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Astron. Astrophys. 364, 706-711 (2000)

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4. HK Dra

4.1. Photometric analysis

In Fig. 5 the entire data set (Hipparcos and APT-80 photometry) is plotted versus time. The modulation amplitude as well as both the brightest and mean magnitudes show a very high variability from season to season.

[FIGURE] Fig. 5. Long-term variability of HK Dra. Observations from 1990 through 1992 are from Hipparcos; observations from 1993 through 1999 are from our APT-80.

Three time intervals of the entire data set, in which the observations are more numerous, have been analysed using the Scargle-Press period search routine to derive the period of the photometric modulation (Scargle 1982; Horne & Baliunas 1986). In Table 3 we report the mean epoch, the HJD date range, the number of observations ([FORMULA]), the photometric period (P) revealed by the periodogram analysis and its uncertainty ([FORMULA]). The photometric period turns out to be variable from season to season. When the entire data interval is analysed a mean photometric period of [FORMULA] d is found along with secondary periods (see Fig. 6). However, such secondary periods have false-alarm-probability (FAP) [FORMULA] and they are never revealed when each of the three time intervals is analysed separately.

[FIGURE] Fig. 6. The periodogram of the entire photometric data set with indication of the most significant frequency (see last row of Table 3).

It proved possible to select ten time intervals during which the optical flux showed an almost stable modulation. The main characteristics of the analysed light curves are reported in Table 2. V-band, along with U-B and B-V, magnitudes (epochs: 1993.63, 1994.76, 1999.44) from APT-80 are plotted versus time in Fig. 7. These show remarkable variations in amplitude and shape, even on time scales of the order of the modulation period.

[FIGURE] Fig. 7. Expanded view of Fig. 5 for three different epochs in which the APT-80 observations were more numerous. V-band light curves (upper panels) along with U-B and B-V colour indices of HK Dra are plotted versus Heliocentric Julian Day (-2449000).


Table 2. Mean epochs, number of observing nights, mean V-band magnitudes, colour indices and peak-to-peak amplitude of the light curves of HK Dra. The photometry data from 1990.66 to 1992.64 are from Hipparcos.


Table 3. Results of the periodogram analysis for HK Dra. The period in the last row is obtained when the entire data set is analysed.

4.2. Spectroscopic analysis

HK Dra is classified by Hipparcos as a double system; however, the WEB catalogue reports a constant radial velocity. In order to check its binarity and to better define both spectral type and the nature of its variability, we carried out spectroscopic observations in the H[FORMULA] and Na I spectral regions. The H[FORMULA] line at the three different observation epochs, along with their residuals from the average spectrum, are plotted in Fig. 8. Taking into account the possibility of slight differences in the normalisation procedure, no clear variation is apparent. The slight asymmetry of the H[FORMULA] profile may be attributed to a blending effect due to the presence of numerous photospheric lines. The H[FORMULA] equivalent width (EW) and the full width half maximum (FWHM) are listed in Table 1.

[FIGURE] Fig. 8. H[FORMULA] spectra of HK Dra (top ) and residuals from average spectrum (bottom ). It is evident the stability of spectral behaviour through our observation date range.

The Na I lines are shown in Fig. 9. As for the H[FORMULA] line, these lines do not show appreciable variations.

[FIGURE] Fig. 9. As in Fig. 8, but for the Na I D1, D2 spectra of HK Dra.

The radial velocity, which was determined as described in Sect. 3.2, appears to be constant with an average value of [FORMULA] km s-1, which is consistent with the [FORMULA] km s-1 given in the WEB catalogue (Duflot et al. 1995), thus indicating that HK Dra is a single star or at least not a member of a close binary system.

4.3. Discussion

From the analysis presented in Sect. 3, the photometric variability of HK Dra is characterised by periodic modulation of the optical-band flux, quasi-sinusoidal light curves, as well as changes from season to season of the photometric period, light curve shape and amplitude. In giant stars these characteristics are commonly found among stars associated with the Asymptotic Giant Branch (AGB), in particular among semi-regular variables of type b (SRb) according to Khopolov et al. (1998) or, on a more astrophysical base, as "blue SRV" in the sense of Kerschbaum & Hron (1992, 1994). Although reported in the Hipparcos Catalogue as a double component system, our spectroscopic analysis of HK Dra in the H[FORMULA] and Na I D1, D2 regions did not reveal any significant variation of the equivalent width or line profile, suggesting that it is unlikely a close binary system.

Further photometric and spectroscopic observations of HK Dra are very important because long time-series of light curves can provide important informations on the pulsational nature and variability of this star.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001