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Astron. Astrophys. 333, 125-140 (1998)

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1. Introduction

Since the very first discovery of stellar line emission in [FORMULA]  Cas by Secchi (1867), Be stars have been subject to disputes about the origin of their numerous spectral peculiarities (Slettebak, 1976; Jaschek & Groth, 1982; Underhill & Doazan, 1982; Slettebak & Snow, 1987; Balona et al., 1994). Only in the past decade could the geometry of the circumstellar envelopes by interferometric methods (Stee et al., 1995; Quirrenbach et al., 1997) be directly observed to be disk-like. Still ongoing discussions focus intensely on the physical mechanism leading to the formation of the circumstellar disk (Lamers & Pauldrach, 1991; Bjorkman & Cassinelli, 1993; Owocki et al., 1996) and on the nature of the short-term variability of the stellar absorption lines and the integral light (Smith, 1989; Baade & Balona, 1994). The primary candidates for the periodic component of the rapid variability are nonradial pulsation and corotating active areas or star spots.

Not considering the apparently required rapid rotation (Slettebak, 1982; Hanuschik, 1989, e.g.), which however is not sufficient as a defining criterion, the Be phenomenon covers a large parameter space, in the Hertzsprung-Russell diagram as well as in metallicity (Galaxy; LMC: Kjeldsen & Baade 1994; SMC: Grebel et al. 1992) and variability patterns. There are stars whose Balmer line emission is stable over decades, and there are others with variability on a time scale of hours, like µ Cen. It is therefore possible that there is more than one way for a B star to become a Be star.

The most enigmatic Be-star variations are the line emission outbursts. Up to now, a stringent definition of an outburst has not been given, and any significantly faster and stronger than average increase in the Balmer emission line strength has been called an outburst. Accordingly, the associated parameter space is again quite large and ranges from the slow (1-2 years) but spectacular outbursts of [FORMULA] Cas in the 1930's (cf. Doazan et al. 1980, Hummel 1998) via drops in equivalent width (we adopt the general convention that an equivalent width is the more negative the stronger the emission is) by [FORMULA] 0.1 Å within 1-3 days (Peters, 1988, e.g.) to even faster glitches that take place within hours (Oudmaijer & Drew, 1997).

Usually, all outbursts are thought to be events or periods of enhanced mass transfer to the disk. Attempts to find a link between the rapid periodic variability in the photosphere and the episodic mass loss usually did not exceed the level of conjectures or gave negative results (Smith, 1989, e.g.). However, Kambe et al. (1993) find weak evidence for the amplitude of the stellar line profile variability being larger than average close to the time of an outburst. In no case (excluding some binaries) has a convincing scheme for the temporal occurrence of outbursts been established which apparently happen at random. The best known examples of stars exhibiting frequent, rapid small or intermediate outbursts are [FORMULA]  Eri (Smith, 1989; Smith et al., 1991) and µ Cen.

With [FORMULA] µ Cen (= HR 5193 = HD 120 321; B2IV-Ve) is one of the apparently brightest and with a HIPPARCOS distance of [FORMULA] pc (Perryman et al., 1997) also most nearby Be stars so that it ought not to be too exotic an object of its kind. From high- [FORMULA] echelle spectroscopy a projected rotational velocity, [FORMULA], of [FORMULA] has been derived (Brown & Verschueren, 1997). This relatively low value (as compared with the statistically established high mean equatorial rotation velocity, v, of Be stars - cf., e.g., Slettebak 1982, Hanuschik 1989, [FORMULA]) as well as the shape of the H [FORMULA] emission during phases, when it has significant strength (Hanuschik, 1989; Hanuschik et al., 1996), both imply an intermediate value of 30-45 degrees for the inclination angle, i, of the rotation axis.

µ Cen was first reported to exhibit H [FORMULA] emission by Fleming (1890). Since then it has been observed during numerous campaigns. The star is known to have lost its emission entirely for two times, once around 1918 for nearly ten years, and the second time from 1977 through 1989. Summaries of this behaviour were given by Peters (1979) and Hanuschik et al. (1993). In the 1977-1989 interval, µ Cen exhibited only flickering emission (Baade et al., 1988; Hanuschik et al., 1993). This activity strengthened after 1989. Although the emission-peak height thereafter rose temporarily up to values around 2 in units of the local continuum (Peters, 1995), it still decayed at least twice (1992 and 1994) to close to unity between two emission episodes. Since 1995, there has been a steady increase in the ratio of the emission peak height to the ambient continuum ([FORMULA] ratio) from [FORMULA] in 1995 to 2.4 in 1996 and 2.9 in 1997. It seems, therefore, that µ Cen is indeed building up a new persistent envelope, as reported by Stahl et al. (1995a) and Hanuschik et al. (1996). We show the run of the [FORMULA] -peak height from 1992 to 1997 in Fig. 1.

[FIGURE] Fig. 1. The [FORMULA] -emission peak height (relative to the local continuum) from 1992 to 1997. Lines show the measurements of this study, open symbols ([FORMULA]) were taken from Hanuschik et al. (1996), and filled symbols ([FORMULA]) from Peters (1995). Crosses mark unpublished spectra taken by M. Thaller (+) and C. Aerts (x). The numerous narrow dips are not due to measuring errors but mostly correspond to the onset of line emission outbursts (cf. Sect.  4.2)

Our detailed investigation of µ Cen will be published in a series of papers. This paper brings a description of our extended data (Sect.  2) and an analysis of emission line-profile variations. In Sect.  3we extract the common features of the emission outbursts but also discuss their heterogeneity, while the individual phases of the emission outburst are described in detail for various groups of representative spectral lines in Sect.  4. The analysis of accompanying [FORMULA] variations is presented in Sect.  5and examples of transient absorption components appearing in connection with the outbursts in Sect.  6. In Sect.  7, we attempt to give a generalized qualitative and mostly kinematic description of an outburst.

Paper II (Rivinius et al., 1998a) concentrates on the time series analysis of the multiperiodic photospheric line-profile variations. Paper III (Rivinius et al., in preparation) will address the nature of this variability, and its relation to the outburst activity will be the subject of Paper IV (Rivinius et al., in preparation). A preliminary overview of this topic has already been published (Rivinius et al., 1998b; Rivinius et al., 1998c).

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Online publication: April 15, 1998