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An, Deokkeun,Terndrup, Donald M.,Pinsonneault, Marc H.,Lee, Jae-Woo IOP Publishing 2015 The Astrophysical journal Vol.811 No.1
<P>We extend our effort to calibrate stellar isochrones in the Johnson-Cousins (BVIC) and the 2MASS (JHK(s)) filter systems based on observations of well-studied open clusters. Using cool main-sequence (MS) stars in Praesepe, we define empirical corrections to the Lejeune et al. color-effective temperature (T-eff) relations down to T-eff similar to 3600 K, complementing our previous work based on the Hyades and the Pleiades. We apply empirically corrected isochrones to existing optical and near-infrared photometry of cool (T-eff less than or similar to 5500 K) and metal-rich ([Fe/H] = +0.37) MS stars in NGC 6791. The current methodology relies on an assumption that color-T-eff corrections are independent of metallicity, but we find that estimates of color excess and distance from color-magnitude diagrams with different color indices converge on each other at the precisely known metallicity of the cluster. Along with a satisfactory agreement with eclipsing binary data in the cluster, we view the improved internal consistency as a validation of our calibrated isochrones at super-solar metallicities. For very cool stars (T-eff less than or similar to 4800 K), however, we find that B - V colors of our models are systematically redder than the cluster photometry by similar to 0.02 mag. We use color-T-eff transformations from the infrared flux method and alternative photometry to examine a potential color-scale error in the input cluster photometry. After excluding B - V photometry of these cool MS stars, we derive E(B - V) = 0.105 +/- 0.014, [M/H] = +0.42 +/- 0.07, (m - M)(0) = 13.04 +/- 0.08, and the age of 9.5 +/- 0.3 Gyr for NGC 6791.</P>
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Analysis of Ship Groundings on Soft Sea Beds
Simonsen, B. Cerup,Redersen, P. Terndrup The Society of Naval Architects of Korea 1997 Journal of ship and ocean technology Vol.1 No.1
The consequences associated with ships running aground depend very much on the soil characteristics of the sea bed and the geometrical shape of the ship bow. The penetration into the sea bed depends on these factors and the penetration is an important factor for the ship motion because it influences the ship heave and pitch motions as well as the friction between the ship and the soil. In this paper a rational calculation model is presented for the sea bed soil reaction forces on the ship bottom. The model is based on the assumption that the penetration of the ship bow generates a flow of pore water through the grain skeleton of the soil. The flow is governed by Darcy\`s law and it is driven by the pressure of the pore water at the bow. In addition to this pore water pressure, the bow is subjected to the effective stresses in the grain skeleton at the bow surface. These stresses are determined by the theory of frictional soils in rupture. Frictional stresses on the bow surface are assumed to be related to the normal pressure by a simple Coulomb relation. The total soil reaction as a function of velocity and penetration is found by integration of normal pressure and frictional stresses over the surface of the bow. The analysis procedure is implemented in a computer program for time domain rigid body analysis of ships running aground and it is verified in the paper through a comparison of calculated stopping lengths, effective coefficients of friction, and sea bed penetrations with corresponding experimental results obtained by model tests as well as large, scale tests.
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Kim, Bokyoung,An, Deokkeun,Stauffer, John R.,Lee, Young Sun,Terndrup, Donald M.,Johnson, Jennifer A. American Astronomical Society 2016 The Astrophysical journal Supplement series Vol.222 No.2
<P>The tension between the Hipparcos parallax of the Pleiades and other independent distance estimates continues even after the new reduction of the Hipparcos astrometric data and the development of a new geometric distance measurement for the cluster. A short Pleiades distance from the Hipparcos parallax predicts a number of stars in the solar neighborhood that are sub-luminous at a given photospheric abundance. We test this hypothesis using the spectroscopic abundances for a subset of stars in the Hipparcos catalog, which occupy the same region as the Pleiades in the color-magnitude diagram. We derive stellar parameters for 170 nearby G-and K-type field dwarfs in the Hipparcos catalog based on high-resolution spectra obtained using KPNO 4 m echelle spectrograph. Our analysis shows that, when the Hipparcos parallaxes are adopted, most of our sample stars follow empirical colormagnitude relations. A small fraction of stars are too faint compared to main-sequence fitting relations by Delta M-V greater than or similar to 0.3 mag, but the differences are marginal at a 2 sigma level, partly due to relatively large parallax errors. On the other hand, we find that the photometric distances of stars showing signatures of youth as determined from lithium absorption line strengths and R'(HK) chromospheric activity indices are consistent with the Hipparcos parallaxes. Our result is contradictory to a suggestion that the Pleiades distance from main-sequence fitting is significantly altered by stellar activity and/or the young age of its stars, and provides an additional supporting evidence for the longdistance scale of the Pleiades.</P>
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