c ^d Siderud, Emelie (2020). Dust emission modelling of AGB stars.
^ McDonald, I.; Zijlstra, A. A.; Watson, R. A. (2017). "Fundamental parameters and infrared excesses of Tycho-Gaia stars". Monthly Notices of the Royal Astronomical Society. 471 (1): 770. arXiv:1706.02208. Bibcode:2017MNRAS.471..770M. doi:10.1093/mnras/stx1433.
^ "V* TW Hor". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 19 March 2016.
^ ^a ^b Streicher, Magda (October 2009). "The Pendulum Clock". Monthly Notes of the Astronomical Society of South Africa. 68 (9–10): 202–206. Bibcode:2009MNSSA..68..202S.
^ de la Fuente Marcos, R.; et al. (September 2013). "NGC 1252: a high altitude, metal poor open cluster remnant". Monthly Notices of the Royal Astronomical Society. 434 (1): 194–208. arXiv:1306.1643. Bibcode:2013MNRAS.434..194D. doi:10.1093/mnras/stt996.
^ Lebzelter, T.; Hron, J. (December 2003). "Technetium and the third dredge up in AGB stars. I. Field stars". Astronomy and Astrophysics. 411 (3): 533–542. arXiv:astro-ph/0310018. Bibcode:2003A&A...411..533L. doi:10.1051/0004-6361:20031458. S2CID 18879265.
^ Ortiz, Roberto; Guerrero, Martín A. (September 2016). "Ultraviolet emission from main-sequence companions of AGB stars". Monthly Notices of the Royal Astronomical Society. 461 (3): 3036–3046. arXiv:1606.09086. Bibcode:2016MNRAS.461.3036O. doi:10.1093/mnras/stw1547.