XO-2b: A HOT JUPITER WITH A VARIABLE HOST STAR THAT POTENTIALLY AFFECTS ITS MEASURED TRANSIT DEPTH

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

10 Scopus citations

Abstract

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
Volume810
Issue number1
DOIs
Publication statusPublished - Sep 1 2015

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Keywords

  • 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

Cite this

Zellem, R. T., Griffith, C., Pearson, K. A., Turner, J. D., Henry, G. W., Williamson, M. H., ... 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