Unified Kraft Break at ~6500 K: A Newly Identified Single-Star Obliquity Transition Matches the Classical Rotation Break

TL;DR

This study redefines the stellar obliquity transition temperature to approximately 6500 K, aligning it with the classical rotational Kraft break, and clarifies the role of binaries in previous measurements, impacting theories of star-planet alignment.

Contribution

The paper identifies the true single-star obliquity transition at about 6447 K, matching the rotational break, by removing binary effects from previous analyses.

Findings

The single-star obliquity transition is at ~6447 K, aligning with the rotational Kraft break.

Binaries cause the previously observed cooler obliquity transition (~6105 K).

Implications for understanding spin-orbit misalignment and tidal dissipation mechanisms.

Abstract

The stellar obliquity transition, defined by a $\textit{T}_{\rm eff}$ cut separating aligned from misaligned hot Jupiter systems, has long been assumed to coincide with the rotational Kraft break. Yet the commonly quoted obliquity transition (6100 or 6250 K) sits a few hundred kelvin cooler than the rotational break (~6500 K), posing a fundamental inconsistency. We show this offset arises primarily from binaries/multiple-star systems, which drive the cooler stellar obliquity transition ($6105^{+123}_{-133}$ K), although the underlying cause remains ambiguous. After removing binaries and higher-order multiples, the single-star stellar obliquity transition shifts upward to $6447^{+85}_{-119}$ K, in excellent agreement with the single-star rotation break ($6510^{+97}_{-127}$ K). This revision has two immediate consequences for understanding the origin and evolution of spin-orbit misalignment. First, the upward shift reclassifies some hosts previously labeled `hot' into the cooler regime; consequently, there are very few RM measurements of non-hot-Jupiter planets around genuinely hot stars ($T_{\rm eff}\gtrsim6500\,\mathrm{K}$), and previously reported alignment trends for these classes of systems (e.g., warm Jupiters and compact multi-planet systems) lose the power to discriminate the central question: are large misalignments unique to hot-Jupiter-like planets that can be delivered by high-$e$ migration, or are hot stars intrinsically more misaligned across architectures? Second, a single-star stellar obliquity transition near $6500\,\mathrm{K}$, coincident with the rotational break, favors tidal dissipation in outer convective envelopes; as these envelopes thin with increasing $T_{\rm eff}$, inertial-wave damping and magnetic braking weaken in tandem.