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

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

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

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

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

From Contemporary Activities in Astronomy, 2nd 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

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)

Is this a variable star?

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

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

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

The LPV stars are red, as expected.

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.