Differential Rotation in Convective Envelopes: Constraints from Eclipsing Binaries

Over time, tides synchronize the rotation periods of stars in a binary system to the orbital period. However, if the star exhibits differential rotation then only a portion of it can rotate at the orbital period, so the rotation period at the surface may not match the orbital period. The difference between the rotation and orbital periods can therefore be used to infer the extent of the differential rotation. We use a simple parameterization of differential rotation in stars with convective envelopes in circular orbits to predict the difference between the surface rotation period and the orbital period. Comparing this parameterization to observed eclipsing binary systems, we find that in the surface convection zones of stars in short-period binaries there is very little radial differential rotation, with $|r\partial_r \ln \Omega| < 0.02$. This holds even for longer orbital periods, though it is harder to say which systems are synchronized at long periods, and larger differential rotation is degenerate with asynchronous rotation.Comment: 19 pages, published in MNRAS. Corrected typos and cases where solar/anti-solar were swappe

Differential Rotation in Convective Envelopes: Constraints from Eclipsing Binaries

Over time, tides synchronize the rotation periods of stars in a binary system to the orbital period. However, if the star exhibits differential rotation, then only a portion of it can rotate at the orbital period, so the rotation period at the surface may not match the orbital period. The difference between the rotation and orbital periods can therefore be used to infer the extent of the differential rotation. We use a simple parametrization of differential rotation in stars with convective envelopes in circular orbits to predict the difference between the surface rotation period and the orbital period. Comparing this parametrization to observed eclipsing binary systems, we find that in the surface convection zones of stars in short-period binaries there is very little radial differential rotation, with |r∂_rln Ω| < 0.02. This holds even for longer orbital periods, though it is harder to say which systems are synchronized at long periods, and larger differential rotation is degenerate with asynchronous rotation

Investigating APOKASC Red Giant Stars with Abnormal Carbon to Nitrogen Ratios

The success of galactic archaeology and the reconstruction of the formation history of our galaxy critically relies on precise ages for large populations of stars. For evolved stars in the red clump and red giant branch, the carbon to nitrogen ratio ([C/N]) has recently been identified as a powerful diagnostic of mass and age that can be applied to stellar samples from spectroscopic surveys such as SDSS/APOGEE. Here, we show that at least 10\% of red clump stars and %$\approx 10\%$ of red giant branch stars deviate from the standard relationship between [C/N] and mass. {We use the APOGEE-\kepler\ (APOKASC) overlap sample to show that binary interactions are %the majority contributors to these responsible for the majority of these outliers and that stars with %any indicators of current or previous binarity should be excluded from galactic archaeology analyses that rely on [C/N] abundances to infer stellar masses. We also show that the %standard DR14 APOGEE analysis overestimates the surface gravities for even moderately rotating giants (vsini$>2$ km/s)}Comment: Accepted at the Astrophysical Journal, in process of publicatio

Observed Extra Mixing Trends in Red Giants are Reproduced by the Reduced Density Ratio in Thermohaline Zones

Observations show an almost ubiquitous presence of extra mixing in low-mass upper giant branch stars. The most commonly invoked explanation for this is the thermohaline instability. One dimensional stellar evolution models include prescriptions for thermohaline mixing, but our ability to make direct comparisons between models and observations has thus far been limited. Here, we propose a new framework to facilitate direct comparison: Using carbon to nitrogen measurements from the SDSS-IV APOGEE survey as a probe of mixing and a fluid parameter known as the reduced density ratio from one dimensional stellar evolution programs, we compare the observed amount of extra mixing on the upper giant branch to predicted trends from three-dimensional fluid dynamics simulations. By applying this method, we are able to place empirical constraints on the efficiency of mixing across a range of masses and metallicities. We find that the observed amount of extra mixing is strongly correlated with the reduced density ratio and that trends between reduced density ratio and fundamental stellar parameters are robust across choices for modeling prescription. We show that stars with available mixing data tend to have relatively low density ratios, which should inform the regimes selected for future simulation efforts. Finally, we show that there is increased mixing at low values of the reduced density ratio, which is consistent with current hydrodynamical models of the thermohaline instability. The introduction of this framework sets a new standard for theoretical modeling efforts, as validation for not only the amount of extra mixing, but trends between the degree of extra mixing and fundamental stellar parameters is now possible.Comment: 19 pages, 7 figures, submitted to Ap