Waiting and watching: Results from seven years of observing the field of open star cluster M 23

1. Waiting and watching:  Results from seven years of observing the field of open star cluster M23 Jeff Wilkerson Luther College RAC July 13, 2010

2. What We Do 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?

3. 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 • 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

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

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

6. Student Participation: ZebadiahHowes Buena Vista Univ. Travis DeJong Dordt College Forrest Bishop Decorah High School Ujjwal Joshi Nathan Rengstorf Andrea Schiefelbein Todd Brown BrajeshLacoul 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 Support: Roy J. Carver Charitable Trust (Grant #00-50) Luther College R.J. McElroy Trust/Iowa College Foundation

7. OUR M23 DATA SETS

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

9. DATA PROCESSING • CALIBRATION • Dark Noise Correction • Flat Fielding • ALIGNMENT • Use a single frame for entire data set • STAR ID & EXTRACTION • Aperture photometry for signal determination • 256 Background regions • INTRA-NIGHT NORMALIZATION • INTER-NIGHT NORMALIZATION • MAGNITUDE CONVERSION All Analysis done with code developed in IDL

10. Frame Normalization • Identify four reference images from throughout the night • Calculate average flux for each star in all four frames – this is the reference signal • Determine the signal of each star in the frame to be normalized – this is the sample signal • Calculate (ref. signal/sample signal) for each star • Normalization factor = median of all ratios in (4)

13. Types of Variable Stars • Pulsating (e.g., Mira, bCephei, dCephei, dScuti, RR Lyrae, aCygni) • Eclipsing (e.g., W UMa, Algol–type, bLyrae) • Cataclysmic • Rotating

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

15. We have identified 7 eclipsing binary systems in the field; they have periods ranging from 5 hours to several days.

16. We see mostly SR and Mira stars In the GCVS SR and Mira stars are about equally common From Variable Stars by M. Petit, Wiley and Sons, 1987 Miras have been better studied than SRs but still not well understood

17. From Mattei & Foster and Aslan & Yeśilyaprak in Variable Stars as Essential Astrophysical Tools (2000)

21. Is this a variable star?

22. 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

24. 169 stars have f-stat >2.0; 95 have f-stat >3.0; 58 have f-stat >4.0; 38 have f-stat > 5.0

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

26. The LPV stars are red, as expected.

28. 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.