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/Hgt-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'Osservatorio http//dipastro.pd.astro.it/glob
ulars/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
7
(No Transcript)
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 candidates magnitude as a
  function of time

9
(No Transcript)
10
(No Transcript)
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)

12
(No Transcript)
13
(No Transcript)
14
(No Transcript)
15
Membership

• To determine the likelihood of a variable stars
  cluster membership we considered each stars
  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
  stars 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?