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19 64Apj. . .139. .4 0 6G 406 NOTES Vol. 139 in View of the Brightness Of 6G 0 .4 406 NOTES Vol. 139 .139. In view of the brightness of this object, it is strange that bright Ha was not detected previously; it may be variable. 64ApJ. 2. Of perhaps greater interest is the presence of strong bright H and K lines in the 19 spectrum of the sixth-magnitude star 12 Camelopardalis on a plate taken on January 13, 1956, by L. V. Wallace. Since similar stars (f And, a Gem, HR 7275, X And) are spectroscopic binaries with periods around 20 days, the radial velocity of this object may prove to be variable, though plates measured thus far (three early Mount Wilson spectrograms) give no evidence of this. Note added in proof: 12 Cam has recently been classified by Slettebak (Ap. 138, 118, 1963) as K0 m. William P. Bidelman August 22, 1963 University of Michigan Observatory THE SOLAR HELIUM ABUNDANCE Since it is not possible to determine the helium abundance in the sun spectroscopically, attempts have been made to do so with the aid of theoretical models of the solar interior, most recently by Osterbrock and Rogerson (1961). An independent check on this deter- mination now exists, in that measurements of the relative abundances of nuclei in solar cosmic rays have been obtained (Biswas, Fichtel, and Guss 1962; Ney and Stein 1962; Biswas, Fichtel, Guss and Waddington 1963) with strong evidence that, except for hydro- gen, these abundances fairly represent those on the solar surface. The ratio by number of helium nuclei to medium nuclei given by Biswas et al. (1963) is He/(C + N + O) = 60+7 p.e.; the same ratio derived from the results of Osterbrock and Rogerson (1961) is only 45. In the following paragraphs we discuss several possible sources of this dis- crepancy. The detailed arguments supporting the assumption that the cosmic-ray-abundances are those of the solar surface can be summarized as follows. Theoretical models for the production of cosmic rays in flares involve temperatures of the order of 1010 ° K, which is more than sufficient to completely ionize all the elements. The nuclei of He, C, N, O, and Ne all have the same ratio of charge to mass, and one therefore expects these ùuclei to preserve their relative abundances throughout both the acceleration process on the sun and the transfer from the sun to the earth. In support of this idea it is ob- served first that the helium and medium nuclei have the same energy spectrum, whereas that for hydrogen is much less steep. Second, the ratio of helium to medium nuclei (in the same interval of momentum per unit charge) remains fairly constant over six meas- urements in three different flares, whereas the ratio of hydrogen to helium or hydrogen to medium nuclei varies by more than a factor of 10. Third, the relative amounts of C, N, O and the group of elements between Na and A agree well with the spectroscopic abundances, and the upper limits set on Be, B, and F are also consistent with them. In deriving the ratio He/(C + N + O) given above, Biswas et al. computed a straight mean of six independent determinations, and computed a probable error by the expres- sion p = (2^/)1/2/6, where pi is the probable error of the individual determinations. This treatment of the data seems incorrect, for it essentially ignores the fact that these measurements are of the same physical quantity, of different weights as indicated by their different probable errors. Instead, we form a weighted average, with weights pro- portional to \/p?. The probable error for such a weighted mean h p — (Sl/^/)~1/2 (Worthing and Geffner 1943). Using this procedure, we obtain from the data of Biswas et al. the number ratio He/(C + N + O) = 54 + 6 p.e. In their determination of the spectroscopic abundance of the heavy elements, Oster- © American Astronomical Society • Provided by the NASA Astrophysics Data System No. 1, 1964 NOTES 407 brock and Rogerson relied on the forbidden lines of O i at X 5577, X 6300, and X 6364 from which they obtained the values 2.25 X 10-3, 0.92 X 10“3, and 1.01 X 10-3, respec- tively, for the O/H number ratio (Rogerson 1963). The latter two values are probably more reliable than the first, for these lines arise from magnetic dipole transitions for which the transition probability depends only on the spin-orbit interaction parameter, whereas the X 5577 line arises from an electric quadrupole transition for which the transition probability depends strongly on an integral over quite inaccurately known wave functions (Garstang 1956). Therefore a value of O/H = 0.96 X 10-3 is more likely correct than the value O/H = 1.4 X 10-3 used by Osterbrock and Rogerson. Fig. 1.—^Abundance parameters for the sun. Solid lines show the X-Z relation derived from Wey- mann’s models for 3 values of the opacity. (Kw refers to the opacity used by Weymann.) Points are computed from the value He/(C + N + O) = 54 ± 6 p.e. (the bars representing this uncertainty) and the tabulated values of O/H and Mz/0. Neon, like helium, is not observable spectroscopically on the sun. The ratio Ne/O used by Osterbrock and Rogerson is 0.8, whereas that found in the cosmic rays is only 0.1. Using the latter value, and more recent abundances of the other elements (Aller 1961), we find that the total mass of heavy elements per oxygen atom (Mz/O) may be as low as 28 atomic units, instead of the value 46 used by the above authors. The most uncertain aspect of the solar model calculations which can affect the helium abundance determination is the opacity. We have compared the opacities used in the models of Weymann (1957) with more accurate computations obtained from Baker © American Astronomical Society • Provided by the NASA Astrophysics Data System 6G 0 .4 408 NOTES Vol. 139 .139. (1963) for two different compositions (X = 0.70, Z — 0.03; X = 0.60, Z = 0.03) and temperatures and densities appropriate for the solar interior. We find that at tempera- 64ApJ. tures above 10 million degrees the Weymann opacities are too high by as much as a 19 factor of two, but at 4 million degrees they are too low, again by a factor of two. In Figure 1 the relation between the two composition parameters X and Z derived from Weymann’s models is shown, and also the relations obtained by adjusting his opacities in both directions by a factor of 1.5, which we take to be the probable uncertainty of the mean opacity. We can derive these parameters X and Z independently of the solar models from the equations Z/X = O/H X Mz/O, F/X = He/(C + N + 0)X(C + N+ 0)/0 X 0/ H X 4, and X + F + Z = 1. The four points shown in Figure 1 are computed from the observed values of He/ (C + N + O) in the cosmic rays and the possible choices of O/H and Mz/0 discussed in the previous paragraphs. It is clear from the figure that none of these points are inconsistent with the present solar models. Therefore a helium abundance as high as that represented by the lower end of the error bar on point B(X = 0.60, F = 0.36, Z = 0.04) cannot be excluded on this basis. However, in view of the discussion above, the true composition of the sun is more likely that represented by point C on the diagram, X = .72, F = .26, Z = .02, which is obtained by adopting a low value of the neon abundance and excluding the X 5577 line in the oxygen determination. Only a small increase in the opacity is needed to bring Weymann’s models into agreement with this composition. This result is consistent with recent work of Peebles (1963), who has shown that the ratio (by weight) of helium to hydrogen in the interior of Jupiter, which is not likely to be less than that in the sun, cannot be much greater than Y/X — 0.3. The author wishes to thank Professor R. H. Dicke for calling his attention to the cosmic-ray measurements. John E. Gaustad July 5, 1963 Princeton University Observatory REFERENCES Aller, L. H. 1961, The Abundance of the Elements (New York: Interscience Publishers), p. 192. Baker, N. 1963, private communication, opacities computed with Los Alamos Code described by Cox (1963). Biswas, S., Fichtel, C. E., and Guss, D. E. 1962, Phys. Rev., 128, 2756. Biswas, S., Fichtel, C. E., Guss, D. E., and Waddington, C. J. 1963, /. Geophys. Res., 68, 3109. Cox, Arthur N. 1963, Stars and Stellar Systems: A Compendium of Astrophysics, ed. G. Kuiper (Chicago: University of Chicago Press), 8, Chap. 2, in press. Garstang, R. H. 1956, The Air glow and the Aurorae, ed. E. B. Armstrong and A. Dalgarno (London: Pergamon Press), p. 324. Ney, E. P., and Stein, W. A. 1962, J. Geophys. Res., 67, 2087. Osterbrock, D. E., and Rogerson, J. B., Jr. 1961, Pub. A.S.P., 73, 129. Peebles, P. J. E. 1963, “Structure and Composition of Jupiter and the Other Major Planets/’ preprint. Rogerson, J. B., Jr. 1963, private communication. Weymann, R. 1957, Ap. J., 126, 208. Worthing, A. G., and Geffner, J 1943, Treatment of Experimental Data (New York: John Wiley & Sons), p. 96. © American Astronomical Society • Provided by the NASA Astrophysics Data System .
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