Aerts et al. (1999) denoted that the same main frequency of 0.6374 c/d was found in the Hipparcos and Geneva photometry and in the first moment (which is equivalent to the radial velocity) of the SiII 4128-4130 Å lines (see Aerts et al. 1992 for a definition of the moments of a line profile). In Aerts et al. (1999) it is pointed out that only one frequency is found in the data of this star, while other SPBs are multiperiodic. Moreover, the first moment is non-sinusoidal: the first harmonic of the frequency found is necessary to obtain a good fit. In fact, this star is not the only one to present these peculiarities. In Briquet et al. (2001) we studied the B-type star HD 105382. We showed that this star is situated in the upper and blue part of the SPB instability strip, that all sets of photometric and spectroscopic data reveal a monoperiodic star, and that the first moment of the SiII 4128-4130 Å lines is non-sinusoidal. We tried to interpret the variability of this star in terms of a non-radial g-mode pulsation model but also in terms of a rotational modulation model, since the observed period is compatible with the rotation period of a star with such a spectral type. Unfortunately, neither of these two models was able to explain the observed line-profile variations.
In this paper we compare the observed photometric data and line-profile variations of HD 131120 with a non-radial pulsation (NRP) model and with a spot model. The paper is organized as follows. In Sect.2 the description of the data and some physical parameters are given. In Sect.3 the result of the frequency analysis on the data is described. In Sect.4 we attempt to perform a mode identification. We do this for the photometric data using the method of photometric amplitudes. For the spectroscopic data we use both the moment method and line-profile fitting. Then we try to model the variations with a rotational modulation model (Sect.5). The outcome of the modelling of the variations of the star is discussed in Sect.6.
Copyright ESO 2001