Variable Stars in the Globular Cluster NGC 6496

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

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

Motivation Behind Research n n n 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

Why NGC 6496? n n Metal Rich; ([Fe/H])= -0. 64 Listed as having no known variables (Clement et al. 2001) n The only study done was photographic (Fourcade & Laborde 1963) n Located towards the bulge, at a galactic longitude of 348° and a latitude of -10° (Harris 1996) NGC 6496 Dipartimento di Astronomia, Vicolo dell'Osservatorio http: //dipastro. pd. astro. it/globulars/databases/ground/onebyone/NGC 6496. html

Observations and Reductions n n n 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

Color Magnitude Diagrams

Variable Star Detection n n 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

Variable Star Detection (cont. ) n n 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)

Membership n n n 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

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

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

QUESTIONS?