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The birthplaces of planets in the massive Cygnus OB2 Association

posted Aug 23, 2013, 9:19 AM by Jeremy Drake   [ updated Aug 23, 2013, 9:19 AM ]
The Cygnus OB2 Association is one of the most extreme and massive stellar associations known in our Galaxy.  It lies only 1.4 kpc away - relatively close on a Galactic scale - and consists of a population of young stars with ages of 2-5 Myr.  We estimate that the total mass of stars it contains amounts to about 30,000 solar masses, including some stars with masses up to 100 times that of the Sun.  It is one of the best locations in which to study the processes of star and planet formation on a massive scale and we are engaged in a large project to study it at different wavelengths, centred on a Chandra X-ray Observatory Legacy megasecond survey.   

One of the key questions for massive stellar clusters is whether or not planets can form within their harsh radiation environments.  Since massive stars are much hotter than the Sun - by factors of up to 10 or more - they pump out much more light and ultraviolet radiation.  This radiation can heat up the disks of gas around lower mass stars in which planets are forming, dispersing the gas much more quickly than it would do otherwise.  It is possible that planet, and planetary system, formation gets interrupted by this evaporation of gas.

Postdoc Mario Guarcello at SAO led a study to find the so-called "protoplanetary disks" in Cygnus OB2 using our own deep optical  observations made with the 10m Gran Telescopio CANARIAS, combined with a Spitzer Space Telescope infrared survey, and data from other ground-based optical and near-infrared surveys. Protplanetary disks glow mostly in infrared light, and can be found by searching for this "infrared excess".  In practice, the excess for most disks is rather faint, and requires an extremely detailed and careful analysis to form a reliable census.  The study uncovered a total of 1843 stars with surrounding circumstellar disks.  The distribution of these stars was also revealed to be quite clumpy, as seen in the image reproduced here, suggesting the Cyg OB2 region is an association comprising several fairly distinct groups of stars.  The next stage in this study will involve comparing the populations of stars with and without disks in order to assess the impact of massive stars on the disk population. Preliminary indicators point to a fairly low fraction of stars with disks close to the massive UV-bright stars, suggesting a difficult environment for planet formation.  This work was published in Volume 773 of the Astrophysical Journal on August 2 2013.