Thursday, 26 May 2016

The perils of star formation in the vicinity of massive stars

Some collaborators and I have recently had an interesting paper published in which we show that stars born in the vicinity of massive OB stars may be less likely to form planetary systems that stars born further away from such stars.

In our Galaxy stars form in a wide variety of environments, from small regions with only a handful of stars, up to massive star clusters and associations with millions of members. The larger regions are also home to tens to thousands of massive OB stars that emit immense amounts of energetic radiation that can affect nearby stars.

The OB association Cygnus OB2 (Credit: CXO).
One of the questions facing astronomers is whether stars born in the vicinity of multiple OB stars might grow up differently to stars born in regions devoid of such objects. In particular it has been suggested that the radiation from these massive OB stars could erode the disks of material that surround young stars.  These protoplanetary disks are thought to be in the process of forming planetary systems just like our Solar System, so its important we understand how they form and evolve.

The study, lead by my colleague Mario Guarcello, focusses on young stars in the OB association Cygnus OB2, a region home to thousands of massive and luminous OB stars, as well as many less massive stars like our Sun. In the paper they study the spatial distribution of stars with and without protoplanetary disks and find that stars nearer to OB stars are less likely to have a disk around them than stars further away from the OB stars (see figure to the right).

The fraction of stars with disks (y-axis) plotted
against the strength of ultraviolet radiation
from massive stars (x-axis). The fraction of stars
with disks is lower when the ultraviolet flux is
higher (Credit: Guarcello et al. 2016).
This suggests that the OB stars are somehow eroding or destroying the protoplanetary disks around stars in their vicinity, most likely due to the photo-evaporation of material in the disks by the harsh ultraviolet radiation that these stars emit.

This result is very important for our understanding of where planetary systems are forming in our galaxy and what factors are hindering this process. As we start to search for planetary systems in distant star clusters we may find that such systems are rarer, or perhaps have fewer planets in them, than those around stars that aren't in star clusters.

This may also tell us something about where our Sun and its Solar System formed. If protoplanetary disks are eroded in massive clusters and associations, then it is unlikely that our Sun formed in an environment such as this.

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