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

OverviewOverview

•Motivation for Research

•Why NGC 6494

•Observations and Reductions

•Color Magnitude Diagrams

•Variable Star Detection

•Membership

•Conclusion

Motivation Behind ResearchMotivation Behind Research

Metal rich clusters ([Fe/H]>-0.8) generally have cool red Metal rich clusters ([Fe/H]>-0.8) generally have cool red giant and horizontal branchesgiant and horizontal branches

In contrast, the metal rich clusters NGC 6388 and NGC 6441 In contrast, the metal rich clusters NGC 6388 and NGC 6441 were found to have extended blue horizontal branches with were found to have extended blue horizontal branches with a large number of RRLs (a large number of RRLs (Pritzl et al. 2001, 2002; Layden et al. 1999)Pritzl et al. 2001, 2002; Layden et al. 1999)

We want to find other metal rich clusters exhibiting the We want to find other metal rich clusters exhibiting the same unusual behaviorsame unusual behavior

Why NGC 6496?Why NGC 6496? Metal Rich; Metal Rich;

([Fe/H])= -0.64([Fe/H])= -0.64 Listed as having no Listed as having no

known variables known variables (Clement et al. 2001)(Clement et al. 2001)

The only study done was The only study done was photographic photographic (Fourcade & Laborde 1963)(Fourcade & Laborde 1963)

Located towards the Located towards the bulge, at a galactic bulge, at a galactic longitude of 348longitude of 348° and a ° and a latitude of -10° latitude of -10° (Harris 1996)(Harris 1996)

NGC 6496

Dipartimento di Astronomia, Vicolo dell'Osservatorio http://dipastro.pd.astro.it/globulars/databases/ground/onebyone/NGC6496.html

Observations and Observations and ReductionsReductions

Images were taken with the direct CCD camera Images were taken with the direct CCD camera on the 0.9-m telescope at Cerro Tololo Inter-on the 0.9-m telescope at Cerro Tololo Inter-American Observatory in May and June of 1996American Observatory in May and June of 1996

The image-processed frames were reduced using The image-processed frames were reduced using Stetson's (1987,1994) DAOPHOT and ALLFRAME Stetson's (1987,1994) DAOPHOT and ALLFRAME photometry software photometry software

Standard procedures for point-spread function Standard procedures for point-spread function generation, iterative star detection, and generation, iterative star detection, and photometry combination were followedphotometry combination were followed

Color Magnitude DiagramsColor Magnitude Diagrams

Variable Star DetectionVariable Star Detection

We selected variable star candidates from our We selected variable star candidates from our ALLFRAME photometry lists based on high values ALLFRAME photometry lists based on high values of the variability index calculated by DAOMASTER of the variability index calculated by DAOMASTER (Stetson 1994)(Stetson 1994)

We then plotted each candidate’s magnitude as a We then plotted each candidate’s magnitude as a function of time function of time

Variable Star Detection Variable Star Detection (cont.)(cont.)

Several of the variable star candidates exhibited Several of the variable star candidates exhibited significant variations of shorter time-scalessignificant variations of shorter time-scales

We fitted their magnitude-time data with a We fitted their magnitude-time data with a sequence of periods and light-curve shaped sequence of periods and light-curve shaped

templatestemplates (Layden & Sarajedini 2000)(Layden & Sarajedini 2000)

MembershipMembership

To determine the likelihood of a variable stars’ To determine the likelihood of a variable stars’ cluster membership we considered each star’s cluster membership we considered each star’s color, magnitude, and XY positioncolor, magnitude, and XY position

If the color or magnitude deviate significantly If the color or magnitude deviate significantly from the expected values, the stars are unlikely from the expected values, the stars are unlikely to be cluster members to be cluster members

Also, there is an inverse relationship between a Also, there is an inverse relationship between a star’s distance from the cluster center and the star’s distance from the cluster center and the likelihood of cluster membershiplikelihood of cluster membership

ConclusionsConclusionsThere 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

ReferencesReferences

Clement et al. 2001, AJ, 82, 2587Clement et al. 2001, AJ, 82, 2587

Fourcade & Laborde 1963, BAAA, 6, 111Fourcade & Laborde 1963, BAAA, 6, 111

Harris 1996, AJ, 112, 148Harris 1996, AJ, 112, 148

Layden, Ritter, Welch & Webb, 1999, AJ, 117, 1313Layden, Ritter, Welch & Webb, 1999, AJ, 117, 1313

Pritzl et al. 2003, AJ, 126, 1381Pritzl et al. 2003, AJ, 126, 1381

Stetson 1987, 1994, PASP, 99, 191, 106, 250Stetson 1987, 1994, PASP, 99, 191, 106, 250

QUESTIONS?QUESTIONS?