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.

Planet-forming discs around young stars truncated by close encounters in Orion

The protoplanetary disk of HL Tau, as observed by the
ALMA Observatory. Discs like those would be continually
eroded by close encounters in dense star clusters like
the Orion Nebula Cluste (Credit: APOD)

Astronomers have for a long time suspected that the environment in which stars form could very strongly influence the formation of planetary systems. A recent paper by Simon Portegies Zwart at the Leiden Observatory shows this to be case and reports results of simulations that explore this effect. The simulations show that close encounters between stars in a dense cluster can truncate the protoplanetary discs around stars where planetary systems are thought to form. His simulations are able to reproduce the properties of protoplanetary discs in the nearby Orion Nebula Cluster, suggesting the stars and discs in this cluster have already felt the impact of their crowded environment. These interactions will not only lead to an erosion of the disc (leaving less material available to form planets) but also reduces the orbital radii at which planets can form, impacting the types of planetary system that might form.

Improving our census of star clusters in the Large Magellanic Cloud

A new paper published last week by Romita et al. presents results of a search for new star clusters in the nearest galaxy to the Milky Way, the Large Magellanic Cloud (LMC).

The LMC is a dwarf satellite galaxy currently orbiting our galaxy that has a number of key differences to the Milky Way. For a start the LMC is much smaller than the Milky Way, but critically it is also less chemically evolved, meaning that it has fewer 'metals', which is the name astronomers give to anything other than hydrogen or helium. By studying the distribution of star clusters in this galaxy we can try to understand whether star formation and the evolution of star clusters has proceeded any differently in this environment compared to in our own Galaxy.

The authors have targeted a 1.65 square degree area of the LMC that includes the massive star forming region 30 Doradus, the largest region of star formation in the LMC, and larger than anything in our own galaxy. It's a rich field of star formation as the images below show, and a good place to be hunting for new star clusters.

Images of the area of the LMC studied in this paper. On the left is a colour image compiled from the infrared observations used in the study, while on the right a black and white image is marked with the positions of the newly-discovered star clusters (red dots) relative to the positions of known molecular clouds (black ellipses). (Credit: Romita et al. 2016)

Using infrared images of the LMC the authors identify 65 embedded star clusters, 45 of which are new discoveries. Using their observations the authors are able to estimate the sizes, masses and luminosities of these clusters, all key properties of star clusters.

The authors compare the distribution of these star cluster properties with their distribution in our own Galaxy, and find that the LMC clusters are generally larger, more massive, and more luminous. Since these three quantities are often well-correlated with each other, it's not a surprise that all three properties are bigger in the LMC, but this does clearly show that LMC star clusters are typically more massive than those in our galaxy.

The authors also find the density of clusters in the LMC is 3 times higher than in the Milky Way, and that the mass of clusters in this area of the LMC is 40 times higher than an equivalent area in out galaxy. Both these results suggest that the LMC is producing star clusters at a much higher rate than in our own Galaxy.

These two results are actually linked. If a galaxy is forming more stars and producing more star clusters then it is likely that it will, on average, produce larger and more massive clusters than a galaxy that it is forming fewer star clusters. It is clear that the LMC is very actively forming stars and clusters at the moment.

However the authors note that this shouldn't surprise us because the LMC contains many more molecular clouds than the Milky Way, and since stars form in molecular clouds then more molecular clouds should mean more star clusters! They find that both galaxies display the same relationship (known as the star formation rate scaling law) between the amount of dense gas and the amount of stars (and star clusters) that are forming.

This means that while the environments of the two galaxies may be different, the star formation process that takes place within them isn't. We can therefore take what we've learnt about star formation in the Milky Way and apply it to other galaxies. This is an important step forward for understanding star formation across the Universe!

Active star formation in the North American and Pelican Nebulae

Today I thought I'd share some new astronomy results from a recently-published paper I read today. The paper is called "Outflows, Dusty Cores, and a Burst of Star Formation in the North America and Pelican Nebulae" by John Bally and collaborators at the University of Colorado. You may be familiar with the North American (NGC 7000) and Pelican (IC 5070) Nebulae (so-named because they resemble the outline of the North American continent and, supposedly, a pelican), they are two of the most famous night sky nebulae, and visible with a pair of binoculars in the constellation of Cygnus.

The North American (left) and Pelican (centre) Nebulae
(Credit: AstroArn Photography)

These two nebulae are actually part of one much larger complex called W80. Though they might appear to be separated into two (or more regions), this separation is actually caused by an obscuring lane of dust (sometimes referred to as the 'Gulf of Mexico') that lies slightly in the foreground compared to the bright emission nebula (dust absorbs and scatters optically visible light as seen in the image above). The obscuring effect of this dust makes the single large nebula appear like two moderately-sized nebulae.

When seen at infrared wavelengths the absorbing lane and the distinctive shape of the nebula disappears. This is because infrared light is less absorbed by interstellar dust, allowing us to see the true shape of the nebula.

The North American Nebula in infrared light
(Credit: NASA's Spitzer Space Telescope)

The authors of this paper use a wide variety of images of these nebulae taken with different detectors and cameras, and using a variety of filters to capture light at certain wavelengths. This is a common practice in astronomy because some important physical processes only emit light at certain wavelengths, and so imaging a nebula at that wavelength can show you where those processes are taking place. The North American Nebula and Pelican Nebula are most well known from images at optical wavelengths, but these authors also used data at near-infrared wavelengths (including images at 2.12μm, which highlights emission from shocks between colliding gas flows, as shown in the image below) and sub-mm wavelengths (images at a wavelength of 1.1mm).

Infrared 2.12μm image of the Pelican Nebula showing some of the
 shocked regions identified (Credit: Figure 6 from Bally et al. 2014).

Using this data they identified hundreds of shocked regions (see the example above) where flows of gas are colliding. These gas outflows originate from stars that are still in the process of forming and are known as 'Herbig-Haro objects' (named after their discoverers, George Herbig and Guillermo Haro), and can therefore be used to trace active sites of star formation. The authors discovered over 50 such objects, allowing them to trace star formation across the region.

The authors then use their sub-mm images to measure the mass of gas and dust in each of the star-forming clouds that they are studying. This can be done by adding up all the sub-mm light coming from each region and performing some simple calculations.

The authors also introduce a new 'activity index' designed to provide an indication of the level of star forming activity in the cloud. They do this by adding up the number of shocks and outflows in each cloud, and dividing this by the mass of the cloud. This index gives an idea of how much star formation activity is taking place in each cloud, compared to how much star formation it could potentially support (which is dictated by the amount of gas mass in the cloud). A high index shows that there is a considerable amount of star formation taking place in the cloud (compared to that which it could support), while a low value suggests a low amount of star formation (possibly suggesting star formation has yet to start). This is an interesting metric similar to the idea of a 'star formation efficiency' that is often used in large-scale studies of star formation.

You can read about this in more detail by checking out the paper, which is free to read on the astronomy pre-print server arXiv.