Spindown of massive main sequence stars in the Milky Way

TL;DR

This study probes wind-driven spin-down in massive Milky Way main-sequence stars using the Brott single-star grid, extended by a new angular-momentum-loss prescription and a population-synthesis tool (Hōkū). By comparing synthetic $v$-distributions to IACOB OB-star data through kernel-density estimates in the spectroscopic HR diagram, it finds good agreement for $M\lesssim 40\ M_\odot$ but substantial overestimation of spindown at higher masses, where fast rotators persist across MS ages. The persistence of rapid rotators, likely tied to binary interactions, suggests that wind spindown alone cannot explain the observed distributions and that close-binary evolution must be incorporated. The work highlights key uncertainties in wind mass loss, bi-stability effects, and envelope inflation, delineating paths for improved modeling of massive-star rotation and its end products.

Abstract

Context. We need to understand the spin evolution of massive stars to compute their internal rotationally induced mixing processes, isolate effects of close binary evolution, and predict the rotation rates of white dwarfs, neutron stars and black holes. Aims. We discuss the spindown of massive main sequence stars imposed by stellar winds. Methods. We use detailed grids of single star evolutionary models to predict the distribution of the surface rotational velocities of core-hydrogen burning Galactic massive stars as function of their mass and evolutionary state. We then compare the spin properties of our synthetic populations with appropriately selected sub-samples of Galactic main sequence OB-type stars extracted from the IACOB survey. Results. We find that below $\sim 40 M_\odot$, observations and models agree in finding that the surface rotational velocities of Galactic massive stars remain relatively constant during their main sequence evolution. The more massive stars in the IACOB sample appear to spin down less than predicted, while our updated angular momentum loss prescription predicts an enhanced spindown. Furthermore, the observations show a population of fast rotators, with $v \sin I \gtrsim 200$ km/s persisting for all ages, which is not reproduced by our synthetic single star populations. Conclusions. We conclude that the wind-induced spindown of massive main sequence stars is yet to be fully understood, and that close binary evolution might significantly contribute to the fraction of rapid rotators in massive stars.

Figures (10)

• Figure 1: A schematic diagram of a model's specific angular momentum as a function of radius, illustrating the angular momentum loss prescriptions corresponding to Eqs. \ref{['eqn:og_angmo']} and \ref{['eqn:new_angmo']}. The width of the hatched rectangle indicates the radius range containing the material that is removed from the model within one time step. The prescription implemented in the Brott grid removes the angular momentum represented by the hatched area. The new prescription removes the angular momentum in the hatched plus the shaded area.
• Figure 2: Top: Illustration of the Hōkū interpolation process. Left panel shows the initial masses and ZAMS velocities of the grid models in purple and turquoise and the simulated star in red. The right panel shows the evolutionary models in a spectroscopic HR diagram and the simulated star's interpolated effective temperature and spectroscopic luminosity. The crosses mark where the effective temperature and spectroscopic luminosity are interpolated from the grid models. Bottom: the resulting evolutionary track of the 27, $v_{\rm ZAMS} = \qty{200}{\kms}$ interpolated star in a spectroscopic HR diagram, along with the grid models used for interpolation.
• Figure 3: Evolution of the rotational velocity from ZAMS to TAMS of the models from the Brott grid, according to the new predictions (derived as discussed in Appendix \ref{['app:AM_loss']}). Each panel shows a different initial mass with the colors indicating the effective temperature over the course of the MS. The $x$-axis shows the current center hydrogen mass fraction ($X_\mathrm{core}$) divided by the initial center hydrogen mass fraction ($X_\mathrm{core,i}$).
• Figure 4: The background represents the two-dimensional KDE of the entire observed sample in the sHRD. The solid lines show the Brott grid evolutionary tracks at 20, 25, 30, 35, 40, 50, 60, 80, and 100. The dotted lines divide the MS into quarters, determined by the fraction of the initial center hydrogen mass fraction that has been burnt. The white line indicates $\log g = \qty{3.7}{dex}$. Only stars to the left of this line are used to create the initial velocity KDE discussed in Section \ref{['sec:vini_pdf']}.
• Figure 5: Distribution of projected rotational velocity for observed stars with $\log g > \qty{3.7}{dex}$, overplotted with 1- and 2-component Gaussian KDEs (solid and dashed lines, respectively).