Robert Wilson (1982) - Interstellar Molecular Clouds

In 1970 Arnold Penzias, Keith Jefferts and I put together a millimetre wave spectral line receiver which we took to Kitt Peak to look for carbon monoxide in interstellar space. The main ingredient of the receiver was a front end that had been developed for communications purposes at Bell Labs. But we integrated it with pieces that the national radio astronomy observatory provided and made the complete spectral line receiver. When we finally got to Kitt Peak, got it all installed on the antenna and were ready to go, the source on our list which was up was the Orion Nebula. This is a fairly ordinary ionised hydrogen region, it happens to be relatively nearby. So that it is a nice thing for people on earth to study. It's excited by some hot stars in this region which are so bright that on this picture which has been exposed for the nebulosity, the ionised gas, you can’t really see them. They are usually referred to as O or B stars and they may have masses as much as fifty times that of the sun. But a star with a large mass burns its material much faster than a low mass star. So such as star will have luminosity perhaps twenty thousand times that of the sun, and a lifetime of only a few million years. So one knows that such a region is a relatively young object. At that time the chemistry of the interstellar medium was really fairly simple. If we ignore possible massive neutrinos which don’t enter into chemistry anyway, Maybe a quarter of it helium. And those are things which were formed in the big bang most likely. And something under 1% of such regions was typically thought to be dust. That’s the only readily observable component of the heavy elements. The things we are made of that were a result of previous generations of star production. At that time, the OH radical had been discovered and was distributed somewhat with neutral hydrogen atoms. A couple of years before ammonia had been found, water vapour and formaldehyde. But the ammonia and water vapour were in very special dense regions. About here in the Orion Nebula, there were a couple of infrared emitting regions that didn’t have optical counterparts. So called Kleinmann-Low and Becklin-Neugebauer nebulae. So this was a somewhat interesting region but we had no way of knowing that there might be any carbon monoxide in it. In fact formaldehyde, which was the only thing that was known which might have one made expect carbon monoxide, was not present in that region. Well, when we got the receiver going, there was a mode in which it would operate for testing purposes in which it continuously looked at the spectrum with about a two second time constant. So that you could see things change. It wasn’t for serious observing but as soon as we looked at the antenna pointed at the Orion Nebula something like this, with of course not the signal and noise ratio, immediately appeared on the screen. After some months of working, not knowing whether one was going to see anything, it was quite a moment of discovery. After we looked at it more carefully, we developed a picture like this of the central spectrum. I probably ought to tell you a little bit more about what that is. Carbon monoxide is a diatomic molecule. You have a carbon and an oxygen and they can spin around one another. And the first rotational transition frequency is at 115 gigahertz or 2.6mm. That’s what we were looking at. A small dipole moment so that as the thing rotates it tends to radiate radio photons. Well, if we were just looking at the intrinsic radiation of carbon monoxide we would see only one frequency of radiation. But of course I can’t imitate Archehelos' good explanation of the Doppler effect, but that’s what’s going on in the source. Some of the molecules are coming toward us, some are going away. The ones that are going away radiate at a lower frequency. And the ones that are coming toward us radiate at a higher frequency. And so this frequency displacement can be expressed as a velocity. Why is the carbon monoxide radiating at all? Something keeps hitting it. We now know that there’s a lot of hydrogen molecules in the same region. And the temperature of the gas there is around 70° and when a carbon monoxide molecule encounters some other molecule, typically a hydrogen molecule, it can be set spinning again. So if its radiated away its rotational energy will be started up again. Radio photons unlike optical photons are very easy to generate. You don’t need thousands of degrees, 2.6mm all you need is a few degrees above absolute zero, a few Kelvin. So this region which we are looking, the temperature is only about 70 Kelvin. At that sort of temperature the soundspeed is about a half a kilometre per second. So it's clear that the velocities in this thing, even the core of this line, the velocities are quite supersonic. That is the beginning, first manifestation of a problem. What is it that’s causing the broadening of this line? Another thing which shows up on this spectrum is the fact that this is not a simple narrow spectral feature, but has emission which continues out to very large velocities. That’s perhaps the hint of a solution to the first problem I mentioned, but I’ll get back to it in a few minutes. If we look at somewhat later results in which we’ve measured three isotopes of carbon monoxide, substituting first the carbon and then the oxygen less abundance species, you start to see the problem in little more detail. We have carbon monoxide on the same scale as 13CO, which should be maybe a sixtieth of the abundance. On the earth it's about a ninetieth of the abundance of the carbon monoxide. So, here we have it appearing at maybe one fifth or one sixth of the intensity. So what’s the explanation of that? Probably there’s so much carbon monoxide that we can’t see it all, it's saturating. The molecules on the near side of the cloud are absorbing the radiation from the molecules on the far side of the cloud, and we can only see radiation from a part of it. Or on thermodynamic terms, the actual temperature of the cloud correspondents to this peak. Then we have, we know that the isotopes ratios are not too messed up because the 13CO and the C18O are in somewhat of a correspondence. Although Arnold will go into that in much more detail, I expect. If one actually had a simple saturation model with such a region with isotropic homogeneous, I guess I mean, turbulence causing the line broadening, then if you multiply a point about here by ninety or by sixty. one would come up away above the picture. But the thermodynamic limit would make it still come about there. If we come out in the wing somewhat down about half as much, we are still going to be up there, because we are still essentially saturated. Only when we get way out where this line has gone down very much, would we expect the line shape to drop down. We would expect a very square topped line, under such conditions. Obviously that’s not what’s going on. Another possibility is large scale motions in the whole cloud. A likely thing in such a cloud might be collapse. However if the clouds were collapsing at the velocities involved the lifetime would only be a few million years, and those clouds would be much less abundant than we see. What was immediately clear especially from looking at the 13CO is that there is a large amount of carbon monoxide in this cloud. When one gets into details of the excitation and how many hydrogen molecules it takes to excite the carbon monoxide, there is a tremendous mass in this cloud. If I may have the first slide? We can see a larger picture of the region that’s been made in carbon monoxide. Here we have the extent of the Orion Nebula which we were looking at before. These are the three stars in Orion’s belt. So you can see the constellation here as it appears on the sky. And now you can see the extent of carbon monoxide in the vicinity of the Orion Nebula. I think that will do for the slide. It's clear that the nebula itself is just the tip of the iceberg. There’s a lot of material behind there which is associated with it. In fact the explanation which is developed is that the Orion Nebula has been created out of the molecular cloud. The so-called giant molecular cloud, which is behind it. This is in essence a new type of object that was discovered with carbon monoxide. The picture I showed a moment ago covering degrees of sky one might term a giant molecular cloud complex, since it actually can be subdivided into separate clouds. The picture though is that somewhere in such a cloud when the densities get great, star production can start. And if a few high density, a few large mass stars are created, they can turn on and start producing a lot of ultraviolet flux. This can ionise the remaining gas around the clouds, heat it up the way the nebula looks and blow apart that part of the cloud. There are various scenarios people have suggested for what might happen following that. A shockwave from such an event could propagate into the cloud causing another generation of stars to form, or there may be other reasons why stars would form in the other parts of the cloud. But in any case, it seems to be common for stars to form in such clouds. In fact for a number of years we found molecular clouds just by looking for the ionised hydrogen regions caused by the bright stars which had been formed from them. If we go back to this picture and the high velocities which we see there, if we were to look away from the centre of a cloud, this part of the line would stay about the same. But the component at very large velocities would disappear. So it's a very localised phenomenon. In recent years it's been realised that this is a new phenomenon. High velocities flows in these clouds. And apparently there are stars there which are in the process of either blowing away their envelops or having a tremendous stellar wind. You are probably aware of the wind of the sun which is just a wind. This is a stellar tornado or something. Because it is extremely high velocities and a lot of mass. The problem I alluded to before with turbulence in the cloud is partly because supersonic turbulence tends to dissipate very quickly, or at least to dissipate a lot of energy. So you need a lot of energy input to keep supersonic turbulence going. Here we have a new source of energy input in such a cloud. There are in fact six such flows within the Orion Nebula. It's not clear that the sum of that is enough to keep the clouds stirred up, but certainly that energy input is contributing to keeping the cloud going. I’d like to make a comment to the students at this point that it's very important that you understand your apparatus if you are doing experiments. And, once you understand it and know what it's doing it's important that you pay attention to things like this, that you weren’t really looking for. Because this was known, existing in 1970 but it was only in 1977 that someone else looked at a similar spectrum and realised that there was new information there. Let's take a look at a different cloud. This is a so-called horse head nebula, named after the obvious horse head here. This is a negative photograph. The stars are appearing black and this white region is actually an absorption region, there’s a lot of dust in there. And in this case we have a dusty cloud on this side of the picture and an ionised hydrogen region over on this side. Now if one looks at this in carbon monoxide, I will superimpose the brightness of the carbon monoxide on the optical picture, I’ll take it away so you can see the optical picture again, then superimpose it. What we are seeing is that the original dark cloud contains the molecules and the ionised hydrogen region is eating away on that. This particular region has a series, not just the single horse head, but a series of bumps on it which are spaced at just about the spacing one might expect from Rayleigh–Taylor instability. We have a light identity gas on this side which is impinging on a low density gas on this side, impinging on high density gas over here. So it's like the overburden of a heavy liquid on a light liquid. These are either instability from that or regions of extra density. Some of the pressure in this region is caused by the rocket effect. The ultraviolet radiation from the stars in this region are hitting the surface of the molecular cloud causing it to evaporate and putting pressure on it. What about cloud lifetimes? We see numerous examples of these clouds. What can we learn about how long they might live? If they were to live a short time and suddenly turn all their material into stars, then there would be many more stars around in our galaxy. However, there is a question then of whether they might live a long time and only have periods of star formation or short periods of star formation or whether they live a short time. Because in spiral galaxies such as the andromeda nebula shown here, one can see a large contrast optically between things that one might call arm regions and inter-arm regions. The star formation seems to be very much concentrated to the arm regions. Well, at the bottom I have a blow up of this little region but let me show you a carbon monoxide picture in more detail. Here we have the same region and you can see that the carbon monoxide is concentrated in an arc there. On the other side you can see a blow up of the photograph and then I will help you by putting this on top. You can see that the dusty region, where the molecules would be expected, shows carbon monoxide. The other regions, the part of the centre shows very little. So it's clear that the molecule clouds at least in M31, the andromeda nebula, are highly concentrated in the arms of the galaxy. In fact there’s at least a 5:1 contrast between the arm region and the inter-arm region. And in the inter-arm region if there were just a simple giant molecular cloud in our beam in any of this region we would see more than we do. So it seems in at least M31 the giant molecular clouds and so-called spiral arms really go together. There’s a difference however. M31 looks a lot like what we think our galaxy looks like. But the carbon monoxide shown in this picture is only about a tenth as much as one would expect if one was seeing our own galaxy from the same perspective. So we can’t take the association completely directly. Well, a simpler sort of thing can be seen in at least parts of our own galaxy. This is some carbon monoxide which has been selected from a region where we think there’s a spiral arm. You see that there are large extents of carbon monoxide. One might at least call this a cloud complex, another big cloud, there are numerous big clouds in this region. This is at 34°, a few degrees away at 36° where we expect an inter-arm region we see only a few very minor clouds. Again the giant molecular clouds and the giant molecular cloud complexes at least in that part of our own galaxy don’t seem to be present between the arms, but are present on the arms or in the arms. That goes along with the picture that probably the giant molecular clouds are what really define the arms from an observationalist point of view. Thus one is left with the problem of the lifetime of the clouds, how they are formed and destroyed. One expects that the same material is going to be used in the next generation of arms. I should have explained earlier that the material in a spiral arm does not stay there. The linear velocity on a galaxy is about constant. So that the inner part rotates with a much higher angular velocity than the outer part and any structure will rotate up and become very flat very quickly. So in order to maintain anything which is not circular requires that the material moves through the structure. That it would be a density wave or something else which is not actually a material structure. I’ve given an overview, I think, of molecular clouds. I would like to close with a discussion of something I’ve been studying recently, and that is the structure of small dark clouds. May I have the second slide please. We are going to see a picture of what’s known as the dark cloud. The next slide please. The dark cloud is known because it's relatively nearby, it's full of dust and it obscures all the stars in the background. So it's not very dramatic but you can see a region that’s somewhat elliptical, in which there are not very many stars to see, compared to the rest of the region. What happens when we look in carbon monoxide? Here is the 12CO, common carbon monoxide, and it's a slightly funny plot in that it has a spacial coordinate this way and a velocity coordinate that way. We’ve taken a cut across the cloud and plotted the intensity and velocity as we go along that cloud. What you see is that as you go across the cloud this way the velocity is changing. That’s the picture of a rotation. This is a rotation which is counter to the general rotation of our galaxy. When we look in the abundance species of carbon monoxide, we see only the outer part of the cloud, as I explained before. If I now show you a slightly different presentation of the same sort of thing with C18O, you can see that the velocity is now horizontal and the same special offsets are vertical but what we’ve shown is several spectra instead of a contour plot. You can see the opposite slope. We are now looking at the species which is perhaps one five hundredth as abundant as the first one. We are looking way into the cloud, and we are seeing a rotation in the opposite direction. If we look at 13 carbon monoxide, we can see both the major, the rotation of the major part of the cloud and the minor part of the cloud. The core of this cloud, the part that is rotating in the same direction as the galaxy as a whole, contains perhaps three hundred solar mass and the outer part maybe eight thousand. The axis along which I took these cuts is the small axis of the cloud. That is the rotation axis seems to be the long axis of the cloud: It's about fifteen light years by thirty light years. The typical picture of something which is collapsed is a pancake which is rotating around an axis perpendicular to the pancake, because it's easier to collapse along the direction of rotation than perpendicular to it in order to conserve angular momentum. In this case the general galactic magnetic field is perpendicular to the direction of rotation. And that may have caused the collapse of the cloud perpendicular to the field to be slowed. That is, there’s enough ionisation even though the cloud is basically neutral, that the material has a hard time crossing the magnetic field. The magnetic field may have been wound up by the cloud as it collapses and rotates. That is like the ice skater as the cloud comes in it will spin more rapidly. Perhaps the core of the cloud spun much more rapidly then the outer part, wound up the magnetic field, storing energy just like in a rubber band. That then stopped the rotation and has accelerated it in the other direction. Or perhaps the core disconnected itself by completely winding up this magnetic field and the outer part has done that. In any case it's an interesting puzzle in cloud structure to see this cloud which is rotating in both directions. Thank you.

Robert Wilson (1982)

Interstellar Molecular Clouds

Robert Wilson (1982)

Interstellar Molecular Clouds

Comment

Robert Wilson came to Lindau in 1982 together with his co-Nobel Laureate, Bell Labs colleague and astronomy research collaborator Arno Penzias. Their two talks did not explicitly touch upon their Nobel discovery, but reported from ongoing radio astronomy research concerning the interstellar medium, the space between the stars. Radio astronomy started up around 1950, but in the beginning the radio astronomers in general “saw” only a diffuse radiation of varying intensity. Then there was the important and promising discovery of a sharp line at 21 cm wavelength, emanating from hydrogen, the most abundant element in the Universe. With the invention of the maser amplifier by Charles Townes in 1954 (rewarded by a Nobel Prize in Physics ten years later), a very rapid development took place and from around 1960 characteristic radiation from a whole series of molecules was detected. Bell Labs were at the forefront in constructing amplifiers using the new technique and Robert Wilson in his lecture describes results from very precise observations of the molecule CO, carbon monoxide, in interstellar space. When this molecule rotates, it emits radio waves at around 2.6 mm wavelength, with small variations depending on what isotopes of C and O enters into the molecule. It turned out that “empty space” was not empty at all, but filled with molecular clouds of varying sizes and densities. Wilson describes how his measurements of three isotopes of CO laid the foundation of a new picture of, e.g., the Orion nebula. Behind the nebula he detected a giant molecular cloud, which made this well known and well studied nebula appear as only the tip of an iceberg! Anders Bárány

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