TW Horologii

TW Horologii is a variable star in the southern constellation of Horologium,[10] near the eastern constellation border with Reticulum. It has a ruddy hue and is dimly visible to the naked eye with an apparent visual magnitude that ranges from 5.52 down to 5.95.[3] Based on parallax measurements, it is located at a distance of approximately 1,370 light years from the Sun.[1] It is drifting further away with a radial velocity of +14 km/s.[2] In the past this star has been considered a member of the open cluster NGC 1252, but this now seems unlikely.[11]

TW Horologii
Observation data
Epoch J2000      Equinox J2000
Constellation Horologium
Right ascension 03h 12m 33.16062s[1]
Declination −57° 19′ 17.5710″[1]
Apparent magnitude (V) 5.71[2] (5.52 to 5.95)[3]
Characteristics
Spectral type C-N4IIIb: (C2 3.5)[4]
U−B color index +2.93[5]
B−V color index +2.419±0.014[2]
Variable type SRb[3]
Astrometry
Radial velocity (Rv)+14.3±2.9[2] km/s
Proper motion (μ) RA: +18.492[1] mas/yr
Dec.: +13.298[1] mas/yr
Parallax (π)2.3755 ± 0.1295[1] mas
Distance1,370 ± 70 ly
(420 ± 20 pc)
Absolute magnitude (MV)−1.79[2]
Absolute bolometric
magnitude (Mbol)		−4.62[6]
Details
Mass3.29±0.65[7] M☉
Radius163.6+18.7
−27.9[1] R☉
Luminosity4,390±281[1] L☉
Surface gravity (log g)0.049[8] cgs
Temperature3,674+360
−194[1] K
Other designations
TW Hor, CD−57°626, FK5 118, HD 20234, HIP 14930, HR 977, SAO 233037[9]
Database references
SIMBADdata

This is an aging red giant star on the asymptotic giant branch[6] with a stellar classification of C-N4IIIb:.[4] It is one of the brightest carbon stars[10] and has a type of C2 3.5.[4] The star is classified as a semiregular variable of type SRb and has a periodicity of 158 days.[3] It has expanded to 164[1] times the radius of the Sun and, on average, is radiating 4,390 times the Sun's luminosity from its swollen photosphere at an effective temperature of 3,674 K.[1] The short-lived element technetium has been observed in the spectrum, an indicator of thermal pulses during helium shell burning.[12]

Based on the detection of excess ultraviolet excess, it is most likely a binary star system.[13] An analysis of the motion of TW Horologii suggests a low-mass companion, although the UV excess suggests it is hot.[7]

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.[2]

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. 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.
  4. 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.
  5. 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.
  6. 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. A120.
  7. 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.
  8. 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.
  9. "V* TW Hor". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 19 March 2016.
  10. 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.
  11. 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.
  12. Lebzelter, T.; Hron, J. (December 2003). "Technetium and the third dredge up in AGB stars. I. Field stars". Astronomy and Astrophysics. 411: 533–542. arXiv:astro-ph/0310018. Bibcode:2003A&A...411..533L. doi:10.1051/0004-6361:20031458.
  13. 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.
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