We report a comprehensive Chemo-dynamical analysis of LAMOST J1109+0754, a relatively bright (V = 12.8), extremely metal-poor ([Fe/H] = $-3.17$), and prograde ($J_ϕ$ and $V_ϕ$ $> 0$) star, with a strong \textit{r}-process enhancement ([Eu/Fe] = $+$0.94 $\pm$ 0.12, [Ba/Fe] = $-$0.52 $\pm$ 0.15). 31 chemical abundances (from Lithium to Thorium) were derived. We suggest a possible progenitor with stellar mass of 13.4-29.5 M$_\odot$. We argue that J1109+0754 is representative of the main \textit{r}-process component due to the well agreement with the scaled-solar \textit{r}-process component. We analyze the orbital history of this star in a {\it time-varying Galactic potential}, based on a Milky-Way analogue model extracted from \texttt{Illustris-TNG} simulations. Using this model, we carry out a statistical estimation of the phase-space coordinates of J1109+0754 at a young cosmic age. Collectively, the calculated motions, the derived chemistry, and the results from the cosmological simulations suggest that LAMOST J1109+0754 most likely formed in a low-mass dwarf galaxy, and belongs to the Galactic outer-halo population.
This study presents a comprehensive chemo-dynamical analysis of LAMOST J1109+0754, a bright (V = 12.8), extremely metal-poor (\abund{Fe}{H} = $-3.17$) star, with a strong \textit{r}-process enhancement (\abund{Eu}{Fe} = +0.94 $\pm$ 0.12). Our results are based on the 7-D measurements supplied by $Gaia$ and the chemical composition derived from a high-resolution ($R\sim 110,000$), high signal-to-noise ratio ($S/N \sim 60)$ optical spectrum obtained by the 2.4\,m Automated Planet Finder Telescope at Lick Observatory. We obtain chemical abundances of 31 elements (from lithium to thorium). The abu…
Low-resolution spectra are proved competitive to high-resolution spectra 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 and 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…
We revisit the moving groups (MGs) in the solar neighborhood with a sample of 91969 nearby stars constructed from LAMOST DR7. Using the wavelet technique and Monte Carlo simulations, five MGs together with a new candidate located at $V \simeq$ -130 km s$^{-1}$ are detected simultaneously in $V-\sqrt{U^2+2V^2}$ space. Taking into account the other known MGs, we conclude that MGs in the Galactic disk are spaced by approximately 15 $\sim$ 25 km s$^{-1}$ along $V$ velocity. The origin of detected MGs is analysed through the distributions of [Fe/H]$-$[Mg/Fe] and ages. Our results support attributin…
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 $α$-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 …
We report that LAMOST-HVS1 is a massive hyper-runaway subgiant star with mass of 8.3 Msun and super-Solar metallicity, ejected from the inner stellar disk of the Milky Way $\sim$ 33 Myr ago with the intrinsic ejection velocity of $568^{+19}_{-17}$ km/s (corrected for the streaming motion of the disk), based on the proper motion data from Gaia Data Release 2 (DR2) and high-resolution spectroscopy. The extremely large ejection velocity indicates that this star was not ejected by the supernova explosion of the binary companion. Rather, it was probably ejected by a 3- or 4-body dynamical interacti…
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" [$α$/Fe]-enhanced stars with stellar ages younger than 6.0 Gyr and [$α$/Fe] ratios greater than 0.15 dex. The stellar ages and [$α$/Fe] ratios are measured from LAMOST spectra, using a machine learning method trained with common stars in the LAMOST-APOGEE fields (for [$α$/Fe]) and in the LAMOST-$Kepler$ fields (for stellar age). The existence of these "young" [$α$/Fe]-enhanced stars is not expected from the classical Galactic chemical evolution mode…
Super metal-rich (SMR) stars in the solar neighborhood are thought to be born in the inner disk and came to present location by radial migration, which is most intense at the co-rotation resonance (CR) of the Galactic bar. In this work, we show evidence for the CR origin of SMR stars in LAMOST-Gaia by detecting six ridges and undulations in the phi versus Lz space coded by median VR, following a similar slope of -8 km/s kpc/deg. The slope is predicted by Monario et al.'s model for CR of a large and slow Galactic bar. For the first time, we show the variation of angular momentum with azimuths f…
Since Gaia DR2 was released, many velocity structures in the disk have been revealed such as large scale ridge-like patterns in the phase space. Both kinematic information and stellar elemental abundances are needed to reveal their evolution history. We have used labels from the APOGEE survey to predict elemental abundances for a huge amount of low resolution spectra from the LAMOST survey. Deep learning with artificial neural networks can automatically draw on physically sensible features in the spectrum for their predictions. Abundances of 12 individual elements: [C/Fe], [N/Fe], [O/Fe], [Mg/…
Based on the first Gaia data release and spectroscopy from the LAMOST Data Release 4, we study the kinematics and chemistry of the local halo stars. The halo stars are identified kinematically with a relative speed of at least 220 km s$^{-1}$ with respect to the local standard of rest. In total, 436 halo stars are identified. From this halo sample, 16 high velocity (HiVel) stars are identified. We studied the metallicity and [$α$/Fe] distribution of these HiVel stars. Though most of HiVel stars are metal-poor, there are several stars that have metallicity above $-0.5$ dex. To understand the or…
Based on the Gaia DR2 catalogue of hot subdwarf star candidates, we identified 1587 hot subdwarf stars with spectra in LAMOST DR7. We present atmospheric parameters for these stars by fitting the LAMOST spectra with {\sc Tlusty/Synspec} non-LTE synthetic spectra. Combining LAMOST radial velocities and Gaia Early Data Release 3 (EDR3) parallaxes and proper motions, we also present the Galactic space positions, velocity vectors, orbital parameters and the Galactic population memberships of the stars. With our He classification scheme, we identify four groups of He rich hot subdwarf stars in the …
We report the discovery of two new unbound hypervelocity stars (HVSs) from the LAMOST spectroscopic surveys. They are respectively a B2V type star of ~ 7 M$_{\rm \odot}$ with a Galactic rest-frame radial velocity of 502 km/s at a Galactocentric radius of ~ 21 kpc and a B7V type star of ~ 4 M$_{\rm \odot}$ with a Galactic rest-frame radial velocity of 408 km/s at a Galactocentric radius of ~ 30 kpc. The origins of the two HVSs are not clear given their currently poorly measured proper motions. However, the future data releases of Gaia should provide proper motion measurements accurate enough to…
We have cross-matched the LAMOST DR3 with the Gaia DR1 TGAS catalogs and obtained a sample of 166,827 stars with reliable kinematics. A technique based on the wavelet transform was applied to detect significant overdensities in velocity space among five subsamples divided by spatial position. In total, 16 significant overdensities of stars with very similar kinematics were identified. Among these, four are new stream candidates and the rest are previously known groups. Both the U-V velocity and metallicity distributions of the local sample show a clear gap between the Hercules structure and th…
The large amount of chemical and kinematic information available in large spectroscopic surveys have inspired the search for chemically peculiar stars in the field. Though these metal-poor field stars ([Fe/H$]<-1$) are commonly enriched in nitrogen, their detailed spatial, kinematic, and chemical distributions suggest that various groups may exist, and thus their origin is still a mystery. To study these stars statistically, we increase the sample size by identifying new CN-strong stars with LAMOST DR3 for the first time. We use CN-CH bands around 4000 Å~to find CN-strong stars, and further se…
With the photometric data from the SDSS survey, the spectroscopic data from the SDSS/SEGUE and the LAMOST surveys, and the astrometric data from the Gaia DR2, we have identified 71 highly-probable member stars of the GD-1 cold stellar stream spread along almost its entire length (i.e. from 126 to 203 degree in Right Ascension). With the accurate spectroscopic (i.e. metallicity and line-of-sight velocity) and astrometric (i.e. proper motions) information, the position-velocity diagrams, i.e. $ϕ_{1}$-$μ_α$, $ϕ_{1}$-$μ_δ$ and $ϕ_{1}$-${v_{\rm gsr}}$, of the GD-1 stream are well mapped. The stream…
Basing on the large volume \textit{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 thousand 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$\pm$0.01 for the whol…
We revisit the diagonal ridge feature (diagonal distributions in the $R, v_ϕ$ plane) found in $Gaia$ and present timing analysis for it between Galactocentric distances of $R=7.5$ and 12 \,kpc, using Main-Sequence-Turn-Off and OB stars selected from the LAMOST Galactic spectroscopic surveys. We recover the ridge pattern in the $R$--$v_ϕ$ plane color coded by mean radial velocity and find this feature is presented from very young (OB stars, few hundred \,Myr) to very old populations ($τ$ = 9$-$14 \,Gyr). Meanwhile, some ridge features are also revealed in the metallicity [Fe/H], [$α$/Fe] and $v…
Moving Groups in the Solar Neighborhood with Gaia, APOGEE, GALAH, and LAMOST: Dynamical Effects Gather Gas and the Ensuing Star Formation Plays an Important Role in Shaping the Stellar Velocity Distributions
Large stellar surveys are revealing the chemodynamical structure of the Galaxy across a vast spatial extent. However, the many millions of low-resolution spectra observed to date are yet to be fully exploited. We employ The Cannon, a data-driven approach to estimating abundances, to obtain detailed abundances from low-resolution (R = 1800) LAMOST spectra, using the GALAH survey as our reference. We deliver five (for dwarfs) or six (for giants) estimated abundances representing five different nucleosynthetic channels, for 3.9 million stars, to a precision of 0.05 - 0.23 dex. Using wide binary p…
We report on the discovery of five carbon-enhanced metal-poor (CEMP) stars in the metallicity range of $-3.3<$ [Fe/H] $<-2.4$. These stars were selected from the LAMOST DR3 low-resolution (R$ \sim$ 2,000) spectroscopic database as metal-poor candidates and followed-up with high-resolution spectroscopy (R$ \sim$110,000) with the LICK/APF. Stellar parameters and individual abundances for 25 chemical elements (from Li to Eu) are presented for the first time. These stars exhibit chemical abundance patterns that are similar to those reported in other literature studies of very and extremely metal-p…
In this work, we present the new catalog of carbon stars from the LAMOST DR2 catalog. In total, 894 carbon stars are identified from multiple line indices measured from the stellar spectra. Combining the CN bands in the red end with \ctwo\ and other lines, we are able to identify the carbon stars. Moreover, we also classify the carbon stars into spectral sub-types of \ch, \CR, and \cn. These sub-types approximately show distinct features in the multi-dimensional line indices, implying that in the future we can use them to identify carbon stars from larger spectroscopic datasets. Meanwhile, fro…
We report two new tidal debris nearby the Sagittarius (Sgr) tidal stream in the north Galactic cap identified from the M giant stars in LAMOST DR2 data. The M giant stars with sky area of $210^\circ<$Λ$<290^\circ$, distance of 10--20kpc, and [Fe/H]$<-0.75$ show clear bimodality in velocity distribution. We denote the two peaks as Vel-3+83 for the one within mean velocity of -3kms$^{-1}$ with respect to that of the well observed Sgr leading tail at the same $Λ$ and Vel+162+26 for the other one with mean velocity of 162kms$^{-1}$ with respect to the Sgr leading tail. Although the projected $Λ$--…
We investigate the kinematic-chemical distribution of Red Giant Branch (RGB) stars from the LAMOST survey crossed matched with Gaia DR2 proper motions, and present time tagging for the well-known ridge structures (diagonal distributions for $V_R$ in the $R$, $V_ϕ$ plane) in the range of Galactocentric distance $R$ = 8 to 15 kpc. We detect six ridge structures, including five ridges apparent in the radial velocity distribution and three ridges apparent in the vertical velocity, the sensitive time of which to the perturbations are from young population (0$-$3 Gyr) to old population (9$-$14 Gyr).…
We search for metal-rich Sausage-kinematic (MRSK) stars with [Fe/H]> -0.8 and -100<Vphi<50 km/s in LAMOST DR5 in order to investigate the influence of the Gaia-Sausage-Enceladus (GSE) merger event on the Galactic disk. For the first time, we find a group of low-alpha MRSK stars, and classify it as a metal-rich tail of the GSE galaxy based on the chemical and kinematical properties. This group has slightly larger Rapo, Zmax and Etot distributions than a previously-reported high-alpha group. Its low-alpha ratio does not allow for an origin resulting from the splash process of the GSE merger even…
With the help of Gaia DR2, we are able to obtain the full 6-D phase space information for stars from LAMOST DR5. With high precision of position, velocity, and metallicity, the rotation of the local stellar halo is presented using the K giant stars with [Fe/H]$<-1$ dex within 4 kpc from the Sun. By fitting the rotational velocity distribution with three Gaussian components, stellar halo, disk, and a counter-rotating hot population, we find that the local halo progradely rotates with $V_T=+27^{+4}_{-5}$ km s$^{-1}$ providing the local standard of rest velocity of $V_{LSR}=232$ km s$^{-1}$. Mean…
Photometric Distances for Metal-poor Giants and a Search for Hypervelocity Stars with LAMOST DR13 and Gaia DR3
Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University（北京师范大学天文与前沿科学研究院）；School of Physics and Astronomy, Beijing Normal University（北京师范大学物理与天文学院）；School of Astronomy and Space Science, University of Chinese Academy of Sciences（中国科学院大学天文与空间科学学院）
Radial velocity (RV) is among the most fundamental physical quantities obtainable from stellar spectra and is rather important in the analysis of time-domain phenomena. The LAMOST Medium-Resolution Survey (MRS) DR7 contains 5 million single-exposure stellar spectra at spectral resolution $R\sim7\,500$. However, the temporal variation of the RV zero-points (RVZPs) of the MRS survey, which makes the RVs from multiple epochs inconsistent, has not been addressed. In this paper, we measure the RVs of the 3.8 million single-exposure spectra (for 0.6 million stars) with signal-to-noise ratio (SNR) hi…
We identify 20 F-type Herbig stars and provide a list of 22 pre-main-sequence candidates from LAMOST DR8. The effective temperature, distance, extinction, stellar luminosity, mass, and radius are derived for each Herbig star based on optical spectra, photometry, Gaia EDR3 parallaxes, and pre-main-sequence evolutionary tracks. According to spectral energy distributions, 19 F-type Herbig stars belong to Class II YSOs, and one belongs to the flat-spectrum class. Four have Spitzer IRS spectra, of which three show extremely weak polycyclic aromatic hydrocarbons emissions, and three with both amorph…
We present 22\,901 OB spectra of 16\,032 stars identified from LAMOST DR5 dataset. A larger sample of OB candidates are firstly selected from the distributions in the spectral line indices space. Then all 22\,901 OB spectra are identified by manual inspection. Based on a sub-sample validation, we find that the completeness of the OB spectra reaches about $89\pm22$\% for the stars with spectral type earlier than B7, while around $57\pm16$\% B8--B9 stars are identified. The smaller completeness for late B stars is lead to the difficulty to discriminate them from A0--A1 type stars. The sub-classe…
In this paper, we report 591 high velocity star candidates (HiVelSCs) selected from over 10 million spectra of the data release seven (DR7) of the Large Sky Area Multi-object Fiber Spectroscopic Telescope and the second Gaia data release, with three-dimensional velocities in the Galactic rest-frame larger than 445 km/s. We show that at least 43 HiVelSCs are unbound to the Galaxy with escape probabilities larger than 50%, and this number decreases to eight if the possible parallax zero-point error is corrected. Most of these HiVelSCs are metal-poor and slightly alpha-enhanced inner halo stars. …
The origin of B-type runaway stars based on kinematics
Yunnan observatories, Chinese Academy of Sciences（中国科学院云南天文台）；International Centre of Supernovae, Yunnan Key Laboratory（超新星国际中心，云南省重点实验室）；CAS Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences（中国科学院国家天文台光学天文重点实验室）；Department of Physics, Hebei Normal University（河北师范大学物理学院）；Shanghai NanHui High School（上海南汇中学）；Department of Astronomy, China West Normal University（中国西部师范大学天文学系）
It is challenging to reliably identify stars that were born together outside of actively star-forming regions and bound stellar systems. However, co-natal stars should be present throughout the Galaxy, and their demographics can shed light on the clustered nature of star formation and the dynamical state of the disk. In previous work we presented a set of simulations of the Galactic disk that followed the clustered formation and dynamical evolution of 4 billion individual stars over the last 5 Gyr. The simulations predict that a high fraction of co-moving stars with physical and 3D velocity se…
Interesting chemically peculiar field stars may reflect their stellar evolution history and their possible origin in a different environment from where they are found now, which is one of the most important research fields in Galactic archaeology. To explore this further, we have used the CN-CH bands around 4000 A to identify N-rich metal-poor field stars in LAMOST DR3. Here we expand our N-rich metal-poor field star sample to ~100 stars in LAMOST DR5, where 53 of them are newly found in this work. We investigate light elements of the common stars between our sample and APOGEE DR14. While Mg, …
We develop a new method StarGO (Stars' Galactic Origin) to identify the galactic origins of halo stars using their kinematics. Our method is based on self-organizing map (SOM), which is one of the most popular unsupervised learning algorithms. StarGO combines SOM with a novel adaptive group identification algorithm with essentially no free parameters. In order to evaluate our model, we build a synthetic stellar halo from mergers of nine satellites in the Milky Way. We construct the mock catalogue by extracting a heliocentric volume of 10 kpc from our simulations and assigning expected observat…
Recent work indicates that the nearby Galactic halo is dominated by the debris from a major accretion event. We confirm that result from an analysis of APOGEE-DR14 element abundances and $\textit{Gaia}$-DR2 kinematics of halo stars. We show that $\sim$2/3 of nearby halo stars have high orbital eccentricities ($e \gtrsim 0.8$), and abundance patterns typical of massive Milky Way dwarf galaxy satellites today, characterised by relatively low [Fe/H], [Mg/Fe], [Al/Fe], and [Ni/Fe]. The trend followed by high $e$ stars in the [Mg/Fe]-[Fe/H] plane shows a change of slope at [Fe/H]$\sim-1.3$, which i…
We study the $\textit{Pontus}$ structure -- a recently discovered merger that brought in $\sim7$ globular clusters in the course of the hierarchical build-up of the Milky Way's halo. Here, we analyse the stellar population of $\textit{Pontus}$ and examine (1) its phase-space distribution using the ESA/$\textit{Gaia}$ dataset, (2) its metallicity and chemical abundances (i.e., [Fe/H], [$α$/Fe], [Mg/Fe], [Al/Fe]) using the spectroscopic catalogue of APOGEE DR17, and (3) the colour-magnitude diagram that shows interesting features, including a possibly double horizontal branch and a small populat…
Evolution and Mass Dependence of UV-to-near-IR Color Gradients up to z=2.5 from HST+JWST
Universiteit Gent（根特大学）；University of Michigan（密歇根大学）；University of Copenhagen（哥本哈根大学）；Université de Liège（列日大学）；University of Massachusetts, Amherst（马萨诸塞大学阿默斯特分校）；University of Bath（巴斯大学）
The low-metallicity, kinematically interesting dwarf stars studied by Stephens \& Boesgaard (2002, SB02) are re-examined using Gaia DR2 astrometry, and updated model atmospheres and atomic line data. New stellar parameters are determined based on the Gaia DR2 parallactic distances and Dartmouth Stellar Evolution Database isochrones. These are in excellent agreement with spectroscopically determined stellar parameters for stars with [Fe/H]$>-2$; however, large disagreements are found for stars with [Fe/H]$\le-2$, with offsets as large as $Δ$T$_{\rm eff}\sim+500$ K and $Δ$log\,$g\sim+1.0$. A sub…
