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Register a new userCo-formation of the thin and thick discs revealed by APOGEE-DR16 and Gaia-DR2
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Added by
Leandro Beraldo e Silva
Publication date
2020
fields
Physics
and research's language is
English
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Since thin disc stars are younger than thick disc stars on average, the thin disc is predicted by some models to start forming after the thick disc had formed, around 10 Gyr ago. Accordingly, no significant old thin disc population should exist. Using 6-D coordinates from Gaia-DR2 and age estimates from Sanders & Das (2018), we select $sim 24000$ old stars (${tau > 10}$ Gyr, with uncertainties $lesssim 15%$) within 2 kpc from the Sun (full sample). A cross-match with APOGEE-DR16 ($sim 1000$ stars) reveals comparable fractions of old chemically defined thin/thick disc stars. We show that the full sample pericenter radius ($r_mathrm{per}$) distribution has three peaks, one associated with the stellar halo and the other two having contributions from the thin/thick discs. Using a high-resolution $N$-body+Smooth Particle Hydrodynamics simulation, we demonstrate that one peak, at $r_mathrm{per}approx 7.1$ kpc, is produced by stars from both discs which were born in the inner Galaxy and migrated to the Solar Neighbourhood. In the Solar Neighbourhood, $sim 1/2$ ($sim 1/3$) of the old thin (thick) disc stars are classified as migrators. Our results suggest that thin/thick discs are affected differently by radial migration inasmuch as they have different eccentricity distributions, regardless of vertical scale heights. We interpret the existence of a significant old thin disc population as evidence for an early co-formation of thin/thick discs, arguing that clump instabilities in the early disc offer a compelling explanation for the observed trends.
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We have obtained high-resolution spectra and carried out a detailed elemental abundance analysis for a new sample of 899 F and G dwarf stars in the Solar neighbourhood. The results allow us to, in a multi-dimensional space consisting of stellar ages, detailed elemental abundances, and full kinematic information for the stars, study and trace their respective origins. Here we briefly address selection criteria and discuss how to define a thick disc star. The results are discussed in the context of galaxy formation.
Line-of-sight kinematic studies indicate that many Galactic globular clusters have a significant degree of internal rotation. However, three-dimensional kinematics from a combination of proper motions and line-of-sight velocities are needed to unveil the role of angular momentum in the formation and evolution of these old stellar systems. Here we present the first quantitative study of internal rotation on the plane-of-the-sky for a large sample of globular clusters using proper motions from Gaia DR2. We detect signatures of rotation in the tangential component of proper motions for 11 out of 51 clusters at a $>$3-sigma confidence level, confirming the detection reported in Gaia collaboration et al. (2018) for 8 clusters, and additionally identify 11 GCs with a 2-sigma rotation detection. For the clusters with a detected global rotation, we construct the two-dimensional rotation maps and proper motion rotation curves, and we assess the relevance of rotation with respect to random motions ($V/sigmasim0.08-0.51$). We find evidence of a correlation between the degree of internal rotation and relaxation time, highlighting the importance of long-term dynamical evolution in shaping the clusters current properties. This is a strong indication that angular momentum must have played a fundamental role in the earliest phases of cluster formation. Finally, exploiting the spatial information of the rotation maps and a comparison with line-of-sight data, we provide an estimate of the inclination of the rotation axis for a subset of 8 clusters. Our work demonstrates the potential of Gaia data for internal kinematic studies of globular clusters and provides the first step to reconstruct their intrinsic three-dimensional structure.
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