1 Introduction

The classic Be phenomenon has been known since the discovery of emission lines in the spectrum of $\gamma$ Cas by Secchi (1867). Although for a long time this phenomenon was considered to be limited to the B spectral type, today its characteristics are recognized to be present in other spectral types and so it could be named the "OBAe phenomenon'', as it is shared by non-supergiant late O to early A spectral type stars that have shown at least once some emission in H Balmer lines (Frost & Conti 1976; Andrillat et al. 1986; Marlborough 2000). Apart from the many spectroscopic, photometric, spectrophotometric and polarimetric characteristics seen in the spectral domain ranging from visible to the IR, which are attributed to both photospheric activity and circumstellar envelope (CE), Be stars stand out by their high rotation, though undercritical with $\Omega/\Omega_{\rm c} \sim 0.8$ (Chauville et al. 2001). For simplicity, we will refer in this paper to all of these stars and phenomena as "Be stars" and "Be phenomenon" respectively. Two main questions concern these objects: what is their nature and evolutionary status in order to generate these characteristics, and how do they produce the CE? In the spatial UV spectrum of Be stars there are signatures indicating the presence of winds with average mass loss of the order of 10^-9 to $10^{-11}~M_{\odot}~\rm ~yr^{-1}$ (Snow 1987). So, neither once assumed critical rotation (Struve 1931) nor a stationary wind alone can account for the total mass flux in the CE where particle densities are $\sim$10¹² cm^-3. Combined wind-rotation mechanisms were put forward to produce the CE (Lamers & Pauldrach 1991; Bjorkman & Cassinelli 1993). Contrasting with the above continuing mass loss related phenomena, evidence is accumulating on recurrent small-scale or moderate $\Delta M \la 10^{-11}~M_{\odot}$ (cf. Hanuschik et al. 1993; Floquet et al. 2000, 2002) to large-scale $\Delta M \ga 10^{-10}~M_{\odot}$ discrete mass ejections (Hubert et al. 2000; Zorec et al. 2000a,b). Ando (1983, 1986), Kambe et al. (1993) and Rivinius et al. (1999) suggested that the coupling of two nonradial pulsations (NRP) modes could supply the energy required to produce discrete ejections. The detection of line profile variations (hereafter lpv) and their interpretation as NRP were made for the Be star $\eta$ Cen (HD 127972, B1.5Vne) for the first time by Janot-Pacheco et al. (1991) in the He  I $\lambda$6678 Å line. Leister et al. (1994) detected at least 6 bumps moving across the same line and attributed it to a tesseral NRP mode with pulsational parameters $\ell = 7$ and $\vert m\vert = 6$, a phenomenon also discussed by Telting & Schrijvers (1997b). Basing their studies of lpv of the He  I $\lambda$6678 Å line on $\eta$ Cen on time series analysis with "Clean" (Roberts et al. 1974) and "Cleanest'' (Foster 1995) algorithms, Janot-Pacheco et al. (1999) showed that the star displayed a multiperiodic character.

 

 

Table 1: Log of HD 127972 spectroscopic observing campaigns.

Epoch	Observing	Telescope	Instrument	Spectral range	No of nights	No of spectra
	season
1	May/June 1996	LNA 1.60 m	Coudé	He  I 6678 Å	7	539
2	May 1997	LNA 1.60 m	Coudé	He  I 6678 Å	3	26
3	June 1998	LNA 1.60 m	Coudé	He  I 6678 Å	2	31
4	April 2000	ESO 1.52 m	FEROS	3560-9200 Å	10	33
5	May 2000	LNA 1.60 m	Coudé	He  I 6678 Å	5	56
6	April 2001	ESO 1.52 m	FEROS	3560-9200 Å	3	59

Long-term (years) photometric variations of $\eta$ Cen were reported by Jaschek et al. (1964) and Feinstein & Marraco (1979). Short time-scale photometric variations of this object were first reported by Cuypers et al. (1989). Though the latter were sometimes interpreted as due to corotating features, they can also be atributed to NRP due to effects of compression/expansion phenomena associated with the local temperature variations caused by the passage of waves through the stellar surface (Smith 1977).

The aim of this paper is to present new Fourier analysis of the lpv in He  I  $\lambda\lambda$4026, 4121, 4144, 4388, 4471, 4922, 6678 Å, Si  II $\lambda$4131 Å, Mg  II $\lambda$4481 Å and Fe  II $\lambda$5169 Å lines. On the other hand, this star also underwent photometric monitoring by the Hipparcos satellite from 1990 to 1992. The variations detected are worth studying in some detail. Observations of $\eta$ Cen were also made in the BCD spectrophotometric system, which allows us to derive an independent set of stellar fundamental parameters unperturbed by CE emission/absorption. Once these parameters are corrected for rotational effects, we can determine the evolutionary status of the central object. Their comparison with fundamental parameters derived using stellar model atmospheres will also help us to discuss the effects induced by the rapid rotation on the stellar surface. Finally, we will report H$\alpha$ emission line profiles obtained from 1996 to 2001 whose variations can give us new insights on CE formation characteristics.