2,898 research outputs found
The Bifurcated Age-Metallicity Relation of Milky Way Globular Clusters and its Implications For the Accretion History of the Galaxy
We use recently derived ages for 61 Milky Way (MW) globular clusters (GCs) to
show that their age-metallicity relation (AMR) can be divided into two
distinct, parallel sequences at [Fe/H] \ga -1.8. Approximately one-third of
the clusters form an offset sequence that spans the full range in age (--13 Gyr), but is more metal rich at a given age by dex in
[Fe/H]. All but one of the clusters in the offset sequence show orbital
properties that are consistent with membership in the MW disk. They are not
simply the most metal-rich GCs, which have long been known to have disk-like
kinematics, but they are the most metal-rich clusters at all ages. The slope of
the mass-metallicity relation (MMR) for galaxies implies that the offset in
metallicity of the two branches of the AMR corresponds to a mass decrement of 2
dex, suggesting host galaxy masses of M_{*} \sim 10^{7-8} \msol for GCs that
belong to the more metal-poor AMR. We suggest that the metal-rich branch of the
AMR consists of clusters that formed in-situ in the disk, while the metal-poor
GCs were formed in relatively low-mass (dwarf) galaxies and later accreted by
the MW. The observed AMR of MW disk stars, and of the LMC, SMC and WLM dwarf
galaxies are shown to be consistent with this interpretation, and the relative
distribution of implied progenitor masses for the halo GC clusters is in
excellent agreement with the MW subhalo mass function predicted by simulations.
A notable implication of the bifurcated AMR, is that the identical mean ages
and spread in ages, for the metal rich and metal poor GCs are difficult to
reconcile with an in-situ formation for the latter population.Comment: 16 pages, 9 figures, accepted for publication in MNRA
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Hybrid Manufacturing: Integrating Direct Write and Stereolithography
A commercial stereolithography (SL) machine was modified to integrate fluid dispensing or
direct-write (DW) technology with SL in an integrated manufacturing environment for
automated and efficient hybrid manufacturing of complex electrical devices, combining threedimensional (3D) electrical circuitry with SL-manufactured parts. The modified SL system
operates similarly to a commercially available machine, although build interrupts were used to
stop and start the SL build while depositing fluid using the DW system. An additional linear
encoder was attached to the SL platform z-stage and used to maintain accurate part registration
during the SL and DW build processes. Individual STL files were required as part of the
manufacturing process plan. The DW system employed a three-axis translation mechanism that
was integrated with the commercial SL machine. Registration between the SL part, SL laser and
the DW nozzle was maintained through the use of 0.025-inch diameter cylindrical reference
holes manufactured in the part during SL. After depositing conductive ink using DW, the SL
laser was commanded to trace the profile until the ink was cured. The current system allows for
easy exchange between SL and DW in order to manufacture fully functional 3D electrical
circuits and structures in a semi-automated environment. To demonstrate the manufacturing
capabilities, the hybrid SL/DW setup was used to make a simple multi-layer SL part with
embedded circuitry. This hybrid system is not intended to function as a commercial system, it is
intended for experimental demonstration only. This hybrid SL/DW system has the potential for
manufacturing fully functional electromechanical devices that are more compact, less expensive,
and more reliable than their conventional predecessors, and work is ongoing in order to fully
automate the current system.Mechanical Engineerin
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