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

3. HD 172468

3.1. Photometric analysis

The photometric observations of HD 172468 were analysed by using a Scargle-Press period search routine (Scargle 1982; Horne & Baliunas 1986). No evidence of periodic modulation was found (see Fig. 1). As shown in Fig. 2-Fig. 3, for most of the time the mean V-band magnitude of HD 172468 was 7.52 mag, with a standard deviation of 0.014 mag, and mean colours U-B=1.11 and B-V=1.28. Triangles and filled dots in Fig. 2 represent the Hipparcos and the APT-80 photometry, respectively. However, during the 1993-1999 observations, the star's brightness did drop several times and randomly, from 1 up to 4 magnitudes (open diamonds of Fig. 2). As described in Sect.  2.1, the Catania APT-80 always observed HD 172468 along with the variable star BY Dra and other comparison stars. All stars in our observing sequence were regularly measured, except HD 1724678, which was not found and, therefore, not measured during several nights (short bars below the dotted line in the top panel of Fig. 2) and, in particular, during none of the 1992 and 1998 observing seasons. A star within an observing sequence is not found only if its magnitude results to be fainter than the APT-80 detection threshold. For example in the 1998 observing season such threshold resulted to be about V=12.6 mag (dotted line in the top panel of Fig. 2). Hence, we infer that during those nights the brightness of HD 172468 was fainter than V=12.6 mag. Moreover, it is interesting to point out that the photometric monitoring of HD 172468 by Hipparcos, whose limiting magnitude is V 12.4 mag, presents a gap during 1992, that corresponds to the interval over which the star was not found by our APT-80. We guess that the gap in the Hipparcos monitoring may be likely due to the faintness of the star below the cited limiting magnitude.

Fig. 1. The periodogram of the entire photometric data set of HD 172468.

Fig. 2. Long-term UBV photometry of HD 172468. The observations from 1990 through the beginning of 1993 are from Hipparcos (open triangles); the observations from 1993 through 1999 are from APT-80 (filled dots). HD 172468 was undetectable, i.e., fainter than V=12.6 mag in the days marked by the short bars below the dotted line (top panel). Open diamonds and vertical bars represent the APT-80 observations of the light fading phases.

Fig. 3. Enlarged view of Fig. 2. The APT-80 observations of the light fading phases have been omitted to better show the detailed light curve.

During the brightness decline, both U-B and B-V colours present a strong "blueing", as already observed for R CrB (Cottrel et al. 1990), up to 0.9 mag and 0.45 mag, respectively.

3.2. Spectroscopic analysis

The H and the Na I D spectral regions of HD 172468 are shown in Fig. 4. The H equivalent width (EW) and full-width half-maximum (FWHM) are listed in Table 1. The radial velocities were obtained by cross-correlating the HD 172468 with the  Ari spectrum. The latter star is a bright slow-rotating primary standard ( km s^-1, Wilson 1953) whose spectrum has been obtained almost contemporaneously with the HD 172468 spectrum. The wavelength ranges for the cross-correlation analysis were selected in order to exclude the H line and the spectral regions heavily affected by telluric lines (e.g., the 6276-6315 Å band of O₂). A RV of km s^-1 was derived.

Fig. 4. H and Na I spectra of HD 172468 (solid line) and the spectra of a K5 III ( Tau) star. The difference is shown in the the lower parts of each panel.

Table 1. Equivalent width (EW) and full width at half maximum (FWHM) of H absorption line for HD 172468 and HK Dra.

Adopting the Hipparcos parallax of 3.11 mas and the V-band magnitude V₀=7.20, after correction for interstellar reddening (Johnson 1965), we determined an absolute magnitude . The spectrum of HD 172468 was then compared with the spectra of two standard stars of spectral type K2 III ( Ari) and of K5 III ( Tau). Although HD 172468 is listed in the SIMBAD database as a K2 star, the best agreement is found with a K5 III spectral type classification. However, the mean dereddened (U-B)₀ = 1.05 and (B-V)₀=1.25 colours appear to be somewhat bluer than for a K5 III spectral type. The dotted lines in Fig. 4 represent the spectrum of  Tau (K5 III), which is wavelength shifted to account for the different radial velocities of the two stars in the plot. The residuals between the spectra of the two stars are also shown in the lower parts of the panels.

3.3. Discussion

The presently available photometric and spectroscopic data do not allow us to confidently infer to which class of variability HD 172468 may belong.

The brightness drops, up to several magnitudes, as well as the "blueing" showed by HD 172468 during the decline phases are typically observed in eruptive-type stars (RCB). The main characteristic of RCB stars is the abrupt drop in brightness, probably due to a sudden ejection of highly absorbent matter, followed by a longer lasting return to normal light (Sterne 1935; Proust & Verdenet 1983). Although HD 172468 has a photometric behaviour similar to an RCB star, however, its spectral type is quite late and the luminosity too low to confidently assign this star to the RCB class. Moreover, from the presently available photometry we did not find evidence of the pulsational modulation i.e., the semi-regular oscillations with amplitude of 0.2-0.4 mag and period between 30 and 100 days, which characterises RCB stars (Khopolov et al. 1998).

The irregular variations of colour and brightness showed by HD 172468 may be alternatively attributed to the variable light extinction due to a circumstellar dust cloud. In this case a classification as young Orion type object may be more appropriate.

We consider it important to present this preliminary work on HD 172468 in order to stimulate further photometric and spectroscopic observations. In fact, a systematic monitoring of the photometric and spectroscopic characteristics, especially close to the light fading, may provide important hints to understand the nature of the variability of HD 172468, as well as the composition and dynamics of the dust cloud, which we suspect to be the cause of the observed variability.

© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001
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