9,757 research outputs found
Development of the weldbond process for joining titanium
High quality resistance spot welds were produced by welding through epoxy adhesive on titanium alloys. Weldbond joints were consistently stronger than those of either mechanical fasteners, structural adhesive bonds, or mechanical fasteners with adhesive at the joint interface. Weldbond joints and/or spot weld joints showed superior strength at all temperature ranges as compared to other joints tested
Near-Earth Supernova Explosions: Evidence, Implications, and Opportunities
There is now solid experimental evidence of at least one supernova explosion within 100 pc of Earth within the last few million years, from measurements of the short-lived isotope ⁶⁰Fe in widespread deep-ocean samples, as well as in the lunar regolith and cosmic rays. This is the first established example of a specific dated astrophysical event outside the Solar System having a measurable impact on the Earth, offering new probes of stellar evolution, nuclear astrophysics, the astrophysics of the solar neighborhood, cosmic-ray sources and acceleration, multi-messenger astronomy, and astrobiology. Interdisciplinary connections reach broadly to include heliophysics, geology, and evolutionary biology. Objectives for the future include pinning down the nature and location of the established near-Earth supernova explosions, seeking evidence for others, and searching for other short-lived isotopes such as ²⁶Al and ²⁴⁴Pu. The unique information provided by geological and lunar detections of radioactive ⁶⁰Fe to assess nearby supernova explosions make now a compelling time for the astronomy community to advocate for supporting multi-disciplinary, cross-cutting research programs
The Primordial Lithium Problem
Big-bang nucleosynthesis (BBN) theory, together with the precise WMAP cosmic
baryon density, makes tight predictions for the abundances of the lightest
elements. Deuterium and 4He measurements agree well with expectations, but 7Li
observations lie a factor 3-4 below the BBN+WMAP prediction. This 4-5\sigma\
mismatch constitutes the cosmic "lithium problem," with disparate solutions
possible. (1) Astrophysical systematics in the observations could exist but are
increasingly constrained. (2) Nuclear physics experiments provide a wealth of
well-measured cross-section data, but 7Be destruction could be enhanced by
unknown or poorly-measured resonances, such as 7Be + 3He -> 10C^* -> p + 9B.
(3) Physics beyond the Standard Model can alter the 7Li abundance, though D and
4He must remain unperturbed; we discuss such scenarios, highlighting decaying
Supersymmetric particles and time-varying fundamental constants. Present and
planned experiments could reveal which (if any) of these is the solution to the
problem.Comment: 29 pages, 7 figures. Per Annual Reviews policy, this is the original
submitted draft. Posted with permission from the Annual Review of Nuclear and
Particle Science, Volume 61. Annual Reviews, http://www.annualreviews.org .
Final published version at
http://www.annualreviews.org/doi/abs/10.1146/annurev-nucl-102010-13044
Massive Compact Halo Objects Viewed from a Cosmological Perspective: Contribution to the Baryonic Mass Density of the Universe
[Abridged] We estimate the contribution of Massive Compact Halo Objects
(Machos) and their stellar progenitors to the mass density of the Universe. If
the Machos that have been detected reside in the Halo of our Galaxy, then a
simple extrapolation of the Galactic population (out to 50 kpc) of Machos to
cosmic scales gives a cosmic density \rho_{Macho} = (1-5) \times 10^9 h \msun
\Mpc^{-3}, which in terms of the critical density corresponds to
. Such a mass density is comparable to
the baryon density implied by Big Bang Nucleosynthesis. If we take the central
values of the estimates, then Machos dominate the baryonic content of the
Universe today, with . However, the
cumulative uncertainties in the density determinations only require that
, where the fraction of
galaxies that contain Machos , and is the Hubble constant
in units of 100 km s Mpc. Our best estimate for
is hard to reconcile with the current best estimates of the baryonic content of
the intergalactic medium indicated by measurements of the Lyman-
forest. We explore the addition constraints that arise if the Machos are white
dwarfs as suggested by the present microlensing data. We discuss the challenges
this scenario presents at both the local and cosmic scales, emphasizing in
particular the constraints on the required mass budget and nucleosynthesis
products (particularly carbon).Comment: 18 pages, LaTeX, uses AASTeX macros. In press, New Astronomy
(submitted Jan. 20, 1998
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