Most stars are typically born from fragments of matter floating freely in the interstellar space, like space dust or gas clouds. It would seem logical that the bigger the star is, the more dense the original materials for its formation should be. However, it is not always true: seems that very massive stars can be born even in relatively low-density environments.

This is a conclusion of a study published at arXiv.org by a team of scientists from United Kingdom, Switzerland and the United States. The scientists examined substructure and mass segregation in the massive stellar association Cygnus OB2 to explore the original conditions that resulted in the formation of stars forming this object. Data from Chandra X-ray observations was used for the study.

A distinctive feature of Cygnus OB2 is that it is not only one of the largest OB associations in Milky Way galaxy, but also contains a large number of massive stars with masses up to approximately 100 solar masses. It is also located quite close to us on a cosmic scale, only 1.4 kpc away, which makes it an ideal candidate for detailed studies.

The authors of this research selected a sample of young stars from this association based on their luminosity in the X-ray frequency range. The young or pre-main-sequence stars are typically from 10 to 1000 times more luminous in X-rays than main-sequence stars due to enhanced magnetic activity and collisions in strong stellar winds. The scientists calculated main parameters related to spatial, mass and age-specific distributions of Cygnus OB2 stars.

The local stellar surface density analysis has shown that there is no significant mass segregation among the stars from the association. The presence of mass segregation usually indicates concentration of massive stars in denser areas of the cluster, which in turn leads to higher local surface densities compared to the general population. Meanwhile, Cygnus OB2 shows no evidence that its massive stars are distributed any differently compared to the low-mass stars.

These facts lead to an interesting conclusion: Cygnus OB2 has preserved its dynamically young nature and thus retains the imprint of its initial formation conditions, according to the authors. The absence of mass segregation shows that the massive stars in this cluster were never grouped together more closely than could be expected from a random selection of low-mass stars, and they were not formed in a locally overdense regions.

So what are the implications for the theories of massive star formation? The findings of this study contradict the ‘classical’ theories, which claim that massive stars can only be formed in dense massive clusters. It appears that gas clouds with relatively low-density are sufficient to give a birth to stars of up to 100 solar masses and perhaps even beyond that. This also complements a previous study, which stated that super-massive stars can form in isolation from their smaller neighbors.

By Alius Noreika, Source: www.technology.org