The luminosity and the photospheric radius increase and the effective temperature decreases,
on average, while following the sequence of increasing variability: Cst, Lb, SRb, SRa and M. The
stars classified as constant (i.e. for which no photometric variations larger than a few
hundredths
of a magnitude could be evidenced) are essentially early HC-giants from HC0 to HC3, many HC4 and
HC5-stars being variables. We obtained HC2 as their equivalent group with good consistency for
mean luminosity, photospheric radius and effective temperature. Most variable carbon
stars belong to a CV-group as initially intended and a strong discontinuity in properties appears
when skipping from Cst to Lb. There is then a slow evolution along the sequence from Lb to M, with
increasing mean luminosities and photospheric radii, and decreasing mean effective temperatures.
Considering dispersions, the regular evolution
along the sequence is more convincing than the differences found between two successive classes.
The mean
i.e.
value obtained for the full sample M(1) of carbon Miras, is similar to that quoted for SRa
variables. From HIPPARCOS data, a few Miras proved to be strongly underluminous (e.g. Bergeat
et al. 1998): this is the case of
Lyn and C5570=RZ Peg which we excluded from
sample M(2). Then we obtained
i.e.
On average, the amplitudes of variations in carbon Miras
are lower than those of their oxygen-rich counterparts. There is no gap between the former
and large-amplitude carbon semiregulars classified SRa. The results
of Table
can be interpreted in terms of increasing variability, on average, along
the evolutive tracks in the TP-AGB region (Sect. 7).
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Figure 6:
The MW-LF of Fig.
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Figure 7:
The MW-LF of Fig.
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Copyright ESO 2002