5,195 research outputs found
High-precision abundances of elements in solar twin stars: Trends with stellar age and elemental condensation temperature
HARPS spectra with S/N > 600 for 21 solar twin stars are used to determine
very precise (sigma ~ 0.01 dex) differential abundances of C, O, Na, Mg, Al,
Si, S, Ca, Ti, Cr, Fe, Ni, Zn, and Y in order to see how well [X/Fe] is
correlated with elemental condensation temperature, Tc. In addition, precise
(sigma < 0.8 Gyr) stellar ages are obtained by interpolating between
Yonsei-Yale isochrones in the logg - Teff diagram. It is confirmed that the
ratio between refractory and volatile elements is lower in the Sun than in most
of the solar twins, but for many stars, the relation between [X/Fe] and Tc is
not well defined. For several elements there is, instead, an astonishingly
tight correlation between [X/Fe] and stellar age with amplitudes up to 0.2 dex
over an age interval of 8 Gyr in contrast to the lack of correlation between
[Fe/H] and age. While [Mg/Fe] increases with age, the s-process element yttrium
shows the opposite behavior so that [Y/Mg] can be used as a sensitive
chronometer for Galactic evolution. [Na/Fe] and [Ni/Fe] are not well correlated
with stellar age, but define a tight Ni-Na relation similar to that previously
found for more metal-poor stars. These results provide new constraints on
supernovae yields and Galactic evolution. Furthermore, it is found that the C/O
ratio evolves very little with time, which is of interest for discussions of
the composition of exoplanets.Comment: 13 pages, 14 figures, and 2 on-line tables. To appear in A&
The problem of the Pleiades distance. Constraints from Stromgren photometry of nearby field stars
The discrepancy between the Pleiades cluster distance based on Hipparcos
parallaxes and main sequence fitting is investigated on the basis of Stromgren
photometry of F-type stars. Field stars with the same metallicity as the
Pleiades have been selected from the m1 index and a technique has been
developed to locate the ZAMS of these field stars in color-magnitude diagrams
based on the color/temperature indices b-y, v-y, and beta. Fitting the Pleiades
to these ZAMS relations results in a distance modulus of 5.61+/-0.03 mag in
contrast to the Hipparcos modulus of 5.36+/-0.06 mag. Hence, we cannot confirm
the recent claim by Grenon (1999) that the distance problem is solved by
adopting a low metallicity of the Pleiades ([Fe/H]=-0.11) as determined from
Geneva photometry. The metallicity sensitivity of the ZAMS determined by the
field stars is investigated, and by combining this sensitivity in all three
color/temperature indices b-y, v-y, and beta we get a independent test of the
Pleiades distance modulus which support our value of 5.61 mag. Furthermore, the
field star sample used for the comparison is tested against theoretical
isochrones of different ages to show that evolutionary effects in the field
star sample are not biasing our distance modulus estimate significantly.
Possible explanations of the Pleiades distance problem are discussed and it is
suggested that the discrepancy in the derived moduli may be linked to a
non-spherical shape of the cluster.Comment: 11 pages, 5 figures, accepted for publication in A&
The lithium isotope ratio in the metal-poor halo star G271-162 from VLT/UVES observations
A high resolution (R = 110.000), very high S/N (>600) spectrum of the
metal-poor turnoff star G271-162 has been obtained in connection with the
commissioning of UVES at VLT/Kueyen. Using both 1D hydrostatic and 3D
hydrodynamical model atmospheres, the lithium isotope ratio has been estimated
from the LiI 670.8 nm line by means of spectral synthesis. The necessary
stellar line broadening (1D: macroturbulence + rotation, 3D: rotation) has been
determined from unblended KI, CaI and FeI lines. The 3D line profiles agree
very well with the observed profiles, including the characteristic line
asymmetries. Both the 1D and 3D analyses reveal a possible detection of 6Li in
G271-162, 6Li/7Li = 0.02 +-0.01 (one sigma). It is discussed if the smaller
amount of 6Li in G271-162 than in the similar halo star HD84937 could be due to
differences in stellar mass and/or metallicity or whether it may reflect an
intrinsic scatter of the Li isotope ratio in the ISM at a given metallicity.Comment: 5 pages with 6 figures. Accepted as a letter in A&
Cosmological Cosmic Rays and the observed Li6 plateau in metal poor halo stars
Very recent observations of the Li6 isotope in halo stars reveal a Li6
plateau about 1000 times above the predicted BBN abundance. We calculate the
evolution of Li6 versus redshift generated from an initial burst of
cosmological cosmic rays (CCRs) up to the formation of the Galaxy. We show that
the pregalactic production of the Li6 isotope can account for the Li6 plateau
observed in metal poor halo stars without additional over-production of Li7.
The derived relation between the amplitude of the CCR energy spectra and the
redshift of the initial CCR production puts constraints on the physics and
history of the objects, such as pop III stars, responsible for these early
cosmic rays. Consequently, we consider the evolution of Li6 in the Galaxy.
Since Li6 is also produced in Galactic cosmic ray nucleosynthesis, we argue
that halo stars with metallicities between [Fe/H] = -2 and -1, must be somewhat
depleted in Li6.Comment: 8 pages, 6 figures, version accepted for publication in Ap
Hypothesis Testing of Edge Organizations: Specifying Computational C2 Models for Experimentation
Beyond electronic disintermediation through multi-agent systems
Supply chain management represents a critical competency
in today's global business environment and has been the
focus of considerable, but mixed, information systems
research. The research described in this paper builds on
work in multi-agent systems to argue that intelligent agents
offer excellent potential and capability for supply chain
management, and contributes to discussion and theory
pertaining to electronic markets and supply chain
disintermediation. Argues that the knowledge associated
with intermediation work represents a key mediating
variable between disintermediating technology and supply
chain efficacy and discusses how intelligent agent
technology can be employed to both intermediate and
disintermediate the supply chain, attaining the cost and
cycle-time benefits of disintermediation without the
attendant loss of human knowledge and expertise. The
paper outlines a number of implications for theory and
practice in information systems, and it formalizes some
important research questions through a contingency
framework to help stimulate and guide future work along
these lines
O/Fe in metal-poor main sequence and subgiant stars
A study of the O/Fe ratio in metal-poor main sequence and subgiant stars is
presented using the [OI] 6300A line, the OI 7774A triplet, and a selection of
weak FeII lines observed on high-resolution spectra acquired with the VLT UVES
spectrograph. The [OI] line is detected in the spectra of 18 stars with -0.5 <
[Fe/H] < -2.4, and the triplet is observed for 15 stars with [Fe/H] ranging
from -1.0 to -2.7. The abundance analysis was made first using standard model
atmospheres taking into account non-LTE effects on the triplet: the [OI] line
and the triplet give consistent results with [O/Fe] increasing quasi-linearly
with decreasing [Fe/H] reaching [O/Fe] ~ +0.7 at [Fe/H] = -2.5. When
hydrodynamical model atmospheres representing stellar granulation in dwarf and
subgiant stars replace standard models, the [O/Fe] from the [OI] and FeII lines
is decreased by an amount which increases with decreasing [Fe/H]. The [O/Fe] vs
[Fe/H] relation remains quasi-linear extending to [O/Fe] ~ +0.5 at [Fe/H] =
-2.5, but with a tendency of a plateau with [O/Fe] ~ +0.3 for -2.0 < [Fe/H] <
-1.0, and a hint of cosmic scatter in [O/Fe] at [Fe/H] ~ -1.0. Use of the
hydrodynamical models disturbs the broad agreement between the oxygen
abundances from the [OI], OI, and OH lines, but 3D non-LTE effects may serve to
erase these differences.Comment: ps file, 18 pages (including 10 figures) - Accepted for publication
in A&
Toward enterprise process engineering: configuration measurement and analysis
The enterprise process has become a central unit of analysis in management. But a dearth of theory and methods to predict process performance is available to the enterprise manager. With the proposition that enterprise process design represents an engineering problem as much as a managerial one, we focus on the process configuration and measurement to overcome design limitations that currently confront the enterprise manager. Building on seminal work for process description and comparison, and employing Graph Theory and Measurement Theory, we integrate this work and demonstrate useful analytical properties associated with the kinds of measures proposed (e.g., ratio scales). This represents a new result, which extends the prior research on enterprise process design and makes a contribution to our understanding in management science. Further, with parallels to the predictive capability enjoyed by engineers who design physical artifacts, we indicate how enterprise process configuration measures can be used to hypothesize relations with respect to enterprise performance. And we suggest how enterprise managers may design processes in a deductive manner-working backward from performance requirements to determine configuration parameters required for designs to meet performance goals. These represent new ca abilities not previously available to the enterprise manager as well as another contribution of the resent research.Approved for public release; distribution is unlimited
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