On the origin of stellar associations

Stellar associations (a term introduced by Ambartsumian, 1947) are unbound loose ensembles of young stars (see Gouliermis, 2018, for an exhaustive recent review on this subject). They are dominated by OB stars, which are so bright to be often visible even at optical wave-lengths, in spite of the patchy and obscured environment in which they form and spend most of their short life. As expected, they are rich in pre-main sequence stars (Damiani, 2018) as well. Being very young, they should keep a vivid memory of the star formation (SF) event from which they formed and therefore are ideal tracers to study different modes of SF. It is unclear whether stellar (OB or T) associations are a direct product of star formation, or, more conservatively, an intermediate dynamical stage during the early evolution of young star clusters, soon after their formation, and before their final dissolution.

In the first scenario the actual spatial distribution of stars is the one at birth. Since they do not show any special shape, SF is expected to occur at any scale, from very large scale, as large as a galaxy, down to scales as small as individual young stellar objects. SF is then hierarchical, and the stellar structures which emerge from it are often fractally organized. Efremov (1989) identifies associations, aggregates, complexes and super-complexes. There is no preferred scale, and whatever density peak in the interstellar medium (ISM) can generate stars, from cluster size all the way to even individual stars.

In the second scenario, stars form in star clusters (Lada & Lada, 2003) which are initially embedded, and made of 50 stars or more. They are formed then in the upper part of the molecular cloud mass distribution. Then stellar winds, UV flux, and eventually SNe explosions from the most massive stars in the pristine cluster remove the gas not processed by SF and, in this way, move the star cluster out of virial equilibrium. The cluster becomes loose and turns into an OB association. Associations in this scenario are then the final stage of the evolution of star clusters before dissolution in the general Galactic field. Being the time scale of this process short compared with the average age of Milky Way open clusters, this is normally referred to it as cluster “infant mortality”. From an observational point of view, the 3D kinematics of stars should show some indication of global expansion, in other words, some relationships between radial velocity and radius.

As mentioned in a recent study by Ward et al. (2020), this monolithic scenario does not seem to be supported by Gaia DR2 (Gaia Collaboration et al., 2018) data. These authors do not find any significant correlation between radial velocity and radius in most OB associations which, therefore, do not show dominant expansion signatures confirming earlier results by (Mel’nik & Dambis, 2018). Instead, they seem to be dominated by other large scale motions, which indicate that the velocity field is highly structured and cannot be reconciled with a monolithic collapse. To cast additional light on this topic, ideally one should study in detail many OB associations, coupling multi-wavelength campaigns with kinematical and astrometric data from Gaia DR2. Studies of this type have been recently performed for some associations, like Cygnus OB2 (Berlanas et al., 2019), Vela OB2 (Beccari et al., 2018; Cantat-Gaudin et al., 2019) and Scorpius OB1 (Damiani, 2018; Yalyalieva et al., 2020), to give a few examples. In all cases, as I will outline below in more detail, the evidence is that SF is highly structured.

The second release of Gaia astrometric data prompted an intense investigation of stellar associations in the Milky Way. I provide here a couple of illuminating examples of recent studies: Cygnus OB2 and Vela OB2. A full section will then be devoted to Scorpius OB1, in the study of which I was personally involved.

Recent studies on this OB association have been performed by Damiani (2018); Kuhn et al. (2019), and Yalyalieva et al. (2020), using Gaia DR2 and multi-wavelength photometry. Sco 1 is a very rich and complex stellar association (Damiani, 2018). The spectacular H ii region G345.45+1.50 is situated in the northern part of the field, while the most prominent young star cluster, NGC 6231, is located in the southern part. I will summarise here the results of Yalyalieva et al. (2020), who are not covering NGC 6231 (see Fig. 2), but concentrate on the northern and central region. Here, the most interesting structure is certainly Trumpler 24. This is thought to be a young open cluster with poorly defined boundaries and complex structure belonging to Sco OB1 (Heske & Wendker, 1984). Besides, the area under investigation is rich in pre-main sequence stars (Heske & Wendker, 1984; Damiani, 2018), which indicates active/recent star formation.

The area shown in Fig. 2 was surveyed with multi-colour UBVI photometry (Yalyalieva et al., 2020) from the Las Campanas Henrietta Swope 1.0-meter telescope. Then, by cross-correlating with Gaia DR2 data, the existence of different physical stellar groupings was searched for.

The clustering algorithm adopted is based on the DBSCAN (Density-Based Spatial Clustering of Applications with Noise) technique. The algorithm uses the clustering module of the machine learning library scikit-learn (Pedregosa et al., 2011) as its basis. Nine different groups were found and characterised (see Table 1 and Fig. 3) in some detail, depending on the number of recovered members.

In this review, I discussed in some detail the two major theories of the formation of OB associations in the Milky Way: the monolithic collapse theory, and the fractal theory of SF. These two theories predict different formation scenarios for the appearance of stellar (OB or T) associations. In the first case, an association is simply a dynamical stage of young star cluster evolution, caused by gas expulsion driven by massive stars evolution. In the second case, associations are the direct product of SF and therefore they reveal the ISM density peak distribution prior to the onset of the SF process. I then described in some detail observational material on three recently studied OB associations, namely Cygnus OB2, Vela OB2, and Scorpius OB1. Multi-colour photometry are typically combined with Gaia DR2 data (parallaxes and proper motion components) which are allowing the investigation of stellar OB associations with much more detail than in the pre-Gaia era. In all three cases the observational data outline the high complexity of these associations. Several stellar groups are found with different spatial, kinematical and age properties. Two groups are found in Cygnus OB2, six groups in Vela OB2, and nine groups in Scorpius OB1. Interestingly, in these cases ages, spatial positions, and kinematics combine in very diverse fashions. Based on these preliminary results it is tempting to conclude that SF occurred in a highly structured ISM, and involved molecular clouds with widely different masses and differently distributed (in number and mass) pre-stellar cores. These are of course preliminary conclusions, since many more OB associations await such detailed studies in the future. In any case, these studies show the impressive impact that Gaia is providing in the investigation of SF and OB associations in the Milky Way. It is anyway recommendable to couple at some point Gaia data with precise radial velocity measurements of a sizeable number of stars, as the Gaia-ESO (Randich et al., 2013) survey has been partially doing. Only in this way, in fact, a real 3D kinematical study can be performed, and much more solid conclusions can be drawn on the origin and early evolution of stellar associations.

I sincerely thank the organisers of the XVI LARIM conference for inviting me in Antofagasta to deliver this review. I also express my gratitude to Lidia Yalialeva (Lomonosov Moscow University, Russian Federation) and Rubén A. Vázquez (Universidad de La Plata, Argentina) for their help in the assembly of this material.