Day 684

Tuesday.

1406.4908
Background sky observation by cluster galaxies as a a source of systematic error for weak lensing
Simet, Mandelbaum

Lensing magnification and stacked shear measurements of galaxy clusters rely on measuring the density of background galaxies behind the clusters.  The most common ways of measuring this quantity ignore the fact that some fraction of the sky is obscured by the cluster galaxies themselves, reducing the area in which background galaxies can be observed.  Discuss the size of this effecting the SDSS and CFHTLenS, finding a minimum 1 % effect at 0.1 Mpc/h from the centers of clusters in SDSS; the effect is an order of magnitude higher in CFHTLenS.  The resulting biases on cluster mass and concentration measurements are of the same order as the size of the obscuration effect, which is below the statistical errors for cluster lensing in SDSS but likely exceeds them for CFHTLenS.  Also forecast the impact of this systematic error on cluster mass and magnification measurements in several upcoming surveys, and find that it typically exceeds the statistical errors.  Conclude that future surveys must account for this effect in stacked lensing and magnification measurements in order to avoid being dominated by systematic error.

1406.5509
Some stars are totally metal: a new mechanism driving dust across star-forming clouds, and consequences for planets, stars, and galaxies
Hopkins

Dust grains in neutral gas behave as aerodynamic particles, so the y can develop large local density fluctuations entirely independent of gas density fluctuations.  Specifically, gas turbulence can drive order-of-magnitude 'resonant' fluctuations in the dust density on scales where the gas stopping/drag timescale is comparable to the turbulent eddy turnover time.  Show that for large grains (>0.1 micron, containing most grain mass) in sufficiently large molecular clouds (>1-10pc, >1e4 Msun), this scale becomes longer than the characteristic sizes of pre-stellar cores (the sonic length), so large fluctuations in the dust-to-gas ratio are imprinted on cores.  As a result, star clusters and protostellar disks formed in large clouds should exhibit substantial abundance spreads in the elements preferentially found in large grains (C, O, Si). This naturally predicts populations of carbon-enhanced stars, certain highly unusual stellar populations observed in nearby open clusters, and may explain the 'UV upturn' in early-type galaxies.  It will also dramatically change planet formation in the resulting protostellar disks, by preferentially seeding disks with an enhancement in large carbonaceous or silicate grains.  The relevant threshold for this behavior scales simply with cloud densities and temperatures, making straightforward predictions for clusters in starbursts and high-redshift galaxies.  Because of the selected sorting by size, this process is not visible in extinction mapping.  Also predict the shape of the abundance distribution.  When these fluctuations occur, a small fraction of the cores are actually seeded with abundances Z~100 Z_mean, such that they are almost 'totally metal' (Z~1)!  Assuming the cores collapse, these to ally metal stars would be rare (1 in 1e4 in clusters where this occurs), but represent a fundamentally new staler evolution channel.