Obliquities of Hot Jupiter host stars: Evidence for tidal interactions and primordial misalignments (1206.6105v2)

Abstract: We provide evidence that the obliquities of stars with close-in giant planets were initially nearly random, and that the low obliquities that are often observed are a consequence of star-planet tidal interactions. The evidence is based on 14 new measurements of the Rossiter-McLaughlin effect (for the systems HAT-P-6, HAT-P-7, HAT-P-16, HAT-P-24, HAT-P-32, HAT-P-34, WASP-12, WASP-16, WASP-18, WASP-19, WASP-26, WASP-31, Gl 436, and Kepler-8), as well as a critical review of previous observations. The low-obliquity (well-aligned) systems are those for which the expected tidal timescale is short, and likewise the high-obliquity (misaligned and retrograde) systems are those for which the expected timescale is long. At face value, this finding indicates that the origin of hot Jupiters involves dynamical interactions like planet-planet interactions or the Kozai effect that tilt their orbits, rather than inspiraling due to interaction with a protoplanetary disk. We discuss the status of this hypothesis and the observations that are needed for a more definitive conclusion.

• The paper presents 14 new RM effect measurements that showcase how tidal forces shape stellar obliquities in hot Jupiter systems.
• The paper finds that stars with rapid tidal interactions exhibit low obliquities, while systems with longer tidal timescales display a wider range of alignments.
• The paper challenges standard migration models by suggesting that primordial misalignments, from dynamic interactions, play a key role in exoplanetary system evolution.

Obliquities of Hot Jupiter Host Stars: Evidence for Tidal Interactions and Primordial Misalignments

The paper conducted by Albrecht et al. investigates the obliquities of stars hosting hot Jupiters, focusing on the underlying causes of the observed alignment—or misalignment—between stellar rotation axes and planetary orbital planes. This research combines new observational data with a critical review of existing literature to provide comprehensive insights into the dynamics of star-planet interactions, particularly those resulting from tidal forces.

Key Findings and Methodology

The paper presents 14 new measurements of the Rossiter-McLaughlin (RM) effect across various hot Jupiter systems, including notable objects such as WASP-12, Kepler-8, and HAT-P series stars. These measurements are pivotal in determining the angle of obliquity—defined as the angle between the star's rotational axis and the orbit of its close-in giant planet. The paper consolidates these new findings with previously available data, offering an enriched dataset for analysis.

Central to the discussion is the role of tidal interactions in shaping the observed obliquities. The paper underscores a pattern where stars exerting quick tidal interactions exhibit relatively low obliquities, while systems with longer tidal timescales present a broader range of obliquities. This suggests that initially, the obliquities were distributed in a nearly random fashion, with subsequent tidal forces driving the systems towards alignment over time.

Implications and Theoretical Considerations

The findings have significant implications for understanding planetary migration theories. The broad range in primordial obliquities supports theories involving dynamic interactions, such as planet-planet scattering or the Kozai mechanism, that perturb orbital inclinations. This stands in contrast to theories positing that hot Jupiters migrate inward through interaction with a protoplanetary disk, which would typically preserve low obliquities.

Albrecht et al. hypothesize that the effectiveness of tidal realignment is contingent upon the internal structure of the host star, particularly the presence of a convective envelope. Stars with effective temperatures exceeding 6250 K, often lacking significant convective envelopes, exhibit less tidal dissipation and thus retain higher obliquities.

Direction for Future Research

This paper paves the way for future research to focus on several areas, including:

1. Extended Observational Campaigns: More comprehensive measurements of RM effects in additional star-planet systems will further substantiate the findings.
2. Multiple Planet Systems: Investigating obliquities in systems with multiple planets could reveal whether the observed misalignments are a feature unique to single hot Jupiter systems or a broader characteristic of planetary formation.
3. Theoretical Models of Tidal Dissipation: Refining and testing models that predict tidal dissipation rates based on stellar structure will be crucial in understanding the longevity and final orbits of close-in giant planets.
4. Effect of Stellar Evolution: Examining how stellar evolution impacts tidal interactions and the resulting planetary system architecture will enhance models of system stability over time.

The research of Albrecht et al. offers a detailed analysis of star-planet obliquity dynamics, emphasizing the importance of tidal interactions while also suggesting the potential for primordial misalignments. This work challenges simplistic models of planetary migration and calls for a nuanced understanding of the factors influencing the early dynamical histories of exoplanetary systems.