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We are making precise determinations of the abundance of the light isotope of helium, 3He. The 3He abundance in Milky Way sources impacts stellar evolution, chemical evolution, and cosmology. The abundance of 3He is derived from measurements of the hyperfine transition of 3He+ which has a rest wavelength of 3.46 cm (8.665 GHz). As with all the light elements, the present interstellar 3He abundance results from a combination of Big Bang Nucleosynthesis (BBNS) and stellar nucleosynthesis. We are measuring the
3He abundance in Milky Way H ii regions and planetary nebulae (PNe). The source sample is currently comprised of 60 H ii regions and 12 PNe. H ii regions are examples of zero-age objects that are young relative to the age of the Galaxy. Therefore their abundances chronicle
the results of billions of years of Galactic chemical evolution. PNe probe material that has been ejected from low-mass (M≤ 2M
⊙) to intermediate-mass (M∼2–5M
⊙) stars to be further processed by future stellar generations. Because the Milky Way ISM is optically thin at centimeter wavelengths,
our source sample probes a larger volume of the Galactic disk than does any other light element tracer of Galactic chemical
evolution. The sources in our sample possess a wide range of physical properties (including object type, size, temperature,
excitation, etc.). The 3He abundances we derive have led to what has been called “The 3He Problem”. 相似文献
604.
无自由参数型混合格式 总被引:2,自引:2,他引:0
针对紧致-加权本质无振荡混合格式中子格式的权重,设计了新的算子.该算子利用模板的光滑因子求解权重,避免引入自由参数,提高了格式的易用性和鲁棒性.利用新的权重算子,分别将两种五阶迎风紧致格式与WENO-Z(Weighted Essentially Non-Oscillatory Scheme)耦合.新权重算子使子格式之间的切换较为光滑.WENO-Z能够保持格式在极点附近的精度,紧致格式在光滑区域耗散低,混合格式因此能够保持高分辨率、高精度特性以及对间断的捕捉能力.数值试验针对若干双曲型方程算例展开,比较了两种混合格式在采用新权重算子与采用其他典型算子时的数值特性.结果表明无自由参数型权重算子性能良好. 相似文献
605.
转子系统碰摩约束模型与振动响应分析 总被引:2,自引:0,他引:2
针对转静子碰摩的典型力学特征,从碰摩对转子产生附加约束的角度提出了碰摩力学模型,基于该约束模型,分析了转子系统碰摩过程中的共振区间扩展、振幅跃迁与不稳定接触等响应特征与机理.研究了力学特征参数对转子碰摩振动响应的影响,结果表明附加约束刚度提升使得系统共振区间与不稳定接触区间增大,而转静子间摩擦系数升高造成系统共振区间减小与振动响应幅值的降低.建立了具有航空发动机结构特征的碰摩分析模型,仿真结果表明,除碰摩约束造成的共振区间扩展外,具有结构特征的转子碰摩响应还与碰摩位置及模态振型密切相关.碰摩程度较轻时,转子瞬态响应表现出拟周期振动的特点,而碰摩程度严重时,转子运动趋向不稳定,响应特征更接近约束模型. 相似文献
606.
Luciana Bianchi Yongbeom Kang Paul Hodge Julianne Dalcanton Benjamin Williams 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2014
We discuss the relevance of UV data in the detection and characterization of hot massive stars and young stellar populations in galaxies. We show results from recent extensive surveys in M31 and M33 with Hubble Space Telescope (HST) multi-wavelength data including UV filters, which imaged several regions at a linear resolution (projected) of less than half a pc in these galaxies, and from GALEX far-UV and near-UV wide-field, low-resolution imaging of the entire galaxies. Both datasets allow us to study the hierarchical structure of star formation: the youngest stellar groups are the most compact, and are often arranged within broader, sparser structures. The derived recent star-formation rates are rather similar for the two galaxies, when scaled for the respective areas. We show how uncertainties in metallicity and type of selective extinction for the internal reddening may affect the results, and how an appropriate complement of UV filters could reduce such uncertainties, and significantly alleviate some parameter degeneracies. 相似文献
607.
M.S.I. Malik Sajjad Asghar 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2014
Recent developments in integrated power and attitude control systems (IPACSs) for small satellite, has opened a new dimension to more complex and demanding space missions. This paper presents a new inverse free steering approach for integrated power and attitude control systems using variable-speed single gimbal control moment gyroscope. The proposed inverse free steering law computes the VSCMG steering commands (gimbal rates and wheel accelerations) such that error signal (difference in command and output) in feedback loop is driven to zero. H∞ norm optimization approach is employed to synthesize the static matrix elements of steering law for a static state of VSCMG. Later these matrix elements are suitably made dynamic in order for the adaptation. In order to improve the performance of proposed steering law while passing through a singular state of CMG cluster (no torque output), the matrix element of steering law is suitably modified. Therefore, this steering law is capable of escaping internal singularities and using the full momentum capacity of CMG cluster. Finally, two numerical examples for a satellite in a low earth orbit are simulated to test the proposed steering law. 相似文献
608.
压电网络板的振动控制原理与控制效果 总被引:1,自引:0,他引:1
在给出压电网络板机电耦合动力学方程的基础上对其进行求解,将压电网络板的响应表示为子模态压电系统响应的线性组合.通过分析压电网络板在脉冲激励下子模态压电系统的响应行为,揭示压电网络板可以通过两种途径控制自身振动.基于对传递函数的分析进一步得到子模态压电系统的最优电学参数;在此基础上研究了针对某一阶子模态压电系统设计最优电学参数时,各阶子模态压电系统的响应行为以及压电网络板的多模态控制效果.研究结果表明,压电网络板具有多模态控制效果,特别地,当对板的某一阶次的振动控制具有较高要求时,通过对最优电学参数进行设计,电感电阻并联型压电网络板可满足要求. 相似文献
609.
610.
以单晶涡轮叶片发生再结晶的榫头进气窗口为研究对象,基于镍基单晶合金再结晶临界应力模型,通过单晶叶片铸造热应力场仿真计算,建立了最大残余应力与结构参数和温度的关系模型;并在此基础上,以最大铸造残余应力不大于再结晶临界应力、冷气通道面积不变和满足强度为约束条件,求出了不发生再结晶条件下的临界应力与结构设计参数和热处理温度的映射模型。最后通过对实际叶片榫头进气窗口的优化设计和试验对比分析,验证了本文方法的有效性。结果表明,优化后榫头进气窗口最大铸造残余应力下降30%以上,原有的再结晶现象消除。 相似文献