238 research outputs found
The plasma-solid transition:some implications in astrophysics
Using a criterion proposed by Salpeter,and standard solid state physics,we
have determined the Debye temperature of a solid in equilibrium with the
electron gas surrounding it.The results obtained can have implications in
studies of the interstellar medium.Comment: plain LaTeX,9 pages,no figure
A synoptic comparison of the MHD and the OPAL equations of state
A detailed comparison is carried out between two popular equations of state
(EOS), the Mihalas-Hummer-Dappen (MHD) and the OPAL equations of state, which
have found widespread use in solar and stellar modeling during the past two
decades. They are parts of two independent efforts to recalculate stellar
opacities; the international Opacity Project (OP) and the Livermore-based OPAL
project. We examine the difference between the two equations of state in a
broad sense, over the whole applicable rho-T range, and for three different
chemical mixtures. Such a global comparison highlights both their differences
and their similarities.
We find that omitting a questionable hard-sphere correction, tau, to the
Coulomb interaction in the MHD formulation, greatly improves the agreement
between the MHD and OPAL EOS. We also find signs of differences that could stem
from quantum effects not yet included in the MHD EOS, and differences in the
ionization zones that are probably caused by differences in the mechanisms for
pressure ionization. Our analysis do not only give a clearer perception of the
limitations of each equation of state for astrophysical applications, but also
serve as guidance for future work on the physical issues behind the
differences. The outcome should be an improvement of both equations of state.Comment: 33 pages, 26 figures. Corrected discussion of Basu & Antia, 2004,
ApJ, 606, L85-L8
Land Cover Inventory of the Niobrara River Watershed
The Niobrara River watershed covers a large area in northern Nebraska, and neighboring parts of South Dakota and Wyoming, and is an important component of statewide natural resources management. Under ongoing agreements with the U.S. Bureau of Reclamation (BoR), the Nebraska Game and Parks Commission are charged with the development of management plans for various BoR projects in the Niobrara River watershed in Nebraska. A detailed delineation of land use and land cover patterns was needed in order to define and implement effective management strategies
Does the Sun shrink with increasing magnetic activity?
It has been demonstrated that frequencies of f-modes can be used to estimate
the solar radius to a good accuracy. These frequencies have been used to study
temporal variations in the solar radius with conflicting results. The variation
in f-mode frequencies is more complicated than what is assumed in these
studies. If a careful analysis is performed then it turns out that there is no
evidence for any variation in the solar radius.Comment: To appear in Astrophys.
Grids of stellar models. VIII. From 0.4 to 1.0 Msun at Z=0.020 and Z=0.001, with the MHD equation of state
We present stellar evolutionary models covering the mass range from 0.4 to 1
Msun calculated for metallicities Z=0.020 and 0.001 with the MHD equation of
state (Hummer & Mihalas, 1988; Mihalas et al. 1988; D\"appen et al. 1988). A
parallel calculation using the OPAL (Rogers et al. 1996) equation of state has
been made to demonstrate the adequacy of the MHD equation of state in the range
of 1.0 to 0.8 Msun (the lower end of the OPAL tables). Below, down to 0.4 Msun,
we have justified the use of the MHD equation of state by theoretical arguments
and the findings of Chabrier & Baraffe (1997).
We use the radiative opacities by Iglesias & Rogers (1996), completed with
the atomic and molecular opacities by Alexander & Fergusson (1994). We follow
the evolution from the Hayashi fully convective configuration up to the red
giant tip for the most massive stars, and up to an age of 20 Gyr for the less
massive ones. We compare our solar-metallicity models with recent models
computed by other groups and with observations.
The present stellar models complete the set of grids computed with the same
up-to-date input physics by the Geneva group [Z=0.020 and 0.001, Schaller et
al. (1992), Bernasconi (1996), and Charbonnel et al. (1996); Z=0.008, Schaerer
et al. (1992); Z=0.004, Charbonnel et al. (1993); Z=0.040, Schaerer et al.
(1993); Z=0.10, Mowlavi et al. (1998); enhanced mass loss rate evolutionary
tracks, Meynet et al. (1994)].Comment: Accepted for publication in A&A Supplement Serie
MAPPING AGRICULTURAL LAND COVER FOR HYDROLOGIC MODELING IN THE PLATTE RIVER WATERSHED OF NEBRASKA
Throughout the western United States, natural resources managers are attempting to address the growing, and often competing, demands that municipal, agricultural and environmental interests have for water. The Platte River Cooperative Hydrology Study (COHYST) is a multi-agency effort that seeks to improve understanding of the ecology, geology, and hydrology of the Platte River watershed in central and western Nebraska. Information regarding the types, areal extent, and locations of crops (especially irrigated crops) is critical for estimating consumptive use of water. Digital land-cover and land-use datasets of the central and western Platte River valley have been prepared for four years: 1982, 1997, 2001, and 2005. Mapping was carried out using multidate Landsat satellite imagery in combination with ancillary geospatial data. The mapping was validated using field observations collected independently. Overall accuracy of the maps developed ranged from 74% to 82.7%. All land-cover maps and full documentation are available online at http://www.calmit.unl.edu/cohyst/
Equation of state SAHA-S meets stellar evolution code CESAM2k
We present an example of an interpolation code of the SAHA-S equation of
state that has been adapted for use in the stellar evolution code CESAM2k. The
aim is to provide the necessary data and numerical procedures for its
implementation in a stellar code. A technical problem is the discrepancy
between the sets of thermodynamic quantities provided by the SAHA-S equation of
state and those necessary in the CESAM2k computations. Moreover, the
independent variables in a practical equation of state (like SAHA-S) are
temperature and density, whereas for modelling calculations the variables
temperature and pressure are preferable. Specifically for the CESAM2k code,
some additional quantities and their derivatives must be provided. To provide
the bridge between the equation of state and stellar modelling, we prepare
auxiliary tables of the quantities that are demanded in CESAM2k. Then we use
cubic spline interpolation to provide both smoothness and a good approximation
of the necessary derivatives. Using the B-form of spline representation
provides us with an efficient algorithm for three-dimensional interpolation.
The table of B-spline coefficients provided can be directly used during stellar
model calculations together with the module of cubic spline interpolation. This
implementation of the SAHA-S equation of state in the CESAM2k stellar structure
and evolution code has been tested on a solar model evolved to the present. A
comparison with other equations of state is briefly discussed. The choice of a
regular net of mesh points for specific primary quantities in the SAHA-S
equation of state, together with accurate and consistently smooth tabulated
values, provides an effective algorithm of interpolation in modelling
calculations. The proposed module of interpolation procedures can be easily
adopted in other evolution codes.Comment: 8 pages, 5 figure
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