Study Of Stellar X-ray Emission Activity

X-ray emission is an important indicator of stellar activity.In this thesis,we will introduce stellar activity indicated by X-ray emission using two independent samples.First,we probed stellar X-ray activity over a wide range of stellar parameters,using Chandra and LAMOST data.We measured the X-ray-to-bolometric luminosity ratio(RX=LX/Lbol)for 484 main-sequence stars,and found a bimodal distribution for G and K types.We interpret this bimodality as evidence of two sub-populations with different coronal temperatures.Using the metallicity and velocity information,we find both of the two sub-populations are mostly located in the thin disk.We find no trend of RX with stellar age,although a trough with the lowest range of RX appears at ages around 2 Gyr.We then examined the correlation between RX and RH?(proxy of chromospheric activity):we find that the two quantities are well correlated.Finally,we selected a sample of twelve stars with X-ray flares,and studied the light-curve morphology of the flares.The variety of flare profiles and timescales observed in our sample suggests the contribution of different processes of energy release.Second,we study stellar X-ray activity using XMM-Newton and LAMOST da-ta for different types of stars.By cross matching the 3XMM-DR5 catalog and the LAMOST DR3 catalog,we provide a sample including 1259 stars.We find a bimodal distribution of X-ray to optical flux ratio(fX/fV)for G and K stars.We interpret that this bimodality is due to two sub-populations with different coronal temperatures.Fur-thermore,using the full widths at half maxima calculated from H? and H? lines,we show that these stars in the inactive peaks have smaller rotational velocities.This is consistent with the magnetic dynamo theory that stars with low rotational velocities have low stellar activities.We also examine the correlation between fX/fV and Ha emission line luminosity,and find a tight relation between the coronal and chromo-spheric activity indicators.