Stellar Spin-Orbit Misalignment in a Multiplanet System

Abstract

The Kepler space telescope detects exoplanets by measuring periodic dimmings of light as a planet passes in front of its host star (1). The majority of the ∼ 150,000 targets observed by Kepler are unevolved stars near the main sequence, because those stars provide the best prospect for detecting habitable planets similar to Earth (2). In contrast, the temperature and surface gravity of indicate that it is an evolved star with exhausted hydrogen in its core, and that it started burning hydrogen in a shell surrounding an inert Helium core. Stellar evolutionary theory predicts that our Sun will evolve into a low-luminosity red giant similar in size to Kepler-56 in roughly 7 billion years.The Kepler planet search pipeline detected two planet candidates orbiting (designated as KOI-1241) (3) with periods of 10.50 and 21.41 days, a nearly 2:1 commensurability. The observation of transit time variations caused by gravitational interactions 2 showed that the two candidates represent objects orbiting the same star, and modeling of these variations led to upper limits on their masses that place them firmly in the planetary regime (4). Kepler-56 is the most evolved star observed by Kepler with more than one detected planet.Transit observations lead to measurements of planet properties relative to stellar properties, and hence accurate knowledge of the host star is required to characterize the system. Asteroseismology enables inference of stellar properties through the measurement of oscillations excited by near-surface convection (5). The power spectrum of the Kepler-56 data after removing the planetary transits shows a regular series of peaks ( Fig. 1), which are characteristic of stellar oscillations. By combining the measured oscillation frequencies with the effective temperature and chemical composition obtained from spectroscopy, we were able to precisely determine the properties of the host star (6). Kepler-56 is more than four times as large as the Sun and its mass is 30% greater (Table 1).Non-radial oscillations in evolved stars are mixed modes, behaving like pressure modes in the envelope and like gravity modes in the core (7,8). Unlike pressure-dominated mixed modes, gravity-dominated mixed modes have frequencies that are shifted from the regular asymptotic spacing. Mixed modes are also approximately equally spaced in period (9). We measured the average period spacing between dipole (l = 1) modes in Kepler-56 to be 50 seconds, consistent with a first ascent red giant (10).Individual mixed dipole modes are further split into multiplets as a result of stellar rotation. Because the modes in each multiplet are on average expected to be excited to very nearly equal amplitudes, the observed relative amplitudes depend only on viewing angle relative to the stellar rotation axis (11). For Kepler-56 several mixed dipole modes show triplets (Fig. 1). A rotation axis perpendicular to the line of sight (inclination i = 90 • for pressure-dominated modes. Simulations confirmed that the inclination measurements are not strongly...

Determining stellar parameters of asteroseismic targets: going beyond the use of scaling relations

“…In this respect a way forward would be to determine the star's mean density by using the full set of observed acoustic modes, not just their average frequency spacing. This approach was carried out in at least two RGB stars (Huber et al 2013;Lillo-Box et al 2014), and led to determination of the stellar mean density which is ∼ 5 − 6 per cent higher than derived from assuming scaling relations, and with a much improved precision of ∼ 1.4 per cent. Furthermore, the impact of surface effects on the inferred mean density is mitigated when determining the mean density using individual mode frequencies rather than using the average large separation (e.g., see Chaplin & Miglio 2013).…”

Asteroseismology of red giants: From analysing light curves to estimating ages

“…This approach was carried out in at least two RGB stars (Huber et al 2013;Lillo-Box et al 2014), and led to determination of the stellar mean density which is ∼ 5-6 % higher than derived from assuming scaling relations, and with a much improved precision of ∼ 1.4 %.…”

Spin-orbit angle distribution and the origin of (mis)aligned hot Jupiters

“…Traditionally, this has been achieved through the Rossiter-McLaughlin effect (Mac Laughlin 1924); however, recently novel techniques involving asteroseismology (Huber et al 2013) and star-spot measurements (Hirano et al 2012) have also been utilised to this end. A wide variety of measured angles have been reported to date, and many exo-planets are apparently characterised by spin-orbit misalignment: the orbital obliquity differs significantly from 0.…”

“…Contrary to the violent category of migration mechanisms -e.g., planet-planet scattering (Rasio & Ford 1996;Ford & Rasio 2008) and Kozai resonance (Wu & Murray 2003;Fabrycky & Tremaine 2007;Naoz et al 2011) -this process predicts that all the planets of the system can share the same misalignment. The recent discovery of significant misalignment among a multi-transiting system (Huber et al 2013) therefore puts emphasis on this model. Using analytical arguments and Monte Carlo simulations, we find that the current observational aggregate can be well explained by the disk-torquing effect, implying that disk-driven planet migration in (torqued) disks could be the dominant source of (mis)aligned hot Jupiters.…”