1. Simultaneous Photometry: Vetting Exoplanets via the Transit Method Ethan McGee, Robert Morehead
Identifying eclipsing binary stars is an obstacle to astronomers studying exoplanets via the transit method.
Eclipsing binaries are two stars, often of different spectral types, in orbit around each other. As one star passes in front of the other, we detect the change in brightness via the transit method. The resulting light curve of an EB’s eclipse can be very similar to that of an exoplanet. When astronomers use this method to discover new exoplanet candidates, they must validate their data to ensure the object is a planet and not a false positive due to an EB. The positioning and orientation of EB stars, relative to our perspective from Earth, may also affect how the data appears.
We attempted this process to see if the Preston Gott Skyview Observatory is capable of performing such measurements.
Improvements in our technique during future observations may enable us to contribute to ongoing efforts in validating exoplanet candidates.
Both plots show that we are close to the expected depth. This tells us that we are able to take good data with some effort. Even with generally poor data such as WASP-33 b, we are still able to detect the entire transit. These results are very encouraging to future projects and research in this area.
After several attempts to gather useful data, we realized a number of errors are likely to occur on any given night. The following list of potential problems are things to consider when planning future observations:
Timing of observation vs. actual event duration
Deteriorating viewing conditions (increasing airmass over time)
Weather changes despite predicted patterns
Equipment malfunctions
Inconsistent exposure times
Guiding error
Moving forward with this project, we will use our previous shortcomings to better prepare for future observations. As we continue to make improvements in our technology and resources, we may even operate the equipment remotely from indoors. Our goal is to attain truly simultaneous photometry using multiple telescopes at the Preston Gott Skyview Observatory.
The implications of this project could potentially enable amateur astronomers to contribute to future exoplanet surveys. Further study in this area of research may include collaboration with student observers in the mission to vet exoplanet candidates and eliminate false positive results.
We began our project by first using a single telescope to gather data. We targeted bright stars as a best case scenario to ensure that our equipment operated sufficiently at the most basic level. After observing both types of events, we used AstroImageJ to reduce and analyze the data. The plots shown below are detrended using parameters such as airmass, total counts on the CCD chip, saturation of pixels, etc. On the left is the complete transit of an exoplanet. On the right is the partial secondary eclipse of an EB star. Both observations occurred over approximately 6 hours.
Detect obvious transits
Bright star with known exoplanet
Analyze and detrend data
Known eclipsing binary star
WASP-33b on 1/24/18
XZ CMi on 2/7/18
Four astrophysical scenarios that produce planet-like light curves
Simultaneous multi-color photometry has been used to reduce the rate of false positive data resulting from EBs.
Acknowledgements
I would like to thank the Honors College Undergraduate Research Scholars Program supported by the CH and Helen Jones Foundations.
I would also like to thank my research mentor Dr. Morehead. I’ve learned more than I ever knew before about exoplanets and astronomy in general through his teachings, both in and out of class. I certainly would not be here if not for his guidance and support.
Research assistance was provided by students in the ASTR 2401 Observational Astronomy course. These students helped gather data and conduct multiple observations. Their efforts were tantamount to the process of refining and improving our research.
Poster design is based on templates provided by the TTU Teaching, Learning, & Professional Development Center.
In studying Kepler exoplanet candidates, Colón et al (2015) used multicolor filters on a single large telescope (GTC) to conduct differential photometry. This involved switching filters between images to simultaneously take measurements in different colors of light.
The results of this study indicate that exoplanet candidates may be validated or rejected by using simultaneous multicolor photometry.
Our goal is to replicate this method at our own observatory. Though the telescopes are small (12-inch), we have several that could be used at once. We plan to use multicolor filters on multiple telescopes to achieve simultaneous photometry.
Multicolor photometry of an exoplanet candidate
Multicolor photometry of an eclipsing binary star, note the difference in measured depth for both colors
Photometry of an exoplanet transit
Photometry of an EB secondary eclipse (partial)
Both EB and planet transits are visible in the data displayed above, despite minor setbacks in data collection.
Over the course of many attempted observations throughout the Fall semester, we experienced multiple setbacks each time we tried to gather data. One issue we encountered was a malfunctioning light panel, used to produce flat field images that uniformly illuminate the CCD. Another problem that occurred consistently was faulty autoguiding on the telescopes. This forced us to manually adjust the scope’s position to keep the target in view, moving very slightly every so often. If the frames are misaligned repeatedly, the software has difficulty recognizing objects across multiple exposures. However, as we refined our process and accounted for previous errors, we were able to achieve more successful results in the following Spring semester.
EXOPLANET
WASP-33 is a bright 8th magnitude star with a confirmed “hot Jupiter”. Due to worsening conditions and increasing airmass, this observation had multiple increases in exposure time as the transit occurred. Guiding error and streaking occurred in early images, resulting in the gap of missing data points on the left side of the graph. Following these “bad frames”, we were able to correct the guiding and stay on target for the duration of the transit.
EB STAR
XZ CMi is an eclipsing binary of Algol type, meaning its component stars are mostly spherical in shape and thus its eclipses duplicate the geometry of a planet transit. The graph depicts the latter half of this EB’s secondary eclipse. The plot only shows a partial eclipse due to an error in timing. For this observation, autoguiding was functional and performed consistently for the entire duration of the event. Only a small portion on the right side of the plot is missing due to misaligned frames and bad exposures.
References
Cameron, A. C. 2012, Nature, 492, 48
Collins, K. A., Kielkopf, J. F., Stassun, K. G., & Hessman, F. V. 2017, AJ, 153, 77
Colón, K. D., Morehead, R. C., & Ford, E. B. 2015, MNRAS, 452, 3001