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Variable Stars in NGC 6304

Variable Stars in NGC 6304. Nathan De Lee (MSU) Horace Smith (Advisor) (MSU) Barton Pritzl (Macalester College) Marcio Catelan (PUC) Allen Sweigart (GSFC). Overview. Basic Properties of RR Lyrae Stars The Oosterhoff Dichotomy NGC 6388 and NGC 6441 Methods for Analysis NGC 6304 Results

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Variable Stars in NGC 6304

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  1. Variable Stars in NGC 6304 Nathan De Lee (MSU) Horace Smith (Advisor) (MSU) Barton Pritzl (Macalester College) Marcio Catelan (PUC) Allen Sweigart (GSFC)

  2. Overview • Basic Properties of RR Lyrae Stars • The Oosterhoff Dichotomy • NGC 6388 and NGC 6441 • Methods for Analysis • NGC 6304 Results • Where to Go From Here Michigan State University

  3. RR Lyrae Variables • Population II stars (Age > 10 Gyr) • Horizontal Branch Stars • Radially Pulsating • Standard Candles MV .6 (Smith 1995) Michigan State University

  4. Bailey Types • Based on light curve shape • RRab Fundamental Mode • RRc First Overtone Michigan State University

  5. The Oosterhoff Dichotomy • In 1939, Oosterhoff noticed a division in GC RR stars. OOI OOII <Pab> .55d .64d <Pc> .32d .37d NRRc/Ntotal .17 .44 (Oosterhoff 1939) Michigan State University

  6. Other Properties • The Oosterhoff types are also metallicity groups. • Oosterhoff type I have [Fe/H] > -1.7 • Oosterhoff type II have [Fe/H] < -1.7 Michigan State University

  7. [Fe/H] Dichotomy (Smith 1995) Michigan State University

  8. Do we fully understand the Oosterhoff Groups? There are a several issues that have appeared in the story of the Oosterhoff groups. First, the Oosterhoff dichotomy may be particular to the Milky Way. Michigan State University

  9. Milky Way GC The Oosterhoff gap in this version of the Period Metallicity Graph is filled with GCs from the LMC. (Catelan & Prizl 2004) Michigan State University

  10. Other Issues • In general, metal rich GCs should have a stubby red clump that doesn’t cross the instability strip. • Hence, few to no RR Lyrae stars. • It appears, however, that some metal rich GCs have extended HB. Michigan State University

  11. 2nd Parameter Problem in Metal Rich GCs • The existence of these GCs suggests that something beyond metallicity affects the morphology of the HB. • One set of possibilities involve helium enrichment (Sweigart & Catelan 1998) through various mechanisms. • This leads to brighter HB and thus longer RR Lyrae Periods. Michigan State University

  12. NGC 6388 and 6441 • NGC 6388 and 6441 are metal rich GCs that have extended HB that cross the instability strip. • Thus, they have significant numbers of RR Lyrae stars. Michigan State University

  13. NGC 6388 • [Fe/H] = -.60 ± .15 • Total RRL = 14 • <Pab> = .71d • <Pc> = .36d • Nc/NTotal = .57 Values from Pritzl et al. 2002 Michigan State University

  14. NGC 6441 • [Fe/H] = -.53 ± .11 • Total RRL = 38 • <Pab> = .759d • <Pc> = .375d • Nc/NTotal = .33 Values from Pritzl et al. 2003 Michigan State University

  15. The Big Picture • Both NGC 6388 and 6441 represent deviations from the Oosterhoff Dichotomy. • Metal rich and long average periods. • Contain RRab stars with periods .8d (Catelan 2003) Michigan State University

  16. Why NGC 6304? • NGC 6304 is very metal rich [Fe/H] = -.59 (Zinn &West 1984). • Several Previous Studies have found some RR Lyrae stars near NGC 6304. Rosino 1962, Terzan 1966, 1968, Hesser & Hartwick 1976, Hartwick, Barlow & Hesser 1981). • More recent studies (Valenti et al. 2003) have found new variables. Michigan State University

  17. Hartwick, Barlow & Hesser Found 31 RR Lyrae stars near and around NGC 6304 Although all of these were within the tidal radius, most were probably not members. Valenti et al. Found 4 RRab stars and 2 RRc that she called likely cluster members. One of the RRab stars has the high period of .856d Previous Work Michigan State University

  18. NGC 6304 • [Fe/H] = -.59 • Total RRL = ? • <Pab> = ? • <Pc> = ? • Nc/NTotal = ? In 2002, we got data using ANDICAM on the YALO 1-m telescope at CTIO Michigan State University

  19. A Tale of Two Methods • To reduce this data, I will use two methods. • Peter Stetson’s Daophot/Allframe method fits pseudogaussian point spread functions to each star. • C. Alard’s ISIS method uses image subtraction to identify variable stars. Michigan State University

  20. ISIS 2.1 • In image subtraction, variable stars do not need to be fully resolved in order to find them. • This allows us to get much closer into the center of a GC. Michigan State University

  21. Allframe and ISIS ISIS Allframe Period .394d .338d Michigan State University

  22. CM-Diagram Using Allframe Michigan State University

  23. A Visual Picture • The circle shows which stars were included in the CM Diagram. • The numbered stars are the RR Lyrae stars. Michigan State University

  24. RR Lyrae Light Curves RR 11685 P = .467d RR 5835 P=.394d RR 9056 P=.338d Michigan State University

  25. More Light Curves LPV 7980 LPV 62966 RR 11563 P = .270 Michigan State University

  26. Light Curves for RR 1932 b P = .812 v Michigan State University

  27. Case For Same V level as HB Within tidal radius of GC B and V are only averaged over 4 nights each Case Against Too Blue, at least in preliminary CM It is physically distant from the GC RR 1932 Cluster Membership Michigan State University

  28. Where to Go From Here? • Recalibrate the variables on the CM diagram. • Use deeper variability cuts to find more variables. • Use Image Subtraction to try to get closer into the core of NGC 6304. Michigan State University

  29. References Alard, C. 2000, AA&S, 144, 363 Alard, C. & Lupton, R. H. 1998, ApJ, v. 503, p. 325 Freedman, W. L., et al. 2001, ApJ, 553, 47 Catelan, M 2003, astro-ph/0310159 Hartwick, F. D. A., Barlow D. J., & Hesser, J. E. 1981, AJ, 86, 1044 Hesser, J. E. & Hartwick, F. D. A. 1976, ApJ, 203, 113 Layden, A. C., Ritter, L.A., Welch, D. L., & Webb, T. M. A. 1999, AJ, 117, 1313 Pritzl B., Smith, H. A., Catelan, M, & Swigart, A. V. 2000, ApJ, 530, L41 Pritzl B., Smith, H. A., Catelan, M., & Swigart, A.V. 2001, AJ, 122, 2600 Pritzl B., Smith, H. A., Catelan, M., & Swigart, A.V. 2002, AJ, 124, 949 Pritzl B., Smith, H. A., Stetson, P. B., Catelan, M., Swigart A. V., Layden, A. C., & Rich, R. M. 2003, AJ, 126, 1381 Rosino, Asiago Contr 132 1962 Terzan, Publications de l'Observatoire de Haute-Provence, v. 9, no 1 1966 Terzan, Publications de l'Observatoire de Haute-Provence, v. 9, no 24 1968 Silbermann, N. A., Smith, H. A., Bolte, M., & Hazen, M. L. 1994, AJ, 107, 1764 Smith, H. A. RR Lyrae Stars, Cambrigde University Press, 1995 Stetson, P. B. 1987, PASP, 99, 191 Stetson, P. B. 1994, PASP, 106, 250 Stetson, P. B., et al. 1998, ApJ, 508, 491 Sweigart, A. V. & Catelan, M. 1998, ApJ, 501, L63 Valenti, E., Bellazzini, M., & Cacciari, C. 2003, in ASP Conf. Ser., 296, 404 Zinn, R. & West, M. J. 1984, ApJ, 55, 45 Michigan State University

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