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Astron. Astrophys. 349, 151-168 (1999)

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1. Introduction

In a recent work, Moujtahid et al. (1998, hereafter MZH) have studied the long-term spectrophotometric (SPh) variations of 49 Be stars using SPh data, photometric magnitudes and colour indices observed over about 50 years. Photometric magnitudes and colour indices collected by different authors in six different photometric systems were homogenized. This enabled us to derive relations between the V magnitude and the [FORMULA] (visual gradient of energy distribution) and D (total Balmer discontinuity: BD, D = [FORMULA]; [FORMULA] constant stellar component; [FORMULA] variable circumstellar component) for each program star [gradient [FORMULA] and Balmer discontinuity D are defined as in the BCD SPh system (Chalonge & Divan 1952)]. We first sum up the conclusions obtained in MZH and add some results concerning the SPh behaviours obtained by other authors.

1.1. Brief review of results

Line and continuum spectrum variations of Be stars may show some correlation, although not necessarily implying a one to one relation. To avoid possible misunderstandings, in what follows we designate SPh-E the "spectrophotometric phase of Be stars when the second BD is in emission : ([FORMULA])", and SPh-A the "spectrophotometric phase of Be stars when the second BD is in absorption ([FORMULA])". In the same way, we can also use the designation "SPh-N" for phases where the visible continuum energy distribution of a Be star resembles that of a normal B star, whatever the momentary aspect of the line spectrum. In MZH we could use SPh-Be and SPh-shell respectively for SPh-E and SPh-A. Hence, the main results obtained in MZH may be summed up as follows:

(1) A given Be star, depending on the observing epoch, may be seen in any of the above mentioned SPh phases. Relations between V, [FORMULA] and D parameters appear different when they refer to a SPh-E or a SPh-A phase. SPh-E phases are characterized by slopes [FORMULA] [FORMULA], positive or negative, depending on the star and epoch: the most frequent combination is [FORMULA] (brightening) and [FORMULA] (reddening) as emission increases ([FORMULA]). SPh-A phases more likely show [FORMULA] [FORMULA] and [FORMULA]. Similar results were previously obtained by Divan (1979), Divan et al. (1982), Zorec (1986) for a small number of early type Be stars using only BCD data. As all individual SPh variations studied in MZH are published in electronic form 1, we reproduce in Fig. 1a typical SPh-A behaviour observed in HD 37202 and in Fig. 1b that observed in HD 200120 as an example of a SPh-E phase.

[FIGURE] Fig. 1a and b.  Spectrophotometric behaviour of Be stars. a  HD 37202 (SPh-A phase); b  HD 200120 (SPh-E phase). Symbols are: [FORMULA] BCD data; + UBV data; [FORMULA] UBVRI data; [FORMULA] Geneva photometry; [FORMULA] uvby data; [FORMULA] 13 colour photometry

(2) Depending on the star, the ([FORMULA]) relations are unique or double valued in a given SPh phase. When they are single, they do not change (or very rarely) after the star has spent some time in another SPh phase.

(3) For most studied stars, the collected observations revealed only one characteristic SPh phase. However, in 8 stars SPh-E [FORMULA] SPh-A phase changes were detected. Unfortunately, in some Be stars where spectroscopic Be [FORMULA] shell [FORMULA] B-normal phase transitions are well known, the scarce photometric data, which are sometimes also heavily marred by errors, did not allow us to study the corresponding SPh behaviour.

(4) SPh-A phases, transitory or more permanent, were detected in some stars with low [FORMULA] values.

In this context it is worth mentioning the results derived from uvby Strömgren photometry of 13 Be stars by Mennickent et al. (1994), who showed that Be-shell stars have [FORMULA] [FORMULA]. These authors also found that: (a) the total amplitude of long-term variations is correlated with the average amplitude of short-term variations in each photometric band; (b) there seems to be a correlation between the slope of the (c[FORMULA]) relation and [FORMULA].

For some stars in typical spectroscopic Be phases, Dachs (1982), Dachs & Hanuschik (1984), Dachs et al. (1988) obtained relations between the V magnitude and the [FORMULA] colour indices which are characterized by slopes: [FORMULA].

Finally, making use of UBV photometric data spanning about 10 years of a large number of Be stars, it was shown that: (a) brightening is accompanied by reddening in early type Be stars (Hirata & Hubert-Delplace 1981, Hirata 1982), while brightening is correlated with blueing in late type Be stars (Kogure & Hirata 1982); (b) the slope [FORMULA] has the same sign as [FORMULA] and the latter is steeper when the [FORMULA] is higher (Hirata & Hubert-Delplace 1981, Hirata 1982).

1.2. Aim of the present work

The above-mentioned results of MZH refer to a relatively large number of Be stars. So it is worth studying this data, first as a function of fundamental stellar parameters to check the SPh-like correlations obtained in previous works, and then to find new incidences of stellar characteristics on the observed SPh behaviours. We shall also use these data to discuss global properties of CE regions responsible for the observed SPh variations.

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© European Southern Observatory (ESO) 1999

Online publication: August 25, 1999