1 Introduction

A significant number of upper Main Sequence stars show spectral peculiarities, which reflect abundance anomalies (Preston 1974). Microscopic diffusion is considered to be the main physical process that creates these anomalies by the building of an abundance stratification in the stable layers of the star (Michaud 1970; Michaud et al. 1976). The stratification and the abundances vary with time (Michaud & Charland 1986; Alecian 1986, 1996). Among the Chemically Peculiar stars, most studies on abundance variation due to diffusion concern Am stars (see Alecian 1996; Richer et al. 2000). However, the simplest diffusion models lead to anomalies much larger than observed (Michaud et al. 1976). More sophisticated models, which include competing processes, are needed. The abundance evolution of Ca (and Sc but with less reliable atomic data) was investigated by Alecian (1996) taking into account mass-loss and an extension of the superficial mixing zone below the convective zone. The abundance evolution with time is very sensitive to these competing processes. A phase of overabundance may occur for very young stars. Richer et al. (2000) simulated the evolution of Am stars considering consistently the radiative diffusion of 28 elements and isotopes, and turbulence. An important result is that the strength of the abundance anomalies depends strongly on the amount of material affected by the turbulent mixing.

To constrain these models, abundance analyses of stars with various ages are needed: cluster stars or field stars with a precise location in the H-R diagram. Hence, we present a study of field stars belonging to the HIPPARCOS sample. The abundance analysis is performed using the same instrument and method as for the cluster stars of Hui-Bon-Hoa and coworkers and completes these previous works towards the end of the Main Sequence lifetime.

 

 

Table 1: Basic data for the programme stars

Name	HD	Sp. type	V mag	Remarks
28 And	2628	A7III (1)	5.23	$\delta$ Scuti, p = 0.07d
9 Aur	32537	F0V (1)	5.00	SBO, p = 392d
14 Aur	33959	A9IV$\delta$Del (1)	5.02	SBO, p = 3.79d; $\delta$ Scuti, p = 0.09d
6 Mon	43760	F0$\delta$Del (1)	6.75	Var Vr?
RR Lyn	44691	A3Vm (1)	5.64	SB2O, p = 9.95d
$\rho$ Pup	67523	F6IIp$\delta$Del (1)	2.81	kF3hF5mF5(Ib-II) (2); $\delta$ Scuti, p = 0.14d
$\tau$ UMa	78362	Am (1)	4.67	SBO, p = 1062d
32 Aqr	209625	A5m (1)	5.30	SB1O, p = 7.83d
o Peg	214994	A1IV (1)	4.79	Am (3); Magnetic field (4)

• References: (1) Hoffleit (1982); (2) Gray & Garrison (1989); (3) Conti & Strom (1968); (4) Mathys & Lanz (1990).