HIPPARCOS Results for Solar Analogs
Giusa Cayrel de Strobel (Observatoire de Paris/Meudon) & Eileen D. Friel (NSF/Boston Univ.)
5. Results and their interpretation by means of the observational
(log Teff, Mbol) diagram
The results of the above selection of solar analogs, as of mid-1996, are
presented in Cayrel de Strobel (1996), and yield a sample of 99 objects that
were the basis of the discussion in that review article. We summarize the
pre-Hipparcos situation for these stars in the H-R diagram of Figure 1. This
figure represents the observational (log Teff, Mbol) diagram of 99 analogs with
an overplot of a theoretical diagram computed by Lebreton (see, for
instance, Lebreton et al. 1997) for a solar chemical composition
(Y= 0.28, Z= 0.018, alpha = 1.7)
and showing the ZAMS, the 1 solar mass evolutionary track, and
6 isochrones for 2, 4, 6, 8, 10, 12 Gyr. In Figure 1 the underluminosity for the
stars lying below the solar composition ZAMS can come from incorrect parallax
determinations, but also to a smaller extent it may derive from chemical
composition effects. Indeed, in the sample considered here, a non-negligible
number of stars have a metallicity significantly different from that of the Sun. Also, we
see that a significant number of analogs are more evolved than the Sun; this
conclusion is confirmed by the spectroscopic gravities determined from detailed
analyses of these stars. It is striking that many of the supposedly solar analogs in the sample
seem older than the Sun, falling on isochrones with ages of 6 to 12 Gyr.
FIGURE 1:Observational (log Teff, Mbol) diagram of 99 solar
analogs having ground-based parallaxes as the basis of
their Mbol determination with an overplot of
a theoretical diagram computed by Lebreton (1996) for a solar chemical
composition (Y=0.28, Z=0.018), and for a mixing length parameter,
alpha = 1.7. Note that a significant number of stars are underluminous
(lying below the ZAMS) and some other stars older than 12 Gyr (not compatible
with their belonging to the disk population of the galaxy).
[First | Prev | Next ]
[Contents]