The evolution of the Milky Way: new insights from open clusters

Abstract

We have collected high-dispersion echelle spectra of red giant members in the twelve open clusters (OCs) and derived stellar parameters and chemical abundances for 26 species by either line equivalent widths or synthetic spectrum analyses. We confirm the lack of an age−metallicity relation for OCs but argue that such a lack of trend for OCs arise from the limited coverage in metallicity compared to that of field stars which span a wide range in metallicity and age. We confirm that the radial metallicity gradient of OCs is steeper (flatter) for R gc < 12 kpc (>12 kpc). We demonstrate that the sample of clusters constituting a steep radial metallicity gradient of slope −0.052±0.011 dex kpc −1 at R gc < 12 kpc are younger than 1.5 Gyr and located close to the Galactic midplane (| z| < 0.5 kpc) with kinematics typical of the thin disc. Whereas the clusters describing a shallow slope of −0.015±0.007 dex kpc −1 at R gc > 12 kpc are relatively old, thick disc members with a striking spread in age and height above the midplane (0.5 < | z| < 2.5 kpc). Our investigation reveals that the OCs and field stars yield consistent radial metallicity gradients if the comparison is limited to samples drawn from the similar vertical heights. We argue via the computation of Galactic orbits that all the outer disc clusters were actually born inward of 12 kpc but the orbital eccentricity has taken them to present locations very far from their birthplaces.

The Open Cluster Chemical Abundances and Mapping Survey. IV. Abundances for 128 Open Clusters Using SDSS/APOGEE DR16

“…Open clusters have been used to study Galactic chemical trends as far back as Janes (1979), where the author showed open clusters to be a reliable tracer of a Galactic radial metallicity gradient. More recently, this trend has been consistently considered a 2-function gradient (e.g., Sestito et al 2008;Bragaglia et al 2008;Friel et al 2010;Carrera & Pancino 2011;Yong et al 2012;Frinchaboy et al 2013;Reddy et al 2016;Magrini et al 2017), with the break falling between R GC ≈ 10 kpc and R GC ≈ 16 kpc. This gradient has become an important observable constraint for models of Galactic Chemical Evolution.…”

“…Our results are consistent with Yong et al (2012) open clusters in the range 6 ≤ R GC ≤ 11 kpc, although both slopes are much shallower when they include more clusters from the literature. Carrera & Pancino (2011) and Reddy et al (2016) report [α/Fe] vs R GC gradients of 0.004 ± 0.001 dex kpc −1 and 0.014 ± 0.005 dex kpc −1 , respectively. Our results are therefore in good agreement with the literature, except perhaps for Si which appears to be almost completely flat in our case.…”

“…It has become common in the literature, when measuring Galactic metallicity gradients, to divide the sample somewhere between R GC ≈ 10 kpc and R GC ≈ 13 kpc and fit two separate lines to the data (e.g., Twarog et al 1997;Sestito et al 2008;Friel et al 2010;Jacobson et al 2011;Carrera & Pancino 2011;Yong et al 2012;Frinchaboy et al 2013;Reddy et al 2016;Magrini et al 2017), with a much shallower trend in the outer galaxy than in the inner galaxy. Since the OCCAM sample includes open clusters as far away as R GC ≈ 19 kpc, we can investigate if the Galactic metallicity gradient becomes significantly shallower at a given R GC .…”

“…Galactic trends in elements besides iron have been reported (e.g., Yong et al 2005;Friel et al 2010;Jacobson et al 2011). Trend lines are commonly fit for α-elements (e.g., Carrera & Pancino 2011;Yong et al 2012;Reddy et al 2016), and in some cases for other elements , such as [Ni/Fe], [Cr/Fe], and [V/Fe] (Casamiquela et al 2019) or [Na/Fe] and [Al/Fe] (Yong et al 2012). There is a growing consensus that there is a mild positive [α/Fe] versus R GC trend in the inner galaxy, similar to some chemodynamical model predictions (see Minchev et al 2014).…”

Astronuclear Physics: A tale of the atomic nuclei in the skies

“…Massive (Cats. 3 and 4) stars, and more specifically their He-burning cores and, to some extent, their C-burning shells, are also predicted to be s-nuclide producers through the operation of the 22 Ne (α , n) 25 Mg . This neutron source can indeed be active in these locations that are hotter than the He shell of AGB stars.…”