Part 4
TABLE X DROP IN INTENSITY, FROM BACKGROUND TO LINE CENTER, FOR ELEVEN STARS, EXPRESSED IN STELLAR MAGNITUDES ┌──────┬──────┬──────┬──────┬──────┬──────┬──────┬──────┬──────┬──────┐ │ Star │Class │ M │ Hβ │ Hγ │ Hδ │ Hε │ K │ 4227 │ 4215 │ ├──────┼──────┼──────┼──────┼──────┼──────┼──────┼──────┼──────┼──────┤ │ε Ori │ B0│ │ .23│ .40│ .37│ .32│ │ │ │ │ß Ori │ cB8│ 5:│ .22│ .45│ .43│ .46│ .17│ │ │ │α Lyr │ A0│ 0.6│ .95│ 1.43│ 1.60│ 1.68│ .23│ │ │ │α CMa │ A0│ 1.2│ .96│ 1.39│ 1.46│ 1.47│ │ │ │ │α Cyg │ cA2│ 4:│ .33│ .63│ .63│ .70│ .54│ │ │ │α Aql │ A5│ 2.4│ .71│ .81│ .71│ 1.50│ .79│ │ │ │δ Cas │ A5│ 1.6│ 1.49│ 1.46│ 1.54│ 2.15│ 1.48│ │ │ │δ CMa │ cF8│ 3:│ .61│ 1.12│ .86│ >2.25│ >2.25│ .70│ .51│ │α Aur │ G0│ 0.0│ .43│ .76│ .57│ 1.62│ 1.67│ .57│ .36│ │α Boo │ K0│ 0.3│ │ .54│ .74│ │ │ 1.34│ .70│ │α Cas │ K0│ 0.0│ .25│ .42│ .56│ 2.50│ 2.47│ .83│ .71│ └──────┴──────┴──────┴──────┴──────┴──────┴──────┴──────┴──────┴──────┘
10. The material contained in Table X is reproduced in Table XI, where the intensity at the line center is expressed in terms of percentage of the background intensity, instead of in stellar magnitudes. The “background intensity,” as defined in Section 6, is the intensity that the background would have if the line were not present. It is noteworthy that, for the great majority of the lines, the residual intensity at the line center is greater than 30 per cent of the background intensity.
11. The material presented above constitutes the first systematic study of the contours of strong absorption lines. In view of the preliminary nature of the work the discussion has been devoted for the most part to presentation of method. Extended discussion seems at present to be premature, and only a few points need be mentioned.
Probably the chief interest of Table X lies in the result that the maximum intensity drop from background to line recorded for any of these stars is 2.50 magnitudes, corresponding to a light loss of ninety per cent. Except for the supergiant cF8 star and the Ca+ absorption for α Cassiopeiae and Hε for δ Cassiopeiae, the light remaining at the center of the line is at least fifteen per cent of the background intensity. On the average for all these strong absorption lines there is something like twenty-five per cent of the background light remaining at the center of the lines. The significance of these residual intensities will be discussed in a later publication, when the forms of the lines as shown by the data of Table IX will also be considered.
TABLE XI RESIDUAL INTENSITIES AT LINE CENTERS, EXPRESSED AS PERCENTAGES OF BACKGROUND INTENSITY ┌───────┬───────┬───────┬───────┬───────┬───────┬───────┬──────┬──────┐ │ Star │ Class │ Hβ │ Hγ │ Hδ │ Hε │ K │ 4227 │ 4215 │ ├───────┼───────┼───────┼───────┼───────┼───────┼───────┼──────┼──────┤ │ε Ori │ B0│ 81│ 69│ 71│ 74│ │ │ │ │β Ori │ cB8│ 82│ 66│ 67│ 65│ 86│ │ │ │α Lyr │ A0│ 42│ 27│ 23│ 21│ 81│ │ │ │α CMa │ A0│ 41│ 28│ 26│ 26│ │ │ │ │α Cyg │ cA2│ 74│ 56│ 56│ 52│ 61│ │ │ │α Aql │ A5│ 51│ 47│ 51│ 25│ 48│ │ │ │δ Cas │ A5│ 25│ 26│ 24│ 14│ 26│ │ │ │δ CMa │ cF8│ 57│ 36│ 45│ <13│ <13│ 52│ 63│ │α Aur │ G0│ 67│ 50│ 59│ 22│ 21│ 59│ 72│ │α Boo │ K0│ │ 61│ 51│ │ │ 29│ 52│ │α Cas │ K0│ 79│ 68│ 60│ 10│ 10│ 47│ 52│ └───────┴───────┴───────┴───────┴───────┴───────┴───────┴──────┴──────┘
For the wider lines, especially those that are strong and heavily winged, the intensities derived in this paper are probably of the right order. Probably, however, the dispersion used is too small to reproduce satisfactorily the detail at the centers of lines as narrow as those of such stars as α Cygni and β Orionis. The difficulty introduced does not involve inaccuracy of plates, microphotometer, or process of measurement; it is concerned solely with the fact that the spectral region examined is so narrow that, with the dispersion used, the grain of the plate is not fine enough to reproduce the spectral detail. The same difficulty would prevent any recognition of the double reversal of the solar H and K lines, if they were studied with the present dispersion.
Whatever the dispersion used, the same qualification must be made in discussing the results; probably the dispersion would have to be greatly increased before the measured effective line depth becomes much greater for narrow line stars.
Relative effective line depth, derived from numerous spectra made with the same dispersion, is still, however, of considerable significance. It permits us to recognize differences of surface gravity, and to form an idea of relative chromospheric depths for different classes of stars.
SUMMARY
1. The investigation deals with the determination of the depth and contour of prominent absorption lines in the spectra of stars of various classes.
2. The spectra used were made with the 16-inch refractor of the Harvard Observatory, using two prisms and a special set of apertures.
3. Results are presented for eleven stars, of spectral class ranging from B0 to K0.
4. The spectra were analyzed under uniform conditions by means of the Moll thermoelectric microphotometer. The resolving power of this instrument is such that no integrating effect need be considered in discussing the results.
5. The microphotometer tracings were measured with reference to fiducial lines representing “darkness” and “clear film,” and to a line, representing the continuous background, drawn across the absorption lines.
6. The intensity drop from continuous background to line was deduced graphically from the measures.
7. The accuracy of the results is discussed in detail.
a. The reliability of the plates, as judged from qualitative reproduction of detail, and from the consistency of the numerical results, is satisfactory. Effects of stray light are of negligible magnitude, and in this respect slit spectra appear to have no advantage over objective prism spectra.
b. Effects of poor focus are measurable, but small. Spectra that are in such poor focus as to cause appreciable inaccuracy would be rejected from visual inspection.
c. The accuracy of the microphotometer tracings is in general satisfactory. Tracings showing abnormal deflections from “darkness” to “clear film” are not susceptible of correction, and are omitted in deriving results.
d. The measures upon the tracings are also of satisfactory accuracy.
8. The differences in intensity between the continuous background and various points along the line contour are tabulated for the eleven stars under discussion.
9. The general results for the intensities at the centers of lines show an interesting relation to absolute brightness; the brighter stars have, in general, lines that cut more deeply into the background. A result of considerable interest is that the average residual intensity in the strong wide absorption lines is more than 30 per cent of the background intensity.
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