Y. C. Liang

The Wolf-Rayet features and mass–metallicity relation of long-duration gamma-ray burst host galaxies

Aims. We gather optical spectra of 8 long-duration GRB host galaxies selected from the archival data of VLT/FORS2. We investigate whether or not Wolf-Rayet (WR) stars can be detected in these GRB host galaxies. We also estimate the physical properties of GRB host galaxies, such as metallicity. Methods. We identify the WR features in these spectra by fitting the WR bumps and WR emission lines in blue and red bumps. We identify the subtypes of the WR stars, estimate the numbers of stars in each subtype, and calculate the WR/O star ratios. The (O/H) abundances of GRB hosts are inferred from both the electron temperature (Te) and the metallicity-sensitive strong-line ratio (R23), for which we break the R23 degeneracy. We compare the environments of long-duration GRB host galaxies with those of other galaxies in terms of their luminosity (stellar mass)‐metallicity relations (L−Z, M∗−Z). Results. We detect WR stars in 5 GRB host galaxies with spectra of relatively high signal-to-noise ratios (S /N). In the comparison of L−Z, M∗−Z relations, we show that GRB hosts have lower metallicities than other samples of comparable luminosity and stellar mass. The presence of WR stars and the observed high WR/O star ratio, together with the low metallicity, support the “core-collapsar” model and imply that we are witnessing the first stage of star formation in the host regions of GRBs.

Oxidation behavior of laser remelted plasma sprayed NiCrAlY and NiCrAlY–Al2O3 coatings

Abstract Two types of plasma sprayed coatings (NiCrAlY and NiCrAlY–Al 2 O 3 ) were remelted by continuous wave CO 2 laser. A homogeneous dense remelted layer without voids, cavities, unmelted particles and microcracks was formed. As a result of isothermal oxidation tests at 1000°C, the weight gains of laser remelted coatings, especially laser remelted NiCrAlY–Al 2 O 3 coatings, were obviously lower than plasma sprayed coatings. The effects of both laser remelting and incorporation of Al 2 O 3 second phase in NiCiAlY matrix on high temperature oxidation resistance of plasma sprayed coatings was discussed.

Stellar Abundance and Galactic Chemical Evolution through LAMOST Spectroscopic Survey

A project of a spectroscopic surveyof Galactic structure and evolutionwith a Large sky Area Multi-Object fiber Spectroscopic Telescope (LAMOST) is presented. The spectro- scopic survey consists of two observational modes for various targets in our Galaxy. One is a major survey of the Milky Way aimed at a systematic study of the stellar abundance and Galactic chemical evolution through low resolution (R = 1000 − 2000) spectroscopy. Another is a follow-up observation with medium resolution (R = 10000) spectrographs aimed at detailedstudies of the selected stars with differentchemicalcomposition,kinematics and dynamics. The universe consists of millions of galaxies, in which there is a special one - the Milky Way - where all we stay. The Milky Way is also the subject most concerned by astronomers since it links our knowledge of stellar evolution and some important problems of formation of the universe. It can be used as an ideal laboratory for the studies of large scale structure of the Universe and gas distribution. We can observe more easily the various objects in the Milky Way than those in other galaxies. The modern study of the Milky Way by the method of star counting was started in late 19th century. Other galaxies were identified to be similar stellar systems as the Milky Way at beginning of 20th century. Considering that the Milky Way is a typical spiral galaxy in which our solar system is located, it is the best sample with which various theoretical models can be checked when we study the formation and evo- lution of the galaxies. The structure and chemical evolution of a galaxy can be represented by the different distribution and motion of the stars with different metallicities. There was no exciting progress in the field of Galactic study before 1980s due to the lack of sufficient observational data. Thanks to the rapid development of high resolution spectrographs and some large scale

Significant evolution of the stellar mass-metallicity relation since z ~ 0.65

We present the stellar mass-metallicity relation for 34 0.4 < z < 1 galaxies selected from CFRS and Marano fields, and compare it to those derived from three local samples of galaxies (NFGS, KISS and SDSS). Our metal abundance estimates account for extinction effects, as estimated from IR/optical ratios and Balmer line ratios. All three comparisons show that the intermediate mass galaxies at z ∼ 0.65 are more metal-deficient by 0.3 dex at a given M K or stellar mass relative to z = 0. We find no evidence that this discrepancy could be related to different methods used to derive mass and metallicity. Assuming a closed box model predicts a gas fraction converted into stars of 20-25% since z ∼ 0.65, if the gas fraction is 10-20% in present-day galaxies with intermediate masses. This result is in excellent agreement with previous findings that most of the decline of the cosmic star formation density is related to the population of intermediate mass galaxies, which is composed of 75% spirals today. We find no evidence for a change of the slope of the M star -Z relation from z ∼ 0.65 to z = 0 within the intermediate mass range (10.5

Automatic determination of stellar atmospheric parameters and construction of stellar spectral templates of the Guoshoujing telescope (LAMOST)

A number of spectroscopic surveys have been carried out or are planned to study the origin of the Milky Way. Their exploitation requires reliable automated methods and softwares to measure the fundamental parameters of the stars. Adopting the ULySS package, we have tested the effect of different resolutions and signal-to-noise ratios (SNR) on the measurement of the stellar atmospheric parameters (effective temperature Teff, surface gravity log g, and metallicity [Fe/H]). We show that ULySS is reliable for determining these parameters with medium-resolution spectra (R ~ 2000). Then, we applied the method to measure the parameters of 771 stars selected in the commissioning database of the Guoshoujing Telescope (LAMOST). The results were compared with the SDSS/SEGUE Stellar Parameter Pipeline (SSPP), and we derived precisions of 167 K, 0.34 dex, and 0.16 dex for Teff, log g and [Fe/H] respectively. Furthermore, 120 of these stars are selected to construct the primary stellar spectral template library (Version 1.0) of LAMOST, and will be deployed as basic ingredients for the LAMOST automated parametrization pipeline.

The direct oxygen abundances of metal-rich galaxies derived from electron temperature

Context. Direct measurement of oxygen abundance for metal-rich galaxies from electron temperature is difficult or impossible since temperature-sensitive auroral lines generally become too weak to be measured. Aims. We aim to derive the electron temperature (Te) in the gas of metal-rich star-forming galaxies, which can be obtained from their ratios of auroral lines [O ii]λλ7320, 7330 to nebular lines [O ii]λ3727, in order to establish a more robust massmetallicity relationship, and compare the Te-based (O/H) abundances with those from empirical strong-line calibrations, such as R23 (=([O ii]λ3727+[O iii]λλ4959, 5007)/Hβ). Methods. We obtained 27 spectra by stacking the spectra of several hundred (even several thousand) star-forming galaxies selected from the SDSS-DR4 in each of the 27 stellar mass bins from log(M∗) ∼ 8.0 to 10.6 (in units of log(M� )). This “stack” method sufficiently improves the signal-to-noise ratio of the auroral lines [O ii]λλ7320, 7330. Using a two-zone model for the temperature structure, we derive the electron temperature t2 in the low ionization region from the [O ii]λλ7320, 7330/[O ii]λ3727 ratio, and then use a relation derived by fitting H ii region models to estimate the electron temperature t3 in the high ionization region from t2. Then, the direct (O/H) abundances are obtained from t2, t3 and the related line-ratios. The emission lines have been carefully corrected for dust extinction using the Balmer line ratio after correcting for the underlying stellar absorption. Results. Combining our results with those from the literature with lower metallicities, we are able to provide a new relationship between the direct measurements of (O/H) and R23, which still shows an upper and a lower branch with the transition around 12 + log(O/H) ∼ 8.0−8.2. It also shows that the empirical R23 method will overestimate log(O/H) by 0.2 to 0.6 dex. The new metal-mass relationship of the galaxies with moderate metallicities is fitted by a linear fit (12 + log(O/H) = 6.223 + 0.231 × log(M∗)) confirming that empirical methods significantly overestimate (O/H). We also derived their (N/O) abundance ratios on the basis of the Te method, which are consistent with the combination of the primary and secondary components of nitrogen. Conclusions. This study provides for the first time a method to calibrate direct O/H abundances (from Te) for a large range of galaxies

Sources of carbon and the evolution of the abundance of CNO elements

Using the standard infall model of Galactic chemical evolution, we explore the origin of carbon and calculate the abundance evolution of CNO elements for 8 dierent models of stellar nucleosynthesis yields. The results show that, in the early stage of the Galaxy, massive stars are the main producer of carbon, and that as our Galaxy evolves to the late stage, the longer lived intermediate- and low-mass stars play an increasingly important role, while at the same time, metal-rich Wolf-Rayet stars eject a signicant amount of carbon into the ISM by radiative-driven stellar winds. However, from the present published nucleosynthesis yields we cannot distinguish whether the main source of carbon in the late Galactic stage is just the massive stars ( M> 8 M) alone, or just the intermediate-, low-mass stars and M 40 M massive stars that do not go through the Wolf-Rayet stage. The 12 C(; ) 16 O reaction rate is very important in the stellar nucleosynthesis calculations: a lower rate will give a higher yield of carbon. The contribution to nitrogen is dominated by intermediate- and low-mass stars, and the secondary source of massive stars cannot explain the observed (N/Fe) in metal-poor stars. Most of oxygen is produced by massive stars. The fact that a higher O abundance in metal-poor stars is derived from the O i 7771 7775 A triplet than from the forbidden (O i) line at 6300 A poses a problem.

Comparing six evolutionary population synthesis models by performing spectral synthesis for galaxies

Aims. We compare six popularly used evolutionary population synthesis (EPS) models by fitting the full optical spectra of six representative types of galaxies (star-forming and composite galaxies, Seyfert 2s, LINERs, E+A, and early-type galaxies) taken from the Sloan Digital Sky Survey (SDSS). We explore the dependence of stellar population synthesis results on the main ingredients of the EPS models and study whether there is an age sequence among these types of galaxies. Methods. We use the simple stellar populations (SSPs) of each EPS model and the software STARLIGHT to perform our fits. Firstly, we explore the dependence of stellar population synthesis on EPS models by fixing the age, metallicity, and initial mass function (IMF) to construct a standard SSP group. We then use the standard SSP group of each EPS model (BC03, CB07, Ma05, GALEV, GRASIL, and Vazdekis/Miles) to fit the spectra of star-forming and E+A galaxies. Secondly, we fix the IMF and alter the age and metallicity to construct eight additional SSP groups. We then use these SSP groups to fit the spectra of star-forming and E+A galaxies to verify the effects of age and metallicity on stellar populations. Finally, we study the effect of stellar evolution tracks and stellar spectral libraries on our results, and present a possible age sequence among these types of galaxies. Results. Using different EPS models, the numerical values of contributing light fractions obviously change, even though the dominant populations are unaltered. The stellar population synthesis does depend on the selection of age and metallicity, but does not depend significantly on the stellar evolution track. The importance of young populations decreases from star-forming, composite, Seyfert 2, LINER, to early-type galaxies, and the properties of E+A galaxies are between composite galaxies and Seyfert 2s in most cases. Conclusions. Different EPS models infer different stellar population parameters, so that it is not reasonable to directly compare stellar populations estimated from different EPS models. To obtain reliable results, we should use the same EPS model to derive the parameter values that we compare.

A large sample of low surface brightness disc galaxies from the SDSS – I. The sample and the stellar populations

We present the properties of a large sample (12 282) of nearly face-on low surface brightness (LSB) disc galaxies selected from the main galaxy sample of SDSS-DR4. These properties include B-band central surface brightness μ 0 (B), scalelengths h, integrated magnitudes, colours and distances D. This sample has μ 0 (B) values from 22 to 24.5 mag arcsec -2 with a median value of 22.42 mag arcsec -2 , and disc scalelengths ranging from 2 to 19 kpc. They are quite bright with M B taking values from -18 to -23 mag with a median value of -20.08 mag. There exist clear correlations between log h and M B , log h and log D, log D and M B . However, no obvious correlations are found between μ 0 (B) and log h, colours, etc. The correlation between colours and log h is weak even though it exists. Both the optical-optical and optical-NIR colour-colour diagrams indicate that most of them have a mixture of young and old stellar populations. They also satisfy colour-magnitude relations, which indicate that brighter galaxies tend generally to be redder. The comparison between the LSBGs and a control sample of nearly face-on disc galaxies with higher surface brightness (HSB) with μ 0 (B) from 18.5 to 22 mag arcsec -2 show that, at a given luminosity or distance, the observed LSB galaxies tend to have larger scalelengths. These trends could be seen gradually by dividing both the LSBGs and HSBGs into two subgroups according to surface brightness. A volume-limited subsample was extracted to check the incompleteness of surface brightness. The only one of the property relations having an obvious change is the relation of log h versus μ 0 (B), which shows a correlation in this subsample.

THE DIFFERENT NATURE OF SEYFERT 2 GALAXIES WITH AND WITHOUT HIDDEN BROAD-LINE REGIONS

We compile a large sample of 120 Seyfert 2 galaxies (Sy2s) which contains 49 hidden broad-line region (HBLR) Sy2s and 71 non-HBLR Sy2s. From the difference in the power sources between two groups, we test whether HBLR Sy2s are dominated by active galactic nuclei (AGNs) and whether non-HBLR Sy2s are dominated by starbursts. We show that (1) HBLR Sy2s have larger accretion rates than non-HBLR Sy2s; (2) HBLR Sy2s have larger [Ne V] λ14.32/[Ne II] λ12.81 and [O IV] λ25.89/[Ne II] λ12.81 line ratios than non-HBLR Sy2s; and (3) HBLR Sy2s have smaller IRAS f 60/f 25 flux ratios, which show the relative strength of the host galaxy and nuclear emission, than non-HBLR Sy2s. Consequently, we suggest that HBLR Sy2s and non-HBLR Sy2s are AGN dominated and starburst dominated, respectively. In addition, non-HBLR Sy2s can be classified into luminous (L [O III]>1041 erg s–1) and less luminous (L [O III] < 1041 erg s–1) samples, when considering only their obscuration. We suggest that (1) the invisibility of polarized broad lines (PBLs) in the luminous non-HBLR Sy2s depends on the obscuration and (2) the invisibility of PBLs in the less luminous non-HBLR Sy2s depends on the very low Eddington ratio rather than the obscuration.