Tidally Delayed Spin-Down of Very Low Mass Stars

TL;DR

This paper proposes that tidal interactions with sub-stellar companions, such as brown dwarfs, can delay the spin-down of very low-mass stars, explaining observed rotation period trends and affecting stellar activity and aging methods.

Contribution

It introduces a model coupling magnetic spin-down with tidal spin-up from sub-stellar companions, providing a new explanation for rotation trends in low-mass stars.

Findings

Tidal forces from brown dwarf companions delay stellar spin-down.

Tides influence stellar X-ray activity evolution.

Model can help constrain the distribution of companion orbital separations.

Abstract

Very low-mass main-sequence stars reveal some curious trends in observed rotation period distributions that require abating the spin-down that standard rotational evolution models would otherwise imply. By dynamically coupling magnetically mediated spin-down to tidally induced spin-up from close orbiting substellar companions, we show that tides from sub-stellar companions may explain these trends. In particular, brown dwarf companions can delay the spin-down and explain the dearth of field, late-type M dwarfs with intermediate rotation periods. We find that tidal forces also strongly influence stellar X-ray activity evolution, so that methods of gyrochronological aging must be generalized for stars with even sub-stellar companions. We also discuss how the theoretical predictions of the spin evolution model can be used with future data to constrain the population distribution of companion orbital separations.