Li Evolution and the Open Cluster NGC 6819: A Correlation between Li Depletion and Spindown in Dwarfs More Massive Than the F-Dwarf Li-Dip
Li Evolution and the Open Cluster NGC 6819: A Correlation between Li Depletion and Spindown in Dwarfs More Massive Than the F-Dwarf Li-Dip
复制标题
DOI：
10.3847/1538-3881/ab3fad
复制
发表时间：
2019-09
期刊：
影响因子：
--
通讯作者：
C. Deliyannis;B. Anthony-Twarog;Donald B. Lee-Brown;B. Twarog
中科院分区：
文献类型：
--
作者：
C. Deliyannis;B. Anthony-Twarog;Donald B. Lee-Brown;B. Twarog
利用333颗NGC6819恒星的光谱和Gaia天体测量学绘制了从巨大的分支尖端到Li倾角以下0.5mag的Li演化图。根据神经网络光谱分析，用[Fe/H]=−0.04进行等时比较，得到E(B−V)=0 16(0 14)和(m−M)=12 40(12 2 9)的年龄为2 2 5(2 4)Gyr。尽管起源于李氏指数之外，但只有10%的单个亚巨人/巨人拥有可测量的李氏指数。在Li倾角以上，单星的极限A(Li)为3.2±0.1，但其下限范围与倾角内的范围相当。F-矮星Li倾角剖面与向前演化的Hyade/Praesepe剖面一致。位于高原比Li倾角弱的恒星的Li能级为A(Li)=2.83±0.16；仅从光谱误差的角度来看，色散比预期的要大。对比了NGC7789、NGC2506、NGC3680和NGC6819四个转折星的Li和VROT分布，结果表明，从主层序开始的旋转自旋下降是确定Li倾角边界的关键。对于质量较高的矮星，自旋下降同样与锂的耗尽相关，造成第二次下降，但在较高质量和更长的时间尺度上。与年轻的NGC 7789相比，NGC 6819演化恒星中的Li分布更能代表较老的M67，在M67中，亚巨星和巨星从Li DIP中产生，而年轻的NGC 7789中的Vrot恒星范围很大，可能会在巨星中产生质量范围。
Spectroscopy of 333 NGC 6819 stars and Gaia astrometry are used to map Li evolution from the giant branch tip to 0.5 mag below the Li dip. Isochrone comparison with [Fe/H] = −0.04, based upon neural network spectroscopic analysis, produces an age of 2.25 (2.4) Gyr for E(B − V) = 0.16 (0.14) and (m − M) = 12.40 (12.29). Despite originating outside the Li dip, only 10% of single subgiants/giants have measurable Li. Above the Li dip, the limiting A(Li) for single stars is 3.2 ± 0.1 but the lower range is comparable to that found within the dip. The F-dwarf Li dip profile agrees with the Hyades/Praesepe, evolved forward. The Li level among stars populating the plateau fainter than the Li dip is A(Li) = 2.83 ± 0.16; the dispersion is larger than expected from spectroscopic error alone. Comparison of Li and VROT distributions among turnoff stars in NGC 7789, NGC 2506, NGC 3680, and NGC 6819 indicates that rotational spindown from the main sequence is critical in defining the boundaries of the Li dip. For higher-mass dwarfs, spindown is likewise correlated with Li depletion, creating a second dip, but at higher mass and on a longer timescale. The Li distribution among evolved stars of NGC 6819 is more representative of the older M67, where subgiant and giant stars emerge from within the Li dip, than the younger NGC 7789, where a broad range in VROT among the turnoff stars likely produces a range in mass among the giants.