Giants in the globular cluster ω Centauri: dust production, mass-loss and distance     

Iain McDonald  Jacco Th. van Loon  Leen Decin  Martha L. Boyer  rea K. Dupree  Aneurin Evans  Robert D. Gehrz  Charles E. Woodward 《Monthly notices of the Royal Astronomical Society》2009,394(2):831-856

  相似文献   

Polymictic pool behaviour in a montane meadow,Sierra Nevada,CA          下载免费PDF全文

Ryan G. Lucas  Francisco Suárez  Scott W. Tyler  Jean E. Moran  Martha H. Conklin 《水文研究》2016,30(18):3274-3288

We observed polymictic behaviour in stream pools in Long Meadow, Sequoia National Park, California—part of the Southern Sierra Critical Zone Observatory. Stream pools stratified thermally during the day time and were isothermal at night—this pattern persists from the middle of summer into the fall. We found that four characteristics typical of a mountain meadow environment—low stream flow, open sky, cold groundwater discharge, and elevated organic carbon concentrations—are particularly conducive to pool stratification. Incoming shortwave radiation was the dominant energy input to heat pool water while nighttime emitted longwave radiation was the major cooling mechanism. Relatively cold groundwater discharge into the pool bottom increased density stratification within the pool. Elevated DOC concentrations increased the capacity of the pool to absorb photosynthetically active radiation and also promoted stratification. Stream velocities in the meadow were generally insufficient to meet threshold Richardson numbers and mix the pools during the daytime; smaller stream cross sectional areas would have potential for destabilizing pools in the daytime. We propose a conceptual model for describing polymictic stream pools and assessing the potential for polymictic pools to occur. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

Facies estimation through data assimilation and structure parameterization     

Martha Lien  Trond Mannseth 《Computational Geosciences》2014,18(5):869-882

  相似文献   

A 2D study of the effects of the size of a solid core on the equatorial flow in giant planets     

Martha Evonuk  Gary A. Glatzmaier 《Icarus》2006,181(2):458-464

Zonal winds simulated in two-dimensional computer models of turbulent convection in the equatorial plane of giant planets have greater surface amplitudes for cases with smaller solid cores, and therefore larger buoyancy driving, all other properties being equal. This differential rotation in radius is maintained by the convergence of angular momentum flux, which occurs because of the convective flow that develops due to the effects of planetary rotation and density stratification. The superposition of the convective flow and the stronger zonal flow produces wave-like, instead of cellular convection.  相似文献   

Microstructures of phosphatic porcelain from sintering to vitrification: Evidence from sherds excavated in Charleston,South Carolina     

J. Victor Owen  Robert Hunter  Rod Jellicoe  Martha Zierden 《Geoarchaeology》2011,26(2):292-313

Eight phosphatic porcelain sherds recovered from various historical sites in Charleston were analyzed by electron microprobe. Some sherds contain sulfur (2.3–3.1wt.% SO₃); others contain only traces of this component. The analytical data suggest that the sulfurous sherds are Bow porcelain (London, Bowcock period, ca. 1755–1769). The origin(s) of the low‐S samples remains unidentified; one compositionally resembles “gold‐anchor period” (phosphatic) Chelsea porcelain (London, ca. 1756–1769) but its decoration is inconsistent with known wares produced by this factory during that era. The degree of vitrification is highly variable, particularly among the SO₃‐poor samples. The melt phase is strongly enriched in incompatible elements (Ti, Fe, Na, K). The phosphate phase [calcined bone ash (hydroxyapatite)] in poorly vitrified samples hosts minute melt blebs, but remains porous. With increasing vitrification, these melt blebs increase in size and begin to coalesce, ultimately forming ameboid patches up to ∼10 μm in diameter. In the most vitrified samples, the coalesced melt “leaks” into the matrix, leaving behind a phosphate phase that lacks pores and melt and has a lower CaO/P₂O₅ ratio (=2.7, molecular proportions) than either hydroxyapatite (3.3) or β‐whitlockite (3.0). The two varieties of phosphate occur in some poorly vitrified samples, suggesting the recycling of high‐fired wasters (as “grog”) in their ceramic pastes. Melt compositions vary with contiguous mineralogy, accounting for their divergence from the ternary eutectic in the Ca₃(PO₄)₂‐CaAl₂Si₂O₈‐SiO₂ system. The resorption of phosphate by the matrix melt virtually precludes recognition of anorthite formed by the “non‐phosphate glass equation.” © 2011 Wiley Periodicals, Inc.  相似文献   

INFRARED (JHK) PHOTOMETRY OF METEORITES AND ASTEROIDS     

Martha Leake  Jonathan Gradie  David Morrison 《Meteoritics & planetary science》1978,13(1):101-120

We present JHK colors observed for ten asteroids and synthesized JHK colors for seven meteorite groups, samples of iron and nickel metal, pyroxene, olivine, feldspar, a lunar anorthite and some terrestrial mineral samples. Pronounced differences are apparent between the chondritic and achondritic meteorite classes; the chondritic classes show less subdued trends in J-H color which reflect their metamorphic grade We find small but significant differences between the JHK colors of the predominant C and S classes of asteroids. All JHK colors of asteroids observed here fall within the limited domain defined by the various chondritic and iron-rich meteorites but are strikingly different from those of most achondritic meteorites  相似文献   

Postglacial vegetation, climate, and fire history along the east side of the Andes (lat 41-42.5°S), Argentina     

Cathy Whitlock  Maria Martha Bianchi  Vera Markgraf  Megan Walsh 《Quaternary Research》2006,66(2):187-201

The history of the low-elevation forest and forest-steppe ecotone on the east side of the Andes is revealed in pollen and charcoal records obtained from mid-latitude lakes. Prior to 15,000 cal yr BP, the vegetation was characterized by steppe vegetation with isolated stands of Nothofagus. The climate was generally dry, and the sparse vegetation apparently lacked sufficient fuels to burn extensively. After 15,000 cal yr BP, a mixture of Nothofagus forest and shrubland/steppe developed. Fire activity increased between 13,250 and 11,400 cal yr BP, contemporaneous with a regionally defined cold dry period (Huelmo/Mascardi Cold Reversal). The early-Holocene period was characterized by an open Nothofagus forest/shrubland mosaic, and fire frequency was high in dry sites and low in wet sites; the data suggest a sharp decrease in moisture eastward from the Andes. A shift to a surface-fire regime occurred at 7500 cal yr BP at the wet site and at 4400 cal yr BP at the dry site, preceding the expansion of Austrocedrus by 1000-1500 yr. The spread of Austrocedrus is explained by a shift towards a cooler and wetter climate in the middle and late Holocene. The change to a surface-fire regime is consistent with increased interannual climate variability and the onset or strengthening of ENSO. The present-day mixed forest dominated by Nothofagus and Austrocedrus was established in the last few millennia.  相似文献   

A note on the top-down and bottom-up gradient functions over a forested site     

Weiguo Wang  Kenneth J. Davis  Chuixiang Yi  Edward G. Patton  Martha P. Butler  Daniel M. Ricciuto  Peter S. Bakwin 《Boundary-Layer Meteorology》2007,124(2):305-314

The dimensionless bottom-up and top-down gradient functions in the convective boundary layer (CBL) are evaluated utilizing long-term well-calibrated carbon dioxide mixing ratio and flux measurements from multiple levels of a 447-m tall tower over a forested area in northern Wisconsin, USA. The estimated bottom-up and top-down functions are qualitatively consistent with those from large-eddy simulation (LES) results and theoretical expectations. Newly fitted gradient functions are proposed based on observations for this forested site. The integrated bottom-up function over the lowest 4% of the CBL depth estimated from the tower data is about five times larger than that from LES results for a ‘with-canopy’ case, and is smaller than that from LES results for a ‘no-canopy’ case by a factor of 0.7. We discuss the uncertainty in the evaluated gradient functions due to stability, wind direction, and uncertainty in the entrainment flux and show that while all of these have a significant impact on the gradient functions, none can explain the differences between the modelled and observed functions. The effects of canopy features and atmospheric stability may need to be considered in the gradient function relations.  相似文献   

可持续发展研究方法国际进展——脆弱性分析方法与可持续生计方法比较   总被引：43，自引：3，他引：43  

杨国安 《地理科学进展》2003,22(1):11-21

自从以布伦特兰德夫人领导的委员会正式提出可持续发展概念以来,可持续发展受到社会各界的密切关注,可持续发展思想已经深入人心。但由于问题的复杂性,既使是今天,可持续的实现依然困难。因此,围绕可持续发展提出了各式各样的研究工具,其中可持续生计方法和脆弱性分析方法可谓是众多方法中的佼佼者。前者强调贫困的消除,后者强调脆弱性的缓解,可持续发展又是其共同的目的。本文对两种方法在概念和实施方案方面进行了比较分析,在明确它们的差异和相似之处的基础上,认为两种方法通过联合可以在实践中相互加强和相互促进。  相似文献   

Revised life history analysis for large grazing copepods in the subarctic Pacific Ocean     

Charles B. Miller  Martha J. Clemons 《Progress in Oceanography》1988,20(4)

Discovery that the subarctic Pacific copepods previously grouped as Neocalanus plumchrus belong to two species required reanalysis of the life histories of both. After correction of the abundance estimates for N. plumchrus s.str., our concept of its life history remains much as previously described, because it makes up about 90% of the summed populations. Fifth copepodites of the new species, Neocalanus flemingeri, descend from the surface layer in late May to early June and mature immediately. Males are only present for about two months, and females carrying spermatophores are found during that period. Throughout the summer and autumn the entire population is constituted of females with small, dormant ovaries. This appears to be a diapause phase. Ovarian development begins in November, and spawning occurs at the end of January. Copepodite stages develop in surface layers from February through May.  相似文献