Evidence of Possible Spin-Orbit Misalignment Along the Line of Sight in Transiting Exoplanet Systems

Abstract

There has lately been intense interest in the degree of alignment between the orbits of transiting exoplanets and the spin of their hosts stars. Indeed, of 26 transiting exoplanet systems with measurements of the Rossiter-McLaughlin (RM) effect, eight have now been found to be significantly spin-orbit misaligned in the plane of the sky. Unfortunately, the RM effect only measures the angle between the orbit of a transiting exoplanet and the spin of its host star projected in the plane of sky, leaving unconstrained the compliment misalignment angle between the orbit of the planet and the spin of its host star along the line of sight. I use a simple model of stellar rotation benchmarked with observational data to statistically identify ten exoplanet systems from a sample of 75 for which there is likely a significant degree of misalignment along the line of sight between the orbit of the planet and the spin of its host star. I find that HAT-P-7, HAT-P-14, HAT-P-16, HD 17156, Kepler-5, Kepler-7, TrES-4, WASP-1, WASP-12, and WASP-14 are likely spin-orbit misaligned along the line of sight. All ten systems have host stellar masses M * in the range 1.2 M ⊙ M * 1.5 M ⊙ , and the probability of this occurrence by chance is less than one in ten thousand. In addition, the planets in the candidate misaligned systems are preferentially massive and eccentric. I combine the distribution of misalignment in the plane of the sky from RM measurements with the distribution of misalignment along the line of sight from this analysis, and the result suggests that transiting exoplanets are more likely to be aligned along the line of sight than expected given predictions for the spin-orbit misalignment distribution of a population produced entirely by planet-planet scattering or Kozai cycles and tidal friction. For that reason, there are likely two populations of close-in exoplanet systems: a population of aligned systems and a population of apparently misaligned systems in which the processes that lead to misalignment or to the survival of misaligned systems operate more efficiently in systems with massive stars and planets.

Spin-orbit measurements and refined parameters for the exoplanet systems WASP-22 and WASP-26

“…The predicted amplitude of the RM effect of WASP-26b is smaller than that of WASP-22b due to the higher impact parameter and, to a lesser extent, the slower stellar rotation velocity. The similarity of our estimates of v sin I and v (Table 1), which agrees with the prediction of Schlaufman (2010) that v = 3.1 ± 0.3 km s −1 , suggests that the spin axis of WASP-26 is not significantly inclined relative to the sky plane. Thus it is unlikely that the non-detection of the RM effect is due to the orientation of WASP-26 being close to pole-on.…”

“…3a). The similarity of our estimates of v sin I and v (Table 1), which agrees with the prediction of Schlaufman (2010) that v = 3.2 ± 1.0 km s −1 , suggests that the spin axis of WASP-22 is not significantly inclined relative to the sky plane (i.e., Ψ ∼ λ). With T eff = 6000 ± 100 K, WASP-22 is consistent with the observation of Winn et al (2010a) that the orbits of planets around stars cooler than ∼6250 K tend to be aligned.…”

“…With T eff = 6000 ± 100 K, WASP-22 is consistent with the observation of Winn et al (2010a) that the orbits of planets around stars cooler than ∼6250 K tend to be aligned. Conversely, the orbits of planets around hotter stars tend to be misaligned, as noted by both Winn et al (2010a) and Schlaufman (2010). A few possible expections to this trend are now known, including HAT-P-9 with T eff = 6350 K and λ = −16 ± 8 • (Moutou et al 2011).…”

Planets transiting non-eclipsing binaries

“…Earlier considerations aside, it is likely that the ultimate orientation of the disc does not completely determine the inclinations of planetary orbits, as epitomised by the existence of inclined and even retrograde hot Jupiters (Hébrard et al 2008;Winn et al 2009;Triaud et al 2010;Schlaufman 2010;Triaud 2011a;Brown et al 2012;Albrecht et al 2012). Even if circumbinary planets can ultimately only form in coplanar discs, concluding that planets will also be coplanar does not take into account post-formation dynamical processes, such as planetplanet scattering.…”

The Rossiter-McLaughlin effect for exoplanets

“…Figure 5 shows that the hot stars have a wide range of obliquities, and the cold stars generally have low obliquities except for those with the weakest tidal interactions. In addition, Schlaufman (2010) presented evidence that hot stars have high obliquities, based on a completely different technique. His idea was to compare the v sin i distribution of stars with transiting planets with those of random stars of the same mass and evolutionary state.…”