Spectroscopic phenomena in the system of Algol

Spectroscopic Phenomena in the System of Algol OTTO S T R U V E Berkeley Astronomical D e p a r t m e n t , University of California* ~UMMARY Recent observations of the s p e c t r u m of Algol confirm the existence of duplicities in the llne profile of Mg H 4481 n e a r the phase of totality. I n this article, w r i t t e n in 1953, the suggestion is a d v a n c e d t h a t t h e m a g n e s i u m a t o m s m a y be located preferentiall~ in an equatorial belt between ~= 50 ° of latitude.

IN 1931 C. T. ELVEY and I noticed that the absorption line Mg II 4481 in the spectrum of Algol, near mid-eclipse, consisted of two unequal components. However, since later plates did not appear to confirm this duplicity, we regarded it as accidental. Four years later, MORGAN(1935) re-examined the entire collection of Yerkes Observatory spectrograms of Algol taken with a dispersion of 10 A/mm at 44500, and concluded that "all of the material now leaves no doubt that the line is actually double between phases -- 0.d04 and ÷ 0.d04''. Soon after the appearance of MORGAN'S paper I received a letter from the late J. H. MOORE, informing me that the duplicity could not be seen on Lick Observatory spectrograms having only a slightly smaller dispersion than the Yerkes plates. On the other hand, MELNIKOV (1940) found that "the doubling of the line Mg II near the primary minimum is more or less established, while its duplicity at other phases needs further confirmation." He attributed "the appearance of the second line . . . to the second faint star" in the eclipsing system. In the meantime a misunderstanding has arisen with regard to the double MgII line. In SCHNELLER'S recent bibliography of Algol (1952) the appearance of two absorption components at approximately constant separation is taken to mean that the line does not originate in the reversing layer of the B8 star. This unwarranted conclusion is probably due to a remark by KOPAL (1942) that the "arithmetical means of the radial velocities of the two components (measured b y MOROA~¢)showed variation conforming to the velocity curve of the primary star, as derived from the hydrogen lines, but without exhibiting the rotational effect. If so, the lines could scarcely originate in the reversing layer of the B8 star." Although this erroneous conclusion was not repeated in EGGEN'S discussion of the orbit of Algol (1948), the fact that it has reappeared in SCHNELLER'S discussion renders it desirable to re-examine the problem from the beginning. A series of spectrograms was obtained with the Coud6 spectrograph attached to the 100-in. reflector on 9th December, 1952, U.T. According to the latest discussion of the period by PAVEL (see SC~NELLEI~, 1952), mid-eclipse was expected to occur at 4h54 m U.T. The tracings of He I 4472 and Mg H 4481 as recorded directly on a Leeds and Northrup recording mierophotometer are shown in Fig. 1. The original dispersion was 2.85 A per ram, and the exposure times were of the order of 20 min. The geocentric times shown in the figure may be converted into heliocentric through the addition of 6 rain. The existence of two components is strikingly demonstrated for the plate taken * The observational material used in this paper was obtained at tim Mount Wilson Observatory. 1371

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Speetroseopic phenomena in the system of Algol

nearest to mid-eclipse; and it is still clearly noticeable at 3h'2(.}m U.T. and at 5h43 m U.T. Since the relative intensities of the components vary in such a way that the red component predominates before mid-eclipse, while the violet component is the Hei4472

1952

t~lI 4481

DEC.7 6:09 U.T.

t DEC. 9. 2:38 U.T

t DECg. 3:29 U.T

DEc

f

4:34 U.T

~,,,,,,_,.,,~

DEC.9. 5:43 UT.

t DEC.q. 6:46UT

t DECg. 7:58UT.

I

I

I ANGSTROM

t

Fig. 1. The system of Algol: mierophotometer tracings of He i 4472 and Mg IT 4481

stronger after mid-eclipse, there can be no doubt whatever t h a t the phenomenon is due to the Rossiter-McLaughlin effect of axial rotation of the B8 component. The separation between the components is only approximately constant: it appears to reach a maximum about 1½ hours after mid-eclipse, and it is possible, though not clearly brought out by either the tracings or by direct measurement of the

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original plates, that the separation reached a similar large value about 2 hours before mid-eclipse. The geometrical circumstances of the eclipse are well known : the orbital elements recently derived by PLAUT (see SCHNELLER, 1952) agree closely with earlier determinations. Fig. 2 shows the relative positions of the two stars at mid-eclipse. Fig. 3 shows the manner in which the edge of the invisible giant F8 star encroaches upon the disk of the bright B8 star. The predicted profile of a line produced from atoms uniformly distributed over the visible disk of the B8 star does not show double lines. These predicted profiles are shown in Fig. 2 of my paper with ELVEY (1931).

Fig. 2. The Algol system in eclipse

Fig. 3. Rotation effect in Algol

In order not to tamper with the best available geometrical picture of the eclipse, it is therefore necessary to assume that the absorbing atoms of Mg II are concentrated in an equatorial belt that extends about to latitude 4- 50 ° of the B8 component. I t seems to me that this evidence is very strong and that it proves, once again, t h a t the non-uniform distribution of the absorbing atoms and ions over the surfaces of rapidly rotating stars is a phenomenon of major significance in astrophysics. The tracings of the line He I 4472 are also interesting. These lines participate in showing the rotational disturbance, as was already shown by McLAuGHLIN in 1934. But while the peaked minima of Mg H 4481 render this line very conspicuous before and after mid-eclipse, there is nothing like it in the ease of I-Ie I 4472. On the contrary, this line becomes progressively weaker, and almost disappears at mid-eclipse. Yet, it shows the full range of the rotational disturbance. My own direct measurements tend to make the full range of the rotational disturbance in He I (75 km per sec) almost as great as that in Mg II (96 km per sec), while McLAUGHLINfound t h a t the range shown by He I and Si II was even greater than was obtained for the mean of all lines. The tracings in Fig. 1 show a slight asymmetry of the rotation effect in He I, as compared to that in Mg IL The arrows at the bottom of each tracing of Mg II mark the deepest points of each curve. The arrow at the bottom of He 1 4472 on the tracing for 7th December marks the deepest point as observed outside of the eclipse. The arrows below He i 4472 in the tracings for 9th December mark the distance between

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Spectroscopic phenomena in the system of Algol

Mg II 4481 and He I 4472, as observed on 7th December. It is clear that after mideclipse the He I line was relatively more displaced toward the red than was the deepest point of Mg II. This asymmetry is similar in character to that observed bv MCLAUGHLIN, but it is opposite in sign. The apparent weakness of He I 4472 near mid-eclipse suggests that it is even more closely concentrated to the equator of the B8 component than is Mg II, and that, in effect, nearly the entire He I belt is eclipsed at mid-eclipse. A uniform distribution of He I within a belt of ~: 30 ° in latitude might do the trick. Such a distribution would play havoc with our usual interpretation of stellar line-profiles. Yet, the proposed distribution does not apply to stars with sharp lines like y Pegasi, ~ Scorpii or 10 Lacertae. These stars probably rotate slowly, and they may not be expose(] to the latitude effect. There remains the question of the "extra" lines of Fe I, F e i I , etc, which were first observed by F. SCItLESINGER, and later exhaustively studied by Miss IDA BAR~EY (1923). These lines are quite conspicuous on my plates, especially on a set taken during the 9th December eclipse, with a dispersion of 10A per mm on Eastman Process emulsion. The lines become more conspicuous at mid-eclipse, and they are then definitely narrower than they are several hours before and after mid-eclipse. They do not clearly show the rotation effect of the H, Mg II, Ca II, Si u, and He I lines, but they seem to shift slightly toward the violet at mid-eclipse, by perhaps 5 km per sec, in order to regain their normal position 2 hours before and after mideclipse. Perhaps the strangest observation (or rather lack of it) is the absence of any trace of a broad line of Ca II near mid-eclipse. We observe only the normal line of the B8 star, with its rotational disturbance. The photometric discussion by EGGEN has shown that the companion in the eclipsing system is nearly two magnitudes fainter in photographic light than the eclipsed portion of the B8 star. Hence, it should be invisible in the spectra. EGGEN concludes from STEBBI~S' observations t h a t this star is an F8 subgiant, similar to the companions of U Cep, U Sge, and TX UMa. There is no light from the third and fourth, distant, companions. PAVEL'S more recent discussion assumes the existence of six members of the system, and we may have to leave undetermined the contribution to the total light by these hypothetical objects. It is certain that the metallic lines are too narrow to belong to the equatorial belt of the B8 star. Neither can they be attributed to an atmospheric eclipse of a portion of the B8 star by the edge of the gF8 companion. The latter would show a strong line of Ca II, and would presumably show a rotation effect similar in character to t h a t observed in ~ Aurigae. This follows immediately from the geometry of Fig. 3, provided we assume t h a t the F8 star's rotation has the same period as its orbital revolution. The narrowness of the metallic lines also precludes their coming from the entire disk of the F8 component. Perhaps they originate in the polar regions of the B8 star. This would satisfactorily explain m y own observations, but would leave unexplained Miss BARI~EY'S conclusion that the amplitude of the velocity curve, as determined from the "extra" metallic lines, is only about one-third of the amplitude found from the lines of H, Mg II, etc. If, as now seems most probable, we should attribute the " e x t r a " lines to the polar regions of the B8 star, it is quite possible t h a t their distribution with latitude may be closely related to their equivalent widths. Thus, it is entirely possible t h a t the

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strongest of these lines, as, for example, ~4549, may originate in a relatively wide belt of latitude, and may therefore show vestiges of the rotational disturbance, as was actually surmised by MORGAN (1935). It is, however, also possible that the extra lines are blends from the B8 star and from one of the other components.

REFERENCES

BARNEY, I .

. . ELVEY, C. T. a n d KOPAL, Z . . . MELNIKOV, O. A .

EGGEN, O. J .

.

. . . . . . . . . . STRUVE, O . . . . . . . . . . . .

. .

. . .

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. .

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. .

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. .

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. . . . . . .

1923 1948 1931 1942 1940

Ap. J., 35, 95. Ap. J., 108, 1. M.N., 91, 663. Ap. J . , 96, 406. Tzirkuliary G1. Astron. Obs., Pulkovo, No.

1934 1935 1952

Pub. Obs., Univ. of Michigan, 6, 16. Ap. J., 81, 348.

30, p. 65. McLAUGHLI~, D. B . . . . . . . . . MGROAN, W . W . . . . . . . . . . SCHNELLER, H . . . . . . . . . .

. . .

Geschichte und Literatur der Ver~inderlichen Sterne. Zweite Ausgabe, Vol. I I I (OrionVulpecula), Berlin, p. 85.

Stars w i t h E x p a n d i n g A t m o s p h e r e s PAUL W.

MERRILL

lV[ount Wilson and Palomar Observatories, Pasadena, California ~UMMARY Most stars are stabilized spheres whose outer strata are nearly quiescent. Those exceptional objects in whose atmospheres outward motions have been observed may bo placed in three groups: p u l s a t i n g stars, stars with occasional outbursts, and stars with a continual outflow of atoms. E x a m p l e s of pulsating stars are Cepheid variables, with periods of a few hours to a few days; possibly red variables with periods about 300 d a y s ; shell stars with periods of 8-10 years.

Solar prominences, the irregular behaviour of Be stars and of SS Cygai variables, and the tremendous explosions of novae are described under the heading stars with occasional outbursts. Among stars exhibiting a continual outflow of atoms ar~ P Cygni stars which can maintain a steady outward flow of matter for long intervals ; certain interesting binary systems; arLd,perhaps most raysterious of all, stars whose spectra have multiple displaced lines indicating outward motions with several discrete velocities.

EVERY star makes a determined effort to cool itself by dispatching from its surface outward into space vast amounts of radiant energy. In this endeavour it has little success, for the supply of energy, no matter how fast the light-beams carry it away, is steadily replenished by some reliable process that maintains the radiating layers at a constant temperature. Nature has, of course, had a long time in which to bring about this adjustment. The energy fed into the photosphere at the proper rate to balance the expenditure comes from one or both of two basic sources: (1) gravitational energy released by slow contraction of the whole star; (2) nuclear energy released in the interior by conversion of hydrogen into helium. Our interest in this stellar equilibrium is not wholly academic; should our Sun fail to maintain its happy balance, we might suffer considerable inconvenience. The outgoing stream of energy from the photosphere of a star must pass through the comparatively thin and tenuous atmosphere lying above the photosphere. Here again the adjustment is good, for the atmosphere has learned how to live in contact 39