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The treatment of chemical mixing in the radiative envelopes of intermediate-mass stars has hardly been calibrated so far. Recent asteroseismic studies demonstrated that a constant diffusion coefficient in the radiative envelope is not able to explain the periods of trapped gravity modes in the oscillation spectra of $\gamma$ Doradus pulsators. We present a new generation of MESA stellar models with two major improvements. First, we present a new implementation for computing radiative accelerations and Rosseland mean opacities that requires significantly less CPU time. Second, the inclusion of shear mixing based on rotation profiles computed with the 2D stellar structure code ESTER is considered. We show predictions for the mode periods of these models covering stellar masses from 1.4 to 3.0${\rm M_\odot}$ across the main sequence (MS), computed for different metallicities. The morphology of the chemical mixing profile resulting from shear mixing in combination with atomic diffusion and radiative levitation does allow for mode trapping, while the diffusion coefficient in the outer envelope is large ($>10^{6}\,{\rm cm^2\,s^{-1}}$). Furthermore, we make predictions for the evolution of surface abundances for which radiative accelerations can be computed. We find that the N/C and C/O abundance ratios correlate with stellar age. We predict that these correlations are observable with precisions $\lesssim 0.1$ dex on these ratios, given that a precise age estimate can be made., Comment: Accepted for publication in The Astrophysical Journal

New observations of the resolved stellar population of the extremely metal-poor Magellanic dwarf irregular galaxy Leo A in Thuan-Gunn r, g, i, and narrowband Ha filters are presented. Using the recent Cepheid variable star distance determination to Leo A by Hoessel et al., we are able to create an accurate color-magnitude diagram (CMD). We have used the Bavesian inference method described by Tolstoy & Saha to calculate the likelihood of a Monte Carlo simulation of the stellar population of Leo A being a good match to the data within the well understood errors in the data. The magnitude limits on our data are sensitive enough to look back at similar to 1 Gyr of star formation history at the distance of Leo A. To explain the observed ratio of red to blue stars in the observed CMD, it is necessary to invoke either a steadily decreasing star formation rate toward the present lime or gaps in the star formation history. We also compare the properties of the observed stellar population with the known spatial distribution of the H I gas and H II regions to support the conclusions from CMD modeling. We consider the possibility that currently there is a period of diminished star formation in Leo A, as evidenced by the lack of very young stars in the CMD and the faint H Ir regions. How the chaotic II I distribution, with no observable rotation, fits into our picture of the evolution of Leo A is as yet unclear.

New observations of the resolved stellar population of the extremely metal-poor Magellanic dwarf irregular galaxy Leo A in Thuan-Gunn r, g, i, and narrowband Ha filters are presented. Using the recent Cepheid variable star distance determination to Leo A by Hoessel et al., we are able to create an accurate color-magnitude diagram (CMD). We have used the Bavesian inference method described by Tolstoy & Saha to calculate the likelihood of a Monte Carlo simulation of the stellar population of Leo A being a good match to the data within the well understood errors in the data. The magnitude limits on our data are sensitive enough to look back at similar to 1 Gyr of star formation history at the distance of Leo A. To explain the observed ratio of red to blue stars in the observed CMD, it is necessary to invoke either a steadily decreasing star formation rate toward the present lime or gaps in the star formation history. We also compare the properties of the observed stellar population with the known spatial distribution of the H I gas and H II regions to support the conclusions from CMD modeling. We consider the possibility that currently there is a period of diminished star formation in Leo A, as evidenced by the lack of very young stars in the CMD and the faint H Ir regions. How the chaotic II I distribution, with no observable rotation, fits into our picture of the evolution of Leo A is as yet unclear.