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Astron. Astrophys. 339, 150-158 (1998)

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1. Introduction

The influence of rotation on the excitation of particular pulsation modes is unknown. Rapidly rotating [FORMULA] Cephei stars ([FORMULA] [FORMULA]70km/s) are rarely found to pulsate in radial or low-order non-radial modes [FORMULA]. This might be due to intrinsic rotational damping of low-order modes, to observational selection effects, or to the fact that rotation modifies the shape of the eigenfunctions. To investigate this problem we are searching for new [FORMULA] Cephei stars using high-resolution spectroscopy, focusing on stars that have high [FORMULA]. This way we hope to complement the group of [FORMULA] Cephei stars that have been found photometrically, and hence have low-order pulsations, with new variables that might have higher-order pulsations. Ultimately, we intend to establish the differences in the pulsational properties between rapidly-rotating and slowly-rotating [FORMULA] Cephei stars.

For rapidly rotating stars, pulsation modes with intermediate pulsational degrees, 3 [FORMULA] [FORMULA] [FORMULA] 20, show up as line-profile variations in which alternate 'emission' bumps and 'absorption' troughs move from red to blue through the line profiles (Vogt & Penrod 1983). These variations are due to the combined effects of the pulsational velocity field (Doppler shifts, e.g. Kambe & Osaki 1988, or Gies & Kullavanijaya 1988) and the pulsational temperature fluctuations (equivalent width and surface brightness variations, e.g. Lee et al. 1992). The line-profile variations hold information on the pulsation modes that are present; good estimates of the pulsational degree [FORMULA], the azimuthal order m and the velocity amplitude of the pulsations can be derived (Schrijvers et al. 1997, Telting & Schrijvers 1997a, Schrijvers & Telting 1998).

Here we present a spectral time series of a new, bright, rapidly rotating [FORMULA] Cephei star: [FORMULA] Sco (HD144470, [FORMULA], B1V (Hoffleit & Jaschek 1982), Hipparcos parallax [FORMULA]). This star, a member of Sco OB2 association (Upper Scorpius, Blaauw 1964; Brown & Verschueren 1997), has served as the primary standard star of the Walraven photometric system (Lub & Pel 1977), and is therefore expected to show no photometric variations with an amplitude higher than [FORMULA]3 mmag in the visual. In the Hipparcos catalogue [FORMULA] Sco is marked as constant with a standard error of the median of 51 observations of [FORMULA], with [FORMULA]=3.92 and [FORMULA]=3.94 as 5th and 95th percentiles. Besides being a photometric standard, this star is also one of the rotational velocity standards used by Slettebak et al. (1975, [FORMULA]=110km/s; [FORMULA]=100km/s, Brown & Verschueren 1997). No line-profile variations have been reported to date, however.

In Sect. 2 we will discuss the data taking and data reduction. In Sect. 3 we present the Fourier analysis of the time series, and in Sect. 4 we identify the mode that is causing the line-profile variations. Our concluding remarks are in Sect. 5.

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© European Southern Observatory (ESO) 1998

Online publication: September 30, 1998