George Dransfield

Abstract:

Binary stars are ubiquitous; yet it remains unclear how wide-orbit stellar companions influence the formation of hot Jupiters, particularly around M dwarfs. Here, we first report the discovery of TOI-5628Ab, a giant planet transiting a midtype M dwarf (M* = 0.36 ± 0.02 M⊙) every 4.34 days, accompanied by an associated white dwarf TOI-5628B (MWD = 0.59 ± 0.16 M⊙) at a projected distance of about 2500 au. Using TESS, ground-based photometry, and SPIRou radial velocities, we constrain the planet radius to 0.74 ± 0.04 RJ and mass to 0.09 ± 0.04 MJ, with a 3σ upper limit of 0.22 MJ. Building on this system, we further conduct a homogeneous systematic search for comoving stellar companions with projected semimajor axes between 100 and 10,000 au around all M dwarfs with confirmed giant planets with periods smaller than 10 days and radii larger than 0.7 RJ, as well as a group of field M stars with stellar properties similar to the planet sample, based on the stellar kinematics from Gaia DR3. We measure a stellar multiplicity of 34.2% ± 9.5% for M dwarfs hosting short-period giant planets, which is substantially higher than the fraction of 5.3% ± 3.7% for the field M stars by approximately a factor of 6. Our results suggest that wide-orbit stellar companions tend to promote the formation of short-period gas giants around M stars with masses 0.21 ≤ M* ≤ 0.64 M⊙, and high-eccentricity migration may play an important role in producing such systems.

Abstract:

Context . We present the analysis of five long-period TESS Objects of Interest (TOIs), all orbiting Sun-like stars, with orbital periods exceeding one month. Initially identified by the Transiting Exoplanet Survey Satellite (TESS), we extensively monitored these targets with the Antarctic Search for Transiting Exoplanets (ASTEP), supported by other facilities in the TESS Follow-up Observing Program (TFOP) network. Aims . These targets occupy a relatively underexplored region of the period-radius parameter space, offering valuable primordial probes for planetary formation and migration as warm planets better maintain their evolutionary fingerprints. Methods . To characterise these systems, we leveraged high-resolution speckle imaging to search for nearby stellar companions, and refine stellar parameters using both reconnaissance spectroscopy and spectral energy distribution (SED) fitting. We combined TESS photometry with high-precision ground-based observations from ASTEP, and when available, included additional photometry and radial velocity data. We applied statistical validation to assess the planetary nature of each candidate and used allesfitter to jointly model the photometric and spectroscopic datasets. Results . We validate the planetary nature of three TOIs, including the two warm Saturns TOI-4507 b (8.2 R ⊕ , 104 d) and TOI-3457 b (10.0 R ⊕ , 32.6 d), as well as the warm sub-Neptune TOI-707 b (2.4 R ⊕ , 52.8 d). The remaining two candidates are most consistent with eclipsing binaries, namely TOI-2404 and TOI-4404. Conclusions . These results help populate the sparse regime of warm planets, which serve as key tracers of planetary evolution, and demonstrate ASTEP’s effectiveness as a ground-based follow-up instrument for long-period systems.

Abstract:

ABSTRACT Gas giant planets with periods $20~\lt ~P~\lt ~300~\rm d$ orbiting Sun-like stars are a relatively uncommon outcome of planetary formation, and key questions about the nature and formation of this subpopulation remain unanswered. Theoretical models for the location of their formation (in situ or ex situ) and for their subsequent migration predict different outcomes in terms of planet masses and eccentricities, indicating that observations have a key role to play in disentangling their histories. In this work, we present the discovery and confirmation of a pair of long-period Jupiter-sized planets transiting an F7 star: TOI-791 b is a $0.993\pm 0.033\rm ~R_{Jup}$ planet on a $139.29931_{-0.00012}^{+0.00011}~\rm d$ orbit, and TOI-791 c, a $1.155\pm 0.040\rm ~R_{Jup}$ planet on a $232.01570_{-0.00071}^{+0.00067}~\rm d$ orbit. The two planets are within 0.07 per cent of a second-order 5:3 period commensurability leading to transit timing variations (TTVs) of up to 50 min. We confirm their planetary nature using ground-based photometry, including multiple full detections of the $\gt 11~\rm h$ transits of both TOI-791 b and c from Antarctica with ASTEP, making these the longest-duration transits ever observed in their entirety from the ground. Our detailed analysis of the TTV signal allows us to measure dynamical masses for both planets, which yield densities of $\rho _{\rm b}=0.038\pm 0.008 \rm ~g~cm^{-3}$ and $\rho _{\rm c}=0.047\pm 0.006 \rm ~g~cm^{-3}$, indicating that TOI-791 b and c are two of the lowest density giant planets ever detected. While these measurements are robust, further follow-up is needed to fully characterize the TTV signal and the architecture of the system.