Variable Stars in the Globular Cluster NGC 6496

1. Variable Stars in the Globular Cluster NGC 6496 Katherine Guldenschuh & Andrew Layden (Bowling Green State University) Douglas Welch (McMaster University), and Tracy Webb (Leiden Observatory) Presented by Katherine Guldenschuh April 3, 2004

2. Overview • Motivation for Research • Why NGC 6494 • Observations and Reductions • Color Magnitude Diagrams • Variable Star Detection • Membership • Conclusion

3. Motivation Behind Research • Metal rich clusters ([Fe/H]>-0.8) generally have cool red giant and horizontal branches • In contrast, the metal rich clusters NGC 6388 and NGC 6441 were found to have extended blue horizontal branches with a large number of RRLs (Pritzl et al. 2001, 2002; Layden et al. 1999) • We want to find other metal rich clusters exhibiting the same unusual behavior

4. Why NGC 6496? • Metal Rich; ([Fe/H])= -0.64 • Listed as having no known variables (Clement et al. 2001) • The only study done was photographic (Fourcade & Laborde 1963) • Located towards the bulge, at a galactic longitude of 348° and a latitude of -10° (Harris 1996) NGC 6496 Dipartimento di Astronomia, Vicolo dell'Osservatoriohttp://dipastro.pd.astro.it/globulars/databases/ground/onebyone/NGC6496.html

5. Observations and Reductions • Images were taken with the direct CCD camera on the 0.9-m telescope at Cerro Tololo Inter-American Observatory in May and June of 1996 • The image-processed frames were reduced using Stetson's (1987,1994) DAOPHOT and ALLFRAME photometry software • Standard procedures for point-spread function generation, iterative star detection, and photometry combination were followed

6. Color Magnitude Diagrams

8. Variable Star Detection • We selected variable star candidates from our ALLFRAME photometry lists based on high values of the variability index calculated by DAOMASTER (Stetson 1994) • We then plotted each candidate’s magnitude as a function of time

11. Variable Star Detection (cont.) • Several of the variable star candidates exhibited significant variations of shorter time-scales • We fitted their magnitude-time data with a sequence of periods and light-curve shaped templates(Layden & Sarajedini 2000)

15. Membership • To determine the likelihood of a variable stars’ cluster membership we considered each star’s color, magnitude, and XY position • If the color or magnitude deviate significantly from the expected values, the stars are unlikely to be cluster members • Also, there is an inverse relationship between a star’s distance from the cluster center and the likelihood of cluster membership

16. Conclusions • There does not appear to be an extended blue horizontal branch present in this cluster • We were able to discover four or more LPVs along the red giant branch in this cluster and also a possible RRL belonging to this cluster • The other SPVs detected during this search most likely are not cluster members • Our study of NGC 6496 has helped improve the variable star inventory of metal rich globular clusters

17. References Clement et al. 2001, AJ, 82, 2587 Fourcade & Laborde 1963, BAAA, 6, 111 Harris 1996, AJ, 112, 148 Layden, Ritter, Welch & Webb, 1999, AJ, 117, 1313 Pritzl et al. 2003, AJ, 126, 1381 Stetson 1987, 1994, PASP, 99, 191, 106, 250

18. QUESTIONS?