Radial mixing and the transition between the thick and thin Galactic discs

Abstract

The analysis of the kinematics of solar neighbourhood stars shows that the low‐ and high‐metallicity tails of the thin disc are populated by objects which orbital properties suggest an origin in the outer and inner Galactic disc, respectively. Signatures of radial migration are identified in various recent samples, and are shown to be responsible for the high‐metallicity dispersion in the age–metallicity distribution. Most importantly, it is shown that the population of low‐metallicity wanderers of the thin disc (−0.7 < [Fe/H] < −0.3 dex) is also responsible for the apparent hiatus in metallicity with the thick disc (which terminal metallicity is about −0.2 dex). It implies that the thin disc at the solar circle has started to form stars at about this same metallicity. This is also consistent with the fact that ‘transition’ objects, which have α‐element abundance intermediate between that of the thick and thin discs, are found in the range [−0.4, −0.2] dex. Once the metal‐poor thin disc stars are recognized for what they are – wanderers from the outer thin disc – the parenthood between the two discs can be identified on stars genuinely formed at the solar circle through an evolutionary sequence in [α/Fe] and [Fe/H]. Another consequence is that stars that can be considered as truly resulting of the chemical evolution at the solar circle have a metallicity restricted to about [−0.2, +0.2] dex, confirming an old idea that most chemical evolution in the Milky Way have preceded the thin disc formation.

CCD UBV(RI)C photometry of twenty open clusters

“…13. As discussed in the literature, this is caused by inhomogeneous chemical enhancements in the Galactic disc (Haywood , 2008;Jacobson et al , 2011), orbital diffusion, and radial migration and mixing of OCs (Schönrich & Binney , 2009). These OCs have probably originated from differing Galactic radii and/or from various star forming regions (Lepine et al , 2011).…”

“…The significant scatter in the ages and metallicities dominates and hides any possible small correlation. The reason for this scatter is due to orbital diffusion, radial mixing, and radial migration (Schönrich & Binney , 2009), as well as inhomogeneous chemical evolution in the Galactic disc (Haywood , 2008). …”

A newα-enhanced super-solar metallicity population

“…Stellar radial migration could give an explanation of this link (e.g. Haywood 2008b;Schönrich & Binney 2009a). It has been proposed that metal-rich stars found in the solar vicinity may have been formed in the inner Galactic disk regions(e.g.…”

Radial migration in galactic disks caused by resonance overlap of multiple patterns: Self-consistent simulations

“…However, Quillen et al (2009) showed that small satellites on radial, in-plane orbits can cause mixing in the outer disk and thus account for the fraction of low-metallicity stars present in the solar neighborhood (Haywood 2008). Moreover, we have recently demonstrated (Minchev & Famaey 2010, hereafter MF10) that a strong exchange of angular momentum occurs when a stellar disk is perturbed by a central bar and SS simultaneously: our test-particle simulations allowed us to attribute this effect to the overlap or first and second order resonances of each perturber.…”

“…In the past few decades, discrepancies in the solar neighborhood age-metallicity relation have implied that effective radial migration (i.e., redistribution of angular momentum) must be taking place in the Milky Way disk (Edvardsson et al 1993;Haywood 2008;Schönrich & Binney 2009;see Minchev & Famaey 2010, for a comprehensive discussion). In parallel, there is now considerable observational evidence that the non-axisymmetry of the Galactic potential can cause significant perturbations in the motion of stars of all ages.…”