Download Variable Stars as Essential Astrophysical Tools

Waiting and watching: Results from seven years of
observing the field of open star cluster M23
Jeff Wilkerson
Luther College
RAC
July 13, 2010
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?
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
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2.
3.
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5.
6.
7.
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
Student Participation:
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
OUR M23 DATA SETS
Duration (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
1.
CALIBRATION
•
•
2.
ALIGNMENT
•
3.
Dark Noise Correction
Flat Fielding
Use a single frame for entire data set
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
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.
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
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.