Mapping the Galactic Metallicity Gradient with Open Clusters: The State-of-the-Art and Future Challenges

Abstract

In this paper, we make use of data collected for open cluster members by high-resolution spectroscopic surveys and programmes (i.e., APOGEE, Gaia-ESO, GALAH, OCCASO, and SPA). These data have been homogenised and then analysed as a whole. The resulting catalogue contains [Fe/H] and orbital parameters for 251 Galactic open clusters. The slope of the radial metallicity gradient obtained through 175 open clusters with high-quality metallicity determinations is −0.064 ± 0.007 dex kpc−1. The radial metallicity distribution traced by open clusters flattens beyond RGal = 12.1 ± 1.1 kpc. The slope traced by open clusters in the [Fe/H]-Lz diagram is −0.31 ± 0.02 × 103 dex km−1 kpc−1 s, but it flattens beyond Lz = 2769 ± 177 km kpc s−1. In this paper, we also review some high-priority practical challenges around the study of open clusters that will significantly push our understanding beyond the state-of-the-art. Finally, we compare the shape of the galactic radial metallicity gradient to those of other spiral galaxies.

New Open-cluster Candidates Found in the Galactic Disk Using Gaia DR2/EDR3 Data

“…For the cluster candidates younger than 50 Myr, the metallicity gradient is about −0.092 dex kpc −1 , which is comparable with the predictions of previous studies (Magrini et al 2009;Spina et al 2021). More interestingly, there is a suspicious steeper decrease near R ∼ 9 kpc, and here the younger candidates exhibit a lower metallicity distribution than older ones; this is also consistent with recent spectroscopic observations of OCs (Spina et al 2017(Spina et al , 2021(Spina et al , 2022. However, it should be noticed that these new samples are mainly located on the Local, Sagittarius, and Perseus Arms near the solar system, and whether the gradient has a wider applicability needs more relevant research using larger OC samples.…”

New Open Cluster candidates Found in Galactic Disk Using Gaia DR2/EDR3 Data

“…For the cluster candidates younger than 50 Myr, the metallicity gradient is about -0.092 dex kpc −1 , which is comparable with the predictions of previous studies (Magrini et al 2009;Spina et al 2021). More interesting, there is a suspicious steeper decrease near R ∼ 9 kpc, and here the younger candidates exhibit a lower metallicity distribution than older ones; this is also consistent with recent spectroscopic observations of OCs (Spina et al 2017(Spina et al , 2021(Spina et al , 2022. However, it should be noticed that these new samples are mainly located on the Local, Sagittarius, and Perseus arms near the Solar System, and whether the gradient has a wider applicability needs more relevant research using larger OC samples.…”

Reliability and limitations of inferring birth radii in the Milky Way disk

“…We also note that the metallicity gradient in the Galaxy is still not well constrained (e.g. Spina et al 2022;Eilers et al 2022) We can also connect the ISM evolution and 𝜎 Rbirth with the "orderliness" of the stars orbit (a proxy for circularity of the star's orbits), < 𝑣 𝜙 > /𝜎 𝑣 𝜙 , for the ISM, where < 𝑣 𝜙 > is the average azimuthal velocity, and 𝜎 𝑣 𝜙 is the azimuthal velocity dispersion. Figure 3 left column plots show the time evolution of < 𝑣 𝜙 > /𝜎 𝑣 𝜙 for the 4 simulated galaxies.…”