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Astron. Astrophys. 330, 222-224 (1998)

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2. Analysis of the Hvar UBV observations

Between 1982 and 1990, 370 UBV observations of HD 6226 were secured with the 0.65-m reflector of the Hvar Observatory, Croatia with 3 additional observations obtained on one night in 1995. The star was originally observed as one of the check stars in the course of the Be-star observing program, initiated and coordinated by Harmanec, Horn & Koubský (see Harmanec, Horn & Koubský 1981 and the consecutive reports in the Be star Newsletters Nos. 3 to 12; for the final report on the Be-star observations at Hvar, see Pavlovski et al. 1997).

When we noted that HD 6226 is variable, we continued its observations along with the Be stars o Cas = HD 4180 and [FORMULA]  And = HD 6811, relative to HR 189 (HD 4142). HR 289 (HD 6114) was chosen as the new check star and was usually observed with the same frequency as the variable. The only exception was the night HJD 2446689 when 4 Per (HD 12303) and 2 Per (HD 11291) served as the comparison and check, respectively.

The observations were carefully reduced to the standard UBV system by Harmanec, Horn & Juza (1994) with the help of the reduction program HEC22 and the Hvar data archives of individual differential observations were made available in digital form by Harmanec et al. (1997).

Perryman et al. (1997) have now published 89 individual photometrically reliable Hipparcos [FORMULA] measurements of the star which also show clear evidence of variability and can be conveniently combined with our own data since the [FORMULA] magnitude is close to Johnson V.

Fig. 1 is the plot of the V and [FORMULA] magnitudes and the [FORMULA] and [FORMULA] colour indices of HD 6226 vs. time, based on all individual measurements. Corresponding data for the check star HR 289 are also plotted there. The overlap between Hvar and Hipparcos measurements shows that there is no significant zero-point shift between these two data sets. It is seen that there are periods when the brightness of HD 6226 varies little (within some 0. [FORMULA] 04) but these stable periods are occasionally interrupted by brightenings of a variable amplitude amounting up to 0. [FORMULA] 2. This is the behaviour reminiscent of some other Be stars like QR Vul = HD 192685 or o  Cas = HD 4180 (Horn et al. 1985, Pavlovski et al. 1983, 1997). Having no spectral observations at our disposal, we cannot say whether or not the brightenings of HD 6226 were accompanied by the appearance of the Balmer emission lines as it was the case of QR Vul. It is clear, however, from Fig. 1 that the brightening of the star is accompanied by the reddening of [FORMULA], and blueing of the [FORMULA] index. This formally indicates an increase of the photometrically derived radius since in the colour-colour diagram the star moves away from the main sequence towards to giant sequence. This behaviour is typical for the formation of an envelope around a Be star seen under some intermediate inclination (not too close to equator-on view) - see Harmanec (1983, 1994). Fig. 1 also shows that the comparison or check star itself could be suspected of microvariability, but we warn that individual 10-s integrations are shown for the Hvar data, without any data editing. For HD 6226 it seems that a slow cyclic variation can be suspected during the periods between large brightenings.

[FIGURE] Fig. 1. UBV observations of HD 6226 plotted vs. time: Hvar data are shown by dots, Hipparcos [FORMULA] observations by plusses

Since little is still known about the character of similar brightenings, we carried out a period analysis to see whether the variations of HD 6226 are (semi-)regular or not. To this end, we used both, the PDM technique (Stellingwerf 1978) (10 bins and 4 "covers") and Fourier periodograms (Deeming 1975) to search for periodicity over the range of periods from 50 to 5000 d. Fig. 2 show the periodograms for the V magnitude and [FORMULA] colour index.

[FIGURE] Fig. 2. Deeming's (1975) and Stellingwerf's (1978) PDM periodograms of the light and colour variations of HD 6226: Upper panel: Fourier (Deeming) periodogram of combined V data; the dotted line shows the spectral window of the data, normalized to the highest peak of the periodogram and shifted for 0.012; Cetral and bottom panels: PDM periodograms of combined V and Hvar [FORMULA] data

It is seen that the results of the two period-searching techniques differ. This is not surprising since Fourier technique prefers roughly sinusoidal variations while the PDM is more sensitive to non-sinusoidal light curves. Yet, one can find some common periods in both periodograms. If one ignores the longest detected periods (these basically sort the data in phase in such a way as to avoid their phase overlaps), one of the best detected periods is 481. [FORMULA] 294 which brings the two major brightenings into the same phase interval. The phase diagram for this period is shown in Fig. 3.

[FIGURE] Fig. 3. A phase plot of photometric variations of HD 6226 for the 481-d period: Hvar data are shown by dots, Hipparcos [FORMULA] measurements by plusses

Other possible periods include 935 d, 240 d (a half of the 481 d period if one wants to bring all brightenings into the same interval of phases) and 149.4 d.

There may also be some rapid variations of a small amplitude but our data contain too few night series to allow us a meaningful investigation of their reality and possible nature.

It is clear that systematic spectroscopic monitoring of HD 6226 is urgently needed. Without spectroscopic observations at hand it seems meaningless to speculate about the nature and/or causes of the light and colour variations of HD 6226. Rather, we appeal to other observers to continue observations of this interesting star and especially to check on the possible regularity of its brightenings. If the phenomenon occurs regularly with the 481-d period, then the nearest brightening should occur around Nov. 27, 1998.

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© European Southern Observatory (ESO) 1998

Online publication: January 8, 1998