X-ray emission is an important indicator of stellar activity. In this paper, we study stellar Xray activity using the XMM-Newton and LAMOST data for different types of stars. We provide a sample including 1259 X-ray-emitting stars, of which 1090 have accurate stellar parameter estimations. Our sample size is much larger than those used in previous works. We find a bimodal distribution of the X-ray to optical flux ratio(log(fX/fV)) for G and K stars. We interpret that this bimodality is due to two subpopulations with different coronal heating rates. Furthermore, using the full widths at half maxima calculated from Hα and Hβ lines, we show that these stars in the inactive peaks have smaller rotational velocities. This is consistent with the magnetic dynamo theory that presumes stars with low rotational velocities have low levels of stellar activity. We also examine the correlation between log(fX/fV) and luminosity of the excess emission in the Hα line, and find a tight relation between the coronal and chromospheric activity indicators. 相似文献
Ocean Dynamics - Based on a non-hydrostatic two-dimensional and high-resolution model, evolution of internal solitary waves (ISWs) on the typical slope-shelf topography in the northern South China... 相似文献
Atlantic salmon reared in recirculating aquaculture system (RAS) may lead to inappropriately high stocking density, because fish live in a limited space. Finding the suitable stocking density of Atlantic salmon reared in RAS is very important for RAS industry. In this paper, the influence of stocking density on growth and some stress related physiological factors were investigated to evaluate the effects of stocking density. The fish were reared for 220 days at five densities (A: 24 kg/m3; B: 21 kg/m3; C: 15 kg/m3; D: 9 kg/ m3 and E: 6 kg/m3 ). The results show that 30 kg/m3 might be the maximum density which RAS can afford in China. The stocking densities under 30 kg/m3 have no effect on mortality of Atlantic salmon reared in RAS. However, the specific growth rate (SGR), final weight and weight gain in the high density group were significantly lower than the lower density groups and middle density groups. Moreover, feed conversion rate (FCR) had a negative correlation with density. Plasma hormone T3 and GH showed significant decrease with the increase of the stocking density of the experiment. Furthermore, thyroid hormone (T3), GH (growth hormone) activities were decreased with stocking density increase. However, plasma cortisol, GOT (glutamic oxalacetic transaminase) and GPT (glutamic pyruvic transaminase) activities were increase with stocking density increase. And the stocking density has no effects on plasma lysozyme of Atlantic salmon reared in RAS. These investigations would also help devise efficient ways to rear adult Atlantic salmon in China and may, in a way, help spread salmon mariculture in China.
Changes of subaqueous topography in shallow offshore water pose safety risks for embankments,navigation,and ports.This study conducted measurements of subaqueous topography between Datong and Xuliujing in the Yangtze River using a Sea Bat 7125 multi-beam echo sounder,and the channel change from 1998 to 2013 was calculated using historical bathymetry data.The study revealed several important results:(1)the overall pattern of changes through the studied stretch of the river was erosion–deposition–erosion.Erosion with a volume 700×10~6m~3occurred in the upper reach,deposition of about 204×10~6m~3occurred in the middle reach,and erosion of about 602×10~6m~3occurred in the lower reach.(2)Dunes are the most common microtopographic feature,accounting for 64.3%of the Datong to Xuliujing reach,followed by erosional topography and flat river topography,accounting for 27.6%and 6.6%,respectively.(3)Human activities have a direct impact on the development of the microtopography.For instance,the mining of sand formed holes on the surface of dunes with lengths of 20–35 m and depths of 3–5 m.We concluded that the overall trend of erosion(net erosion volume of 468×10~6m~3)occurred in the study area mainly because of the decreased sediment discharge following the closure of the Three Gorges Dam.However,other human activities were also impact factors of topographic change.Use of embankments and channel management reduced channel width,restricted river meandering,and exacerbated the erosion phenomenon. 相似文献
This paper analyzes the characteristics of time sequence changes of gravity points near the epicenter, different changes of measuring lines and gravity changes of measuring areas in point-line-area manner respectively with the 5-period mobile gravity data through densified observation by the South Xinjiang Observation Network after the 2015-2016 Akto earthquake in Xinjiang. The gravity observation results before the earthquake indicate that the Wuqia-Bulungkol area near the epicenter presented the trend of gravity value increasing since 2015, but the gravity value decreased half a year before the earthquake, and witnessed a high gradient zone of gravity changes during some periods before the earthquake. The gravity observation results after the earthquake show that there is a trend of opposite changes in gravity difference on the northern and southern sides of Bulunkou, and good correspondence exists between the characteristics of gravity field changes near the epicenter before and after the earthquake and the geologic structure distribution in the area. 相似文献
In this paper, a comprehensive study on simulating the shearing behavior of frictional materials is performed. A set of two explicit equations, describing the relationship among the shear stress ratio and the distortional strain and the volumetric strain, are formulated independently. The equations contain three stress parameters and three strain parameters and another parameter representing the nonuniformity of stress and strain during softening. All the parameters have clear physical significance and can be determined experimentally. It is demonstrated that the proposed equations have the capacity of simulating the complicated shearing behavior of many types of frictional materials including geomaterials. The proposed equations are used to simulate the stress–strain behavior for 27 frictional materials with 98 tests. These materials include soft and stiff clays in both reconstituted and structured states, silicon sands and calcareous sands, silts, compacted fill materials, volcanic soils, decomposed granite soils, cemented soils (both artificially and naturally cemented), partially saturated soils, ballast, rocks, reinforced soils, tire chips, sugar, wheat, and rapeseed. It has been demonstrated that the proposed explicit constitutive equations have the capacity to capture accurately the shearing behavior of frictional materials both qualitatively and quantitatively. A study on model parameters has been performed. 相似文献