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591.
Star formation within galaxies occurs on multiple scales, from spiral structure, to OB associations, to individual star clusters, and often as substructure within these clusters. This multitude of scales calls for objective methods to find and classify star-forming regions, regardless of spatial size. To this end, we present an analysis of star-forming groups in the Local Group spiral galaxy M33, based on a new implementation of the Minimum Spanning Tree (MST) method. Unlike previous studies, which limited themselves to a single spatial scale, we study star-forming structures from the effective resolution limit (~20 pc) to kpc scales. Once the groups have been identified, we study their properties, such as their size and luminosity distributions, and compare these with studies of young star clusters and giant molecular clouds (GMCs). We find evidence for a continuum of star-forming group sizes, which extends into the star cluster spatial-scale regime. We do not find a characteristic scale for OB associations, unlike that found in previous studies, and we suggest that the appearance of such a scale was caused by spatial resolution and selection effects. The luminosity function of the groups is found to be well represented by a power law with an index of ?2, as has also been found for the luminosity and mass functions of young star clusters, as well as for the mass function of GMCs. Additionally, the groups follow a similar mass-radius relation as GMCs. The size distribution of the groups is best described by a log-normal distribution, the peak of which is controlled by the spatial scale probed and the minimum number of sources used to define a group. We show that within a hierarchical distribution, if a scale is selected to find structure, the resulting size distribution will have a log-normal distribution. We find an abrupt drop of the number of groups outside a galactic radius of ~4 kpc (although individual high-mass stars are found beyond this limit), suggesting a change in the structure of the star-forming interstellar medium, possibly reflected in the lack of GMCs beyond this radius. Finally, we find that the spatial distribution of H?ii regions, GMCs, and star-forming groups are all highly correlated. 相似文献
592.
R.D. Cess Barbara E. Carlson John Caldwell I.G. Nolt F.C. Gillett A.T. Tokunaga 《Icarus》1981,46(2):249-255
Ground-based observations of Jupiter show that the planet's stratospheric and tropospheric thermal emission are anticorrelated. The observations can possibly be explained by latitudinal variations in cloud altitude. These variations cause differential stratospheric heating by sunlight which is reflected off the clouds and then absorbed within the stratosphere by visible and near-infrared bands of methane. 相似文献
593.
594.
Janez K. Lapajne Barbara Sket Motnikar Blaz Zabukovec Polona Zupancic 《Journal of Seismology》1997,1(1):73-85
A progress report on the mapping effort for construction of a peak ground acceleration (PGA) map of Slovenia for 475-year return period for rock and firm soil is presented. The methodology is similar to that recently applied in Central and Eastern United States. It is based on historical seismicity spatially smoothed to different length scales. The procedure is described by Frankel (1995). He uses the characteristic earthquake recurrence relationship and in his firt version four different seismicity models. We also use four seismicity models. But instead of characteristic earthquake recurrence, we use the doubly truncated exponential magnitude-frequency relationship; no evidence of characteristic earthquakes in Slovenia has been found yet. Three of our models are similar to Frankel's first three models. Model 1 uses spatially smoothed activity rate based on magnitude 3.7 and above events since 1880. Model 2 deals with spatially smoothed activity rate based on magnitude 5.0 abd above events since 1690. Model 3 smoothes the observed activity over the entire region; it represents a uniform seismicity zone. Frankel skipped this model in his latest version (Frankel et al. 1996). In model 4, we introduce a new approach of calculating seismic activity rate taking into account released seismic energy. The ground motion attenuation model of Pugliese and Sabetta (1989) is used for all models. PGA maps for models 1, 2, 3 and 4 have been calculated, and a weighted mean map derived from them. A map of model 1 has been compared with the corresponding source zone map; the two maps do not differ significantly. A worst-case map derived from all four models has also been produced. 相似文献
595.
596.
Artificial substrate collectors were used to sample settled brachyuran megalopae in Terrebonne Bay, Louisiana, northern Gulf
of Mexico. Three taxa,Callinectes sapidus, Uca spp., andRhithropanopeus harrisii, were the most common settlers. Settlement occurred almost year-round, although settlement densities were highest in late
summer and early fall. In 1990, all three taxa had several simultaneous settlement peaks; in 1991, one simultaneous settlement
peak occurred. FewerC. sapidus andUca spp. settled in 1991 than in 1990, but settlement abundance ofR. harrissii was similar between years. Local weather events, such as heavy precipitation, may have influenced the lower abundances ofC. sapidus andUca spp. in 1991. The megalopae ofC. sapidus andUca spp., which re-invade the estuary from higher salinity offshore waters where larval development occurs, may have been more
affected by the lower 1991 estuarine salinity values than the larvae ofR. harrissii, which are retained within the estuary throughout larval development. Settlement ofC. sapidus andUca spp. was correlated with maximum tidal height, tidal amplitude, salinity, temperature, and lunar phase. In 1990, settlement
ofC. sapidus was correlated with lunar declination cycles; settlement peaks occurred during equatorial minimum amplitude tides. The settlement
ofR. harrisii was associated with tidal amplitude and lunar phase. Associations between environmental variables and settlement of megalopae
were not consistent between the two years sampled. The opposing or reinforcing effects of various environmental variables
on settlement, e.g., tidal height and rainfall, are superimposed on long-term hydrologic cycles so that dramatically altered
cycles of settlement occur among years. 相似文献
597.
598.
Barbara Louise Endemao Walker 《The Professional geographer》2001,53(2):160-177
As processors and marketers of fish, women fishtraders in the Fanti town of Cape Coast, Ghana have become powerful financers and owners of canoes, nets, and other fishing equipment. Since the 1960s, when motors were first introduced to Ghana's artisanal canoe fleet, two interrelated processes have occurred in Cape Coast. First, Ghana's fisheries have become increasingly exploited and—in the case of some species—overfished. Second, the social relations of production in the artisanal sector have shifted from being socially embedded to being more market‐based and impersonal. I argue that two recent Women in Development (WID) projects in particular have contributed to the breakdown of fishtraders’ traditional economic networks and livelihood strategies: (a) loan schemes that target women's associations, and (b) the 1985 Intestate Succession Law, which reconfigured inheritance rights. These WID projects, based on western notions of gender and the household, have created disharmony and mistrust among Cape Coast's fishtraders rather than promoting their “development.” The breakdown of fishtraders’ labor and marketing organizations has resulted in increasingly desperate strategies to get fish, increased degradation of Ghana's marine environment, and uncertainty for the future of the coastal economy. 相似文献
599.
B. Lafrance Barbara E. John James S. Scoates 《Contributions to Mineralogy and Petrology》1996,122(4):431-440
Plagioclase recrystallization microstructures and petrofabrics in the unmetamorphosed, 1.43 Ga Poe Mountain anorthosite,
Wyoming, are indicative of very high-temperature deformation and recrystallization during the emplacement of the anorthosite
body. The Poe Mountain anorthosite consists of a core of recrystallized, massive anorthosite transitional with a series of
layered anorthositic cumulates at the margin of the intrusion. Irregular grain boundaries and dissected grain microstructures
in the massive core and transitional anorthosites suggest that the anorthositic rocks recrystallized by “fast” grain boundary
migration and possibly subgrain rotation recrystallization, at very high temperatures (≈1050°C) during emplacement of the
intrusion in the mid-crust (3 kbar). The deformation and recrystallization of the Poe Mountain anorthosite was continuous
from subliquidus to subsolidus temperature conditions during the emplacement of the intrusion. Anorthosites with the lowest
modal percentages of ferromagnesian minerals and Fe-Ti oxides are always the most recrystallized. This suggests that melt
interstitial to the plagioclase-crystal framework was removed during deformation and recrystallization of the intrusion. Bulging
of plagioclase grain boundaries around Fe-Ti oxides together with deformed oikocrystic ferromagnesian minerals and plagioclase
chadacrysts indicate that the deformation and recrystallization of the intrusion continued after the crystallization of the
interstitial melt minerals.
Received: 28 February 1995/Accepted: 20 July 1995 相似文献
600.
Estuarine nursery areas are critical for successful recruitment of tautog (Tautoga onitis), yet they have not been studied over most of this species' range. Distribution, abundance and habitat characteristics of young-of-the-year (YOY, age 0) and age 1+juvenile tautog were evaluated during 1988–1992 in the Narragansett Bay estuary, Rhode Island, using a 16-station, beach-seine survey. Estuary-wide abundance was similar among years. Greatest numbers of juveniles were collected at northern Narragansett Bay stations between July and September. Juvenile abundances varied with density of macroalgal and eelgrass cover; abundances ranged from 0.03 fish per 100 m2 to 8.1 fish per 100 m2. Although juveniles use eelgrass, macroalgae is the dominant vegetative cover in Narragansett Bay. Macroalgal habitats play a previously unrealized, important role and contribute to successful recruitment of juvenile tautog in Narragansett Bay. Juvenile abundances did not vary with sediment type or salinity, but were correlated with surface water temperature. Fish collected in June were age 1+ juveniles from the previous year-class (50–167 mm TL) and these declined in number after July or August. The appearance of YOY (25–30 mm TL) in July and August was coincident with the period of their greatest abundances. A precipitous decline in abundance occurred by October because of the individual or combined effects of mortality and movement to alternative habitats. Based on juvenile abundance, a previously unidentified spawning area was noted in Mount Hope Bay, a smaller embayment attached to the northeastern portion of Narragansett Bay. In August 1991, Hurricane Bob disrupted juvenile sise distribution and abundance, resulting in reduced numbers of YOY collected after the storm and few 1+ juveniles in 1992. 相似文献