Variable Stars in NGC 6304

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

3
RR Lyrae Variables

• Population II stars (Age gt 10 Gyr)
• Horizontal Branch Stars
• Radially Pulsating
• Standard Candles MV ? .6

(Smith 1995)
4
Bailey Types

• Based on light curve shape
• RRab Fundamental Mode
• RRc First Overtone

5
The Oosterhoff Dichotomy

• In 1939, Oosterhoff noticed a division in GC RR
  stars.
• OOI OOII
• ltPabgt .55d .64d
• ltPcgt .32d .37d
• NRRc/Ntotal .17 .44

(Oosterhoff 1939)
6
Other Properties

• The Oosterhoff types are also metallicity groups.
• Oosterhoff type I have Fe/H gt -1.7
• Oosterhoff type II have Fe/H lt -1.7

7
Fe/H Dichotomy
(Smith 1995)
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.

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)
10
Other Issues

• In general, metal rich GCs should have a stubby
  red clump that doesnt cross the instability
  strip.
• Hence, few to no RR Lyrae stars.
• It appears, however, that some metal rich GCs
  have extended HB.

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.

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.

13
NGC 6388

• Fe/H -.60 .15
• Total RRL 14
• ltPabgt .71d
• ltPcgt .36d
• Nc/NTotal .57
• Values from Pritzl et al. 2002

14
NGC 6441

• Fe/H -.53 .11
• Total RRL 38
• ltPabgt .759d
• ltPcgt .375d
• Nc/NTotal .33
• Values from Pritzl et al. 2003

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)
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.

17
Previous Work

• 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

18
NGC 6304

• Fe/H -.59
• Total RRL ?
• ltPabgt ?
• ltPcgt ?
• Nc/NTotal ?

In 2002, we got data using ANDICAM on the YALO
1-m telescope at CTIO
19
A Tale of Two Methods

• To reduce this data, I will use two methods.
• Peter Stetsons Daophot/Allframe method fits
  pseudogaussian point spread functions to each
  star.
• C. Alards ISIS method uses image subtraction to
  identify variable stars.

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.

21
Allframe and ISIS
ISIS
Allframe
Period
.394d
.338d
22
CM-Diagram Using Allframe
23
A Visual Picture

• The circle shows which stars were included in the
  CM Diagram.
• The numbered stars are the RR Lyrae stars.

24
RR Lyrae Light Curves
RR 11685 P .467d
RR 9056 P.338d
RR 5835 P.394d
25
More Light Curves
LPV 7980
LPV 62966
RR 11563 P .270
26
Light Curves for RR 1932
b
P .812
v
27
RR 1932 Cluster Membership

• 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

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.

29
References

• Alard, C. 2000, AAS, 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