Relating turbulent pressure and macroturbulence across the HR diagram with a possible link toγDoradus stars

Abstract

A significant fraction of the envelope of low-and intermediate-mass stars is unstable to convection, leading to sub-surface turbulent motion. Here, we consider and include the effects of turbulence pressure in our stellar evolution calculations. In search of an observational signature, we compare the fractional contribution of turbulent pressure to the observed macroturbulent velocities in stars at different evolutionary stages. We find a strong correlation between the two quantities, similar to what was previously found for massive OB stars. We therefore argue that turbulent pressure fluctuations of finite amplitude may excite high-order, high-angular degree stellar oscillations, which manifest themselves at the surface an additional broadening of the spectral lines, i.e., macroturbulence, across most of the HR diagram. When considering the locations in the HR diagram where we expect high-order oscillations to be excited by stochastic turbulent pressure fluctuations, we find a close match with the observational γ Doradus instability strip, which indeed contains high-order, non-radial pulsators. We suggest that turbulent pressure fluctuations on a percentual level may contribute to the γ Dor phenomenon, calling for more detailed theoretical modeling in this direction.

Near-uniform internal rotation of the main-sequence γ Doradus pulsator KIC 7661054

“…Hence, pulsational broadening is taken into account via our macroturbulence term. In other words, some of the vmacro we measured actually arises from the g-mode pulsations, while some most likely originates from turbulence in the convective zones (Grassitelli et al 2015).…”

“…As they warned, neglecting macroturbulence can lead to a gross overestimate of v sin i in slow rotators. Macroturbulence is a significant line-broadening mechanism in late-A and early-F stars associated with a surface velocity field that can be as high as the local sound speed (Grassitelli et al 2015). In the case of KIC 7661054, this is a large fraction of the published v sin i value.…”

“…At the molecular level, the inviscid assumption is valid, but turbulent viscosity is nonnegligible. Indeed, turbulent pressure contributes as much as 15 per cent of the total pressure in the convective zone of early-F stars (Grassitelli et al 2015), and the associated viscosity should be accounted for. Perhaps viscosity works efficiently at reducing rotation gradients, and this is why near-uniform rotation profiles tend to be observed, but this requires turbulent motion to be ubiquitous in the radiative layers between the core and the envelope.…”

Asteroseismic test of rotational mixing in low-mass white dwarfs

“…Using the latter version would increase the contribution of convective driving and, presumably, widen the band of excited periods and the width of the instability strip. Moreover, the additional pressure arising from the turbulent convective motion can be as high as a few percent, which could lead to more structural differences and observational effects (Grassitelli et al 2015). We are thus optimistic that the case of J0756+6704 can be solved.…”

The IACOB project

“…considering a similar sample of stars in both Magellanic Clouds), and other stellar domains in the HR diagram (see, e.g. Grassitelli et al 2015b) will certainly provide interesting clues to further support/dismiss scenarios about macroturbulent broadening in a more general context.…”