Solar-Type Stars: Basic Information on Their Characterization and Classification
David R. Soderblom & Jeremy R. King (Space Telescope Science Institute)

3. Defining Samples of Solar-Type Stars

The Hipparcos Output Catalog makes it easy by providing high-quality astrometric and photometric data for nearly all stars to V~9. Figure 5 shows a first cut, namely 0.50 to 1.00 in B-V for any parallax. Note the vertical bands, especially every 0.1 in B-V, due to round-off for stars with poor photometry. We can also define a distance-limited sample, say all the stars within 60 pc (Figure 6). For a typical Hipparcos parallax error of 2 mas, the distance is good to about 12%, and MV is good to 24%, or 0.24 magnitude. Note that there are few evolved stars in this distance-limited sample, as compared to the huge number of evolved stars in the magnitude-limited sample of Figure 5. Note also that many stars appear to fall far below the main sequence.

FIGURE 5:(left) All the stars in the Hipparcos catalog that fall between 0.50 and 1.00 in B-V.
FIGURE 6:(right) Stars from the Hipparcos catalog that fall between 0.50 and 1.00 in B-V and which are closer than 60 pc. The two lines represent theoretical ZAMS relations transformed to these coordinates with two different color-temperature relations.

Now let's concentrate on solar analogs. Figure 7 shows an enlarged portion of Figure 6, with the position of the Sun shown, as well as theoretical ZAMSs for two different color-temperature relations. Clearly there is no shortage of candidate solar twins to study.

FIGURE 7:An enlargement of a portion of Figure 6. The large star represents the position of the Sun in this diagram.}

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