Tracing the evolution of short-period binaries with super-synchronous fast rotators

The initial distribution of rotational velocities of stars is still poorly
known, and how the stellar spin evolves from birth to the various end points of
stellar evolution is an actively debated topic. Binary interactions are often
invoked to explain the existence of extremely fast-rotating stars ($v\sin\,i$
$\gtrsim$ 200 $km\,s^{-1}$). The primary mechanisms through which binaries can
spin up stars are tidal interactions, mass transfer, and possibly mergers. To
evaluate these scenarios, we investigated in detail the evolution of three
known fast-rotating stars in short-period spectroscopic and eclipsing binaries,
namely HD 25631, HD 191495, and HD 46485, with primaries of masses of 7, 15,
and 24 $M_{\odot}$, respectively, with companions of $\sim1\,M_\odot$ and
orbital periods of less than 7 days. These systems belong to a recently
identified class of binaries with extreme mass ratios, whose evolutionary
origin is still poorly understood. We evaluated in detail three scenarios that
could explain the fast rotation observed in these binaries: it could be
primordial, a product of mass transfer, or the result of a merger within an
originally triple system. We computed grids of single and binary MESA models
varying tidal forces and initial binary architectures to investigate the
evolution and reproduce observational properties of these systems. We find
that, because of the extreme mass-ratio between binary components, tides have a
limited impact, regardless of the prescription used, and that the observed
short orbital periods are at odds with post-mass-transfer scenarios. The most
likely scenario to form such young, rapidly rotating, and short-period binaries
is primordial rotation, implying that the observed binaries are pre-interaction
ones. These binaries show that the initial spin distribution of massive stars
can have a wide range of rotational velocities.