9,625 research outputs found
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
Numerical Study of Heterogeneous Reactions in an SOFC Anode with Oxygen Addition
Previous experimental studies have shown that addition of small amounts of oxygen to a hydrocarbon fuel stream can control coking in the anode, while relatively large amounts of oxygen are present in the fuel stream in single-chamber solid oxide fuel cells (SOFCs). In order to rationally design an anode for such use, it is important to understand the coupled catalytic oxidation/reforming chemistry and diffusion within the anode under SOFC operating conditions. In this study, the heterogeneous catalytic reactions in the anode of an anode-supported SOFC running on methane fuel with added oxygen are numerically investigated using a model that accounts for catalytic chemistry, porous media transport, and electrochemistry at the anode/electrolyte interface. Using an experimentally validated heterogeneous reaction mechanism for methane partial oxidation and reforming on nickel, we identify three distinct reaction zones at different depths within the anode: a thin outer layer in which oxygen is nearly fully consumed in oxidizing methane and hydrogen, followed by a reforming region, and then a water–gas shift region deep within the anode. Both single-chamber and dual-chamber SOFC anodes are explored
Dietary variations in three co-occurring rockfish species off the Pacific Northwest during anomalous oceanographic events in 1998 and 1999
Stomach samples from three rockfish species, yellowtail
(Sebastes f lavidus), widow (S. entomelas), and canary (S. pinniger) rockfish, seasonally collected off the Pacific Northwest in 1998 and 1999, provided quantitative information on the food habits of these species during and after the 1997–98 El Niño event. Although euphausiids were the most common major prey of all three predators, gelatinous zooplankton and fishes were the most commonly
consumed prey items during some seasonal quarters. The influence of the El Niño event was evident in the diets. Anomalous prey items, including the southern euphausiid species Nyctiphanes simplex and juveniles of Pacific whiting (Merluccius productus) frequently appeared in the diets in the spring and summer of 1998. The results of stomach contents analyses, based on 905 stomach samples from 49 trawl hauls during seven commercial fishing trips and from 56 stations during research surveys, were consistent with the timing of occurrence and the magnitude of change in biomass of some zooplankton species reported
from zooplankton studies in the northern California Current during the 1997–98 El Niño. Our findings indicate that the observed variations of prey groups in some rockfish diets
may be a function of prey variability related to climate and environment changes
Two-Layered Superposition of Broadcast/Multicast and Unicast Signals in Multiuser OFDMA Systems
We study optimal delivery strategies of one common and independent
messages from a source to multiple users in wireless environments. In
particular, two-layered superposition of broadcast/multicast and unicast
signals is considered in a downlink multiuser OFDMA system. In the literature
and industry, the two-layer superposition is often considered as a pragmatic
approach to make a compromise between the simple but suboptimal orthogonal
multiplexing (OM) and the optimal but complex fully-layered non-orthogonal
multiplexing. In this work, we show that only two-layers are necessary to
achieve the maximum sum-rate when the common message has higher priority than
the individual unicast messages, and OM cannot be sum-rate optimal in
general. We develop an algorithm that finds the optimal power allocation over
the two-layers and across the OFDMA radio resources in static channels and a
class of fading channels. Two main use-cases are considered: i) Multicast and
unicast multiplexing when users with uplink capabilities request both
common and independent messages, and ii) broadcast and unicast multiplexing
when the common message targets receive-only devices and users with uplink
capabilities additionally request independent messages. Finally, we develop a
transceiver design for broadcast/multicast and unicast superposition
transmission based on LTE-A-Pro physical layer and show with numerical
evaluations in mobile environments with multipath propagation that the capacity
improvements can be translated into significant practical performance gains
compared to the orthogonal schemes in the 3GPP specifications. We also analyze
the impact of real channel estimation and show that significant gains in terms
of spectral efficiency or coverage area are still available even with
estimation errors and imperfect interference cancellation for the two-layered
superposition system
Improvement and extension of a radar forest backscattering model
Radar modeling of mangal forest stands, in the Sundarbans area of Southern Bangladesh, was developed. The modeling employs radar system parameters such as wavelength, polarization, and incidence angle, with forest data on tree height, spacing, biomass, species combinations, and water content (including slightly conductive water) both in leaves and trunks of the mangal. For Sundri and Gewa tropical mangal forests, five model components are proposed, which are required to explain the contributions of various forest species combinations in the attenuation and scattering of mangal vegetated nonflooded or flooded surfaces. Statistical data of simulated images (HH components only) were compared with those of SIR-B images both to refine the modeling procedures and to appropriately characterize the model output. The possibility of delineation of flooded or non-flooded boundaries is discussed
Woodford goes to Africa
This paper analyses the effects of inflation shocks, demands shocks, and aid shocks on low-income, quasi-emerging-market economies, and discusses how monetary policy can be used to manage these effects. We make use of a model developed for such economies by Adam et al. (2007). We examine the e¤ects of four things which this model features, which we take to be typical of such economies. These are: the existence of a tradeables/non-tradeables production structure, the fact that international capital movements are - at least initially - confined to the effects of currency substitution by domestic residents, the use of targets for financial assets in the implementation of monetary policy, and the pursuit, in some countries, of a fixed exchange rate. We then modify the model to examine the effect on such economies of three major changes, changes which we take to be part of the transition by such economies towards more fully- fledged emerging-market status: an opening of the capital account so that uncovered- interest-parity comes to hold, a move to floating exchange rates, and the replacement of fixed stocks of financial aggregates by the pursuit of a Taylor rule in the conduct of monetary policy.currency substitution, emerging market macroeconomics, interactions between fiscal and monetary policy, Taylor rule
Thermoelectric effects in quantum Hall systems beyond linear response
We consider a quantum Hall system with an antidot acting as an energy
dependent scatterer. In the purely charge case, we find deviations from the
Wiedemann-Franz law that take place in the nonlinear regime of transport. We
also discuss Peltier effects beyond linear response and describe both effects
using magnetic-field asymmetric transport coefficients. For the spin case such
as that arising along the helical edge states of a two-dimensional topological
insulator, we investigate the generation of spin currents as a result of
applied voltage and temperature differences in samples attached to
ferromagnetic leads. We find that in the parallel configuration the spin
current can be tuned with the leads' polarization even in the linear regime of
transport. In contrast, for antiparallel magnetizations the spin currents has a
strict nonlinear dependence on the applied fields.Comment: 17 pages, 8 figure
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