Twin Suns of Different Mothers

HIP 56948 (HD 101364)—an 8.7 magnitude star in Draco—is more like our Sun than any other star yet discovered. It is 194 light years away and located at α2000 = 11h 40m 28s and δ2000 = +69° 00′ 31″, near Gianfar (λ Draconis) and the Draco-Ursa Major border, above the Big Dipper’s bowl.

Solar twin HIP 56948 (circled) in Draco near Gianfar

With the exception of lithium, the elemental abundances are identical to that found in the Sun, within the observational uncertainties. As expected, lithium is severely depleted in HIP 56948, but not as much as in the Sun. This is to be expected for a solar twin about 1 Gyr younger than the Sun.

The temperature, luminosity, mass, and rotation of HIP 56948 almost exactly match that of the Sun. For example, HIP 56948 is only 17 ± 7 K hotter than the Sun, and its mass is 1.02 ± 0.02 M. Given all these similarities, it appears its most recently determined (1993) spectral type of G5 is incorrect. Or is it the spectral type of our Sun that is wrong (G2V)? Actually, it is quite difficult to make measurements of our Sun “as a star” because it is so incredibly close and bright.

HIP 56948 harbors no giant planets or “hot Jupiters” within or interior to its habitable zone, so there remains the enticing possibility that it may host a planetary system similar to our own, though no planets have yet been detected.

Incidentally, the next time you’ve got a good view of the Head of Draco and the “box” of Cepheus, cast your eyes toward a point halfway between the two. You’re looking towards where the rotational axis of the Sun points north. Like HIP 56948, it’s in Draco.

North Solar Pole in relation to HIP 56948

References
“The remarkable solar twin HIP 56948: a prime target in the quest
for other Earths”
J. Meléndez, et. al., A&A 543, A29 (2012)
https://www.aanda.org/articles/aa/pdf/2012/07/aa17222-11.pdf

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