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

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.

Figures (4)

• Figure 1: The $\lambda$ and $v\sin{i_\star}$ distributions, along with derived $T_{\rm eff}$ boundaries. Panel a:$\lambda$ distribution for hot-Jupiter systems with confirmed or candidate stellar companions. The $T_{\rm eff}$ boundary of ${6105^{+123}_{-133}}$ K is marked with a black dashed line, with shaded 1 $\sigma$ interval. Panel b: Same as the top panel, but for single-star hot-Jupiter systems with $T_{\rm eff}$ boundary = ${6447^{+85}_{-119}}$ K. Panel c:$v\sin{i_\star}$ distribution for single stars, compiled following the updated prescription described in Beyer2024. The resulting $T_{\rm eff}$ boundary is ${6510^{+97}_{-127}}$ K, consistent with that derived from the single-star $\lambda$ distribution. Panel d: same as panel c, but for binary and multiple-star systems.
• Figure 2: Stellar effective temperature ($T_{\rm eff}$) and Gaia DR3 $G$ magnitude for the stellar obliquity sample with stellar companions. Planet-host stars and their corresponding stellar companions are shown as orange and blue dots, respectively. The gray lines connect each host-companion pair. The $T_{\rm eff}$ distributions of the two populations are shown in the right panel. Interestingly, in our stellar-obliquity sample (consisting of hot Jupiters), the planet host is always the more massive component of a binary.
• Figure 3: The $\lambda$ distributions for hot-Jupiter ($3\times10^{-4} \leq m_{\rm p}/M_\star \leq 2\times10^{-3}$, $a/R_\star$$<$10), sub-Saturn ($3\times10^{-5} \leq m_{\rm p}/M_\star \leq 3\times10^{-3}$), warm-Jupiter ($3\times10^{-4} \leq m_{\rm p}/M_\star \leq 2\times10^{-3}$, $a/R_\star$$>$11), compact multiplanet (period ratio of adjacent planets, $P_{i+1}/P_{i}<6$), massive-planet systems ($2\times10^{-3} \leq m_{\rm p}/M_\star$), and eccentric Jupiters ($3\times10^{-4} \leq m_{\rm p}/M_\star \leq 2\times10^{-3}$, $e>0.1$), along with the Kraft break and its associated uncertainty derived in this work. In terms of $T_{\rm eff}$ coverage, only the hot-Jupiter systems have sufficient measurements ($>10$) in the hot-star regime. Sub-Saturns with nearby companions are marked in blue. The predicted relation between $T_{\rm eff}$ and $\lambda$ from secular resonance crossing Petrovich2020Dugan2025 is shown as the light-blue shaded region.
• Figure 4: Evolution of solar-composition MESA stellar models on either side of the convective-core transition. In both panels, the blue curves show the time evolution of $1.11~M_\odot$ stellar models, which do not exhibit a long-lived convective core on the main sequence, while the orange curves show those of $1.15~M_\odot$ stellar models, where a convective core persists throughout the main sequence until core hydrogen exhaustion. The upper panel shows the time evolution of the average Brunt-Väisälä frequency (or, equivalently, the g-mode undertone spacing), \ref{['eq:bv']}, compared to a reference epoch of $t_0 = 1$ Gyr, as in zanazzi2024damping. The lower panel shows how these stellar models evolve on the Hertzsprung-Russell diagram.