ACTA ASTRONOMICA Vol. 51 (2001) pp. 49–63 Search for Variable Stars in Two Old Open Clusters: NGC 2506 and NGC 2420 by S.-L. Kim 1 , M.-Y. Chun1 , B.-G. Park1 , S.C. Kim2 , S.H. Lee3 , M.G. Lee2 , H.B. Ann3 , H. Sung1 , Y.-B. Jeon1 and I.-S. Yuk1 1 Korea Astronomy Observatory, Taejon, 305-348, Korea e-mail: [email protected] 2 Astronomy Program, SEES, Seoul Nat. Univ., Seoul, 151-742, Korea 3 Dep. of Earth Sciences, Pusan Nat. Univ., Pusan, 609-735, Korea Received February 9, 2001 ABSTRACT We present results of photometric observations of two old open clusters: NGC 2506 and NGC 2420. V -band time-series and UBVI absolute CCD photometric observations were carried out to search for variable stars and to investigate their physical properties. From the UBVI photometry µ = : ¦ : of NGC 2506, we obtain interstellar reddening of E ´B V 0 04 0 03 mag, distance modulus µ = : ¦ : ´ = µ = : ´V MV 0 12 5 0 1 mag, and age log t yr 9 25. From monitoring of 590 stars with 304 CCD frames taken for ten nights, three δ Sct stars and one eclipsing binary star are discovered in NGC 2506. For the other cluster, NGC 2420, we have examined light variations of 505 stars using 347 time-series data obtained for five nights, finding no variable star. It should be noted that we have found no γ Dor star among main-sequence stars near F0 in these two old open clusters, which is consistent with the suggestion that γ Dor-type phenomenon occurs = µ = : in stars younger than log ´t yr 8 4. On the other hand, it has been known that color-magnitude diagrams of these two clusters show well-established binary sequences, implying high incidence of binary systems: 20% for NGC 2506 and 50% for NGC 2420. However, only one eclipsing binary star was found in these two clusters. Key words: Stars: variables: general – open clusters and associations: individual: NGC 2506, NGC 2420 1. Introduction We have performed a long-term project of CCD photometry of open clusters. Its primary goal is to search for variable stars, in particular short-period (less than a few days) pulsating stars such as δ Sct, γ Dor, and slowly pulsating B-type stars (SPBs). These pulsating stars are recognized as important objects in studying stel- lar structure and testing evolution theory of intermediate-mass (1.3–7.0 M ¬ ) main sequence stars (see Kim 1998 for a project background). We have obtained time- series CCD data for eleven clusters (NGC 457, NGC 663, NGC 2099, NGC 2301, 50 A. A. NGC 2420, NGC 2506, NGC 2539, NGC 6633, NGC 6709, NGC 7654 and Mel 71) from October 1996 to November 2000. Four δ Sct stars in Mel 71 (Kim et al. 1999), three SPBs in NGC 7654 (Choi et al. 1999), two γ Dor candidates in NGC 2301 (Kim et al. 2001) and one δ Sct candidate in NGC 2539 (Choo et al. 2001) have been discovered. In addition, eleven new eclipsing binary stars were found in these four clusters. γ Dor stars are the main-sequence or sub-giant stars with spectral types ranging from A7 to F5, near the red edge of δ Sct instability strip in the HR diagram (Handler 1999, Kaye et al. 1999). They have pulsation periods of 0.d 3–3.d 0 and amplitudes less than 0:1 mag in the V passband. On the basis of photometric and spectroscopic observations, their light variations are considered to be attributed to excitation of high-order low-degree nonradial g-mode oscillations. They have observational properties very similar to δ Sct stars, except for the large difference in pulsation periods. δ Sct stars have short periods of 0.d 02–0.d 2 and their variability is ascribed to excitation of radial/non-radial p-mode oscillations. Several γ Dor candidates have been detected in young or intermediate age open clusters such as eight candidates in NGC 2516 (Zerbi et al. 1998), two candidates in M34 (Krisciunas and Patten 1999), two candidates in the Pleiades cluster (Martín and Rodríguez 2000) and another two candidates in NGC 2301 (Kim et al. 2001). However, they were not found in relatively old Hyades cluster (Krisciunas et al. 1995). Krisciunas and Patten (1999) suggested that γ Dor type phenomena may occur in stars younger than about 250 Myr. However, Handler (1999) noted that among known and candidate γ Dor stars several objects exist far from the ZAMS, i.e., they can have rather old ages. He proposed instead that metallicity could play a role for γ Dor type pulsation. No γ Dor stars have been reported in relatively metal-rich clusters. In this paper, we present CCD photometric results for two old open clusters, NGC 2506 and NGC 2420. Previously, Kaluzny and Shara (1988) carried out time- series CCD photometry of these clusters in order to search for long-period eclipsing binary stars, but found no variable star. There are a number of stars within γ Dor instability strip in the HR diagram of these clusters and most of probable cluster members are moderately concentrated within our field. Thus these clusters are ideal targets to investigate whether γ Dor type variability occurs in old open clusters or not. The main purpose of this paper is to search for the existence of γ Dor type variability in these old open clusters. 2. Observation and Data Reduction We performed time-series observations of two open clusters using the Bohyun- san Optical Astronomy Observatory (BOAO) 1.8 m telescope in Korea. The ob- ¼ ¼ servations were done with a TeK 1K CCD camera (field of view 5.8 ¢ 5.8) for ¼ ¼ NGC 2506 and a SITe 2K CCD camera (11.6 ¢ 11.6) for NGC 2420 (our CCD Vol. 51 51 Table1 Log of observations Target and Date of Start HJD Run Number of Mode of Observer instrument observations time CCD frames observations1 NGC 2506 Dec. 11, 1996 2450429.17 0.h4 10 TS(V) S.-L.Kim Dec. 12, 1996 430.15 2.h2 87 TS(V) S.-L.Kim BOAO Dec. 13, 1996 431.20 1.h3 35 TS(V) S.-L.Kim 1.8m Tel. Dec. 14, 1996 432.15 0.h7 21 TS(V) S.-L.Kim 1K CCD Jan. 20, 1997 469.15 0.h2 4 TS(V) S.-L.Kim Jan. 29, 1997 478.21 0.h6 15 TS(V) S.-L.Kim Feb. 1, 1997 481.01 0.h6 10 TS(V) S.-L.Kim Feb. 2, 1997 482.00 0.h2 4 TS(V) M.-Y.Chun Feb. 3, 1997 483.00 0.h7 19 TS(V)+DEEP S.-L.Kim +STD(UBVI) Mar. 5, 1997 512.93 3.h5 99 TS(V)+DEEP S.-L.Kim NGC 2420 Mar. 5, 1998 2450878.05 2.h5 57 TS(V) S.C.Kim Mar. 6, 1998 879.09 1.h8 54 TS(V) S.C.Kim BOAO Apr. 2, 1998 905.95 3.h1 110 TS(V) B.-G. Park 1.8m Tel. Apr. 3, 1998 906.97 3.h0 93 TS(V) B.-G.Park 2K CCD Apr. 6, 1998 909.98 3.h0 33 TS(V) M.-Y.Chun Feb. 15, 1999 DEEP+STD(BV) S. H. Lee 1 TS(V) – Time-series photometry (V filter), DEEP – Deep photometry, STD – absolute UBVI photometry. camera system was upgraded from 1K CCD to 2K CCD on September 1997). De- tailed observing log is given in Table 1. Typical photometric seeing (FWHM) was less than 2.¼ ¼ 0. In order to minimize position-dependent external errors (Frandsen et al. 1989), we carefully controlled the telescope pointing trying to locate stars at the same position in CCD frames during our observing run. We also carried out UBVI absolute photometry on photometric nights, in order to estimate physical parame- ters of the clusters. The Johnson UBV and Cousins I filters were used. Several tens of UBVI standard stars from the Landolt (1992) catalog were observed to transform instrumental magnitudes and colors to the standard system. Using IRAF/CCDRED package, we processed CCD images to correct overscan regions, trim unreliable subsections, subtract bias frames and correct the pixel-to- pixel inhomogeneity of quantum efficiency (flat fielding). Instrumental magnitudes were obtained from the empirical point spread function fitting photometry as re- turned by IRAF/DAOPHOT packages (Massey and Davis 1992). To obtain standard magnitudes of stars in NGC 2506 we used the same trans- formation equation as in our previous study of Mel 71 (Kim et al. 1999), which was observed on the same night as NGC 2506. The magnitude difference between 52 A. A. Fig. 1. Differences of magnitude and colors for the stars in NGC 2506 between our data and photom- etry of Marconi et al. (1997). large aperture for standard stars and small aperture for stars in the cluster was cor- rected. Four deeply exposed images for each filter were averaged to obtain more accurate photometry. We have compared our photometry of NGC 2506 with the photometry given by Marconi et al. (1997); the results are shown in Fig. 1. Our UBV data are found to be in good agreement with the previous CCD results by Marconi et al. (1997), only the I -band data show somewhat larger differences; : ¦ : ∆´ µ : ¦ : ∆´ ∆ VOUR M97 0 01 0 04 mag, B V OUR M97 0 01 0 04 mag, U µ : ¦ : ∆´ µ : ¦ : B OUR M97 0 02 0 04 mag and V I OUR M97 0 12 0 04 mag.