【云台学术报告】2020年1月6日（周一）上午10:30-11:30 主楼二楼228会议室----中国科学院云南天文台

【学术报告】

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报告题目：　　The discovery of a new substellar population shown by theoretical models but ignored by observers

报告人：　　　张曾华博士

报告人单位：南京大学

报告时间：　　 2020年1月6日（周一）上午10:30-11:30

报告地点：　　主楼二楼228会议室

报告语言：　中文

报告摘要：

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The transition zone of unsteady hydrogen fusion is also the substellar transition zone (STZ) that separates very low-mass stars, transitional and degenerate brown dwarfs. Transitional brown dwarfs (T-BD) have unsteady hydrogen fusion in their cores to replenish the dissipation of their initial thermal energy, thus have extremely slow cooling rate. Degenerate brown dwarfs (D-BD) are the majority of brown dwarfs. D-BDs have no energy supply from hydrogen fusion thus cool continuously. The STZ been shown by evolutionary models (Burrows & Liebert 1993) before the discovery of BDs in 1995. However, the STZ was ignored by observers. Because first observers was looking for a clear mass boundary (the hydrogen burning minimum mass, e.g. HBMM) between stars and brown dwarfs (BDs). Secondly, the precision of BD mass measurement is larger or similar to the mass range of the STZ. Thirdly, the temperature range of very low-mass stars, T-BD, and D-BD are overlapped because of mass/age degeneracy of BDs in the field. Therefore, the STZ could only be resolved in the metal poor BD population, which are all very old (10 Gyr) but extremely rare. My work is focused on the discovery and characterisation of metal-poor BDs. In this talk, I will discuss the spectral type-colour correlations, spectral type-absolute magnitude correlations, colour-colour plots, and HR diagrams of L and T subdwarfs, in comparison to these of L and T dwarfs. I will show how metal-poor brown dwarfs lead us to new interpretation of evolutionary models, which help us reach a better understanding of brown dwarf population. This talk is partially based on works published in a series titled 'Primeval very low-mass stars and brown dwarfs' ( https://ui.adsabs.harvard.edu/public-libraries/gVGomDWcQGyKPWw2CGg3dg ).

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12月19日