Search

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., 29 pages, 14 figures, accepted by Astronomy and Computing

The Jurkevich method is a useful method to explore periodicity in the unevenly sampled observational data. In this work, we adopted the method to the light curve of Cyg X-1 from 1996 to 2012, and found that there is an interesting period of 370 days, which appears in both low/hard and high/soft states. That period may be correlated with black hole physics and accretion disk geometry.

We test the possible evolutionary tracks of stars with various masses (1.8 M� , 1.9 M� , 2.0 M� , 2.1 M� , 2.2 M� ) and metallicities Z (0.008, 0.010, 0.012), including both models with and without convective core overshooting. At a given mass and metallicity, the models with a larger overshoot predict a larger radius and age of the star. Based on the observedfrequencyof oscillations and the position of � Oph on the H-R diagram, we obtain two distinct better-fitting models: the solutions with mass M =1 .9 Mfavor a radius in the range 10.55 ± 0.03 Rwith an age of 1.01 ± 0.08Gyr; the solutions with mass M =2 .0 Mfavor a radius in the range 10.74 ± 0.03 Rwith an age of 0.95 ± 0.11Gyr. Furthermore, we investigate the in- fluence of overshooting on the internal structure and the pulsation properties, and find that increasing the convective core overshoot significantly decreases non-radial mode inertia, while also increasing the mode amplitude. Therefore, the estimation of stellar mass and age might be modified by convective core penetration.

Solar-like oscillations have recently been observed in the red giant ɛ Ophiuchi (G9.5III). The large frequency separation is found to be 4.8 μHz, non-radial oscillation mode has been shown to exist. Based on the observed frequency of oscillations and the position of ɛ Ophiuchi on the Hertzsprung-Russell diagram and by adopting a method of combined calculation of stellar evolution and oscillations, some preliminary constraints on the mass, metal abundance, age and radius of this star have been obtained.

Taking into account the helium and metal diffusion, we explore the possible evolutionary status and perform seismic analysis of MOST target: the star $\epsilon$ Eridani. We adopt the different input parameters to construct the models by fitting the available observational constraints: e.g., $T_{eff}$, $L$, $R$, $[Fe/H]$. From computation, we obtain the average large spacings of $\epsilon$ Eridani about $194\pm 1 \mu$Hz. The age of the diffused models has been found to be about 1 Gyr, which is younger than one determined previously by models without diffusion. We found that the effect of pure helium diffusion on the internal structure of the young low-mass star is slight, but the metal diffusion influence is obvious. The metal diffusion leads the models to have much higher temperature in the radiation interior, correspondingly the higher sound speed in the interior of the model, thereby the larger frequency and spacings.

Taking into consideration the effects of rotation and interior magnetic field during the lifetime of the star, we reconstruct the model of a Cen B to satisfy the latest non-asteroseismic and asteroseismic observational constraints. We find that the effects can induce a change of about 0.3 mu Hz in the large frequency spacings and can speed up the star's evolution. The model of a Cen B has thereby been improved.

Reconstructing the evolution history of the dark energy equation of state parameter $w(z)$ directly from observational data is highly valuable in cosmology, since it contains substantial clues in understanding the nature of the accelerated expansion of the Universe. Many works have focused on reconstructing $w(z)$ using Type Ia supernova data, however, only a few studies pay attention to Hubble parameter data. In the present work, we explore the merit of Hubble parameter data and make an attempt to reconstruct $w(z)$ from them through the principle component analysis approach. We find that current Hubble parameter data perform well in reconstructing $w(z)$; though, when compared to supernova data, the data are scant and their quality is worse. Both $\Lambda$CDM and evolving $w(z)$ models can be constrained within $10\%$ at redshifts $z \lesssim 1.5$ and even $5\%$ at redshifts 0.1 $\lesssim$ z $\lesssim$ 1 by using simulated $H(z)$ data of observational quality., Comment: 25 pages, 11 figures

The Large Sky-Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) Data Release 3 provided 341 691 K giant stars with stellar parameters. Based on the models of Law & Majewski, we identified 252 K giant stars in the leading stream associated with the Sagittarius (Sgr) dwarf galaxy. We obtained 132 K giant stars belonging to the trailing arm of Sgr using the model of Belokurov et al. We studied the metallicity distribution of member stars along the streams and found a flat gradient in the first wrap of the leading stream, –(0.88±0.3) × 10 −3 dex/( ° ) in the second wrap of the leading stream and –(1.2±0.3) × 10 −3 dex/( ° ) in the first wrap of the trailing stream. Moreover, we obtained a combined metallicity gradient with our sample and data from the literature. We also analyzed the properties of an overdensity, which is located in the leading stream of the Sgr.

The basic intent of this paper is to model 70 Ophiuchi A using the latest asteroseismic observations as complementary constraints and to determine the fundamental parameters of the star. Additionally, we propose a new quantity to lift the degeneracy between the initial chemical composition and stellar age. Using the Yale stellar evolution code (YREC7), we construct a series of stellar evolutionary tracks for the mass range $M$ = 0.85 -- 0.93 $M_{\odot}$ with different composition $Y_{i}$ (0.26 -- 0.30) and $Z_{i}$ (0.017 -- 0.023). Along these tracks, we select a grid of stellar model candidates that fall within the error box in the HR diagram to calculate the theoretical frequencies, the large- and small- frequency separations using the Guenther's stellar pulsation code. Following the asymptotic formula of stellar $p$-modes, we define a quantity $r_{01}$ which is correlated with stellar age. Also, we test it by theoretical adiabatic frequencies of many models. Many detailed models of 70 Ophiuchi A have been listed in Table 3. By combining all non-asteroseismic observations available for 70 Ophiuchi A with these seismological data, we think that Model 60, Model 125 and Model 126, listed in Table 3, are the optimum models presently. Meanwhile, we predict that the radius of this star is about 0.860 -- 0.865 $R_{\odot}$ and the age is about 6.8 -- 7.0 Gyr with mass 0.89 -- 0.90 $M_{\odot}$. Additionally, we prove that the new quantity $r_{01}$ can be a useful indicator of stellar age., 23 pages, 5 figures, accepted by New Astronomy