Back in business

It's been a long and busy summer, hence my absence from this blog. I've had quite a few conferences to attend, as well as the necessary presentations at those conferences that I've had to prepare.

However, that's all over now, I've got lots to tell you about from my busy summer, and it's also the first Gaia data release this week, so hopefully they'll be lots of interesting science coming out of that. Stay tuned!

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!

The new generation of multi-object spectrographs

Last month I visited Barcelona for a short science meeting to discuss plans to use a new multi-object spectrograph currently being constructed. The instrument is called WEAVE, which stands for WHT Enhanced Area Velocity Explorer, and its going to be constructed on the William Herschel Telescope (WHT) in La Palma.

The William Herschel Telescope in
La Palma (Credit: ING)

The name of the instrument comes from the fact that one of its main goals will be to measure the velocities of stars over large areas, covering millions of stars over huge swathes of the sky. In doing so it will complement a lot of the work that Gaia is doing to measure the distances to stars and their proper motions.

The meeting was very productive and there's lots of interesting science that we're planning to do with this instrument. However, this isn't the only multi-object spectrograph (so-called because they can obtain spectra of multiple objects at the same time, thus making them very efficient instruments!) being constructed at the moment.

In addition to WEAVE there is also 4MOST for the VISTA telescope, MOONS for the VLT, and PFS for the Subaru telescope. There are also plans to build a multi-object spectrograph for the upcoming European Extremely Large Telescope (the E-ELT) called MOSAIC. This won't be constructed for almost a decade, but once operational it'll allow scientists to obtain spectra of hundreds of stars in other galaxies, allowing us to study how stars and star clusters evolve in other environments.

Each of these new instruments will be able to take spectra of hundreds of stars at the same time, allowing scientists to efficiently study the properties of lots of stars. This sort of approach is critical for building up large samples with which to do exciting science. I'm really excited to be part of one of the teams that hopes to do this, there's going to be lots of wonderful science to do!

How do you form a massive black hole binary? (Part 3)

This post brings to an end a three-part series of posts about how massive black hole binary systems form, such as those recently observed to merge by the LIGO gravitational wave detector. The first two posts discussed how such massive black holes could form and how a binary system of black holes could form in isolation. Here I'm going to talk about a much more efficient way to form binary black holes, and that's in a star cluster!

As we discussed last week, forming a binary black hole in isolation is very hard. There are lots of processes that could disrupt the binary system before both stars form black holes, and once the binary has fallen apart it's hard to put it back together. But not so in star clusters! In star clusters there are lots of (gravitational) interactions between stars (or black holes) that, under the right circumstances, can lead to the formation of new binary systems. In fact there have been many simulations to explore exactly this sort of process!

The dense and massive star cluster R136 in the 30 Doradus nebula. The bright blue dots are predominantly massive
OB-type stars, many of which will end their lives as black holes (Credit: HST).

The basic idea is a simple 3-step process first put forward by Sigurdsson & Hernquist in 1993. The first step is that star clusters are full of massive stars, and these massive stars are heavier than normal stars and so they naturally sink to the centres of the clusters (we observe this process, called mass segregation, in many star clusters). The second step is that once these stars reach the ends of their lives they explode as supernovae and leave behind stellar remnants such as black holes. This we also know to regularly take place.

Simulation showing the development of mass segregation in a star cluster after only 2 Myrs (right)
compared to a lack of mass segregation at the beginning of the simulation (left). The massive stars (big circles)
are clearly more centrally concentrated in the right-hand panel (Credit: Marc Freitag).

The third step, which is where the clustered environment of the star cluster is critical, is that close encounters between stars (or black holes) in the centres of star clusters can quickly lead to the formation of binary systems, and therefore potentially black hole binary systems. Outside of star clusters the density of stars in space is very low and close encounters are very rare (which is fortunate for our Sun and solar system otherwise we'd always be bumping into other stars!). But star clusters are dense and the stars within them are moving very fast, meaning that close encounters between stars are common. If the stars come close enough to each other in just the right way then a binary system forms.

Many studies have confirmed this general picture and have estimated that it should take (on average) about 1 Giga-year (1000 million years) for enough interactions to occur between stars and black holes to form a binary black hole system in a dense cluster. This means that such binary black hole systems are most likely to be found in older clusters, such as globular clusters, though it shouldn't be impossible to find them in younger clusters.

Does the Omega Centauri globular cluster house
a central black hole? (Credit: ESO)

There is good evidence that this happens quite often. A recent study of the massive globular cluster Omega Centauri found that the dynamics of its stars suggest a particularly massive central black hole, while another study even found evidence that there might be a black hole within the nearby Orion Nebula Cluster. Confirming these theories is difficult because directly detecting black holes isn't always easy, but the evidence is there to suggest they might be occupying many of our favourite star clusters!

Further dynamical interactions within the star cluster can lead to two important effects for the binary black hole system. The first is that the binary will shrink over time due to dynamical interactions with individual stars - this brings the black holes in the binary closer together, making an eventual merger more likely. The second effect is that these black hole binary systems can be easily ejected from the star cluster, often at quite high velocities, through interactions with other binary systems.

The most likely clusters to produce binary black hole mergers are intermediate age clusters with ages of a few Giga-years. Younger clusters are unlikely to have had enough time to form the binary black hole systems, while older clusters will have formed them but they might have all merged by now.

This process implies that a typical star cluster, which contains many massive stars, will easily produce many black holes, and most likely a few black hole binary systems that will one day merge and release gravitation waves just like those observed in September.

Given how many star clusters there are in the Universe its hardly surprisingly that such a black hole merger was observed so easily!