Astron. Astrophys. 362, 1020-1040 (2000)

1. Introduction

The physical conditions giving rise to the so-called "Be phenomenon" that is, to the presence of a circumstellar envelope around some OB stars are still unknown in spite of considerable efforts by the international community in recent years. These stars also show light and line-profile variations with time scales ranging from minutes to years. In fact, three main time scales can be distinguished schematically:

• i) short-term variability (from minutes to tens of hours) due to local photospheric and sub-photospheric changes in temperature, velocity and density, possibly related to non-radial pulsations (nrp), to local magnetic fields which still remain to be detected, and to other unidentified processes.

• ii) medium-term variability (weeks, months) mainly detected in early Be stars characterized by short-lived and/or long-lived light outbursts somewhat analogous to "flares". Hipparcos photometry has allowed the detection of long-lived outbursts with a gradual brightness increase over about 100 days, followed by a rather slow ( 400 days) relaxation phase (Hubert & Floquet 1998). Outbursts in emission lines have also been observed. In the case of µ Cen, monitored each year for several months, a pattern of short-lived outburst cycles associated to a nrp beat phenomenon was derived by Rivinius et al. (1998a).

• iii) long-term variability (years, tens of years) observed in both light and circumstellar lines, which is the consequence of global changes in physical and geometrical parameters of the envelope, induced by episodic mass loss enhancements. Several models (propagation of density waves, successively ejected layers with different density and progressive pile-up, partial infall of matter) are competing to explain the observations. The evidence is increasing, however, that the above processes are present with different relative intensity in individual stars.

Multiperiodicity has been detected mainly in line profile variations (lpv), and there has been continuous progress in spectroscopic modelling codes of pulsationally perturbed stellar photospheres (see Schrijvers et al. 1997; Telting & Schrijvers 1997; Townsend 1997a and b). Low degree g-modes seem to be present in Be stars which would include them among Slow Pulsating B (SPB) variables. A few stars have periods in time scales as short as 1.5-2 hours ( Cen, Janot-Pacheco et al. 1999; 48 Lib, Floquet et al. 1996; 27 CMa, Balona & Rozowsky 1991), which could be attributed to p-modes; these stars would be among "hybrid" stars showing both p- and g- modes (Mathias & Waelkens 1995). However numerous, closely spaced and narrow moving features observed in line profile series are often irregular in intensity and time evolution and have been thought to originate in lower density gas above the photosphere (Gies 1994).

EW Lac (HD 217050, B3 III, V = 5.35, = 340 km s^-1) is a Be star seen under a high inclination angle. In addition to emission contribution, its spectrum presents typical shell characteristics (narrow absorption features), commonly observed when envelope density is high. Moreover, this star is well known for alternating the quiescent and active shell phases. It has been in an active shell phase since 1976. Multiperiodicity has been detected in both light and photospheric line profiles, so this star has been considered as a good candidate for testing the relation between short-term line profiles and light variation, and for studying circumstellar activity. First, a quasi-periodic (0.7 d) photometric variation was obtained by various observers, among them Lester (1975) who argued that the colour variations could be explained by temperature changes in the stellar photosphere. Second, a period of 0.7228d was deduced from the international photometric campaign in September/October 1983 (Stagg et al. 1988). Three periods were obtained by Pavlovski (1987) from Hvar data in 1982 and Walker data in 1950-51, and by Pavlovski et al. (1993) from a new compilation of the 1983 campaign data and from Yonsei observatory measurements obtained in the same season. All three periods coincide with the periods deduced from spectroscopy: 0.7 d in the V/R ratio of H found from Calar Alto data by Pavlovski & Schneider (1990), and 3 periods (0.80, 0.625 and 0.36 d) in He I 6678 found by Floquet et al. (1992) from OHP data collected in an 8-night run in 1989. As the state of the envelope of EW Lac has varied continuously since 1976, one of the essential points was to confirm the multi-periods and their origin.

The star was observed again in 1993 during of a series of multi-site, multi-wavelength and multi-technique campaigns devoted to bright Be stars (see Hirata 1993). A photometric campaign managed by J. Percy took place simultaneously with spectroscopic observations. Polarimetric data were also obtained both at the Crimean Astrophysical Observatory by A. Tarasov and at McDonald Observatory by D. McDavid. Independent of the optical multi-site campaign, UV spectroscopy was performed on 1993 September 8/9 over 24 hours by G.J. Peters. In this paper we report on the visual spectroscopic campaign which in turn confirms our previous detection of multiperiodicity in photospheric line profiles. Evidence is given of temporarily orbiting clouds around the star and we develop arguments for the existence of a highly variable pseudo-photosphere from simultaneous spectroscopic and polarimetric behaviour. Finally we propose a tentative model of mass loss events which contribute to feed the anisotropic envelope.

© European Southern Observatory (ESO) 2000

Online publication: October 30, 2000
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