• Rose Waugh

Let's talk about stellar prominences (long read)


Prominences on the Sun:

Those of you who follow me on Instagram have probably heard me refer to “prominences” as “stellar clouds”. Here’s a post with a little more detail on the physics of prominences.

photo credit: NASA SOHO

Prominences were first discovered on the Sun in the 14th century during a solar eclipse. “Prominence” is the term used to describe regions of cool, dense material in the Solar Corona (atmosphere) that get trapped by the magnetic field lines. Material from the surface of the Sun is blown up the “footpoints” of magnetic loops or “flux tubes” and collects at the summit of the loops, shown in the cartoon below.

picture credit: www.rosefpwaugh.com


On the Sun, prominences can also be called “filaments”, which is the chosen term when the prominence occult’s (is in front of) the Solar Disc. See the beautiful picture below.

photo credit: Leonard Mercer

Prominences on other stars:

Prominences also exist on other stars, but are much more difficult to spot since other stars are so far away from us and appear only as points of light in the sky. To see a prominence on a star other than the Sun, the prominence needs to pass in front of the star and block out enough light to be noticeable. This isn’t a small ask. Below, on the right hand side, is an example of the "photographs" we have of prominences on another star (adapted from Dunstone et. al, MNRAS 365(2), 2006):

picture credit: data from Dunstone et al 2006 and drawing from www.rosefpwaugh.com

It isn’t quite the same as a beautiful photograph of the Sun, but is equally exciting to a nerd like me! Observers watch for the prominence to pass in front of the star and block out light (for any astronomy nerds out there, they usually observe in the Hydrogen-alpha line). Because stars rotate, the emitted light has a different wavelength depending on where on the surface it has come from and this means that when observers look for one type of emission like H-alpha (which produces light only in one wavelength) they in fact see a range of wavelengths rather than the one wavelength that is actually produced (there are some doodles below to help visualise this...)

picture credit: www.rosefpwaugh.com

(A star that is rotating, now the light that leaves that star all over the surface with the same wavelength will not have the same wavelength once it reaches us. Instead, we'll see a distribution of wavelengths.)

picture credit: www.rosefpwaugh.com

(star that is rotating, now light that leaves that star all over the surface with the same wavelength will not have the same wavelength once it reaches us. Instead, we'll see a distribution of wavelengths.)

Light from the centre of the stellar disc comes directly towards us. Light from the side of the star that is rotating towards us gets a small kick, meaning its wavelength is shortened so that it looks bluer. Light from the side of the star rotating away from us will get its wavelength lengthened, so it looks redder. Overall this leads to the distribution of wavelengths observed for a particular "emission line".

This is called “Doppler broadening”; the rotation of the star causes the wavelength of a line to become a distribution rather than a single wavelength. If a prominence is passing in front of the star, this distribution will have a dip in it, since the prominence is blocking out some of that light. Where in the distribution the dip is, tells us where on the star the prominence is:

picture credit: www.rosefpwaugh.com


Let's look at the observational data again (I don't say that very often!). What is the graph showing? Well, the horizontal axis shows the position on the stellar disc - the left-hand side is the velocity of the edge of the star moving towards us, so this part of the graph shows what is happening at this edge of the star. The right-hand side shows us then what is happening on the edge of the star moving away from us. The vertical axis is time (or phase). Prominences can be seen passing from one side of the star to the other as the dark lines, that I've highlighted in red – something is blocking out the light from the star and moves from one side to the other over time. We can deduce how fast the prominence is moving from this, and therefore how far away it is from the star's surface. We can also predict how massive they might be.

picture credit: data from Dunstone et al 2006 and drawing from www.rosefpwaugh.com


The observations I’ve shown here, and the stars I work on in my research, are stars that are “young-suns”. This means they are similar in mass (and therefore size) to the Sun, but they’re a lot younger and rotate very quickly. The Sun does a full rotation about its own axis in about a month (measuring from its equator) whereas these stars rotate in a few days, or sometimes less than a day! These stars can be good indicators about the Sun in its youth (maybe we could talk about this in more depth another time?).

From observations of these prominences, we’ve learnt that prominences are much bigger than those on the Sun (10-100 times the mass) and they tend to collect very far out from the surface of the star. This depends on how fast the star is rotating but for these stars typically, a few stellar radii above the surface (so on the order of 10^8 m). For the Sun, prominences are a few thousand km above the surface (10^6m). Prominences on these young-suns are monsters when compared to the Sun now.


How do we know it’s a prominence and not something else?

Excellent question! I’m glad you asked! Well whilst they form very far away from the star compared to solar prominences, they’re not far enough away to be a planet. Is that what you had in mind? Take Mercury, it lives 5.8x10^10 m away from the Sun. That’s 100 times the distances we’re seeing. So no, not likely to be a planet. We would also expect to see a planet over and over again but prominence signatures disappear. The most likely explanation is a clump of stellar material that is somehow trapped at these distances.

Speaking of which, how are they trapped?

Well, the magnetic field of these stars is very strong. The prominences are held in place by strong magnetic loops, that support the cloud and stop it being flung out into space. Think of twirling a sock of porridge around your head - the porridge wants to escape but the sock, that you are holding onto, is preventing it from doing so. The magnetic loop is fixed at the surface of the star and as the star rotates, so too does the magnetic loop. This means the prominence that is trapped within is forced to rotate with the star and doesn’t fly off… except for when they do (see the last section!)

The science behind how the prominence is trapped in the magnetic field goes like so; prominences are made from hydrogen plasma that is blown into the loops from the stellar surface. The material cools and forms into a cloud of mostly neutral hydrogen (H2). Some of the material in the cloud is still ionised though (H+ and e-). Charged particles are trapped by magnetic field lines – Maxwell tells us that charged particles experience a force that makes them gyrate around field lines in a helix shape:

picture credit: www.schoolphysics.co.uk

So these charged particles in our cloud can only move along field lines and not perpendicular to them, which means they can’t cross the magnetic field line that outlines our cloud so they can’t escape! But what about the neutral particles? Well, they are colliding all the time with the charged particles. In fact, they collide so often that this prevents them from crossing the field line too. This keeps our prominence intact, trapped by the magnetic field and orbiting the star. The fact that we see prominences on these young-suns at really large distances above the stellar surface tells us two things;

1. these stars must have really strong magnetic fields in order to support such large prominences at such large heights from the surface,

2. there must be some closed magnetic loops up to these very large heights. This either means that the magnetic field structure is still "closed" to at these heights i.e. we are still within the stellar corona, or that the magnetic field is "open" i.e. this is within the stellar wind and that magnetic reconnection allows some closed loops to form within the open field.


I'm going to come back to this second point in a later post, so keep your eyes peeled if you're interested in this!


#2020 #prominences #science #research #stars

 
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