# How to estimate/calculate stellar wind from a star?

#### AotrsCommander

How do you estimate/calculate how much solar wind a star produces? (Can we even make a passable attempt?)

I'm trying to work out whether a fictional exoplanet in orbit around R Coronae Borealis would need a magnetic field (as there appears to now be some debate on a cursory google search on how much effect one has), and if so, whether it would need to be stronger or weaker than Earth's. (The planet is orbiting at a distance of roughly 104AU.)

I sort of assume that solar wind would have the same reduction in effect as the stellar flux over distance... But I have no idea whether that's true or not, or what we know (if anything) of the theorhetical stellar wind an RCB yellow supergiant might produce; whether it would be considerably more (if in proportion to the stellar flux, for example, it would be about 80ish% more than Earth's at that distance... presumably requiring a proportionally stronger magnetic field) or less or whether it's just a function of output verses distance or whether the type of star also has an effect.

Can anyone throw any light on the subject for me, please?

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#### EpicAll Rounder

How do you estimate/calculate how much solar wind a star produces? (Can we even make a passable attempt?)

I'm trying to work out whether a fictional exoplanet in orbit around R Coronae Borealis would need a magnetic field (as there appears to now be some debate on a cursory google search on how much effect one has), and if so, whether it would need to be stronger or weaker than Earth's. (The planet is orbiting at a distance of roughly 104AU.)

I sort of assume that solar wind would have the same reduction in effect as the stellar flux over distance... But I have no idea whether that's true or not, or what we know (if anything) of the theorhetical stellar wind an RCB yellow supergiant might produce; whether it would be considerably more (if in proportion to the stellar flux, for example, it would be about 80ish% more than Earth's at that distance... presumably requiring a proportionally stronger magnetic field) or less or whether it's just a function of output verses distance or whether the type of star also has an effect.

Can anyone throw any light on the subject for me, please?

Stellar Flux and solar wind would become less concentrated as it 'expands' out, so a star several light years away would have very little of its solar wind hitting earth. A magnetic field from a planet there could pick up some of that material and fling it another direction, (this is also a technique used for exoplanet hunters) so the amount of solar wind detected would be quite variable. So if you could take solar wind sensitive images of that star to see how much solar wind is hitting earth, you could determine if the planet has a magnetic field or not.

I hope I helped with this question!

#### AotrsCommander

I've basically had to give up on getting anything other very loose estimates on this, unfortunately - there just isn't enough data around (and what maths I did find was beyond me). Using the inverse square law, I estimated from the distance the compartive strength to Sol based on the luminoisty (which was about 2 Sol at that distance and luminosty of R Coronea Boralis, and I managed to find an equation for mass loss from a star, which gave me about a hundred times. I think, unfortunately, that's about as close as I can get, being a layperson. I thus have assumed the planet would need a magnetic field of two plus times Earth's and then looked at how to generate a magnetic field on a tide-locked planet. (Short answer is, you basically can't, without inventing some form of unobtainium. So I'm having to more or less abandon that bit of hard physics.)

#### Ken G

Gold Member
Bear in mind that there are many types of stars, including our own Sun, that we do not know how to calculate the strength of their winds, we can only observe it (in some cases). Scaling to the stellar flux is one thing you can do, but it's extremely uncertain-- it would be a bit like trying to predict the length of someone's hair by scaling to their height. All else being equal, that scaling would work, but there are many other factors that go into how long someone's hair is-- and many other factors that go into a stellar wind. This is especially true of R Cor Bor, which has a history of eruptions in which its wind got spectacularly more dense than it normally is (see http://en.wikipedia.org/wiki/R_Coronae_Borealis). This situation is made even worse by the fact that it is not even known what the basic structure of R Cor Bor is, it's a real oddball of a star! But since R Cor Bor's wind is strong (and variable), perhaps there are observed estimates on the strength of its wind, and how it has varied over time. One thing is certain, it has a really whopping wind, relative even to its radiative flux, when it is doing an outburst-- you could use that in your story.

#### AotrsCommander

Perils of picking R Cor Bor, I think...! In some ways, the lack of information works for me (it's a little easier to blag this particular has stayed in its current state longer and burns "slower" (because Exotic Materials...1) so that the periods of dimming last centuries, not months (which is basically the central conceit) when we don't exactly know how it works), and others... Not so much!

It's point I had to consider though, since obviously a strong stellar wind (which I think, on balance, seems to be the best indication - though it's perhaps only a guess at how strong, since it seems fairly clear at this point that we don't really know the true answer) would require an equally powerful magnetic field. Which of course is a pretty major concern for any prospective life on the planet - you'd expect, with a strong magnetic field, magnetosense would likely be a much more common sense, for example.

It's just me compounding the issue (because aslo part of the central conceit) in having a tide-locked planet, which makes it somewhat harder to justify a "natural" magnetic field..!

#### Ken G

Gold Member
I think you can pretty much assume any magnetic field for the star, or the planet, that you want-- there is plenty of uncertainty in both.

#### TEFLing

I think there are scaling relations between the mass and luminosity of stars

Perhaps there are also relations between luminosity and mass loss / stellar wind ?

What about age, cp gyro chronology?

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