Enhanced and reduced solute transport and flow strength in salt finger convection in porous media

We report a pore-scale numerical study of salt finger convection in porous media, with a focus on the influence of the porosity in the non-Darcy regime, which has received little attention in previous research. The numerical model is based on the lattice Boltzmann method with a multiple-relaxation-time scheme and employs an immersed boundary method to describe the fluid-solid interaction. The simulations are conducted in a two-dimensional, horizontally-periodic domain with an aspect ratio of 4, and the porosity is varied from 0.7 to 1, while the solute Rayleigh number ranges from 4*10^6 to 4*10^9. Our results show that, for all explored Rayleigh number, solute transport first enhances unexpectedly with decreasing porosity, and then decreases when porosity is smaller than a Rayleigh number-dependent value. On the other hand, while the flow strength decreases significantly as porosity decreases at low Rayleigh number, it varies weakly with decreasing porosity at high Rayleigh number and even increases counterintuitively for some porosities at moderate Rayleigh number. Detailed analysis of the salinity and velocity fields reveals that the fingered structures are blocked by the porous structure and can even be destroyed when their widths are larger than the pore scale, but become more ordered and coherent with the presence of porous media. This combination of opposing effects explains the complex porosity-dependencies of solute transport and flow strength. The influence of porous structure arrangement is also examined, with stronger effects observed for smaller porosity and higher Rayleigh number. These findings have important implications for passive control of mass/solute transport in engineering applications

The formation of blue large-amplitude pulsators from white-dwarf main-sequence star mergers

Blue large-amplitude pulsators (BLAPs) are hot low-mass stars which show large-amplitude light variations likely due to radial oscillations driven by iron-group opacities. Period changes provide evidence of both secular contraction and expansion amongst the class. Various formation histories have been proposed, but none are completely satisfactory. \citet{Zhang2017} proposed that the merger of a helium core white dwarf with a low-mass main-sequence star (HeWD+MS) can lead to the formation of some classes of hot subdwarf. We have analyzed these HeWD+MS merger models in more detail. Between helium-shell ignition and full helium-core burning, the models pass through the volume of luminosity -- gravity-- temperature space occupied by BLAPs. Periods of expansion and contraction associated with helium-shell flashes can account for the observed rates of period change. We argue that the HeWD+MS merger model provides at least one BLAP formation channel.Comment: 13 pages, 8 figures, accepted by Ap

Solar-type Stars Observed by LAMOST and Kepler

Obtaining measurements of chromospheric and photometric activity of stars with near-solar fundamental parameters and rotation periods is important for a better understanding of solar-stellar connection. We select a sample of 2603 stars with near-solar fundamental parameters from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST)-Kepler field and use LAMOST spectra to measure their chromospheric activity and Kepler light curves to measure their photospheric activity (i.e., the amplitude of the photometric variability). While the rotation periods of 1556 of these stars could not be measured due to the low amplitude of the photometric variability and highly irregular temporal profile of light curves, 254 stars were further identified as having near-solar rotation periods. We show that stars with near-solar rotation periods have chromospheric activities that are systematically higher than stars with undetected rotation periods. Furthermore, while the solar level of photospheric and chromospheric activity appears to be typical for stars with undetected rotation periods, the Sun appears to be less active than most stars with near-solar rotation periods (both in terms of photospheric and chromospheric activity).Comment: 7 pages, 6 figure

Stellar Parameters of Main Sequence Turn-off Star Candidates Observed with the LAMOST and Kepler

Main sequence turn-off (MSTO) stars have advantages as indicators of Galactic evolution since their ages could be robustly estimated from atmospheric parameters. Hundreds of thousands of MSTO stars have been selected from the LAMOST Galactic sur- vey to study the evolution of the Galaxy, and it is vital to derive accurate stellar parameters. In this work, we select 150 MSTO star candidates from the MSTO stars sample of Xiang that have asteroseismic parameters and determine accurate stellar parameters for these stars combing the asteroseismic parameters deduced from the Kepler photometry and atmospheric parameters deduced from the LAMOST spectra.With this sample, we examine the age deter- mination as well as the contamination rate of the MSTO stars sample. A comparison of age between this work and Xiang shows a mean difference of 0.53 Gyr (7%) and a dispersion of 2.71 Gyr (28%). The results show that 79 of the candidates are MSTO stars, while the others are contaminations from either main sequence or sub-giant stars. The contamination rate for the oldest stars is much higher than that for the younger stars. The main cause for the high contamination rate is found to be the relatively large systematic bias in the LAMOST surface gravity estimates.Comment: accepted by RA