Jinliang Hou

On the Galactic Disk Metallicity Distribution from Open Clusters. I. New Catalogs and Abundance Gradient

We have compiled two new open cluster catalogs. In the first one, there are 119 objects with ages, distances, and metallicities available, while in the second one, 144 objects have both absolute proper motion and radial velocity data, of which 45 clusters also have metallicity data available. Taking advantage of the large number of objects included in our sample, we present an iron radial gradient of about -0.063 ± 0.008 dex kpc-1 from the first sample, which is quite consistent with the most recent determination of the oxygen gradient from nebulae and young stars, about -0.07 dex kpc-1. By dividing clusters into age groups, we show that the iron gradient was steeper in the past, which is consistent with the recent result from Galactic planetary nebulae data, and also consistent with inside-out galactic disk formation scenarios. Based on the cluster sample, we also discuss the metallicity distribution, cluster kinematics, and space distribution. A disk age-metallicity relation could be implied by those properties, although we cannot give conclusive result from the age- metallicity diagram based on the current sample. More observations are needed for metal-poor clusters. From the second catalog, we have calculated the velocity components in cylindrical coordinates with respect to the Galactic standard of rest for 144 open clusters. The velocity dispersions of the older clusters are larger than those of young clusters, but they are all much smaller than that of the Galactic thick disk stars.

Milky Way vs Andromeda: a tale of two disks

Aims. We study the chemical evolution of the disks of the Milky Way (MW) and of Andromeda (M 31), to identify the common properties and differences between the two major galaxies of the Local Group. Methods. We use a large set of observational data for M 31, including observations of the star formation rate (SFR) and gas profiles, as well as stellar metallicity distributions along its disk. When expressed in terms of the corresponding disk scale lengths, we show that the observed radial profiles of MW and M 31 exhibit interesting similarities, suggesting the possibility of a description within a common framework. Results. We find that the profiles of stars, gas fraction, and metallicity of the two galaxies, as well as most of their global properties, are well described by our model, provided that the star formation efficiency in M 31 disk is twice as high as in the MW. We show that the star formation rate profile of M 31 cannot be described by any form of the Kennicutt-Schmidt law (KS Law) for star formation. We propose that these discrepancies are caused by the fact that M 31 has an active star formation history in the recent past, consistent with the hypotheses of a “head-on” collision with the neighboring galaxy (most probably M 32) about 200 Myr ago. Conclusions. The MW has most probably experienced quiescent secular evolution, making possible a fairly successful description with a simple model. If M 31 is more typical of spiral galaxies, more complex models, involving galaxy interactions, will be required for the description of spirals.

Origin and Evolution of the Abundance Gradient Along the Milky Way Disk

Based on a simple model of the chemical evolution of the Milky Way disk, we investigate the disk oxygen abundance gradient and its time evolution. Two star formation rates (SFRs) are considered, one is the classical Kennicutt-Schmidt law (Psi = 0.25 Sigma(1.4)(gas), hereafter C-KS law), another is the modified Kennicutt law (Psi = alpha Sigma(1.4)(gas) (V/r), hereafter M-KS law). In both cases, the model can produce some amount of abundance gradient, and the gradient is steeper in the early epoch of disk evolution. However, we find that when C-KS law is adopted, the classical chemical evolution model, which assumes a radially dependent infall timescale, cannot produce a sufficiently steep present-day abundance gradient. This problem disappears if we introduce a disk formation timescale, which means that at early times, infalling gas cools down onto the inner disk only, while the outer disk forms later. This kind of model, however, will predict a very steep gradient in the past. When the M-KS law is adopted, the model can properly predict both the current abundance gradient and its time evolution, matching recent observations from planetary nebulae and open clusters along the Milky Way disk. Our best model also predicts that outer disk ( artificially defined as the disk with R(g) >= 8 kpc) has a steeper gradient than the inner disk. The observed outer disk gradients from Cepheids, open clusters, and young stars show quite controversial results. There are also some hints from Cepheids that the outer disk abundance gradient may have a bimodal distribution. More data is needed in order to clarify the outer disk gradient problem. Our model calculations show that for an individual Milky Way-type galaxy, a better description of the local star formation is the M-KS law.

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.

First measurement of Mg isotope abundances at high redshifts and accurate estimate of Δα/α

Aims. Abundances of the Mg isotopes 24 Mg, 25 Mg, and 26 Mg can be used to test models of chemical enrichment of interstellar/intergalactic gas clouds. Additionally, because the position of the Mg ii λλ2796, 2803 A lines is often taken as a reference in computations of possible changes of the fine-structure constant α, it should be clarified to which extent these lines are affected by isotopic shifts. Methods. We use a high-resolution spectrum (pixel size ≈1.3 km s −1 ) of the quasar HE0001–2340 observed with the UVES/VLT to measure Mg isotope abundances in the intervening absorption-line systems at high redshifts. Line profiles are prepared taking into account possible shifts between the individual exposures. In the line-fitting procedure, the lines of each ion are treated independently. Because of the unique composition of the selected systems – the presence of several transitions of the same ion – we can test the local accuracy of the wavelength scale calibration, which is the main source of errors in the sub-pixel line position measurements. Results. In the system at zabs = 0.45, which is probably a fragment of the outflow caused by SN Ia explosion of highmetallicity white dwarf(s), we measured velocity shifts of Mg ii and Mg i lines with respect to other lines (Fe i ,F eii ,C ai ,C aii): ΔVMg II = −0.44 ± 0.05 km s −1 ,a ndΔVMg I = −0.17 ± 0.17 km s −1 . This translates into the isotopic ratio 24 Mg: 25 Mg: 26 Mg = (19 ± 11):(22 ± 13):(59 ± 6) with a strong relative overabundance of heavy Mg isotopes, ( 25 Mg+ 26 Mg)/ 24 Mg = 4, as compared to the solar ratio 24 Mg: 25 Mg: 26 Mg = 79:10:11, and ( 25 Mg+ 26 Mg)/ 24 Mg = 0.3. In the systems at zabs = 1.58 and zabs = 1.65 enriched by AGB-stars we find only upper limits on the content of heavy Mg isotopes ( 25 Mg+ 26 Mg)/ 24 Mg < 0. 7a nd ( 25 Mg+ 26 Mg)/ 24 Mg < 2.6, respectively. At zabs = 1.58, we also put a strong constraint on a putative variation of α: Δα/α = (−1.5 ± 2.6) × 10 −6 , which is one of the most stringent limits obtained from optical spectra of QSOs. We reveal that the wavelength calibration in the range above 7500 A is subject to systematic wavelength-dependent drifts.

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

Chemical evolution and depletion pattern in Damped Lyman alpha systems

Dust depletion plays a key role in understanding the nature of Damped Lyman or systems (DLAs). In this paper we point out a previously unnoticed anticorrelation between the observed abundance ratio [X/Zn] (where Zn is assumed to be undepleted and X stands for the refractories Fe, Cr and Ni) and metal column density ([Zn/H] + log(N-HI)) in DLAs. We suggest that this trend is an unambiguous sign of dust depletion, since metal column density is a measure of the amount of dust along the line of sight. Assuming that DLAs are (proto-) galactic disks and using detailed chemical evolution models with metallicity dependent yields we study chemical evolution and dust depletion patterns for ct and iron-peak elements in DLAs. When observational constraints on the metal column density of DLAs are taken into account las suggested in Boisse et al. 1998) we find that our models reproduce fairly well the observed mild redshift evolution of the abundances of 8 elements (Al, Si, S, Cr, Mn, Fe, Zn and Ni) as well as the observed scatter at a given redshift. By considering the aforementioned dependence of abundance ratios on metal column density, we further explore the general dust depletion pattern in DLAs, comparing to our model results and to a solar reference pattern. We find that for low metal column densities (no depletion), our models compare fairly well to the data, while a solar pattern has difficulties with Mn. At high metal column densities (amount of depletion similar to0.5 dex), the solar pattern describes the data quite well, while our models have difficulties with S. We suggest that further measurements of those key elements, i.e. Zn, S and Mn, will help us to gain more insight into the nature of DLAs. The presently uncertain nucleosynthesis of Zn in massive stars Ion which a large part of these conclusions is based) should be carefully scrutinised.

An improved model for the dynamical evolution of dark matter subhaloes

Using an analytical model, we study the evolution of subhalo, including its mass, angular momentum and merging time-scale. This model considers the dominant processes governing subhalo evolution, such as dynamical friction, tidal stripping and tidal heating. We find that in order to best match the evolution of angular momentum measured from N-body simulation, mass stripping by tidal force should become inefficient after subhalo has experienced a few passages of pericentre. It is also found that the often used Coulomb logarithm ln M/m has to be revised to best fit the merging time-scales from simulation. Combining the analytical model with the extended Press-Schechter (EPS) based merger trees, we study the subhalo mass function, and their spatial distribution in a Milky Way (MW) type halo. By tuning the tidal stripping efficiency, we can gain a better match to the subhalo mass function from simulation. The predicted distribution of subhaloes is found to agree with the distribution of MW satellites, but is more concentrated than the simulation results. The radial distribution of subhaloes depends weakly on subhaloes mass at both present day and the time of accretion, but strongly on the accretion time. Using the improved model, we measure the second moment of the subhalo occupation distribution, and it agrees well with the results of Kravtsov et al. and Zheng et al.

THE NEAREST HIGH-VELOCITY STARS REVEALED BY LAMOST DATA RELEASE 1

We report the discovery of 28 candidate high-velocity stars (HVSs) at heliocentric distances of less than 3 kpc, based on the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) Data Release 1. Our sample of HVS candidates covers a much broader color range than the equivalent ranges discussed in previous studies and comprises the first and largest sample of HVSs in the immediate solar neighborhood, at heliocentric distances less than 1-3 kpc. The observed as well as the derived parameters for all candidates are sufficiently accurate to allow us to ascertain their nature as genuine HVSs, of which a subset of 12 objects represents the most promising candidates. Our results also highlight the great potential of discovering statistically large numbers of HVSs of different spectral types in LAMOST survey data. This will ultimately enable us to achieve a better understanding of the nature of Galactic HVSs and their ejection mechanisms, and to constrain the structure of the Galaxy.

Colour–magnitude relations of late-type galaxies

We use a large sample of galaxies drawn from the Sloan Digital Sky Survey (SDSS) and Two-Micron All-Sky Survey (2MASS) to present colour-magnitude relations (CMRs) for late-type galaxies in both optical and optical-infrared (optical-IR) bands. A sample from SDSS Data Release 4 (DR4) is selected to investigate the optical properties. Optical-IR colours are estimated from a position matched sample of DR4 and the 2MASS, in which the photometric aperture mismatch between these two surveys is carefully corrected. It is shown that, after correcting the dust attenuation, the optical colours for faint galaxies (i.e.M-r > -21) have a very weak correlation with the luminosity, whereas the optical colours for bright galaxies (i.e. M-r < -21) are redder than those for more-luminous galaxies. All (optical, optical-IR and IR) colours show similar but stronger correlations with stellar mass than with absolute magnitude. The optical colours correlate more strongly with stellar mass surface density than with stellar mass, whereas optical-IR and IR colours show stronger correlations with stellar mass. By comparing the observed colours of our sample galaxies with the colours predicted by stellar population synthesis model, we find that massive late-type galaxies have older and higher-metallicity stellar population than that of less-massive galaxies. This suggests that CMRs for late-type galaxies are trends defined by the combination of stellar mean age and metallicity. Moreover, our results suggest that the stellar mean metallicity of late-type galaxy is mainly determined by its stellar mass, whereas the star formation history is mainly regulated by the stellar mass surface density.