What is special with HD 70642 stars and HD 101364 (HIP 56948)?

Solar-type star, like twin of the Sun, have three definitions given to stars that have similarities to the Sun, according to photometric and physical characteristics such as spectral class, mass, age, cromospheric activity, temperature, metallicity, Equatorial rotation, abundance of lithium, kinematics and of considerable importance is  whether or not they are individual and non-binary stars.


Diagram of a Solar-Type Star


A star similar to the Sun has a better chance to have planetary systems like the solar system, and consequently to possess planets similar to Earth where it can be developed forms of life.  The order is ascending according to similarities with our star, but a solar type star has only some points in common with the Sun, while the term twin star of Sun is assigned only to a small number of stars with physical characteristics almost identical to those similarities with the solar Sun, defining the three categories that reflect the evolution of astronomical observation techniques over time.  Astronomers originally referred to the stars by “type” (solar-type) stars, compared to the other, were more similar to the Sun.  As a result, the improvement of the instruments and observational techniques have allowed greater accuracy in the measurement of stellar characteristics key details, such as the temperature, allowing the creation of a category, a narrower one, “particularly” stars similar to the Sun.  With the continuous improvement of the accuracy of the measurements a category is finally created that includes very few stars that, for its characteristics, are indistinguishable from the Sun, much to receive the appellation of “twins”.

“Solar-type star” is less restrictive, and includes stars similar to the Sun in mass and stage of evolution.  There are main sequence stars, with an index of color between 0.48 and 0.80, not too distant from the Sun’s color index, equal to 0.65.  Alternatively, similar to the Sun are considered the stars ranging from spectral type F8 at K2, which would correspond to larger values of the index of color, from 0.50 to 1.00.  In the table on Wikipedia, there are listed some solar-type stars within 50 light years from Earth, which do not meet all the criteria to be considered analogous or solar twins.  The Red value is the one that does not respond to the definition of a solar analog.  These include Epsilon Eridani, Tau Ceti, Delta Pavonis, Gliese 785, Gamma Pavonis, 54 Piscium, V538 Aurigae, HR 4587 14412, HD, HD 17205, 72 Herculis and HD 196761. Solar analog defines similar or analogous solar, a main sequence star similar to the Sun, having the following characteristics: temperature of ± 500 K compared to the Sun (approximately 5200 to 6300 K); metallicity between 50% and 200% that of the Sun, implying that the star’s protoplanetary disc must have had similar quantities of powders for the formation of the planets; no Stellar Companion close (i.e. with orbital period of ten days or less), since the closeness would stellar activity.

In the table on Wikipedia, there are listed some solar-like within 50 light years from the Sun: Beta Canum Venaticorum among them, 61 Virginis, Zeta Tucanae Beta Comae Berenices,, HR 4523 A, 61 Ursae Majoris, HR 4458 A, HR 511, Alfa table, 55 Cancri, HD 69830, HD 10307, HD 147513, 58 Eridani, Upsilon Andromedae A, 47 Ursae Majoris, 20 Leonis Minoris, Nu Phoenicis and 51 Pegasi Sun Twin. These stars have features that make them even more similar to the Sun of the solar-like; the criteria to define a twin star of Sun would be: temperature of ± 10 K than the Sun age of ± 1 Gyr compared to the Sun (from 3.5 to 5.6); metallicity of 89 — 112% that of the Sun, which would mean that the star’s proplyd would have had the same amount of dust for the formation of planets.

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