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211.
The Primordial Helium-4 Abundance from Observations of a Large Sample of Blue Compact Dwarf Galaxies
We use a sample of 45 low-metallicity H II regions in blue compact dwarf (BCD) galaxies to determine the primordial helium abundance YP with a precision better than 5%. We have carefully investigated the physical effects which may make the He I line intensities deviate from their recombination values such as collisional and fluorescent enhancements, underlying He I stellar absorption and absorption by Galactic interstellar Na I. By extrapolating the Y vs. O/H and Y vs. N/H linear regressions to O/H = N/H = 0, we obtain YP = 0.244±0.002 and 0.245±0.001, respectively, higher than previous determinations (YP = 0.230 - 0.234). Part of the difference comes from the fact that previous investigators have not taken into account underlying He I stellar absorption, especially in the NW component of the BCD I Zw 18 which, because of its extremely low metallicity plays a key role in the determination of YP. We derive a slope dY/dZ = 2.3±1.0, considerably smaller than those derived before. With this smaller slope and taking into account the errors, chemical evolution models with an outflow of well-mixed material can be built for star-forming dwarf galaxies which satisfy all the observational constraints. Our YP gives bh
50
2
= 0.058±0.007,f consistent with the lower limit set by dynamical measurements and X-ray observations of clusters of galaxies. It is also consistent, within the framework of standard big bang nucleosynthesis theory, with measurements of primordial 7Li in galactic halo stars and with the D/H abundance measured in absorption systems toward quasars by Burles and Tytler (1997). 相似文献
212.
S.A. Melachroinos F.G. Lemoine N.P. Zelensky D.D. Rowlands S.B. Luthcke O. Bordyugov 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
We compute a series of Jason-2 GPS and SLR/DORIS-based orbits using ITRF2005 and the std0905 standards ( Lemoine et al., 2010). Our GPS and SLR/DORIS orbit data sets span a period of 2 years from cycle 3 (July 2008) to cycle 74 (July 2010). We extract the Jason-2 orbit frame translational parameters per cycle by the means of a Helmert transformation between a set of reference orbits and a set of test orbits. We compare the annual terms of these time-series to the annual terms of two different geocenter motion models where biases and trends have been removed. Subsequently, we include the annual terms of the modeled geocenter motion as a degree-1 loading displacement correction to the GPS and SLR/DORIS tracking network of the POD process. Although the annual geocenter motion correction would reflect a stationary signal in time, under ideal conditions, the whole geocenter motion is a non-stationary process that includes secular trends. Our results suggest that our GSFC Jason-2 GPS-based orbits are closely tied to the center of mass (CM) of the Earth consistent with our current force modeling, whereas GSFC’s SLR/DORIS-based orbits are tied to the origin of ITRF2005, which is the center of figure (CF) for sub-secular scales. We quantify the GPS and SLR/DORIS orbit centering and how this impacts the orbit radial error over the globe, which is assimilated into mean sea level (MSL) error, from the omission of the annual term of the geocenter correction. We find that for the SLR/DORIS std0905 orbits, currently used by the oceanographic community, only the negligence of the annual term of the geocenter motion correction results in a – 4.67 ± 3.40 mm error in the Z-component of the orbit frame which creates 1.06 ± 2.66 mm of systematic error in the MSL estimates, mainly due to the uneven distribution of the oceans between the North and South hemisphere. 相似文献
213.
Y.S. Ge C.T. RussellT.-S. Hsu 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008,41(8):1243-1251
In 2001, 2002 and 2003, the Polar spacecraft probed the near equatorial plasma sheet at 9 RE near local midnight. Using the magnetic field observations, the signatures at substorm onsets are studied. Close to the flux pile-up region, the Polar spacecraft readily detects the dipolarization front, especially for pseudo onsets. An event with two distinct onsets has been examined. The signatures are found to be consistent with the multiple-onset model suggested by Russell [Russell, C.T. How northward turnings of the IMF can lead to substorm expansion onsets. Geophys. Res. Lett. 27, 3257–3259, 2000] which is a modified Near-Earth Neutral Line (NENL) model. Another similar event is also examined showing the effects of different Interplanetary Magnetic Field (IMF) conditions upon substorms. Moreover, ground effects can be very weak compared to in situ observations, especially for pseudo onsets, because these signatures appear to be localized and not global. 相似文献
214.
X. Wang J.K. Shi G.J. Wang G.A. Zherebtsov O.M. Pirog 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008,41(4):556-561
Responses of low-latitude ionospheric critical frequency of F2 layer to geomagnetic activities in different seasons and under different levels of solar activity are investigated by analyzing the ionospheric foF2 data from DPS-4 Digisonde in Hainan Observatory during 2002–2005. The results are as follows: (1) the response of foF2 to geomagnetic activity in Hainan shows obvious diurnal variation except for the summer in low solar activity period. Generally, geomagnetic activity will cause foF2 to increase at daytime and decrease at nighttime. The intensity of response of foF2 is stronger at nighttime than that at daytime; (2) seasonal dependence of the response of foF2 to geomagnetic activity is very obvious. The negative ionospheric storm effect is the strongest in summer and the positive ionospheric storm effect is the strongest in winter; (3) the solar cycle has important effect on the response of foF2 to geomagnetic activity in Hainan. In high solar activity period, the diurnal variation of the response of foF2 is very pronounced in each season, and the strong ionospheric response can last several days. In low solar activity period, ionospheric response has very pronounced diurnal variation in winter only; (4) the local time of geomagnetic activities occurring also has important effect on the responses of foF2 in Hainan. Generally, geomagnetic activities occurred at nighttime can cause stronger and longer responses of foF2 in Hainan. 相似文献
215.
Yihua Zheng Anthony T.Y. Lui Mei-Ching Fok Brian J. Anderson Pontus C. Brandt Donald G. Mitchell 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008,41(8):1234-1242
One essential component of magnetosphere and ionosphere coupling is the closure of the ring current through Region 2 field-aligned current (FAC). Using the Comprehensive Ring Current Model (CRCM), which includes magnetosphere and ionosphere coupling by solving the kinetic equation of ring current particles and the closure of the electric currents between the two regions, we have investigated the effects of high latitude potential, ionospheric conductivity, plasma sheet density and different magnetic field models on the development of Region 2 field-aligned currents, and the relationship between R2 FACs and the ring current. It is shown that an increase in high latitude potential, ionospheric conductivity or plasma sheet density generally results in an increase in Region 2 FACs’ intensity, but R2 FACs display different local time and latitudinal distributions for changes in each parameter due to the different mechanisms involved. Our simulation results show that the magnetic field configuration of the inner magnetosphere is also an important factor in the development of Region 2 field-aligned current. More numerical experiments and observational results are needed in further our understanding of the complex relationship of the two current systems. 相似文献
216.
首先提出双向信息不对称问题是制约B2C电子商务发展的关键,通过对信号传递理论的研究成果进行归纳,总结出建立B2C电子商务中信号机制需要符合的条件,并建立起B2C中信号机制的模型,即企业传递信号—消费者甄别的信号机制和消费者传递信号—企业甄别的信号机制。 相似文献
217.
218.
本文通过介绍一种客车顶盖蒙皮、顶骨架低位作业自动焊接方法,综合CO2气体保护焊和单面双点电阻焊,找出影响期焊接质量的原因及解决方法,以充分发挥其焊接变形小、焊接强度高、节能、高效的特点. 相似文献
219.
Ravi Sood Sean Farrell Paul ONeill Ravi Manchanda N.M. Ashok 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2006,38(12):2779-2781
We have analysed 9 years of data from the All Sky Monitor on the Rossi X-ray Timing Explorer for 2S 0114+650 to study the evolution of its spin, binary, and super-orbital periods. The spin history of the neutron star in this system exhibits torque reversals lasting 1 year. The newly discovered super-orbital period has remained stable over the 9-year span, making 2S 0114+650 the fourth known system to exhibit stable super-orbital modulation. We compare its super-orbital period evolution with those of the other three such systems. 相似文献
220.
载人航天器空气环境参数控制非定常仿真分析 总被引:2,自引:0,他引:2
为支持乘员在轨驻留,载人航天器需通过空气环境控制系统将众多设计参数和空气环境参数控制在指标范围内。文章建立了一种载人航天器空气环境非定常控制仿真分析模型,包括舱体模块、航天员模块、舱压控制模块、温湿度控制模块以及CO2净化模块。利用该模型分析了载人航天器空气环境参数随乘员代谢水平的非定常变化趋势,并评估了控制系统的工作性能。结果表明:乘员代谢水平变化对空气环境参数有显著影响,通过调节控制系统运行参数可将各空气参数控制在有效指标范围内。人区温度与O2分压、CO2分压和人区湿度有密切的影响关系,不可孤立地进行分析。为载人航天器空气环境参数控制系统的设计和流程改进提供了依据。 相似文献