Stellar Winds As a Mechanism to Tilt the Spin Axes of Sun-like Stars

Abstract

The rotation axis of the Sun is misaligned from the mean angular momentum plane of the Solar system by about 6 degrees. This obliquity significantly exceeds the ∼ 1 − 2 degree distribution of inclinations among the planetary orbits and therefore requires a physical explanation. In concert, Sun-like stars are known to spin down by an order of magnitude throughout their lifetimes. This spin-down is driven by the stellar wind, which carries angular momentum from the star. If the mean angular momentum axis of the stellar wind deviates from that of the stellar spin axis, it will lead to a component of the spin-down torque that acts to tilt the star. Here, we show that Solar-like tilts of 6 degrees naturally arise during the first 10-100 Myr after planet formation as a result of stellar winds that deviate by about 10 degrees from the star's spin axis. These results apply to the idealized case of a dipole field, mildly inclined to the spin axis. Time-variability in the misalignment between the magnetic and spin poles is modeled as stochastic fluctuations, autocorrelated over timescales comparable to the primordial spin-down time of several million years. In addition to wind direction, time-variability in mass-loss rate and magnetic topology over the stellar lifetime may alternatively generate obliquity. We hypothesize that the gaseous environments of young, open clusters may provide forcing over sufficient timescales to tilt the astrospheres of young stars, exciting modest obliquities. The more extreme, retrograde stellar obliquities of extrasolar planetary systems likely arise through separate mechanisms.

Tidal erasure of stellar obliquities constrains the timing of hot Jupiter formation

“…However, it is also possible that a hot Jupiter arrives on a well-aligned orbit, via disk-driven migration (or even through coplanar high-eccentricity migration; Petrovich 2015), and the misalignment occurs later. The proposed scenarios that have this property all depend upon the internal dynamics of the star, including internal angular momentum transport within the star due to internal gravity waves (Rogers et al 2012, the fluid elliptical instability (Cébron et al 2013), and asymmetric angular momentum loss from stellar winds (Spalding 2019). Each of these mechanisms depends upon an array of assumptions, the validity of which remains difficult to ascertain.…”

The distribution of mutual inclinations arising from the stellar quadrupole moment

“…Nevertheless, we see that the critical aspect of incorporating migration into the oblate tilted star framework lies in the relative timescales of migration and stellar spin-down. Both of these time-scales are poorly constrained during the earliest 100 Myr of the system's evolution (Bouvier et al 2014;Spalding 2019).…”

“…Initially, the spin-down time-scales of Sun-like stars may range within 10-100 Myr (Garraffo et al 2018;Spalding 2019). However, over longer time-scales, stars across a range of spectral types converge on to the so-called 'Skumanich' curve of P ∝ t 1/2 (Skumanich 1972;Garraffo et al 2018), with the spin-down time-scale eventually reaching Gyr.…”

The distribution of mutual inclinations arising from the stellar quadrupole moment

“…Nevertheless, we see that the critical aspect of incorporating migration into the oblate tilted star framework lies in the relative timescales of migration and stellar spin-down. Both of these timescales are poorly constrained during the earliest 100 Myrs of the system's evolution (Bouvier et al 2014;Spalding 2019).…”

“…Initially, the spin-down timescales of Sun-like stars may range between 10 − 100 Myr (Garraffo et al 2018;Spalding 2019). However, over longer timescales, stars across a range of spectral types converge onto the socalled "Skumanich" curve of P ∝ t 1/2 (Skumanich 1972;Garraffo et al 2018), with the spin-down timescale eventually reaching Gyr.…”