TOI-6692b: An eccentric 130 day period giant planet with a single transit from TESS

TL;DR

TOI-6692 b is a long-period giant planet discovered as a single TESS transit and subsequently confirmed and characterized through extensive RV monitoring and ground-based photometric follow-up. The authors perform a joint EXOFASTv2 analysis of TESS data, RVs from PFS, CHIRON, FEROS, and CORALIE, Gaia parallax, and SED constraints to derive $M_p \approx 0.62\,M_J$, $R_p \approx 1.04\,R_J$, $P \approx 131$ days, and $e \approx 0.54$, with a transit duration of about $T_{14} \approx 11.1$ hours and a stellar temperature around 5890 K. A long-term RV trend indicates a possible exterior companion, with speckle imaging constraining companions to $\lesssim 400\,M_J$ at $\sim 20$ AU, illustrating the dynamical complexity of the system. The work demonstrates the viability of recovering transits for single-transit TESS candidates using night-to-night ground-based photometric networks and provides a crucial data point for understanding migration pathways, favoring planet-planet scattering for this system.

Abstract

We report the discovery and characterization of TOI-6692 b, an eccentric (e~0.54) Jupiter on a 130-day orbit. TOI-6692 b was first detected as a community TESS Object of Interest (cTOI) by the Visual Survey Group and the Planet Hunters group as a single transit candidate via TESS observation. The period was subsequently confirmed via radial velocity monitoring from the Planet Finder Spectrograph on the 6.5m Magellan telescope. Additional radial velocities were acquired with the CHIRON, FEROS, and CORALIE spectrographs. LCOGT ground-based photometric follow-up was conducted over 2 weeks to detect another transit and refine the period. Although we did not detect an ingress or egress of the 11.04 hr transit, we did detect a possible in-transit signal in the multi-night data and provide an updated ephemeris for future monitoring. TOI-6692 b is one of few planets with orbital periods longer than 100 days that have a secure mass, radius, and eccentricity detection. As with most giant planets at these orbital periods, the eccentricity of TOI-6692 b is lower than that expected of planets undergoing high-eccentricity tidal migration, but is more consistent with the expectations of planet-planet scattering outcomes. A long-term radial velocity trend was detected and further monitoring is warranted to determine the outer companion period. TOI-6692 b is also one of few TESS single transit targets that have its period eventually confirmed via follow-up photometric campaigns timed to capture transits despite the relatively large ephemeris uncertainties. Such efforts highlight the capabilities of night-to-night stability on ground-based photometric facilities today.

Figures (9)

• Figure 1: Per-sector Normalized TESS PDCSAP light curves for TOI-6692. The target star was observed over six TESS sectors. The observing cadence was 600 seconds in Sector 27 and 120 seconds in Sectors 39, 66, 67, 93, and 94. Due to the long period, the planet had only a single TESS transit in Sector 39.
• Figure 2: Multi-night LCOGT 1.0 m ground-based follow-up lightcurve. The differential lightcurve was extracted using a common set of 22 reference stars and was normalized over the full dataset. No lightcurve detrending was applied. Grey symbols show the unbinned data. Black symbols show the data in 8 minute bins. The vertical red dashed line shows the nominal mid-transit time based on the updated ephemeris from the global fit (Section \ref{['sec:global']}), which is based on the single TESS transit detection and all available radial velocities. The blue shaded region indicates the mid-transit time $1 \sigma$ uncertainty window. The apparent 6 ppt in-transit detection is just outside the $1 \sigma$ window. The bounds on orbital period based on the TESS single transit, RVs, and the apparent LCOGT in-transit detection is $P=131.125\pm 0.012$ days.
• Figure 3: The figure shows $5\sigma$ magnitude contrast curves in both filters as a function of the angular separation out to 1.2 arcsec. The inset shows the reconstructed 832 nm image of TOI-6692 with a 1 arcsec scale bar. TOI-6692 was found to have no close companions from 0.1 to 1.2 arcsec to within the magnitude contrast levels achieved.
• Figure 4: Spectral energy distribution of the target star TOI-6692. Magnitudes from Gaia$G$, $B_p$, $R_p$, 2MASS $J$, $H$, $Ks$, and WISE $W1$, $W2$, $W3$, and $W4$ are included in the global modeling of the system and are shown in dark blue.
• Figure 5: Radial velocity observations phase folded to the ephemeris. The red line shows the $\texttt{EXOFASTv2}$ best model fit to the RVs. Section \ref{['subsec:spec']} details the radial velocities observations.