Stellar Winds and Their Interaction with the Variable ISM #1 | Brian Wood/Jeffrey Linsky

All right, we can begin now. Good morning, everyone. I'd start by introducing myself. My name is Brian Wood. I work at the Naval Research Laboratory in Washington, DC. So I do both observational astrophysics and observational solar physics. So, in terms of astrophysics, I'm interested in the atmospheres of cool main-sequence stars and in the interstellar medium. And in terms of solar physics, I'm mostly interested in coronal mass ejections, especially observations of CMEs from the STEREO spacecraft.

So, even though I'm currently at NRL, I'm very familiar with Boulder. I was an undergraduate at the University of Colorado, I was a graduate student at the University of Colorado, and later I even had an extended postdoc here as well. So my thesis advisor at the University of Colorado was Jeff Linskey. And Jeff and I have actually worked together on the book chapter and on this lecture. So I'll be giving the first half of the lecture, and Jeff will be giving the second part. So we're going to be talking about stellar winds and their interaction with the interstellar medium.

So my part, I'll be talking about heliospheric and astrospheric evolution associated with solar/stellar wind evolution. And specifically, I've listed four specific things that I'll be talking about: I'll introduce basic heliospheric structure; I'll discuss how we can detect stellar winds and/or astrospheres around other stars; and how we infer wind evolution from the astrospheric data; and if I have time, I'll get into planetary implications for the wind evolution. And if I don't have time, I think other speakers here will be dealing with that topic as well.

So after the break, Jeff will be talking more about the interstellar medium and how ISM variability effects astrospheric and heliospheric evolution. Okay, so the natural place to start when talking about solar winds interacting with the interstellar medium is to discuss our basic understanding of how the wind of our own star interacts with the interstellar medium. The solar wind interstellar interaction defines the large-scale structure of our heliosphere; defines the boundary between the region of space carved out by the solar wind and our galactic environment, if you will. So this is a hydrodynamic model of the global heliosphere. So the solar wind is expanding radially from the Sun's position here. The Sun is moving with respect to the interstellar medium. So the Sun sees a flow of interstellar material from the right in this figure. So the interstellar medium is partly ionized and partly neutral. So the top panel is showing the plasma temperature; in the bottom panel is neutral hydrogen density. So the solar wind expands until its ram pressure drops to that of the interstellar medium, and at that point it is shocked to slower speeds at this termination shock. The interstellar wind also possibly encounters a shock on its way towards the heliosphere. There's actually a vigorous debate in the heliospheric community about whether the bow shock exists or not. I discuss that a bit in the book chapter; I won't talk about that much here. In between the termination shock and the bow shock, there's a third discontinuity called the heliopause. The heliopause divides the plasma flows of the interstellar plasma and the solar wind. So the plasmas of the solar wind and that of the heliosphere and the interstellar medium don't mix. Then interstellar plasma is deflected around the heliosphere like that, and the solar wind is deflected backwards into this long heliotail in the downwind direction.

So there are two spacecraft, two important spacecraft that are directly exploring the characteristics of the global heliosphere. Those are the two Voyager spacecraft, Voyager 1 and Voyager 2, and the rough trajectories are shown on this figure. These spacecraft were launched way back in 1977, back before most of you were even born, and I'm sure. So these are now the most distant man-made objects in the universe. Voyager 1 is 128 astronomical units away from us; that's roughly four times the distance to Neptune. So way out there, so far out it takes 18 hours for a signal from Voyager 1 to travel back to Earth, traveling at the speed of light. So Voyager 1, in 2004, for Voyager 1 crossed the termination shock at a distance of 94 astronomical units from the Sun. In 2007, Voyager 2 crossed the termination shock at a distance of 84 AU. A couple of years ago, Voyager 1 started seeing particle signatures that many lead many to believe that Voyager 1 has actually now crossed the heliopause. That's exciting because if Voyager 1 has indeed crossed the heliopause, it is essentially mankind's first true interstellar probe, because once you cross the heliopause, you're surrounded by a plasma that is fundamentally interstellar in nature and not solar wind. Okay, so enough about the Voyagers; it's back to talking about the neutrals, which I'll be talking a lot about in this.

So unlike the plasma, though, the neutrals in the interstellar medium are able to penetrate to the inner heliosphere. The neutrals interact within the heliosphere primarily through charge exchange reactions. So charge exchange is a relatively simple process where we have a neutral atom that interacts with an ion, and an electron jumps; changes partners; it jumps from the neutron to the ion. So, for example, you have a neutral hydrogen atom encounters a proton, and the electron from the formerly neutral hydrogen atom jumps to the proton. So but the mean free paths for these reactions are large enough that you can actually get interstellar neutrals permeating this entire region, and charge exchange within different parts of the heliosphere can create different populations of neutrals, and that has important observational consequences, which I'll now discuss.

So this is a schematic picture of our heliosphere with the interstellar flow now coming from the left. So I've already talked about the Voyager spacecraft and how they're exploring the global heliosphere. The other important NASA mission that's that's that's currently studying the global heliosphere is IBEX, the Interstellar Boundary Explorer. So IBEX orbits the Earth, but it studies the global heliosphere by observing energetic neutral atoms streaming from the outer heliosphere, and it can make maps of those energetic neutral atoms. So the primary population of energetic atoms, neutral atoms that IBEX was designed to study, is is created by two separate charge exchange processes, two separate charge exchange interactions, which I'll now describe. So an interstellar neutral can penetrate inside the termination shock, charge exchange with a solar wind proton; that essentially creates a solar wind neutral and an inflowing interstellar proton, which is then picked up by the solar wind and it's taken out with it. And these pickup ions create a superthermal particle population within the solar wind, and this energetic particle population is energized even further when it hits the termination shock. So after it goes to the termination shock, this really energized pickup ion population can charge exchange again with with with an interstellar neutral that happens to be passing by, and this creates this population of energetic neutrals, some of which can be directed back to the inner heliosphere, and this is what IBEX observes. So this so-called distributed component of energetic neutrals is what IBEX was designed to study; it's a remote diagnostic of the plasma properties in this region outside the termination shock. So Voyager 1 and Voyager 2 only kind of provide point probes for the properties in particular locations; IBEX provides a more global picture of the inner heliosheath. So Voyager and IBEX are kind of complementary in that respect. But Interstellar Boundary Explorer, where it's in Earth orbit, it's a highly elliptical orbit; it needs to be in a highly elliptical orbit to carry it out of the magnetosphere. So I think it goes almost most of the way towards the Moon, actually. So yeah, it was launched in 2009, I believe, and so and it's still operating. So so yeah, the Voyager and IBEX are two, I think the two big NASA missions that are focused on studying the global heliosphere.

Oh, it's still contentious. Yes, yes, yes, trust me. Right. Yeah, right. Yeah. Okay. The question was, has the question of whether there's a bow shock around the heliosphere been resolved? And in the end, the answer, the short answer is no. You could do a whole lecture on this, like, no, it's still very contentious. So these are the ENAs that IBEX was designed to observe. But when IBEX started taking data in 2009, it observed something else as well, something that was not expected; it observed a ribbon of energetic neutral atoms completing almost a full circle in the sky. And so the source of this ribbon of energetic neutrals is still somewhat of a mystery; we don't know exactly where they're coming from. I think it is is commonly believed by almost everyone that it has to have something to do with the orientation of the interstellar magnetic field. The orientation of the interstellar field is the only thing that provides a preferential axis on which you could build something like this. So the idea is that you have an interstellar field that it alters the shape of the heliosphere somewhat, and somehow that leads to ENAs coming from a plane perpendicular to the interstellar field direction. And whether that the emission, these ENAs are being produced at the termination shock or the inner heliosheath, in between the termination shock and heliopause, or the heliopause, or the outer heliosheath beyond the heliopause, is still uncertain at this point. So that's an active area of research.

So another currently operating spacecraft that can be used to study interstellar neutrals is the Hubble Space Telescope, and this now gets into work that I am heavily involved with. And specifically, the relevant Hubble data is UV spectra of stellar Lyman-alpha lines. So when you observe a spectrum of a stellar Lyman-alpha line, you always observe, you get always get something that looks like this. So the intrinsic stellar Lyman-alpha profile should look something like this, with a self-reversal in the middle. The solar Lyman-alpha profile looks something like that. But this, this even for the nearest stars, the Lyman-alpha profile is mutilated by lots of absorption by hydrogen and deuterium in between us and the star. So this big broad absorption and this absorption in the wings, too, is it's due to hydrogen in between us and the star. This little dip here, that's due to deuterium in between us and the star. So most of this absorption, most of this material is interstellar material, but a detailed analysis of this line profile shows that at least in some cases you see excess absorption on the right side of the line that cannot be interstellar, and in some cases you'll see excess on the left side that cannot be interstellar. So the excess on the right side is heliospheric absorption, specifically heliospheric absorption caused by neutrals created by charge exchange beyond the heliopause and what has been called the hydrogen wall in between the bow shock and heliopause. And as this figure kind of suggests, we only see this absorption in upwind directions; we do not see it in downwind directions. So what is the absorption only on the left side? Well, that's the astrospheric analog; that's the hydrogen wall around the observed star. So if we can observe absorption surrounding the Sun, you might expect we should be able to observe it around the star, too. Yes, before taking data like this, right? Jeff will be talking about, you know, the question was how would we know the amount of interstellar material between us and a star. Yeah, Jeff will be talking a lot more about interstellar stuff in the next talk. I'll defer to him.

So the importance of this astrospheric hydrogen wall absorption, you know, the astrosphere, of course, the corresponding astrospheric erosion of this, is that this is the only currently, the only way we have to observationally study solar-like stellar winds and solar-like astrospheres. There is no other way. And so a lot of this talk will be devoted to this, this absorption. I okay, so we are, the question is why why is the heliospheric absorption on the left side? So we are observing the heliospheric absorption from inside the heliosphere; we are observing the astrospheric absorption from outside the heliosphere. So the Doppler shift works in opposite ways; it's just it's just the heliosphere absorption one way and the astrosphere absorption the other way. Ultimately, you know, as the interstellar absorption, you know, enters the hydrogen wall region, it's decelerated and deflected. And so for the heliosphere case, that that ends up being a redshift, and for the astrosphere case, it's a blueshift because our perspective is reversed; we're seeing it from outside instead of inside. But the Doppler shift effect. All right, there's one last Lyman-alpha diagnostic I'll show, and that's that's in very downwind directions; you see a different Lyman-alpha absorption, very very broad and very shallow. And this is due to a population of neutrals in the heliosphere caused by charge exchange between the termination shock and heliopause. So this population is formed by charge exchange outside the heliopause. This population of neutral hydrogen is formed by charge exchange in between the termination shock and heliopause, and it's only in very very downwind directions with long path lengths down the heliotail that that you get enough column density to see absorption from this component. So in some, I would argue this is the first detection of the heliotail. But when I talk about heliospheric and astrospheric absorption after this, that will all be of the hydrogen wall variety. Yes. But the astrosphere, astrosphere is the stellar analogue of this, so there the star is there instead of the Sun. Excuse me. Whatever star you're observing. Yes. So so this is an observation of the Lyman-alpha line from the star called Alpha Cen. And so our line of sight to that star passes through the interstellar medium; it passes through our own heliosphere, and it passes through the stellar astrosphere. And so you can get absorption from all three, and disentangling them is not easy, but I would argue it can be done. Yes. Well, there's a flow. I mean, so so that there's the there's if the if there was no interstellar flow and the the the solar wind was just sitting there, then you would get an expanding bubble, basically. Oh, well, yeah. So imagine that, so imagine that the interstellar medium is just like the air in this room; it's just sitting there, and and the Sun is moving through it. So so so it's so so you see a flow from one direction; you don't see anything from the other direction. Okay. Okay. Well, so so that that thus ends my introduction to basic heliospheric structure and my introduction to how the Voyager spacecraft, IBEX, and Hubble Space Telescope are studying the heliosphere. But the topic of this talk is beyond just the heliosphere; it's also about astrospheres. Yes. Yes. Yeah. Yes. Yeah. Yes. Cassini also observes a band of energetic neutral atoms stretching across the sky, which is different than this, and that's because it's a higher particle energy; Cassini observes that higher energetic neutral particle energies than IBEX does. And so the characteristics of the ribbon are different. Yeah. Okay. So astrospheres. So I've described how we discuss, discuss the do explore the heliosphere. How can we explore astrospheres around other stars? Well, I've mentioned one way; there's the Lyman-alpha absorption diagnostic, which I will certainly be coming back to. But can't we just image astrospheres? In some cases, the answer is yes, and here are some examples of images of astrospheres for different types of stars. So so, for example, this is a very young star, newly born star with a strong wind interacting with the high-density nebula from which the star just formed. I think this this image is on the cover of your heliospheric physics books. So there are a couple of examples of red supergiant winds interacting with the interstellar medium; two famous stars, Mira, a famous variable star, and Betelgeuse, the brightest star in Orion. You see the wind interactions for those. This is a massive hot star interacting with the interstellar medium. Finally, this is a pulsar bow shock. Pulsars don't have high mass loss rates, but but they're relativistic, so they're really fast and really energetic, and so you can get visible astrospheres around pulsars. But none of these stars or stellar winds is much like the Sun, or much like the solar wind at all. These are much stronger stellar winds, much different types of stars than the Sun that unfortunately we just cannot detect the astrosphere of a sun-like star in such a manner. Yes. Oh, dear. Um, let's see. That this, I think I think these are all infrared; I could be wrong. I think this is H-alpha, optical H-alpha. This is this is an oddball; this is an ultraviolet image taken by a spacecraft called GALEX. I think it's almost unique in being able to observe an astrosphere in the ultraviolet; that's not normal. Yes. Oh, dear. So that's their, I think this is oh, this it's overexposed; it's overexposed. So yes, and that the stars are never resolved. So these are, you know, I think this is there's 15 arcseconds, arcminutes. Saying this is oh, this is big. I think this is arcminutes, and these don't have a scale to them, but yeah. Yes. Sure. Sure. Right. Great. Yeah. Okay. So the stellar winds themselves of solar-like stars are extremely difficult to detect. For the more massive winds, they provide nice spectroscopic diagnostics. So this is the this is the line profile of a line observed from it from a hot massive star, and this big broad absorption that you see is from the stellar wind, and it's blueshifted from the rest frame of the line. So the blueshift tells you roughly the wind velocity; the amount of absorption tells you roughly the wind mass loss rate. So there's a lot of information there in line profiles like this. And likewise, this broad absorption features from the wind of this red giant star, Aldebaran is its proper name. Unfortunately, the solar wind and solar-like winds around other stars provide no such nice diagnostics, and the basic problem can be perceived here in this table comparing typical stellar wind properties for these different types of stars, including hot massive stars, O & B type stars, and red supergiant's and red giant stars, and finally the solar wind. So the properties that are listed here are ionization temperature, typical wind velocity, typical mass loss rate, and typical characteristic densities at 1 AU and at the stellar surface. And so the mass loss rate of the solar wind and its characteristic densities are just orders of magnitude less than these other types of winds; that just makes it much harder to detect an analogue for the solar wind around another star. We don't have any nice spectroscopic diagnostics like this to go on. It's at the stellar surface. Yeah. And by the way, if I talk about O and B type stars, how many of you do not know what I'm talking about? Okay. Yeah, this is my background is astronomy, so so I just take this stuff for granted. I have, I mean, I majored in astrophysics; that's what I got my PhD in. So but I realized that a lot of you are come from a more solar physics background and are familiar with these these these spectral types. So O and B type stars are very hot massive stars. Okay. If you think about it, the solar wind itself isn't terribly easy to detect or study remotely. So this this figure shows some some ways that we we study the solar wind. So the aurorae and the behavior of comet tails were the first two indications that there was a particle flow from the Sun. These days we can use coronagraphic imaging to reveal the corona, and we can see mass streaming away from the Sun and then precede the solar wind that way. And of course, obviously, we have spacecraft that operate directly within the solar wind, and we can observe its properties directly. But none of these mechanisms that we used to study the solar wind can apply, it can be applied just detecting analogous winds around other stars.

So one of the reasons why we would want to detect winds around other stars is if we can detect winds around stars of different ages and activity levels; we can figure out how winds evolve with time; we can figure out what that what the solar wind has been doing during the course of its four-and-a-half billion-year lifetime. So what was the solar wind like 3 billion years ago? Well, if we could observe the solar wind of a young solar-like star that's 3 billion years younger than the Sun, and that could tell us. So we know a lot, so we back up. So the solar wind arises from the solar corona. I mean, the solar wind, the existence of the solar wind could be most simply as simple thermal expansion from this hot corona. So these are X-ray images of the Sun, from well, UV images of the Sun from a solar max period and a solar minimum period. So so you can study stellar coronae by observing stars in X-rays, and we know a lot about how stellar coronal X-ray

Emission evolves with time for four sun-like stars, and that's what this figure is showing. So this figure shows the evolution of coronal X-ray and EUV flux as a function of age for very sun-like stars. The Sun is there, and you can see that the X-ray coronal activity declines with time, declines with age for sun-like stars.

So the basic idea here is that stars begin their lives rotating very rapidly. This rapid rotation induces a creates a strong magnetic dynamo in the stellar interior. So that creates a lot of a lot of magnetic activity on the surface and therefore a lot of X-ray and UV emission. So stars begin their lives very bright in X-rays and very currently active, but with time stellar rotation slows with time because the the magnetic field of the star will drag against the wind of the star and slowing the the rotation with time. Therefore, the dynamo weakens and therefore the coronal activity weakens. That's the the basic idea here: the coronal activity declines with time.

But the the big question is what does this mean for the stellar wind? Does this mean that the solar wind does this mean that the winds of solar-like stars also behave in a similar manner? We're not and specifically what does it mean for what the solar wind was like, you know, four billion years ago? We know from the stellar data that it was more currently active, but does that mean that its wind was was was much stronger in the distant past? So you can argue multiple ways. So the case for a very strong wind from the young Sun: so we know that the young Sun was was was criminally very active, very bright in X-rays, rapidly rotating because there was more material heated to coronal temperatures. You might think it means more material available to expand into a wind; therefore, you'd expect the young Sun to have a strong wind. Furthermore, young stars are also known to be frequent flares and and and with flares much more energetic than flares that we see from the Sun on the Sun. The energetic flares are accompanied by coronal mass ejections, and one might imagine that the the more frequent flaring could lead to a wind that's dominated by by a coronal by material and coronal mass ejection associated with it with the flares. So that's another argument for there being a strong wind from the Sun that could be seen. He dominated this figure here is from an attempt to take the solar flare energy coronal mass ejection mass relation seen for the Sun and extrapolating it to younger, more active, more frequently flaring stars to infer how the how the CME mass loss rate should change with with X-ray luminosity and therefore with age, essentially. And the suggestion is, yeah, you get orders of magnitude stronger mass loss rates just due to CME mass loss alone. So these this seems to be a strong argument in favor of a strong wind for for the young Sun.

But you can also make the opposite argument. So during the course of the solar activity cycle, the solar X-ray flux varies by quite a bit. So the question is, does that mean the solar wind varies by quite a bit over the course of an activity cycle? And the answer is no, and that's what these these three plots are are showing. So if you plot mass loss rate versus X-ray flux, there's just no relation whatsoever. And Voyager 2 has been monitoring the solar wind for, you know, 30 years, and there's no clear activity cycle variation. So this is a case for there's an argument for there being no relation at all between X-ray emission, coronal activity, and stellar wind strength. You could also make a case for there being an anticorrelation between coronal activity and then stellar wind strength. One problem with the argument from the previous slide is that coronal X-ray emission comes from closed magnetic field regions; it comes from coronal loops. The wind comes from open magnetic field regions. So one could imagine that if you had a really really active star, it would be completely covered by closed magnetic fields and there wouldn't be any room for any open magnetic fields and therefore no room for the for the wind to escape. So that's an argument for an anticorrelation between the coronal activity and and winds.

So the bottom line here is is without guidance from observations, we really don't know very much. Yes, this plot so this is these are actual mass loss rates measured at one AU by spacecraft observations. These are observations, and this is X-ray fluxes from the GOES, I think. Now this is over space [Laughter] yes, is your after your afternoon speakers. This is his work. So these are X-ray fine, so these are both observational things. So in Andover was looking for a a correlation between the two and found nothing. So what's so what's here is is so so this is an attempt to this is more complicated. So this is it: we know how for a report on the Sun the relation between flare energy and mass of coronal mass ejection, and so that creates some power law relation between those things. Now we know flare rate for stars and flare energy; you have some flare distribution for the stars or for more active stars. So if you extrapolate the solar flare CME relation to more active stars with more frequent flaring rates, we you can get at what their mass loss rate from CMEs should be. So yeah, yeah, right, right, yeah. Yes, is the fraction of the solar wind that occurs in the appears in the slow and fast solar wind change with time? You you would certainly think so. Oh, do you want to answer that over? Oh no, this this this is for I mean this data, this mass loss rates were taken from one AU. I don't know from Wind or ACE, and this this upper ones are from from drum Ulysses. This is from high latitudes, and this is from near Earth. All right.

So the bottom line from all this was that we really do need observations of stellar winds to tell us what they're doing, and this brings us back to the astrospheric Lyman-alpha absorption diagnostic, which at the moment is the only game in town. So this figure from the book chapter shows what happens to the stellar Lyman-alpha profiles that travels from the star to the Sun. You get absorption from the astrosphere, from the interstellar medium, and from our own heliosphere. And the bottom line is something that looks like this, which I showed before with where most of this absorption but from hydrogen and deuterium is from the interstellar medium, but there is an excess on in this case both sides of the line. The excess on the right on the right side is Heliospheric absorption, and the excess from the the on the left is astrospheric absorption, which can be used as a diagnostic of the stellar wind. And yes, mm-hmm, if they're in the same direction. Yes, it okay. So so the question was you expect the the amount of Heliospheric absorption to be the same for all observed stars, and the answer is yes, if it's in the same direction. I mean, obviously, at the amount of Heliospheric absorption that you observe will depend on on where it is relative to the Heliospheric structure, and you really only see this absorption strongly in upwind directions. That's where the the it's easiest to see. All right. So this is this is so that the Lyman-alpha data that I've been showing you is for Alpha Centauri, which is a binary, two stars, Alpha Cen A and Alpha Cen B. This is the closest star system to us, only 1.3 parsecs away, about four light-years. There is a very distant companion to the Alpha Cen system called Proxima Centauri, a teeny-weeny little M dwarf star, not much bigger than Jupiter. Anyway, so this the green line here is the Alpha Cen data again. The dashed line is the IM absorption. So once again, you see the excess absorption on both sides of the line profile. In red is over plotted the the Lyman-alpha profile of Proxima Cen. Notice that this gets to your question: you see the exact same Heliospheric absorption for Alpha Cen and Proxima Cen in the same direction. So so yes, you see the same absorption, but the truth the same is not true where the astrosphere of absorption is. The excess absorption that you see towards Alpha Cen is not observed at all towards Proxima Cen. So let's demonstrate, you know, Proxima Cen really is far away; it's like 10,000 AU away. This demonstrates that this excess absorption is due to circumstances and that doesn't extend as far as the distant companion Proxima Cen. And this also suggests that the Alpha Cen must have a stronger wind than Proxima Cen to create a larger astrosphere, a thicker hydrogen wall, and more absorption. And so this points the way towards how the amount of astrospheric absorption that you observe can be a diagnostic for wind strength. Okay. So both these L facin and Proxima Centauri different vectors, you'd be you'd be absolutely correct; they could in principle be different, but Proxima Centauri Alpha Cen A they're a they're a very they're all their assistants, so they have the same space motions, and so they see the same interstellar flow. So the geometry is the same in this case. Yes. Here? Well, I haven't even got to the actual measurements yet, but that's my next slide. Let me come back to that. So in order to actually quantify what the mass loss rate is, you do need assistance from from models of the astrosphere like this. So this is for models of the astrosphere the Alpha Cen astrosphere with assuming for different mass loss rates ranging from 0.2 times the solar mass loss rate up to twice the solar mass loss rate. Naturally, as you increase the mass loss rate, the astrosphere gets bigger, the hydrogen wall gets thicker; that's the red region, and therefore the amount of absorption that you predict from from from the astrosphere increases. So for all these nearby stars, we know what their space motion vector is very well. We know that their proper motion is the radial velocity. We know basically what the interstellar flow vector is. So we can compute what's interstellar flow velocity each star sees in its rest frame and take that into account in this kind of modeling. We also know what our line of sight is through the structure; in this case, it's about 79 degrees from the upwind direction. So using these models, we can get predicted absorption for the appropriate direction, and this is the same figure as I showed earlier, but now I also show the prediction the predicted astrospheric absorption of those four models, and the model with twice the solar mass loss rate fits the the Alpha Cen data reasonably well, and the model with point two it represents kind of an upper limit for Proxima Cen. And to get to your question of what kind of uncertainties I would attach to these, at least a factor of two kinds of uncertainties because of these uncertain the assumptions that you have to make and then this kind of modeling. I'd be thrilled with factor of two uncertainties or Rachel. You basically have to assume right. So so the first task that you'd have to do when you get into this game is you have to develop a Heliospheric model that you know reproduces the Heliospheric absorption because this is a case where you know what the input parameters should be and then so okay, you've got a model that if you assume a mass loss rate of 1, it can reproduce the Heliospheric absorption, and then you just keep the interstellar parameters the same. You have to assume that the interstellar medium is similar between from where the Sun is to where the star is. So that's that's one assumption. What? Well, it's hard to quantify, but I mean one argument and its favor is that the interstellar medium was kind of venturing and adjusted territory again, but but the interstellar medium is supposed to be in pressure equilibrium, and so there could be different ionization fractions of them, you know, that these local neutral clouds, but it should have roughly the same pressure, and if it has roughly the same pressure, you should get similar results, and we have done some testing about that. But what I didn't understand the question. Sure, sure. I mean, yeah, I mean the the the the Sun exists within a region of space called the local bubble, and now we're really venturing into Jeff territory, but nevertheless. So the local bubble stretches for maybe 100 per second, 100 parsecs in all directions, and most of the volume of the local bubble is supposed to be fully ionized, and if you have an interstellar medium that's fully ionized, you shouldn't detect any astrospheric absorption at all because there's no neutrals to create such a thing. So but if you do detect astrospheric absorption, the assumption the implicit assumption that we're making is that it's due because there is a cloud somewhat similar to the one that the round surrounds the Sun surrounding the star as well. So that's the implicit assumption. Exactly what kind of errors that that translates to and this kind of it's hard to quantify because well, how do you define what a reasonable assumption is for the the cloud? Okay. So we can do this exercise for all stars with with detected astrospheric absorption, and these are some examples. The one point I wanted to make in this slide is that these are the models of those 66 astrospheres, and these can be quite big. The Epsilon Eridani astrosphere, Epsilon Eridani is only 3 parsecs away, and the astrosphere is thousands of AU big, and it's comparable to the size of the full moon in the night sky if you could see it. So this gets to the there was a question about how big these things can be. So they can be quite big. Okay, I won't get it into that. This is a list of all the astrosphere measurements that we made, and yes, the absorption. Yeah, yeah. Well, to win parameters that matter, it's only the wind parameters that matter. What we do is is we vary the assumed density of the stellar wind. Another uncertainty that it goes into this is that we assume that the velocity of the wind is is is the same as the solar wind. One hand-waving argument in favor of this is that observationally, if you look at different types of stellar winds, that the wind speeds that you see for different types of stellar winds tend to be close to the surface escape speed, no matter what type of wind you're talking about: red giant, red supergiant, hot OB star, solar winds. Escapes B to 600 kilometers per second, which is, you know, a reasonable for an estimate for the storm. And so the assumption it's that's another assumption that gets me. Those are the two things that matter; it's the ram pressure that matters; that's rho V squared. So it's density and velocity. It's actually density that we're varying, but we could do velocity too in principle, but I explain the reason why we do density. So this is the full list, and as I talked about early, so these are the mass loss rates that we've measured for these of these stars. I'm gonna focus on the the solar-like main-sequence stars, and these are X-ray coronal X-ray luminosities for these stars, and for the reasons I talked about before, since since coronal winds arise from stellar coronae, one might expect there to be some relation between X-ray and mass loss rate, and if you plot mass loss per unit surface area versus X-ray coronal surface kronole X-ray surface flux, this is what you get for the main-sequence stars. So the most solar-like stars are shown in red. Teeny-weeny little M dwarf stars are shown; there's two of them; those are shown in green. So at least at lower activity levels, you you seem to see evidence for an increase in mass loss rate with coronal activity in a manner consistent with this parallel relation here, but there's also a suggestion that something this this relation fails at some activity level, and and the one problem is that the two of the stars in this these high activity regime are these teeny-weeny little M dwarfs, and do the teeny-weeny little M dwarfs have this issue? Should they have the same relation as the more solar-like G and K dwarfs? Debatable. Pi-1 Uma, however, is an excellent young Sun analog, basically the same spectral type; it's 500 million years old, and it has a low mass loss rate of only half times. So this is a very new measurement by the way, published this year. Psi Boo is a binary, and we don't know which of the two stars the wind is coming from, but what's being noted here is the only way to make Psi Boo be consistent with the stars that it's similar to and in terms of activity level and Psi Boo similar to consistent with with the star that it's similar to, Pi-1 Uma is to assign almost all the mass although all the wind to Psi Boo B. So the assumption is that Psi Boo B is 90% accounts for 90% of the wind and Psi Boo A accounts for 10% of the wind from the binary. There's no observation; there's no observational way to tell if this this assumption is correct. So let me skip this. So at least for the lower activity levels, you have that power law relation between mass loss and X-ray surface flux. This can combined with with relations between X-ray activity and root stellar rotation and stellar rotation and stellar age. There's been a lot of work to do in the past few decades to determine what these relations are. It's and if you combine these three parallel relations, you can get a relation between mass loss and stellar age, not proportional to T to the minus 2.33, and this is what that suggests for the mass loss rate history of the solar wind. And so I'll land with this. So the Sun currently has a mass loss rate of about 2 x 10^-14 solar masses per year, and that power law relation suggested as you go backwards in time, the solar wind gets stronger and stronger and stronger up to about eighty times the strength of the current solar wind. But because the mass loss activity relation is is truncated, this one is as well, and perhaps the Pi-1 Uma measurement is indicative of surprisingly low mass loss rates at very early times. So this is this is a what we would infer for the mass loss evolution for for solar-like stars based on the astrospheric climate outflow diagnostic, and I'll stop there and take any more questions. [Applause] Yes. Oh, we don't have any astrosphere detections for brown dwarfs. So wait, we we barely have enough detections of normal stars, but yes. So I don't think we know anything about what the winds of brown dwarfs or astrospheres are. Brown dwarfs, if they have, I don't know. I saw back there. Yeah, I wouldn't. It would have to be yeah, it could. Let me put it this way: if you had I mean stars are generally pretty widely separated. For example, the two Alpha Cen stars, Alpha Cen A and B, we're talking about 20 to 30 AU separation. So that's that's normally the kind of binary separations that we're talking about in the kinds of there were some binary stars there, but but widely separated, not really really close things like like your I mean I could imagine if you had two stars close together and they were both contributing equally equal wind, there would be some momentum lost to the due to the collision, which would cause us to underestimate the strength of the stellar wind if we do the kind of analysis that we did. Now they probably yeah, that's, you know, okay. So so okay, so for the four for hot massive stars, there was one that's actually go to the figure, my pretty astrosphere figure. Okay. So so this is a hot massive star. So hot stars have had very massive winds driven by radiation pressure, so very fast, thousands of kilometers per second, and it's driven by radiation pressure, so a completely different acceleration mechanism from the solar wind. So those are those types of stars. Red giants and supergiants um and these are supergiants. These have very low surface gravities on very low surface escape speed, so these winds tend to be very slow and very cool. The wind acceleration mechanism is not known very well at all for for such stars. Various I'll feign wave ideas out there, but the the idea is because the surface gravity is so low. I mean, let me put it this way: when the Sun goes through a Mira phase, it might actually end up swallowing up the Earth, and they expand that much. And so the idea is the surface gravity is so low that you can get a lot of material escaping fairly easily, whatever the acceleration mechanism is. So so red supergiants and giants have relatively cool, slow winds, but they can be quite massive. So this is a young star that's just formed, so probably still has an accretion disk around it, and so there may be a contribution to the wind from from the accretion process somehow, and it's still embedded in the nebula from which it forms. So that's another reason why an astrosphere for your star like this might be detectable is because the the

Is M density around it is particularly high. The Pulsar is really exotic. So do you know? Does anyone know what a pulsar is? Or how many of you know what a pulsar is? Okay, so so you know, so it's a rapidly rotating neutron star, or really, so it's something the mass of the Sun packed into the size of a city. So it, so the surface escape speed from a neutron star is, you don't have to speed a light or something like that. So, so the wind from a pulsar wouldn't be very massive, but it's really, really fast, so it's really energetic.

So I cover the types of stars you're interested in. Yes, I'm an alpha. Yes, yes. Oh-wee, the so we really would like to have another stellar wind diagnostic. That, that's certainly true. The one hope that's kind of on the horizon that people talk about is radio emission, and winds should be source user ionize coronal winds; they should be a source of free-free emission at some level. However, current radio telescopes are only able to give us upper limits that are, you know, orders of magnitude higher than the strength of the sol, the solar wind, but that's, that's kind of the hope, to find some other way of more directly detecting stellar winds. Ancient care of the star. Oh, we know any any activity diagnostic of that sort? Yeah, we know, we know everything for these well-observed stars, but yes, it's not the activity diagnostic that's the problem; it's a wind diagnostic that's, that's what we're missing. I mean, these various activity diagnostics like calcium 2 and x-rays, UV emission, even flare rate or something, these things are all pretty highly correlated. So, so, so we know we've got that side of the equation figured out pretty well, but that's the wind side that we're lacking. Pre-mission surprise, other way. Maybe we could ask the, how many, how many of you know what a type 2 radio burst is? Okay, so type 2 radio emission comes from when you see a very, very fast, see any from the Sun with a shock, and so the emission comes from the shock. So Jeff is proposing how we could detect the analog of that around.

Yep, how, how will the stellar wind affect the evolution of magnetospheres? Yes, I mean that's definitely, you know, I mentioned I was going to get into if I had time, and I didn't get into the planetary implications, but yes, people are very interested in, and so in how solar-like winds evolve with time, and how does that affect? I even have a visualization of, of a star, of a hot Jupiter planet interacting with the, the wind of a star, and so what effect does the evolution of the stellar wind have on the atmosphere of the planet and on the planet's magnetosphere? To what degree did, can a planetary magnetosphere protect the planet from losing its atmosphere like this artist's conception is showing? So yes, yeah, that's, that, that question is of interest, but I'm not the best person talked about talking about the planet. Okay, great. Yeah, I'll be around today and tomorrow. Yeah