Tuesday, 8 November 2016

What sort of environment do stars form in?

Last week we talked about the initial spatial distribution of young stars and how their distribution follows that of the dense gas in molecular clouds. But we also know that stars form in groups with a wide variety of sizes and densities, which astronomers think is really important for determining the type and sizes of the star clusters that form.

Distribution of young stars in the Perseus Molecular Cloud
(red, green and blue dots) projected against the gas
distribution (Credit: Evans et al. 2009)
The image on the right shows the distribution of young stars across the Perseus Molecular Cloud. These young stars were all detected by the Spitzer Space Telescope, an infrared telescope that was particularly effective in detecting young stars due to the copious amounts of infrared light they emit.

The molecular cloud is very elongated, as the image clearly shows, but even within that elongated structure the young stars are not evenly distributed, they're clumped into groups. Many of these groups represent the well-studied embedded star clusters typically found in molecular clouds, such as IC 348 and NGC 1333.

In addition to these dense and compact clusters there are also smaller groups, such as the clumps of young stars labelled B1 and B5, as well as numerous young stars that appear relatively isolated.

It appears that while young stars do like to form in groups, there are almost as many young stars that form alone - so is there a typical group size and density that stars form in? And if so, what is it?

One way that astronomers have attempted to tackle this problem is to study the distribution of densities that stars are forming at. To do this astronomers have measured the density of stars surrounding each young star. The distribution of densities is usually referred to as the surface density distribution of young stars.

The figure below shows such a distribution compiled from Spitzer Space Telescope observations of numerous nearby star forming regions. Mid-infrared observations from the Spitzer Space Telescope were chosen for this because it allows astronomers to peer deep within molecular clouds and hopefully identify all the young stars that are present. Hopefully this means no stars were missed!

The surface density distribution of young stars (both Class I and Class II young stars) identified from
Spitzer Space Telescope observations (Credit: Bressert et al. 2010)

The figure shows the fraction of stars born at various densities, from low densities on the left (surface densities of 1 star per square parsec) to high densities on the right (hundreds to thousands of stars per square parsec). The former represent stars that have formed in relative isolation, while the latter represent stars that have formed in dense groups or clusters.

Most notable in this figure is the fact that there is a smooth distribution from low to high densities, which suggests that stars don't just form at low and high densities (in isolation and in clusters), but at a wide range of densities, with groups and clusters existing over a variety of densities.

This is important for our understanding of star formation because it tells us about the conditions under which stars form, as well as the sort of environment where planetary systems form. A planet forming in a dense cluster faces very different conditions compared to one born around a relatively isolated star. In a dense cluster there could be multiple interactions or collisions between stars and planets, as well as a very powerful radiation field due to the close proximity of so many other stars, which could damage a forming planet's atmosphere.

Hopefully as we start to learn more about the various types of planetary system that exist, and especially once we start studying the atmospheres of these planets, we can hopefully address the question of what impact the birth environment has on a forming planetary system.

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