Day 1393

Wednesday.

1804.00659
Where are the most ancient stars in the Milky Way?
El-Badry, et al

The oldest stars in the MW bear imprints of the Galaxy's early assembly history.  Use FIRE cosmological zoom-in simulations of 3 MW-mass disk galaxies to study the spatial distribution, chemistry, and kinematics of the oldest surviving stars (z_form >~5) in MW-like galaxies.  Predict the oldest stars to be less centrally concentrated at z=0 than stars formed at later times as a result of two processes.  First the majority of the oldest stars are not formed in situ but are accreted during hierarchical assembly.  These ex situ stars are deposited on dispersion-supported, halo-like orbits but dominate over old stars formed in situ in the solar neighborhood, and in some simulations, even in the galactic center.  Secondly, old stars formed in situ are driven outwards by bursty star formation and energetic feedback processes that create a time-varying gravitational potential at z>~2, similar to the process that creates DM cores and expands stellar orbits in bursty dwarf galaxies.  The total fraction of stars that are ancient is more than an order of magnitude higher for sight lines away from the bulge and inner halo than for inward-looking sightlines.  Although the task of identifying specific stars as ancient remains challenging, anticipate that million-star spectral surveys and photometric surveys targeting metal-poor stars already include hundreds of stars formed before z=5.  Predict most of these targets to have higher metallicity (-3<[Fe/H]<-2) than the most extreme metal-poor stars.


1804.00676
CLUMP-3D: three-dimensional shape and structure of 20 CLASH galaxy clusters from combined weak and strong lensing
Chiu, Umetsu, et al

Perform a 3d triaxial analysis of 16 -ray regular and 4 high-magnification galaxy clusters selected from the CLASH survey by combining 2d WL and central SL constraints.  In a Bayesian framework, constrain the intrinsic structure and geometry of each individual cluster assuming a triaxial NFW halo with arbitrary orientations, characterized by the mass M_200c, halo concentration C_200c, and triaxial axis ratios (q_a<=q_b), and investigate scaling relations between these halo structural parameters.  From triaxial modeling of the X-ray-selected subsample, find that the halo concentration decreases with increasing cluster mass, with a mean concentration of C_200c=4.82±0.30 at the pivot mass M_200c=1e15 Msun/h.  This is consistent with the result from spherical modeling, C_200c=4.51±0.14.  Independently of the priors, the minor-to-major axis ratio q_a of the full sample exhibits a clear deviation from the spherical configuration (q_a=0.52±0.04 at 1e15Msun/h with uniform priors), with a weak dependence on the cluster mass.  Combining all 20 clusters, obtain a joint ensemble constraint on the minor-to-major axis ratio of q_a=0.652+-0.162-0.078 and a lower bound on the intermediate-to-major axis ratio of q_b>0.63 at the 2 sigma level from an analysis with uniform priors.  Assuming priors on the axis ratios derived from numerical simulations, constraint the degree of triaxiality for the full sample to be T=0.79±).03 at 1e15 Msun/h, indicating a preference for a prolate geometry of cluster halos.  Find no statistical evidence for an orientation bias (f_geo=0.93±0.07).


1804.00699
The direct imaging search for Earth 2.0: quantifying biases and planetary false positives
Guimond, Cowan

Direct imaging is likely the best way to characterize the atmospheres of Earth-Sized exoplanets in the habitable zone of Sun-like stars.  Previously, Stark+(2014,15,16) estimated the Earth twin yield of future direct imaging missions, such as LUVOIR and HabEx.  Extend this analysis to other types of planets, which will act as false positives for Earth twins.  Define an Earth twin as any exoplanet within half an e-folding of 1AU in semi-major axis and 1 R_Earth in planetary radius, orbiting a G-dwarf.  Using Monte Carlo analyses, quantify the biases and planetary false positive rates of Earth searches.  That is, given a pale dot at the correct projected separation and brightness to be a candidate Earth, what are the odds that it is, in fact, an Earth twin?  The notional telescope has a diameter of 10m, and inner working angle of 3 lambda/D, and an outer working angle of 10 lambda/D (62 was and 206 was at 1.0um).  With no precursor knowledge and one visit per star, 77% of detected candidate Earths are actually un-Earths; their mean radius is 2.3 R_Earth, a sub-Neptune.  The odds improve if every planet is imaged at its optimal orbital phase, either by relying on precursor knowledge, or by performing multi-epoch direct imaging.  In such a targeted search, 47% of detected Earth twin candidates are false positives, and they have a mean radius of 1.7 R_Earth.  The false positive rate is insensitive to stellar spectral type and the assumption of circular orbits.