Analysis of the high temperature region in Be starsPresented at the meeting of the Working Group on Active B Stars during the 26th IAU General Assembly in Prague, Czech Republic on 2006 August 18 A. Ringuelet1 & A. Torres1,2
1
FCAG, Universidad Nacional de La Plata, Paseo del Bosque S/N,
1900 La Plata, Argentina;
ringuele@fcaglp.unlp.edu.ar
Received: 2007 March 26; Accepted: 2007 April 10. Introduction In IAU Symposium No 70 (1976), Marlborough suggested the High Temperature Region (HTR) to be at the star's disc surface in the PM model. Marlborough (1987) reviewed other suggestions, namely, by Thomas (1983) in relation to thermodynamically open systems and non-radiative heating, and the possibility of heating up the inner part of the disc by non-radial pulsations. When the WCD model (Bjorkman & Cassinelli, 1993) was challenged by the results of Cranmer & Owocki (1995) the suggestion of a collisionally heated disc was abandoned. At present, if we consider the observational results of Kogure & Hirata (1982), Slettebak (1994), and Grady et al. (1987), and the theoretical requirements posed by Porter (1999) for the existence of Keplerian discs and the discussion by Thomas (1983) on the existence of a heated region, it is absolutely necessary to analyze observational features that would 1) provide hints as to the existence of material dissipating kinetic energy, 2) permit to establish the geometry of the HTR, and 3) sustain a discussion on the measured expansion velocities in Be stars, which is significant in relation to the applicability of different physical processes. We have attempted to attain this by measuring equivalent widths, expansion velocities and residual intensities of the resonance lines (in absorption) of Si IV, C IV and Al III and the transition λ1640 of He II. Results Expansion velocities: We measured expansion velocities at 0.7 of the residual intensity of the lines. We support this choice on the results by Cidale & Ringuelet (1993), where Hα profiles have been computed applying the "co-moving frame" method (Kunasz & Hummer, 1974) in order to obtain a rigorous solution. In this solution and according to the parameters of B stars, terminal velocities do not correspond to the end of the blue wing in the line profile. Consequently, terminal velocities in Be stars are much lower than generally attributed. Together with the finding of Grady et al. (1987) on the merging of the DACs in the blue wing of the resonance lines of Si IV and C IV, we consider that the wind or fluid velocity is better represented by the line core. On these bases, the expansion velocities we measured are not larger at the poles. This result poses limits of applicability to models based on bi-stability and to the geometry of the magnetic fields. Equivalent widths: We measured equivalent widths after carefully determining the continuum. In Figure 1, which is taken from Torres & Ringuelet (2006), equivalent widths are represented as a function of inclination angle, for transitions of the ions in the HTR. Fig. 1 includes 19 stars measured by Slettebak (1994). All values are corrected for possible photospheric contributions. It readily appears that the contribution of the HTR to the equivalent widths arises from all inclination angles. Mean values show a tendency to increase with the inclination angle. This strengthens the results related to the HTR obtained in spherical geometry. We have also verified that representing mean values of the equivalent widths versus kinetic energy, as yielded by our measured expansion velocities, a similar correlation like the one obtained with the inclination angles is present. This correlation is important in connection to the formation of Keplerian rings (Porter 1999) and the heating of the region by dissipation of mechanical energy (Thomas 1983). The velocity curve: We suggest a velocity curve that increases with radius inside the wind and decreases after the temperature maximum. This behaviour is in keeping with the presence of Keplerian rings (Porter 2000) and with Kogure & Hirata's (1982) statement that high temperature regions display large expansion velocities and cool regions display low expansion velocities. The resonance lines of Al III turn out to be an interface between the stellar wind and the cool envelope. The results we have presented suggest a model of a B star surrounded by a hot wind which form a thermodynamical open system interacting with a cool envelope or a Keplerian ring; X-ray models are independent (in a first approximation) of the existence of a chromosphere since these models refer to regions far outside the HTR. The chromospheric model for the regions above the stellar photosphere provides new possibilities for explaining i) the alternation of phases: B-shell-Be, ii) the relation between Hα emission and IR emission (van Kerkwijk et al. 1995), iii) the shape of the Balmer continuum when in emission, and iv) the WUPPE observations of polarization which do not satisfy existing models (Bjorkman et al. 1991). A thorough discussion of these results is to be published elsewhere.
Figure 1. Equivalent widths vs. References
Bjorkman, K. S., Nordsieck, K. H., Code, A. D. et al. 1991,
ApJ, 383, L 67
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