Hot Stars with Hot Jupiters Have High Obliquities (1006.4161v2)

Abstract: We show that stars with transiting planets for which the stellar obliquity is large are preferentially hot (T_eff > 6250 K). This could explain why small obliquities were observed in the earliest measurements, which focused on relatively cool stars drawn from Doppler surveys, as opposed to hotter stars that emerged later from transit surveys. The observed trend could be due to differences in planet formation and migration around stars of varying mass. Alternatively, we speculate that hot-Jupiter systems begin with a wide range of obliquities, but the photospheres of cool stars realign with the orbits due to tidal dissipation in their convective zones, while hot stars cannot realign because of their thinner convective zones. This in turn would suggest that hot Jupiters originate from few-body gravitational dynamics, and that disk migration plays at most a supporting role.

• The paper demonstrates that stars hotter than 6250 K hosting hot Jupiters have a 75% misalignment rate, compared to 18% for cooler stars.
• The study links high obliquity in hot stars to weak tidal realignment due to their thin convective envelopes, suggesting distinct formation paths.
• The findings challenge conventional disk migration theories and call for refined tidal models and further exploration of alternative orbital evolution mechanisms.

Analysis of "Hot Stars with Hot Jupiters Have High Obliquities"

This paper by Winn et al. investigates the relationship between stellar obliquities and the effective temperature of stars hosting hot Jupiters. The paper posits that stars with higher effective temperatures are more likely to exhibit misaligned spin-orbit configurations with their hot Jupiter exoplanets, a trend initially noted in a small sample of systems.

Key Findings

The paper reports that for stars with effective temperatures greater than 6250 K, there is a substantial tendency toward high stellar obliquity. This parameter, representing the angle between a star's rotation axis and the orbital plane of its planets, has been measured via the Rossiter-McLaughlin effect. In particular, the paper asserts that:

• Among systems observed, stars cooler than 6250 K exhibit a much lower rate of spin-orbit misalignment compared to hotter stars. Specifically, 75% of systems with hot Jupiters associated with hot stars are misaligned, whereas only 18% of the cooler stars show this misalignment.
• The paper suggests two contrasting formation and evolutionary pathways for hot Jupiters. For cooler stars, possibly due to their thicker convective envelopes, tidal dissipation can realign the stellar rotation axis with the orbit of the planet. Conversely, hotter stars, with thinner convective zones, lack this realigning capability, thereby maintaining their native misaligned states.

Discussion on Formation Mechanisms

The findings lead to speculative discussions on the mechanisms by which hot Jupiters attain their orbits. The paper challenges the conventional disk migration theory, which predicts aligned orbits. Instead, it considers the potential roles of planet-planet scattering and Kozai-Lidov cycles triggering orbital dynamics that result in high obliquities.

Tidal Realignment and Core-Envelope Decoupling

A central hypothesis explored concerns tidal interactions, primarily affecting stars with substantial convective envelopes. The authors speculate about a scenario where only the stellar convective zone, rather than the radiative interior, is subjected to dissipative interactions with the planetary orbit. This could allow the surface of cooler stars to align with planetary orbits without inducing significant orbital decay.

The paper acknowledges the complexity and uncertainty regarding tidal dissipation constants and mechanisms, suggesting that theoretical models should be refined to distinguish between surface convective and deeper radiative effects.

Implications for the Exoplanetary Research

The paper points out the importance of considering stellar effective temperature in developing predictive models of exoplanetary alignment. A significant implication is that surveys may underrepresent well-aligned systems around hotter stars due to detection biases, such as limitations in radial velocity measurements for rapidly rotating hot stars.

Future observational campaigns and theoretical work are encouraged to further assess the $\lambda$--$T_{\rm eff}$ correlation's permanence and significance. Investigations could explore correlations with other parameters like stellar mass, drawing from a broader sample to bolster statistical validity and understand the underlying physics.

In summary, the paper provides a nuanced examination of alignment patterns in hot Jupiter systems, urging further scrutiny of low obliquity predictions from disk migration theories and encouraging a re-evaluation of tidal interaction models in stellar evolution.