Study of Stellar Clusters Containing Massive Stars

Costado, Teresa; Alfaro, E. J.; Delgado, A. J.; Djupvik, A. A.; Maíz Apellániz, J.
Massive Stars: From α to Ω, held 10-14 June 2013 in Rhodes, Greece; Online at, id.61 (2013).


Most stars form in clusters, but the percentage of stars born in dense stellar systems is currently matter of controversy and depends very much on the own definition of cluster. The cluster definition and hence the morphologies of individual clusters appear to vary significantly from region to region, as well as with age, which suggests that either, star formation in clusters is not universal and may depend on the local environment, or that all clusters form with the same morphology but early dynamical evolution quickly modifies the structure of the phase space distribution.

In addition, young populated clusters containing massive stars are excellent labs for the study of the formation of the massive stellar component of the Galactic disk. Three main scenarios have been proposed for the formation of high-mass stars (M > 7-8 M_{⊙}): a) monolithic collapse of proto-stellar nuclei; b) competitive accretion inside the proto-cluster molecular cloud; and c) coalescence of proto-stellar nuclei and low-mass stars in very dense atmospheres.

Both scientific questions: a) cluster formation and b) formation of high mass stars in clusters are intimately connected via the structural description of the phase space distribution of cluster stars and their Mass Function (MF).

Models of static clusters with different initial spatial and kinematic distributions show how the spatial distribution dynamically evolves with time, allowing a characterization of their dynamical state from snapshots of their spatial distribution. Four are the main variables (and their distribution with mass and position) needed for a reliable characterization of the cluster dynamical state: a) Mass segregation parameter; b) Mapping of surface density for different ranges of masses; c) Q morphological parameter based on the minimum spanning tree graph and its variation with mass and cluster age, and d) MF of the cluster members.

Two years ago, the Stellar System Group of IAA has begun an observational programme of stellar clusters containing massive stars, whose main objective is to answer the two scientific questions. In order to perform this study we need to measure the main physical variables of the cluster, determine cluster membership and mass and age of cluster members. The obtaining of this is best carried out through determination of the Spectral Energy Distribution (SED) of stars in the field, from multiband UBVRIHα + JHKs photometry. In this conference, we want to present some results for clusters currently under study.