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Register a new userChemical and Kinematic Properties of the Galactic Disk from the LAMOST and Gaia Sample Stars
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We determined the chemical and kinematic properties of the Galactic thin and thick disk using a sample of 307,246 A/F/G/K-type giant stars from the LAMOST spectroscopic survey and Gaia DR2 survey. Our study found that the thick disk globally exhibits no metallicity radial gradient, but the inner disk ($R le 8$ kpc) and the outer disk ($R>8$ kpc) have different gradients when they are studied separately. The thin disk also shows two different metallicity radial gradients for the inner disk and the outer disk, and has steep metallicity vertical gradient of d[Fe/H]/d$|z|$ $=-0.12pm0.0007$ dex kpc$^{-1}$, but it becomes flat when it is measured at increasing radial distance, while the metallicity radial gradient becomes weaker with increasing vertical distance. Adopting a galaxy potential model, we derived the orbital eccentricity of sample stars and found a downtrend of average eccentricity with increasing metallicity for the thick disk. The variation of the rotation velocity with the metallicity shows a positive gradient for the thick disk stars and a negative one for the thin disk stars. Comparisons of our observed results with models of disk formation suggest that radial migration could have influenced the chemical evolution of the thin disk. The formation of the thick disk could be affected by more than one processes: the accretion model could play an indispensable role, while other formation mechanisms, such as the radial migration or heating scenario model could also have a contribution.
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The perturbation mechanism of the Galactic disk has puzzled us for a long time. The imprints from perturbations provide important diagnostics on the disk formation and evolution. Here we try to constrain when the vertical perturbation took place in the disk by tracking the phase mixing history. Firstly, we clearly depict the spiral structures of radial ($v_R$) and azimuthal ($v_{phi}$) velocities in the phase space of the vertical position and velocity ($z$-$v_z$) with 723,871 LAMOST-Gaia combined stars. Then, we investigate the variation of the spirals with stellar age ($tau$) by dividing the sample into seven stellar age bins. Finally, we find that the spirals explicitly exist in all the bins, even in the bin of $tau<0.5$,Gyr, except for the bin of $tau>6.0$,Gyr. This constrains the vertical perturbation probably starting no later than 0.5,Gyr ago. But we can not rule out whether the young stars ($tau<0.5$,Gyr) inherit the oscillations from the perturbed ISM where they born from. This study provides some important observational evidences to understand the disk perturbation mechanisms, even the formation and evolution of our Galaxy.
Using a sample of nearly 140,000 primary red clump stars selected from the LAMOST and $Gaia$ surveys, we have identified a large sample of young [$alpha$/Fe]-enhanced stars with stellar ages younger than 6.0 Gyr and [$alpha$/Fe] ratios greater than 0.15 dex. The stellar ages and [$alpha$/Fe] ratios are measured from LAMOST spectra, using a machine learning method trained with common stars in the LAMOST-APOGEE fields (for [$alpha$/Fe]) and in the LAMOST-$Kepler$ fields (for stellar age). The existence of these young [$alpha$/Fe]-enhanced stars is not expected from the classical Galactic chemical evolution models. To explore their possible origins, we have analyzed the spatial distribution, and the chemical and kinematic properties of those stars and compared the results with those of the chemically thin and thick disk populations. We find that those young [$alpha$/Fe]-enhanced stars have distributions in number density, metallicity, [C/N] abundance ratio, velocity dispersion and orbital eccentricity that are essentially the same as those of the chemically thick disk population. Our results clearly show those so-called young [$alpha$/Fe]-enhanced stars are not really young but $genuinely$ $old$. Although other alternative explanations can not be fully ruled out, our results suggest that the most possible origin of these old stars is the result of stellar mergers or mass transfer.
Based on the second Gaia data (Gaia DR2) and spectroscopy from the LAMOST Data Release 5, we defined the high-velocity (HiVel) stars sample as those stars with $v_{mathrm{gc}} > 0.85 v_{mathrm{esc}}$, and derived the final sample of 24 HiVel stars with stellar astrometric parameters and radial velocities. Most of the HiVel stars are metal-poor and $alpha$-enhanced. In order to further explore the origin of these HiVel stars, we traced the backwards orbits of each HiVel star in the Galactic potential to derive probability parameters which are used to classify these HiVel stars. Of these, 5 stars are from the tidal debris of disrupted dwarf galaxy and 19 stars are runaway-star candidates which originate from the stellar disk.
Using a sample of 96,201 primary red clump (RC) stars selected from the LAMOST and Gaia surveys, we investigate the stellar structure of the Galactic disk. The sample stars show two separated sequences of high-[{alpha}/Fe] and low-[{alpha}/Fe] in the [{alpha}/Fe]-[Fe/H] plane. We divide the sample stars into five mono-abundance populations (MAPs) with different ranges of [{alpha}/Fe] and [Fe/H], named as the high-[{alpha}/Fe], high-[{alpha}/Fe] & high-[Fe/H], low-[Fe/H], solar, high-[Fe/H] MAPs respectively. We present the stellar number density distributions in the R R Z plane, and the scale heights and scale lengths of the individual MAPs by fitting their vertical and radial density profiles. The vertical profiles, the variation trend of scale height with the Galactocentric radius, indicate that there is a clear disk flare in the outer disk both for the low-[{alpha}/Fe] and the high-[{alpha}/Fe] MAPs. While the radial surface density profiles show a peak radius of 7 kpc and 8 kpc for the high-[{alpha}/Fe] and low-[{alpha}/Fe] MAPs, respectively. We also investigate the correlation between the mean rotation velocity and metallicity of the individual MAPs, and find that the mean rotation velocities are well separated and show different trends between the high-[{alpha}/Fe] and the low-[{alpha}/Fe] MAPs. At last, we discuss the character of the high-[{alpha}/Fe] & high-[Fe/H] MAP and find that it is more similar to the high-[{alpha}/Fe] MAP either in the radial and vertical density profiles or in the rotation velocity.
We have studied the kinematic properties of young pre-main-sequence stars. We have selected these stars based on data from the Gaia DR2 catalogue by invoking a number of photometric infrared surveys. Using 4564 stars with parallax errors less than 20%, we have found the following parameters of the angular velocity of Galactic rotation: $Omega_0 =28.84pm0.10$ km s$^{-1}$ kpc$^{-1}$, $Omega^{}_0=-4.063pm0.029$ km s$^{-1}$ kpc$^{-2}$ and $Omega^{}_0=0.766pm0.020$ km s$^{-1}$ kpc$^{-3}$, where the Oort constants are $A=16.25pm0.33$ km s$^{-1}$ kpc$^{-1}$ and $B=-12.58pm0.34$ km s$^{-1}$ kpc$^{-1}$. The circular rotation velocity of the solar neighborhood around the Galactic center is $V_0=230.7pm4.4$ km s$^{-1}$ for the adopted Galactocentric distance of the Sun $R_0=8.0pm0.15$ kpc. The residual velocity dispersion for the stars considered is shown to be low, suggesting that they are extremely young. The residual velocity dispersion averaged over three coordinates is $sim$11 km s$^{-1}$ for Herbig Ae/Be stars and $sim$7 km s$^{-1}$ for T Tauri stars.
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