2,643 research outputs found
Engagement Amid Austerity: A Bipartisan Approach to Reorienting the International Affairs Budget
Examines trends that influence U.S. foreign aid and proposes reforms to better target economic and security assistance, transition ownership of PEPFAR to host countries, overhaul food aid, and establish an international affairs realignment commission
Twenty Years of Collapse and Counting: The Cost of Failure in Somalia
Analyzes the human and financial costs of Somali conflicts and famines, including costs of humanitarian and development aid; peacekeeping, military responses and aid, anti-terrorism, and diplomacy; piracy; and international crime and illicit money flows
Near-Field Cosmology with Metal-Poor Stars
The oldest, most metal-poor stars in the Galactic halo and satellite dwarf
galaxies present an opportunity to explore the chemical and physical conditions
of the earliest star forming environments in the Universe. We review the fields
of stellar archaeology and dwarf galaxy archaeology by examining the chemical
abundance measurements of various elements in extremely metal-poor stars. Focus
on the carbon-rich and carbon-normal halo star populations illustrates how
these provide insight into the Population III star progenitors responsible for
the first metal enrichment events. We extend the discussion to near-field
cosmology, which is concerned with the formation of the first stars and
galaxies and how metal-poor stars can be used to constrain these processes.
Complementary abundance measurements in high-redshift gas clouds further help
to establish the early chemical evolution of the Universe. The data appear
consistent with the existence of two distinct channels of star formation at the
earliest times.Comment: 126 pages, 12 figures, Annual Review of Astronomy and Astrophysics
(ARA&A), in pres
The Most Metal-Poor Stars. V. The CEMP-no Stars in 3D and Non-LTE
We explore the nature of carbon-rich ([C/Fe]_{1D,LTE} > +0.7), metal-poor
([Fe/H_{1D,LTE}] < -2.0) stars in the light of post 1D,LTE literature analyses,
which provide 3D-1D and NLTE-LTE corrections for iron, and 3D-1D corrections
for carbon (from the CH G-band, the only indicator at lowest [Fe/H]).
High-excitation C~I lines are used to constrain 3D,NLTE corrections of G-band
analyses. Corrections to the 1D,LTE compilations of Yoon et al. and Yong et al.
yield 3D,LTE and 3D,NLTE Fe and C abundances. The number of CEMP-no stars in
the Yoon et al. compilation (plus eight others) decreases from 130 (1D,LTE) to
68 (3D,LTE) and 35 (3D,NLTE). For stars with -4.5 < [Fe/H] < -3.0 in the
compilation of Yong et al., the corresponding CEMP-no fractions change from
0.30 to 0.15 and 0.12, respectively.
We present a toy model of the coalescence of pre-stellar clouds of the two
populations that followed chemical enrichment by the first zero-heavy-element
stars: the C-rich, hyper-metal-poor and the C-normal, very-metal-poor
populations. The model provides a reasonable first-order explanation of the
distribution of the 1D,LTE abundances of CEMP-no stars in the A(C) and [C/Fe]
vs. [Fe/H] planes, in the range -4.0 < [Fe/H] < -2.0.
The Yoon et al. CEMP Group I contains a subset of 19 CEMP-no stars (14% of
the group), 4/9 of which are binary, and which have large [Sr/Ba]_{1D,LTE}
values. The data support the conjectures of Hansen et al. (2016b, 2019) and
Arentsen et al. (2018) that these stars may have experienced enrichment from
AGB stars and/or "spinstars".Comment: ApJ in pres
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