Stars Like Our Sun – II

Last time we looked at the brightest G2V stars in the nighttime sky.

Now, we’ll focus on a more sophisticated approach to identify stars that are most like our Sun.  A solar twin is currently defined as a star with the following characteristics (Adibekyan et al. 2017):

Teff = 5777 ± 100 K

log g = 4.44 ± 0.10 dex

[Fe/H] = 0.00 ± 0.10 dex

Teff is the effective temperature of the star.  The effective temperature is the uniform temperature of a black body (which stars closely approximate) that would have the same radiant energy at all wavelengths as the star.

log g is the surface gravity, the base-10 logarithm of the gravitational acceleration, at the photosphere of the star.  The surface gravity is presented logarithmically because the gravitational acceleration at the surface of a star ranges over many orders of magnitude depending on the type of star (for example, a red dwarf vs. a white dwarf or neutron star).

[Fe/H] is the metallicity of the star, giving the ratio of iron to hydrogen atoms in logarithmic units relative to the Sun.  So measured, metallicity as the iron content of a star’s photosphere is often a reasonable proxy for the total metal content of the star (all elements except for hydrogen and helium).

Looking at a recent list of 21 solar twins in the solar neighborhood (Nissen  2016), we find that HD 20782 has the closest Teff match to the Sun, HR 2318 has the closest log g match to the Sun, and HD 222582 has the closest [Fe/H] match to the Sun.  The star with the closest match to all three solar twin characteristics is 18 Scorpii.

HD 20782
Fornax
Teff = 5776K, log g = 4.345, [Fe/H] = -0.058
Age = 8.1 ± 0.4 Gyr, Mass = 0.97 M
mv = 7.38, mb = 8.03, B-V = 0.65, G1.5V
α2000 = 03h 20m 04s, δ2000 = -28° 51′ 15″
116 – 118 ly
Single star with one known planet, 1.4 – 2.4 MJ, 592d orbital period, in a highly eccentric orbit (e = 0.97).

HR 2318
Canis Major
Teff = 5871 K, log g = 4.445, [Fe/H] = 0.047
Age = 2.7 ± 0.5 Gyr, Mass = 1.05 M
mv = 6.39, mb = 7.01, B-V = 0.62, G1.5V
α2000 = 06h 24m 44s, δ2000 = -28° 46′ 48″
71 – 72 ly
Single star with one known planet, 87% the mass of Uranus, 5.89d orbital period, in a mildly eccentric orbit (e = 0.3).

HD 222582
Aquarius (below the Circlet of Pisces)
Teff = 5784 K, log g = 4.361, [Fe/H] = -0.004
Age = 7.0 ± 0.4 Gyr, Mass = 1.00 M
mv = 7.69, mb = 8.34, B-V = 0.65, G5V
α2000 = 23h 41m 52s, δ2000 = -05° 59′ 09″
136 – 140 ly
Single star with one known planet, 7.1 – 8.4 MJ, 572d orbital period, in a very eccentric orbit (e = 0.725).

18 Scorpii (18 Sco)
Scorpius (just below the “coffee pot” asterism of Ophiuchus)
Teff = 5809 K, log g = 4.434, [Fe/H] = 0.046
Age = 4.0 ± 0.5 Gyr, Mass = 1.03 M
mv = 5.50, mb = 6.15, B-V = 0.65, G5V
α2000 = 16h 15m 37s, δ2000 = -08° 22′ 10″
45.1 – 45.6 ly
Single star, very similar to our Sun.

An additional solar twin in the solar neighborhood has been added recently (Yana Galarza 2016): HD 195034.  It has an even closer match to the Sun’s [Fe/H] than HD 222582 does.

HD 195034
Vulpecula
Teff = 5818 K, log g = 4.49, [Fe/H] = -0.003
Age = 2.0 ± 0.4 Gyr, Mass = 1.03 M
mv = 7.09, mb = 7.74, B-V = 0.65, G5
α2000 = 20h 28m 12s, δ2000 = +22° 07′ 44″
91 – 92 ly
Single star.

References
Adibekyan, V., Delgado-Mena, E., Feltzing, S., et al. 2017, arXiv:1701.05737
Nissen, P.E. 2016, A&A, 593, A65
Yana Galarza, J., Meléndez, J., Ramírez, I., et al. 2016, A&A, 589, A17

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