Saturday, 27 September 2014

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.

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