THE CHEMICAL COMPOSITION OF THE INTERSTELLAR MEDIUM

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<ul><li><p>THE CHEMICAL COMPOSITION OF THE INTERSTELLAR MEDIUM* </p><p>Lawrence H. Aller University of California Los Angeles, Calif. </p><p>The physics and chemistry of the interstellar medium present one of the most challenging problems in contemporary astrophysics. The problems are intimately intertwined: we must know the physical situation, temperature density, and SO on in the emitting material, but these in turn depend upon its chemical composition. </p><p>For a long time progress was painfully slow. It is nearly seventy years since Hartmann found the interstellar H and K lines of ionized calcium, and more than half a century has elapsed since Miss Heger discovered the sodium D lines. In the twenties, it was customary to speak of the interstellar calcium clouds - a terminology quantitatively as accurate as referring to rain clouds as nitrogen oxide clouds. In those days it was also customary to speak of the calcium chromo- sphere. The lines of ionized calcium were well placed for observation in the optical spectrum and were prominent in many celestial sources. Astrophysics moved slowly out of the calcium age. </p><p>By the end of the thirties, what appears to be the correct picture, began to emerge. The studies of Struve and his collaborators of the faint H I1 regions, by means of a specially designed nebular spectrograph, revealed the overwhelming importance of interstellar hydrogen; DunhamslO careful investigation of the curve of growth for interstellar lines observed in the spectrum of 5 Ophiuchi yielded additional clues to the physical nature of the medium, dilution of the radiation field, and so forth, while W y ~ e s ~ ~ classical investigation of the spectrum of the Orion Nebula indicated that it had essentially the same chemical composi- tion as stars such as the sun. Since then, enormous progress has been made in interpreting interstellar lines of the optical spectral region,lg in studies of bright H I1 regions such as the Orion Nebula, and particularly by the opening of new windows on the electromagnetic spectrum in the infrared, radio-frequency region, and ultraviolet. </p><p>Observations of interstellar lines yield rather specialized information. In the optical region they tell something about the kinematics and motion of the material in the direction of specific bright stars. In the ultraviolet region of the spectrum, where fall the resonance absorptions of several abundant elements and also important molecular lines, the information yield is considerably greater. </p><p>Radio-frequency observations have provided a wealth of intriguing data. Types of interstellar lines include high-level, n - 100, atomic recombination transitions of hydrogen, helium, and ionized carbon, and transitions of diatomic molecules and of polyatomic molecules. The latter appear to originate mostly in small con- densations with dimensions comparable with that of the solar system. They show remarkable time variations and maser effects and, for the time being at least, they are more likely to cast light on processes pertinent to the origin of solar systems and stars than directly to abundance problems, that is, except for data pertaining to isotope ratios that can be found from these interstellar molecular microwave lines. </p><p>For example, Snyder and B ~ h 1 ~ ~ have detected both H12C14N and H13C14N at wavelengths of roughly 3 mm in Orion A and Sgr A. They found N ( W) /N(I3C) </p><p>* This research was supported in part by National Science Foundation grant GP-23460. 45 </p></li><li><p>46 Annals New York Academy of Sciences = 8.9 for Orion and 4.7 for Sgr A. Penzias has detected CO in three isotopic forms, 12C160, 13C160, 12C1*0, for example in Sgr B2, but the isotopic ratio is a function of radial velocity indicating possible saturation effects. </p><p>Formaldehyde has been detected in two forms, H212C1E0 and H Z ~ ~ C ~ ~ O , and has given isotope ratios N( 12C) /N(13C), equal to 8.6 for Sgr B2, and 50 for W51 as compared with the terrestrial value of 89. Optical depth effects may complicate things. To date, there have been few interferometer observations which could verify the optical depth explanation of the discrepancy. Possibly, the 13C/12C ratio in the interstellar medium may actually differ from the terrestrial ratio, although a number of workers have suggested it is the same. </p><p>Infrared spectroscopic observations are extremely important in that they pro- vide data on certain forbidden lines of [NeII], [SII], and other ions that cannot be observed in conventional spectral regions. The ultraviolet interstellar extinction law also gives some information on the probable qualitative chemical composition of the solid grains-graphites, magnesium, silicates-but the data are mostly in- direct and can cast little light on the quantitative chemical composition of the interstellar medium. </p><p>Bright H I1 emission regions, that is, diffuse nebulae, offer some very special ad- vantages. A representative portion of the interstellar medium is heated to incan- descence and caused to fluoresce by the radiation of hot stars embedded within it. The emergent radiation is principally that of abundant permanent gases- hydrogen, helium, nitrogen, oxygen, neon, and argon-and also of sulfur, carbon, and occasionally weak lines of silicon, magnesium, and iron. An understanding of the relevant physical processes was developed in the twenties and thirties, and the necessary quantitative ideas and equations were expounded by Menzel and his associate^.^' </p><p>Later, I shall discuss the uses that may be made of gaseous nebular data in more detail, but I must mention some of the restrictions and limitations that are en- countered. First, the chemical elements whose abundances can be found from their emission line spectra in gaseous nebulae are limited to the more abundant ones. Second, most elements are observed in only one or two stages of ionization although they may actually exist in several (invisible) stages, and third, the analysis is enormously complicated by the clumping tendency of the material. There exist huge fluctuations in density, and possibly also in temperature, from point to point in most gaseous nebulae. </p><p>On the other hand, it must be remarked that the He /H ratio can probably be established much more accurately for the interstellar medium than for any star. The He/H ratio is an extremely important datum for many popular cosmological theories. It is fortunate that it can be measured in so many places throughout our universe. </p><p>Another trick is to use the Fabian approach, that is, to study the chemical com- positions of the atmospheres of stars that have but recently been formed from the interstellar medium. Almost any early type B star will do, since its age can scarcely exceed a few tens of millions of years. Stars of spectral class BO and 0 stars are likely to be less useful for our purposes-since we cannot even pretend to know enough about their atmospheric structures-and spectral lines must certainly be formed under conditions departing severely from local thermo- dynamic equilibrium. We must refer here to investigations by Underhill, Strom, Avrett, and their associates, and by Mihalas and Auer.l* </p><p>An account of the principles of abundance determination from stellar atmos- pheres would be quite extraneous to this presentation. A few general statements </p></li><li><p>Aller : Composition of the Interstellar Medium 47 </p><p>may suffice. We interpret the spectrum of a star with the aid of a model atmos- phere, often by an iterative process involving the chemical composition; however, in any event, we must know the effective temperature and surface gravity. The model atmosphere predicts the emergent radiation from a star, in the continuum as well as the lines. As one example, I might mention the work of Mrs. Petersz8 on the B3V star iota Herculis, for which available energy distributions, the measured ultraviolet flux, and determinations of angular diameters of similar stars indicate an effective temperature of 17,000 K. The surface gravity is near 1 04cm-2. The derived relative abundances are remarkably similar to those found for the sun. Somewhat similar results have been found for other stars by a variety of investigators, supporting a claim made some years ago that there is no estab- lished difference between the present chemical composition of the interstellar medium and that of the sun which was formed 4.5 X lo9 years ago. I feel that this statement can be taken only as a general guide. We must look for small but perhaps significant differences. </p><p>The questions to which we would like to have answers are the following: 1. Does the interstellar medium have the same mixture of elemental abun- </p><p>dances now as it had several aeons ago? Has its chemical composition changed appreciably with time? </p><p>2. Does the chemical composition of the interstellar medium change from point to point within the galaxy? How does the interstellar medium of our galaxy compare with that of other stellar systems? </p><p>3. What can be said about elemental abundances including isotope ratios in the interstellar medium in the neighborhood of the sun? </p><p>Of course, we can give no direct answer to the first question since we cannot find an uncontaminated sample from that period. All we can do is to examine the spectra of stars that are believed to be of very great age. I shall not discuss this topic here other than to recall that some very ancient clusters such as the globular cluster 47 Tucanae and the galactic cluster M67 (also of very great age) appear to have essentially the same chemical composition as the sun, while other globular clusters such as M92 appear to be metal-deficient. Apparently, the com- position of the interstellar medium changed very rapidly sometime during the early history of the stellar system, and relatively slightly since the time of the formation of the sun. But I think it is very important to find out just how much the interstellar medium changed from the epoch of formation of the solar system to the present time. Of course, if the composition of the medium changes from point to point in the galactic system, the pertinent datum is a comparison of the composition of the present-day interstellar medium at the point where the sun was formed. Since the sun is moving in a nearly circular orbit and since the composition of the interstellar medium changes slowly, if at all, with distance from the galactic center, it seemed legitimate to regard the local medium as corresponding to the medium of the region of the formation of the solar system, wherever that was. </p><p>The He/H ratio can be measured from line ratios in the optical region of the spectrum, e.g., 1(5876)/ (HP) , or 1(4471)/I(HP) or from radio-frequency lines. In each instance we are concerned with lines produced by recombination and subsequent cascade. TABLE 1 gives the He /H ratio for several diffuse galactic nebulae regions as measured by various observers using both optical and radio methods. </p><p>What about the stellar results? We note that here the values depend on what you assume about theories of line broadening. Shipman and S t r ~ m ~ ~ obtained </p></li><li><p>48 Annals New York Academy of Sciences </p><p>N(He/N(H) = 0.10 from the spectra of B stars, while Leckrone,15 from a series of B-star spectra carefully obtained for this specific purpose at Lick Observatory with the 120-inch CoudC, found 0.12. From main sequence eclipsing binaries in the spectral range A-G, Popper and coworkers30 obtained 0.12 and concluded that the ratio is unlikely to lie outside the range 0.09-0.14. They identified seven of thirteen of these main sequence binaries as unevolved and used the theoretical mass luminosity relationship to get the helium/ hydrogen ratio. </p><p>For the sun, one cannot determine the He/H ratio by spectroscopic methods. From a solar model with improved opacity and nuclear-reaction data, Bahcall and Ulrich' found N(He)/N(H) = 0.089. On the other hand, solar cosmic ray data predict a rather low value, -0.06 * 0.01 for this ratio. At the present time, we should accept probably a value of 0.1 1 as the most likely value for the helium/ hydrogen ratio in our part of the galaxy, remarking that the solar data are not yet properly explained . </p><p>Turning, to external galaxies, numerous measurements have been made of the He/H ratio in the local group of galaxies (TABLE 2) . </p><p>Peimbert and SpinradZ8 find a mean ratio N(He)/N(H) = 0.10 in several more remote external galaxies, NGC 4449,5461, 5471, and 7679. Thus, we reach the interesting conclusion that helium/hydrogen ratio is remarkably similar from one galaxy to another in our local region of the universe. </p><p>What about other elements? From a study of the nuclei of M51 and M81, Peim- bert argues that both galaxies have an overabundance of nitrogen in their central regions. He attributes most of the variation of the [NII] / H a intensity ratio from the spiral arms to the nuclei to an actual variation in the nitrogen abundance rather than to a change in physical conditions. Peimbert and Spinrad conclude </p><p>TABLE 1 THE H E L I U M ~ ~ Y D R O G E N RATIO FOR SEVERAL GALACTIC NEBULAE. </p><p>Object N(He)/N(H) Method Reference </p><p>Orion 0.12 optical 11, 4a, 16 0.12 2 0.01 optical 23 0.083 I?r 0.004 radio 23 0.11 optical 27 </p><p>M 8 0.113 optical 16 0.11 optical 27 </p><p>M17 0.107 optical 21 0.09 I?r 0.01 radio 23 </p><p>M20 0.097 optical 16 Carinae 0.122 f 0.02 optical 11 </p><p>W5l 0.081 I?r 0.008 radio 23 NGC 6357 0.088 k 0.018 radio 23 </p><p>IC 1795 0.081 k 0.016 radio 23 </p><p>Notice that ratios measured by radio-frequency methods tend to be smaller than those found by optical methods. One must, of course, allow for the fact that in some H I1 regions much of the helium exists as neutral gas and, therefore, its recombination lines are not ob- servable. Particularly in the extended regions measured by the radio techniques, there are likely to be large zones in which the helium is neutral so that the measured ratio will tend to be too low. We prefer, therefore, the results of the optical studies and adapt them for our local region of the galactic system: N(He)/N(H) = 011. </p></li><li><p>Aller : Composition of the Interstellar Medium 49 TABLE 2 </p><p>THE HELIUM/HYDROGEN RATIO IN EXTERNAL GALAXIES </p><p>Nebulosity Galaxy Ratio Reference </p><p>NGC 604 M33 0.106 16 0.12 1. 0.13 28 </p><p>NGC 346 SMC 0.076 f 0.02 11. </p><p>30 Doradus LMC 0.082 f 0.02 11. *The value 0.12 has been augmented to 0.17 by Peimbert and Spinrad to allow for the </p><p>presence of neutral helium, but I believe this correction to be too large. Possibly, the results for NGC 346 and 30 Doradus might have to be similarly increased. However, it will be difficult to make such corrections until detailed structural models of these nebulae have been developed. Johnson1* obtained 0.1 3 for 30 Doradus by photographic photometry. </p><p>that the irregular galaxy NGC 6822 is deficient in nitrogen and oxygen by factors of 6 and 1.7 respectivel,, whereas the helium/hydrogen ratio is similar to that found in other systems. Further investigation of the physical conditions in such systems is necessary be...</p></li></ul>