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.

The Serpens South Cluster

Continuing our series on nearby star forming regions that are interesting and important to the history of astronomy I want to turn this week to one of the most recently-discovered regions, the Serpens South Cluster.

Serpens South is not as famous as some of the other nearby star regions such as Taurus and Ophiuchus, predominantly because it was only discovered in the last decade. The cluster is very faint in the optical part of the spectrum because it is still heavily embedded within its molecular cloud, so it wasn't seen by previous surveys that were predominantly performed in the optical part of the spectrum. Furthermore, because the Serpens and Aquila regions of the Galactic plane appear relatively unpopulated in young stars, astronomers hadn't studied the area in much detail.

The vicinity of the Serpens Molecular Cloud as
seen on inverted Deep Sky Survey plates.  The
presence of the molecular cloud is seen from its
obscuration of background stars (Credit: Eiroa et al. 2008).

The wider region first came to prominence in the 1970s when a dark cloud was discovered in the vicinity of the bright star VV Ser near the Aquila Rift (a dark cloud of gas and dust that extends along the Galactic plane in this area of the sky). The image to the left shows the night sky in the vicinity of the Serpens Molecular Cloud where its presence can be seen by the lower density of stars. The density of stars appears lower towards the molecular cloud because dust in the cloud obscures the starlight from stars behind the cloud.

A number of small bright nebulae were identified in the area at this time, including Sharpless 68 and the Serpens Reflection Nebulosity, both illuminated by nearby bright young stars.

South of this region lies the Westerhout 40 (W40) HII region, a modest cloud of ionised gas thought to be at a distance of 1500 light years (500pc). The HII region is the visible part of a larger star forming region where stars of all masses are currently forming including O and B-type stars, making this one of the nearest regions where O and B stars are in the process of forming.

Map of the dark clouds in Serpens that form part of the Aquila Rift. The main part of the cloud
is shown in the grey rectangular box, which includes the W40 region, the Serpens South Cluster
(the white star), and the HII region Sh2-62. The Galactic Plane can be seen across the bottom-left
corner of the image. The earlier image covers the region around Serpens Main and Serpens
NH3 at the top of the image (Credit: Bontemps et al. 2010).

When this area of the sky was observed by the Spitzer Space telescope in 2006 astronomers discovered a cluster of stars previously unknown, highly embedded within the molecular cloud and visible only to infrared telescopes such as Spitzer. The cluster is very close to the W40 HII region, as can be seen on the map of the region shown above. It was soon dubbed the Serpens South Cluster, and since then it has been the focus of considerable study.

The Serpens South Cluster as seen by the
Spitzer Space Telescope
(Credit: Spitzer/NASA)

The kinematics of the gas associated with the cluster are very similar to the gas surrounding W40, suggesting that the two structures are part of the same star forming complex and are likely at the same distance. A distance of 1500 light years is also in good agreement with that recently obtained from radio parallax measurements.

Early studies, primarily with the Spitzer Space Telescope (see image to the right) uncovered a cluster of about 50 stars, of which at least 35 were still in the process of forming, suggesting that the cluster is very young. Later studies in the far-infrared with ESA's Herschel Observatory detected even more highly embedded sources at even earlier evolutionary stages, providing evidence for even younger protostars still in the process of collapsing to form stars.

Given its youth, the density of the cluster is very high, with at least a few hundred stars per square parsec on the sky. This suggests that either the stars formed in a very dense and clustered state (as we see them now) or that clusters like this can form very quickly out of stars that form in a low density distribution. This is one of the key questions astronomers are trying to answer when they study young star clusters.

The infrared observations have also revealed an intricate network of filaments emanating from the cluster with a hub-like morphology. These filaments are thought to play an important role not just in how stars form and build up their masses, but also in how star clusters grow to their present sizes so quickly. Collisions between filamentary structures in molecular clouds may play a critical role in the formation of such dense clusters.

Studies of young and dense clusters such as Serpens South are important for understanding how young star clusters form and how this is related to the formation of stars within them. It seems that the two processes are critically connected and so to study one we must also study the other!