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

1. Introduction

The influence of rotation on the excitation of particular pulsation modes is unknown. Rapidly rotating  Cephei stars ( 70km/s) are rarely found to pulsate in radial or low-order non-radial modes . 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  Cephei stars using high-resolution spectroscopy, focusing on stars that have high . This way we hope to complement the group of  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  Cephei stars.

For rapidly rotating stars, pulsation modes with intermediate pulsational degrees, 3 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 , 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  Cephei star: Sco (HD144470, , B1V (Hoffleit & Jaschek 1982), Hipparcos parallax ). 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 3 mmag in the visual. In the Hipparcos catalogue Sco is marked as constant with a standard error of the median of 51 observations of , with =3.92 and =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, =110km/s; =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.

© European Southern Observatory (ESO) 1998

Online publication: September 30, 1998
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