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Astron. Astrophys. 348, 831-842 (1999)

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1. Introduction

In the past two decades, snap shot observations of B and Be stars have revealed that a few of them exhibit a special type of line profiles, in which the central part of the line core exhibits a weak local flux maximum. These so-called central quasi emission peaks (CQE's) were as a distinct phenomenon first noted by Baade (1990).

Five of the six stars, in which CQE's have been found to date (cf. Table 1), are known Be stars. But there is also one object, namely [FORMULA] Pup, that did not so far have a record of observed emission or shell lines. Furthermore, the features were first identified in lines of HeI (early-B sub-types) and of MgII (late B stars) considered to be primarily of photospheric nature. Therefore, all previously mechanisms suggested for their formation assumed a photospheric origin of the CQE's, which is unrelated to the Be nature (Baade 1990, Jeffery 1991, Zorec 1994).


[TABLE]

Table 1. Journal of observations of CQE stars. HEROS , FEROS , and CES observations are marked with H, F, and CES, respectively. The aperture of the telescope used and the observatory (ESO and Calar Alto) are also indicated. Spectral type and [FORMULA] were adopted from Simbad, where available; for [FORMULA] Car and [FORMULA] Pup [FORMULA] was taken from the Yale Bright Star Catalogue (4th edition)


These models are based either on the variation of local line strengths caused by the equator-to-pole variation of effective gravity and temperature in the presence of rapid rotation (Baade, Jeffery), which is a distinctive property of Be stars, or on differential rotation (Zorec). However, in his detailed model atmosphere calculations, Jeffery could reproduce CQE's only with the additonal assumption of thermal polar caps, for which there is presently no physical justification. Differential rotation would be less of an ad hoc ingredient but has not so far been required for the explanation of other observations of Be and shell stars. These early models (see also Baade 1990 had already in common that they did not seek an explanation of CQE's in terms of line emission but rather hypothesized a local deficit in absorption.

A first hint, that the search for an explanation of CQE's should probably be extended beyond the photosphere, was given by 4 Her: In this star, CQE-like features were observed only in shell lines (Koubský et al. 1993). However, whether the two phenomena can be identified with one another, so far remained open.

Since the detection of CQE's requires relatively large spectral resolution, most of the available small datasets only cover few spectral lines. Accordingly, they probe a limited range of atmospheric conditions and do not permit a full assessment of the possible origin(s) of CQE's. Therefore, series of new cross-dispersed echelle spectra were obtained of all 6 stars currently known to display these features; they are described in Sects. 2 and 3. Empirical criteria for the occurrence of CQE's are established in Sect. 4. Since it turns out that a proper calculation of line transfer in stellar disks in the presence of rotation is essential for their understanding, the introduction of the model (Hanuschik 1995) used for this purpose is deferred to Sect. 5. Sect. 6 adds in other indications from the literature that disks of Be stars are, in fact, rotationally supported, and Sect. 7 briefly examines this point and the significance of CQE's in connection with models for the formation of Be star disks. The main conclusions are summarized in Sect. 8.

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© European Southern Observatory (ESO) 1999

Online publication: August 13, 199
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