XO-2b: a hot Jupiter with a variable host star that potentially affects its measured transit depth

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${}_p$/R${}_s\approx 0.015$). 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 $0.0049\pm 0.0007$~R-mag and a period of $29.89\pm 0.16$~days for the 2013--2014 observing season and a peak-to-peak amplitude of $0.0035\pm 0.0007$~R-mag and $27.34\pm 0.21$~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.

published in ApJ, 9 pages, 11 figures, 3 tables; updated figures with more ground-based monitoring, added more citations to previous works

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• Forecasting the Impact of Stellar Activity on Transiting Exoplanet Spectra
• Utilizing Small Telescopes Operated by Citizen Scientists for Transiting Exoplanet Follow-up
• Investigating the physical properties of transiting hot Jupiters with the 1.5-m Kuiper Telescope
• The Influence of Stellar Contamination on the Interpretation of Near Infrared Transmission Spectra of Sub-Neptune Worlds around M-Dwarfs
• Investigation of the environment around close-in transiting exoplanets using CLOUDY
• High-resolution, high-sensitivity, ground-based solar spectropolarimetry with a new fast imaging polarimeter
• Variation in the pre-transit Balmer line signal around the hot Jupiter HD 189733 b
• Evaluating Climate Variability Of The Canonical Hot Jupiters Hd 189733b & Hd 209458b Through Multi-epoch Eclipse Observations
• Constraining Exoplanet Metallicities and Aerosols with ARIEL: An Independent Study by the Contribution to ARIEL Spectroscopy of Exoplanets (CASE) Team
• Ground-based Spectroscopy of the Exoplanet XO-2b using a Systematic Wavelength Calibration
• Larger and faster: revised properties and a shorter orbital period for the WASP-57 planetary system from a pro-am collaboration
• Quantifying the Impact of Spectral Coverage on the Retrieval of Molecular Abundances from Exoplanet Transmission Spectra
• The GAPS Programme at TNG. LIII. New insights on the peculiar XO-2 system
• Precision Time-series Photometry in the Thermal Infrared with a 'Wall-Eyed' Pointing Mode at the Large Binocular Telescope