621 research outputs found
Rare White dwarf stars with carbon atmospheres
White dwarfs represent the endpoint of stellar evolution for stars with
initial masses between approximately 0.07 msun and 8-10 msun, where msun is the
mass of the Sun (more massive stars end their life as either black holes or
neutron stars). The theory of stellar evolution predicts that the majority of
white dwarfs have a core made of carbon and oxygen, which itself is surrounded
by a helium layer and, for ~80 per cent of known white dwarfs, by an additional
hydrogen layer. All white dwarfs therefore have been traditionally found to
belong to one of two categories: those with a hydrogen-rich atmosphere (the DA
spectral type) and those with a helium-rich atmosphere (the non-DAs). Here we
report the discovery of several white dwarfs with atmospheres primarily
composed of carbon, with little or no trace of hydrogen or helium. Our analysis
shows that the atmospheric parameters found for these stars do not fit
satisfactorily in any of the currently known theories of post-asymptotic giant
branch evolution, although these objects might be the cooler counterpart of the
unique and extensively studied PG1159 star H1504+65. These stars, together with
H1504+65, might accordingly form a new evolutionary sequence that follow the
asymptotic giant branch.Comment: 7 pages, 1 figure, to appear in Nov 22nd 2007 edition of Natur
Abundance analyses of helium-rich subluminous B stars
The connection between helium-rich hot subdwarfs of spectral types O and B
(He-sdB) has been relatively unexplored since the latter were found in
significant numbers in the 1980's. In order to explore this connection further,
we have analysed the surface composition of six He-sdB stars, including LB
1766, LB 3229, SB 21 (= Ton-S 137 = BPS 29503-0009), BPS 22940-0009, BPS
29496-0010, and BPS 22956-0094. Opacity-sampled line-blanketed model
atmospheres have been used to derive atmospheric properties and elemental
abundances. All the stars are moderately metal-poor compared with the Sun
([Fe/H] ~ -0.5). Four stars are nitrogen-rich, two of these are carbon-rich,
and at least four appear to be neon-rich. The data are insufficient to rule out
binarity in any of the sample. The surface composition and locus of the N-rich
He-sdBs are currently best explained by the merger of two helium white dwarfs,
or possibly by the merger of a helium white dwarf with a post-sdB white dwarf.
C-rich He-sdBs require further investigation.Comment: Accepted 2010 July
Simulation Analysis of DFIG Integrated Wind Turbine Control System
Recently, scientists and academics are discovering progressive improvements in the arena of wind power technology economically and reliably, allowing them to produce electricity focusing on renewable energy resources. Wind turbines (WT) using the Doubly Fed Induction Generators (DFIGs) have attracted particular attention because of their advantages such as variable speed constant frequency (VSCF) operation, independent control capabilities for maximum power point tracking (MPPT), active and reactive power controls, and voltage control strategy at the point of common coupling (PCC). When such resources have to be integrated into the existing power system, the operation becomes more challenging, particularly in terms of stability, security, and reliability. A DFIG system with its control strategies is simulated on MATLAB software. This entails the rapid control prototype testing of grid-connected, variable speed DFIG wind turbines to investigate the WT’s steady-state and dynamic behavior under normal and disturbed wind conditions. To augment the transient stability of DFIG, the simulation results for the active and reactive power of conventional controllers are compared with the adaptive tracking, self-tuned feed-forward PI controller model for optimum performance. Conclusive outcomes manifest the superior robustness of the feed-forward PI controller in terms of rising time, settling time, and overshoot value
Greener, Safer and Better Performing Aqueous Binder for Positive Electrode Manufacturing of Sodium Ion Batteries
P2-type cobalt-free MnNi-based layered oxides are promising cathode materials for sodium-ion batteries (SIBs) due to their high reversible capacity and well chemical stability. However, the phase transformations during repeated (dis)charge steps lead to rapid capacity decay and deteriorated Na+ diffusion kinetics. Moreover, the electrode manufacturing based on polyvinylidene difluoride (PVDF) binder system has been reported with severely defluorination issue as well as the energy intensive and expensive process due to the use of toxic and volatile N-methyl-2-pyrrolidone (NMP) solvent. It calls for designing a sustainable, better performing, and cost-effective binder for positive electrode manufacturing. In this work, we investigated inorganic sodium metasilicate (SMS) as a viable binder in conjunction with P2-Na0.67Mn0.55Ni0.25Fe0.1Ti0.1O2 (NMNFT) cathode material for SIBs. The NMNFT-SMS electrode delivered a superior electrochemical performance compared to carboxy methylcellulose (CMC) and PVDF based electrodes with a reversible capacity of ~161 mAh/g and retaining ~83 % after 200 cycles. Lower cell impedance and faster Na+ diffusion was also observed in this binder system. Meanwhile, with the assistance of TEM technique, SMS is suggested to form a uniform and stable nanoscale layer over the cathode particle surface, protecting the particle from exfoliation/cracking due to electrolyte attack. It effectively maintained the electrode connectivity and suppressed early phase transitions during cycling as confirmed by operando XRD study. With these findings, SMS binder can be proposed as a powerful multifunctional binder to enable positive electrode manufacturing of SIBs and to overall reduce battery manufacturing costs
Quantitative spectroscopy of extreme helium stars - Model atmospheres and a non-LTE abundance analysis of BD+102179?
Extreme helium stars (EHe stars) are hydrogen-deficient supergiants of
spectral type A and B. They are believed to result from mergers in double
degenerate systems. In this paper we present a detailed quantitative non-LTE
spectral analysis for BD+102179, a prototype of this rare class of
stars, using UVES and FEROS spectra covering the range from 3100 to 10
000 {\AA}. Atmosphere model computations were improved in two ways. First,
since the UV metal line blanketing has a strong impact on the
temperature-density stratification, we used the Atlas12 code. Additionally, We
tested Atlas12 against the benchmark code Sterne3, and found only small
differences in the temperature and density stratifications, and good agreement
with the spectral energy distributions. Second, 12 chemical species were
treated in non-LTE. Pronounced non-LTE effects occur in individual spectral
lines but, for the majority, the effects are moderate to small. The
spectroscopic parameters give = 17 300300 K and
= 2.800.10, and an evolutionary mass of 0.550.05 . The star
is thus slightly hotter, more compact and less massive than found in previous
studies. The kinematic properties imply a thick-disk membership, which is
consistent with the metallicity Fe/H and -enhancement.
The refined light-element abundances are consistent with the white dwarf merger
scenario. We further discuss the observed helium spectrum in an appendix,
detecting dipole-allowed transitions from about 150 multiplets plus the most
comprehensive set of known/predicted isolated forbidden components to date.
Moreover, a so far unreported series of pronounced forbidden He I components is
detected in the optical-UV.Comment: Accepted for publication in MNRAS, 26 pages, 19 Figure
DNA Of A Successful BPO
This paper identifies some of the critical elements of a successful BPO through a case study of an U.S. financial services firm that had BPO operations in India. The important dimensions of selection, training, motivation, respecting cultural differences, treating quality as paramount in all operations, and understanding how appropriate control has to be exercised in the area of knowledge-intensive services, were identified as being critical to success. Specific propositions are developed based on these critical elements, and provide a potentially new direction to service outsourcing research
Main-Sequence and sub-giant stars in the Globular Cluster NGC6397: The complex evolution of the lithium abundance
Thanks to the high multiplex and efficiency of Giraffe at the VLT we have
been able for the first time to observe the Li I doublet in the Main Sequence
(MS) stars of a Globular Cluster. At the same time we observed Li in a sample
of Sub-Giant (SG) stars of the same B-V colour. Our final sample is composed of
84 SG stars and 79 MS stars. In spite of the fact that SG and MS span the same
temperature range we find that the equivalent widths of the Li I doublet in SG
stars are systematically larger than those in MS stars, suggesting a higher Li
content among SG stars. This is confirmed by our quantitative analysis. We
derived the effective temperatures, from H fitting, and NLTE Li
abundances of the stars in our the sample, using 3D and 1D models. We find that
SG stars have a mean Li abundance higher by 0.1dex than MS stars, using both 1D
and 3D models. We also detect a positive slope of Li abundance with effective
temperature. These results provide an unambiguous evidence that the Li
abundance changes with evolutionary status. The physical mechanisms responsible
for this behaviour are not yet clear, and none of the existing models seems to
describe accurately these observations. Based on these conclusions, we believe
that the cosmological lithium problem still remains an open question.Comment: Proceedings of the contributed talk presented at the IAU Symposium
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Three carbon-enhanced metal-poor dwarf stars from the SDSS - Chemical abundances from CO^5BOLD 3D hydrodynamical model atmospheres
The origin of carbon-enhanced metal-poor stars enriched with both s and r
elements is highly debated. Detailed abundances of these types of stars are
crucial to understand the nature of their progenitors. The aim of this
investigation is to study in detail the abundances of SDSS J1349-0229, SDSS
J0912+0216 and SDSS J1036+1212, three dwarf CEMP stars, selected from the Sloan
Digital Sky Survey. Using high resolution VLT/UVES spectra (R ~ 30 000) we
determine abundances for Li, C, N, O, Na, Mg, Al, Ca, Sc, Ti, Cr, Mn, Fe, Co,
Ni and 21 neutron-capture elements. We made use of CO^5BOLD 3D hydrodynamical
model atmospheres in the analysis of the carbon, nitrogen and oxygen
abundances. NLTE corrections for C I and O I lines were computed using the Kiel
code. We classify SDSS J1349-0229 and SDSS J0912+0216 as CEMP-r+s stars. SDSS
J1036+1212 belongs to the class CEMP-no/s, with enhanced Ba, but deficient Sr,
of which it is the third member discovered to date. Radial-velocity variations
have been observed in SDSS J1349-0229, providing evidence that it is a member
of a binary system. The chemical composition of the three stars is generally
compatible with mass transfer from an AGB companion. However, many details
remain difficult to explain. Most notably of those are the abundance of Li at
the level of the Spite plateau in SDSS J1036+1212 and the large over-abundance
of the pure r-process element Eu in all three stars.Comment: 12 pages, 15 figures. Accepted for publication in A&
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