arXiv subjects

Songmei Qin

Publications and source records attributed to Songmei Qin.

At least 19 recordsLinked to original sources

arXiv2026

Discovery of an Unbound Flyby Companion of UBC 63: In the Immediate Aftermath of a Close Encounter

We re-investigate the open cluster UBC 63 using the Gaia DR3 data and show that, rather than being a single cluster as previously classified, it is a compelling candidate for a double cluster undergoing an unbound flyby interaction. A GMM decomposition performed in the 5D astrometric space reveals the two statistically distinct components, namely UBC 63A (98 members, Age = 21 $\pm$ 4 Myr) and UBC 63B (148 members, Age = 562 $\pm$ 43 Myr). A significant age difference of $\Delta \mathrm{Age} = 541 \pm 43$ Myr between the clusters, rules out coeval formation. Their 3D separation of $60 \pm 29$ pc at the birth-epoch of the younger cluster, indicates that the clusters might have originated from the same molecular cloud complex. At present, the system exhibits a 3D separation of $26 \pm 8$ pc, with a relative velocity of $3.60 \pm 1.80$ km s$^{-1}$. Orbital integrations and \textit{N}-body simulations of the pair suggest that the systems had a close encounter, reaching a separation of $7 \pm 2$ pc only $\sim$~6 Myr ago and predict a rapid divergence to a separation of $491 \pm 213$ pc within the next $\sim$100 Myr. The low escape velocity ($V_{\rm esc} = 0.51 \pm 0.12$ km s$^{-1}$) of the system compared to the relative 3D velocity indicates that they are gravitationally unbound. Their low tidal factors, elongated structures and populations extending beyond the Jacobi radii may reflect a strong transient tidal interaction between the clusters.

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arXiv2026

Revisiting the Radial Velocities of Nearby Open Clusters using Gaia DR3

Open clusters (OCs) are essential laboratories for probing stellar dynamics and tracing the structure and evolution of the Milky Way. Accurate measurements of their average radial velocities (RVs) and RV dispersions are crucial for estimating dynamical masses, orbital evolution, and overall kinematic states. Gaia Data Release 3 (DR3) provides an unprecedented volume of high-precision RVs. However, when applied to OCs, Gaia DR3 RVs often yield unusually large and overestimated RV dispersions. This inflation is primarily driven by RV measurement systematics for hot and faint stars, as well as unresolved binary contamination. To mitigate this, we revisit the average RVs and RV dispersions of OCs within 500 pc of the Sun using Gaia DR3. We evaluate the reliability of RV measurements and employ a color-based filtering method. By selecting member stars within an intrinsic color range of $0.2 \le (BP-RP)_{0} \le 1.2$ mag, we exclude hot and cool stars with less reliable RVs. This filtering significantly reduces the inferred RV dispersions while maintaining average RVs consistent with previous literature. Specifically, the median RV dispersion decreases by 26%, dropping from 3.76 km s$^{-1}$ prior to filtering to 2.79 km s$^{-1}$ afterward. These filtered RV dispersions remain systematically larger than tangential velocity dispersions, likely due to residual biases and undetected binaries. However, RV dispersions measured exclusively from red clump giants (found in 6 clusters) are remarkably small ($\lesssim 1.6$ km s$^{-1}$) and align closely with tangential dispersions ($\lesssim 1$ km s$^{-1}$). Ultimately, we provide a practical, Gaia-only strategy to derive more realistic RV dispersions for OCs, identifying red clump giants as exceptionally high-fidelity kinematic tracers for robust cluster studies.

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arXiv2026

Hot Degenerate Components in Blue Stragglers: A Multi-Wavelength SED Analysis of Nine Open Clusters with Swift/UVOT

We present a homogeneous multi-wavelength analysis of 35 blue straggler star (BSS) candidates in nine open clusters, combining Swift/UVOT near-ultraviolet data with Gaia DR3 astrometry and optical-to-infrared photometry. We construct spectral energy distributions (SEDs) to search for signatures of hot companions associated with past mass transfer. Among the sample, 15 BSSs (~43%) show ultraviolet excesses that are better described by two-component SED fits. The inferred companions are consistent with hot white dwarfs and pre-extremely low-mass (pre-ELM) white dwarf candidates, suggesting systems observed at different stages following mass transfer. We examine the radial distribution of the BSSs and find evidence for mass segregation in dynamically evolved clusters, a result that is broadly consistent with the estimated half-mass relaxation timescales of the host systems. To place the clusters in a Galactic context, we compute their orbits using galpy, obtaining low eccentricities (e <= 0.1) and disk-like trajectories. We also find a positive relation between the half-number radius of the BSS population (r50) and the total number of BSSs. Overall, our results are consistent with a scenario in which the BSS population in these clusters is dominated by binary evolution. The systems identified here provide observational constraints on post-mass-transfer evolutionary phases. While the number of robust detections is limited and intrinsic degeneracies remain in SED-based decomposition, these results provide a useful foundation for future spectroscopic confirmation.

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arXiv2025

Stellar masses and mass ratios for Gaia open cluster members

Context: Unresolved binaries in star clusters can bias stellar and cluster mass estimates, making their proper treatment essential for studying cluster dynamics and evolution. Aims: We aim to develop a fast and robust framework for jointly deriving stellar masses and multiplicity statistics of member stars, together with optimal cluster parameters. Methods: We use Gaia DR3 parallaxes together with multi-band photometry of open cluster (OC) members to infer stellar masses and binary mass-ratios through simulation-based inference (SBI), while iteratively fitting the cluster parameters. The validation of our SBI framework on simulated clusters demonstrates that the inclusion of infrared photometry significantly improves the detection of low mass-ratio binaries. The minimum mass-ratio threshold for reliably identifying unresolved binaries depends on cluster properties and the available photometry, but typically lies below $q=0.5$. Results: Applying our method to 42 well-populated OCs, we derive a catalogue of stellar masses and mass-ratios for 27201 stars, achieving typical uncertainties of 0.08 in $q$ and $0.01\,\mathrm{M}_\odot$ in the primary stellar mass. We analyse the archetype OCs M67 and NGC 2360 in detail, including mass segregation and mass-ratio distribution among other characteristics, while deriving multiplicity fractions for the rest of the sample. We find evidence that the high mass-ratio ($q\geq 0.6$) binary fraction shows a strong correlation with the age and a weak anti-correlation with the cluster metallicity. Furthermore, the variation of the binary fraction with stellar mass in OCs shows strong accordance with the observed dependence for field stars heavier than $\gtrsim0.6\,\mathrm{M}_\odot$. Conclusions: Our work paves a path for future population-level investigations of multiplicity statistics and precision stellar masses in extended samples of OCs.

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arXiv2025

A catalog of new blue stragglers in open clusters with Gaia DR3

The high-precision {\it Gaia} data release 3 (DR3) enables the discovery of numerous open clusters in the Milky Way, providing an excellent opportunity to search for blue straggler stars in open clusters and investigate their formation and evolution in these environments. Using the member stars from literature open cluster catalogs, we visually inspected the color-magnitude diagram (CMD) of each cluster and selected cluster candidates that potentially host blue stragglers. We then reassessed cluster memberships using the {\tt pyUPMASK} algorithm with {\it Gaia} DR3 and performed isochrone fitting to derive physical parameters for each cluster, including age, distance modulus, mean reddening, and metallicity. Finally, we empirically identified straggler stars based on their positions relative to the best-fitting isochrone, zero-age main sequence (ZAMS), and equal-mass binary sequence on the CMD. In total, we identified 272 new straggler stars in 99 open clusters, comprising 153 blue stragglers, 98 probable blue stragglers, and 21 yellow stragglers. Compared to the reported blue straggler catalogs based on earlier {\it Gaia} data, our results increase the number of open clusters with stragglers in the Milky Way by 22.2\%, and the total number of blue stragglers by 11.2\%.

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arXiv2025

Binary clusters in the Galactic disk I: Systematic identification and classification using Gaia DR3

Aims. We aim to identify and classify BCs using high-precision astrometric and kinematic data, and to investigate their physical properties, mutual gravitational interactions, and formation rates. Methods. We used a comprehensive star cluster catalog that contains 4,084 high-quality clusters. Based on spatial and kinematic proximity, we identified 400 cluster pairs involving 686 unique clusters. These pairs were classified into three types: primordial BCs, systems formed through tidal capture or resonant trapping, and hyperbolic encounter pairs. For each system, we calculated the tidal factor to quantify the strength of mutual tidal interaction. Additionally, we constructed multi-cluster systems by identifying transitive connections among cluster pairs. Results. Among the 400 identified cluster pairs, nearly 60.8% (243 pairs) are probably primordial BCs, exhibiting both similar ages and motions. This supports a scenario where they formed together in the same giant molecular cloud. We find that 82.5% of the cluster pairs have strong mutual tidal forces. In addition, 278 star clusters are identified as members of 82 multi-cluster systems, including 27 newly reported groups. Cross-matching with the literature confirms the recovery of previously reported systems and leads to the discovery of 268 new cluster pairs. In our sample, about 16.8% of star clusters are involved in some type of interaction with another cluster, and 9.94% of star clusters are likely born in primordial BCs. Conclusions. Our results provide a comprehensive, homogeneously identified sample of Galactic BCs. The high fraction of primordial BCs and their mutual tidal interaction suggest that cluster formation in pairs is a main outcome of star formation. This work offers new observational constraints on the formation and dynamical evolution of multiple star cluster systems.

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arXiv2025

Selected open cluster sample for validating atmospheric parameters: Application to Gaia and other surveys

Reliable stellar atmospheric parameters are essential for probing stellar structure and evolution, and for stellar population studies. However, various deviations appear in comparisons with different ground-based spectroscopic surveys. We aim to select high-quality open cluster members and employ the atmospheric parameters provided by the theoretical isochrones of open clusters as a benchmark to assess the quality of stellar atmospheric parameters from Gaia DR3 and other ground-based spectroscopic surveys, such as LAMOST DR11, APOGEE DR17, and GALAH DR4. We selected 130 open clusters with well-defined main sequences within 500 pc of the solar neighborhood as a benchmark sample to estimate the reference atmospheric parameters of the members from the best-fit isochrones of those clusters. By comparing the atmospheric parameters provided by different spectroscopic surveys to the theoretical parameters, we found that the atmospheric parameter deviation and the corresponding dispersions exhibit different variations. The atmospheric parameter deviations of F, G, and K-type stars are smaller than those of B, A, and M-type stars for most surveys. For most samples, the dispersion of Teff decreases as temperature decreases, whereas the dispersion of logg shows the opposite trend.

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arXiv2024

3D projection analysis: characterizing the morphological stability of nearby open clusters

Context. The study of open cluster morphology is pivotal for exploring their formation and evolutionary processes. Aims. We manage to assess the morphological stability of 105 nearby open clusters within tidal radii on the X-Y, X-Z, and Y-Z planes of the heliocentric Cartesian coordinate system, utilizing member catalogs from the literature. Meanwhile, we also delve into factors potentially impacting the clusters' morphological stability on these projection planes. Methods. We used the rose diagram method by constructing the 3D projected morphology of sample clusters to quantify the morphological stability of their 3D projected morphology. Results. Our analysis indicates there is a demonstrated linear positive correlation between the number of sample clusters' member stars within tidal radii and their morphological stability in different 3D projection planes. This may suggest that the more member stars there are within the tidal radius of a cluster, the stronger its own gravitational binding capacity is, resulting in strong morphological stability. We find a direct link between the clusters' morphological stabilities in the X-Z plane within tidal radii and their velocity dispersion in the same plane, suggesting that the morphological stabilities in the X-Z plane may be dependent on internal dynamics. Moreover, the morphological stability of the clusters' 3D projection is influenced by their spatial positions along the Y axis, but not linearly, indicating that the environmental changes at the clusters' location may affect their morphological stability. Likewise, specific external forces can have an effect on their morphological stability. Conclusions. This research introduces a novel perspective for understanding the morphological stability of open clusters, with a particular focus on their 3D projected morphologies.

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arXiv2024

Unveiling the Binary Nature of NGC 2323

As a well-known open cluster, NGC 2323 (also called M50) has been widely investigated for over a hundred years and has always been considered a classical single cluster. In this work, with the help of Gaia DR3, we study the binary structure nature of this cluster. Although indistinguishable in the spatial space, the small but undeniable difference in the proper motion indicates that they may be two individual clusters. After investigating the properties of the two clusters, it is found that they have very close positions (three-dimensional $Δ$pos = 12.3 pc, $σ_{Δ\mathrm{pos}} = 3.4$ pc) and similar tangential velocities (two-dimensional $Δ$V = 2.2 km s$^{-1}$, $σ_{Δ\mathrm{V}} = 0.02$ km s$^{-1}$), indicating the existence of their physical association. Moreover, the best isochrone fitting ages of the two clusters are the same (158 Myr), further proving their possibly common origin. To comprehensively understand the formation and evolution of this binary cluster, we employ the PETAR $N$-body code to trace back their birthplace and deduce their dynamical evolutionary fate. With observational mean cluster properties, the simulations suggest that they may form together, and then orbit each other as a binary cluster for over 200 Myr. After that, because of their gradual mass loss, the two clusters will eventually separate and evolve into two independent clusters. Meanwhile, the numerical $N$-body simulation suggests that the less massive cluster is unlikely to be the cluster tidal tails created by the differential rotation of the Milky Way.

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arXiv2024

On the Determination of Stellar Mass and Binary Fraction of Open Clusters within 500 pc from the Sun

We investigated the stellar mass function and the binary fraction of 114 nearby open clusters (OCs) using the high-precision photometric data from Gaia Data Release 3 (Gaia DR3). We estimated the mass of member stars by using a ridge line (RL) that is better in line with the observed color-magnitude diagram (CMD), thus obtaining more accurate stellar mass and binary mass ratio ($q$) at the low-mass region. By analyzing the present-day mass function (PDMF) of star clusters, we found that 108 OCs follow a two-stage power-law distribution, whereas 6 OCs present a single power-law PDMF. Subsequently, we fitted the high(low)-mass index of PDMF ($dN/dm \propto m^{-α}$), denoted as $α_{\rm h}$($α_{\rm l}$), and segmentation point $m_{\rm c}$. For our cluster sample, the median values of $α_{\rm h}$ and $α_{\rm l}$ are 2.65 and 0.95, respectively, which are approximately consistent with the initial mass function (IMF) results provided by Kroupa (2001). We utilized the cumulative radial number distribution of stars with different masses to quantify the degree of mass segregation. We found a significant positive correlation between the state of dynamical evolution and mass segregation in OCs. We also estimated the fraction of binary stars with $q \geq 0.5$, ranging from 6% to 34% with a median of 17%. Finally, we provided a catalog of 114 nearby cluster properties, including the total mass, the binary fraction, the PDMF, and the dynamical state.

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arXiv2024

Exploration of morphological coherence in open clusters with a "core-shell'' structure

The study of their morphological coherence allows us to obtain a better understanding of the morphological evolution of open clusters. We employed the ellipsoid fitting method to delineate the 3D spatial structure of the sample clusters while using the morphological dislocation (MD) defined in our previous work and the ellipticity ratio (ER) of the clusters' inner and outer structures to characterize the morphological coherence of the sample clusters. The results show an inverse correlation between the ER of the sample clusters and the number of their members, indicating that sample clusters with a much more elliptical external morphology than internal shape generally tend to host a large number of members. Meanwhile, a slight shrinking of the MD of the sample clusters with their members' number may shed light on the significant role of the gravitational binding of the sample clusters in maintaining their morphological stability. Moreover, there are no correlations between the MD and ER of the sample clusters and their age. They are also not significantly correlated with the X-axis, the Y-axis, their orbital eccentricities, and the radial and vertical forces on them. However, the ER of the sample clusters displays some fluctuations in the distributions between it and the above covariates, implying that the morphologies of the sample clusters are sensitive to the external environment if sample effects are not taken into account. Finally, the analysis of the 3D spatial shapes of sample clusters with a small ER or a large ER demonstrates that the number of members lays an important foundation for forming a dense internal system for sample clusters. At the same time, the MD of the sample clusters can serve well as an indicator of their morphological stability, which is built upon a certain amount of member stars.

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arXiv2024

A fast-rotating blue straggler star in the tidal tail of the open cluster NGC 752

NGC 752 is a famous Galactic open cluster of intermediate age. In recent works, a very long and asymmetric tail was newly revealed. A blue straggler star (BSS) at the periphery of the tidal tail of the cluster has been identified subsequently. We aim to perform a detailed analysis of the newly detected BSS based on the available comprehensive spectroscopic and photometric data. We also explored this BSS's possible formation pathway and age limitation based on the collected spectroscopic and photometric data. We estimated the projected rotational velocity $v\ \mathrm{sin}i$ and the mass of the BSS from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope low-resolution spectra and multiband photometric data from various catalogs, respectively. The newly discovered BSS is confirmed as a genuine member of NGC 752. The lack of ultraviolet excess in the SED and no significant variations in the light curve imply that this BSS is likely a single star ($mass=1.86^{+3.62}_{-0.94}\ M_{\odot}$) formed through stellar mergers. The fast rotation velocity ($v\ \mathrm{sin}i=206.9\pm4.9$~km $\rm s^{-1}$) of the BSS may provide constraints on its age (less than a hundred million years), but more formation details require further investigation.

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arXiv2023

The role of tidal interactions in the formation of slowly rotating early-type stars in young star clusters

The split main sequences found in the colour-magnitude diagrams of star clusters younger than ~600 Myr are suggested to be caused by the dichotomy of stellar rotation rates of upper main-sequence stars. Tidal interactions have been suggested as a possible explanation of the dichotomy of the stellar rotation rates. This hypothesis proposes that the slow rotation rates of stars along the split main sequences are caused by tidal interactions in binaries. To test this scenario, we measured the variations in the radial velocities of slowly rotating stars along the split main sequence of the young Galactic cluster NGC 2422 (~90 Myr) using spectra obtained at multiple epochs with the Canada-France-Hawai'i Telescope. Our results show that most slowly rotating stars are not radial-velocity variables. Using the theory of dynamical tides, we find that the binary separations necessary to fully or partially synchronise our spectroscopic targets, on time-scales shorter than the cluster age, predict much larger radial velocity variations across multiple-epoch observations, or a much larger radial velocity dispersion at a single epoch, than the observed values. This indicates that tidal interactions are not the dominant mechanism to form slowly rotating stars along the split main sequences. As the observations of the rotation velocity distribution among B- and A-type stars in binaries of larger separations hint at a much stronger effect of braking with age, we discuss the consequences of relaxing the constraints of the dynamical tides theory.

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arXiv2023

The new detection of blue straggler stars in 50 open clusters using Gaia DR3

The particularly abundant presence of blue straggler stars (BSS) in Galactic open clusters offers favorable conditions for detailed studies on the statistical properties and the origin of the blue straggler population. With the help of Gaia DR3, the number of identified open clusters continuously increases, and the determination of star cluster members is more reliable. We performed a more thorough search for BSS in newly found open clusters based on Gaia data. We implemented a uniform membership determination for over one thousand newly identified open clusters with larger sky coverage based on the astrometric and photometric data from Gaia DR3. The membership probabilities of stars were assigned by the pyUPMASK algorithm. Then we estimated the physical parameters of these clusters by isochrone fitting on their CMDs and picked out BSS in the specific region of these CMDs. We identified 138 BSS that had not been reported before in 50 open clusters. Compared with recent catalogs that present more than 1500 BSS in 339 open clusters, our new catalog increased the number of BSS in Galactic open clusters by about 10%, and the number of open clusters with BSS by nearly 17%. In the future, more accurate abundance measurements are anticipated to better probe the origin of BSS in open clusters.

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arXiv2023

Value-added catalog of M-giant stars in LAMOST DR9

In this work, we update the catalog of M-giant stars from the low-resolution spectra of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) Data Release 9. There are 58,076 M giants identified from the classification pipeline with seven temperature subtypes from M0 to M6. The 2471 misclassified non-M-giant stars are white dwarf binaries, early types, and M dwarfs. And the contamination rate is 4.2$\%$ in the M-giants sample. A total of 372 carbon stars were identified by CaH spectral indices, and were further confirmed by the LAMOST spectra. We update the correlation between the $(W1-W2)_0$ color and [M/H] from APOGEE DR17. We calculate the radial velocities of all M giants by applying cross-correlation to the spectra between 8000 and 8950 Åwith synthetic spectra from ATLAS9. Taking star distances less than 4 kpc from Gaia EDR3 as the standard, we refitted the photometric distance relation of M giants. And based on our M-giant stars, we select a group of Sagittarius stream members, whose sky and 3D velocity distributions are well consistent with K-giant Saggitarius stream members found in Yang et al. With our M giants, we find that the disk is asymmetric out to R = 25 kpc, which is 5 kpc further out than detected using K giants.

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arXiv2022

Hunting for Neighboring Open Clusters with Gaia DR3: 101 New Open Clusters within 500 pc

We systematically searched for open clusters in the solar neighborhood within 500 pc using pyUPMASK and HDBSCAN clustering algorithms based on {\it Gaia} DR3. Taking into consideration that the physical size for most open clusters is less than 50 pc, we adopted a slicing approach for different distance shells and identified 324 neighboring open clusters, including 223 reported open clusters and 101 newly discovered open clusters (named as OCSN, Open Cluster of Solar Neighborhood). Our discovery has increased the number of open clusters in the solar neighborhood by about 45\%. In this work, larger spatial extents and more member stars were attained for our cluster sample. We provided the member stars and the membership probabilities through the pyUPMASK algorithm for each cluster and derived their astrophysical, age, and structural parameters.

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arXiv2022

The role of binarity and stellar rotation in the split main sequence of NGC 2422

In addition to the extended main-sequence turnoffs widely found in young and intermediate-age (~ 600 Myr-2 Gyr-old) star clusters, some younger clusters even exhibit split main sequences (MSs). Different stellar rotation rates are proposed to account for the bifurcated MS pattern, with red and blue MSs (rMS and bMS) populated by fast and slowly rotating stars, respectively. Using photometry from Gaia Early Data Release 3, we report a Galactic open cluster with a bifurcated MS, NGC 2422 ( ~ 90 Myr). We exclude the possibilities that the bifurcated MS pattern is caused by photometric noise or differential reddening. We aim to examine if stellar rotation can account for the split MSs. We use spectra observed with the Canada-France-Hawaii Telescope and the Southern African Large Telescope, and directly measured v sin i, the projected rotational velocities, for stars populating the bMS and rMS. We find that their v sin i values are weakly correlated with their loci in the color-magnitude diagram because of contamination caused by a large fraction of rMS stars with low projected rotational velocities. Based on the spectral energy distribution fitting method, we suggest that these slowly rotating stars at the rMS may hide a binary companion, which breaks the expected v sin i-color correlation. Future time-domain studies focusing on whether these slowly rotating stars are radial velocity variables are crucial to test the roles of stellar rotation and binarity in generating the split MSs.

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arXiv2022

New insights into the structure of open clusters in the Gaia era

With the help of Gaia data, it is noted that in addition to the core components, there are low-density outer halo components in the extended region of open clusters. To study the extended structure beyond the core radius of the cluster ($\sim$ 10 pc), based on Gaia EDR3 data, taking up to 50 pc as the searching radius, we use the pyUPMASK algorithm to re-determine the member stars of the open cluster within 1-2 kpc. We obtain the member stars of 256 open clusters, especially those located in the outer halo region of open clusters. Furthermore, we find that most open clusters' radial density profile in the outer region deviates from the King's profile. To better describe the internal and external structural characteristics of open clusters, we propose a double components model for description: core components with King model distribution and outer halo components with logarithmic Gaussian distribution, and then suggest using four radii ( $r_c$, $r_t$, $r_o$, $r_e$) for describing the structure and distribution profile of star clusters, where $r_t$ and $r_e$ represent the boundaries of core components and outer halo components respectively. Finally, we provide a catalog of 256 clusters with structural parameters. In addition, our study shows the sizes of these radii are statistically linear related, which indicates that the inner and outer regions of the cluster are interrelated and follow similar evolutionary processes. Further, we show that the structure of two components can be used to better trace the cluster evolution properties in different stages.

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