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Astron. Astrophys. 338, 435-441 (1998)

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3. Isochrones: metallicity and age

3.1. Calibration

The photometric calibration of the AGB-phase of the isochrones is a cumbersome process. Ng (1997) gave in his analysis no consideration to the possibility that the photometric estimate of the metallicity might have been too high, which could be due to the uncertainties of the various transformations applied. In particular, the photometric calibration of the AGB-phase of the isochrones in the ESO photometric system (Bouchet et al. 1991). The photometric calibration of this phase was not specifically based on carbon stars. The dusty atmospheres of carbon stars should result in redder colours for a particular metallicity and NG (1997) LIKELY OVERESTIMATED THE METALLICITY OF THE ALRW91 C-STARS .

3.2. Metallicity from `dusty' isochrones

Preferably one ought to calibrate correctly the AGB phase for the carbon stars. Unfortunately, an empirical calibration cannot be established in the ESO photometric system, because not enough data is available.

An indication of the correction to be applied to the metallicity estimated by Ng (1997) for the ALRW91 C-stars can be obtained from the relative shift between the isochrones with and without dusty envelopes computed by Bressan et al. (1998; mixture B with amorphous carbon grains) for the SAAO photometric system (Carter 1990). The shift is the same for the ESO photometric system.

Fig. 1 indicates that in the thermally pulsing (TP) AGB phase the Z = 0.004 `dusty' isochrone with a K-magnitude brighter than MK [FORMULA] - [FORMULA] is comparable to the Z = 0.008 non-dusty isochrone. The isochrones start to diverge from each other when the superwind phase sets in at MK [FORMULA] - [FORMULA]. The transition from the E-AGB (early-AGB) to the TP-AGB phase is likely smoother than presently modeled for the isochrones 2. Improvements are however beyond the scope of this paper. For our purpose Fig. 1 sufficiently demonstrates that the metallicity of the carbon stars in the TP-AGB phase are overestimated with non dusty isochrones. It is therefore argued that THE PHOTOMETRIC METALLICITY ESTIMATED BY NG (1997) FOR THE ALRW91 C-STARS SHOULD BE REVISED DOWNWARD TO Z [FORMULA] 0.004 .

[FIGURE] Fig. 1. A comparison between isochrones of 1 Gyr with and without dusty envelopes (carbon grain mixture B; Bressan et al. 1998). A dotted and a solid line are used for the dust free envelopes for Z = 0.004 and Z = 0.008; for respectively the same chemical compositions a dot, long dashed and a long dashed line are used for the isochrones with dusty envelopes

3.3. Age

Low metallicity carbon stars with an extended carbon envelope might be confused with stars with less extended envelopes, which are either older but have the same metallicity or younger/same age and metal-richer. In addition, the envelope of the carbon star is enriched progressively during each thermal pulse. As a result the star becomes metal-richer (redder) after each thermal pulse for virtually the same age, i.e. the metallicity enrichment takes precedence over aging. The red edge of the carbon stars in the (K,J - K) CMD, see Fig. 2b (Ng 1997), is therefore due to metal enriched carbon stars with the same age and initial metallicity as those found at the blue edge. This implies that the upper age limit of 1 Gyr given by Ng (1997) for the ALRW91 C-stars is wrong.

THE AGE OF THE ALRW91 C-STARS IS ABOUT 0.1 GYR 3.

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© European Southern Observatory (ESO) 1998

Online publication: September 14, 1998
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