Isotopic Composition of Some Metals in the Sun
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SNSTITUTE OF THEORETICAL ASTROPHYSICS BLINDERN - OSLO REPORT .No. 35 ISOTOPIC COMPOSITION OF SOME METALS IN THE SUN by ØIVIND HAUGE y UNIVERSITETSFORLAqET • OSLO 1972 Universitetsfc lagets trykningssentral, Oslo INSTITUTE OF THEORETICAL ASTROPHYSICS BLINDERN - OSLO REPORT No. 35 ISOTOPIC COMPOSITION OF SOME METALS IN THE SUN by ØIVIND HAUGE UNIVERSITETSFORLAGET • OSLO 1972 Universitetsforlagets tryknlngssentral, Oslo CONTENTS Abstract 1 1. Introduction 2 2. Fine structure in spectral lines from atoms 5 1. Isotope shift 5 2. Hyperfine structure 6 3. Applications to atomic lines in photospheric spectrum .... 8 1. Elements with one odd isotope , 9 2. Elements with two odd isotopes 9 3. Elements with one odd and several even isotopes 11 k. Elements with several odd and even isotopes 11 h. Studies of elements in the Sun with two odd isotopes 1. Isotopes of rubidium 12 A. Observations lk B. Calculations 16 C. The Rb I line at 78OO Å 1. The continuum level 16 2. Line profiles and turbulent velocities 18 3. The asymmetry of the Si I line 19 h. Isotope investigations 21 P. The Rb I line at 79^7 A 28 E. The isotope ratio of rubidium 31 F. The abundance of rubidium 3k 2. Isotopes of antimony 35 A. Spectroscopic data 35 B. The Sb I lines at 3267 and 3722 A 37 3* Isotopes of europium 1*0 A. Observations and methods of analysis ^1 B. Spectroscopic data 1*1 C. Spectral line investigations 1. Investigations of four Eu II lines **3 2. The Eu II lines at Ul29 and U205 k ^6 D. The isotope ratio of europium 50 E. The abundance of europium 51 5. Study of an element in the Sun with one odd and several even isotopes 1. The upper limit of the solar Sr 87 content 53 A. Observations 5^ B. Spectroscopic data 56 C. Isotope investigations 58 D. The abundance of strontium » 60 6. The isotope ratio of copper in the Sun studied from Cu I and CuH lines 1. Studies of molecular lines from other elements 62 2, The isotope ratio of copper , 63 7- Summary and final comments 67 Acknowledgements * 70 References ,., ,, 71 - 1 - Abstract Information on the isotopic composition of chemical elements in the Sun may be obtained by analysis of phctospheric and sunspot spectra. It is shown that besides investigations performed on molecular lines, information on isotope abundances of some heavier elements may also be derived from atomic line studies. A discussion of the elements Kb, Sr, Sb and Eu are described. Studies of the solar isotopic composition of Cu from analysis of Cu I lines in photospheric spectra and CuH lines in sunspot spectra are described. The results obtained are: Cu65/Cu65 = (70+T)/(30+T) Rb85/Rbo7 = (73+1*)/(27+1») Eul5l/Eul53 - {52+6)/(kQ+6) Sr87/Sr <_ 0.25 No conclusions could be drawn about isotopes of Sb in the Sun. Although the results may indicate a solar over-abundance of Eu 151 as compared with terrestrial composition, the investigations provide no proof of non-terrestrial isotope ratios in the Sun of any of the elements listed. The abundances of Kb. Sr and Eu were determined and the results agree closely with previously determined values. 1. INTRODUCTION Increased knowledge of isotope abundances is of vital importance for a "better understanding of the processes that led to the formatien of the chemical elements and to the formation of celestial bodies. Conclusions about the evolution of different celestial bodies and their internal relation might easier be dravn when the isotopic composition cf their constituting elements is knovn. Accordingly large efforts have been made in order to increase our knowledge of isotopic compositions in different celestial bodies. During the last few decades our know ledge of the composition of bodies belonging to the Solar System has increased enormously. The composition of the terrestrial crust, meteorites and surface layers on the Moon are nov well known. Data on the isotopic composition of elements in the Sun are important for theories of the creation and the evolution of this» our nearest star in space. But the methods one can use to determine isotope abundances in the Sun are not as direct and sensitive as these that can be applied when analysing stone samples. The solar results must therefore he given with considerably larger error limits. Some information on isotopes in the Sun may be obtained from direct study of the composition of the solar wind and of solar cosmic rays. But the results obtained -from such investigations do not always reflect the average composition in the Sun's photosphere. Orbits of ionized particles are perturbed by electromagnetic fields and in cosmic rays, secondary emmission is mixed up with the primary rays. Moreover, in the solar wind and cosmic rays the concentration of many elements is too small to permit investigations. - 3 - Information on solar isotopic composition is obtained from spectral line studies. The wavelength of lines from molecules and atoms is isotope dependent. In order to interprete the observed spectra, the physical conditions in the line forming region together with various atomic or molecular data must be known. The isotope displacements are often very smell and disappear in line broadening due to high temperature and velocity fields in the region of line formation. In sunspot spectra also magnetic fields act on the profile. In molecular bands spectral lines from different isotopic compounds are often well resolved. But only few elements exist in molecules in the photosphere. Investigations often have to be undertaken on spectra from sunspots where the lower temperature leads to an increase in the number of molecules. It is often necessary to know the direction and strength of magnetic fields when sunspot spectra are examined. The setting of the continuum level may he difficult as spectral lines from spots are very numerous and overlap. Ho undisturbed spectral region may be found. In isotope studies of very light elements atomic line spectra are most frequently used. The isotope shifts in spectral lines from light elements are large enough to affect line profiles. The reason for this is to be found in the mass dependence of the Rydcerg constant. Considering heavier elements, this effect is negligible. But the isotope shifts also depend on other properties of the nuclei, and isotope shifts and hyperfine structure may lead to information on the isotopic composition of heavier elements. These two effects are usually undetectable, but in some cases minor influences on solar line profiles occur. Recent developements in observational technique and new methods to calculate line profiles, however, make it possible to determine the composition of some - It - heavier elements from observations of atomic line profiles. Vhen recording the photospheric spectrum using extremely long observing time, the noi Be in an averaged spectrum is brought down to a very lov limit. And when spectra are taken with an integrating lens in front of the slit, an average of the emission from an extended area is recorded. Local variations in physical conditions in the photosphere do not affect the result. The number of elements for which information on isotopes in the Sun may be obtained from molecular line studies must be limited, as only few elements exist in molecular compounds. Also atomic line investigations may give information only on a very small number of elements, but it is fortunate that amongst these there are some e3ements which are not expected to exist in molecular compounds. Solar isotope ratios are always given with large error limits as compared with results obtained from stone sample analysis. Nevertheless, it should be pointed out that the derived isotope ratios may be given with considerably smaller errors than the derived abundance ratios between different elements. Isotopes from the same elements have many physical constants in common and errors in their values or in the model atmosphere do not substantially affect the final result. In the following the solar isotopic composition of some heavier elements is discussed. A short bibliography referring to results obtained by different authors on other elements was given by Hauge and Engvold (1970). - 5 - 2. FIHE STRUCTURE IN SPECTRAL LINES FROM ATOMS When spectral lines are examined vith spectrographs of very- high resolution, it is frequently found that the line is split into several components lying close together. As an example the fine structure of a Tb II line is demonstrated in Figure 2-1. The photospheric spectrum at this wavelength has earlier heen studied by Engvold and Hauge (1970). The fine structure of the Yb II line can be considered as the result of two effects- Even isotopes appear as single lines. One odd isotope haB several, components shoving the hyper fine structure. And, as seen from Figure 2-ls two odd isotopes may have different hyperfine structure. The spectroscopic behaviour of even and odd isotopes is fundamentally different and of vital importance for the discussion given in Chapter 3- a,p> -ni Q,b/.,d - 173 Fig. 2-1. Fine structure of the Yb II line at 36*9^ A as determined by Krebs and Nelkovski (1955). 2.1. Isotope shift Golovin and Striganov (196*8) have given, an extended survey of experimental results, theoretical interpretations and a comprehensive bibliography of isotope effects in Bpectra from heavy elements. Only a fev characteristic behaviours of the isotope shift are referred in the following. - 6 - 1. The energy of certain levels is more isotope sensitive than other. The consequences are that different spectral lioes from an element show quite different isotope shifts. The heavier isotope may he situated on the red side or on the blue. 2. As seen in Figure 2-1, the wavelength interval between Yb 170 and Yb 172 is larger than the interval between Yb 17^ and Yb 176.