The Dedicated Monitor of Exotransits (DEMONEX): Seven Transits of XO-4b

The DEdicated MONitor of EXotransits (DEMONEX) was a 20-inch robotic and automated telescope to monitor bright stars hosting transiting exoplanets to discover new planets and improve constraints on the properties of known transiting planetary systems. We present results for the misaligned hot Jupiter XO-4b containing seven new transits from the DEMONEX telescope, including three full and four partial transits. We combine these data with archival light curves and archival radial velocity measurements to derive the host star mass {M}_*={1.293}_-0.029^+0.030 {M}_⊙ and radius {R}_*={1.554}_-0.030^+0.042 {R}_⊙ , the planet mass {M}_P={1.615}_-0.099^+0.10 {M}_{{J}} and radius {R}_P={1.317}_-0.029^+0.040\\{R}_{{J}}, and a refined ephemeris of P=4.1250687+/- 0.0000024 days and {T}₀=2,4547,58.18978+/- 0.00024 {{BJD}}_{TDB}. We include archival Rossiter-McLaughlin measurements of XO-4 to infer the stellar spin-planetary orbit alignment of λ =-{40.0}_-7.5^+8.8 degrees. We test the effects of including various detrend parameters, theoretical and empirical mass-radius relations, and Rossiter-McLaughlin models. We infer that detrending against CCD position and time or airmass can improve data quality but can have significant effects on the inferred values of many parameters—most significantly {R}_P/{R}_* and the observed central transit times T_C. In the case of {R}_P/{R}_* we find that the systematic uncertainty due to detrending can be three times that of the quoted statistical uncertainties. The choice of mass-radius relation has little effect on our inferred values of the system parameters. The choice of Rossiter-McLaughlin models can have significant effects on the inferred values of v{sin}{I}_* and the stellar spin-planet orbit angle λ.