The Globular Cluster NGC5285 Color-Magnitude Diagram and Variable Stars

M. Zorotovic, M. Catelan (PUC), H. A. Smith (MSU), B. J. Pritzl (Macalester),A. Layden, J. Gregorsok (BGSU), D. L. Welch (McMaster), T. Webb (Leiden)

AbstractWe present BV photometry and the results of a search for stellar variability in the globular cluster NGC5286. We found 57 variable stars, 31 of which being new discoveries.

M69 is a fairly metal-rich globular cluster ([Fe/H]= -0.71) that is considered part of the bulge population of GCs. Although it has a red HB, some recent studies have suggested a blue extension on its HB. This means that it may have an unknown RR Lyrae star population.

Fig. 1: Color-magnitude diagram for M69

Fig. 2: Comparison between M69 CMD and the ridgeline of 47 Tucanae. Note the cleaner CMD than Fig. 1.

Fig. 3: V-band light curves for three of the 10 newly discovered eclipsing variable stars toward M69.

Our work is based on B,V images collected with the Warsaw 1.3m telescope in Las Campanas over a one-week run in April 2003. The images were taken using the 8kCCD camera, a 8-chip array. Here we present the results of the first three chips, covering a field of 27’x17’. The photometry was performed using DAOPHOT II/ALLFRAME (Stetson 1987) and the variability search was made with ISIS 2.2 (Alard 2000) and the task TRIAL of ALLFRAME.

Color-Magnitude Diagram: Due to the location of M69 near the Galactic center, its Color-Magnitude Diagram (CMD) shows severe contamination by field stars (Fig. 1).

Variable Stars: We detected 61 variable stars, 8 of which had already been detected and/or catalogued. Between the 53 newly discovered variable stars we have 8 RR Lyrae (6 RRab and 2 RRc) (Fig. 4), 4 SX Phe (Fig. 5), 10 eclipsing variables (Fig. 3), and 15 LPV candidates.

We can see a deep CMD, but the field star contamination prevents us from seeing the characteristic sequences of a GC CMD. Making a star selection by distance from the cluster center and by photometric error, we obtain a cleaner CMD (Figs. 2 and 6). We can see a deep CMD that shows a stubby red HB. This CMD is not clear enough to confirm or discard a small blue HB extension. Since M69 and 47 Tucanae ([Fe/H] = -0.76) share similar CMD morphology and metalicity, we compared our calibrated photometry with 47 Tuc’s ridgeline as published by Hesser et al. (1987) (Fig. 2). The reasonable match between the M69 data and the 47 Tuc ridgeline confirms that the clusters have similar metallicities and ages.

We located the variable stars in the M69 CMD using the ALLFRAME magnitudes for 60 of the 61 variable star candidates (Fig. 6). This magnitude is not the most accurate one but still represents a reasonable first approximation. The location of the RR Lyrae stars is not at the HB level. We have one star (top curve in Fig. 4) that is located slightly brighter than the HB, reminding us of the case of V9 in 47 Tuc and of the bright RR Lyrae stars in NGC 6388/NGC 6441. In the case of the other two RRab presented in Fig. 4, they are located below the HB level, and in only one of these cases (middle curve) does the star fall within the published M69 tidal radius.

The classification of the remainder of the variable star candidates is still in progress.

Fig. 5: B-band light curves for three of the 4 newly discovered SX Phe stars in M69.

Fig. 4: V-band light curves for three of the 8 newly discovered RR Lyrae stars toward M69.

Acknowledgments: MEE and MC were supported by Proyecto Fondecyt Regular #1030954; HAS by NSF grant AST 02-05813; and BJP by a CAREER award AST 99-84073.

Fig. 6: M69 CMD indicating the location of 60 variable stars detected.