植物物候是地表植被对气候变化响应的指示器,研究植物物候时空变化规律对理解其对气候变化的响应机制、预测植被变化具有重要意义.基于2001—2014年MODIS植被物候数据,经过图像拼接和转投影等预处理操作后,分别统计分析了5个自然区的森...植物物候是地表植被对气候变化响应的指示器,研究植物物候时空变化规律对理解其对气候变化的响应机制、预测植被变化具有重要意义.基于2001—2014年MODIS植被物候数据,经过图像拼接和转投影等预处理操作后,分别统计分析了5个自然区的森林、灌木和草原物候的时空变化.结果表明:①在高纬和高海拔地区,我国自然植被物候多年平均生长季节始期(begin of growing season,BGS)较晚,末期(end of growing season,EGS)较早,这一现象很好地反映了地形和气候的地域分异规律;森林、灌木和草原BGS之间的差异在气候温度低的区域大,森林、灌木和草原EGS之间的差异在气候干旱的区域大.②从植被物候的变化趋势来看,灌木的BGS以推迟趋势为主,森林和草原的BGS以提前趋势为主,森林、灌木和草原的EGS均以推迟为主.从平均结果看,我国5个自然区植被BGS呈现提前趋势,植被EGS呈现推迟趋势,但变化速率变缓,仅有16%的像元通过了显著性检验(P<0.1).③植被物候在年际上的波动程度远小于其在空间上的变化.植被BGS时间标准差约1~3 d,植被EGS的时间标准差略大,约为2~6 d.展开更多
The abundances of long-lived radioactive elements Th and U observed in metal-poor halo stars can be used as chronometers to determine the age of individual stars, and hence set a lower limit on the age of the Galaxy a...The abundances of long-lived radioactive elements Th and U observed in metal-poor halo stars can be used as chronometers to determine the age of individual stars, and hence set a lower limit on the age of the Galaxy and hence of the universe. This radioactive dating requires the zero-decay productions of Th and U, which involves complicated r-process nucleosynthesis calculations. Several parametric r-process models have been used to calculate the initial abundance ratios of Th/Eu and U/Th, but, due to the sharp sensitivity of these models to nuclear physics inputs, the calculations have relatively large uncertainties which lead to large uncertainties in the age determinations. In order to reduce these uncertainties, we present a simple method to estimate the initial productions of Th and U, which only depends on the solar system abundances and the stellar abundances of stable r-process elements. From our calculations of the initial abundance ratios of Th/Eu and U/Th, we re-estimate the ages of those very metal-poor halo stars with published abundances of Th and U. Our age estimates are consistent, within the errors, with the other age determinations derived from r-process models, and offer useful constrains for r-process theoretical calculations. The advantages and limitations of our simple method of radioactive dating are discussed.展开更多
We compile a new sample of 89 open clusters with ages, distances and metallicities available. We derive a radial iron gradient of about -0.099±0.008 dex kpc^(-1) (unweighted) for the whole sample, which is somewh...We compile a new sample of 89 open clusters with ages, distances and metallicities available. We derive a radial iron gradient of about -0.099±0.008 dex kpc^(-1) (unweighted) for the whole sample, which is somewhat greater than the most recent determination of oxygen gradient from nebulae and young stars. By dividing the clusters into age groups, we show that the iron gradient was steeper in the past and has evolved slowly in time. Current data show a substantial scatter of the cluster metallicities indicating that the Galactic disk has undergone a very rapid, inhomogeneous enrichment. Also, based on a simple, but quite successful model of chemical evolution of the Milky Way disk, we make a detailed calculation of the iron abundance gradient and its time evolution. The predicted current iron gradient is about -0.072 dex kpc^(-1). The model also predicts a steady flattening of the iron gradient with time, which agrees with the result from our open cluster sample.展开更多
The stellar halo is one of the major components in the Milky Way. Research on its age can provide critical constraints on the origin of the stellar halo and further on the formation of our Galaxy.So far, different app...The stellar halo is one of the major components in the Milky Way. Research on its age can provide critical constraints on the origin of the stellar halo and further on the formation of our Galaxy.So far, different approaches and samples have been used to estimate the age of the Galactic halo. In our previous paper, we carefully selected 63 field halo turn-off stars within 1 kpc from the literature using a kinematic approach, then estimated the age of the halo. In this following work, we not only update the data from LAMOST DR4 and Gaia DR1, but also try a different method to select a clean halo sample by combining the metallicity and orbital parameters. Then we compare this halo turn-off sample with the GARSTEC model in the B-V vs. metallicity plane. After Monte Carlo simulations are performed, the age is estimated to be 10.5±1.4 Gyr, highly consistent with our previous result and other studies. However,due to the limited common sources between LAMOST DR4 and Gaia DR1, the final sample in this paper is still quite small. The estimated age will be more robust with the much larger Gaia DR2.展开更多
文摘植物物候是地表植被对气候变化响应的指示器,研究植物物候时空变化规律对理解其对气候变化的响应机制、预测植被变化具有重要意义.基于2001—2014年MODIS植被物候数据,经过图像拼接和转投影等预处理操作后,分别统计分析了5个自然区的森林、灌木和草原物候的时空变化.结果表明:①在高纬和高海拔地区,我国自然植被物候多年平均生长季节始期(begin of growing season,BGS)较晚,末期(end of growing season,EGS)较早,这一现象很好地反映了地形和气候的地域分异规律;森林、灌木和草原BGS之间的差异在气候温度低的区域大,森林、灌木和草原EGS之间的差异在气候干旱的区域大.②从植被物候的变化趋势来看,灌木的BGS以推迟趋势为主,森林和草原的BGS以提前趋势为主,森林、灌木和草原的EGS均以推迟为主.从平均结果看,我国5个自然区植被BGS呈现提前趋势,植被EGS呈现推迟趋势,但变化速率变缓,仅有16%的像元通过了显著性检验(P<0.1).③植被物候在年际上的波动程度远小于其在空间上的变化.植被BGS时间标准差约1~3 d,植被EGS的时间标准差略大,约为2~6 d.
基金Supported by the National Natural Science Foundation of China.
文摘The abundances of long-lived radioactive elements Th and U observed in metal-poor halo stars can be used as chronometers to determine the age of individual stars, and hence set a lower limit on the age of the Galaxy and hence of the universe. This radioactive dating requires the zero-decay productions of Th and U, which involves complicated r-process nucleosynthesis calculations. Several parametric r-process models have been used to calculate the initial abundance ratios of Th/Eu and U/Th, but, due to the sharp sensitivity of these models to nuclear physics inputs, the calculations have relatively large uncertainties which lead to large uncertainties in the age determinations. In order to reduce these uncertainties, we present a simple method to estimate the initial productions of Th and U, which only depends on the solar system abundances and the stellar abundances of stable r-process elements. From our calculations of the initial abundance ratios of Th/Eu and U/Th, we re-estimate the ages of those very metal-poor halo stars with published abundances of Th and U. Our age estimates are consistent, within the errors, with the other age determinations derived from r-process models, and offer useful constrains for r-process theoretical calculations. The advantages and limitations of our simple method of radioactive dating are discussed.
基金the National Natural Science Foundation ofChina (No. 19873014) and NKBRSFG19990754, and partly by SRF for ROCS, SEM.
文摘We compile a new sample of 89 open clusters with ages, distances and metallicities available. We derive a radial iron gradient of about -0.099±0.008 dex kpc^(-1) (unweighted) for the whole sample, which is somewhat greater than the most recent determination of oxygen gradient from nebulae and young stars. By dividing the clusters into age groups, we show that the iron gradient was steeper in the past and has evolved slowly in time. Current data show a substantial scatter of the cluster metallicities indicating that the Galactic disk has undergone a very rapid, inhomogeneous enrichment. Also, based on a simple, but quite successful model of chemical evolution of the Milky Way disk, we make a detailed calculation of the iron abundance gradient and its time evolution. The predicted current iron gradient is about -0.072 dex kpc^(-1). The model also predicts a steady flattening of the iron gradient with time, which agrees with the result from our open cluster sample.
基金supported by the National Natural Science Foundation of China (Grant Nos. 11473001 and 11078006)support by the National Key Basic Research Program of China (Grant No. 2014CB845700)+2 种基金supported by the China Postdoctoral Science Foundation (Grant No. 2017M610695)supported by Special Funding for Advanced Users,budgeted and administrated by the Center for Astronomical Mega-Science, Chinese Academy of Sciencesa National Major Scientific Project which is built by the Chinese Academy of Sciences,funded by the National Development and Reform Commission, and operated and managed by the National Astronomical Observatories, Chinese Academy of Sciences
文摘The stellar halo is one of the major components in the Milky Way. Research on its age can provide critical constraints on the origin of the stellar halo and further on the formation of our Galaxy.So far, different approaches and samples have been used to estimate the age of the Galactic halo. In our previous paper, we carefully selected 63 field halo turn-off stars within 1 kpc from the literature using a kinematic approach, then estimated the age of the halo. In this following work, we not only update the data from LAMOST DR4 and Gaia DR1, but also try a different method to select a clean halo sample by combining the metallicity and orbital parameters. Then we compare this halo turn-off sample with the GARSTEC model in the B-V vs. metallicity plane. After Monte Carlo simulations are performed, the age is estimated to be 10.5±1.4 Gyr, highly consistent with our previous result and other studies. However,due to the limited common sources between LAMOST DR4 and Gaia DR1, the final sample in this paper is still quite small. The estimated age will be more robust with the much larger Gaia DR2.
