2. Individual objects
The light curves of MZ Vir, HV Lib, V4401 Sgr and HD 113410 are plotted in Fig. 1. HD 200512 has been observed at a different time and is therefore plotted in Fig. 2. Stellar parameters and parameters of the light change deduced from our observations are listed in Table 1. Additional data were taken from the HIPPARCOS (ESA 1997) and IRAS (1988) catalogue (see references therein).
Table 1. Stellar properties and light curve parameters for the four new variables. Light curve parameters were derived from the data presented in Figs. 1 and 2. Column 6 gives the variability amplitude listed in the Hipparcos catalogue. The periods given in column 5 have to be seen as typical time scales of the variation, where the period in brackets is the result of the Fourier analysis (see text).
A misidentification by the automatic telescope or influence of the measurement by a nearby object can be excluded as none of these objects is located in a crowded field in the sky or has a star of comparable brightness nearby. Brightness of the stars was determined using a second comparison star. These check stars were either early type K stars or G stars. All of them were observed by HIPPARCOS without detection of any variability.
At a first glance all light curves show more or less irregular light changes suggesting that these stars are either semiregular or irregular variables. However, the light curves show a number distinct maxima and minima. These were used to estimate a timescale of variability. None of the objects showed a single distinct period but a period range around 30-60 days. Therefore I will use the term period in the sense of `timescale of variability' in this context. A Fourier analysis of the data was done to see, whether the derived timescale could also be found in the Fourier spectrum. Our Fourier analysis algorithm of unequally spaced data has been discussed by Deeming (1975). We used it coded in an improved way as proposed by Kerschbaum (1988). An example for the result of a Fourier analysis is given in Fig. 3 for HV Lib. It is obvious that the same problem occurs noted already from visual inspection of the light curves. A large number of peaks of comparable height make the identification of a single period difficult. Several of these peaks are secondary maxima of a real peak as can be seen from the spectral window (Fig. 3, upper panel). However, this is what one would have expected due to the large irregularities and asymmetries in the shape of the light curves. Therefore the Fourier analysis alone is not an appropriate tool to derive a period for these kinds of objects. But it provides a possibility to check or quantify periods identified by measuring the time difference between two observed maxima or minima. In this paper I will use it for this aspect only. Analyzed with these two approaches the four stars gave the following results:
MZ Vir: The amplitude of the variation is quite large. The light curve shows a lot of bumps and other irregularities over the whole time of observation. From measuring maxima and minima we derive a period of 61.67 days. We thereby excluded some small `intermediate' maxima, which would suggest half the period. A regularity of the occurrence of these intermediate maxima could not be found. Fig. 4 (top panel) shows the result of the Fourier analysis for MZ Vir. The highest amplitude is reached for a period of 69 days, close to the value derived by visual inspection. A further peak is visible around 35 days which coincides with the variations found from to the small `intermediate' maxima.
HV Lib: For this star determination of a typical timescale is very difficult. The amplitude of the light cycles changes strongly making a clear separation of different cycles problematic. Therefore only a rough estimate of the period was done giving about 45 days. It is interesting to note a significant shift in the mean brightness of the star after the break in observations where the telescope was closed. This might indicate a secondary period with a much longer time scale as discussed below. As expected the Fourier analysis (Fig. 3 and 4) does not really help to improve the result. The highest peak is found at 735 d. This might be due to a possible secondary period, but it is badly resolved and close to the total time covered by the observation. Still this long period peak is not that expressed in the other three objects. It also has the same amplitude as the mean brightness difference before and after the break in observations. The second highest peak lies at 48 days, i.e. close to our estimated value. However, the amplitude of this peak does not differ much from peaks at 37 and 90 days. Unless a longer time series of observations reveals a dominant period, this star has to be classified as irregular.
V4401 Sgr: The number of maxima and minima, which can be clearly identified, is small. 4 maxima and 2 minima were chosen resulting in a mean period of 257 days. Close to this value two peaks can be found in the Fourier spectrum (Fig. 4) at 23 and 31 d. The amplitude of all peaks is very small. Compared with the other three objects, the time coverage is less dense for this star. The spectral window is therefore not very clean so that the detection of the correct frequencies is difficult.
V4401 Sgr is the only object of our sample with several notations in the literature. Eggen (1976) suspected the star to be variable. From an inspection of the HIPPARCOS light curve one might suspect an overlapped variation on a significantly longer timescale indicated by a change in mean brightness between JD 2448000 and 2448500. Such a variability is not found in our observations. The HIPPARCOS catalogue lists furthermore variations on a very short time scale of 3.4 days for this star.
HD113410: Maxima and minima are clearly defined in this object. Five maxima and five minima were selected giving a mean timescale of variability of 32.56 days. Maxima and minima give very similar results. Fourier analysis shows three periods with comparable amplitude: 25, 27 and 39 days. The peak at 27 days is thereby probably influenced by the peak at 25 days.
HD200512: As the time of observation of this object is slightly different from the other four stars it is plotted separately in Fig. 2. Variations are on a short time scale but maxima and minima are clearly defined. 8 maxima and 5 minima were selected giving a mean period of 23.46 days. The amplitude is the smallest found among the five objects investigated. The Fourier analysis gives two peaks at 26 and at 30 days, respectively, both of them not very well expressed. The mean brightness of this star seems to vary slowly over the time observations were obtained. Therefore we suspect the presence of a second, significantly longer period. For this star we have also information on the flux in the Johnson I filter relative to the comparison star. The I-amplitude is approximately by a factor of 2 smaller than the V-amplitude.
For these five stars, which are probably quite good examples of short period semiregular and/or irregular variables, the results of Fourier analysis and visual inspection lead to similar results in the way that periods from visual inspection are detectable as those peaks in the Fourier analysis, which show the maximum amplitude. But as noted above the large number of peaks does not allow to draw a conclusion on the period from the Fourier spectrum alone for the presented set of data. Light curve parameters for all four objects are summarized in Table 1. The period in brackets is thereby the result of the highest peak in the Fourier spectrum (except for HV Lib). The variability amplitude given in the HIPPARCOS catalogue differs slightly from our value in the case of HV Lib and MZ Vir, while good agreement is found for V4401 Sgr.
A separation between semiregular and irregular (Lb) variables is difficult for these objects. As a typical timescale of the variations can still be defined we would suggest to classify MZ Vir, V4401 Sgr, HD 113410 and HD 200512 as semiregular variables. On the other hand first results from our monitoring program of irregular variables classified by the General Catalogue of Variable Stars (GCVS, Kholopov et al. 1985-88) show that such typical timescales can be found for several Lbs, too. Therefore we suspect that a division in semiregular and irregular is not meaningful. This is consistent with results from near infrared photometry by Kerschbaum et al. (1996) showing that the O-rich Lbs are undistinguishable from SRVs with the same chemistry. An investigation of this question with a larger sample of Lbs is planned.
© European Southern Observatory (ESO) 1999
Online publication: May 21, 1999