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?