Peremennye Zvezdy (Variable Stars) 39, No. 2, 2019 Received 19 August; accepted 16 September.

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BVI_c Observations of 64 Classical Cepheids

L. N. Berdnikov¹, A. Yu. Kniazev^1,2,3, A. K. Dambis¹, V. V. Kravtsov¹

1. Sternberg Astronomical Institute, Moscow State University, Universitetskij pr. 13, Moscow 119234, Russia; berdnik@sai.msu.ru

2. South Africa Astronomical Observatory, Cape Town, South Africa

3. Southern African Large Telescope, Cape Town, South Africa

1. Introduction

High luminosities and reliable absolute-magnitude calibration make classical Cepheids highly important distance indicators. As rather young objects (no older than  years), they concentrate toward the Galactic plane and therefore serve as ideal tracers of the Galactic disk structure.

We showed in our recent paper (Berdnikov et al., 2014) CCD observations to be preferred over photoelectric photometry for determining Cepheid distances, because the former allow systematic observation errors to be reduced substantially. It therefore appears obvious that CCD observations should be performed for all Cepheids (or, at least, for the faint ones). In this paper, we report the results of such B-, V-, and I_c-band observations for 64 classical Cepheids and also present the results of our yet unpublished photoelectric observations of these Cepheids.

2. Observations

We carried out our CCD observations during 11 observing seasons in 2005-2014 (JD 2453483-56791) with the 76-cm telescope of the South African Astronomical Observatory (SAAO) in South Africa and with the 40-cm telescope of Observatorio Cerro Armazones of Universidad Catolica del Norte (OCA, Chile). We used a SBIG CCD ST-10XME camera equipped with Kron-Cousins filters (Cousins, 1976).

Photoelectric observations were carried out using a single-channel photometer mounted on the 60-cm telescope of Mount Maidanak Observatory in 1982 and 76-cm SAAO telescope in 1998-2008. A description of the technique of the reductions of photoelectric observations can be found in Berdnikov and Turner (2004).

When reducing CCD data, we first reduced the observations obtained using "all-sky" technique during photometric nights exclusively to obtain a catalog of positions and PSF magnitudes of all objects found on the best CCD frames. We then selected, from this catalog, the constant stars, which we used for differential photometry of all stars in all CCD frames including those acquired during non-photometric nights. For a complete description of the observation and reduction technique employed, see Berdnikov et al. (2011).

3. Results and Discussion

We acquired a total of 8225 CCD frames and 903 photoelectric measurements for 64 Cepheids. The results of our reductions are presented in Table 1 (its complete version is available in the file attached to the html version of this paper) and shown graphically in Figs. 1-4, where open and filled circles denote photoelectric and CCD observations, respectively. The scatter of data points in the plots shows that observational errors are close to 0.01.

Small changes of Cepheid periods, which have practically no effect on their computed distances, are often conspicuous even on short time scales of several years (Berdnikov, 1994; Berdnikov & Pastukhova, 1994ab,1995); as a result, for most of the Cepheids the times of maximum light deviate appreciably from the zero-phase times given by published light elements (ephemerides) of most of the Cepheids including those reported in recent catalogs. That is why, for plots in Figs. 1-4, we used the current light elements from Table 2, which we determined based on the times of maximum light computed using the Hertzsprung (1919) method whose computer implementation is described in Berdnikov (1982). We determined the current light elements from an analysis of - and -band observations from this paper combined with published photoelectric and CCD observations including the data acquired in Hipparcos (ESA, 1997), ASAS-3 (Pojmanski, 2002), ASAS-SN (Yayasinghe et al., 2018), and INTEGRAL-OMC (Alfonso-Garcon et al., 2012). All light elements in Table 2 refer to the -band filter.

Figures 5-7 show the O-C diagrams for all the 64 Cepheids. These diagrams can be used to determine the corrections to the ephemerides from Table 2 and, in particular, compute the phases of spectroscopic observations of Cepheids or when determining the velocity from a single velocity measurement.

Table 3 lists the normalized Fourier coefficients (cosine expansion) (Petersen 1986) for Cepheid light curves, and in Fig. 8, we plot as a function of period for the 547 Cepheids that we observed; the circles show the data from Table 3.

4. Conclusions

(1) A total of 9128 magnitude measurements were made for 64 Cepheids in 1982-2014 in the B-, V-, and I_c-band filters.

(2) We analyzed our data combined with all published photoelectric and CCD observations of these Cepheids using the Hertzsprung method and constructed the O-C diagrams based on 1384 times of maximum light. These diagrams can be used to compute the corrections to ephemerides in Table 2, e.g., in order to determine the phases of spectroscopic observations of Cepheids or when determining the  velocity from a single radial-velocity measurement for a Cepheid.

(3) We fitted the light curves of 64 Cepheids by Fourier series (cosine expansion) and computed the corresponding normalized Fourier coefficients , , , , , and . The plot of vs. period is used to validate the classification of a variable as a classical Cepheid. Figure 8 shows such a plot for the 547 Cepheids that we observed earlier. Here the positions of the circles, which correspond to the data from Table 3, corroborate the correctness of the classification of the 64 Cepheids.

We will use our new data to study the structure and kinematics of the Galactic disk and the properties of the Cepheids, in particular, to search for evolutionary changes of their periods.

Acknowledgements: This work was supported by the Russian Foundation for Basic Research (projects Nos. 18-02-00890 and 19-02-00611).

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