Spectral variability of P Cyg

Abstract

In careful analysis of the CCD-spectra of P Cyg it was possible to identify 10 of the 43 lines that have been unidentified up to now. The majority of the latter belong to multiply ionized atoms. The conclusion is reached that there is anomalous heating in certain layers of the stellar wind. We give facts that suggest variation in the degree of ionization of the stellar wind. 1. Introduction. The star P Cyg has attracted the attention of astronomers for almost 400 years. Since its discovery in 1600 by Blau [1] it has been the subject of many papers, a survey of which can be found in [2, 3]. Its spectrum has been studied in the range of electromagnetic waves accessible to modern receivers. Despite this, many questions relating to this star remain unresolved. The terminal velocity is still uncertain (Y~ = 200km/sec according to [4, 5], 300km/sec according to [6], and 400km/sec according to [3]), as is the effective temperature (Tefr = 19300K [7] or 12200K [3]). The evolutionary status of this star is unknown, as is the mechanism of formation of discrete absorption components. The uncertainty in the determination of the basic parameters of the star such as V~, M, R, Tefr, and M makes it impossible to construct an adequate theoretical model of its atmosphere and wind. It is important to note that P Cyg is a peculiar star, and therefore the standard methods of determining these parameters do not apply to it, since they are based on the use of statistical material. The following properties of the star can be considered peculiar: a) an effective temperature Tef[= 1.22.10 4 K corresponds to the integral flux of the star, yet its spectral class is B1 Ia [3]; b) despite its early spectral class, the spectrum contains no strong lines of the ions CIV, NV, and OIV; c) the star suddenly increased in brightness to magnitude 3 TM in 1600, having been invisible up to that time, yet the remnants of this explosion have not yet been detected; d) the terminal velocity V~ is 7-8 times smaller than in the supergiants of spectral class B1, while tile rate of loss of mass M is 3-10 times larger. 2. Spectral variability in P Cyg. It is known that P Cyg belongs to the recently distinguished class of light-blue variable stars (LBV) [2], which quasiperiodically approach the Humphreys-Davidson [8] limit of luminosity. The bolometric luminosity of LBV-stars remains constant under photometric changes. Spectral variations have been found for all LBV-stars except P Cyg [2]. Nevertheless, comparing the CCD-spectra of P Cyg having the dispersion of 0.8 A/ram obtained by Stahl et al. in 1990 [9], with the spectra of Johnson of 1977 (dispersion 6 A/mm) [10], we have been able to detect certain variations. Thus, for example, the intensive lines Fe II 4433, 4840, and 4868 AA, that occurred in the 1977 spectrum have vanished. At the same time the faint line NII 4802 A remained unchanged. The lines Fe II 5991 5988 AA in the 1977 spectra were more intense in comparison with the line Fe III 5999 A: The latter occurs in the spectrum of 1990, while the first two lines were not observed. Speaking of the Fe lines in general, it should be noted that almost all of the Fe II lines observed in 1977 are absent in the latest spectrum, while the Fe III lines have become more intense. Many more such examples could be given. The variation of degree of ionization in the atmosphere seems to be explainable by the variation in the density of the stellar wind and the surface temperature [7]. One can then assert that the 1977 and 1990 spectra relate to two different states of the star with different rates of mass loss. The variation in the wings in the Balmer lines [11] also suggests this. The problem of the unidentified emission lines is also of particular interest. Johnson counted about 30 such lines [10]. In the 1990 spectrum some of these lines had disappeared, and new ones had appeared in their place. In the general spectrum we counted 43 unidentified lines, of which we succeeded in identifying 10. The lines of [Ni IV] are strongest in the multiplets (cf. Table 1). Some of these unidentified lines occur in the spectra of such objects as Sco X-I, RR Tel, and 7 Cas, whose atmospheres are characterized by an anomalously high degree of excitation.