分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： One of the fundamental issues in cosmic ray physics is to explain the nature of cosmic ray acceleration and propagation mechanisms. Thanks to the precise cosmic ray data measured by recent space experiments, we are able to investigate the cosmic ray acceleration and propagation models more comprehensively and reliably. In this paper, we combine the secondary-to-primary ratios and the primary spectra measured by PAMELA, AMS02, ACE-CRIS and Voyager-1 to constrain the cosmic ray source and transport parameters. The study shows that the $Z>2$ data yield a medium-energy diffusion slope $\delta_{2}\sim\left(0.42, 0.48\right)$ and a high-energy slope $\delta_{3}\sim\left(0.22, 0.34\right)$. The $Z\leq2$ species obtain a looser constraint on $\delta_{2}\sim\left(0.38, 0.47\right)$, but a tighter constraint on $\delta_{3}\sim\left(0.21, 0.30\right)$. The overlaps infer that the heavy and light particles can give compatible results at medium to high energies. Besides, both the light and heavy nuclei indicate a consistent diffusion slope variation $\Delta\delta_{H}$ around $200\sim300$~GV. At low energies, significant disagreements exist between the heavy and light elements. The B/C ratio requires a much larger diffusion slope shift $\Delta\delta_{L}$ around 4 GV or a stronger \textit{Alfv$\acute{e}$n velocity} $v_{A}$ than the low-mass data. This indicates that the heavy and light particles may suffer different low-energy transport behaviors in Galaxy. However, better understanding on the consistency/inconsistency between the heavy and light cosmic rays relies on more precise cross-sections, better constraints on correlations in systematic errors of data, more accurate estimation on Galaxy halo size and more robust description for Solar modulation during the reversal period of HMF.
分类： 生物学 >> 动物学 提交时间： 2017-10-11 合作期刊: 《动物营养学报》
摘要： 近年来由于抗生素在临床上的滥用，使得抗菌药物的抗菌效果大大降低，抗生素滥用还会导致有害菌定殖增加，影响人和动物的健康。因此，很多国家开始禁止用抗生素作为饲料添加剂，且在饲料中添加低剂量的抗生素作为肠道微生态调节剂已不再受欢迎，而探寻新的抗生素替代品来调节畜禽肠道微生物区系和维持肠道健康显得非常重要。人们一直误认为饲粮纤维是一种抗营养因子，指出纤维能稀释饲粮能量水平，影响家禽的生产性能。但最新研究表明，在家禽饲粮中添加适量纤维是改善家禽肠道健康、减少家禽肠道紊乱的一种有效的营养调控措施。本文从家禽肠道发育、肠道黏膜形态及上皮组织健康、肠道消化生理、肠道微生物区系等方面综述饲粮纤维对家禽肠道健康调控的影响，为进一步研究纤维对家禽肠道健康调控机制提供参考，也为今后在家禽饲粮中添加纤维性物质提供一定的科学依据。
分类： 天文学 >> 天文学 提交时间： 2024-03-29 合作期刊: 《Research in Astronomy and Astrophysics》
摘要： How galaxies assemble masses through their own star formation or interaction with the external environment is still an important topic in the field of galaxy formation and evolution. We use Value Added Catalogs with galaxy features that are spatially and temporally resolved from Sloan Digital Sky Survey Data Release 17 to investigate the mass growth histories of early-type galaxies (ETGs) and late-type galaxies (LTGs). We find that the mass growth of ETGs is earlier than that of LTGs for massive galaxies (M* > 1010M⊙), while low-mass (M* ≤ 1010M⊙) ETGs have statistically similar mass assembly histories as low-mass LTGs. The stellar metallicity of all massive galaxies shows a negative gradient and basically does not change with time. However, in low-mass galaxies, the stellar metallicity gradient of elliptical galaxies is negative, and the stellar metallicity gradient of lenticular and spiral galaxies evolves from positive to negative. ETGs are not all in a high-density environment, but exhibit mass dependence. As the tidal strength increases, the star formation rate of low-mass ETGs rapidly decreases. These results support a picture where massive galaxies exhibit inside-out quenching mode, while low-mass galaxies show outside-in quenching mode. Environmental effects play an important role in regulating the mass assembly histories of low-mass ETGs.
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