Obliquities of stars from the study of transiting exoplanets and eclipsing binaries

In this thesis I study stellar obliquities across a range of companion masses, and in new regimes, that aims at constraining theories of planet formation and evolution. I begin in Chapter 1 with an introduction to the state of the field of extrasolar planets, key discoveries that have motivated previous studies on the misalignments between planetary orbits and stellar spins, and highlight the gaps in our knowledge where this thesis aims to make an impact. In Chapter 2, I outline the models and tools that underpin the analysis of transit light curves and high-resolution spectra in subsequent chapters. In Chapter 3, I apply these tools to the discovery of binary systems of various mass ratios. Two such systems are rare brown dwarfs whose discoveries help calibrate models of sub-stellar evolution, and the connection to giant planet formation and evolution. In Chapters 4–6, I present new measurements of stellar obliquities across a range of companion masses. In Chapter 4 I consider two systems hosting small planets. I demonstrate a misalignment the stellar spin and the orbit of a planet twice the size of Earth. This discovery is consistent with some disc-free migration scenarios, and provides the first observational evidence of its kind that super-Earths may form far from their star. In Chapter 5, I consider a sample of 13 giant planets orbiting cool stars in weak-tide regimes. I show that their host stars display a variety of obliquities, contrary to similar planets orbiting closer to their star. Such an effect is consistent with the expectation from tidal evolution, but has not yet been tested on this scale. In Chapter 6, I study the spin angular momentum of the primary component of a binary star hosting a circumbinary planet. I demonstrate that the star is aligned with the binary and planet orbit, providing an important constraint on the formation of binary stars and circumbinary planets. Finally, in Chapter 7, I conclude and offer some thoughts on future prospects

The TESS light curve of the eccentric eclipsing binary 1SWASP J011351.29+314909.7 -- no evidence for a very hot M-dwarf companion

A 2014 study of the eclipsing binary star 1SWASPJ011351.29+314909.7 (J0113+31) reported an unexpectedly high effective temperature for the M-dwarf companion to the 0.95-M$_{\odot}$ primary star. The effective temperature inferred from the secondary eclipse depth was $\sim$600 K higher than the value predicted from stellar models. Such an anomalous result questions our understanding of low-mass stars and might indicate a significant uncertainty when inferring properties of exoplanets orbiting them. We seek to measure the effective temperature of the M-dwarf companion using the light curve of J0113+31 recently observed by the Transiting Exoplanet Survey Satellite (TESS). We use the pycheops modelling software to fit a combined transit and eclipse model to the TESS light curve. To calculate the secondary effective temperature, we compare the best-fit eclipse depth to the predicted eclipse depths from theoretical stellar models. We determined the effective temperature of the M dwarf to be ${\rm T}_{\rm eff,2}$ = 3208 $\pm$ 43 K, assuming $\log g_2$ = 5, [Fe/H] = $-0.4$ and no alpha-element enhancement. Varying these assumptions changes ${\rm T}_{\rm eff,2}$ by less than 100 K. These results do not support a large anomaly between observed and theoretical low-mass star temperatures.Comment: 5 pages, 3 figures, published in MNRA

Orbital misalignment of the super-Earth π\pi Men c with the spin of its star

Planet-planet scattering events can leave an observable trace of a planet's migration history in the form of orbital misalignment with respect to the the stellar spin axis, which is measurable from spectroscopic timeseries taken during transit. We present high-resolution spectroscopic transits observed with ESPRESSO of the close-in super-Earth $\pi$ Men c. The system also contains an outer giant planet on a wide, eccentric orbit, recently found to be inclined with respect to the inner planetary orbit. These characteristics are reminiscent of past dynamical interactions. We successfully retrieve the planet-occulted light during transit and find evidence that the orbit of $\pi$ Men c is moderately misaligned with the stellar spin axis with $\lambda = -24.0^\circ \pm 4.1^\circ$ ($\psi = 26.9^{\circ +5.8^\circ}_{\,-4.7^\circ}$). This is consistent with the super-Earth $\pi$ Men c having followed a high-eccentricity migration followed by tidal circularisation, and hints that super-Earths can form at large distances from their star. We also detect clear signatures of solar-like oscillations within our ESPRESSO radial velocity timeseries, where we reach a radial velocity precision of ${\sim}20$ cm/s. We model the oscillations using Gaussian processes and retrieve a frequency of maximum oscillation, $\nu_\text{max} = 2771^{+65}_{-60}$ $\mu$Hz. These oscillations makes it challenging to detect the Rossiter-McLaughlin effect using traditional methods. We are, however, successful using the reloaded Rossiter-McLaughlin approach. Finally, in an appendix we also present updated physical parameters and ephemerides for $\pi$ Men c from a Gaussian process transit analysis of the full TESS Cycle 1 data.Comment: 20 pages, 11 figures. Published in MNRA

The TESS-SPOC FFI target sample explored with gaia

The Transiting Exoplanet Survey Satellite (TESS) mission has provided the community with high-precision times-series photometry for ∼2.8 million stars across the entire sky via the full frame image (FFI) light curves produced by the TESS Science Processing Operations Center (SPOC). This set of light curves is an extremely valuable resource for the discovery of transiting exoplanets and other stellar science. However, due to the sample selection, this set of light curves does not constitute a magnitude-limited sample. In order to understand the effects of this sample selection, we use Gaia Data Release 2 (DR2) and Data Release 3 (DR3) to study the properties of the stars in the TESS-SPOC FFI light-curve set, with the aim of providing vital context for further research using the sample. We report on the properties of the TESS-SPOC FFI targets in Sectors 1–55 (covering Cycles 1–4). We cross-match the TESS-SPOC FFI targets with the Gaia DR2 and DR3 catalogues of all targets brighter than Gaia magnitude 14 to understand the effects of sample selection on the overall stellar properties. This includes Gaia magnitude, parallax, radius, temperature, non-single star flags, luminosity, radial velocity, and stellar surface gravity. In total, there are ∼16.7 million Gaia targets brighter than G = 14, which when cross-matched with the TESS-SPOC FFI targets leaves ∼2.75 million. We investigate the binarity of each TESS-SPOC FFI target and calculate the radius detection limit from two detected TESS transits that could be detected around each target. Finally, we create a comprehensive main-sequence TESS-SPOC FFI target sample that can be utilized in future studies

Measured Spin-Orbit Alignment of Ultra-Short Period Super-Earth 55 Cancri e

A planet's orbital alignment places important constraints on how a planet formed and consequently evolved. The dominant formation pathway of ultra-short period planets ($P<1$ day) is particularly mysterious as such planets most likely formed further out, and it is not well understood what drove their migration inwards to their current positions. Measuring the orbital alignment is difficult for smaller super-Earth/sub-Neptune planets, which give rise to smaller amplitude signals. Here we present radial velocities across two transits of 55 Cancri e, an ultra-short period Super-Earth, observed with the Extreme Precision Spectrograph (EXPRES). Using the classical Rossiter-McLaughlin (RM) method, we measure 55 Cnc e's sky-projected stellar spin-orbit alignment (i.e., the projected angle between the planet's orbital axis and its host star's spin axis) to be $\lambda=10\substack{+17\\ -20}^{\circ}$ with an unprojected angle of $\psi=23\substack{+14\\ -12}^{\circ}$. The best-fit RM model to the EXPRES data has a radial velocity semi-amplitude of just $0.41\substack{+0.09\\ -0.10} m s^{-1}$. The spin-orbit alignment of 55 Cnc e favors dynamically gentle migration theories for ultra-short period planets, namely tidal dissipation through low-eccentricity planet-planet interactions and/or planetary obliquity tides.Comment: 12 pages, 4 figures, published in Nature Astronom

The EBLM project. VII. Spin-orbit alignment for the circumbinary planet host EBLM J0608-59 A/TOI-1338 A

Funding: This work was inpart funded by the U.S.–Norway Fulbright Foundation and a NASATESSGI grant G022253 (PI: Martin). AHMJT received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant 803193/BEBOP), and from a Leverhulme Trust Research Project grant (RPG-2018-418). VKH is also supported by a Birmingham Doctoral Scholarship, and by a studentship from Birmingham’s School of Physics & Astronomy. DVM received funding from the Swiss National Science Foundation (grant P 400P2 186735). SG has been supported by STFC through consolidated grants ST/L000733/1and ST/P000495/1.A dozen short-period detached binaries are known to host transiting circumbinary planets. In all circumbinary systems so far, the planetary and binary orbits are aligned within a couple of degrees. However, the obliquity of the primary star, which is an important tracer of their formation, evolution, and tidal history, has only been measured in one circumbinary system until now. EBLM J0608-59/TOI-1338 is a low-mass eclipsing binary system with a recently discovered circumbinary planet identified by TESS. Here, we perform high-resolution spectroscopy during primary eclipse to measure the projected stellar obliquity of the primary component. The obliquity is low, and thus the primary star is aligned with the binary and planetary orbits with a projected spin-orbit angle β = 2.°8 ± 17.°1. The rotation period of18.1 ± 1.6 d implied by our measurement of vsinI⋆ suggests that the primary has not yet pseudo-synchronized with the binary orbit, but is consistent with gyrochronology and weak tidal interaction with the binary companion. Our result, combined with the known coplanarity of the binary and planet orbits, is suggestive of formation from a single disc. Finally, we considered whether the spectrum of the faint secondary star could affect our measurements. We show through simulations that the effect is negligible for our system, but can lead to strong biases in vsinI⋆ and β for higher flux ratios. We encourage future studies in eclipse spectroscopy test the assumption of a dark secondary for flux ratios ≳1ppt.Publisher PDFPeer reviewe

BEBOP V. Homogeneous stellar analysis of potential circumbinary planet hosts

Planets orbiting binary systems are relatively unexplored compared to those around single stars. Detections of circumbinary planets and planetary systems offer a first detailed view into our understanding of circumbinary planet formation and dynamical evolution. The BEBOP (binaries escorted by orbiting planets) radial velocity survey plays a special role in this adventure as it focuses on eclipsing single-lined binaries with an FGK dwarf primary and M dwarf secondary allowing for the highest radial velocity precision using the HARPS and SOPHIE spectrographs. We obtained 4512 high-resolution spectra for the 179 targets in the BEBOP survey which we used to derive the stellar atmospheric parameters using both equivalent widths and spectral synthesis. We furthermore derive stellar masses, radii, and ages for all targets. With this work, we present the first homogeneous catalogue of precise stellar parameters for these eclipsing single-lined binaries

BEBOP V. Homogeneous stellar analysis of potential circumbinary planet hosts

Funding: This research is supported from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 803193/BEBOP), and by a Leverhulme Trust Research Project Grant (no. RPG-2018-418). ACC acknowledges support from STFC consolidated grant numbers ST/R000824/1 and ST/V000861/1. PM and NM were supported by STFC grant number ST/S001301/1. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (project SPICE DUNE, grant agreement no. 947634). DJA is supported by UKRI through the STFC (ST/R00384X/1) and EPSRC (EP/X027562/1). AC acknowledge funding from the French ANR under contract number ANR18CE310019 (SPlaSH). This work is supported by the French National Research Agency in the framework of the Investissements d’Avenir program (ANR-15-IDEX-02), through the funding of the ‘Origin of Life’ project of the Grenoble-Alpes University. EW acknowledges support from the ERC Consolidator Grant funding scheme (project ASTEROCHRONOMETRY, grant agreement no. 772293 http://www.asterochronometry.eu). MRS acknowledges support from the UK Science and Technology Facilities Council (ST/T000295/1).Planets orbiting binary systems are relatively unexplored compared to those around single stars. Detections of circumbinary planets and planetary systems offer a first detailed view into our understanding of circumbinary planet formation and dynamical evolution. The BEBOP (Binaries Escorted by Orbiting Planets) radial velocity survey plays a special role in this adventure as it focuses on eclipsing single-lined binaries with an FGK dwarf primary and M dwarf secondary allowing for the highest-radial velocity precision using the HARPS and SOPHIE spectrographs. We obtained 4512 high-resolution spectra for the 179 targets in the BEBOP survey which we used to derive the stellar atmospheric parameters using both equivalent widths and spectral synthesis. We furthermore derive stellar masses, radii, and ages for all targets. With this work, we present the first homogeneous catalogue of precise stellar parameters for these eclipsing single-lined binaries.Peer reviewe

BEBOP V. Homogeneous stellar analysis of potential circumbinary planet hosts

Planets orbiting binary systems are relatively unexplored compared to those around single stars. Detections of circumbinary planets and planetary systems offer a first detailed view into our understanding of circumbinary planet formation and dynamical evolution. The BEBOP (binaries escorted by orbiting planets) radial velocity survey plays a special role in this adventure as it focuses on eclipsing single-lined binaries with an FGK dwarf primary and M dwarf secondary allowing for the highest radial velocity precision using the HARPS and SOPHIE spectrographs. We obtained 4512 high-resolution spectra for the 179 targets in the BEBOP survey which we used to derive the stellar atmospheric parameters using both equivalent widths and spectral synthesis. We furthermore derive stellar masses, radii, and ages for all targets. With this work, we present the first homogeneous catalogue of precise stellar parameters for these eclipsing single-lined binaries