The complicated world of star formation

Hi, I am Ellis Owen, a 3rd Year PhD Student. I work on high energy astrophysics, looking at the role cosmic rays may have in young, star-forming galaxies in the distant Universe. You can see some of my earlier blog posts on this subject, and how early Galaxies can shape the early Universe, here and here.

Supernova Remnant – a candidate source of cosmic rays. Image credit: SNR 0509-67.5, NASA Hubble Space Telescope & Chandra X-ray Observatory

Cosmic rays are very high energy particles which are thought to originate in the remnants of the violent death-throws of massive stars, called supernova remnants (SNRs). These cosmic rays are observed on Earth and propagate by a diffusion process through the magnetic fields of our Milky Way galaxy. The highest energies of these cosmic rays are able escape from the Milky Way into the Universe beyond. It is anticipated that galaxies other than our own, particularly those which play host to abundant SNR populations, can also host cosmic rays like our own Milky Way.

Very young galaxies in the earlier stages of the evolution of the Universe (those around and just after the Epoch of Reionization, for example) are observed to have relatively high rates of star formation. This translates into a high rate of stellar deaths, and therefore supernovae – indicating that these SNR environments which provide the conditions to accelerate cosmic rays to high energies.

One of my interests is to better understand how these cosmic rays can impact on the processes leading to the formation of stars – both in young, distant high-redshift galaxies as well as our own Milky Way, where aspects of the star forming process are likely to be similar. Since observations of the Milky Way are much easier in the high resolutions required to study small star-forming regions, understanding the processes in the Milky Way first is a key first step towards uncovering the mysteries of star formation in the early Universe and how cosmic rays may influence this (if at all).

To learn more about star formation, I am currently on leave from MSSL and visiting the Institute of Astronomy at National Tsing Hua University (NTHU) in Hsinchu, Taiwan. At NTHU there is a very active observational star formation group which looks at many aspects of the star formation in the Milky Way – ranging from the mechanisms of cloud and core collapse to the formation of protoplanetary disks, as well as the role over-arching considerations like intergalactic magnetic fields may have on these processes.

As charged particles, cosmic rays interact with magnetic fields. As mentioned before, these can be the magnetic fields of their host galaxy, but could also be local magnetic fields in particular regions – such as the dense molecular clouds in which star formation occurs. The charged cosmic rays follow a curved path in a magnetic field. The radius of curvature of this path increases with cosmic ray energy, but decreases with magnetic field strength (i.e. a low-energy cosmic ray in a strong magnetic field will be deflected much more than a high-energy cosmic ray and a weak magnetic field). The magnetic fields in some star forming regions can be much stronger than the mean galactic magnetic field, sometimes by a factor of 1000 or more. This means that even relatively high energy cosmic rays may be influenced by these fields along their propagations.

Hubble space telescope of a star-forming molecular cloud (Carina Nebula, 7500 light-years away). The colours correspond to oxygen (blue), hydrogen and nitrogen (green), and sulphur (red). Image credit: NASA/ESA 2010.

An important consideration is how much the cosmic rays are hampered by these strong magnetic fields in star forming regions as they travel through their host galaxy. If they are deflected relatively strongly, it is harder for them to move through these regions and they can end up being caught up there for an extended time. This makes it more likely they may interact with the matter in the molecular clouds, depositing energy and driving a heating effect. In turn, this heating effect can affect the dynamics of the cloud system and have consequences on the way in which stars can then form, maybe even slowing down or distorting the star-formation process.

So, the first thing to look at is the structure of the magnetic fields in these regions. Take a look at some recent observations with ALMA, for instance, which uses measurements of polarization (resulting from light scattering off the preferentially orientated dust grains along the magnetic field) to trace the magnetic field lines…

Magnetic field shape in the W43-MM1 high-mass star forming region as observed with ALMA. The lines show the magnetic field morphology. Figure credit: Cortes et al. (2016).

It can be seen that there are distinctive ‘lines’ in the image which map out the vector field. Interestingly (and to make the problem more complicated), cosmic rays propagate much more easily along these lines, while it is harder for them to move perpendicularly to them. Overall, the morphology of the magnetic field is very complicated, and is actually governed by the dominating physical processes occurring in the star forming region – this could be the gravitational collapse of the cloud, turbulent or thermal support in the cloud, or propagating shocks (among other things).

Close up view of a collapsing star-forming region. The cloud is collapsing under gravity, and pulling the magnetic fields with it into an hourglass shape.  This example – called NGC 1333 IRAS 4A – is around 980 light-years from Earth, in the direction of the Perseus constellation. Figure credit: Harvard-Smithsonian Center for Astrophysics (2006).

In the case of dominating gravitational collapse (see the picture above), we would expect the magnetic field lines to be dragged inwards along with the partly-ionzied matter, creating an ‘hourglass’ type of shape. After this initial gravitational collapse, the magnetic field strength is increased as the lines are dragged together until the extra magnetic support is able to stop further gravitational collapse. This is particularly effective in the direction perpendicular to the field lines, but not so strong in the direction parallel to the lines…

This essentially means that a second phase of gravitational collapse can occur – this time with an angle preference along the magnetic field lines. The result is a pancake shaped cloud, with the field lines cutting through it in a perpendicular manner.

Sketch to outline the collapse of a magnetised cloud – firstly the cloud collapses freely under gravity creating an hourglass shape magnetic field (not shown here), then collapse occurs along the magnetic field lines into a pancake-shaped cloud with magnetic fields threaded through. A point may then be reached where the pancake shaped cloud can no longer support itself and may fragment and collapse further into smaller, denser cores supported by thermal or turbulent pressure.

The stability of the resulting pancake-shaped cloud can be determined by looking at the magnetic flux to mass ratio, which can tell us whether the density of the cloud is enough to overcome the magnetic support to allow further collapse and fragmentation. Looking at how the collapse along the field lines progresses and how the magnetic field slowly slips out of the cloud over time (via ambipolar diffusion), we can estimate the point at which this pancake can go on to fragment and collapse into many, much denser ‘cores’. It is these cores which will eventually harbour the sites of star-formation.

So, the key factors I hope to look at over the next few months of my visit are how the cosmic rays can be directed along these magnetic field structures found in star-forming regions, how the interactions of these cosmic rays can drive a heating effect to alter the dynamics of star formation, and how much this changes the picture I have introduced here…

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