RR Lyrae stars in the southern globular cluster NGC 362

• Published in: Astronomy and astrophysics (Berlin) Vol. 463; no. 2; pp. 589 - 600
• Main Authors: Székely, P., Kiss, L. L., Jackson, R., Derekas, A., Csák, B., Szatmáry, K.
• Format: Journal Article
• Language: English
• Published: Les Ulis EDP Sciences 01.02.2007
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; globular clusters: individual: NGC 362; stars: variables: general; stars: variables: RR Lyr; Eclipsing binary stars; stars: variables: general; Variability; Light curves; Luminosity; Phase analysis; Pulsating variable stars; Small Magellanic Cloud; Stellar magnitude; Classification; Color-magnitude diagrams; stars: variables: RR Lyr; Radial velocity; Age; Photometry; Optical spectrum; Photometric observation; globular clusters: individual: NGC 362; Isochrone; Cepheids; Absolute magnitude; Time series; Abundance; Fourier analysis; Globular clusters; RR Lyrae stars; Metallicity; Physical parameter; Positions
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361:20066242

Aims.NGC 362 is a bright southern globular cluster in the foreground of the Small Magellanic Cloud (SMC), for which no extensive variability survey has ever been done. Finding regularly pulsating RR Lyrae stars in the cluster can lead to improved metallicity and distance estimates of the system, while other types of variable objects may be used to confirm the results. Methods.Time-series CCD photometric observations have been obtained. Light curves have been derived with both profile fitting photometry and image subtraction. We developed a simple method to convert flux phase curves to magnitudes, which allows the use of empirical light curve shape vs. physical parameters calibrations. Periods and light curve parameters of the detected variable stars have been determined with Fourier analysis, phase dispersion minimization and string-length minimization. Using the RR Lyrae metallicity and luminosity calibrations, we have determined the relative iron abundances and absolute magnitudes of the stars. The color–magnitude diagram has been fitted with Yale-Yonsei isochrones to determine reddening and distance independently. For five RR Lyrae stars we obtained radial velocity measurements from optical spectra. Results.We found 45 RR Lyr stars, of which the majority are new discoveries. While most of them are cluster members, as shown by their radial velocities and positions in the color–magnitude diagram, we also see a few stars in the galactic field and in the outskirts of the SMC. About half of the RR Lyraes exhibit light curve changes (Blazhko effect). The RR Lyrae-based metallicity of the cluster is ${\rm [Fe/H]}=-1.16\pm0.25$, the mean absolute magnitude of the RR Lyrae stars is $M_{\rm V}=0.82\pm0.04$ mag implying a distance of $7.9\pm0.6$ kpc. The mean period of RRab stars is $0.585\pm0.081$ days. These properties place NGC 362 among the Oosterhoff type I globular clusters. The isochrone fit implies a slightly larger distance of $9.2\pm0.5$ kpc and an age of $11\pm1$ Gyr. We also found 11 eclipsing binaries, 14 pulsating stars of other types, including classical Cepheids in the SMC and 15 variable stars with no firm classification. Conclusions.NGC 362 hosts a large number of RR Lyrae stars, which makes the cluster a potentially important test object for studying the Blazhko effect in a chemically homogeneous environment.