Using a small telescope to detect variable stars in Globular Cluster NGC 6779

1
Using a small telescope todetect variable stars
in Globular Cluster NGC 6779
AAVSO Spring Meeting 26 March 2005

• Jerry D. Horne
• San Jose, California USA

2
Contents

• Abstract
• Known variables in the cluster
• The Research Study
• The Equipment
• The Observations
• Data Reduction
• Variable Detection Methods
• Detected Variables
• Color-Magnitude Diagrams
• Conclusions

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Abstract

• The nominal feasibility of utilizing small
  instruments, in near-urban environments, to
  detect variable stars in globular clusters is
  demonstrated through the use of modern CCD
  cameras and commercially available image
  processing software.
• Specifically, a 0.25 meter telescope and a
  camera with a 550 x 752 CCD array was used to
  detect and perform photometry of six of the known
  twelve variable stars in NGC 6779 (M56). Two
  commercial image processing and photometric
  analysis software packages were utilized, and
  multiple variable detection techniques were
  examined. The periods of five variables were
  derived and B-V and V-I color magnitude diagrams
  for the entire cluster were developed.

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NGC 6779 (M56)

• R.A. 19 16.6
• Dec. 30 11
• Constellation Lyra
• Distance 32,900 light-years.
• Diameter 8.8 arc minutes
• B-V color index of 0.86
• Visual Brightness 8.3

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NGC 6779
6
NGC 6779 CCD Image
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Known Variables

• Elements of all variables Clement (2001), Wehlau
  Sawyer Hogg (1985) 
• No. x" y" Max. Min. Epoch
  Period Remarks1
• 1 44.69 74.10 15.4 16.6 45252.316
  1.510019 Cep,Sp,V,mem
• 2 18.16 33.09 15.2 15.8 irr
• 3 25.10 91.69 14.3 15.2 SR
  Sp,V,mem
• 4 -112.13 -159.46 16.2 16.9 45250.403
  0.423096?, alt 0.734873
• 5 6.79 -134.78 14.4 15.2 SR
  
• 6 - 2.02 37.06 12.9 14.9 42256.734
  90.00 RVTau,Sp,V,mem
• 7 293.48 -213.24 15.7 16.7
  irr f
• 8 -97.63 -335.90 16.0 16.9
  irr f
• 9 177 525 15.7 16.5
  irr f
• 10 -431.53 88.33 16.8 18.3 45277.308
  0.598890 RR0, f
• 11 -415.58 283.80 15.6 16.7 45250.331
  0.07562534 SX Phe?f
• 12 -243.96 - 95.41 16.3 17.3 45582.594
  0.90608 RR0/Cep?

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A Research Project

• Relatively few formal studies on M56
• All performed with 1m-2m telescopes
• New CCD equipment software
• Greatly expand the capabilities of smaller scopes
• Could this work be done with smaller telescopes?
• Rowe (2000) et al, suggested such possible
  research projects

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The Equipment Software

• Meade 10in (254mm) LX200
• Permanently Mounted
• Starlight Express MX716 CCD
• 550 x 720 pixel array
• Johnson B,V,I Photometric Filters
• MaximDL/CCD (Ver 4) used for imaging
• AiP4Win used for data photometric data extraction

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The Location

• San Jose, CA 37-20N 121-49W

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The Observations

• Taken at f/6.3 and f/10
• Series of 30-second exposures
• Images taken primarily for detection
• Using no filter to maximize photons
• Other images taken
• Using Johnson B, V, and I filters
• All exposures were dark bias subtracted, and
  flat-fielded

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The Observations
13
Data Reduction

• For detection, 16 images were stacked and
  aligned.
• Photometry data was obtained with AIP4WINs
  Extractive Photometry Tool (high sensitivity
  setting)
• Extraction was also performed using the SPS
  Stellar Photometry Software package
• Janes and Heasley (1993)
• Very similar results

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Photometric Transforms

• Landolt standard were observed

15
Photometric Transforms

• Transformation Equations Developed
• (B-V)s 0.92114(B-V)i - 0.0425
• (V-I)s 1.2532(B-V)i 2.3506

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Variable Star Detection

• Elements of all variables Clement (2001), Wehlau
  Sawyer Hogg (1985) 
• No. x" y" Max. Min. Epoch
  Period Remarks1
• 1 44.69 74.10 15.4 16.6 45252.316
  1.510019 Cep,Sp,V,mem
• 2 18.16 33.09 15.2 15.8 irr
• 3 25.10 91.69 14.3 15.2 SR
  Sp,V,mem
• 4 -112.13 -159.46 16.2 16.9 45250.403
  0.423096?, alt 0.734873
• 5 6.79 -134.78 14.4 15.2 SR
  
• 6 - 2.02 37.06 12.9 14.9 42256.734
  90.00 RVTau,Sp,V,mem
• 7 293.48 -213.24 15.7 16.7
  irr f
• 8 -97.63 -335.90 16.0 16.9
  irr f
• 9 177 525 15.7 16.5
  irr f
• 10 -431.53 88.33 16.8 18.3 45277.308
  0.598890 RR0, f
• 11 -415.58 283.80 15.6 16.7 45250.331
  0.07562534 SX Phe?f
• 12 -243.96 - 95.41 16.3 17.3 45582.594
  0.90608 RR0/Cep?

17
Variable Star Detection

• Given coordinates not accurate enough for
  immediate detection of variables
• Because the center coordinates and epochs are not
  entirely clear from previous studies,
• Clement notes the given x-y coordinates can be
  used to aid in identifying the variables, but
  they can not be used for finding the precise
  locations. Clement (2001).
• Other methods needed

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Variable Star Detection

• Several scatter techniques was used to find the
  variables
• (e.g.) Chi-Square variability statistic, (Saha
  Hoessel 1990)

n number of data points (images) i
image number m magnitude of star
average mag (all images)
s
mag uncertainty
n
n -1
gt 1.75 were identified as possible variable star
candidates
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Variable Star Detection

• A computer program was developed that read in the
  text files generated by either SPS or AIP4Win,
  matched stars for each image, and developed the
  variability statistic for each star
• Two detection cycles were run.
• The first, concentrating on detecting long period
  and irregular variability was run over all the
  images
• The second, concentrating on detection the short
  period variables, was run over a consecutive
  three night set of observations
• Possible candidates were confirmed or eliminated
  using standard differential photometry techniques
  on each image

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Variable Star Detection

• Results of Scatter Techniques
• Large number of false positives were seen
• Due in part to artifacts from stacking and
  alignment
• V1, V3, V4, and V6 were easily detected. (large
  chi-square value)
• V2 and V5 present but with low chi-square value
  (confirmed by differential photometry)

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Variable Star Detection

• Another method, Image Subtraction, was attempted
• using the ISIS program developed by Alard (1999).
  
• This technique did not seem as robust in
  determining the faint variables in the images
  from this study, and only V6 was detected via
  this method.

22
Additional Analyses

• Periods of the detected stars were obtained by
  using the period analysis tool Peranso, from the
  CBA Belgium Observatory
• Using the Photometric transform equations, B-V
  and V-I indices were determined for the detected
  variables
• B-V and V-I color-magnitude charts for the entire
  cluster were developed.

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Detected Variables in NGC 6779
24
Detected Variables in NGC 6779
25
Other Variables in NGC 6779

• These variables were either not detected, or were
  out of the field of view of the CCD camera for
  this study.
• Wehlau et al (1985), indicates that variables
  V7, V8, V9, V11 are probable field stars and not
  members of the cluster.
• Rishels analysis (1981), tends to confirm that
  these stars (at extended distances from the
  cluster), are not part of the cluster membership

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Color Magnitude Diagrams
V-I 1.4
B-V 1.16
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Conclusions

• Six Variables Detected
• Four variables easily
• Good agreement with literature on period and
  color indices
• Cluster Color Magnitude Diagrams consistent

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Conclusions

• Much of the work previously done on Globular
  Clusters with larger instruments can be performed
  with much smaller scopes and modern equipment
  and software

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References (1)

• Alard, C. Lupton R.H., 1998 ApJ, 503, 325
• Alard C. 2000, AA, 144, 363
• Clement, C. M., Muzzin, A., et al, 2001,
  Astronomical Journal, 122, 2587-2599
• Clement C. 2000, http//www.astro.utoronto.ca/ccle
  ment/cat/clusters.html
• Davis, H., 1917, PASP 29..210D.
• Hatzidimitriou D., Antoniou V., et al, 2004, Mon
  Not., R Astron Soc, 348, 1157-1163
• Janes K., Heasley J., , 1993, PASP 105 527-537
•  Rawson, D. Macri, L., et al, 1997, ApJ
  490517556
•  Rishel, B., Sanders W. Schroder R., 1981, Astro.
  Astrophys. Suppl. Ser. 45, 443-450.
•  Rosenberg A., Aparicio, A, Saviane, I., and
  Piotto, G., 2000, A A, Suppl. Ser. 144, 5-38.

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References (2)

• Rowe, J.F., 2000, JAAVSO Preprint
•  Russeva T., 1999, IBVS 4727.
•  Russeva T., 2000, IBVS 4846
•  Saha, A., Hoessel, J. G. 1990, AJ, 99, 97
•  Shapley H. 1920, ApJ, 52 73
•  Simbad http//simbad.u-strasbg.fr/sim-fid.pl
•  Wehlau A., Sawyer Hogg, H., 1985, Astro J.
  90(12).