Transcription
I'd like to begin. I'm Jeffrey Lenski. I am a I R I know. I think we know what a rhino is. Rhino means retired and name only. So I'm retired from the University of Colorado, but I still work full-time, or 70 hours a week, or whatever it is, and enjoy doing science with no employer, no supervisor, no responsibilities, and no paycheck. Okay. Well, he graduated a long time ago, so long he's does this thing, but we collaborate quite a bit. Okay, this is a tag team, and Brian gave you the first half of the talk pertaining to essentially asterisk fears interaction with the interstellar medium. I'm going to give the second half. Brian was talking primarily about what you see from the inside of the heliosphere, and I'm going to talk mainly what you see and can learn from the outside of the heliosphere, the interaction of the stuff around us, around the heliosphere, and how it affects the heliosphere itself. And I welcome questions. I'm told I could go over 45 minutes. I've been in this business a long time. I enjoy going longer than the allowed time, and I do it all the time. Okay. There's a lot of material in here that is almost certainly unfamiliar to people in the solar physics domain. What I'm talking about is the interface between solar physics and astrophysics. The two fields have different jargon. I'll attempt, when I'm talking on the astrophysics, I'd use terms that are familiar to solar physicists, and if you're not, don't understand the terms, please ask. And when I'm on the Heliospheric side, I'll try to talk in terms that you know, make sense to the astrophysics side. So this is interphase science. Okay. I have a lot of things to talk about. Some of the slides I'll show I'm going to go through rapidly, just sort of motivation. You can all guess what the motivation might be. And then on a number of these slides that I consider really important, let's stop, look carefully at what's being presented, ask some questions, and try to get it understood. Okay. Briefly, through motivation: The Sun and the heliosphere lives in a highly inhomogeneous environment that's rap that is changing, not rapidly changing, but changing by huge orders of magnitude. So the environment of the heliosphere today is not the same as the environment in the past, and it's not the same as the environment in the future. There are going to be big changes. Right now there have been big changes. Of course, the inner star meaning is very important because we are made out of interstellar medium material. Okay. Everything except hydrogen in our bodies is from stars, supernovae, etc. Okay. I'm going to talk about heliosphere, and I'm talking about the interstellar medium, its various components, its temperatures, density structures, and how it affects the heliosphere. I'll even mention that we may be moving from one element of the interstellar medium into another element, in which case things are going to change. Okay. And the objectives of much of the work that I've been doing with Brian and Seth Redfield at Wesleyan University is basically to characterize the interstellar medium so we can come up with an effective boundary condition to models of of the heliosphere. And I mentioned a few papers at the end. So let me start, and now I'll go slow and try to put this all in context. What you see here is a model of the universe and galactic coordinates. I'll be talking a lot about galactic or galactic coordinates, latitude, longitude relative to the center of the galaxy, which is right here, and longitude. This is the anti-galactic center Direction, 180 degrees away, and that's the North galactic Pole. So all the diagrams are going to see are going to be in these units, these coordinate system. The data we have to play with our observations of stars. Now if you think about what do you see in a star that you don't see in the Sun in this spectrum, there are stars that are almost identical to the Sun. We call them twins. Best example is a star in Scorpius called 18s Co. Its spectrum is almost identical to the Sun, but it differs in a very fundamental way, namely there's the signature of the gas, interstellar gas between us and the star that's superimposed on the stellar spectrum as narrow absorption lines. And as he's narrow absorption lines, which to a stellar astronomer is noise, though an interstellar astronomer is signal. Okay. And we're going to diagnose the signal. Okay. Unfortunately, we cannot sample all directions in space uniformly, like you would like to do in a simple experiment. You have to deal with real stars, and you have to deal with real telescope allocation committee's. So as a result, we don't uniformly sample all space. We sample what we can get, and the observations shown here in black and red, the red are just the newer stars are our sampling of stars out in different directions in galactic coordinates. Well, we can only sample stars for this work with observations that are primarily ultraviolet, high spectral resolution. So we're forced to limit what we're doing to one scientific instrument, Hubble, at one spacecraft, and one instrument on Hubble called the Space Telescope imaging spectrograph. Unfortunately, that's all the data really that we can play with. And when Hubble goes, we're in trouble. Okay. So here's what we do. Eres a high-resolution spectrum of two nearby stars. Actually, forget about the names, they just have astronomical names to them, and say okay, what if we are looking at one star, so we have one sight line, and we have one ion, in this case singly ionized iron, but we have many absorption lines of that ion. All we see is these narrow absorption lines all lined up at the same radial velocity, about 15 kilometers a second. They're all pretty narrow, and they all line up very nicely. Okay. That gives us some information that basically tells us the velocity of the interstellar gas relative to the Sun. And if we can hit a line its equivalent with, we can measure the column density, the number of atoms of ions on that line of sight to the star. Two pieces of information. However, many stars show not one absorption feature, but two different radial velocities, sometimes three, four, or five, and sometimes it gets even more complicated. So if you're looking at this star, you can say, well, there's two pieces of interstellar medium gas in the line of sight, and they have different radio velocities, and they have different column densities. Got that far? Let's make it more complicated. Alright. Let's say we now look at the same line of sight. We have two different ions, iron 2 and deuterium 1, or whatever, we have multiple ions. Then four, we can look at the line width, which is usually characterized by this quantity, but think of it as full width half maximum, which is related to the thermal component, temperature over mass, and a non-thermal component that astronomers called turbulence. Don't ask me what that is. Okay. Now if you have two different ions of different mass, then you can immediately separate a thermal component of the width from the non-thermal component. Okay. So that Amelie stays ah, we can get a temperature of that gas, and we can get its turbulence. And if we look at the same ion, a different species, different, let's say different lines of that particular ion, some of which might be forbidden or semi-forbidden, then we can get densities, whose electron densities, and we can measure abundances, how much there is in that column density, and we can compare those abundances, for example relating say iron to hydrogen, compare it to what's in the Sun, and we can turn determine the difference and call it a depletion, and we can in principle even get iron fractions if you have, let's say iron 2 and iron 1. Okay. That's getting more interesting. All right. So let's go further. Okay. I have to get forward and backwards right on this. Let's say we're not going to look now going to look at not one line of sight over there, but a line of sight over here and over here and over there and up there. So we have multiple lines of sight. Okay. And for each of these lines of sight and for each of these ions, we get all these quantities, but now what additional can we learn by looking in different directions and seeing if things correlate? Well, you get a radial velocity in this direction and a radial velocity in that direction, both of which are projections of the three-dimensional motion along the line of sight. And when you look out into space and say, well, are there regions of space for which the radial velocities follow the projections of a single velocity vector? Whew. The whole thing is moving as if it's a steel ball, coherent through space, even though the densities are 0.1 per cubic centimeter. And guess what? It happens that leads into whole idea of global morphology: How are different parts of the interstellar medium moving with respect to other parts of the interstellar medium? Yes. Okay. When you measure when you measure an absorption line again in the stellar spectrum, you measure a velocity in that line of sight, so it's a radial velocity, not a tangential velocity. It's a Doppler shift. Alright. No problem. Doppler shift. I didn't use the word Doppler. Okay. And if you can go one step further, namely if you have global more morphology and kinematics, these different things moving with respect to each other, then you can begin to come out with some kind of evolution like the Sun was in past in some idea, some part of the interstellar move medium moving in a certain way with certain abundances in certain column densities, and in the future you can project where the Sun is going to be moving through different pieces of the interstellar medium. Okay. So let's get there, at least as far as we can. Okay. That said, we have many lines of sight. We measure the radial velocity and lying on all these different lines of sight, and let's say for take a zeroth-order approximation, what if all of the interstellar medium gas around the Sun were moving coherently with one velocity vector? Okay. All moving together. What if? So you can take all the different radial velocities, fit them with least-squares law for a single velocity vector, and sure enough you can now plot the observed velocities, kilometers per second, against that predicted by this mean least-squares zeroth order fit to all the data, and what do you get? Well, you get a fit. However, each of these data points has an error of about 1 or one and a half kilometers a second. The spread is huge, and what's shown here in color-coded, for example, look at the red, the red points are those points for which the error is 10 Sigma. I'm sorry, the displacement is 10 Sigma compared to the random error of measurement. So if you have a distribution of points for which 45 percent of the points are 10 Sigma away from the mean, you know that this is not a uniform flow, right? What is it? Not uniform, although on the gross dist:0 m o V in the same direction, but the random motions are much much or large compared to the uniform flow. Be 40, 45 percent of the points are 10 Sigma away from the mean. I would like you to be a referee on some of my papers. [Music] No more comment than that. Okay. So what do you get? So let's start taking pieces of the sky here, there, everywhere, and see if they fit a vector, a common vector within the error bars of the individual measurements, right? And we start playing with the Gator, and this Seth Redfuel is the real expert at this, and first of all, you know, if you take points that are very close together, then the different projections tells you nothing. You have to have data points that are, you know, stars that are far apart in order to see if they have a common vector. And sure enough, we look around and we say, ah, have a number of data points, and wow, we find a whole group of data points covering a large piece of the sky that are all moving at the same velocity of velocities corresponding to a common velocity vector. This happens, and the structures are not simple spheres. The interstellar medium is not a whole bunch of spheres. It's, we think, a whole bunch of long, thin structures that are filling, entering. We give one example. I'll show you more. Yeah, structures that line up. Here's going, and then of course you have to rotate around to the other side of the sky, and they fill up. When you start getting down the structures that are only a few data points, we worry about it, but in many cases we get large numbers of data points. Now we have structures. We have to give them names. Okay. We choose names for what constellation must you of lead many of the data points are so they have names of constellations, but I'm sure the IAU, when they get serious, are gonna say you've got to have some kind of telephone, you know, telephone book numbers for these things. Okay. So what are the closest structures to of the interstellar medium in terms of kinematics to the Sun? Okay. One thing we find is roughly forty or fifty percent of the sky, shown in red here, is moving coherently velocity with respect to the Sun, and huge piece of the sky. And if you see essentially half the sky from one vantage point, that means one of two answers: Either you're just inside that structure, or you're just outside that structure. Okay. Because if you're far away, the structure will be small in terms of angle, and if you're close, if you're inside the structure, well, inside the structure, it'll cover the whole sky. That clear. So a real question, which we're going to get to, is is the Sun inside or outside of this thing? And since many people have thought were inside it, it's called the local interstellar cloud. Okay. To be not yet determined whether it really is the cloud we're in. Okay. That's one big structure. Another big structure, sort of centered close to the galactic center direction, seen in projection towards Alpha Cen, the nearest star, is called the G cloud, chief or galactic center. It covers about 20 or 30 percent of the sky, and there's this filament Reetha seen in projection tours another very nearby star, Sirius. Okay. And if we give it a name, it's everything. Yes. Yes. Exactly. Therefore, you're right. Therefore, you're just inside it or just outside it. You're at the edge. Well, the question of whether we're inside or outside at the end of my talk, maybe we'll take a vote. Okay. And I'll show you the data one way and the other. Okay. That'll be like a live homework problem. Okay. But now here's here's a question. What if you know we see actually many of these clouds that are like Dillon entry type structure. If you see a cloud hedge on its filum entering, what if you see it this way? Our cloud could be very thin, narrow sort of thing, but if you see it perpendicular to its axis, it looks large, covering a large piece of sky. You see it this way, it's thin, right? We have only one vantage point where we live, so the question of the Nate, the the real three-dimensional structure of these things is TBD, to be determined. Okay. But the universe is more complicated than that, at least even the local universe. Here's the 15 different structures we've been able to identify so far. All of them have names, and I don't want to go into too much detail in this other than point out, see these little X's with a circle or each of the different clouds, which is color-coded. That's the direction from which the material is coming towards us. Now, of course, the Sun is moving with respect to interstellar gas surrounding it, so the net effect of the inflow of the individual cloud directions, they're all sort of in this direction, coming from here, and the outflow going out the back, all sort of here, which basically is another way of saying zeroth order, the whole interstellar medium around the Sun is all moving sort of coherently, but it has individual structures to it that have different velocities such that the inflow Direction covers a piece of Okay. So I haven't lost you yet. I'll try better. Okay. Since we have many lines of sight through the quote local interstellar cloud, we can determine its three-dimensional structure. Hi, you know, least squares, three dimensions. Okay. So here is the column density through the local interstellar cloud or converted into disk if you assume a number density for the material that'll give you a distance. How how large is this thing in parsecs? Oh, parsecs. Very simple to understand parsecs. Parsec is about 600,000 astronomical units. Okay. So you can make that thing. Okay. So we are now looking out into galactic space, galactic coordinates. This is the direction towards the galactic center, and once you say is Pete a major piece of the sky, more than half the sky has a very low distance to the edge of the cloud, or equivalently very low column density, and you know in other directions we have the much, you know, two or three parsecs. So we're talking about a million astronomical units or perhaps two at the edge of the cloud. Okay. So that's the view from us, but if you have a three-dimensional model for this thing, you can say, let's put us at the geometric center of the cloud and say, well, what do we see? Okay. Let's do it. So we're now at the geometric center of the local interstellar cloud, looking out in all directions, and of course we see the column density or distance to the edge of the cloud in all directions, and you notice something interesting. There's a piece of the sky for which, you know, the edge of the cloud is very close to the geometric center, intro sang. What's there? What's in that direction as seen from the center of the cloud? Okay. What do you think is there? Now we're at the center. We see this region of almost no absorption in the local interstellar cloud. What's off in that direction? Any ideas? Well, the rocket center is actually in this direction. No, that's that's thousands of parsecs away. Hmm. I'll give you a hint. Okay. The disk. Okay. The column densities that we're measuring our neutral hydrogen. In this direction, there is almost no neutral hydrogen, ionized. How would it get ionized? Nars. What kind of stars? Hot stars. Oh, here we go. Hi stars. Okay. What I now they added to this plot are three hot stars. In fact, this one, epsilon Canis Majoris, is the source is the largest source of extreme ultraviolet radiation in our local neighborhood. Okay. As seen by the EU ve satellite, this is the biggest source of extreme ultraviolet photons. Beta CMA is the second biggest source, and Sirius B is the closest hot white dwarf that is a strong source of extreme ultraviolet, extreme ultraviolet radiation, ionizes hydrogen, right? Isn't it interesting that the three strongest sources of ionization of hydrogen just happened to correspond to the region of space as seen from the centre of the local interstellar cloud that doesn't have any or very little nutrition? Okay. Interesting. So I mention we're doing working with this with work with South Redfield. I'm coming up with three-dimensional representations of local cloud. I don't want to spend any significant amount of time saying, you know, we're doing projections. For example, if you're sitting in the galactic center and you look at the local cloud, then you see is sort of split off, and the shapes are not spheres. Nothing in the interstellar medium is the sphere. Okay. We're actually beginning to lay out the locations in three-dimensional space of these different clouds with different names and their orientations, and this is work in progress. There's a real problem because when you see absorption due to a certain cloud in the direction of a star, you don't know where along the line of sight this material is. Is it nearby? Is it close to the star? You know, that's the big issue. So you have to do some guessing, but I do want to point out this arrow. This is the direction that the Sun is moving relative to these interstellar clouds, and how far it'll move, the end the end of the arrow, and about 275,000 years. So you get sort of a feel of a time scale of moving through these structures of warm gas. So let's Okay. What are the properties of these clouds? These this is the environment in which we live. So the environment which the heliosphere lives. Well, there's a very important paper by Johnson, John Slavin, and Priscilla Frisch, 2008. I don't think things have changed much in terms of our understanding of the properties since then, so I quote their numbers: Neutral hydrogen density of about 0.2, and you wonder how can large pieces of space move coherently if their densities are 0.2 for a cubic centimeter? This is not a solid ball of iron moving through space. This is an extremely low density plasma just happy moving coherently. I don't understand what we continue. Electron densities of order point oh seven. So notice that the material is mostly neutral, but it's really personally ionized. Of course, the neutrality dominance of the neutral is very important because if there wasn't neutral material coming into the heliosphere, then there wouldn't be charge exchange that wouldn't produce a hydrogen wall which we see in Lyman-alpha previous talking. Okay. So we're talking about material, it's partially ionized, temperatures six, seven thousand degrees, or
So, which turns out to be a really interesting question. I'll explain later. Fractional ionization of about point two. Heating is primarily by ionization of hydrogen, producing photoelectrons that then heat the plasma. So this is extreme ultraviolet radiation from stars like epsilon C Ma, photoionizing hydrogen, giving these energetic electrons—well, thermal electrons—which then provide the heat and produce the temperature. Cooling is typically by forbidden transitions, for example, of carbon two and so on, sulphur two.
Pressures—interstellar medium. People talk about pressures divided by Boltzmann constant. Okay, so P over Boltzmann constant is about 2,000, and that's an interesting number because often you think in terms of equal partition. Is there the pressure balance there? Pressure balance between the magnetic field and the thermal pressure in the plasma in the interstellar medium? Well, there would be if the interstellar medium had a magnetic field strength of 2.7 micro Gauss. And there are indications that in fact it's larger than that, in which case the magnetic field determines the structure of these partially ionized warm clouds. That's an interesting idea; we haven't developed that.
Okay, the inflow velocity of the material to the Sun—that's including the Sun's motion through the interstellar medium—is about 26 kilometers a second. This number is being debated right now, and I'll show it in this subsequent slide, and it has an important effect in terms of our understanding and these calculations. Okay, I know I have to be honest and tell you these calculations assume local n star cloud is in ionization equilibrium, thermal equilibrium, and dynamic equilibrium—all three of which are almost certainly not true. Yes, a mean free path of the hydrogen atoms or the neutrals or the electrons—you know, I never did that calculation. That's an interesting point; definitely not even for neutral collision back. Thank you. In a comic, it's always great to have a graduate student, even a former graduate student, because they keep you honest. Right.
Okay, then how do you measure a temperature? Okay, and I showed this formula before, namely the width of the line is has a thermal component and a turbulent component. And so you look at different atoms; for example, here's a plot of deuterium as a function of temperature. So the width—take a line like deuterium, which is, of course, thermally broadened because then it's very low-mass—but you can say, well, it's either formed at like about 8,000 degrees, it was purely thermal, or if the plasma has zero temperature, then it's eight kilometers a second—nan, essentially turbulent—and everything in between. And so if you have a light element like deuterium and a heavy element like iron, they have a plot in different directions, so you can actually come up—say, aha, there's a temperature and a turbulence—or to put it differently, there's a plot of the Doppler parameter as a function of the atomic weight of all these different elements from deuterium to iron, and they form a nice quad, and then you fit it with a temperature and a turbulence, and you get numbers like 7,000 degrees and turbulence like 1 or 2 kilometers a second. There are other pieces—the interstellar medium—where you take, do the same experiment, you find they lie along pretty much a straight line, namely the temperature is low, close to zero, and the turbulence is high. Right, so we played that game.
Okay, the kinds of temperatures that we find in interstellar gas near the Sun, they're all in this range—shown in purple are the very best measurements that we have in terms of signal-to-noise. The mean values are like about 7,000 degrees. The turbulence, which is really how the material is moving that in a non-thermal way, is like 1 or 2 kilometers a second typically. So the non-thermal motion is is much less than the thermal speed. Okay, that's fine. Okay, we have these 15 different clouds; we have a range of temperatures. I don't want to go into detail on that. I do want to go into—come back to the question of—are we inside or outside of the local interstellar cloud? So you have to pay attention to this because we're gonna have a vote. Okay, show three slides with have different interval that leads to different answers. Okay, here we go. What's shown here is, first of all, local interstellar cloud and the closest cloud to us, the G cloud towards the black Tech Center. And what's shown here are the velocity of the cloud relative to the Sun and the temperature of the cloud. So for the local interstellar cloud—at least at this—for this paper, we have 79 lines of sight for which we—the mean value of the flow speed was 23.8 kilometers a second, plus or minus 0.9. The G-cloud, much different speed, 29.6, based on 21 lines of sight. Right, temperature 7,500 degrees; this cloud, G-cloud, 5,500. These are real measurements.
Okay, now Ulysses has looked at the inflowing helium from the interstellar medium. Not helium is important because charge exchange is a big thing with hydrogen, but it's extremely weak with helium. So the helium coming in—coming in—is not changed, deflected, whatever, slowed-down, orientation change by charge exchange; it's free flowing in. So it measures the direction of the inflow, and we can measure from the width of the helium line—gets absorbed—its temperature and its velocity. Okay, so according to Ulysses—and original work was by Vitta, and then summarized by Pepper, Hard Mobius, and a paper in 2004—is this temperature is about 6,300 degrees, and its velocity is 26.2 kilometers a second. Okay, now from that set of numbers, are we in the LIC, or are we of the G cloud? How many think we're in the LIC? How many think we're in the G? Okay, one. Well, at this time, Seth and I went off of the limb and said we're not in either; we're sort of in transition from one cloud to the next, right, based on the fact that the temperature is in between—of the helium coming into the heliosphere—in between these two, and the velocity is in between those two. Okay, IBEX went up, and it measured the inflow of helium into the heliosphere. There's not a bad idea—helium into the heliosphere—but, of course, the heliosphere was named because of helium. Okay, and he did analysis of the IBEX data and found that its temperature has a broad large range, 5,000 to 8,200, and this velocity is about 23, with large error bars. So question: Are we in the LIC, or are we in the G cloud, or are we in between? How many think the LIC? How many think the G-cloud? How many think we're in between? Okay, well, let's let's continue; we're not finished yet. Okay, how many think, based on this data set, that we are in the G—we are in the LIC? Oh, that's quite a few numbers. How many think we are in the G cloud? How many think we're in between? Okay, let's continue. I'm sorry, what? Hi. Okay, you really have to go back and read every paper. It turns out that IBEX does not directly measure anything; it measures—there are degeneracies, and there are assumptions, and you can say, well, it's this if you assume a certain temperature, you get a certain velocity; if you assume a velocity, a certain temperature. Okay, please read the paper; I don't want to misquote. Okay. Okay, um, McComas, who's the PI of the IBEX instrument, went through and did his own, you know, comparing what other people and his team were doing and said, okay, let's come up with a consensus of what IBEX is telling us about the inflow of helium, and in this paper—I think it's a Science paper—and came out with a temperature now with much lower error bars, 6,300 plus or minus 390, and a velocity of 20, 3.2 with small error bars. And now are we in the—how many people think we're in the LIC based on this data? How many people think we're in the G cloud? How many think we're in between? Okay, now I don't have a lot of the third piece of data, namely that Brian Wood has done, so I'll put Brian Wood on the spot and ask him to say—he has reanalyzed—they essentially reanalyze—go back—no, no—essentially this data set originally published by Vitta based on the Ulysses—and why don't you tell us if you want to go off on a—we haven't—we haven't orchestrated this yet—but if you want to tell us which way you think the new analysis of the old data is leading us, you know. Okay, this is not yet published. So now let's take another vote. Let us say, okay, according to the reanalysis of, you know, Ulysses and new Ulysses data, actually this number stays the same, and this number goes up to what, 7,200 degrees? Okay, so let's change this to 7,300 degrees. Okay, now are we in the LIC? How many people think we're in the LIC based on this new data? How many people think we're in the G cloud? I mean, people think we're in between. Okay, so I want to leave you with that mixed answer and make it very clear that we're not there yet. Okay, in terms of understanding where the heliosphere is located with respect to the local interstellar cloud or the environment—how the local interstellar cloud. Okay, but now let's let's see what comes next. Where do I think we are? Okay, let me go back. Okay, my answer is: Stay tuned, perhaps in our lifetime. And the reason is the Sun is right there, right up either just inside the LIC or just outside of LIC, and the direction of its motion is out of the LIC. Right, so we can look at one of the strongest absorption lines in the interstellar medium, namely the line—the resonance line—of magnesium two, 2802.796 angstroms, and what we see is absorption towards Alpha Cen, which is sort of in the direction that we're moving—always see is the G cloud absorption, not the LIC absorption. So we can come up with an upper limit of the amount of absorption in the line of sight that we're traveling—that would be LIC. Okay, that's a tiny number—that's the column density; we now have a number density. Okay, that will immediately tell you a distance. We know the speed that the Sun is moving in that direction, so if you have a speed and you have a distance, then you have a time—an upper limit of the time. Okay, so according to that calculation, the Sun will leave the LIC, if we're inside it, in less than 3,000 years—perhaps tomorrow. So I say: Stay tuned; you may know an answer. Okay, now that I've piqued your curiosity and this—so let's march—you know, go in the other direction here. Okay, we played that game. Next game: If space is filled with—partially filled with—these sort of warm clouds, what's—what's in between them? If they don't fill all the space, there has to be something in between which we don't see by looking at neutral hydrogen or ionized iron or ionized magnesium or whatever lowly uh singly ionized—what is it? Okay, and here is a wonderful object lesson how it is that when theory and observation agree, we're in trouble. Okay, you think about the first ideas of what's between these warm clouds was based on a theory and an observation. Theory was: Well, let's compute the theoretical properties in the interstellar medium; we have certain heat inputs or cooling, and what they came up with—this is going back to classic papers—of a three-component interstellar medium in which the hot component is a million degrees. Okay, observation—X-ray satellites look out into space; they say they see diffuse X-ray emission; they say, ah, X-ray emissions must be produced on the in-degree plasma; therefore, the region around the Sun, aside from those areas that are cooler, must be made—you agree—plasma emitting the X-rays. Theory and observation agree. Okay, well, that happens—the funding agencies don't give you money; they observe—the telescopes don't keep you observing time because there's already an answer. Right? Why keep observing? Why keep funding? Problem is the theory is based upon a whole bunch of assumptions that are not correct, and the observations—well, somebody and the XMM satellite decided, let's point this X-ray telescope that our common—now—commissar—cold gas, of course, looked at a comet and saw a bright X-ray emission. How can the comet emit X-rays? It's not million-degree plasma. I only thought of that and said, well, the solar wind is fully ionized; the comet is ejecting all kinds of neutral hydrogen; there's charge exchange between the solar wind protons and the comet's emitting neutral hydrogen; that charge exchange leads to X-rays. Okay, and then they said, well, it's not just calm—instead of X X-rays—but there's a lot of hydrogen in the interplanetary medium, and it sees essentially the solar wind, so it's me—charge exchange—producing X-rays. So you do a calculation, and what you find is that much, if not most, of the diffuse X-rays that we see—you looking in all directions—is local. Right? Well, this was done, and I mentioned some papers by a Snowden, for example, and go to new—tomb—Tron, maybe if I get her Greek name correct, and they basically say there's either little or no evidence of hot gas within a few parsecs of the Sun. What is it? There's a real interesting paper by Barry Welch and Robin Shelton, 2009, in which they argue that it's really highly ionized gas—maybe recombining from a supernova explosion or wholly ionized by hot stars—maybe 20,000 degrees or so—and but no neutral hydrogen. And then other ideas have been proposed that this is really Strömgren sphere gas. So I don't want to get into the what's of Strömgren sphere other than say that near a hot star, you know, fully ionized gas and out to a certain distance sphere, for example, and that was first discussed by Strömgren, and they're called Strömgren sphere gases. Okay, so that's a good question: What is in between the these warm clouds? No, no, no—stop. I tell my students there are no stupid questions; there are only stupid answers, and I'll try not to give one. Yes, I mean, I'm—sharp boundaries—we don't. Okay, it's a great question. There's a paper just coming out—I know it's an Astro-ph—that's not—I don't know if it's yet in the in the Astrophysical Journal—my Cecile Gris and Ed Jenkins, and they argue along your lines, namely you don't have discrete clouds; you have gradual transitions from one property to another, and maybe they're right. In fact, this is an argument that's going to go on for the next several years in the literature, and we're gonna go back and forth on this particular issue. I suspect that's just—it's more likely discrete individual clouds because when we look at nearby stars, we see a velocity structure—velocity of the interstellar absorption—that's, you know, as centered at one velocity and then another velocity but not a whole range of velocities in between. Right, that would be my argument. Are you have to combine it with magnetic fields? I mentioned before that equal partition, you know, balance the thermal pressure against magnetic pressure occurs at about 2.7 micro Gauss. There's evidence that at least locally the magnetic field is three or four or five micro Gauss, though the structure of the magnetic field will determine the structure of these quote clouds, but of course the clouds are only partially ionized, so you're gonna have ambipolar diffusion; it's gonna be interesting out there. Okay, that's not yet settled; there are all kinds of PhD theses that have to be written on that topic. Okay, let's go on. So I mentioned this—what fills space? Okay, that's not yet well determined. Okay, this is sort of the final thing I want to get to. Let's compare the heliosphere today with the heliosphere under different boundary conditions—different environments. Okay, so let's start—there's a plot done by Neurones—you know, they're published in 2009—which he did characterize different interstellar medium interactions with heliosphere. Okay, first let's take the heliosphere today. Okay, there's a plot of temperature versus distance where the Sun is now at 0; this is interstellar medium is coming in from the right. So this is 500 astronomical units coming in from the right and headed out—there's 400 astronomical units in the backward direction. Sun is here; here's temperature; here's neutral—mostly neutral—hydrogen density. What you see is as the interstellar material comes in, there's a bow shock—or maybe there isn't a bow shock; that really depends on the strength of the magnetic field; it might be about a wave—not quite a shock—but in this calculation we'll call it a shock. Then there is the heliopause, which Brian showed was material—it's that's the separation between the flowing interstellar material and the and the solar wind material—and then there is the termination shock where the solar wind goes from supersonic to subsonic. So it's supersonic on inside—subsonic on the outside. Okay, and here's the density for conditions today, namely interstellar medium neutral hydrogen is about 0.2. We have the hydrogen wall; it jumps all the way up to about point four—actually the term wall for a density of 0.4 per cubic centimeter. And then you know we have very little neutral hydrogen. Of course, the reason that the hydrogen wall is charge exchange, which in effect slows down and heats the interstellar material. Okay, fine. So we have this kind of structure—hydrogen wall; we have, you know, low amount of neutral hydrogen and so on. What if the interstellar medium is really hot—fully ionized? Well, then the temperature is up in a million degrees. I don't think there's million-degree plasmas in nearby; it may only be 20,000 degrees. Let's just say okay, fully ionized. If the Sun moves into a region that's fully ionized, then we'll probably have a little bit of a bow shock; there will be a heliopause, but the temperature will be the same on both sides probably, and there'll be a huge termination shock according to these models. There will—in the density of neutral hydrogen—that's going to be this structure here. What we're going to see is—new—truancy—neutral hydrogen in the LIC; there will be essentially no—let's get it right—this—that will be ions. So this is the electron density; then this will be the—no, it can't be the neutral hydrogen density because there isn't one. Okay—Oh, local bubble. Okay, for hot gas, the densities are all gonna be low, and there's no hydrogen wall because there's no hydrogen coming in. Okay, a different world. Okay, let's look—let's go further—keep going in the wrong direction. Okay, people have gone out and looked with the Arecibo radio telescope—measuring hydrogen's spin-flipped transition—this is 21 centimeter. Okay, looking out in all directions of the galaxies that could be seen from Arecibo and looking—finding out where those neutral hydrogen and what temperature it is. Right, individual data points are individual parts of the galaxy or clouds—you might want to call them—that have been studied in these 21 centimeter type measurements, and they're color coded, and I'll describe in a minute, but there may be a hundred or several hundred of data points for which there are not only location but also temperatures. Right, cold regions of the interstellar medium are very small, so when the Sun—the Sun—were to move through a region of very cold gas, it'll go through it quickly; that's—as—small—hotter regions tend to be much larger, so it takes more time for the Sun to move across a fertile region. So what's plotted here is new things: First of all, how much time it takes to go through red—hot regions of the interstellar medium—blue—cold regions—and second, what's the size of the heliopause at the time in which the Sun is moving through some of these regions. These cold regions have very high densities; high densities mean high pressure; the—over—the heliopause is determined by a pressure balance between the interstellar medium and the solar wind. So when you're going through a dense region, the heliopause moves in—it's small—and when you're going through a region of much lower pressure—higher temperature gas—that's much lower density, then the heliopause gets large. Okay, so notice if we're gonna go through cold gas regions, then the heliopause starts coming down to small numbers. Right, that's interesting. Let's take an example. Okay, here's the local bubble. Local bubble is a piece of our galaxy extending out to one or two hundred parsecs or parsec—as 600,000 AU—and it's a region of mostly ionized gas, except we have these warm neutral clouds near the Sun, and it's surrounded in black—color-coded regions—that are cold—higher density gas—seen, for example, in sodium one. Right, and this is the view—let's say—from—okay, this would be the view from one direction—gap—towards the galactic center—towards the north galactic pole—and notice this arrow—this is the direction in which the Sun is moving relative to this piece of the inner self—big piece of the interstellar medium—in the course of 12 million years. So roughly—and you know—maybe something in the order of 10 million years—the Sun is going to move into a region of much of cold—higher density gas—and if you go backwards—roughly 10 million years ago—the Sun was in another region of cold—high pressure gas—in which the heliopause will be compressed—very small. So let's go further. Okay, this is a great study—a separate field is a one.
Of my former students, now an assistant professor—it wasn't in university—and he wrote this paper with Catherine Wyman, one of his undergraduate students. Okay, and they did the following experiment. They say, "Well, we know which direction the Sun is moving through space, so we know the direction which the Sun was moving through space. Let's look in the backward direction and see, towards stars in that direction, how far it is you have to go back in time in order to start seeing cold gas." Cold gas is measured by sodium one, sodium one absorption. So they did it. Okay, and here's a plot, a function of several things. First of all, a distance going back where the Sun was, the time in years—I'm sorry, millions of years—going back in time. So nothing really happened in terms of sodium cold gas absorption until you get to about 10 million years ago, in which case you start seeing significant amounts of, you know, high-density cold gas. And as you go back in time, this is also a function of radial velocity; you can see a lot more of this cold material. So 10 million years ago and also 10 million years in the future, the Sun's going to be in a region of really cold gas in which the heliopause is compressed down, and lots of things are going to happen. No cold gas, neutral hydrogen is going to be flowing in close into the inner solar system, including the Earth. Cosmic ray flux is going to be much, much larger, for example. Cosmic rays destroy ozone; no ozone, no people. Okay, okay.
What is the nearest parcel of cold gas to us? Right. People have been studying this, in particular looking at 21 centimeters out in various directions, and what they find is—this is work by Hylas and Trollin—as they found in this direction, this certain direction, Galactic longitude and latitude, regions of space where there is cold neutral hydrogen, and was shown as these dots are nearby stars in which the light is projected through the cold gas to us. There's a paper by a Meyer and collaborators in which they've looked at several of these stars, and they find that this cold gas in these directions must lie between 11 and 24 parsecs because, at 24 parsecs, you have a star where you see it, and a star in the same direction 11 parsecs away, you don't see it. So the cold gas is somewhere in between; that's relatively nearby. Okay. Now, the Sun is not headed in that direction, fortunately, but what if it were? Okay, what if it were? What would happen? Right. So we have to make an assumption; we don't know the velocity of what the Sun would be in a particular direction, but if we assume the velocity is the same as going through presently through the interstellar medium, the density, neutral hydrogen density in this local Leo cold cloud—Leo because the constellation Leo—roughly 3000 per cubic centimeter compared to the local—so the local interstellar cloud was about 0.2—so it's an enhancement of density of only a factor of 15,000. Okay. So where's the heliopause? Well, the solar wind pressure, this course is going to decrease, is one over distance squared, so the radius of the heliopause will be its present value divided by the square root of 15,000. That's an interesting number: 1.3 astronomical units. There's the whole heliopause gets compressed almost to the Earth's orbit. Right, that will have consequences.
Let's go further. What if there were supernova nearby? Supernovas produce huge shockwaves out into space, and there, Hans Muller has done this calculation. Okay, let's put a supernova and take a typical energy of 10 to the 51 ergs and put it relatively nearby, 8 parsecs, and ask what happens. Okay, well, you can compute the ram pressure of this supernova explosion at the distance of 8 parsecs, and what you find is it's going to compress the heliopause enormously. So here's a plot of density. Well, by the way, first of all, the Earth's orbit is 1 astronomical unit, so there's the circle of the Earth's orbit, the radius in the direction of the supernova explosion and radius perpendicular to that, and what you find is the density structure, of course, is going to come right into the Earth's orbit and even more so. Here's a plot; red indicates the material is purely supernova remnant material; white means it's not. Okay, so this remnant, supernova remnant material is basically going to come into the Earth's orbit. Supernovae produce all kinds of stuff that we're made out of, and one of the interesting things that produces is iron. And there have been measurements of people looking at what is down on the bottom layers of the ocean, and they find—you'll scoop up some of these bottom layers—and you find there's a lot of iron 60. Iron 60 is radioactive; it has a half-life of one and a half million years. You can compare iron 60 to its daughters—I don't know what they are—take the ratio; you can say, "Aha, with that half-life, a supernova of this sort must have occurred and hit the Earth 2.8 million years ago." Right, and there have been more such events. So it's interesting how the interstellar medium could not only interact with the heliosphere but interact with us on the Earth.
Okay, so going backwards here, so let me finish with actually the very beginning of what I said, namely, we live in a very diverse environment: the interstellar medium, huge ranges of density and temperature. Things change since the Earth and the heliosphere moves through the interstellar medium; we're going to see very different structures over timescales of millions of years, but also I mentioned with this interstellar cloud, local interstellar cloud, and lenient, it might be tomorrow or a thousand years. Okay, we live in this changing environment, and the environment is not just interstellar gas; it's also dust, magnetic fields, X-ray cosmic rays, supernova ejecta, etc. Okay, so it's interesting to pursue this, and things will happen. You know, cosmic ray flux affects the Earth; the effects ozone, affects habitability. And I want to leave you with the idea that solar physics, heliospheric physics, and stellar physics have a lot to learn from each other. It's interesting to look at the interface of the two and how one field, interesting things in one field, affects what's going on in another. So I'll end with that and ask for questions. [Applause] Yes, for the local interstellar cloud? Yes. Okay, that's Redfield, and another one of the students actually did some measurements of trying to get the measure the local, the electron is in other of these clouds, these nearby clouds, and there's a range, but I think the real answer to your question is radio scintillation measurements, though people have gone out and looked at, for example, pulsars. Okay, and what you see is variations—that's called scintillation—you see scintillation in the Earth because of, you know, if you look at a star, it twinkles, right? And that's because you're looking through a highly inhomogeneous path through the ionosphere where the electron density changes on rapid timescales, so the starlight keeps looking through different electron densities. So that kind of scintillation measures the variation in the electron density in the ionosphere. The same thing happens in the interstellar medium, and you can, by looking at pulsars and quasars and looking at time variations, you can get an idea of not only the electron density in the interstellar medium but also how it fluctuates and what spatial scales that fluctuates on. Okay, spatial scales can be very small, I mean much less, much less than an astronomical unit. Yes, and I'm not an expert in this literature, but I could point you in the right direction of people who are. Yes, I mean, what star did it? Ah, wow. I've, I don't know of anybody who's actually pursued that, although I haven't gotten into the literature very deeply. Let me point out the following: I showed this local bubble, right, the word inside of—let me go back to that picture—ah, okay. We think the local bubble is produced, was produced by supernova explosions. There were explosions, of course; supernovae were massive star, massive stars, therefore young. Okay, so supernovae II come out of young clusters of stars. Okay, and they have to be massive, more than eight times the mass of the Sun, and these stars will live for, you know, a few million years and then explode. All right. It turns out that, you know, in this neighborhood, for roughly a hundred parsecs away, is what's called the Scorpio-Centaurus Association, okay, a whole group of young stars, many of which—and we also see neutron stars, which are the remnants of supernovae—in this cluster, and you can calculate from the mass distribution of stars in the Scorpio-Centaurus Association that it must have been a number of stars of mass more than eight times the Sun and therefore there must have been a number of supernovae. Okay, the stars in the Association are of the order of 10 million years old, so on that timescale, there would have been a number of supernovae that would be in our neighborhood, but exactly which star did it? I don't know; I don't know the villain of the piece. Okay, again, that's somebody who wants to really get deeply into this topic; it would be a great PhD thesis: identify the villain, right, our proven theory, right or wrong, as the case may be. Yes, other questions? Yes. Yeah, we are safe for the moment. Yes. Well, actually, it's projected that the next star relatively nearby in the galaxy to explode will be Eta Carinae, and that's—so I don't know—about 200 parsecs away. I think we're safe, but hmm, the thing is, it's more than that. Okay, I think we're safe on that one, but you know, the last supernova that went off in the galaxy was Kepler's supernova, which was 1604, and you know, in the last—and of course, there was one in the Large Magellanic Cloud, one of the Magellanic Clouds, in our lifetime, was like 20 years ago, supernova 1987A. Right, that was really bright; that was, you could see with the naked eye, but nothing within 8 parsecs. You can rest assured; you can sleep tonight. I don't know of any—do you know of any? No. One of the problems they tune—maybe we'll come up with an answer to that in the next decade or so. You mean, okay, direct imaging? You've actually wrote a—you had a Hubble program to do direct imaging; there's just no—either was asteroids or otherwise. Any other questions? Okay.