arXiv subjects

J. K. Zhao

Publications and source records attributed to J. K. Zhao.

21 records · Page 2Linked to original sources

arXiv2011

The Chromospheric Activity, Age, Metallicty and Space Motions of 36 Wide Binaries

We present the chromospheric activity (CA) levels, metallicities and full space motions for 41 F, G, K and M dwarf stars in 36 wide binary systems. Thirty-one of the binaries, contain a white dwarf component. In such binaries the total age can be estimated by adding the cooling age of the white dwarf to an estimate of the progenitor's main sequence lifetime. To better understand how CA correlates to stellar age, 14 cluster member stars were also observed. Our observations demonstrate for the first time that in general CA decays with age from 50 Myr to at least 8 Gyr for stars with 1.0 < V-I < 2.4. However, little change occurs in CA level for stars with V-I < 1.0 between 1 Gyr and 5 Gyr, consistent with the results of Pace et al. (2009). Our sample also exhibits a negative correlation between stellar age and metallicity, a positive correlation between stellar age and W space velocity component and the W velocity dispersion increases with age. Finally, the population membership of these wide binaries is examined based upon their U, V, W kinematics, metallicity and CA. We conclude that wide binaries are similar to field and cluster stars in these respects. More importantly, they span a much more continuous range in age and metallicity than is afforded by nearby clusters.

astro-ph.SR↗

arXiv2011

Metallicity calibration for solar type stars based on red spectra

Based on high resolution and high signal-to-noise ratio (S/N) spectra analysis of 90 solar type stars, we have established several new metallicity calibrations in Teff range [5600, 6500] K based on red spectra with the wavelength range of 560-880 nm. The new metallicity calibrations are applied to determine the metallicity of solar analogs selected from SDSS spectra. There is a good consistent result with the adopted value presented in SDSS-DR7 and a small scatter of 0.26 dex for stars with S/N > 50 is obtained. This study provides a new reliable way to derive the metallicity for solar-like stars with low resolution spectra. In particular, our calibrations are useful for finding metal-rich stars, which are missing in SSPP.

astro-ph.IM↗

arXiv2009

The Absolute Magnitudes of Red Horizontal Branch Stars in the ugriz System

Based on photometric data of the central parts of eight globular clusters and one open cluster presented by An and his collaborators, we select red horizontal branch (RHB) stars in the (g-r)0-g0 diagram and make a statistical study of the distributions of their colors and absolute magnitudes in the SDSS ugriz system. Meanwhile, absolute magnitudes in the Johnson VRI system are calculated through the translation formulae between gri and VRI in the literature. The calibrations of absolute magnitude as functions of metallicity and age are established by linear regressions of the data. It is found that metallicity coefficients in these calibrations decrease, while age coefficients increase, from the blue $u$ filter to the red $z$ filter. The calibration of Mi= 0.06[Fe/H]+0.040t+0.03 has the smallest scatter of 0.04 mag, and thus i is the best filter in the $ugriz$ system when RHB stars are used for distance indicators. The comparison of the MI calibration from our data with that from red clump stars indicates that the previous suggestion that the $I$ filter is better than the V filter in distance determination may not be true because of its significant dependence on age.

astro-ph.SR↗