Robert T. Zellem, Caitlin Griffith, Kyle A. Pearson, Jake D. Turner, Gregory W. Henry, Michael H. Williamson, M. Ryleigh Fitzpatrick, Johanna K. Teske, Lauren I. Biddle

Research output: Contribution to journalArticle

11 Scopus citations


The transiting hot Jupiter XO-2b is an ideal target for multi-object photometry and spectroscopy as it has a relatively bright (V-mag = 11.25) K0V host star (XO-2N) and a large planet-to-star contrast ratio (R<inf>p</inf>/R<inf>s</inf> ≈ 0.015). It also has a nearby (31″.21) binary stellar companion (XO-2S) of nearly the same brightness (V-mag = 11.20) and spectral type (G9V), allowing for the characterization and removal of shared systematic errors (e.g., airmass brightness variations). We have therefore conducted a multiyear (2012-2015) study of XO-2b with the University of Arizona's 61″ (1.55 m) Kuiper Telescope and Mont4k CCD in the Bessel U and Harris B photometric passbands to measure its Rayleigh scattering slope to place upper limits on the pressure-dependent radius at, e.g., 10 bar. Such measurements are needed to constrain its derived molecular abundances from primary transit observations. We have also been monitoring XO-2N since the 2013-2014 winter season with Tennessee State University's Celestron-14 (0.36 m) automated imaging telescope to investigate stellar variability, which could affect XO-2b's transit depth. Our observations indicate that XO-2N is variable, potentially due to cool star spots, with a peak-to-peak amplitude of 0.0049 ± 0.0007 R-mag and a period of 29.89 ± 0.16 days for the 2013-2014 observing season and a peak-to-peak amplitude of 0.0035 ± 0.0007 R-mag and 27.34 ± 0.21 day period for the 2014-2015 observing season. Because of the likely influence of XO-2N's variability on the derivation of XO-2b's transit depth, we cannot bin multiple nights of data to decrease our uncertainties, preventing us from constraining its gas abundances. This study demonstrates that long-term monitoring programs of exoplanet host stars are crucial for understanding host star variability.

Original languageEnglish (US)
Article number11
JournalAstrophysical Journal
Issue number1
StatePublished - Sep 1 2015


  • methods: analytical
  • planets and satellites: atmospheres
  • planets and satellites: general
  • planets and satellites: individual (XO-2b)
  • radiative transfer

ASJC Scopus subject areas

  • Space and Planetary Science
  • Astronomy and Astrophysics

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    Zellem, R. T., Griffith, C., Pearson, K. A., Turner, J. D., Henry, G. W., Williamson, M. H., Fitzpatrick, M. R., Teske, J. K., & Biddle, L. I. (2015). XO-2b: A HOT JUPITER WITH A VARIABLE HOST STAR THAT POTENTIALLY AFFECTS ITS MEASURED TRANSIT DEPTH. Astrophysical Journal, 810(1), [11]. https://doi.org/10.1088/0004-637X/810/1/11