Table 5: Age and mass for the programme stars (including the field stars of HBA97)

Name HD Mass ( $M_{\odot}$ ) log t

28 And 2628 1.92 9.01

HR 178 3883 2.44 8.78

9 Aur 32537 1.47 8.92

14 Aur 33959 2.37 8.81

6 Mon 43760 3.14 8.51

RR LynA 44691A 2.10 8.83

RR LynB 44691B 1.77 8.94

Sirius 48915 2.21 $\leq$ 8.50

Procyon 61421 1.48 9.29

$\rho$ Pup 67523 1.99 9.02

$\tau$ UMa 78362 1.84 9.02

32 Aqr 209625 2.09 8.89

o Peg 214994 2.82 8.54

**Table 5:** Age and mass for the programme stars (including the field stars of HBA97)
Name	HD	Mass ( $M_{\odot}$ )	log t
28 And	2628	1.92	9.01
HR 178	3883	2.44	8.78
9 Aur	32537	1.47	8.92
14 Aur	33959	2.37	8.81
6 Mon	43760	3.14	8.51
RR LynA	44691A	2.10	8.83
RR LynB	44691B	1.77	8.94
Sirius	48915	2.21	$\leq$ 8.50
Procyon	61421	1.48	9.29
$\rho$ Pup	67523	1.99	9.02
$\tau$ UMa	78362	1.84	9.02
32 Aqr	209625	2.09	8.89
o Peg	214994	2.82	8.54

Our sample mostly consists of nearby stars (less than 100 pc far) for which we can estimate the absolute magnitude with HIPPARCOS parallaxes (ESA 1997) and rather small uncertainties. Figure 3 shows their location in the H-R diagram (the data for RR Lyn are from Popper 1971). The corrections for absorption (derived from E(b-y)) are generally very small (about 0.04 mag). We do not correct for duplicity since our high S/N spectra do not show any secondary spectrum for the SB1 stars, implying that the companion would contribute to less than 0.05 percents to the total luminosity yielding a shift of at most 0.05 mag. We derived the age and mass by interpolation in the evolutionary tracks of Schaller et al. (1992), assuming that the Am stars follow the same tracks as normal solar composition stars of similar mass, as the chemical anomalies of Am stars only affect the superficial layers. The mass can be overestimated for the stars near the upper boundary of the MS (namely HD 3883, 33959, 67523) since we assume that they are still in a MS stage though one or even two evolutionary tracks corresponding to lighter and more evolved stars also pass by these points (see Sect. 2.1 of Asiain et al. 1997). The resulting ages and masses are shown in Table 5. We could only assign an upper limit for Sirius, being very close to the ZAMS.

The typical pattern (underabundances of Ca and Sc, heavy elements enhanced) is obvious for most Am stars and the anomalies are of comparable strengths as those observed in the oldest cluster stars studied by Hui-Bon-Hoa and coworkers (namely in the Hyades and Praesepe) whose ages are similar to those of our field Am stars (except Sirius). Apart from RR LynB, whose abundances are not reliable enough, two stars show significant differences from this pattern. First, $\rho$ Pup has quasi normal Ca and Sc. Since it is in the very upper part of the Main Sequence strip, the extension of the superficial mixing zone occurring when a star leaves the MS could reduce the strength of the abundance anomalies (Richer et al. 2000). The disappearance of the anomalies occurs sooner for Cr and Fe than for Ca, as suggested by the models (Richer 2000, private communication). The Ca value for o Peg is more than 0.4 dex enhanced in comparison with that of Sirius. As both have similar atmospheric parameters, this discrepancy may be due to a magnetic field in o Peg. Borsenberger et al. (1981) predict normal abundances of Ca for magnetic Ap stars under the effect of diffusion. A careful study of the stratification would help understand the origin of the dependence of the Ca abundance on excitation potential (this trend appears because the abundance determination assumes homogeneous content throughout the atmosphere and LTE).

There seems to be a correlation between [Sc/H] and T_eff (r=0.73, F (0.05)=23.6) in our typical Am stars (Sirius is not included), the underabundances of Sc being more pronounced with increasing temperature (see Fig. 4). An age effect cannot be invoked as our stars are of the same age. This trend is also visible in the oldest cluster Am stars (members of the Hyades and Praesepe, see Fig. 3 of Hui-Bon-Hoa & Alecian 1998). A reverse tendency is followed by Ca in the same stars (r=0.30, F (0.05)=3.7). Such correlations can be understood considering that the visible abundance is the reflect of that present at the bottom of the superficial convective zone (SCZ), whose location is very sensitive to T_eff (it is located upper in the star for increasing temperature). In the case of Ca, the abundance at the bottom of the SCZ grows as T_eff increases. The radiative accelerations also depend on T_eff, but much less sensitively (see Figs. 5 and 6 of Richer et al. 2000) and we can therefore reasonably expect that the shape of the stratification profile is slowly temperature-dependent too at a given age, at least for a limited temperature range (say 1000 K). The abundance variation against T_eff would then roughly reflect the stratification profile in the temperature range of interest. In the case of Ca, the local abundance should be larger when one goes to upper layers, which is consistent with the stratification profile predicted by Alecian (1998) and that of Richer et al. (2000) for their 1.6 $M_{\odot}$ model. Unfortunately, they do not consider up to now the case of Sc. Also, we should not forget that effective temperature is not the only parameter on which depends the location of the bottom of the SCZ and this could explain the abundance dispersion at similar T_eff and age.

As for the normal stars, their quasi-solar patterns may reflect the efficiency of the superficial mixing mechanisms: the He convective should have remained in the MS stars, otherwise they should show obvious abundance anomalies, even the less massive of them (Turcotte et al. 1998). 6 Mon is in a post-MS stage where dredge-up is certainly active.

The pattern of 28 And cannot be safely discussed until a more complete picture of the anomalies is available. However, owing to certain similarities with the anomalies presented by Vega, the diffusion model does not seem to be the relevant one and one should rather consider the models proposed to explain the $\lambda$ Boö phenomenon.

5 Discussion