Day 412

Monday.

1304.3455
LSST's DC bias against planets and galactic-plane science
Gould

LSST-like survey of Galactic plane (deep images every 3-4 days) could probe the Galactic distribution of planets by two distinct methods: gravitational microlensing of planets beyond the snow line and transits by planets very close to their hosts.  Survey would identify over 250 disk-lens/disk-source microlensng events per year that peak at r<19, including 10% reaching the high magnification A>100 that makes them especially sensitive to planets.  Intensive followup of these events would be required to find planets, similar to what is done presently for Galactic bulge microlensing.  The same data would enable a wealth of other science, including detection of isolated BHs, systematic study of brown-dwarf binaries, a pre-explosion lightcurve of the next Galactic SNe, pre-explosion lightcurves of stellar mergers, early nova lightcurves, proper motions of many more stars than can be reached by GAIA, and probably much more.  As usual, the most exciting discoveries from probing the huge parameter space encompassed by Galactic-plane stellar populations might well be serendipitous.  Unfortunately, the LSST collaboration plans to exclude the first and fourth quadrants of the Galactic plane from their "synoptic" observations because the DC image that resulted from repeated observations would be limited by crowding.  I demonstrate that the majority of the science can be recovered by employing well-developed image subtraction analysis methods, and that the cost to other (high Galactic latitude) science would be negligible.

1304.3456
Convergence of AMR and SPH simulations - I. Hydrodynamical resolution and convergence tests
Hubber, Falle, Goodwin

Compare AMR finite volume code MG and SPH code SEREN.  Test suite includes shock tube tests, with and without cooling, the NL thin-shell instability, and the Kelvin-Helmholtz instability.  The main conclusions are: (i) the two methods converge in the limit of high resolution and accuracy in most cases.  All tests show good agreement when numerical effects (e.g. discontinuities in SPH) are properly treated.  (ii) Both methods can capture adiabatic shocks and well-resolved cooling shocks perfectly well with standard prescriptions.  However, they both have problems when dealing with under-resolved cooling shocks, or strictly isothermal shocks, at high Mach numbers.  The finite volume code only works well at 1st order and even then requires some additional artificial viscosity.  SPH requires either a larger value of the artificial viscosity parameter, alpha-AV, or a modified form of the standard artificial viscosity term using the harmonic mean of the density, rather than the arithmetic mean.  (iii) Some SPH simulations require larger kernels to increase neighbor number and reduce particle noise in order to achieve agreement with finite volume simulations.  However, this is partly due to the need to reduce noise that can corrupt the growth of small-scale perturbations.  In contrast, instabilities seeded from large-scale perturbations do not require more neighbors and hence work well with standard SPH formulations and converge with the finite volume simulations.  (iv) For purely hydrodynamical problems, SPH simulations take an order of magnitude longer to run than finite volume simulations when running at equivalent resolutions, i.e., when they both resolve the underlying physics to the same degree.  This requires about 2-3 times as many particles as the number of cells.