The GALAH survey: tracing the Galactic disc with open clusters

Abstract

Open clusters are unique tracers of the history of our own Galaxy’s disk. According to our membership analysis based on Gaia astrometry, out of the 226 potential clusters falling in the footprint of GALAH or APOGEE, we find that 205 have secure members that were observed by at least one of the survey. Furthermore, members of 134 clusters have high-quality spectroscopic data that we use to determine their chemical composition. We leverage this information to study the chemical distribution throughout the Galactic disk of 21 elements, from C to Eu. The radial metallicity gradient obtained from our analysis is −0.076 ± 0.009 dex kpc−1, which is in agreement with previous works based on smaller samples. Furthermore, the gradient in the [Fe/H] - guiding radius (rguid) plane is −0.073 ± 0.008 dex kpc−1. We show consistently that open clusters trace the distribution of chemical elements throughout the Galactic disk differently than field stars. In particular, at given radius, open clusters show an age-metallicity relation that has less scatter than field stars. As such scatter is often interpreted as an effect of radial migration, we suggest that these differences are due to the physical selection effect imposed by our Galaxy: clusters that would have migrated significantly also had higher chances to get destroyed. Finally, our results reveal trends in the [X/Fe]-rguid-age space, which are important to understand production rates of different elements as a function of space and time.

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

“…The slope of the inner metallicity gradient (i.e., b 1 ) is another important parameter which is tightly linked to the evolution of our Galaxy. Our value of −0.064 ± 0.007 kpc −1 is in excellent agreement with many recent works based on open clusters [47,48,97,113,116].…”

“…A similar figure showing flatter gradients for younger populations, including OB stars, was previously presented in Daflon and Cunha [37] for oxygen, keeping in mind that the latter element is measured in H II regions, OB stars, planetary nebulae and Cepheids.) As discussed previously in the literature [36,48,91,97], the young [Fe/H]-R Gal gradient is flatter than the old one. Interestingly, the gradients traced by the youngest clusters are remarkably similar to those traced by Cepheids and young field stars.…”

“…Only this second behaviour is what it is expected from chemo-dynamical models of the Galactic disk [120]. This dichotomy between clusters and field stars is still a topic of debate; however, it is likely the result of a bias that is intrinsic in the populations of open clusters and that is operated by the Galaxy itself that quickly dissipates clusters living most of their lives near the Galactic centre (for more details, see Spina et al [97]). As it is discussed in Section 5.2, we require further studies of these selection effects in order to better understand how the Galaxy is shaping the current demography of open clusters.…”

“…Homogenised chemical abundances from the GALAH+ DR3 and APOGEE DR16 catalogs have been used by Spina et al [97] to derive chemical abundances of 21 elements, from C to Eu, for 134 open clusters, which are publicly available 1 Following the authors' recommendation, we caution against the chemical abundances of open clusters that have been derived though only one stellar member, as their values could not be accurate and the uncertainties underestimated. Therefore, in order to make this distinction clearer to the reader, we call the group of clusters whose metallicity was derived by Spina et al [97] through only one stellar member as the silver sample, while all the other clusters belong to the gold sample.…”

“…Instead, the precise astrometric solutions now produced by Gaia make it possible to capture the entire kinematic information for a broad range of clusters. This now makes L z a quantity that is much better suited than R Gal to characterise the metal content across the Galaxy [97]. Note that the guiding radius can also be used in place of the angular momentum; in fact, the two are quantities carrying the same information.…”