Rose Waugh
Stellar “prominences” are clouds of coronal plasma, supported above the stellar surface by the stellar magnetic field. On young Suns (rapidly rotating low mass stars) they have been observed to be 10-100 times the mass of Solar prominences, and can be found multiple stellar radii above the stellar surface. Their formation in the tops of closed magnetic loops provides information about the local field structure which is difficult to measure. Thus, understanding how and where these clouds form could prove useful for understanding the field structure in the corona of these stars.
We model the formation sites by prescribing a modified dipolar field structure, that opens out to become radial at the “source surface”. Considering force balance, we find the new loop shapes for cooled loops. These cooled loops are the solutions that support prominences. We show solutions for the prominence loop shapes alongside the background field structure for a set cooled loop temperature on the star AB Doradus. Having found the family of cooled solutions (the full set of solutions), the relationship between height and widths of these loops is used to model the distribution of prominences with distance from the stellar rotation axis. Comparing to observations, the shape of the distribution is consistent – with a peak just beyond the co-rotation radius and a secondary peak beyond the source surface. We find a small peak close into the star which is not seen in the observations - this could be due to their illusiveness as they transit the stellar disc quickly and block out such small quantities of light. Thus, it is difficult to compare this region of the distribution. In conclusion, we find that placing the source surface at 3.7Rstar gives a reasonable distribution of modelled slingshot prominences when compared to the observations, and thus it is reasonable that the magnetic field may become open at this distance.
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On the top right-hand panel the motivation for the work is given. It reads that the clouds (prominences) can help to inform us of the local magnetic field structure (since they only form in close magnetic loops). Understanding the field structure is important for many areas of stellar physics, such as in stellar evolution theories and in understanding stellar-planet interactions. The magnetic field of a star is responsible for its activity and CMEs, prominences, flares etc that are associated with this will have important consequences for planets since they are a part of the planetary ecosystem.
The second panel down then shows the method of this work; prescribing a modified dipolar field structure that has a “source surface” at 3.7Rstar (i.e. the field becomes radial at this point). A plot is shown of this field structure. By balancing the forces present, we are able to find the new shape of a loop that has been cooled from the background field (i.e. its environment).
“The very rapid rotation on these stars means that beyond the co-rotation radius (where an object orbiting the star would stay above the same point on the stellar surface), the gas pressure increases with height – driving gas into the tops of loops.” A plot is shown of this change in gas pressure within a loop, and it can be seen that it decreases with height above the stellar surface until the co-rotation radius, at which point the pressure increases again steeply.
The next panel down shows the results.
1. A plot shows a few examples in blue of these prominence loop shapes solutions and the grey field lines represent the background stellar magnetic field that we prescribed. Cooled solutions can be seen that would be embedded in the open field region, and yet they are closed field lines. These prominences would be found within the stellar wind.
2. A histogram is plotted that shows the shape of the distribution of prominences with height above the stellar surface (our model in dark grey and the observations are shown in light blue, confusingly! for comparison). The distribution shape matches the observations quite well, replicating the peak (i.e. most prominences) just beyond the corotation radius and showing a second peak beyond the source surface. The model predicts a greater many at low heights (could be due to their observational illusiveness as they transit the stellar disc quickly and block out such small quantities of light.) and beyond the source surface than the observed data show however. This suggests the source surface of this star could well be nearby to the value used here.
The bottom right corner of the paper gives a link and QR code to the published paper.
https://research-repository.st-andrews.ac.uk/bitstream/handle/10023/16677/Waugh_2018_MNRAS_Magneticsupport_FinalPubVersion.pdf?sequence=1&isAllowed=y