http://www.cell.com/article/S2666675822000200/pdf

Overview of the LAMOST survey in the first decade

The outer Galactic disc contains some features such as the warp and flare, whose origin is still debated. The Gaia data provide an excellent opportunity to probe the Galactic disc at large distances and study these features. We derive the density distributions of the average (old) whole population and the supergiants (representative of a young population), and we use them to constrain their warp and flare. By comparing the results, we study how the properties of these phenomena depend on the studied population. We used Lucy's deconvolution method to recover corrected star counts as a function of distance, from which we derive the density distribution. We find that supergiants have an asymmetric warp, reaching a maximum amplitude of $z_w=0.658$ kpc and minimum amplitude of $z_w=-0.717$ kpc at a distance of $R=[19.5,20]$ kpc, which is almost twice as high as the amplitude of the whole population of the disc. We find a significant flare of the whole population, especially in the thick disc. The scale height increases from $h_{z,thick}\approx 0.8$ kpc and $h_{z,thin}\approx 0.3$ kpc in the solar neighbourhood, to $h_{z,thick} \approx 3$ kpc and $h_{z,thin}\approx 0.7$ kpc in the remote regions of the Milky Way ($R\approx 18$ kpc). The supergiants' population has only a small flare.

/pdf/warp-and-flare-of-the-galactic-disc-revealed-with-2afpsbwl.pdf

Warp and flare of the Galactic disc revealed with supergiants by Gaia EDR3

Context. Mapping the Galactic spiral structure is a di ﬃ cult task since the Sun is located in the Galactic plane and because of dust extinction. For these reasons, molecular masers in radio wavelengths have been used with great success to trace the Milky Way spiral arms. Recently, Gaia parallaxes have helped in investigating the spiral structure in the Solar extended neighborhood. Aims. In this paper, we propose to determine the location of the spiral arms using Cepheids since they are bright, young supergiants with accurate distances (they are the ﬁrst ladder of the extragalactic distance scale). They can be observed at very large distances; therefore, we need to take the Galactic warp into account. Methods. Thanks to updated mid-infrared photometry and to the most complete catalog of Galactic Cepheids, we derived the pa- rameters of the warp using a robust regression method. Using a clustering algorithm, we identiﬁed groups of Cepheids after having corrected their Galactocentric distances from the (small) e ﬀ ects of the warp. Results. We derived new parameters for the Galactic warp, and we show that the warp cannot be responsible for the increased disper- sion of abundance gradients in the outer disk reported in previous studies. We show that Cepheids can be used to trace spiral arms, even at large distances from the Sun. The groups we identify are consistent with previous studies explicitly deriving the position of spiral arms using young tracers (masers, OB(A) stars) or mapping overdensities of upper main-sequence stars in the Solar neighborhood thanks to Gaia data.

/pdf/tracing-the-milky-way-warp-and-spiral-arms-with-classical-1t0mgmzm.pdf

Tracing the Milky Way warp and spiral arms with classical Cepheids

The infall of the Sagittarius (Sgr) dwarf spheroidal galaxy in the Milky Way halo is an unique opportunity to understand how the different components of a dwarf galaxy could be tidally removed. In this work, we reconstruct the Sgr core morphology and kinematics on the basis of a model that has already successfully reproduced the Sgr stream. Here we use a very high resolution model that almost resolves individual stars in the Sgr core. It reproduces most of the observed morphology and kinematic properties, without specific fine tuning. We also show that the dark matter may have been almost entirely stripped by Milky Way tides after two passages at the pericenter. Finally the model predicts that the Sgr core will be fully disrupted within the next 2 Gyr.

The Dark Matter Tidal Stripping History of the Sagittarius Core with N-body Simulations

We present a catalog of 948,216 stars with mass labels and a catalog of 163,105 red clump (RC) stars with mass and age labels simultaneously. The training data set is crossmatched from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope DR5, and high-resolution asteroseismology data, mass, and age are predicted by the random forest (RF) method or a convex-hull algorithm. The stellar parameters with a high correlation with mass and age are extracted and the test data set shows that the median relative error of the prediction model for the mass of the large sample is 3%, and for the mass and age of RC stars is 4% and 7%. We also compare the predicted age of RC stars with recent works and find that the final uncertainty of the RC sample could reach 18% for age and 9% for mass; meanwhile, the final precision of the mass for the large sample with different types of stars could reach 13% without considering systematics. All of this implies that this method could be widely used in the future. Moreover, we explore the performance of different machine-learning methods for our sample, including Bayesian linear regression and the gradient-boosting decision tree (GBDT), multilayer perceptron, multiple linear regression, RF, and support vector regression methods. Finally, we find that the performance of a nonlinear model is generally better than that of a linear model, and the GBDT and RF methods are relatively better.

https://iopscience.iop.org/article/10.3847/1538-4365/ac81be/pdf

Mass and Age Determination of the LAMOST Data with Different Machine-learning Methods

We present analysis of the spatial density structure for the outer disk from 8$-$14 \,kpc with the LAMOST DR5 13534 OB-type stars. We observe similar flaring on north and south sides of the disk implying that the structure is symmetrical about the Galactic plane, for which the scale height is from 0.14 to 0.5 \,kpc in the north and south sides. We reveal that the thickness of the OB stellar disk is similar but flaring is slightly stronger compared to the thin disk as traced by red giant branch stars, possibly implying that secular evolution is not the main contributor to the flaring but perturbation scenarios such as interactions with passing dwarf galaxies should be more possible. When comparing the OB stellar disk of the north and south side with the gas disk, the former one is slightly thicker than the later one by $\approx$ 33 and 9 \,pc for the median value, meaning that one could tentatively use young OB-type stars to trace the gas properties. Meanwhile, we unravel that the radial scale length of the young OB stellar disk is 1.17 $\pm$ 0.05 \,kpc, which is shorter than that of the gas disk, confirming that the gas disk is more extended than stellar disk. What is more, by considering the mid$-$plane displacements ($Z_{0}$) in our density model we find that almost all of $Z_{0}$ are within 100 \,pc with the increasing trend as Galactocentric distance increases, which looks like the warp signal in young stellar population but the significance and scenarios will need to be investigated in more detail.

The Flare and Warp of the Young Stellar Disk Traced with LAMOST DR5 OB-type Stars

The Flare and Warp of the Young Stellar Disk traced with LAMOST DR5 OB-type stars

In this work, we present a catalog of cataclysmic variables (CVs) identified from the Sixth Data Release (DR6) of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). To single out the CV spectra, we introduce a novel machine-learning algorithm called UMAP to screen out a total of 169,509 H$\alpha$-emission spectra, and obtain a classification accuracy of the algorithm of over 99.6$\%$ from the cross-validation set. We then apply the template matching program PyHammer v2.0 to the LAMOST spectra to obtain the optimal spectral type with metallicity, which helps us identify the chromospherically active stars and potential binary stars from the 169,509 spectra. After visually inspecting all the spectra, we identify 323 CV candidates from the LAMOST database, among them 52 objects are new. We further discuss the new CV candidates in subtypes based on their spectral features, including five DN subtype during outbursts, five NL subtype and four magnetic CVs (three AM Her type and one IP type). We also find two CVs that have been previously identified by photometry, and confirm their previous classification by the LAMOST spectra.

Yang Yu

Papers

The Flare and Warp of the Young Stellar Disk Traced with LAMOST DR5 OB-type Stars

The Flare and Warp of the Young Stellar Disk traced with LAMOST DR5 OB-type stars

A catalogue of 323 cataclysmic variables from LAMOST DR6