Search for Variable Stars in Two Old Open Clusters: NGC 2506 and NGC 2420

ACTA ASTRONOMICA Vol. 51 (2001) pp. 49–63

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 parameters 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 photometry 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;

: ¦ : ∆ : ¦ : ∆ ∆

VOUR M97 0 01 0 04 mag, B V OUR M97 0 01 0 04 mag, U

: ¦ : ∆ : ¦ :

B OUR M97 0 02 0 04 mag and V I OUR M97 0 12 0 04 mag. Twa-

rog et al. (1999) also noted an offset of about 0.1 mag in (V I ) color when com- paring (V I ) vs. (B V ) relation among several open clusters. Therefore, the differences might be caused by zero point error in Marconi et al. (1997) I -band data. The UBVI photometric procedures and results for NGC 2420 will be given elsewhere. Vol. 51 53

We applied the ensemble normalization technique (Gilliland and Brown 1988) in order to normalize instrumental magnitudes between V time-series CCD frames. Normalization equation with color and position dependent correction terms was

used. Detailed normalizing process was described by Kim et al. (1999, 2001). :

Bright stars near V 14 0 mag have signal-to-noise ratio large enough to detect : light variations as small as about 0.01 mag. Fainter stars, V 18 0 mag, have somewhat larger photometric errors, about 0.1 mag, what is sufﬁcient, however, to search for eclipsing binary stars.

3. Results for NGC 2506

3.1. Physical Parameters Positions of the observed 590 stars in a central ﬁeld of NGC 2506 are drawn in Fig. 2. In Fig. 3 we show color-color and color-magnitude diagrams for the cluster, using the UBVI CCD photometric data. In order to avoid ﬁeld star con- tamination and obtain more accurate physical parameters of the cluster we adopted the membership information from the proper motion study by Chiu and van Altena (1981). Stars with a membership probability (Pµ ) greater than 0.7 were assumed to

be probable cluster members.

¼ ¼

Fig. 2. Observed CCD ﬁeld (5.8 ¢ 5.8) of the open cluster NGC 2506. Four variable stars discovered in this study are represented by open circles. North is up and East is to the left. Scale is 0.¼ ¼ 68/pixel. 54 A. A.

Fig. 3. Color-color and color-magnitude diagrams of the open cluster NGC 2506. The thick lines

represent the adopted empirical ZAMS by Sung and Bessell (1999) for solar metal abundance. The

= µ = : = :

thin lines indicate the theoretical isochrone for log ´t yr 9 25 and Z 0 008 (Girardi et al. 2000). : Large open circles: probable cluster members ( Pµ 0 7); crosses: non-members, deduced from the membership probability by Chiu and van Altena (1981).

The best ﬁt of the empirical ZAMS in the color-color diagram is obtained

: ¦ :

with an interstellar reddening E B V 0 04 0 03 mag, if we adopt E U

= : : ¦ :

B E B V 0 72. The reddening value of E V I 0 05 0 03 mag is

= :

derived adopting E V I E B V 1 25 (Dean et al. 1978). We obtained

: ¦ :

a distance modulus V MV 0 12 5 0 1 mag from the ZAMS ﬁtting on the :

color-magnitude diagrams and assuming RV 3 1. The age of NGC 2506 de-

= :

rived from the isochrone ﬁtting is log t yr 9 25. Observed metallicity of the

= ℄ :

cluster is Fe H 0 55 dex (McClure et al. 1981, from UBV photometry) or

= ℄ : Fe H 0 38 dex (Twarog et al. 1997, from DDO photometry). Thus we made

Vol. 51 55

: = ℄ : use of the theoretical isochrones with Z 0 008 ( Fe H 0 40 dex) from mod- els with convective overshooting by Girardi et al. (2000). Physical parameters obtained in this study are consistent with the previous results (Table 2).

Table2

Physical parameters of NGC 2506

µ Reference E ´B V V0 MV log Age McClure et al. (1981) 0.05 12.15 9.53 Marconi et al. (1997) 0.05 12.5 9.23 Twarog et al. (1999) 0.04 12.58 9.3 This study 0.04 12.5 9.25

3.2. Variable Stars We examined light variations of 590 stars in the observed field using 304 time- series CCD frames. Four new variable stars were discovered. Their positions in Fig. 2 are marked as V1, V2, V3 and V4. Light variations of the variable stars are shown in Fig. 4. It is obvious that the brightness of the three variable stars (V1, V2 and V3) changes periodically. Their light curves indicate that they are short-period and low-amplitude pulsating stars. Brightness of V4 decreases by about 0.1 mag near HJD 2450431.21 and 0.15 mag near HJD 2450512.95, and remains constant at other time, implying that V4 might be a long-period Algol-type (detached) eclipsing binary star. Kaluzny and Shara (1988) also monitored stars of the cluster in wider area than our observations but could not find any variable star, probably due to their limited data set (only nine exposures for three nights). Using Fourier analysis (Kim and Lee 1996) we estimated pulsation periods of three pulsating stars. Power spectra are displayed in Fig. 5. One dominant frequency exists for each star: 14.742 c/d (0.d 0678) for V1, 10.854 c/d (0.d 0921) for V2 and 12.264 c/d (0.d 0815) for V3. Phase diagrams of the stars are shown in Fig. 6. The objects are located within δ Sct instability strip in the color-magnitude diagram as shown in Fig. 7. Considering their positions in the color-magnitude diagram, character of their light curves and pulsating frequencies, we can classify them as δ Sct stars. It should be noted that V1, a probable cluster member as deduced from the proper motion (Chiu and van Altena 1981), is located at a bluer and brighter region than the main-sequence turn-off point of the cluster, i.e., it might be identified as a pulsating blue straggler. Pulsating blue stragglers have been also discovered in other old open clusters (e.g., two stars in M67, Gilliland and Brown 1992). In Fig. 8 we displayed the empirical period-luminosity relation for δ Sct stars 56 A. A.

Fig. 4. Light variations of four variable stars in the open cluster NGC 2506. It is obvious that the brightness of three variable stars, V1, V2 and V3, changes periodically. That of V4 decreases by about 0.1 mag near HJD 2450431.21 and 0.15 mag near HJD 2450512.95 and remains constant at other time.

(Fernie 1992) and observational data (López de Coca et al. 1990). The positions of two δ Sct stars (V1 and V2) – probable cluster members – are consistent with

the relation. We calculated absolute magnitudes of the two stars using the distance :

modulus and interstellar reddening of the cluster, obtaining MV 1 07 mag for V1 : and MV 1 88 mag for V2. From Fig. 8, small amplitude variable star V1 can be identiﬁed as 3H-mode pulsator, and larger amplitude V2 – as F-mode one. How- ever, the pulsating modes might be uncertain because we could not estimate their physical parameters (effective temperature, gravity, etc.), required to determine pulsation constants and modes. Observational parameters of four new variable stars we derived are summarized in Table 3. Vol. 51 57

Fig. 5. Power spectra of three new pulsating stars. The spectral window is shown in the top panel. Dominant frequencies for each variable star are 14.742 c/d for V1, 10.854 c/d for V2 and 12.264 c/d for V3, respectively. 58 A. A.

Fig. 6. Phase diagrams of three pulsating stars (V1, V2 and V3). Data points are differently marked for each observing night. Typical observation errors are represented by error bars. Vol. 51 59

Fig. 7. Positions of four variable stars in the color-magnitude diagram of NGC 2506. A thin line denotes empirical ZAMS (Sung and Bessell 1999). Thick solid and dashed bars represent δ Sct instability strip (Breger 1979) and γ Dor instability strip (Handler 1999), respectively. Three pulsating stars are marked by star symbols and one eclipsing binary star by ﬁlled square.

Table3 Observational parameters of four new variable stars

1 2 ∆ 1 ID VB V Period Epoch VPµ Type [mag] [mag] [d] [mag] δ V1 5462 13.69 0.21 0.0678 431.242 0.03 0.71 Sct star δ V2 5467 14.50 0.29 0.0921 430.209 0.17 0.91 Sct star δ

V3 5589 14.75 0.35 0.0815 513.082 0.11 0.00 Sct star

V4 5474 14.72 0.34 0.16 0.95 eclipsing binary 1: Identiﬁcation and membership probability (Mermilliod 1992)

2 : : Epoch at maximum brightness (HJD. 2450000 0). 60 A. A.

Fig. 8. Empirical period-luminosity relation for δ Sct stars. The solid and dashed lines represent the empirical P-L relations by Fernie (1992) for the fundamental radial mode (F-mode) and the ﬁrst overtone mode (1H-mode), respectively. We also plot the observational data using the δ Sct star catalog (López de Coca et al. 1990); ﬁlled circles for F-mode, asterisks for 1H-mode, open squares for 2H-mode and open triangles for 3H-mode. Two δ Sct stars (V1 and V2) are marked by large star symbols.

4. Results for NGC 2420

Location of 505 stars observed in NGC 2420 is shown in Fig. 9. Color- magnitude diagram of the cluster is given in Fig. 10. Detailed UBVI CCD photometric results such as physical parameters, luminosity function, initial mass function and dynamical evolution will be given elsewhere. A total of 347 time-series CCD frames were collected for ﬁve nights. We examined carefully their light variations, especially for stars near the red boundary of δ Sct instability strip (i.e., within γ Dor instability strip). However, we found no variable star. Kaluzny and Shara

(1988) also monitored this cluster for three nights (about 3.5 hours for each night) : but could not detect any variability in 357 stars with V 18 5 mag.

Vol. 51 61

¼ ¼

Fig. 9. Observed CCD ﬁeld (11.6 ¢ 11.6) of the open cluster NGC 2420. North is up and East is to the left. We monitored 505 stars in the central region of the cluster. The pixel scale is 0.¼ ¼ 34/pixel.

5. Conclusion

We performed time-series CCD photometry of two old open clusters to search for new variable stars. Three δ Sct stars and one eclipsing binary star were discovered in NGC 2506. No variable stars were found in NGC 2420. As can be seen from Fig. 7 (NGC 2506) and Fig. 10 (NGC 2420), there are a number of stars within γ Dor instability strip. However, we could not detect any γ Dor type variability in these two clusters. Our results support Krisciunas and

Patten (1999) suggestion that γ Dor-type phenomenon may occur in stars younger : than logt 8 4 (250 Myr). Well established binary sequences in color-magnitude diagrams of both clus-

ters suggested existence of numerous binaries among cluster members ( 20% for

NGC 2506, Marconi et al. 1997, and 50% for NGC 2420, Anthony-Twarog et al. 1990). However, only one eclipsing binary star was found. The incidence of eclipsing binary stars in these two clusters seems to be very low as compared to other old open clusters: 9 eclipsing binary stars in NGC 7789 (Jahn et al. 1995), 12 stars in Be 39 (Kaluzny et al. 1993) and 44 stars in Cr 261 (Mazur et al. 1995). As a possible reason of this difference we suggest that binary stars in these clusters 62 A. A.

Fig. 10. Color-magnitude diagram of observed 505 stars in the cluster NGC 2420. A thin line

denotes the empirical ZAMS (Sung and Bessell 1999). Thick dashed bars represent γ Dor instability

µ = :

strip (Handler 1999). We adopted interstellar reddening E ´B V 0 05 mag and distance modulus

µ = : ´V MV 0 11 9 mag of the cluster, estimated by Lee et al. (1999). might have low orbital inclination to the line of sight. Since this kind of ordering of orbital axes is very important in a problem of dynamical evolution of open clusters, further spectroscopic observations of binary star candidates (i.e., stars within binary sequence) would be required.

Acknowledgements. This work has been performed as a part of ”BOAO photometric survey of the Galactic open clusters and time-series observations”.

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