The Strange Case of the MetalRich Cluster NGC 6304

1
The Strange Case of the Metal-Rich Cluster NGC
6304

• Nathan De Lee (MSU)
• Horace Smith (Advisor) (MSU)
• Barton Pritzl (Macalester College)
• Marcio Catelan (PUC)
• Allen Sweigart (GSFC)
• Andy Layden (BGSU)
• D. L. Welch (McMaster)

2
A Quick Roadmap

• Techniques used in astronomy
• What are Globular Clusters (GC) and RR Lyrae
  Stars (RRL)
• The Oosterhoff Dichotomy
• Properties of metal rich GC
• NGC 6304
• Results and Conclusions

3
Photometry

• Most of the information that we can derive about
  the intrinsic state of a star comes from its
  spectra.
• Unfortunately, spectra are time consuming to get.
• Photometry uses a series of filters to derive
  major properties of a spectra more efficiently.
• By comparing the amount of flux in different
  filters, we can derive properties about a star.

4
Magnitudes and Color

• The luminosity of a star is often measured in
  magnitudes where
  represents the magnitude of the star in the V
  filter.
• Usually MV denotes absolute magnitude and V or mV
  denotes apparent magnitude.
• One way to compare two filters is to subtract
  them creating a color. For example (B-V).
• The spectrum of a star is to first order a
  blackbody emitter. Thus the color of a star is
  directly related to the temperature of a star.

5
HR Diagrams

• An Hertzsprung-Russell diagram plots both
  Luminosity and Color i.e. surface temperature of
  a star.
• Since the temperature and the luminosity of a
  star is governed by its mass and evolutionary
  state, its position in the HR diagram can be a
  powerful diagnostic tool for any given star.

6
Globular Clusters

• Globular clusters are collections of 10,000 to a
  million stars all within a very small area.
• Stars within globular clusters were born from the
  same gas cloud and are thus roughly the same age
  and composition.
• HR diagrams of a cluster provide insight into the
  evolution of stars.

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RR Lyrae Stars

• RR Lyrae stars are a type of variable star that
  changes in V magnitude between .2 and 2
  magnitudes with a period less than a day.
• They are intrinsically variable and are located
  in instability strip.

Smith (1995)
8
RR Lyrae Star Properties

• Old stars (Age gt 10 Gyr)
• Burns helium in its core for fuel through the
  triple alpha process.
• RRLs are Radially Pulsating
• A primary use of RRLs is as standard candles i.e.
  they give us a way to measure distances in the
  galaxy. They are identifiable through light
  curve shape and all have an MV ? .6
• Understanding how RRLs vary as function of
  environment will allows to more fully understand
  those environments. Example Globular Cluster
  morphology.

9
Bailey Types

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

10
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)
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Other Properties

• The Oosterhoff types are also metallicity groups.
• Metallicity is a measure of the abundance of
  metals (all elements heavier than He).
• Metallicity is often measured relative to solar
  abundance with Fe/H Log10 (Fe/H)star Log10
  (Fe/H)sun.
• Oosterhoff type I have Fe/H gt -1.7
• Oosterhoff type II have Fe/H lt -1.7

12
Oosterhoff Fe/H Dichotomy
Smith (1995)
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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.

14
Milky Way Globulars

• The Oosterhoff gap in this version of the
  Period Metallicity Graph is filled with GCs from
  the LMC.

15
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.

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2nd Parameter Problem

• 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.

17
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.

18
NGC 6388

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

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NGC 6441

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

20
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)
21
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.

22
NGC 6304

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

23
Data Sets

• Smarts Data
• B and V filter data
• Taken 2002, we got data using ANDICAM
• YALO 1-m telescope at CTIO
• .3 arcsec/pixel 10 view
• 32 nights
• Bad left side

• Andy Layden and D. L. Welchs Data
• V and I filter data
• Taken May and June 1996 using Tek2K3
• CTIO .9m
• .396 arcsec/pixel 13.2 view
• 22 nights

24
Methods of Reduction

• Peter Stetsons Daophot/Allframe method fits
  pseudogaussian point spread functions to each
  star.
• Pros Can be transformed to the standard system
  easily.
• Cons Cannot see deep into the cluster.

• C. Alards ISIS method uses image subtraction to
  identify variable stars.
• Pros Does not need to fully resolve a star to be
  able to get a light curve.
• Cons Is very sensitive to image defects and does
  not transform to the standard system easily.

25
A Color View of the Images
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Problems with Smarts Data

• There is some sort of vignetting on the left side
  of the image.
• This seems to have had serious effects on the
  zero points of our images in B and V.

27
A More Analytic View
28
A More Analytic View
29
The RRLs Near NGC 6304
30
RRab Lightcurves
Note Only appears in Layden et al. data, thus I
could not get good Fourier coeff.
31
.8 Day RRab Lightcurves
32
RRc Lightcurves
Note May Be an EV not an RRc.
33
RRc within Half-Mass Radius
34
A Question of Membership

• Although all of these RR Lyrae were found within
  the tidal radius of the cluster, it does not mean
  they are all members.
• The sky is a two-dimensional projection of three
  dimensional space, so I used four methods to
  determine membership.
• Fourier Coefficients
• Period Amplitude Diagrams
• De-reddened HR diagram position
• OGLE Variable Populations

35
Fourier Coefficients

• The shape of a lightcurve can be described by a
  discrete Fourier series.
• In my work, Ive used a cosine series up to order
  8.
• To compare different coefficients Ill use the
  following definitions

36
Fourier Coefficient Diagrams
Note The circles and triangles in Schmidts
graph are RRab. XZ Ceti and BL Boo are anomalous
Cepheids.
(Schmidt 2002)
37
Fourier Coefficient Diagrams
Note The circles and triangles in Schmidts
graph are RRab. XZ Ceti and BL Boo are anomalous
Cepheids.
(Schmidt 2002)
38
Period Amplitude Diagrams
Note The filled circles in Pritzls graph NGC
6441, Open squares M3, and Filled stars M15. The
box denotes the helium mixing scenario by
Sweigart Catelan 1998.
Note Red for RRab Blue for RRc.
(Pritzl 2001)
Note 73593 does not appear because its period is
too low.
39
Magnitudes and Colors

• Before I can make a color magnitude diagram, I
  have to be able to determine the colors and
  magnitudes of RRLs.
• I used a template fitting program (Layden 2000)
  to determine the average magnitudes and colors
  for my RRL.

40
HR Diagrams
Note Red for RRab Blue for RRc the red arrow is
the direction of a reddening vector.
41
An Issue of Reddening

• NGC 6304 lies in the direction of the galactic
  bulge, and as a result suffers from a fair amount
  of reddening.
• Reddening is caused by intervening dust clouds
  that scatter blue light.
• In particular, it looks like there is
  differential reddening, which further complicates
  this issue.
• This is a plot of average color.

42
Correcting for Reddening
Note The RRab universal color relation may not
hold for high metallicities and high period.

• The average reddening for the cluster has been
  determined by several studies to be E(B-V) .53.
• This is okay for the cluster, but I can do better
  for the RRab stars.
• RRab stars have a universal color at minimum
  light in (V-I) of 0.569 ? .012, which allows me
  to derive reddening for them (Layden).
• E(V-I) (V-I) - .569
• I can also get the extinction in V using AV
  1.938E(V-I).

43
DeReddend CM with RRab
Note Red for RRab Blue for RRc.
44
HR Diagrams with all RRLs
Note Red for RRab Blue for RRc.
45
Thoughts on the HR Diagrams

• Who are the members?
• It is not cut and dry, but there is good evidence
  from height in the CMD and color that both .8 day
  variables are members.
• As for the RRc, it is less clear.
• 9056 Very Likely (Near cluster good V mag)
• 73593 Not Likely (Bad color, probably EV)
• 11563 Possible (Far from cluster)
• 5835 Possible (Near cluster but too dim)
• 5819 Not Likely (Far from cluster and dim)

46
OGLE Population Histogram

• We can look at the overall population of RRLs in
  the bulge by looking at results from
  gravitational lensing experiments.
• Likely members are RRab within .2 magnitudes in
  V of the Red clump height and RRc within .5 mags.

Mizerski (2002)
47
A Civil Case Two Scenarios

• Type III Oosterhoff
• The period amplitude diagram is similar to the
  previous Type III clusters.
• The most likely RRab stars are .8 day ones.
• Differential reddening complicates the HR
  diagram.
• The dereddened RRabs are near the HB level and
  one RRc is very likely a member.
• .8 day RRLs are rare and there are two in the in
  the field.

• Ordinary GC
• The number of RRL in the field is small compared
  to the other Type III clusters.
• Neither of the .8 day RRab stars are within the
  half-mass radius or even close.
• Many of the RRLs must belong to the field given
  their position in the HR diagram.
• .8 day RRLs do exist in bulge fields and one of
  the .8 day RRabs may be a cepheid.

48
Where to Go From Here

• To go further with the determination
  membership, there are several routes we can take
• Try new methods for correcting for the
  differential reddening.
• Schlegels maps
• Color-color diagrams for segments of the image
• Determine radial velocities and metallicities
  from spectra for the RRLs.
• We can use Macho data to try and get better
  population statistics.

49
Summary

• The Oosterhoff groups are an important diagnostic
  for galactic formation.
• There is strong evidence that the classical
  dichotomy is not the whole story, and that a
  metal rich type III group exists.
• NGC 6304 has a very reasonable possibility of
  being an Oosterhoff type III cluster, although
  its horizontal branch is much less populated than
  either NGC 6441 or NGC 6388.

50
References

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