Formation and evolution of star clusters

(E. Franciosini, F. Massi, S. Randich, G.G. Sacco, N. Sanna)

 

 

Stars do not form in isolation but in clusters composed of a number of siblings ranging from a few tens to several thousands. Understanding the physical mechanisms driving the formation of star clusters and their evolution is fundamental to investigate both the origin and the evolution of our Galaxy and the formation of planetary systems (e.g. Krumholz et al. 2019). Despite the relevance of this topic in contemporary astronomy, several issues are still open. It is not clear what are the initial conditions of young stellar systems. Several authors suggest that most of the stars form in very dense and massive clusters, where the evolution of protoplanetary disks could be affected by two-body interactions and from the radiation field of high-mass stars. However, recent studies on nearby loose young stellar associations show that stars may also form in low density environment and massive clusters could represent only the tip of the iceberg.

The physical processes driving the evolution of star clusters and their dissolution in the Galactic field is also debated. According to one scenario the energy feedback from high mass stars associated to supernova explosions, stellar winds and radiation pressure is the main driver of the cluster evolution. In particular, the high mass stars by sweeping out the molecular cloud where stars form stop the star formation and make the clusters unbound leading to their dissolution (e.g. Baumgardt & Kroupa 2007). An alternative scenario suggests that the effect of the feedback is negligible and the cluster evolution depends on the stellar dynamical interactions and the effect of the outer gravitational field (e.g. Parker & Dale, 2013; Ward et al. 2020).

 

Kinematics of young clusters and associations

 

In the last few years our group has carried out several observational studies to investigate these issues. In particular, we analysed the structural and kinematic properties of nearby star forming regions using astrometric and spectroscopic data from the ESA space mission Gaia and its associated spectroscopic survey Gaia-ESO (Gaia Collaboration 2016, Randich & Gilmore 2013). The combined use of these two datasets allowed us to study the dynamical evolution of stellar systems in the 6-dimensional phase space and to unveil the presence within the same regions of multiple substructures (Sacco et al. 2015, Sacco et al. 2017, Bravi et al. 2018, Franciosini et al. 2018, Roccatagliata et al. 2018, Roccatagliata et al. 2020). In the next future, we plan to extend our studies to the youngest region of the star clusters, which are still embedded in the molecular clouds and are not visible in the optical band. For these studies we will use MOONS the new multi-object spectrograph at the Very Large Telescope (Cirasuolo et al. 2011), that will operate in the infrared bands, and therefore will allow us to penetrate dusty clouds.

 

Fig. 1: Relation between radial velocities, parallaxes and proper motions in the young clusters Gamma Velorum (taken from Franciosini et al. 2018, A&A, 616, L12).

 

 

The role of massive stars  

 

Another debated topic concerns the relation between young stellar clusters and high-mass stars. Do all massive stars form inside clusters or are they able to form in isolation, as well? Are massive stars the last to form in a cluster, then inhibiting further star formation through feedback? Or does their feedback trigger star formation in nearby areas thus originating a sequence of stellar populations? We have been carrying out multi-wavelength observations of young stellar clusters hosting massive stars from the optical to sub-mm to study the evolution of stellar clusters in their earliest Myr of life. An example is shown in Fig. 2, where the young stellar clusters Pis 24 is clearly shown in the near-infared image and the cold dust condensations possibly nursing the next generation of stars are evident in the submm image. The feedback from the high-mass stars of Pis 24 is eroding the molecular gas in the north, but may be responsible for triggering star formation further north.

 

 Fig. 2: Near-infrared (K) image of the young stellar cluster Pis 24 (left) and the submm emission (0.85 mm) from cold dust cores embedded in the parental gas (right), possibly nursing the next generation of stars in the region (taken from Massi et al. 2015, A&A, 573, A95 and Brand et al., in preparation).

 

 

REFERENCES:

 

Baumgardt & Kroupa 2007, MNRAS, 380, 1589

Bravi, L. Zari, E., Sacco, G.G., et al. 2018, A&A, 615, A37

Cirasuolo, M., Alfonso, J., Bender, R. et al. 2011, The Messanger, 145, 11

Franciosini, E., Sacco, G.G., Jeffries, R.D., et al. 2018, A&A, 616, L12

Gaia Collaboration, 2016, A&A, 595, A2

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Parker, R.J. & Dale, J. E., 2013, MNRAS, 432, 986

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Roccatagliata, V., Sacco, G.G., Franciosini, E., et al., 2018, A&A, 617, L4

Roccatagliata V., Sacco, G., Franciosini, E., et al. 2020, A&A, 638, A85

Sacco, G.G., Jeffries, R. D., Randich, S. et al. 2015, A&A, 574, L7

Sacco, G.G., Spina, L., Randich, S. et al. 2017, A&A, 601, A97

Ward, J.L., Kruijssen, J.M.D., Rix H-M., 2020, MNRAS, 495,663