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Spectroscopy Team

Faculty of Physics

Department of Astrophysics Astronomy & Mechanics

UNIVERSITY OF ATHENS


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INTRODUCTION TO Be_Oe_STARS
Introduction

Peton (1974) first pointed out, in the visual spectrum of the double system AX Mon (HD 45910), the existence of a secondary component of the absorption line Fell i.e wave length 4233A, which, depending on the phase, appeared in the violet or in the red side of the main spectral line. For this reason the secondary component was named "satellite component"

Lamers et al. (1982) noted the possibility of the presence of satellite components superimposed on the wide P Cygni profile of the UV resonance lines of the Oellf star HD 175754. Franco et al. (1983) studied the P Cygni profiles of the above mentioned resonance lines of HD 175754 observed at different epochs and they reported variability at the secondary satellite component. They proposed for this star two different mechanisms for the explanation of the variability, namely, a thermal mechanism in a hot region at 200000 K which produces the principal stationary component and a mechanism which gives rise to the secondary by ionization of cooler high velocity stellar material from X-rays coming from inner coronal region.

Danezis (1984) studied the UV spectra of the gaseous envelope of AX Mon taken by the IUE satellite (at phase 0.568) and noted that the absorption lines of many ionization potential ions, not only of those presenting P Cygni profile, are accompanied by two strong absorption components of the same ion and the same wavelength in the violet side of each main spectral line. This means that the edges variability is the result of a number of absorption components and the whole profile of any spectral line is formed by a composition of several independent lines. For historical reasons, these lines are named "satellite components" (Peton, 1974) and they are defined as all the components (absorption or emission) of the same ion, which originate from independent density shells. Such lines are present not only in P Cygni profile lines but also in the case of classical lines.

This fact is interpreted by the existence of three independent density shells in the region in which the main spectral lines are created.

This observation could supply an answer to the existence of a large number of absorption lines in the UV region of the Be star AX Mon, with large intensity and unknown origin. These components have displacements that lead to expansion velocities-almost invariant for all lines of most ions- of 0 km/sec, -80 km/sec and -260 km/sec (Danezis, 1986).

This fact enabled Danezis (1984) and Danezis et al. (1991) to propose for AX Mon a structure of one non-continuous gaseous envelope, which is formed from three independent layers of matter. The presence of these three density layers in the outer regions of the UV envelope, as well as the presence of the absorption spectral lines of CIV, SilV and NV, led Danezis (1984) and Danezis et al. (1986) to accept a coronal model for the structure of the envelope (Doazan, 1982), that must allow the existence of many absorption shells or many independent density regions (Danezis, 1984 and Danezis et al. 1991) that will expand with different velocities.

The existence of satellite components in the UV spectrum of AX Mon has been verified by Sahade et al. (1984) and Sahade and Brandi (1985) at the phase of 0.095. Also, Hutsemekers (1985) in the UV spectrum of another Be star, HD 50138, noticed a number of satellite components that accompanied the main spectral lines.

Bates (1986), naming the satellite components discrete absorption components (DACs), constructed a model of ejection of gas parcels from above the star's photosphere, accelerated by radiation pressure. Willis et al. (1989) argued against these components being formed in discrete mass-conserving density enhancements, such as shells or blobs. They proposed that they result from "largely chaotic structures in the wind" and that "they are formed by different material at different times", because of radiative instability (Mulan 1984, Owocki et al. 1986, Prinja et al. 1988). Bates and Gilheany (1990) studied twelve B5 - B9 supergiants and concluded to the non-simultaneous appearance of DACs in different ions and they attributed the presence of DACs to mass - loss (Burki et al. 1982).

However, Danezis and Theodossiou (1988, 1990) could not find satellite components (DACs) of the main spectral lines of another Be star, 88 Herculis Laskarides et al. (1992a) observed one more satellite component in the spectral lines of ions with low ionization potential in the UV spectrum of AX Mon, this in the red side of the main lines. The existence of the spectral lines in the red side of the main lines has been proposed by Doazan in 1983. This fact indicates contraction of the outer layers of the gaseous envelope. Danezis et al. (1991, 1995, 1997a, 1997b), Theodossiou et al. (1993,1997), Laskarides et al. (1992a, 1992b, 1993a, 1993b), and Stathopoulou et al. (1995, 1997) have studied the UV spectrum of several Oe stars and noticed the same phenomena, not only for the spectral lines of low ionization potential, but also for the resonance lines of NV, CIV, SilV and the spectral line NIV λ 1718.55A. All these facts led Danezis et al (1995) to believe that Oe and Be stars, members of binary systems, as a rule present many density shells in their gaseous envelopes.

Waldrom et al. (1994) in their study on the O4I(n)f star HD 66811 (z, Puppis) proposed a model for the full time-dependent hydrodynamic response of stellar wind to spherically symmetric propagating density shell. They concluded to DACs being the result of the density shells (Lamers et al. 1978, Henrichs et al. 1980, 1983), which are hydrodynamically stable and propagate through the supersonic wind structure as stable solitary waves.

Cranmer and Owocki (1996) proposed a hydrodynamical model considering an azimuthally inhomogeneous radiation-driven wind for the formation of large-scale "corotating interaction regions" (CIRs). They suggested that DACs could derive by CIRs resulting from a magnetic field (Mulan 1984a,b, 1986), with closed and open magnetic loops above the stellar surface (Underhill and Fahey 1984).

Rivinius et al. (1997) studied the optical spectra of B hypergiants and attributed DACs to non-spherical density perturbations such as blobs, originating not in the wind but in the photosphere or in even deeper layers.

Kaper et al. (1996, 1999) studied a series of 10 bright O stars and described the shape of the individual absorption components by an exponential Gaussian. They calculated the same velocity in resonance lines formed by different ions and proposed that DACs must originate from interacting fast or slow moving regions of higher density, corotating with the star (CIRs). They proposed that the edge variability is directly related to the DACs, which actually have an impact on the position of the edge.


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