分类： 天文学 >> 恒星和银河系 提交时间： 2023-12-31
Hong-Liang Yan
Jian-Rong Shi
Yu-Tao Zhou
Yong-Shou Chen
Er-Tao Li
Suyalatu Zhang
Shao-Lan Bi
Ya-Qian Wu
Zhi-Hong Li
Bing Guo
Wei-Ping Liu
Qi Gao
Jun-Bo Zhang
Ze-Ming Zhou
Hai-Ning Li
Gang Zhao
摘要： About 1% of giants have anomalously high Li abundances in their atmospheres, conflicting directly with the prediction of the standard stellar evolution models. This finding makes the production and evolution of Li in the Universe intriguing, not only in the sense of Big Bang nucleosynthesis or the interstellar medium, but also for the evolution of stars. Decades of efforts have been put into explaining why such extreme objects exist, yet the origins of Lirich giants are still being debated. Here we report the discovery of the most Li-rich giant known to date, with a very high Li abundance of 4.51. This rare phenomenon was observed coincidentally with another short-term event: the star is experiencing its luminosity bump on the red giant branch. Such a high Li abundance indicates that the star might be at the very beginning of its Li-rich phase, which provides a great opportunity to investigate the origin and evolution of Li in the Galaxy. A detailed nuclear simulation is presented with up-to-date reaction rates to recreate the Li enrichment process in this star. Our results provide tight constraints on both observational and theoretical points of view, suggesting that low-mass giants can internally produce Li to a very high level through 7Be transportation during the red giant phase.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： With the calculated guiding center radius $R_{guiding}$ and birth radius $R_{birth}$, we investigate the role of radial migration on the description of lithium evolution in the Galactic disk based on the upper envelope of the A(Li) vs. [Fe/H] diagram. Using migration distances, we find that stars in the solar neighborhood are born at different locations in the galactic disk, and cannot all be explained by models of chemical evolution in the solar neighborhood. It is found that the upper envelope of the A(Li) vs. [Fe/H] diagram varies significantly with $R_{birth}$, which explains the decrease of Li for super-metal-rich (SMR) stars because they are non-young stars born in the inner disk. The upper envelope of Li-$R_{birth}$ plane fits very well with chemical evolution models by Grisoni et al. for $R_{birth} = 7 - 12$ kpc, outside which young stars generally lack sufficient time to migrate to the solar neighborhood. For stars born in the solar neighborhood, the young open clusters and the upper envelope of field stars with age $<$ 3 Gyr fit well with theoretical prediction. We find that calculations using stars with ages less than 3 Gyr are necessary to obtain an undepleted Li upper envelope, and that stars with solar age (around 4.5 Gyr) have depleted around 0.3 dex from the original value based on the chemical evolution model of Grisoni et al.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： With the calculated guiding center radius $R_{guiding}$ and birth radius $R_{birth}$, we investigate the role of radial migration on the description of lithium evolution in the Galactic disk based on the upper envelope of the A(Li) vs. [Fe/H] diagram. Using migration distances, we find that stars in the solar neighborhood are born at different locations in the galactic disk, and cannot all be explained by models of chemical evolution in the solar neighborhood. It is found that the upper envelope of the A(Li) vs. [Fe/H] diagram varies significantly with $R_{birth}$, which explains the decrease of Li for super-metal-rich (SMR) stars because they are non-young stars born in the inner disk. The upper envelope of Li-$R_{birth}$ plane fits very well with chemical evolution models by Grisoni et al. for $R_{birth} = 7 - 12$ kpc, outside which young stars generally lack sufficient time to migrate to the solar neighborhood. For stars born in the solar neighborhood, the young open clusters and the upper envelope of field stars with age $<$ 3 Gyr fit well with theoretical prediction. We find that calculations using stars with ages less than 3 Gyr are necessary to obtain an undepleted Li upper envelope, and that stars with solar age (around 4.5 Gyr) have depleted around 0.3 dex from the original value based on the chemical evolution model of Grisoni et al.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Radial migration is an important process in the evolution of the Galactic disk. The metallicity gradient of open clusters and its outliers provide an effective way to probe for this process. In this work, we compile metallicity, age, and kinematic parameters for 225 open clusters and carry out a quantitative analysis of radial migration via the calculated migration distances. Based on clusters with age $ 11$ kpc and a steeper metallicity gradient from the inner disk of $R_g < 7$ kpc, which tells a complicated evolution history of the Galactic disk by different effects of stellar radial migration.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： While it is well recognized that both the Galactic interstellar extinction curves and the gas-phase abundances of dust-forming elements exhibit considerable variations from one sight line to another, as yet most of the dust extinction modeling efforts have been directed to the Galactic average extinction curve, which is obtained by averaging over many clouds of different gas and dust properties. Therefore, any details concerning the relationship between the dust properties and the interstellar environments are lost. Here we utilize the wealth of extinction and elemental abundance data obtained by space telescopes and explore the dust properties of a large number of individual sight lines. We model the observed extinction curve of each sight line and derive the abundances of the major dust-forming elements (i.e., C, O, Si, Mg and Fe) required to be tied up in dust (i.e., dust depletion). We then confront the derived dust depletions with the observed gas-phase abundances of these elements and investigate the environmental effects on the dust properties and elemental depletions. It is found that for the majority of the sight lines the interstellar oxygen atoms are fully accommodated by gas and dust and therefore there does not appear to be a "missing oxygen" problem. For those sight lines with an extinction-to-hydrogen column density $A_V/N_H$ $>$ 4.8$\times$10$^{-22}$ mag cm$^2$/H there are shortages of C, Si, Mg and Fe elements for making dust to account for the observed extinction, even if the interstellar C/H, Si/H, Mg/H and Fe/H abundances are assumed to be proto-solar abundances augmented by Galactic chemical evolution.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： 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 attributing the origin to the continuous resonant mechanisms probably induced by the bar or spiral arms of the Milky Way.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We report the discovery of emerged tidal tails around open cluster IC 4756 ($\sim$ 1 Gyr) based on 644 members identified from {\it Gaia} EDR3. Three-dimensional spatial positions, two-dimensional tangential velocities $\left( x, y, z, \kappa \cdot \mu_{\alpha}^{*}/\varpi, \kappa \cdot \mu_{\delta}/\varpi \right)$ are utilized to determine the co-moving member candidates of IC 4756. Using a Bayesian method, we correct the distance for each cluster member. Two tidal tails extend up to 180 pc and display a S-shape in $X^{\prime}Y^{\prime}$ space (Cartesian coordinates focused on cluster center). A clean sequence of our members in Color-Absolute-Magnitude Diagram (CAMD) indicates the coeval population and matches perfectly with the PARSEC isochrone with age from Bossini et al. (2019). Mass segregation is detected in this cluster as well. Finally, we derive the tidal radius and core radius of IC 4756 about $12.13$ pc and $4.33 \pm 0.75$ pc, respectively.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We report the detection of a large sample of high-$\alpha$-metal-rich stars on the low giant branch with $2.60.2$ than the comparison group. These properties strongly indicate its connection with the outer bar/bulge region at $R=3-5$ kpc. A tentative interpretation of this special group is that its stars were formed in the X-shaped bar/bulge region, close to its corotation radius, where radial migration is the most intense, and brings them to present locations at 9 kpc and beyond. Low eccentricities and slightly outward radial excursions of its stars are consistent with this scenario. Its kinematics (cold) and chemistry ($[\alpha/Fe]$ $\sim 0.1$) further support the formation of the instability-driven X-shaped bar/bulge from the thin disk.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We examine the vertical structure of Galactic disk kinematics over a
Galactocentric radial distance range of $R=5-15$ $\rm{kpc}$ and up to $3$
$\rm{kpc}$ away from the Galactic plane, using the K-type giants surveyed by
LAMOST. Based on robust measurements of three-dimensional velocity moments, a
wobbly disk is detected in a phenomenological sense. An outflow dominates the
radial motion of the inner disk, while in the outer disk there exist alternate
outward and inward flows. The vertical bulk velocities is a combination of
breathing and bending modes. A contraction-like breathing mode with amplitudes
increasing with the distance to the plane and an upward bending mode dominate
the vertical motion outside $R_0$, and there are reversed breathing mode and
bending mode at $R
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We investigate the three open clusters near Aquila Rift cloud, named as UPK 39 (\texttt{c1} hereafter), UPK 41 (\texttt{c2} hereafter) in Sim et al. (2019) and PHOC 39 (\texttt{c3} hereafter) in Hunt \& Reffert (2021), respectively. Using photometric passpands, reddening, and extinction from Gaia DR3, we construct the color-absolute-magnitude diagram (CAMD). Using isochrone fits their ages are estimated as $6.3\pm0.9$, $8.1\pm1.4$ and $21.8\pm2.2$ Myr, respectively. Their proper motions and radial velocities, estimated using data from Gaia and LAMOST are very similar. From their orbits, relative distances among them at different times, kinematics, ages, and metallicities, we conclude that \texttt{c1} and \texttt{c2} are primordial binary open cluster, which are likely to have been formed at the same time, and \texttt{c3} may capture \texttt{c1}, \texttt{c2} in the future.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We report the discovery of a new stream (dubbed as Yangtze) detected in $Gaia$ Data Release 3. The stream is at a heliocentric distance of $\sim$ 9.12 kpc and spans nearly 27$\deg$ by 1.9$\deg$ on sky. The colour-magnitude diagram of Yangtze indicates a stellar population of Age $\sim$ 11 Gyr and [M/H] $\sim$ -0.7 dex. It has a number density of about 5.5 stars degree$^{-2}$ along with a surface brightness of $\Sigma_G \simeq$ 34.9 mag arcsec$^{-2}$. The dynamics and metallicity estimate suggest that Yangtze may be closely related to Palomar 1 and the Anticenter stream.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We report the discovery of a cold stream near the southern Galactic pole (dubbed as SGP-S) detected in $Gaia$ Early Data Release 3. The stream is at a heliocentric distance of $\sim$ 9.5 kpc and spans nearly 58$^\circ$ by 0.6$^\circ$ on sky. The colour-magnitude diagram of SGP-S indicates an old and metal-poor (age $\sim$ 12 Gyr, [M/H] $\sim$ -2.0 dex) stellar population. The stream's surface brightness reaches an exceedingly low level of $\Sigma_G \simeq$ 36.2 mag arcsec$^{-2}$. Neither extant globular clusters nor other known streams are associated with SGP-S.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： 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 from -10 deg to 20 deg for two outer and broad undulations with negative VR around -18 km/s following this slope. The wave-like pattern with large amplitude outside CR and a wide peak of the second undulations indicate that minor merger of the Sagittarius dwarf galaxy with the disk might play a role besides the significant impact of CR of the Galactic bar.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We explore the contribution of the Gaia-Sausage to the stellar halo of the Milky Way by making use of a Gaussian Mixture model (GMM) and applying it to halo star samples of LAMOST K giants, SEGUE K giants, and SDSS blue horizontal branch stars. The GMM divides the stellar halo into two parts, of which one represents a more metal-rich and highly radially biased component associated with an ancient, head-on collision referred to as the Gaia-Sausage, and the other one is a more metal-poor and isotropic halo. A symmetric bimodal Gaussian is used to describe the distribution of spherical velocity of the Gaia-Sausage, and we find that the mean absolute radial velocity of the two lobes decreases with Galactocentric radius. We find that the Gaia-Sausage contributes about $41\%-74\%$ of the inner (Galactocentric radius $r_\mathrm{gc} < 30$ kpc) stellar halo. The fraction of stars of the Gaia-Sausage starts to decline beyond $r_\mathrm{gc} \sim$ $25-30$ kpc, and the outer halo is found to be significantly less influenced by the Gaia-Sausage than the inner halo. After the removal of halo substructures found by integrals of motion, the contribution of the \textit{Gaia}-Sausage falls slightly within $r_\mathrm{gc} \sim$ 25 kpc, but is still as high as $30\%-63\%$. Finally, we select several possible Sausage-related substructures consisting of stars on highly eccentric orbits. The GMM/Sausage component agrees well with the selected substructure stars in their chemodynamical properties, which increases our confidence in the reliability of the GMM fits.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Yong Yang
Jing-Kun Zhao
Miho N. Ishigaki
Masashi Chiba
Cheng-Qun Yang
Xiang-Xiang Xue
Xian-Hao Ye
Gang Zhao
摘要： Context. Several dynamically cold streams have been associated with certain globular clusters (GCs) based on orbital energies and angular momenta. Some of these streams are surprisingly far apart from their progenitors and one such pair is Triangulum and NGC 5824. Triangulum can be considered as a piece of NGC 5824 leading tail since the cluster's future orbit matches with the stream's track well. The existence of the leading tail for NGC 5824 is the motivation behind the search for its trailing tail. Aims. Our goal is to confirm the connection between Triangulum and NGC 5824 and seek the trailing tail of the cluster. Methods. The selection of member stars of Triangulum is made through various cuts in metallicity, proper motions (PMs), radial velocity and color-magnitude diagram (CMD). The selected members are compared in phase space to a mock stream which models the disruption of NGC 5824. We then try to detect the trailing tail of the cluster based on a modified matched-filter technique. Stars are assigned weights using their color differences from the cluster's locus in CMD. These weights are further scaled based on stars' departures from expected PMs of the model stream. Results. A total of 26 member stars for Triangulum are obtained and 16 of them are newly identified. These members are consistent with the mock stream in the phase space and their metalicity and position on the CMD are in good agreements with NGC 5824. By applying the matched-filter, a tenuous trailing tail of the cluster is detected, spanning $\sim$ 50$^\circ$ long on sky. The signature matches with the mock stream's trajectory well. Conclusions. Our results support that Triangulum stream acts as a part of the leading tail for NGC 5824. On the trailing side, we have detected a 50$^\circ$ tail extended from the cluster. The existence of both leading and trailing tails for the GC NGC 5824 is verified.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We report the discovery of a new stream (dubbed as Yangtze) detected in $Gaia$ Data Release 3. The stream is at a heliocentric distance of $\sim$ 9.12 kpc and spans nearly 27$\deg$ by 1.9$\deg$ on sky. The colour-magnitude diagram of Yangtze indicates a stellar population of Age $\sim$ 11 Gyr and [M/H] $\sim$ -0.7 dex. It has a number density of about 5.5 stars degree$^{-2}$ along with a surface brightness of $\Sigma_G \simeq$ 34.9 mag arcsec$^{-2}$. The dynamics and metallicity estimate suggest that Yangtze may be closely related to Palomar 1 and the Anticenter stream.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We investigate how the existence of hydrogen molecules on grain surfaces may affect H$_2$ formation efficiency in diffuse and translucent clouds. Hydrogen molecules are able to reduce the desorption energy of H atoms on grain surfaces in models. The detailed microscopic Monte Carlo method is used to perform model simulations. We found that the impact of the existence of H$_2$ on H$_2$ formation efficiency strongly depends on the diffusion barriers of H$_2$ on grain surfaces. Diffuse cloud models that do not consider surface H$_2$ predict that H atom recombination efficiency is above 0.5 over a grain temperature (T) range 10 K and 14 K. The adopted H$_2$ diffusion barriers in diffuse cloud models that consider surface H$_2$ are 80$\%$ H$_2$ desorption energies so that H$_2$ can be trapped in stronger binding sites. Depending on model parameters, these diffuse cloud models predict that the recombination efficiency is between nearly 0 and 0.5 at 10 K $\leq$ T $\leq$ 14 K. Translucent cloud model results show that H$_2$ formation efficiency is not affected by the existence of surface H$_2$ if the adopted average H$_2$ diffusion barrier on grain surfaces is low (194 K) so that H$_2$ can diffuse rapidly on grain surfaces. However, the recombination efficiency can drop to below 0.002 at T $\geq$ 10 K if higher average H$_2$ diffusion barrier is used (255 K) in translucent cloud models.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Haining Li
Wako Aoki
Tadafumi Matsuno
Qianfan Xing
Takuma Suda
Nozomu Tominaga
Yuqin Chen
Satoshi Honda
Miho N. Ishigaki
Jianrong Shi
Jingkun Zhao
Gang Zhao
摘要： We present homogeneous abundance analysis of over 20 elements for 385 very metal-poor (VMP) stars based on the LAMOST survey and follow-up observations with the Subaru Telescope. It is the largest high-resolution VMP sample (including 363 new objects) studied by a single program, and the first attempt to accurately determine evolutionary stages for such a large sample based on Gaia parallaxes. The sample covers a wide metallicity range from [Fe/H]=-1.7 down to [Fe/H]=-4.3, including over 110 objects with [Fe/H] 0.7) is in the range of 20-30% for turn-off stars depending on the treatment of objects for which C abundance is not determined, which is much higher than that in giants (~8%). Twelve Mg-poor stars ([Mg/Fe] 1.0) have been discovered in -3.4 <[Fe/H]< -2.0, enlarging the sample of r-process-enhanced stars with relatively high metallicity.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We present a study of the influence of the Gaia-Sausage-Enceladus (GSE) on the density shape of the Galactic stellar halo using 11624 K giants from the LAMOST survey. Every star is assigned a probability of being a member of the GSE based on its spherical velocities and metallicity by a Gaussian Mixture Model. We divide the stellar halo into two parts by the obtained probabilities, of which one is composed of the GSE members and defined as the GSE-related halo, and the other one is referred to as the GSE-removed halo. Using a non-parametric method, the radial number density profiles of the two stellar halos can be well described by a single power law with a variable flattening $q$ ($r = \sqrt{R^2+[(Z/q(r))]^2}, \nu = {\nu_0}r^{-\alpha}$). The index $\alpha$ is $4.92\pm0.12$ for the GSE-related halo and $4.25\pm0.14$ for the GSE-removed halo. Both the two stellar halos are vertically flattened at smaller radii but become more spherical at larger radii. We find that the GSE-related halo is less vertically flattened than the GSE-removed halo, and the difference of $q$ between the two stellar halos ranges from 0.07 to 0.15. However, after the consideration of the bias, it is thought to be within 0.08 at most of the radii. Finally, we compare our results with two Milky Way analogues which experience a significant major merger in the TNG50 simulation. The study of the two analogues also shows that the major merger-related stellar halo has a smaller ellipticity than the major merger-removed stellar halo.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Yong Yang
Jing-Kun Zhao
Miho N. Ishigaki
Jian-Zhao Zhou
Cheng-Qun Yang
Xiang-Xiang Xue
Xian-Hao Ye
Gang Zhao
摘要： By mining the data from $Gaia$ EDR3, SDSS/SEGUE DR16 and LAMOST DR8, 11 member stars of the NGC 5466 tidal stream are detected and 7 of them are newly identified. To reject contaminators, a variety of cuts are applied in sky position, color-magnitude diagram, metallicity, proper motion and radial velocity. We compare our data to a mock stream generated by modeling the cluster's disruption under a smooth Galactic potential plus the Large Magellanic Cloud (LMC). The concordant trends in phase-space between the model and observations imply that the stream might have been perturbed by LMC. The two most distant stars among 11 detected members trace the stream's length to $60^\circ$ of sky, supporting and extending the previous length of $45^\circ$. Given that NGC 5466 is so distant and potentially has a longer tail than previously thought, we expect that NGC 5466 tidal stream could be a useful tool in constraining the Milky Way gravitational field.
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