Praesepe

1 Introduction

Among the most promising targets for successful application of asteroseismology are the $\delta$ Scuti stars. They have rich sets of oscillation modes which in many cases are readily observable. Unfortunately, each $\delta$ Scuti star is only found to oscillate in a seemingly random subset of possible modes, making it difficult to identify exactly which modes are observed. To further complicate matters, these stars tend to be rapid rotators, which causes displacement of the frequencies.

One approach to the problem is to observe a number of stars in a cluster sharing basic parameters like distance, metallicity and age in order to restrict the degrees of freedom in the model calculations. The Praesepe open cluster contains at least 14 $\delta$ Scuti stars, many of which have been the targets of a number of observing campaigns.

The stars BN Cnc and BV Cnc have both been studied extensively. The variability of BV Cnc was firmly established by Arentoft et al. (1998), who also found that BN Cnc has at least eight pulsation modes. The two stars are located only 2.52$^\prime$ apart, which makes them excellent targets for differential CCD photometry.

The STACC network organised an international multi-site campaign in the first half of 1998 on BV Cnc and BN Cnc (Frandsen et al. 2001, hereafter Paper I). The aim of this campaign was, with a very long time-base of $\sim$3 months to provide accurate frequencies of the oscillation modes in these stars. During a few weeks in February 1998, BN Cnc (A7V, V=7.8, $v\sin i=115$ kms^-1) was also the target of a spectroscopic campaign aimed at providing enough information to be able to get a preliminary classification of the pulsation modes in this star using line-indices of spectral lines. This paper deals with the results of the spectroscopic campaign on BN Cnc. See Paper I for the results of the photometric campaign.

The technique is described in Sect. 2, after a short overview of the spectroscopic techniques used for observing $\delta$ Scuti stars. The observations from all sites are presented in Sect. 3, the data reduction in Sect. 4, and the combination of all data is presented in Sect. 5. In Sect. 6 the frequency content and the amplitudes are derived, and Sect. 7 discusses the implications and attempts to identify the pulsation modes. Some concluding remarks are given in Sect. 8.

Praesepe