分类： 天文学 >> 天文学 提交时间： 2023-02-19
Yan Lu
A-Li Luo
Li-Li Wang
Li Qin
Rui Wang
Xiang-Lei Chen
Bing Du
Fang Zuo
Wen Hou
Jian-Jun Chen
Yan-Ke Tang
Jin-Shu Han
Yong-Heng Zhao
摘要： To study the quality of stellar spectra of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) and the correctness of the corresponding stellar parameters derived by the LASP (LAMOST Stellar Parameter Pipeline), the outlier analysis method is applied to the archived AFGK stars in the fifth data release (DR5) of LAMOST. The outlier factor is defined in order to sort more than 3 million stellar spectra selected from the DR5 Stellar Parameter catalog. We propose an improved Local Outlier Factor (LOF) method based on Principal Component Analysis and Monte Carlo to enable the computation of the LOF rankings for randomly picked sub-samples that are computed in parallel by multiple computers, and finally to obtain the outlier ranking of each spectrum in the entire dataset. Totally 3,627 most outlier ranked spectra, around one-thousandth of all spectra, are selected and clustered into 10 groups, and the parameter density distribution of them conforms to the parameter distribution of LAMOST DR5, which suggests that in the whole parameter space the probability of bad spectra is uniformly distributed. By cross-matching the 3,627 spectra with APOGEE, we obtain 122 common ones. The published parameters calculated from LASP agree with APOGEE for the 122 spectra although there are bad pixels or bad flux calibrations in them. On the other hand, some outlier spectra show strong nebular contamination warning the corresponding parameters should be carefully used. A catalog and a spectral atlas of all the 3,627 outliers can be found at the link http://paperdata.china-vo.org/LY_paper/dr5Outlier/dr5Outlier_resource.zip.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Li Qin
A-Li Luo
Wen Hou
Yin-Bi Li
Kai-Ming Cui
Fang Zuo
Shuo Zhang
Rui Wang
Jin-Shu Han
Li-Li Wang
Yan Lu
Xiang-Lei Chen
摘要： Rotation is a critical physical process operating in the formation of Am stars. There is a strong correlation between low-velocity rotation and chemical peculiarity. However, the existence of many non-CP slow rotators challenges the understanding of Am stars. The purpose of our work is to search for low-velocity rotating non-CP A-type stars and Am stars and to make a comparative analysis. In this paper, we pick out a sample from the LAMOST-Kepler project, including 21 Am stars, 125 non-CP slow rotators, and 53 non-CP fast rotators. We calculate the rotational frequencies through periodic change of light curves caused by inhomogeneous stellar surfaces and then obtain the rotational velocities. For slow rotators, the age of Am stars is statistically younger than that of non-CP stars in the same temperature bin. In the comparison of the period, the average amplitude, and stellar mass of Am and non-CP stars, we discover that there is no difference in the photometric variability between Am and non-CP stars, which implies similar inhomogeneities on the surfaces. The average amplitude of non-CP stars has a downward trend with the increase of effective temperature and stellar mass, which is consistent with the theoretical prediction caused by weak dynamo-generated magnetic fields in A-type stars. In addition, we confirm four non-non-CP stars which have flares by checking FOV images, 24 pixel images, and pixel-level light curves.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Li-Li Wang
Shi-Yin Shen
A-Li Luo
Guang-Jun Yang
Ning Gai
Yan-Ke Tang
Meng-Xin Wang
Li Qin
Jin-Shu Han
Li-Xia Rong
摘要： We firstly derive the stellar population properties: age and metallicity for $\sim$ 43,000 low redshift galaxies in the seventh data release (DR7) of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) survey, which have no spectroscopic observations in the Sloan Digital Sky Survey(SDSS). We employ a fitting procedure based on the small-scale features of galaxy spectra so as to avoid possible biases from the uncertain flux calibration of the LAMOST spectroscopy. We show that our algorithm can successfully recover the average age and metallicity of the stellar populations of galaxies down to signal-to-noise$\geq$5 through testing on both mock galaxies and real galaxies comprising LAMOST and their SDSS counterparts. We provide a catalogue of the age and metallicity for $\sim$ 43,000 LAMOST galaxies online. As a demonstration of the scientific application of this catalogue, we present the Holmberg effect on both age and metallicity of a sample of galaxies in galaxy pairs.
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