Solar-Type Stars: Basic Information on Their Characterization and Classification
David R. Soderblom & Jeremy R. King (Space Telescope Science Institute)
7. Do We Understand the Basic Physics of Stellar Evolution?
This question is raised by the Hipparcos results for the distances
to the nearby open clusters. For the Pleiades, for example, the
Hipparcos distance modulus is (m-M) = 5.3, as compared to the
"traditional'' estimate of 5.6 magnitudes, which is based on comparing the
Pleiades CMD to nearby stars. The Pleiades is about 100 Myr old, placing
1 Msun stars squarely on the Zero-Age Main Sequence.
It appears to have solar composition to within ~10%.
- Solar-type stars within the Pleiades exhibit a huge range of rotation
rates (20X) and lithium abundances (~10X), but they form
a tight main sequence in a color-magnitude diagram.
- An 0.3 magnitude difference is a big deal! The Hipparcos results,
taken at face value, mean that some ZAMS stars are 30% fainter than our
models predict. Those models are built and calibrated to reproduce the
current state of the Sun (radius, temperature, composition, luminosity).
- The Hipparcos results for other nearby open clusters are also
discordant with the "usual'' numbers. Also, not all the ZAMS clusters
yield the same result, so Hipparcos is telling us that there is a
parameter or parameters above and beyond the usual ones we consider that
is influencing the observed state of stars in a very significant way.
Hypothesis 1: If the Hipparcos distance to the Pleiades is correct,
then surely the stars of the Pleiades are not totally unique or bizarre,
and there should be examples of similar stars close to the Sun, so close
that their distances have no significant error.
Question: If the Hipparcos distance is correct, how have all our
models been so consistently wrong when they cover a broad range of mass?
Hypothesis 2: If the Hipparcos parallaxes contain a systematic
error, then a problem will show up in examining the apparent density versus
distance of luminous stars.
This is work in progress, but, first, there is no secondary ZAMS
displaced 0.3 magnitude below the usual ZAMS in a color-magnitude
diagram of nearby stars. Spectroscopy of those few
stars below the ZAMS has been done to show they are not as young
as the Pleiades and that they are, in fact, Old Disk stars with
[Fe/H] ~ -0.3 (see Figure 6).
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