Waiting and watching: Results from seven years of observing the field of open star cluster M23
Jeff WilkersonLuther College
RAC
July 13, 2010
We image 3 clusters per year: M23 and two others
Image durations: 2 to 12 seconds, unfilteredCampaign durations: 5 to 7 monthsReturn to a cluster at least once
BVRI photometry at least once for color correction to magnitude conversion and knowledge of variable star colors
Result: tens of thousands of images per cluster per year
How did we get here? What are our goals?
Equipment: 12” Meade Schmidt-Cassegrain; Apogee AP6E camera; SBIG STL-1001E camera
What We Do
Our Observational Goals:I. Brief changes in apparent stellar flux
Occultation and microlensing events Flare stars
II. Very long timescale changes in stellar luminosity Luminosity stability Solar-like cycles6
Low-amplitude, ultra-long period pulsation
III. Traditional Stellar Variability Surveys of new variable stars Locate detached and semi-detached eclipsing binaries in clusters1
Locate contact eclipsing binaries in clusters2
Period/amplitude variations in contact systems3
Period-to-period variability in long period variables Search for cataclysmic variables in clusters4
Search for transiting planets5
Rotating variable star periods in young clusters7
1. Wyithe, J.S.B, and Wilson, R.E. 2002, ApJ, 571, 2932. Rucinski, S.M. 1998, AJ, 116, 29983. Paczynski, B., et al. 2006, MNRAS, 368, 13114. Mochejska, B.J., et al. 2004, AJ, 128, 3125. Mochejska, B.J., and Stanek, K.Z. 2006, AJ, 131,10906. Lockwood G.W., et al. 1997 ApJ, 485, 780-8117. Herbst W. and Mundt R., 2005, ApJ, 633, 967-985
Student Participation:Ujjwal JoshiNathan RengstorfAndrea SchiefelbeinTodd BrownBrajesh LacoulKari FrankAlex NugentDrew DoescherAlex Sperry
Robyn SiedschlagSiri ThompsonMatt FitzgeraldHeather LehmannAmalia AndersonHilary TeslowSteve DignanKirsten StrandjordDonald Lee-Brown
Zebadiah HowesBuena Vista Univ.
Travis DeJongDordt College
Forrest BishopDecorah High School
Support: Roy J. Carver Charitable Trust (Grant #00-50) Luther College
R.J. McElroy Trust/Iowa College Foundation
OUR M23 DATA SETSDuration (s) # Nights Total Images Date Range
3.5 25 45,000 19 June 2003 – 8 Sep. 2003
2.5 20 45,000 23 June 2005 – 30 Aug. 2005
5.0 37 49,000 28 Mar. 2006 – 25 Sep. 2006
2.8 49 91,000 9 Mar. 2007 – 27 Sep. 2007
3.5 53 82,000 3 Mar. 2008 – 16 Sep. 2008
3.5 45 50,000 11 Mar. 2009 – 17 Sep. 2009
3.5 ~30 ~32,000 24 Feb. 2010 – present
DATA PROCESSING
All Analysis done with code developed in IDL
1. CALIBRATION• Dark Noise Correction• Flat Fielding
2. ALIGNMENT • Use a single frame for
entire data set
3. STAR ID & EXTRACTION• Aperture photometry for
signal determination
• 256 Background regions
4. INTRA-NIGHT NORMALIZATION
5. INTER-NIGHT NORMALIZATION
6. MAGNITUDE CONVERSION
Frame Normalization
1. Identify four reference images from throughout the night
2. Calculate average flux for each star in all four frames – this is the reference signal
3. Determine the signal of each star in the frame to be normalized – this is the sample signal
4. Calculate (ref. signal/sample signal) for each star
5. Normalization factor = median of all ratios in (4)
Types of Variable Stars
• Pulsating (e.g., Mira, b Cephei, d Cephei, d Scuti, RR Lyrae, a Cygni)
• Eclipsing (e.g., W UMa, Algol–type, b Lyrae)
• Cataclysmic
• Rotating
We have identified 7 eclipsing binary systems in the field; they have periods ranging from 5 hours to several days.
From Variable Stars by M. Petit, Wiley and Sons, 1987
We see mostly SR and Mira stars
In the GCVS SR and Mira stars are about equally common
Miras have been better studied than SRs but still not well understood
From Mattei & Foster and Aslan & Yeśilyaprak in Variable Stars as Essential Astrophysical Tools (2000)
We search for correlation in the signal using a modified f-test.
Define :f = variance of full data set/variance of consecutive night differences
81 times we have data on a night when we had data the previous night
Restrict our work to stars that appeared in our data at least 50% of the time 1566 stars.
Compute f for stars in chunks of ~100 stars of similar brightness; define f-stat = (f-m)/s
CONCLUSION
At least 50 to 100 (3 to 6%) of the stars in our field are classically variable.
SR stars outnumber Miras by a large margin.
The distribution of periods might be bi-modal.
Stars with secular variations in measured signal appear to have gotten brighter more commonly than dimmer. Results are uncertain.
Need better color measures and spectra. Need to monitor the field for several more years to understand secular variations and changes in our variable stars.
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