Numerical simulation of a possible origin of the positive radial metallicity gradient of the thick disk

We analyze the radial and vertical metallicity and [α/Fe] gradients of the disk stars of a disk galaxy simulated in a fully cosmological setting with the chemodynamical galaxy evolution code GCD+. We study how the radial abundance gradients vary as a function of height above the plane and find that the metallicity([α/Fe]) gradient becomes more positive(negative) with increasing height, changing sign around1.5 kpc above the plane. At the largest vertical height(2 |z| 3 kpc), our simulated galaxy shows a positive radial metallicity gradient. We find that the positive metallicity gradient is caused by the age-metallicity and age-velocity dispersion relation, where younger stars have higher metallicity and lower velocity dispersion. Due to the agevelocity dispersion relation, a greater fraction of younger stars reaches |z| 2 kpc at the outer region, because of the lower gravitational restoring force of the disk, i.e.flaring. As a result, the fraction of younger stars with higher metallicity due to the age-metallicity relation becomes higher at the outer radii, which makes the median metallicity higher at the outer radii. Combining this result with the recently observed age-metallicity and age-velocity dispersion relation for the Milky Way thick disk stars suggested by Haywood et al., we argue that the observed(small) positive radial metallicity gradient at large heights of the Milky Way disk stars can be explained by flaring of the younger thick and/or thin disk stars.     

On the metallicity gradients of the Galactic disk as revealed by LSS-GAC red clump stars

Using a sample of over 70 000 red clump(RC) stars with 5%–10% distance accuracy selected from the LAMOST Spectroscopic Survey of the Galactic Anti-center(LSS-GAC), we study the radial and vertical gradients of the Galactic disk(s) mainly in the anti-center direction, covering a significant volume of the disk in the range of projected Galactocentric radius 7 ≤ RGC ≤ 14 kpc and height from the Galactic midplane 0 ≤ |Z | ≤ 3 kpc. Our analysis shows that both the radial and vertical metallicity gradients are negative across much of the volume of the disk that is probed, and they exhibit significant spatial variations. Near the solar circle(7 ≤ RGC ≤ 11.5 kpc), the radial gradient has a moderately steep, negative slope of-0.08 dex kpc-1near the midplane(|Z | 0.1 kpc), and the slope flattens with increasing |Z |. In the outer disk(11.5 RGC ≤ 14 kpc), the radial gradients have an essentially constant, much less steep slope of-0.01 dex kpc-1at all heights above the plane, suggesting that the outer disk may have experienced an evolutionary path different from that of the inner disk. The vertical gradients are found to flatten largely with increasing RGC. However, the vertical gradient of the lower disk(0 ≤ |Z | ≤ 1 kpc)is found to flatten with RGC quicker than that of the upper disk(1 |Z | ≤ 3 kpc).Our results should provide strong constraints on the theory of disk formation and evolution, as well as the underlying physical processes that shape the disk(e.g. gas flows,radial migration, and internal and external perturbations).     

Radial stellar populations of AGN-host dwarf galaxies in SDSS-Ⅳ MaNGA survey

Based on MaNGA integral field unit(IFU) spectroscopy we search 60 AGN candidates,which have stellar masses M_*≤5 × 10~9 M_⊙ and show AGN ionization signatures in the BPT diagram.For these AGN candidates,we derive the spatially resolved stellar population with the stellar population synthesis code STARLIGHT and measure the gradients of the mean stellar age and metallicity.We find that the gradients of mean stellar age(metallicity) of individual AGN-host dwarfs are diverse in 0-0.5 Re,0.5-1 Re and 0-1 Re.However,the overall behavior of the mean stellar age(metallicity) profiles tend to be flat,as the median values of the gradients are close to zero.We further study the overall behavior of the mean stellar age(metallicity) by plotting the co-added radial profiles for the AGN sample and compare with a control sample with similar stellar mass.We find that the median values of light-weighted mean stellar ages of the AGN sample are as old as 2-3 Gyr within 2 Re,which are about 4-7 times older than those of the control sample.Meanwhile,most of the AGN candidates are low-level AGNs,as only eight sources have L_([O)_(Ⅲ])10~(39.5) erg s~(-1).Hence,the AGNs in dwarf galaxies might accelerate the evolution of galaxies by accelerating the consumption of the gas,resulting in an overall quenching of the dwarf galaxies,and the AGNs also become weak due to the lack of gas.The median values of mass-weighted mean stellar age of both samples within 2 Re are similar and as old as about 10 Gyr,indicating that the stellar mass is mainly contributed by old stellar populations.The gradients of co-added mean stellar metallicity for both samples tend to be negative but close to zero,and the similar mean stellar metallicity profiles for both samples indicate that the chemical evolution of the host galaxy is not strongly influenced by the AGN.     

A Study of Binary Stellar Population Synthesis of Elliptical Galaxies

We determined the relative stellar ages and metallicities of about 80 elliptical galaxies in both low and high density environments using the latest binary stellar population (BSP) synthesis model and tested the predictions of a recent hierarchical formation model that adopted the new ACDM cosmology. The stellar ages and metallicities were estimated from two high-quality published spectra line indices, the Hβand [MgFe] indices. The results show that the stellar populations of elliptical galaxies are older than 3.9 Gyr and more metal rich than 0.02. Most of our results are in agreement with the model predictions: (1) elliptical galaxies in denser environment are redder and have older populations than field galaxies; (2) elliptical galaxies with more massive stellar components are redder and have older and more metal rich populations than less massive ones; (3) the most massive galaxies have the oldest and most metal rich stars. However, some of our results differ from the model predictions on the metallicity distributions of low- and high-density elliptical galaxies and the dependence on the distance to the cluster center.     

Estimation of ages and masses via carbon and nitrogen abundances for 556 007 giants from LAMOST

Estimating ages for a large number of giants is of great importance for studying Galactic evolution.In this work,we determine stellar ages and masses for 556 007 giants from LAMOST Data Release 5(DR5)with empirical relations estimated from chemical [C/N] abundance ratios.Our sample reveals the two wellknown sequences in the age-[α/M] relation.The high-α sequence is composed of stars older than 8 Gyr and low-α sequence is composed of stars with age ranging from 0 Gyr to 13.8 Gyr.Our sample also shows a flat age-[M/H] relation up until 12 Gyr.We compare these distributions with Galactic Chemical Evolution models for reference.When looking at the spatial distribution of stars in 2 Gyr age bins,we find that young stars are concentrated towards the Galactic plane and older stars extend to higher height above and below the disk.We find a smooth transition of median Galactic height for different age bins,which suggests a strong age-dependence on Galactic scale height.     

Galactic halo age estimated from LAMOST DR4 and Gaia DR1

The stellar halo is one of the major components in the Milky Way. Research on its age can provide critical constraints on the origin of the stellar halo and further on the formation of our Galaxy.So far, different approaches and samples have been used to estimate the age of the Galactic halo. In our previous paper, we carefully selected 63 field halo turn-off stars within 1 kpc from the literature using a kinematic approach, then estimated the age of the halo. In this following work, we not only update the data from LAMOST DR4 and Gaia DR1, but also try a different method to select a clean halo sample by combining the metallicity and orbital parameters. Then we compare this halo turn-off sample with the GARSTEC model in the B-V vs. metallicity plane. After Monte Carlo simulations are performed, the age is estimated to be 10.5±1.4 Gyr, highly consistent with our previous result and other studies. However,due to the limited common sources between LAMOST DR4 and Gaia DR1, the final sample in this paper is still quite small. The estimated age will be more robust with the much larger Gaia DR2.     

Information from the Kinematics of F and G Stars in the Solar Neighborhood

We have calculated the orbital parameters for 90 stars in Chen et al. and updated the kinematic data for stars in Edvardsson et al. by using the accurate Hipparcos parallaxes and proper motions, and recalculated the \\\\\\\\\\\\-element abundances in Edvardsson et al. in a way consistent with Chen et al. The two sets of data are combined in a study of stellar populations and characteristics of F & G stars in the solar neighborhood. We confirm the result of Chen et al. that a distinguishable group of stars may belong to the thick disk rather than the thin disk. The ages for the stars are determined using the theoretical isochrones of VandenBerg et al. The age-metallicity relation is investigated for different subgroups according to distance from the sun and galactic orbital parameters. It is found that a mixing of stars with different orbital parameters significantly affect the age-metallicity relation for the disk. Stars with orbits confined to the solar circle all have metallicities [Fe/H] > -0.3 irresp     

Kinematics of solar neighborhood stars and its dependence on age and metallicity

We have constructed a catalog containing the best available astrometric,photometric, radial velocity and astrophysical data for mainly F-type and G-type stars(called the Astrometric Catalog associated with Astrophysical Data, ACAD). This contains 27 553 records and is used for the purpose of analyzing stellar kinematics in the solar neighborhood. Using the Lindblad-Oort model and compiled ACAD, we calculated the solar motion and Oort constants in different age–metallicity bins. The evolution of kinematical parameters with stellar age and metallicity was investigated directly. The results show that the component of the solar motion in the direction of Galactic rotation(denoted S2) linearly increases with age, which may be a consequence of the scattering processes, and its value for a dynamical cold disk was found to be 8.0 ± 1.2 km s-1. S2 also linearly increases with metallicity, which indicates that radial migration is correlated to the metallicity gradient. On the other hand, the rotational velocity of the Sun around the Galactic center has no clear correlation with ages or metallicities of stars used in the estimation.     

Variable Stars in the 50BiN Open Cluster Survey. Ⅲ. NGC 884

Open clusters are the basic building blocks that serve as a laboratory for the study of young stellar populations in the Milky Way.Variable stars in open clusters provide a unique way to accurately probe the internal structure,temporal and dynamical evolutionary stages of individual stars and the host cluster.The most powerful tool for such studies is time-domain photometric observations.This paper follows the route of our previous work,concentrating on a photometric search for variable stars in...     

Red Clump Stars from LAMOST Ⅱ: the outer disc of the Milky Way

We present stellar density maps of the Galactic outer disc with red clump stars from LAMOST data.These samples are separated into younger(mean age~2.7 Gyr)and older(mean age~4.6 Gyr)populations so that they can trace the variation of structures with ages in the range of Galactocentric radius R from 9 to 13.5 kpc.We show that scale heights for both of the two populations increase with R and display radial gradients of 48±6 and 40±4 pc kpc~(-1)for the older and younger populations,respectively.It is evident that flaring occurs in the thin disc populations with a wide range of ages.Moreover,the intensity of flaring does not seem to be significantly related to the age of the thin disc populations.On the other hand,scale lengths of the radial surface density profiles are 4.7±0.5 kpc for the younger population and 3.4±0.2 kpc for the older population,meaning that the younger disc population is more radially extended than the older one.Although the fraction of the younger population mildly increases from 28% at R~9 to about 35% at R~13 kpc,the older population is prominent with a fraction of no less than 65%in the outer disc.     

On the Performances of Estimating Stellar Atmospheric Parameters from CSST Broad-band Photometry

Deriving atmospheric parameters of a large sample of stars is of vital importance to understand the formation and evolution of the Milky Way. Photometric surveys, especially those with near-ultraviolet filters, can offer accurate measurements of stellar parameters, with the precision comparable to that from low/medium resolution spectroscopy. In this study, we explore the capability of measuring stellar atmospheric parameters from Chinese Space Station Telescope (CSST) broad-band photometry (par...     

Exploring the spectral information content in the LAMOST medium-resolution survey(MRS)

Although high-resolution stellar spectra allow us to derive precise stellar labels(effective temperature, metallicity, surface gravity, elemental abundances, etc.) based on resolved atomic lines and molecular bands, low-resolution spectra have been proved to be competitive in determining many stellar labels at comparable precision. It is useful to consider the spectral information content when assessing the capability of a stellar spectrum in deriving precise stellar labels. In this work, we quantify the information content brought by the LAMOST-II medium-resolution spectroscopic survey(MRS) using the gradient spectra as well as the coefficients-of-dependence(CODs). In general, the wavelength coverage of the MRS well constrains the stellar labels but the sensitivities of different stellar labels vary with spectral types and metallicity of the stars of interest. Consequently, this affects the performance of the stellar label determination from the MRS spectra. By applying the SLAM method to the synthetic spectra which mimic the MRS data, we find that the precision of the fundamental stellar parameters Teff, log g and [M/H] are better when combining both the blue and red bands of the MRS. This is especially important for warm stars because the Hα line located in the red part plays a more important role in determining the effective temperature for warm stars. With blue and red parts together, we are able to reach similar performance to the low-resolution spectra except for warm stars. However, at [M/H] ~-2.0 dex, the uncertainties of fundamental stellar labels estimated from MRS are substantially larger than that from low-resolution spectra. We also tested the uncertainties of T_(eff), log g and [M/H] from MRS data induced from the radial velocity mismatch and find that a mismatch of about 1 km s~(-1), which is typical for LAMOST MRS data, would not significantly affect the stellar label estimates. Finally, reference precision limits are calculated using synthetic gradient spectra, according to which we expect abundances of at least 17 elements to be measured precisely from MRS spectra.     

Galactic disk bulk motions as revealed by the LSS-GAC DR2

We report a detailed investigation of the bulk motions of the nearby Galactic stellar disk, based on three samples selected from the LSS-GAC DR2: a global sample containing 0.57 million FGK dwarfs out to ～2 kpc, a local subset of the global sample consisting of ～5400 stars within 150 pc, and an anti-center sample containing ～4400AFGK dwarfs and red clump stars within windows a few degrees wide centered on the Galactic Anti-center. The global sample is used to construct a three-dimensional map of bulk motions of the Galactic disk from the solar vicinity out to ～2 kpc with a spatial resolution of ～250 pc. Typical values of the radial and vertical components of bulk motion range from-15 km s-1to 15 km s-1; in contrast, the lag behind the circular motion dominates the azimuthal component by up to ～15 km s-1. The map reveals spatially coherent, kpc-scale stellar flows in the disk, with typical velocities of a few tens of km s-1. Bending- and breathing-mode perturbations are clearly visible,and vary smoothly across the disk plane. Our data also reveal higher-order perturbations, such as breaks and ripples, in the profiles of vertical motion versus height. From the local sample, we find that stars from different populations exhibit very different patterns of bulk motion. Finally, the anti-center sample reveals a number of peaks in stellar number density in the line-of-sight velocity versus distance distribution, with the nearer ones apparently related to the known moving groups. The "velocity bifurcation" reported by Liu et al. at Galactocentric radii 10–11 kpc is confirmed. However,just beyond this distance, our data also reveal a new triple-peaked structure.     

The age of the halo as determined from halo field stars

The age of the Galactic halo is a critical parameter that can constrain the origin of the stellar halo.In general, the Galactic stellar halo is believed to be very old. However, different independent measurements and techniques based on various types of stars are required so that the age estimates of the Galactic halo are accurate, robust, and reliable. In this work, we provide a novel approach to determine the age of the halo with turn-off stars. We first carefully select 63 field halo turn-off stars from the literature. Then, we compare them with the GARSTEC model, which takes the process of atomic diffusion into account in the B- V vs. metallicity plane. Finally, we run Monte Carlo simulations to consider the uncertainty of the color index and obtain the age of 10.5 ± 1.5 Gyr. This result is in agreement with previous studies. Future works are needed to collect more turn-off samples with more accurate photometry to reduce the uncertainty of the age.     

Star formation properties of galaxy cluster A1767

Abell 1767 is a dynamically relaxed, c D cluster of galaxies with a redshift of 0.0703. Among 250 spectroscopically confirmed member galaxies within a projected radius of 2.5r200, 243 galaxies( Digital Sky Survey. Based on this ～97%) are spectroscopically covered by the Sloanhomogeneous spectral sample, the stellar evolutionary synthesis code STARLIGHT is applied to investigate the stellar populations and star formation histories of galaxies in this cluster. The star formation properties of galaxies, such as mean stellar ages, metallicities, stellar masses, and star formation rates, are presented as functions of local galaxy density. A strong environmental effect is found such that massive galaxies in the high-density core region of the cluster tend to have higher metallicities, older mean stellar ages, and lower specific star formation rates(SSFRs), and their recent star formation activities have been remarkably suppressed. In addition, the correlations of the metallicity and SSFR with stellar mass are confirmed.     

Mapping the Milky Way with LAMOST Ⅰ: method and overview

We present a statistical method to derive the stellar density profiles of the Milky Way from spectroscopic survey data,taking into account selection effects.We assume the selection function,which can be altered during observations and data reductions,of the spectroscopic survey is based on photometric colors and magnitude.Then the underlying selection function for a line-of-sight can be recovered well by comparing the distribution of the spectroscopic stars in a color-magnitude plane with that of the photometric dataset.Subsequently,the stellar density profile along a line-of-sight can be derived from the spectroscopically measured stellar density profile multiplied by the selection function.The method is validated using Galaxia mock data with two different selection functions.We demonstrate that the derived stellar density profiles reconstruct the true ones well not only for the full set of targets,but also for sub-populations selected from the full dataset.Finally,the method is applied to map the density profiles for the Galactic disk and halo,using the LAMOST RGB stars.The Galactic disk extends to about R=19 kpc,where the disk still contributes about 10% to the total stellar surface density.Beyond this radius,the disk smoothly transitions to the halo without any truncation,bending or breaking.Moreover,no over-density corresponding to the Monoceros ring is found in the Galactic anti-center direction.The disk shows moderate north–south asymmetry at radii larger than 12 kpc.On the other hand,the R–Z tomographic map directly shows that the stellar halo is substantially oblate within a Galactocentric radius of 20 kpc and gradually becomes nearly spherical beyond 30 kpc.     

Physical properties of γ Doradus pulsating stars and their relationship with long-period δ Scuti variables

We present LAMOST data on 168 γ Doradus(γ Dor) pulsating stars including stellar atmospheric parameters of 137 variables and spectral types for all of the samples. The distributions of period(P), temperature(T), gravitational acceleration(log(g)) and metallicity [Fe/H] are shown. It is found that most γ Dor variables are main-sequence stars with early F spectral types and temperatures from 6880 K to7280 K. They are slightly more metal poor than the Sun with a metallicity range from-0.4 to 0. On the H-R and log g-T diagrams, both the γ Dor and δ Scuti(δ Sct) stars occupy in the same region and some are beyond the borders predicted by current stellar pulsation theories. It is discovered that the physical properties of γ Dor stars are similar to those of long-period δ Sct(P 0.3 d) stars. The stellar atmospheric parameters are all correlated with the pulsation period for short-period δ Sct variables(P 0.3 d), but there are no such relations for γ Dor or long-period δ Sct stars. These results reveal that γ Dor and long-period δ Sct are the same group of pulsating stars and they are different from short-period δ Sct variables. Meanwhile, 33γ Dor stars are identified as candidates of binary or multiple systems.     

Morphology of the ~(13)CO(3–2) millimetre emission across the gas disc surrounding the triple protostar GG Tau A using ALMA observations

Observations by the Atacama Large Millimetre/sub-millimetre Array of the dust continuum and13 CO(3–2) millimetre emissions of the triple stellar system GG Tau A are analysed,giving evidence for a rotating gas disc and a concentric and coplanar dust ring. The present work complements an earlier analysis(Tang et al.) by exploring detailed properties of the gas disc. A 95% confidence level upper limit of 0.24′′(34 au) is placed on the disc scale height at a distance of 1′′(140 au) from the central stars. Evidence for Keplerian rotation of the gas disc is presented,with the rotation velocity reaching~3.1 km s~(-1) at 1′′ from the central stars,and a 99% confidence level upper limit of 9% is placed on relative contribution from a possible in-fall velocity. Variations of the intensity across the disc area are studied in detail and confirm the presence of a hot spot in the south-eastern quadrant. However several other significant intensity variations,in particular a depression in the northern direction,are also revealed. Variations of the intensity are found to be positively correlated to variations of the line width. Possible contributions to the measured line width are reviewed,suggesting an increase of the disc temperature and opacity with decreasing distance from the stars.     

Quasars in the Galactic Anti-Center Area from LAMOST DR3

We present a sample of quasars discovered in an area near the Galactic Anti-Center covering150?≤ l ≤ 210?and |b| ≤ 30?,based on LAMOST Data Release 3(DR3).This sample contains 151 spectroscopically confirmed quasars.Among them 80 are newly discovered with LAMOST.All these quasars are very bright,with i magnitudes peaking around 17.5 mag.All the new quasars were discovered serendipitously from objects that were originally targeted with LAMOST as stars having bluer colors,except for a few candidates targeted as variable,young stellar objects.This bright quasar sample at low Galactic latitudes will help fill the gap in the spatial distribution of known quasars near the Galactic disk that are used to construct an astrometric reference frame for the purpose of accurate proper motion measurements that can be applied to,for example,Gaia.They are also excellent tracers to probe the kinematics and chemistry of the interstellar medium in the Milky Way disk and halo via absorption line spectroscopy.     

Stellar mass estimation based on IRAC photometry for Spitzer SWIRE-field galaxies

We analyze the feasibility of estimating the stellar mass of galaxies by mid-infrared luminosities based on a large sample of galaxies cross-identified from Spitzer SWIRE fields and the SDSS spectrographic survey.We derived the formulae to calculate the stellar mass by using IRAC 3.6 μm and 4.5 μm luminosities.The massto-luminosity ratios of IRAC 3.6 μm and 4.5 μm luminosities are more sensitive to the star formation history of galaxies than to other factors,such as the intrinsic extinction,metallicity and star formation rate.To remove the effect of star formation history,we used g-r color to recalibrate the formulae and obtain a better result.Researchers must be more careful when estimating the stellar mass of low metallicity galaxies using our formulae.Due to the emission from dust heated by the hottest young stars,luminous infrared galaxies present higher IRAC 4.5 μm luminosities compared to IRAC 3.6 μm luminosities.For most of type-Ⅱ AGNs,the nuclear activity cannot enhance 3.6 μm and 4.5 μm luminosities compared with normal galaxies.Star formation in our AGNhosting galaxies is also very weak,almost all of which are early-type galaxies.