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Astron. Astrophys. 334, 558-570 (1998)

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4. An overview of photometric and emission-line variations of [FORMULA] CMa

Given the evidence of the slow cyclic change of the 1 [FORMULA] 372 period, it was deemed important to identify the timescales of spectral and light changes of [FORMULA]  CMa and investigate their possible relations.

4.1. Photometry

The V -magnitude time variations are compared with variations seen in some other quantities in Fig. 6 while Fig. 7 is a plot of the B observations vs. time. Using some overlapping B and b observations, I decreased all b magnitudes for 0 [FORMULA] 1 to bring them on the scale of B magnitudes. It is seen that the long-term variations of [FORMULA]  CMa are characterized by rather regular major brightenings, with an amplitude as large as 0 [FORMULA] 4, and by occasional smaller brightenings. It is important to stress that these smaller brightenings, taking place within less than a few weeks, are no doubt real. This is clearly documented by Fig. 8 where the independent ESO and Hipparcos observations of one such episode are compared. This very fact shows how difficult the search for the periodic components of the light changes of Be stars can be.


[FIGURE] Fig. 6. Several measured or deduced quantities plotted vs. time. From top to bottom: a The O-C of locally derived epochs calculated for a constant period of P = 1 [FORMULA] 371906: The O-C deviations are expressed in fractions of the period, i.e. as phases of the local RV maxima with respect to ephemeris (1); b The V magnitude: Differential observations are shown by dots, all-sky measurements by crosses; c Peak intensity of the H [FORMULA] emission line; d The semi-amplitude of the locally fitted 1 [FORMULA] 3719 RV curves (only data from spectrograms with a dispersion better than 20 Å mm-1 are shown); The last data point at JD 2450100 comes from a preliminary report by Balona et al. (1998) who give [FORMULA]  km s-1

[FIGURE] Fig. 7. Plots of the B magnitude of [FORMULA]  CMa vs. time. Differential observations are shown by dots, all-sky measurements by crosses. Strömgren b magnitudes were decreased by 0 [FORMULA] 1 to get them on a scale comparable to Johnson B magnitudes.

[FIGURE] Fig. 8. An example of a smaller temporal brightnening of [FORMULA]  CMa recorded by two independent instruments. The ESO y observations and the Hipparcos [FORMULA] observations are denoted by circles and crosses, respectively. See the Appendix for details

There is some indication that the luminosity of the object in the periods outside the brightenings is secularly decreasing in time over the whole period of about 44 years covered by observations. Besides all that, there are also lower-amplitude cyclic brightness variations of the star, first explicitly noted by Mennickent, Vogt & Sterken (1994).

4.2. Balmer line profiles

The most numerous peak-intensity measurements of H [FORMULA] are also plotted vs. time in Fig. 6. One can see that the intensity of the Balmer lines seems to vary on a time scale similar to the cyclic variations of the 1 [FORMULA] 3719 period and brightness of the object. A series of H [FORMULA]   profiles obtained by Baade (1984) indicates (as pointed out by the author and as seen in Table 3) that the emission strength is fairly constant on shorter time scales. This is not quite true for the recent Heros H [FORMULA] data.

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© European Southern Observatory (ESO) 1998

Online publication: May 15, 1998

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