arXiv subjects

Cuihua Du

Publications and source records attributed to Cuihua Du.

At least 37 records · Page 2Linked to original sources

arXiv2022

60 candidate high-velocity stars originating from the Sagittarius dwarf spheroidal galaxy in Gaia EDR3

Using proper motions from Gaia Early Data Release 3 (Gaia EDR 3) and radial velocities from several surveys, we identify 60 candidate high-velocity stars with total velocity greater than 75\% escape velocity that probably origin from Sagittarius dwarf spheroidal galaxy (Sgr) by orbital analysis. Sgr's gravity has little effect on the results and the Large Magellanic Cloud's gravity has non-negligible effect on only a few stars. The closest approach of these stars to the Sgr occurs when the Sgr passed its pericenter ($\sim$ 38.2 Myr ago), which suggest they were tidally stripped from the Sgr. The positions of these stars in the HR diagram and the chemical properties of 19 of them with available [Fe/H] are similar with the Sgr stream member stars. This is consistent with the assumption of their accretion origin. Two of the 60 are hypervelocity stars, which may also be produced by Hills mechanism.

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arXiv2021

Gaia EDR3 proper motions of Milky Way dwarfs I: 3D Motions and Orbits

Based on Gaia Early Data Release 3 (EDR3), we estimate the proper motions for 46 dwarf spheroidal galaxies (dSphs) of the Milky Way. The uncertainties in proper motions, determined by combining both statistical and systematic errors, are smaller by a factor 2.5, when compared with Gaia Data Release 2. We have derived orbits in four Milky Way potential models that are consistent with the MW rotation curve, with total mass ranging from $2.8\times10^{11}$ $M_{\odot}$ to $15\times10^{11}$ $M_{\odot}$. Although the type of orbit (ellipse or hyperbola) are very dependent on the potential model, the pericenter values are firmly determined, largely independent of the adopted MW mass model. By analyzing the orbital phases, we found that the dSphs are highly concentrated close to their pericenter, rather than to their apocenter as expected from Kepler's law. This may challenge the fact that most dSphs are Milky Way satellites, or alternatively indicates an unexpected large number of undiscovered dSphs lying very close to their apocenters. Between half and two thirds of the satellites have orbital poles that indicate them to orbit along the Vast Polar Structure (VPOS), with the vast majority of these co-orbiting in a common direction also shared by the Magellanic Clouds, which is indicative of a real structure of dSphs.

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arXiv2021

Element abundance analysis of the metal-rich stellar halo and high-velocitythick disk in the galaxy

Based on the second Gaia data release (DR2) and APOGEE (DR16) spectroscopic surveys, wedefined two kinds of star sample: high-velocity thick disk (HVTD) with $vϕ>90km/s$ and metal-richstellar halo (MRSH) with $vϕ<90km/s$. Due to high resolution spectra data from APOGEE (DR16),we can analyze accurately the element abundance distribution of HVTD and MRSH. These elementsabundance constituted a multidimensional data space, and we introduced an algorithm method forprocessing multi-dimensional data to give the result of dimensionality reduction clustering. Accordingto chemical properties analysis, we derived that some HVTD stars could origin from the thin disk,and some MRSH stars from dwarf galaxies, but those stars which have similar chemical abundancecharacteristics in both sample may form in-situ.

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arXiv2020

Two substructures in the nearby stellar halo found in Gaia and RAVE

We use the second Gaia data release (Gaia DR2), combined with RAVE spectroscopic surveys, to identify the substructures in the nearby stellar halo. We select 3,845 halo stars kinematically and chemically, and determine their density distribution in energy and angular momentum space. To select the substructures from overdensities, we reshuffle the velocities and estimate their significance. Two statistically significant substructures, GR-1 and GR-2, are identified. GR-1 has a high binding energy and small $z$-angular momentum. GR-2 is metal-rich but retrograde. They are both new substructure, and may be accretion debris of dwarf galaxies.

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arXiv2020

Existence of the Metal-Rich Stellar Halo and High-velocity Thick Disk in the Galaxy

Based on the second Gaia data release (DR2), combined with the LAMOST and APOGEE spectroscopic surveys, we study the kinematics and metallicity distribution of the high-velocity stars that have a relative speed of at least 220 ${\rm km\ s^{-1}}$ with respect to the local standard of rest in the Galaxy. The rotational velocity distribution of the high-velocity stars with [Fe/H] $>-1.0$ dex can be well described by a two-Gaussian model, with peaks at $V_ϕ\sim +164.2\pm0.7$ and $V_ϕ\sim +3.0\pm0.3$ ${\rm km\ s^{-1}}$, associated with the thick disk and halo, respectively. This implies that there should exist a high-velocity thick disk (HVTD) and a metal-rich stellar halo (MRSH) in the Galaxy. The HVTD stars have the same position as the halo in the Toomre diagram and but show the same rotational velocity and metallicity as the canonical thick disk. The MRSH stars have basically the same rotational velocity, orbital eccentricity, and position in the Lindblad and Toomre diagram as the canonical halo stars, but they are more metal-rich. Furthermore, the metallicity distribution function (MDF) of our sample stars are well fitted by a four-Gaussian model, associated with the outer-halo, inner-halo, MRSH, and HVTD, respectively. Chemical and kinematic properties and age imply that the MRSH and HVTD stars may form in situ.

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arXiv2019

New nearby hypervelocity stars and their spatial distribution from Gaia DR2

Base on about 4,500 large tangential velocity ($V_\mathrm{tan}>0.75V_\mathrm{esc}$) with high-precision proper motions and $5σ$ parallaxes in Gaia DR2 5D information derived from parallax and proper motion, we identify more than 600 high velocity stars with $50\%$ unbound probability. Of these, 28 nearby (less than 6 kpc) late-type Hypervelocity stars (HVSs) with over $99\%$ possibility of unbound are discovered. In order to search for the unbound stars from the full Gaia DR2 6D phase space information derived from parallax, proper motion and radial velocity, we also identify 28 stars from the total velocity ($V_\mathrm{gc}>0.75V_\mathrm{esc}$) that have probabilities greater than $50\%$ of being unbound from the Galaxy. Of these, only three have a nearly $99\%$ probabilities of being unbound. On the whole HVSs subsample, there is 12 sources reported by other surveys. We study the spatial distribution of angular positions and angular separation of HVSs. We find the unbound HVSs are spatially anisotropic that is most significant in the Galactic longitude at more than $3σ$ level, and lower unbound probability HVSs are systematically more isotropic. The spatial distribution can reflect the origin of HVSs and we discuss the possible origin link with the anisotropy.

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arXiv2019

Chemical and Kinematic Properties of the Galactic Disk from the LAMOST and Gaia Sample Stars

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$|z|$ $=-0.12\pm0.0007$ 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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arXiv2019

Galactic Stellar Populations from Photometric Metallicity Distribution Functions

Based on Sloan Digital Sky Survey (SDSS) photometric data, Gu developed a new Monte-Carlo-based method for estimating the stellar metallicity distribution functions (MDFs). This method enables a more reliable determination of MDFs compared with the conventional polynomial-based methods. In this work, MDF determined from the method are well fit by three-Gaussian model, with peaks at ${\rm [Fe/H]}$=$-0.68$, $-1.38$, and $-1.90$, associated with the thick disk, inner halo, and outer halo, respectively. The vertical metallicity gradient within $1<Z<5\,{\rm kpc}$ is ${\rm d}\langle{\rm [Fe/H]}\rangle/{\rm d}Z\approx -0.19\,{\rm dex}\cdot{\rm kpc}^{-1}$ around $R=8.25\,{\rm kpc}$. But the mean radial gradient is almost negligible. The density profile of the thick disk is fitted with modified double exponential law decaying to a constant at far distance. The scale height and scale length thus estimated are $H\approx 1.13\,{\rm kpc}$ and $L\approx 3.63\,{\rm kpc}$, which are in consistent with the results determined from star-counts method in previous studies. The halos are described with two-axial power-law ellipsoid and the axis ratios of both inner halo and outer halo, inferred from stellar number density in $R$-$Z$ plane, are $q_{ih}\approx 0.49$ and $q_{oh}\approx 0.61$, respectively. It also manifests that the outer halo is a more spherical than inner halo. Moreover, the halo power-law indices estimated are $n_{ih}\approx 3.4$ and $n_{oh}\approx 3.1$, indicating that the stellar number density of inner halo changes more steeper than that of outer halo.

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arXiv2019

The substructures in the local stellar halo from Gaia and LAMOST

Based on the second Gaia data release (Gaia DR2) and spectroscopy from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) Data, we identified 20,089 halo stars kinematically and chemically. The halo streams in the solar neighborhood could be detected in the space of energy and angular momentum. We reshuffle the velocities of these stars to determine the significance of substructure. Finally, we identify 4 statistically significant substructures that are labeled GL-1 through 4. Among these substructures, GL-1 is previously known stream ("N2" stream) and the rest 3 substructures are new. These substructures may be the debris of dwarf galaxies accretion event, their dynamical and chemical information can help to understand the history of the Milky Way.

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arXiv2018

The origin of high velocity stars from Gaia and LAMOST

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 are from the tidal debris of disrupted dwarf galaxy and 19 stars are runaway-star candidates which originate from the stellar disk.

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arXiv2018

The high velocity stars in the Local Stellar Halo from Gaia and LAMOST

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 origin of high velocity stars, we evolve the trajectory of the star backwards along the orbit in our adopted Galaxy potential model to determine the orbital parameters and assess whether the star could have originated in the Galactic center. We found that some high velocity stars could have originated from the Galactic center, but other stars were probably kicked up from the Galactic disk.

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arXiv2018

Metallicity and Kinematics of the Galactic halo from the LAMOST sample stars

We study the metallicity distribution and kinematic properties of 4,680 A/F/G/K-type giant stars with $|z|>$ 5 kpc selected from the LAMOST spectroscopic survey. The metallicity distribution of giant stars with 5 $<|z|\leqslant$ 15 kpc can be described by a three-peak Gaussian model with peaks at [Fe/H] $\sim-0.6\pm0.1$, $-1.2\pm0.3$ and $-2.0\pm0.2$, corresponding to the ratio of 19$\%$, 74$\%$ and 7$\%$, respectively. The $\rm{[α/Fe]}$ is used to associate the three peaks with the thick disk, inner-halo and outer-halo components of the Galaxy. The metallicity distribution of these giant stars, which is fit with Gaussians corresponding to the three components, show a growing fraction of inner-halo component and declining fraction of the thick-disk component with increasing distance from the Galactic plane. Adopting a galaxy potential model, we also derive the orbital parameters of the sample stars, such as orbit eccentricity and rotation velocity. The peak values of derived orbital eccentricity for stars covering different metallicity regions maintain $e\sim$ 0.75, independent of height above the plane, within the range 5$<|z|<$ 15 kpc. By comparing the MDFs of stars in different rotation velocity intervals, we find that the majority of the retrograde stars are more metal-poor than the prograde stars.

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arXiv2017

Ages and structural and dynamical parameters of two globular clusters in the M81 group

GC-1 and GC-2 are two globular clusters (GCs) in the remote halo of M81 and M82 in the M81 group discovered by Jang et al. using the {\it Hubble Space Telescope} ({\it HST}) images. These two GCs were observed as part of the Beijing--Arizona--Taiwan--Connecticut (BATC) Multicolor Sky Survey, using 14 intermediate-band filters covering a wavelength range of 4000--10000 Å. We accurately determine these two clusters' ages and masses by comparing their spectral energy distributions (from 2267 to 20000~Å, comprising photometric data in the near-ultraviolet of the {\it Galaxy Evolution Explorer}, 14 BATC intermediate-band, and Two Micron All Sky Survey near-infrared $JHK_{\rm s}$ filters) with theoretical stellar population-synthesis models, resulting in ages of $15.50\pm3.20$ for GC-1 and $15.10\pm2.70$ Gyr for GC-2. The masses of GC-1 and GC-2 obtained here are $1.77-2.04\times 10^6$ and $5.20-7.11\times 10^6 \rm~M_\odot$, respectively. In addition, the deep observations with the Advanced Camera for Surveys and Wide Field Camera 3 on the {\it HST} are used to provide the surface brightness profiles of GC-1 and GC-2. The structural and dynamical parameters are derived from fitting the profiles to three different models; in particular, the internal velocity dispersions of GC-1 and GC-2 are derived, which can be compared with ones obtained based on spectral observations in the future. For the first time, in this paper, the $r_h$ versus $M_V$ diagram shows that GC-2 is an ultra-compact dwarf in the M81 group.

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arXiv2016

Kinematics of the Galactic disk from LAMOST Dwarf sample

Based on the LAMOST survey and Sloan Digital Sky Survey (SDSS), we use low-resolution spectra of 130,043 F/G-type dwarf stars to study the kinematics and metallicity properties of the Galactic disk. Our study shows that the stars with poorer metallicity and larger vertical distance from Galactic plane tend to have larger eccentricity and velocity dispersion. After separating the sample stars into likely thin-disk and thick-disk sub-sample, we find that there exits a negative gradient of rotation velocity $V_ϕ$ with metallicity [Fe/H] for the likely thin-disk sub-sample, and the thick-disk sub-sample exhibit a larger positive gradient of rotation velocity with metallicity. By comparing with model prediction, we consider the radial migration of stars appears to have influenced on the thin-disk formation. In addition, our results shows that the observed thick-disk stellar orbital eccentricity distribution peaks at low eccentricity ($e \sim 0.2$) and extends to a high eccentricity ($e \sim 0.8$). We compare this result with four thick-disk formation simulated models, and it imply that our result is consistent with gas-rich merger model.

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arXiv2016

A Monte-Carlo Method for Making SDSS $u$-Band Magnitude more accurate

We develop a new Monte-Carlo-based method to convert the SDSS (Sloan Digital Sky Survey) $u$-band magnitude to the SCUSS (South Galactic Cap of $u$-band Sky Survey) $u$-band magnitude. Due to more accuracy of SCUSS $u$-band measurements, the converted $u$-band magnitude becomes more accurate comparing with the original SDSS $u$-band magnitude, in particular at the faint end. The average $u$ (both SDSS and SCUSS) magnitude error of numerous main-sequence stars with $0.2<g-r<0.8$ increase as $g$-band magnitude becomes fainter. When $g=19.5$, the average magnitude error of SDSS $u$ is 0.11. When $g=20.5$, the average SDSS $u$ error is up to 0.22. However, at this magnitude, the average magnitude error of SCUSS $u$ is just half as much as that of SDSS $u$. The SDSS $u$-band magnitudes of main-sequence stars with $0.2<g-r<0.8$ and $18.5<g<20.5$ are converted, therefore the maximum average error of converted $u$-band magnitudes is 0.11. The potential application of this conversion is to derive more accurate photometric metallicity calibration from SDSS observation, especially for those distant stars. Thus, we can explore stellar metallicity distributions either in the Galactic halo or some stream stars.

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arXiv2016

A Monte-Carlo Method for Estimating Stellar Photometric Metallicity Distributions

Based on the Sloan Digital Sky Survey (SDSS), we develop a new monte-carlo based method to estimate the photometric metallicity distribution function (MDF) for stars in the Milky Way. Compared with other photometric calibration methods, this method enables a more reliable determination of the MDF, in particular at the metal-poor and metal-rich ends. We present a comparison of our new method with a previous polynomial-based approach, and demonstrate its superiority. As an example, we apply this method to main-sequence stars with $0.2<g-r<0.6$, $6$ kpc$<R<9$ kpc, and in different intervals in height above the plane, $|Z|$. The MDFs for the selected stars within two relatively local intervals ($0.8$ kpc$<|Z|<1.2$ kpc, $1.5$ kpc$<|Z|<2.5$ kpc) can be well-fit by two Gaussians, with peaks at [Fe/H] $\approx-0.6$ and $-1.2$ respectively, one associated with the disk system, the other with the halo. The MDFs for the selected stars within two more distant intervals ($3$ kpc$<|Z|<5$ kpc, $6$ kpc$<|Z|<9$ kpc) can be decomposed into three Gaussians, with peaks at [Fe/H] $\approx-0.6$, $-1.4$ and $-1.9$ respectively, where the two lower peaks may provide evidence for a two-component model of the halo: the inner halo and the outer halo. The number ratio between the disk component and halo component(s) decreases with vertical distance from the Galactic plane, consistent with the previous literature.

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arXiv2015

Photometric Metallicity Calibration with SDSS and SCUSS and its Application to distant stars in the South Galactic Cap

Based on SDSS g, r and SCUSS (South Galactic Cap of u-band Sky Survey) $u$ photometry, we develop a photometric calibration for estimating the stellar metallicity from $u-g$ and $g-r$ colors by using the SDSS spectra of 32,542 F- and G-type main sequence stars, which cover almost $3700$ deg$^{2}$ in the south Galactic cap. The rms scatter of the photometric metallicity residuals relative to spectrum-based metallicity is $0.14$ dex when $g-r<0.4$, and $0.16$ dex when $g-r>0.4$. Due to the deeper and more accurate magnitude of SCUSS $u$ band, the estimate can be used up to the faint magnitude of $g=21$. This application range of photometric metallicity calibration is wide enough so that it can be used to study metallicity distribution of distant stars. In this study, we select the Sagittarius (Sgr) stream and its neighboring field halo stars in south Galactic cap to study their metallicity distribution. We find that the Sgr stream at the cylindrical Galactocentric coordinate of $R\sim 19$ kpc, $\left| z\right| \sim 14$ kpc exhibits a relative rich metallicity distribution, and the neighboring field halo stars in our studied fields can be modeled by two-Gaussian model, with peaks respectively at [Fe/H]$=-1.9$ and [Fe/H]$=-1.5$.

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arXiv2015

An Investigation of the Absolute Proper Motions of the SCUSS Catalog

Absolute proper motions for $\sim$ 7.7 million objects were derived based on data from the South Galactic Cap u-band Sky Survey (SCUSS) and astrometric data derived from uncompressed Digitized Sky Surveys that the Space Telescope Science Institute (STScI) created from the Palomar and UK Schmidt survey plates. We put a great deal of effort into correcting the position-, magnitude-, and color-dependent systematic errors in the derived absolute proper motions. The spectroscopically confirmed quasars were used to test the internal systematic and random error of the proper motions. The systematic errors of the overall proper motions in the SCUSS catalog are estimated as -0.08 and -0.06 mas/yr for μα cos δ and μδ, respectively. The random errors of the proper motions in the SCUSS catalog are estimated independently as 4.2 and 4.4 mas/yr for μα cos δ and μδ. There are no obvious position-, magnitude-, and color-dependent systematic errors of the SCUSS proper motions. The random error of the proper motions goes up with the magnitude from about 3 mas/yr at u < 18.0 mag to about 7 mas/yr at u = 22.0 mag. The proper motions of stars in SCUSS catalog are compared with those in the SDSS catalog, and they are highly consistent.

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