Introduction

It is now well known that the propagation of shock waves in the atmosphere of most radially pulsating stars leads to the occurrence of emission lines. This phenomenon has been observed in pratically all pulsating stars classes: Miras, RV tauri, W Virginis, RR Lyrae, classical Cepheids, etc. $\delta$Scuti stars, which are pulsators of weak amplitude, do not show hydrogen or metallic emission lines which can be associated with the presence of a shock wave. A chromospheric activity can be find in $\delta$Scuti stars and sometimes a permanent UV Mgii h and k emissions are observed (e.g. Fracassini et al. [1983]). Thus the modelling of the pulsation of the atmosphere of $\delta$Scuti stars requires to take into account the chromospheric heating by acoustic or magnetic wave dissipation in addition to transient heating due to pulsational shocks. The calculation of such models, including a non-linear and non-adiabatic treatment of the pulsation within an extented atmosphere, does not yet exist in the framework of a self-consistent approach. Only, Sasselov & Lester ([1994]) have constructed an atmospheric pulsation model including a chromosphere for classical Cepheids but assuming a driving piston.

For the $\delta$Scuti star having the largest amplitude (both in radial velocity and magnitude), $\rho$Puppis, Dravins et al. (1977, hereafter DLS) reported the presence of a transient CaiiK emission just before the maximum acceleration. They invoked a shock-wave mechanism to interpret this emision feature. Their observations were done with photographic plates with a reciprocal dispersion of 12.3Å.mm^-1. The very short emission (see their Fig.3) was observed in the blue wing of the CaiiK absorption line which is classically consistent with the presence of a shock wave emerging from the photospheric layers. Moreover, because they only detect the emission during one pulsation cycle over the two observed, they conclude to the transient nature of the phenomenon. This means that the shock intensity is not always large enough to produce an emission component.

Up to now, this observation is unique and there is no confirmation of its detection. In a recent paper (Mathias et al. 1997, hereafter PaperI), we have tried to put into evidence the presence of a shock wave in the atmosphere of $\rho$Puppis with high resolution CCD observations of some metallic lines and H$\alpha$. These new observations were done over four consecutive nights but the CaiiK absorption line was not in the spectral domain considered. Despite the very high quality of the data, no evidence was found for a shock wave mechanism associated with the star pulsation.

In this paper, we present new high temporal and spectral resolution observations of the CaiiK line to try to detect the emission feature. Section 2 presents observations and data reductions and the analysis of radial velocities and profile variations are given in Sect.3. A short discussion about this shock wave scenario and some concluding remarks are presented in the last section.