Solar analog

Solar-type, solar analog, and solar twin stars are those stars that are particularly similar to the Sun. The classification is a hierarchy with solar twin being most like the Sun followed by solar analog and then solar-type. Observations of these stars are important for understanding better the properties of the Sun in relation to other stars and the habitability of planets.

Contents

By similarity to the Sun

Defining the three categories by their similarity to the Sun reflects the evolution of astronomical observational techniques. Originally, solar-type was the closest that similarity to the Sun could be defined. Later, more precise measurement techniques and improved observatories allowed for greater precision of key details like temperature, enabling the creation of a solar analog category for stars that were particularly similar to the Sun. Later still, continued improvements in precision allowed for the creation of a solar twin category for near-perfect matches.

Similarity to the Sun allows for checking derived quantities — like temperature, which is derived from the color index — against the Sun, the only star whose temperature is confidently known. For stars which aren't similar to the Sun, this cross-checking can't be done.[1]

Solar-type

These stars are broadly similar to the Sun. They are main sequence stars with a B-V color between 0.48 and 0.80, the Sun having a B-V color of 0.65. Alternatively, a definition based on spectral type can be used, such as F8 V through K2 V, which would correspond to B-V color of 0.50 to 1.00.[1] This definition fits approximately 10% of stars, so a list of solar-type stars would be quite extensive.

Solar-type stars show highly correlated behavior between their rotation rates and their chromospheric activity (e.g. Calcium H & K line emission) and coronal activity (e.g. X-ray emission). As solar-type stars spin-down during their main sequence lifetimes due to magnetic braking, these correlations allow rough ages to be derived. Mamajek & Hillenbrand (2008)[2] have estimated the ages for the 108 solar-type (F8V-K2V) main sequence stars within 16 parsecs of the Sun based on their chromospheric activity (as measured via Ca H & K emission lines).

The following table shows a sample of solar-type stars within 50 light years that nearly satisfy the criteria for solar analogs, based on current measurements.

Sample of solar-type stars
Identifier Coordinates[3] Distance[3]
(ly)		 Stellar
Class[3]		 Temperature
(K)		 Metallicity
(dex)		 Notes
Right ascension Declination
Tau Ceti 01h 44m 04.1s −15° 56′ 15″ 11.9 G8V 5,344 –0.52 [4]
40 Eridani A 04h 15m 16.3s −07° 39′ 10″ 16.5 K1V 5,126 –0.31 [4]
82 Eridani 03h 19m 55.7s −43° 04′ 11.2″ 19.8 G8V 5,338 –0.54 [5]
Delta Pavonis 20h 08m 43.6s −66° 10′ 55″ 19.9 G8IV 5,604 +0.33 [6]
HR 7722 20h 15m 17.4s −27° 01′ 59″ 28.8 K0V 5,166 –0.04 [6]
Gliese 86 A 02h 10m 25.9s −50° 49′ 25″ 35.2 K1V 5,163 −0.24 [4]
54 Piscium 00h 39m 21.8s +21° 15′ 02″ 36.1 K0V 5,129 +0.19 [5]
V538 Aurigae 05h 41m 20.3s +53° 28′ 51.8″ 39.9 K1V 3,500-5,000 -0.20 [5]
HD 14412 02h 18m 58.5s −25° 56′ 45″ 41.3 G5V 5,432 −0.46 [5]
HR 4587 12h 00m 44.3s -10° 26′ 45.7″ 42.1 G8IV 5,538 0.18 [5]
HD 172051 18h 38m 53.4s −21° 03′ 07″ 42.7 G5V 5,610 −0.32 [5]
72 Herculis 17h 20m 39.6s +32° 28′ 04″ 46.9 G0V 5,662 −0.37 [5]
HD 196761 20h 40m 11.8s −23° 46′ 26″ 46.9 G8V 5,415 -0.31 [6]
Nu² Lupi 15h 21m 48.1s −48° 19′ 03″ 47.5 G4V 5,664 −0.34 [6]

Solar analog

These stars are photometrically similar to the Sun, having the following qualities:[1]

Solar analogs not meeting the stricter solar twin criteria include, within 50 light years and in order of increasing distance:

Identifier Coordinates[3] Distance[3]
(ly)		 Stellar
Class[3]		 Temperature
(K)		 Metallicity
(dex)		 Notes
Right ascension Declination
Alpha Centauri A 14h 39m 36.5s −60° 50′ 02″ 4.37 G2V 5,847 +0.24 [7]
Alpha Centauri B 14h 39m 35.0s −60° 50′ 14″ 4.37 K1V 5,316 +0.25 [7]
70 Ophiuchi A 18h 05m 27.3s +02° 30′ 00″ 16.6 K0V 5,314 –0.02 [8]
Sigma Draconis 19h 32m 21.6s +69° 39′ 40″ 18.8 K0V 5,297 –0.20 [9]
Eta Cassiopeiae A 00h 49m 06.3s +57° 48′ 55″ 19.4 G0V 5,941 –0.17 [10]
107 Piscium 01h 42m 29.8s +20° 16′ 07″ 24.4 K1V 5,242 –0.04 [5][11]
Beta Canum Venaticorum 12h 33m 44.5s +41° 21′ 27″ 27.4 G0V 5,930 −0.30 [5]
61 Virginis 13h 18m 24.3s −18° 18′ 40″ 27.8 G5V 5,558 –0.02 [6]
Zeta Tucanae 00h 20m 04.3s –64° 52′ 29″ 28.0 F9.5V 5,956 –0.14 [4]
Chi¹ Orionis A 05h 54m 23.0s +20° 16′ 34″ 28.3 G0V 5,902 –0.16 [5]
Beta Comae Berenices 13h 11m 52.4s +27° 52′ 41″ 29.8 G0V 5,970 –0.06 [5]
HR 4523 A 11h 46m 31.1s –40° 30′ 01″ 30.1 G5V 5,629 –0.29 [6]
61 Ursae Majoris 11h 41m 03.0s +34° 12′ 06″ 31.1 G8V 5,483 –0.12 [5]
HR 4458 A 11h 34m 29.5s –32° 49′ 53″ 31.1 K0V 5,629 –0.29 [6]
HR 511 01h 47m 44.8s +63° 51′ 09″ 32.8 K0V 5,333 +0.05 [5]
Alpha Mensae 06h 10m 14.5s –74° 45′ 11″ 33.1 G5V 5,594 +0.10 [4]
Zeta1 Reticuli 03h 17m 46.2s −62° 34′ 31″ 39.5 G3-5V 5,733 −0.22 [4]
Zeta2 Reticuli 03h 18m 12.8s −62° 30′ 23″ 39.5 G2V 5,843 −0.23 [4]
55 Cancri 08h 52m 35.81s +28° 19′ 51″ 40.3 G8V 5,235 +0.25 [10]
HD 69830 08h 18m 23.9s −12° 37′ 56″ 40.6 K0V 5,410 −0.03 [4]
HD 10307 01h 41m 47.1s +42° 36′ 48″ 41.2 G1.5V 5,848 −0.05 [5]
HD 147513 16h 24m 01.3s −39° 11′ 35″ 42.0 G1V 5,858 +0.03 [6]
58 Eridani 04h 47m 36.3s −16° 56′ 04″ 43.3 G3V 5,868 +0.02 [4]
Upsilon Andromedae A 01h 36m 47.8s +41° 24′ 20″ 44.0 F8V 6,212 +0.13 [4]
HD 211415 A 22h 18m 15.6s –53° 37′ 37″ 44.4 G1-3V 5,890 −0.17 [4]
47 Ursae Majoris 10h 59m 28.0s +40° 25′ 49″ 45.9 G1V 5,954 +0.06 [4]
Alpha Fornacis A 03h 12m 04.3s −28° 59′ 21″ 46.0 F8IV 6,275 −0.19 [4]
Psi Serpentis A 15h 44m 01.8s +02° 30′ 55″ 47.9 G5V 5,636 −0.03 [5]
HD 84117 09h 42m 14.4s –23° 54′ 56″ 48.5 F8V 6,167 –0.03 [4]
HD 4391 00h 45m 45.6s –47° 33′ 07″ 48.6 G3V 5,878 –0.03 [4]
20 Leonis Minoris 10h 01m 00.7s +31° 55′ 25″ 49.1 G3 V 5,741 +0.20 [5]
Nu Phoenicis 01h 15m 11.1s –45° 31′ 54″ 49.3 F8V 6,140 +0.18 [4]
51 Pegasi 22h 57m 28.0s +20° 46′ 08″ 50.9 G2.5IVa 5,804 +0.20 [4]

Solar twin

These stars are more similar to the Sun still, having the following qualities:[1]

The following are the known stars that come closest to satisfying the criteria for a solar twin. (The Sun is listed for comparison.)

Identifier Coordinates[3] Distance[3]
(ly)		 Stellar
Class[3]		 Temperature
(K)		 Metallicity
(dex)		 Age
(Gyr)		 Notes
Right ascension Declination
Sun — — 0.00 G2V 5,778 +0.00 4.6 [12]
18 Scorpii 16h 15m 37.3s –08° 22′ 06″ 45.1 G2Va 5,835 +0.04 4.2 [13]
HD 44594 06h 20m 06.1s −48° 44′ 29″ 84 G3V 5,840 +0.15 4.1 [14]
HD 195034 20h 28m 11.8s +22° 07′ 44″ 92 G5 5,760 -0.04 5.1 [15]
HD 138573 15h 32m 43.7s +10° 58′ 06″ 101 G5IV-V 5,710 –0.03 7.8 [16]
HD 142093 15h 52m 00.6s +15° 14′ 09″ 103 G2V 5,841 –0.15 5.0 [16]
HD 98618 11h 21m 29.1s +58° 29′ 04″ 126 G5V 5,851 +0.03 4.7 [13]
HD 143436 16h 00m 18.8s +00° 08′ 13″ 141 G0 5,768 +0.00 3.8 [16]
HD 129357 14h 41m 22.4s +29° 03′ 32″ 154 G2V 5,749 –0.02 8.2 [16]
HD 133600 15h 05m 13.2s +06° 17′ 24″ 171 G0 5,808 +0.02 6.3 [13]
HD 101364 11h 40m 28.5s +69° 00′ 31″ 217 G5V 5,783 +0.01 5.4 [13][17]

Some other stars are sometimes mentioned as promising solar twin candidates, particularly: Beta Canum Venaticorum (see references for Turnbull & Tarter), 37 Geminorum (see references for Turnbull & Tarter) and 16 Cygni B (Porto de Mello et al. 2000). However, all three have temperatures and/or luminosities that are too high for true solar twins. Furthermore, Beta Canum Venaticorum and 37 Geminorum have too low metallicities for solar twins. Finally, 16 Cygni B is part of a (very wide) binary system and is very old for a solar twin (at least 7 to 8 Gyr). Beta Canum Venaticorum is mentioned above as a nearby solar analog.

By potential habitability

Another way of defining solar twin is as a "habstar" — a star with qualities believed to be particularly hospitable to an Earth-like planet. Qualities considered include variability, mass, age, metallicity, and close companions.[18]

The requirement that the star remain on the main sequence for at least 3 Ga sets an upper limit of approximately 1.5 solar masses, corresponding to a hottest spectral type of F5 V. Such stars can reach an absolute magnitude of 2.5, or 8.55 times as bright as the Sun, at the end of the main sequence.[18]

Non-variability is ideally defined as variability of less than 1%, but 3% is the practical limit due to limits in available data. Variation in irradiance in a star's habitable zone due to a companion star with an eccentric orbit is also a concern.[18]

Terrestrial planets in multiple star systems, those containing three or more stars, are not likely to have stable orbits in the long term. Stable orbits in binary systems take one of two forms: S-Type (satellite or circumstellar) orbits around one of the stars, and P-Type (planetary or circumbinary) orbits around the entire binary pair. Eccentric Jupiters may also disrupt the orbits of planets in habitable zones.[18]

Metallicity of at least 40% solar ([Fe/H] = -0.4) is required for the formation of an Earth-like terrestrial planet. High metallicity strongly correlates to the formation of hot Jupiters, but these are not absolute bars to life, as some gas giants end up orbiting within the habitable zone themselves, and could potentially host Earth-like moons.[18]

One example of such a star is HD 70642.[19]

See also

References

  1. ^ a b c d D. R. Soderblom; J. R. King (1998). "Solar-Type Stars: Basic Information on Their Classification and Characterization". Solar Analogs : Characteristics and Optimum Candidates. http://www.lowell.edu/users/jch/workshop/drs/drs-p1.html. Retrieved 2008-02-26. 
  2. ^ E. E. Mamajek; L. A. Hillenbrand (2008). "Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics". Astrophysical Journal 687 (2): 1264. Bibcode 2008ApJ...687.1264M. doi:10.1086/591785. 
  3. ^ a b c d e f g h i "SIMBAD Astronomical Database". SIMBAD. Centre de Données astronomiques de Strasbourg. http://simbad.u-strasbg.fr/simbad/. Retrieved 2009-01-14. 
  4. ^ a b c d e f g h i j k l m n o p q Santos, N. C.; Israelian, G.; Randich, S.; García López, R. J.; Rebolo, R. (October 2004). "Beryllium anomalies in solar-type field stars". Astronomy and Astrophysics 425 (3): 1013–1027. arXiv:astro-ph/0408109. Bibcode 2004A&A...425.1013S. doi:10.1051/0004-6361:20040510. 
  5. ^ a b c d e f g h i j k l m n o p Holmberg J., Nordstrom B., Andersen J. (July 2009). "The Geneva-Copenhagen survey of the solar neighbourhood. III. Improved distances, ages, and kinematics". Astronomy and Astrophysics 501 (3): 941–947. Bibcode 2009A&A...501..941H. doi:10.1051/0004-6361/200811191.  See Vizier catalogue V/130.
  6. ^ a b c d e f g h 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. Bibcode 2008A&A...487..373S. doi:10.1051/0004-6361:200809698.  See VizieR catalogue J/A+A/487/373.
  7. ^ a b Porto de Mello, G. F.; Lyra, W.; Keller, G. R. (September 2008). "The Alpha Centauri binary system. Atmospheric parameters and element abundances". Astronomy and Astrophysics 488 (2): 653–666. Bibcode 2008A&A...488..653P. doi:10.1051/0004-6361:200810031. 
  8. ^ Casagrande, Luca; Flynn, Chris; Portinari, Laura; Girardi, Leo; Jimenez, Raul (December 2007). "The helium abundance and ΔY/ΔZ in lower main-sequence stars". Monthly Notices of the Royal Astronomical Society 382 (4): 1516–1540. arXiv:astro-ph/0703766. Bibcode 2007MNRAS.382.1516C. doi:10.1111/j.1365-2966.2007.12512.x. 
  9. ^ Boyajian, Tabetha S. et al. (August 2008). "Angular Diameters of the G Subdwarf μ Cassiopeiae A and the K Dwarfs σ Draconis and HR 511 from Interferometric Measurements with the CHARA Array". The Astrophysical Journal 683 (1): 424–432. Bibcode 2008ApJ...683..424B. doi:10.1086/589554. 
  10. ^ a b Valenti, Jeff A.; Fischer, Debra A. (July 2005). "Spectroscopic Properties of Cool Stars (SPOCS). I. 1040 F, G, and K Dwarfs from Keck, Lick, and AAT Planet Search Programs". The Astrophysical Journal Supplement Series 159 (1): 141–166. Bibcode 2005ApJS..159..141V. doi:10.1086/430500.  See VizieR catalogue J/ApJS/159/141.
  11. ^ Kovtyukh, V. V.; Soubiran, C.; Belik, S. I.; Gorlova, N. I. (2003). "High precision effective temperatures for 181 F-K dwarfs from line-depth ratios". Astronomy and Astrophysics 411 (3): 559–564. arXiv:astro-ph/0308429. Bibcode 2003A&A...411..559K. doi:10.1051/0004-6361:20031378. 
  12. ^ Williams, D.R. (2004). "Sun Fact Sheet". NASA. http://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html. Retrieved 2009-06-23. 
  13. ^ a b c d Meléndez, Jorge; Ramírez, Iván (November 2007). "HIP 56948: A Solar Twin with a Low Lithium Abundance". The Astrophysical Journal 669 (2): L89–L92. Bibcode 2007ApJ...669L..89M. doi:10.1086/523942. 
  14. ^ Sousa, S. G.; Fernandes, J.; Israelian, G.; Santos, N. C. (March 2010). "Higher depletion of lithium in planet host stars: no age and mass effect". Astronomy and Astrophysics 512: L5. Bibcode 2010A&A...512L...5S. doi:10.1051/0004-6361/201014125. 
  15. ^ Takeda, Y.; Tajitsu, A.; Tajitsu (2009). "High-Dispersion Spectroscopic Study of Solar Twins: HIP 56948, HIP 79672, and HIP 100963". Publications of the Astronomical Society of Japan 61: 471. arXiv:0901.2509T. Bibcode 2009PASJ...61..471T. 
  16. ^ a b c d King, Jeremy R.; Boesgaard, Ann M.; Schuler, Simon C. (November 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. 
  17. ^ Vázquez, M.; Pallé, E.; Rodríguez, P. Montañés (2010). "Is Our Environment Special?". The Earth as a Distant Planet: A Rosetta Stone for the Search of Earth-Like Worlds. Astronomy and Astrophysics Library. Springer New York. pp. 391–418. doi:10.1007/978-1-4419-1684-6. ISBN 978-1-4419-1683-9.  See table 9.1.
  18. ^ a b c d e Turnbull, M. C.; Tarter, J. C. (2002). "Target Selection for SETI. I. A Catalog of Nearby Habitable Stellar Systems". The Astrophysical Journal Supplement Series 145: 181. arXiv:astro-ph/0210675. Bibcode 2003ApJS..145..181T. doi:10.1086/345779. 
  19. ^ "Solar System 'twin' found". BBC News. 2003-07-03. http://news.bbc.co.uk/1/hi/sci/tech/3041220.stm. 

Further reading