Kinematics of the Milky Way Thick disk in solar neighborhood

Using GAIA EDR3 catalog, we present the detailed analysis of the two-component Milky Way stellar disk in the solar neighborhood. To determine the kinematical properties of the thin and of the Thick disks, we select the complete sample of about 278,000 evolved red giant branch (RGB) stars distributed in the cylinder of 1 kpc radius and 0.5 kpc height centered at the Sun. We measured the following mean velocities and dispersions for the thin and the Thick disks, respectively: [Formula: see text][Formula: see text]km s[Formula: see text] with [Formula: see text][Formula: see text]km s[Formula: see text], and [Formula: see text][Formula: see text]km s[Formula: see text] with [Formula: see text][Formula: see text]km s[Formula: see text]. Errors in mean velocities and dispersions are all less than 1[Formula: see text]km s[Formula: see text]. Same values were computed on much smaller subsamples of our Gaia data with RAVE DR5 [Fe/H] values, from which a metallicity selection was added. Results are basically the same. We find that up to 500 pc height above/below the galactic plane, Thick disk stars comprise about half the stars of the disk. We also find evidence of a substructure in [Formula: see text] versus [Formula: see text] in the thick disk population mostly that would give support to the accretion scenario for the formation of the thick disk.

Binary Fractions of G and K Dwarf Stars Based on Gaia EDR3 and LAMOST DR5: Impacts of the Chemical Abundances

Based on the large volume Gaia Early Data Release 3 and LAMOST Data Release 5 data, we estimate the bias-corrected binary fractions of the field late G and early K dwarfs. A stellar locus outlier method is used in this work, which works well for binaries of various periods and inclination angles with single-epoch data. With a well-selected, distance-limited sample of about 90,000 GK dwarfs covering wide stellar chemical abundances, it enables us to explore the binary fraction variations with different stellar populations. The average binary fraction is 0.42 ± 0.01 for the whole sample. Thin-disk stars are found to have a binary fraction of 0.39 ± 0.02, thick-disk stars have a higher one of 0.49 ± 0.02, while inner halo stars possibly have the highest binary fraction. For both the thin- and thick-disk stars, the binary fractions decrease toward higher [Fe/H], [α/H], and [M/H] abundances. However, the suppressing impacts of [Fe/H], [α/H], and [M/H] are more significant for the thin-disk stars than those for the thick-disk stars. For a given [Fe/H], a positive correlation between [α/Fe] and the binary fraction is found for the thin-disk stars. However, this tendency disappears for the thick-disk stars. We suspect that it is likely related to the different formation histories of the thin and thick disks. Our results provide new clues for theoretical works on binary formation.

The Flattening Metallicity Gradient in the Milky Way’s Thin Disk

We calculate the ages, orbits and phase-space coordinates for a sample of ∼4 million LAMOST and Gaia stars. The ages are cross-matched and compared with values from two other popular age catalogs, which derive the ages using different methods. Of these ∼4 million stars, we select a sample of 1.3 million stars and investigate their radial metallicity gradients (as determined by orbital radii) as a function of their ages. This analysis is performed on various subsets of the data split by chemistry and orbital parameters. We find that commonly used selections for “thin disk” stars (such as low-α chemistry or vertically thin orbits) yield radial metallicity gradients that generally grow shallower for the oldest stars. We interpret this as a hallmark feature of radial migration (churning). Constraining our sample to very small orbital Z max (the maximal height of a star’s integrated orbit) makes this trend most pronounced. A chemistry-based “thin disk” selection of α-poor stars displays the same trend, but to a lesser extent. Intriguingly, we find that “thick disk” selections in chemistry and Z max reveal slightly positive radial metallicity gradients, which seem similar in magnitude at all ages. This may imply that the thick disk population is well mixed in age, but not in radius. This finding could help constrain conditions during the early epochs of Milky Way formation and shed light on processes such as the accretion and reaccretion of gases of different metallicities.

Search for the metal-weak thick disk from the LAMOST DR5

Based on the data release of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope survey (LAMOST DR5) and the Gaia Early Data Release 3 (Gaia EDR3), we construct a sample containing 46,109 giant (log g 3.5 dex) stars with heliocentric distance d 4 kpc, and the sample is further divided into two groups of the inner (RGC < 8.34 kpc) and outer region (RGC > 8.34 kpc). The LZ distributions of our program stars in the panels with different [Fe/H] and [α/Fe] suggest that the thick-disk consists of two distinct components with different chemical compositions and kinematic properties. For the inner region, the metal-weak thick disk (MWTD) contributes significantly when [α/Fe] > +0.2 dex and [Fe/H] < −0.8 dex, while the canonical thick-disk (TD) dominates when [Fe/H]> −0.8 dex. However, MWTD clear appears only when [α/Fe] > +0.2 dex and [Fe/H] < −1.2 dex for the outer region, and its proportion is lower than that of the inner region within the same metallicity. Similar result can be obtained from the Vφ distribution.

Most “Young” α-rich Stars Have High Masses but are Actually Old

Recent observations have revealed a population of α-element abundances, enhanced giant stars with unexpected high masses (≳1 M ⊙) from asteroseismic analysis and spectroscopy. Assuming single-star evolution, their masses imply young ages (τ < 6 Gyr) incompatible with the canonical Galactic chemical evolution scenario. Here we study the chemistry and kinematics of a large sample of such α-rich, high-mass red giant branch (RGB) stars drawn from the LAMOST spectroscopic surveys. Using LAMOST and Gaia, we found these stars share the same kinematics as the canonical high-α old stellar population in the Galactic thick disk. The stellar abundances show that these high-α massive stars have α- and iron-peak element abundances similar to those of the high-α old thick-disk stars. However, a portion of them exhibit higher [(N+C)/Fe] and [Ba/Fe] ratios, which implies they have gained C- and Ba-rich materials from extra sources, presumably asymptotic giant branch (AGB) companions. The results support the previous suggestion that these RGB stars are products of binary evolution. Their high masses thus mimic “young” single stars, yet in fact they belong to an intrinsic old stellar population. To fully explain the stellar abundance patterns of our sample stars, a variety of binary evolution channels, such as main-sequence (MS) + RGB, MS + AGB, RGB + RGB, and RGB + AGB, are required, pointing to diverse formation mechanisms of these seemly rejuvenated cannibals. With this larger sample, our results confirm earlier findings that most, if not all, α-rich stars in the Galactic disk seem to be old.

Modelling the chemical evolution of the Milky Way

In this review, I will discuss the comparison between model results and observational data for the Milky Way, the predictive power of such models as well as their limits. Such a comparison, known as Galactic archaeology, allows us to impose constraints on stellar nucleosynthesis and timescales of formation of the various Galactic components (halo, bulge, thick disk and thin disk).

How many thick disk subsystems are there in the Milky Way Galaxy?

It was shown that stellar populations of the Galaxy with the thick disk kinematics, namely globular clusters, field RR Lyrae variables (lyrids), and nearby field F - G stars, have different chemical compositions. Based on the analysis of the nature of the dependencies of [alpha/Fe] on [Fe/H] for these objects, it was concluded that the thick disk subsystem in the Galaxy is composite, and at least three components independently exist inside it. The oldest subsystem consists of the metal-rich globular clusters which were formed from a single protogalactic cloud shortly after onset of the explosions of type Ia supernova in it. Then a subsystem of the field thick disk stars was formed as a result of the "heating" of the stars of the primary thin disk already formed in the Galaxy by a rather massive dwarf satellite galaxy which fell onto the Galaxy. And, finally, the subsystem of field stars (including the lyrids) with the kinematics of not only the thick, but even the thin disk that fell onto the Galaxy from this captured satellite galaxy.