Periodic transit timing variations and refined system parameters of the exoplanet XO-6b

Abstract

Only a few exoplanets are known to orbit around fast rotating stars. One of them is XO-6b, which orbits an F5V-type star. Shortly after the discovery, we started multicolor photometric and radial-velocity follow-up observations of XO-6b, using the telescopes of Astronomical Institute of the Slovak Academy of Sciences. Our main scientific goals were to better characterize the planetary system and to search for transit timing variations. We refined several planetary and orbital parameters. Based on our measurements, the planet XO-6b seems to be about 10% larger, which is, however, only about 2σ difference, but its orbit inclination angle, with respect to the plane of the sky, seems to be significantly smaller, than it was determined originally by the discoverers. In this case we found about 9.5σ difference. Moreover, we observed periodic transit timing variations of XO-6b with a semi-amplitude of about 14 min and with a period of about 450 days. There are two plausible explanations of such transit timing variations: (1) a third object in the system XO-6 causing light-time effect, or (2) resonant perturbations between the transiting planet XO-6b and another unknown low-mass planet in this system. From the O-C diagram we derived that the assumed third object in the system should have a stellar mass, therefore significant variations are expected in the radial-velocity measurements of XO-6. Since this is not the case, and since all attempts to fit radial velocities and O-C data simultaneously failed to provide a consistent solution, more realistic is the second explanation.

TESS Observations of the Hot Jupiter Exoplanet XO-6b: No Evidence of Transit Timing Variations

“…Notably, we did not find any evidence of transit timing variations in the case of XO-6b, despite the TESS data having sufficient precision and time baseline to clearly reveal the TTVs reported by Garai et al (2020). Specifically, the 3σ upper-limit on TTVs allowed by the TESS 10 https://github.com/pavolgaj/OCFit observations is 2.5 minutes, while their LiTE model predicts TTVs of 12-14 minutes in Sectors 19 and 20 and 5-9 minutes in Sector 26.…”

“…The analysis of Garai et al (2020) utilized 15 transits, of which 8 were observed by the authors themselves with telescopes at the Astronomical Institute of the Slovak Academy of Sciences and 7 were adopted from the ETD after having been observed by others. While the data obtained from the ETD do not exhibit stronger TTVs than their own observations (see Figure 8 in Appendix D) and they only used data from the ETD that had clearly indicated timing epochs (either JD or HJD) and good quality, it is still possible these data points introduced unaccounted for uncertainties that contributed to the discrepancy.…”

“…The time stamps of the data products produced by the TESS SPOC pipeline are TESS Julian Dates (TESS JD = JD−2457000.0) in the Barycentric Dynamical Time standard, BJD T DB 7 , which is usually the most accurate time standard to use as it accounts for many different timing corrections, including leap seconds (e.g., Eastman et al 2010). This best fit light-time effect (LiTE) model from Garai et al (2020). To compare our transit times to this model, we used the publicly available OCFit package 10 (Gajdoš & Parimucha 2019) to reproduce it using their fitted parameters shown in Table 3 in Appendix C. While this model is a good fit to their reported TTVs, they did not observe radial velocities consistent with the XO-6 starplanet system having a stellar mass companion capable of inducing these light-time effects.…”

“…It orbits a fast-rotating (vsini = 48 kms −1 ), bright (V = 10.25 mag), hot (T ef f = 6720 K) star and has an orbital period of 3.8 days (Crouzet et al 2017). To characterize XO-6b better, Garai et al (2020) observed its transits with telescopes at the Astronomical Institute of the Slovak Academy of Sciences and downloaded transit light curves from the Exoplanet Transit Database (ETD) 1 (Poddaný et al 2010). From these data, they reported that it exhibited intriguing periodic TTVs with a semiamplitude of 14 minutes and period of about 450 days.…”

“…Motivated by the intriguing TTVs of XO-6b reported by Garai et al (2020), we investigated this system further by analysing its TESS light curves. Our paper is structured as follows.…”