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تسجيل مستخدم جديدRadial Migration from Metallicity Gradient of Open Clusters and Outliers
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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 $< 0.5$ Gyr, we obtain the present-day metallicity gradient of $-0.074 pm 0.007$ dex/kpc. Along this gradient distributes three sequences, and clusters in the upper, the middle, and the lower groups are found to be old outward-migrators, in-situ clusters, and inward-migrators, respectively. The migration distance increases with age, but its most effective time is probably less than 3 Gyr. The metallicity gradient breaks out at $R_g$ (guiding center radius) $sim11.5$ kpc, which is caused by the lack of young open clusters in the outer disk and the presence of old outward-migrators in the upper sequence. It shows that this boundary is related to the different effects of radial migration between the inner and outer disks. We also found many special open clusters in and near the outer disk of $R > 11$ 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.
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Radial migration is an important process in the Galactic disk. A few open clusters show some evidence on this mechanism but there is no systematic study. In this work, we investigate the role of radial migration on the Galactic disk based on a large
sample of 146 open clusters with homogeneous metallicity and age from Netopil et al. and kinematics calculated from Gaia DR2. The birth site Rb, guiding radius Rg and other orbital parameters are calculated, and the migration distance |Rg-Rb| is obtained, which is a combination of metallicity, kinematics and age information. It is found that 44% open clusters have |Rg-Rb|< 1 kpc, for which radial migration (churning) is not significant. Among the remaining 56% open clusters with |Rg-Rb|> 1 kpc, young ones with t<1.0 Gyr tend to migrate inward, while older clusters usually migrate outward. Different mechanisms of radial migration between young and old clusters are suggested based on their different migration rates, Galactic locations and orbital parameters. For the old group, we propose a plausible way to estimate migration rate and obtain a reasonable value of 1.5(+-0.5) kpc/Gyr based on ten intermediate-age clusters at the outer disk, where the existence of several special clusters implies its complicate formation history.
Open clusters (OCs) are crucial for studying the formation and evolution of the Galactic disc. However, the lack of a large number of OCs analyzed homogeneously hampers the investigations about chemical patterns and the existence of Galactocentric ra
dial and vertical gradients, or an age-metallicity relation. To overcome this, we have designed the Open Cluster Chemical Abundances from Spanish Observatories survey (OCCASO). We aim to provide homogeneous radial velocities, physical parameters and individual chemical abundances of six or more Red Clump stars for a sample of 25 old and intermediate-age OCs visible from the Northern hemisphere. To do so, we use high resolution spectroscopic facilities (R> 62,000) available at Spanish observatories. We present the motivation, design and current status of the survey, together with the first data release of radial velocities for 77 stars in 12 OCs, which represents about 50% of the survey. We include clusters never studied with high-resolution spectroscopy before (NGC~1907, NGC~6991, NGC~7762), and clusters in common with other large spectroscopic surveys like the Gaia-ESO Survey (NGC~6705) and APOGEE (NGC~2682 and NGC~6819). We perform internal comparisons between instruments to evaluate and correct internal systematics of the results, and compare our radial velocities with previous determinations in the literature, when available. Finally, radial velocities for each cluster are used to perform a preliminar kinematic study in relation with the Galactic disc.
The spatial distribution of elemental abundances in the disc of our Galaxy gives insights both on its assembly process and subsequent evolution, and on the stellar nucleogenesis of the different elements. Gradients can be traced using several types o
f objects as, for instance, (young and old) stars, open clusters, HII regions, planetary nebulae. We aim at tracing the radial distributions of abundances of elements produced through different nucleosynthetic channels -the alpha-elements O, Mg, Si, Ca and Ti, and the iron-peak elements Fe, Cr, Ni and Sc - by using the Gaia-ESO idr4 results of open clusters and young field stars. From the UVES spectra of member stars, we determine the average composition of clusters with ages >0.1 Gyr. We derive statistical ages and distances of field stars. We trace the abundance gradients using the cluster and field populations and we compare them with a chemo-dynamical Galactic evolutionary model. Results. The adopted chemo-dynamical model, with the new generation of metallicity-dependent stellar yields for massive stars, is able to reproduce the observed spatial distributions of abundance ratios, in particular the abundance ratios of [O/Fe] and [Mg/Fe] in the inner disc (5 kpc<RGC <7 kpc), with their differences, that were usually poorly explained by chemical evolution models. Often, oxygen and magnesium are considered as equivalent in tracing alpha-element abundances and in deducing, e.g., the formation time-scales of different Galactic stellar populations. In addition, often [alpha/Fe] is computed combining several alpha-elements. Our results indicate, as expected, a complex and diverse nucleosynthesis of the various alpha-elements, in particular in the high metallicity regimes, pointing towards a different origin of these elements and highlighting the risk of considering them as a single class with common features.
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The study of radial metallicity gradients in the disc of the Milky Way is a powerful tool to understand the mechanisms that have been acting in the formation and evolution of the Galactic disc. In this proceeding, I will put the eye on some problems
that should be carefully addressed to obtain precise determinations of the metallicity gradients.
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