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Astronomy 211. Planetary and Stellar Astronomy, Spring 2016

Variable Star Discovery Lab

Introduction

In the Spring 2016 section of Astronomy 211 we studied the life cycles of stars. Many stars exhibit time variable light intensity in certain phases of their life cycle. While the celestial coordinates of more than a billion stars have been cataloged, only a tiny fraction of these have had their brightness tracked over time sufficiently to determine whether and how they are variable. Therefore, as a multipart laboratory, student teams were assigned 1) to discover variability in a star not previously known to vary, and 2) to follow up on the discovery until the period is known and the nature of the variability is clear. Upon completion, all results were reported to the Variable Star Index, a refereed international clearinghouse of variable star information maintained by the American Association of Variable Star Observers. The table below summarizes the discoveries. A brief description of the laboratory procedure follows the table.

 

Star Name

Period

Variable Type

Discoverer(s)

Light curve

GSC 03739-00703

6.7958(11) hour

contact binary star

Evan Cook

V0352+2135 light curve

2MASS J05044878+5300179

18.8447(8) hour

eclipsing binary star

Larry Molnar

V0432+2845 light curve

2MASS J05060657+5446162

7.9243(12) hour

contact binary star

Evan Cook

V0434+2442 light curve

2MASS J05070515+5329095

15.0946(15) hour

eclipsing binary star

Carson Williams & Nathan Jansen

2MASS 05114205+5835459

7.1014(5) hour

contact binary star

Evan Cook

V0652+2104 light curve

2MASS J05140940+5825526

33.0605(26) hour

eclipsing binary star

Kenton Greene & Nathan Breese

V1205+3500 light curve

2MASS J05160249+5852446

11.6818(25) hour

close binary star

Evan Cook & Alexis Bonnema

V1205+3500 light curve

Notes on the table:

  • Click on any star name to see the full entry in the VSX catalog.
  • The number in parenthesis following the value of the period is the one-sigma uncertainty on the final digit of the period
  • The light curves are plots of system brightness (in units of relative astronomical magnitude) versus phase of the periodic cycle.
  • Click on any figure to see a full size version of the light curve. The legends indicate which observing nights are associated with which color data.

Laboratory Procedure

  1. Discovery: Undiscovered variable stars could be anywhere! We chose six positions in the sky to search that were relatively convenient. They were far to the north (declination > 50 degrees) so that they would be up a long time and to the east so that they would not set too early. A mosaic of 3x4 images were made around each of the six positions, yielding 72 fields of view to study. Students analyzed the images with software written by former Calvin student Dan Van Noord. The locations of all stars in the image were mapped, light curves (plots of brightness versus time) were generated and sorted by the likelihood of showing intrinsic variation. Students learned to sift out a variety of false positives and identify genuinely variable stars. The position of the variable was checked in the Variable Star Index to see if its variability had been previously noted.
  2. Follow up data: We needed additional data for each star covering multiple cycles in order to establish whether the variations were periodic and to determine the shape of the cycle and length of the period. For each discovery, a total of 6 to 11 nights of data were accumulated, as much as was needed to fully define the period and the shape of the light curve.
  3. Determining periods: We used the FALC algorithm (Fourier Analysis of Light Curves) implemented by Peranso to determine the period and its confidence interval. The first step of this analysis was determing whether the period was unambiguous or whether more data were needed to rule out possible alternative periods. All of the stars we found did turn out to have strictly periodic variations although there are classes of variable stars that vary aperiodically.
  4. Variability type: All seven systems showed variability due to orbital motion of a binary system. The second, fourth, and sixth plots show well defined eclipses from stars well separated from each other. Varying light in between eclipses in the second plot likely indicates reflection of light originating from the brighter star off of the near side of the fainter star. The remaining four systems are from binaries so close that their atmospheres touch each other. Their individual shapes are distorted by tides. The light varies both because of mutual eclipsing and because of rotation of the elongated objects. The depth of variation depends on the orientation of the orbital plane relative to the line of sight.
  5. Submission to the VSX and discovery credit: With the conclusion of the semester, all seven stars were submitted to the VSX along with the light curves to document their type and period. Star names come from entries in the Hubble Guide Star Catalog (GSC) or the Two Micron All Sky Survey (2MASS). In every case, discovery credit was given to the class.

posted 6/22/2016 by L. Molnar

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