A massive open cluster hiding in full sight

TL;DR

This paper reports the discovery of a previously undetected massive young star cluster in the Milky Way's Sagittarius arm, using Gaia and other data, revealing new insights into Galactic structure.

Contribution

It uncovers Valparaiso 1, a massive, young open cluster hidden by dust and confusion, combining astrometry, photometry, and spectroscopy for its characterization.

Findings

Valparaiso 1 is about 2.3 kpc away and 75 million years old.

It contains a classical Cepheid and an AGB star, rare in young clusters.

Estimated initial mass is close to 10,000 solar masses.

Abstract

Obscuration and confusion conspire to limit our knowledge of the inner Milky Way. Even at moderate distances, the identification of stellar systems becomes compounded by the extremely high density of background sources. Here we provide a very revealing example of these complications by unveiling a large, massive, young cluster in the Sagittarius arm that has escaped detection until now despite containing more than 30 stars brighter than $G=13$. By combining Gaia DR2 astrometry, Gaia and 2MASS photometry and optical spectroscopy, we find that the new cluster, which we name Valparaiso~1, located at $\sim2.3\:$kpc, is about 75~Ma old and includes a large complement of evolved stars, among which we highlight the 4~d classical Cepheid CM~Sct and an M-type giant that probably represents the first detection of an AGB star in a Galactic young open cluster. Although strong differential reddening renders accurate parameter determination unfeasible with the current dataset, direct comparison to clusters of similar age suggests that Valparaiso~1 was born as one of the most massive clusters in the Solar Neighbourhood, with an initial mass close to $10^{4}\:$M$_{\odot}$.