23
Jeff Wilkerson
Luther College
RAC
July 13, 2010

We image 3 clusters per year: M23 and two others
Image durations: 2 to 12 seconds, unfiltered
Campaign durations: 5 to 7 months
Return 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
Equipment: 12” Meade Schmidt-Cassegrain;
Apogee AP6E camera; SBIG STL-1001E camera
How did we get here? What are our 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 cycles 6
Low-amplitude, ultra-long period pulsation
III. Traditional Stellar Variability
Surveys of new variable stars
Locate detached and semi-detached eclipsing binaries in clusters 1
Locate contact eclipsing binaries in clusters 2
Period/amplitude variations in contact systems 3
Period-to-period variability in long period variables
Search for cataclysmic variables in clusters 4
Search for transiting planets 5
Rotating variable star periods in young clusters 7
4.
5.
6.
7.
1.
2.
3.
Wyithe, J.S.B, and Wilson, R.E. 2002, ApJ, 571, 293
Rucinski, S.M. 1998, AJ, 116, 2998
Paczynski, B., et al. 2006, MNRAS, 368, 1311
Mochejska, B.J., et al. 2004, AJ, 128, 312
Mochejska, B.J., and Stanek, K.Z. 2006, AJ, 131,1090
Lockwood G.W., et al. 1997 ApJ, 485, 780-811
Herbst W. and Mundt R., 2005, ApJ, 633, 967-985

SkyandTelescope.com -
News Blog - A KBO in the
Crosshairs
Posted By Kelly Beatty, June 29, 2010

All images acquired with a
12” Meade LX200 and
Apogee AP6E camera or
SBIG STL-1001E camera

Ujjwal Joshi
Nathan Rengstorf
Andrea Schiefelbein
Todd Brown
Brajesh Lacoul
Kari Frank
Alex Nugent
Drew Doescher
Alex Sperry
Robyn Siedschlag
Siri Thompson
Matt Fitzgerald
Heather Lehmann
Amalia Anderson
Hilary Teslow
Steve Dignan
Kirsten Strandjord
Donald Lee-Brown
Zebadiah Howes
Buena Vista
Univ.
Travis DeJong
Dordt College
Forrest Bishop
Decorah High
School
Support: Roy J. Carver Charitable Trust (Grant #00-50)
Luther College
R.J. McElroy Trust/Iowa College Foundation

Duration (s) # Nights Total Images
3.5
25 45,000
Date Range
19 June 2003 – 8 Sep. 2003
2.5
5.0
2.8
3.5
3.5
3.5
49
53
20
37
45
~30
45,000
49,000
91,000
82,000
50,000
~32,000
23 June 2005 – 30 Aug. 2005
28 Mar. 2006 – 25 Sep. 2006
9 Mar. 2007 – 27 Sep. 2007
3 Mar. 2008 – 16 Sep. 2008
11 Mar. 2009 – 17 Sep. 2009
24 Feb. 2010 – present

From http://rst.gsfc.nasa.gov/Sect20/sun_mw+.jpg

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
All Analysis done with code developed in IDL

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)

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

From Contemporary Activities in Astronomy, 2 nd ed.
by Hoff and
Wilkerson, Kendall-Hunt, 2003

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
Miras have been better studied than SRs but still not well understood
We see mostly SR and Mira stars
In the GCVS SR and
Mira stars are about equally common

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 = (fm
)/ s

Many more semi-regular than Mira stars; perhaps a break in the distribution.

The LPV stars are red, as expected.

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.