18 Scorpii

18 Scorpii is a solitary star located at a distance of some 46.1 light-years (14.13 parsecs) from the Sun at the northern edge of the Scorpius constellation. It has an apparent visual magnitude of 5.5,[2] which is bright enough to be seen with the naked eye outside of urban areas. The star is drifting further away with a radial velocity of +11.6.[2]

18 Scorpii
Diagram showing star positions and boundaries of the Scorpius constellation and its surroundings
Location of 18 Scorpii (circled)
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Scorpius
Right ascension 16h 15m 37.27028s[1]
Declination –08° 22′ 09.9821″[1]
Apparent magnitude (V) 5.503[2]
Characteristics
Spectral type G2 Va[3]
U−B color index +0.18[4]
B−V color index +0.64[4]
Variable type Sun-like[5]
Astrometry
Radial velocity (Rv)11.71±0.12[1] km/s
Proper motion (μ) RA: 232.230 mas/yr[1]
Dec.: −495.378 mas/yr[1]
Parallax (π)70.7371 ± 0.0631 mas[1]
Distance46.11 ± 0.04 ly
(14.14 ± 0.01 pc)
Absolute magnitude (MV)4.77[2]
Details[6]
Mass1.03±0.03 M☉
Radius1.010±0.009 R☉
Luminosity1.0438±0.0120 L☉
Surface gravity (log g)4.448±0.012 cgs
Temperature5,817±4 K
Metallicity [Fe/H]0.04±0.024[7] dex
Rotation22.7±0.5 d[8]
Age2.9±0.5[9] Gyr
5.64 or 7.18 Gyr
Other designations
BD−07°4242, GC 21864, GJ 616, HD 146233, HIP 79672, HR 6060, SAO 141066, CCDM 16156-0822[4]
Database references
SIMBADdata
Exoplanet Archivedata

18 Scorpii has some physical properties in common with the Sun, a G-type star. Cayrel de Strobel (1996) included it in her review of the stars most similar to the Sun,[10] and Porto de Mello & da Silva (1997) identified it as a younger solar twin.[11][12] Some scientists therefore believe the prospects for life in its vicinity are good.

Characteristics
18 scorpii is on the northern boundary of the constellation. Its high proper motion positions it in Ophiuchus before 1700.
The age of 18 Scorpii, shown relative to the Sun, the older solar twin HD 197027, here annotated as HIP 102152 and the formation of the Milky Way

18 Scorpii is a main sequence star of spectral and luminosity type G2 Va,[12] with the luminosity class of 'V' indicating it is generating energy through the nuclear fusion of hydrogen in its core region. Sousa et al. (2008) found its metallicity to be about 1.1 times that of the Sun, which means the abundance of elements other than hydrogen or helium is 10% greater.[13][14] The radius of this star, as measured using interferometry by Bazot et al. (2011), is 101% the radius of the Sun. When combined with the results of asteroseismology measurements, this allows the mass of the star to be estimated as 102% of the Sun's mass.[15] This star is radiating 106% of the Sun's luminosity from its outer atmosphere at an effective temperature of 5,433 K.[16] It is this heat that gives the star the yellow-hued glow of a G-type star.[17]

According to Lockwood (2002), it has a temporal photometric behavior very similar to the Sun.[18] Its brightness variation over its entire activity cycle is 0.09%, about the same as the Sun's brightness variations during recent solar cycles.[19] Using the technique of Zeeman-Doppler imaging, Petit et al. (2008) have detected its surface magnetic field, showing that its intensity and geometry are very similar to the large-scale solar magnetic field.[20] The estimated period for the activity cycle of 18 Scorpii is about seven years,[5] which is significantly shorter than the Sun's, and its overall chromospheric activity level is noticeably higher.[19][21] Like the Sun, it has a hot corona with a temperature in the range of 1.5–2 MK and an X-ray luminosity of 8 ± 1.5 ergs s−1.[8]

Though 18 Scorpii is only slightly more metal-rich overall than the Sun, its lithium abundance is about three times as high; for this reason, Meléndez & Ramírez (2007) have suggested that 18 Scorpii be called a "quasi solar twin", reserving the term solar twin for stars (such as HIP 56948) that match the Sun, within the observational errors, for all parameters.[22]

18 Scorpii is a young star at 2.9 Gyr (2.9 Billion years old). 18 Scorpii has not yet entered its stable burning stage. The Sun at 4.7 Gyr is at its most stable stage. Due to 18 Scorpii age, it is at the edge of range for a solar twin, and is more of a Solar analog.[23] 18 Scorpii was thought to be 5.0 billion years old in the past, new measurements in 2013 found 18 Scorpii to be younger at 2.9 billion years old.[24]

Search for planets

18 Scorpii was identified in September 2003 by astrobiologist Margaret Turnbull from the University of Arizona in Tucson as one of the most promising nearby candidates for hosting life, based on her analysis of the HabCat list of stars. This is a solitary star,[25] and does not display the level of excess infrared emission that would otherwise suggest the presence of unconsolidated circumstellar matter, such as a debris disk.[26]

In a paper published in April 2017, a candidate planet was found orbiting 18 Scorpii (HD 146233) with a period of 2,529 days (6.92 yr),[27] but subsequent studies in 2020 and 2023 found that the radial velocity signal originates from a stellar activity cycle.[28][29]: 38  However, in 2023 evidence of a different candidate planet was found, which would be of super-Earth mass with a period of 19.9 days.[29]: 38 

The 18 Scorpii planetary system[29]
Companion
(in order from star)		 Mass Semimajor axis
(AU)		 Orbital period
(days)		 Eccentricity Inclination Radius
(unconfirmed) ≥6.77±0.86 M🜨 — 19.8777±0.0062 0.38±0.16 — —

References
  1. 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.
  2. Nordström, B.; et al. (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.
  3. Keenan, Philip C.; McNeil, Raymond C. (1989). "The Perkins catalog of revised MK types for the cooler stars". Astrophysical Journal Supplement Series. 71: 245. Bibcode:1989ApJS...71..245K. doi:10.1086/191373.
  4. "18 Sco -- Variable Star", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2011-10-13 The ubv information is per Compilation of Eggen's UBV data, transformed to UBV (unpublished) 1986. See the Measurements section.
  5. Hall, Jeffrey C.; et al. (July 2009). "The Activity and Variability of the Sun and Sun-Like Stars. II. Contemporaneous Photometry and Spectroscopy of Bright Solar Analogs". The Astronomical Journal. 138 (1): 312–322. Bibcode:2009AJ....138..312H. CiteSeerX 10.1.1.216.9004. doi:10.1088/0004-6256/138/1/312. S2CID 12332945.
  6. Bazot, M.; Creevey, O.; Christensen-Dalsgaard, J.; Meléndez, J. (November 2018). "Modelling the solar twin 18 Scorpii". Astronomy & Astrophysics. 619: 15. arXiv:1810.01387. Bibcode:2018A&A...619A.172B. doi:10.1051/0004-6361/201834058. S2CID 119413533. A172.
  7. Meléndez, Jorge; Ramírez, Iván (2007). "HIP 56948: A Solar Twin with a Low Lithium Abundance". The Astrophysical Journal. 669 (2): L89–L92. arXiv:0709.4290. Bibcode:2007ApJ...669L..89M. doi:10.1086/523942.
  8. Coughlin, Jared; et al. (January 2010). "The Night Time Sun: X-Ray Observations of the Solar Twin 18 Scorpii". Bulletin of the American Astronomical Society. 42: 333. Bibcode:2010AAS...21542417C.
  9. "Identification of oldest solar twin may help locate rocky exoplanets". NASA. August 28, 2013. Retrieved 2020-11-10.
  10. Cayrel de Strobel, G. (1996). "Stars Resembling the Sun". The Astronomy and Astrophysics Review. 7 (3): 243–288. Bibcode:1996A&ARv...7..243C. doi:10.1007/s001590050006. S2CID 189937884.
  11. "The life cycle of a Sun-like star (annotated)]". ESO. 28 August 2013. Retrieved 2020-11-10.
  12. Porto de Mello, G. F.; da Silva, L. (1997). "HR 6060: The Closest Ever Solar Twin?". The Astrophysical Journal. 482 (2): L89–L92. Bibcode:1997ApJ...482L..89P. doi:10.1086/310693.
  13. Sousa, S. G.; et al. (August 2008). "Spectroscopic parameters for 451 stars in the HARPS GTO planet search program. Stellar [Fe/H] and the frequency of exo-Neptunes". Astronomy and Astrophysics. 487 (1): 373–381. arXiv:0805.4826. Bibcode:2008A&A...487..373S. doi:10.1051/0004-6361:200809698. S2CID 18173201.
  14. A metallicity of [Fe/H] = 0.04 dex indicates that the star has 100.04 = 1.096, or 110% of the abundance of elements heavier than helium, compared to the Sun.
  15. Bazot, M.; et al. (February 2011). "The radius and mass of the close solar twin 18 Scorpii derived from asteroseismology and interferometry". Astronomy and Astrophysics. 526 (526): L4. arXiv:1209.0217. Bibcode:2011A&A...526L...4B. doi:10.1051/0004-6361/201015679. S2CID 13523885.
  16. Boyajian, Tabetha S.; et al. (February 2012). "Stellar Diameters and Temperatures. I. Main-sequence A, F, and G Stars". The Astrophysical Journal. 746 (1): 101. arXiv:1112.3316. Bibcode:2012ApJ...746..101B. doi:10.1088/0004-637X/746/1/101. S2CID 18993744.. See Table 10.
  17. "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, archived from the original on February 22, 2012, retrieved 2012-01-16
  18. Lockwood, G. W.; et al. (May 2002). "Gauging the Sun: Comparative photometric and magnetic activity measurements of sunlike stars, 1984-2001" (PDF). Bulletin of the American Astronomical Society. 34: 651. Bibcode:2002AAS...200.0709L. Archived from the original (PDF) on 2006-09-05.
  19. Hall, J. C.; Lockwood, G. W. (2007). "The Sun-Like Activity of the Solar Twin 18 Scorpii". The Astronomical Journal. 133 (5): 2206–2208. arXiv:astro-ph/0703450. Bibcode:2007AJ....133.2206H. doi:10.1086/513195. S2CID 8790446.
  20. Petit, P.; et al. (2008). "Toroidal versus poloidal magnetic fields in Sun-like stars: a rotation threshold". Monthly Notices of the Royal Astronomical Society. 388 (1): 80. arXiv:0804.1290. Bibcode:2008MNRAS.388...80P. doi:10.1111/j.1365-2966.2008.13411.x. S2CID 8173734.
  21. Hall, Jeffrey C.; Lockwood, G. W. (2000). "Evidence of a Pronounced Activity Cycle in the Solar Twin 18 Scorpii". The Astrophysical Journal. 545 (2): L43–L45. Bibcode:2000ApJ...545L..43H. doi:10.1086/317331.
  22. Meléndez, J.; Ramírez, I. (2007). "HIP 56948: A Solar Twin with a Low Lithium Abundance". The Astrophysical Journal. 669 (2): L89–L92. arXiv:0709.4290. Bibcode:2007ApJ...669L..89M. doi:10.1086/523942. S2CID 15952981.
  23. King, Jeremy R; Boesgaard, Ann M; Schuler, Simon C (2005). "Keck/HIRES Spectroscopy of Four Candidate Solar Twins". The Astronomical Journal. 130 (5): 2318–2325. arXiv:astro-ph/0508004. Bibcode:2005AJ....130.2318K. doi:10.1086/452640. S2CID 6535115.
  24. NASA NEWS, Identification of oldest solar twin may help locate rocky exoplanets, August 29, 2013
  25. Marcy, Geoffrey W.; et al. (2005). "Five New Extrasolar Planets". The Astrophysical Journal. 619 (1): 570–584. Bibcode:2005ApJ...619..570M. CiteSeerX 10.1.1.516.6667. doi:10.1086/426384. S2CID 5803173.
  26. Lawler, S. M.; et al. (2009). "Explorations Beyond the Snow Line: Spitzer/IRS Spectra of Debris Disks Around Solar-type Stars". The Astrophysical Journal. 705 (1): 89–111. arXiv:0909.0058. Bibcode:2009ApJ...705...89L. doi:10.1088/0004-637X/705/1/89. S2CID 1272803.
  27. Butler, R. Paul; et al. (13 April 2017). "The LCES HIRES/Keck Precision Radial Velocity Exoplanet Survey". The Astronomical Journal. 153 (5): 208. arXiv:1702.03571. Bibcode:2017AJ....153..208B. doi:10.3847/1538-3881/aa66ca. hdl:2299/18220. S2CID 14954371. Planet candidate is in Table 2; full table available here.
  28. Hirsch, Lea A.; et al. (2021), "Understanding the Impacts of Stellar Companions on Planet Formation and Evolution: A Survey of Stellar and Planetary Companions within 25 pc", The Astronomical Journal, 161 (3): 134, arXiv:2012.09190, Bibcode:2021AJ....161..134H, doi:10.3847/1538-3881/abd639, S2CID 229297873.
  29. Laliotis, Katherine; Burt, Jennifer A.; et al. (February 2023). "Doppler Constraints on Planetary Companions to Nearby Sun-like Stars: An Archival Radial Velocity Survey of Southern Targets for Proposed NASA Direct Imaging Missions". The Astronomical Journal. 165 (4): 176. arXiv:2302.10310. Bibcode:2023AJ....165..176L. doi:10.3847/1538-3881/acc067.
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