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Creators/Authors contains: "Ye, Xianhao"

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  1. ; ; ; ; ; (, The Astrophysical Journal)
    Abstract We construct a sample of nearly 30,000 main-sequence stars with 4500 K < Teff < 5000 K and stellar ages estimated by the chromospheric activity−age relation. This sample is used to determine the age distribution in theR–Zplane of the Galaxy, whereRis the projected Galactocentric distance in the disk midplane andZis the height above the disk midplane. As ∣Z∣ increases, the percentage of old stars becomes larger. It is known that scale-height of Galactic disk increases asRincreases, which is called a flare. A mild flare fromR ∼ 8.0 to 9.0 kpc in stellar age distribution is found. We also find that the velocity dispersion increases with age as confirmed by previous studies. Finally we present spiral-shaped structures inZ–υZphase space in three stellar age bins. The spiral is clearly seen in the age bin of [0, 1] Gyr, which suggests that a vertical perturbation to the disk probably took place within the last ∼1.0 Gyr. 
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  2. ; ; ; ; ; ; ; (, The Astrophysical Journal)
    Abstract We identify member stars of more than 90 open clusters in the LAMOST survey. With the method of Fang et al., the chromospheric activity (CA) indices,for 1091 member stars in 82 open clusters andfor 1118 member stars in 83 open clusters, are calculated. The relations between the average,in each open cluster and its age are investigated in differentTeffand [Fe/H] ranges. We find that CA starts to decrease slowly from logt = 6.70 to logt = 8.50, and then decreases rapidly until logt = 9.53. The trend becomes clearer for cooler stars. The quadratic functions between logR′ and logtwith 4000 K  < Teff < 5500 K are constructed, which can be used to roughly estimate ages of field stars with accuracy about 40% forand 60% for. 
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