Stars with expanding atmospheres

PAUL W. MERRII~,

1375

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 .

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

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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

1376

Stars with expanding atmospheres

with the photosphere without being bothered too much by the incessant passage of the departing light-beams. It does, however, exact from them a part of their substance--certain small fees known as absorption lines, the manner of whose payment is very precisely specified. In a few stars the adjustment is not quite perfect and the atmosphere may sometimes depart from its precisely uniform existence. The surface temperature and the intensity of radiation may vary; the outer layers may leave abruptly, e n m a s s e ; or even with a nearly constant photosphere some of the atmospheric atoms may become unhappy and drift away slowly one by one. Stars in whose atmospheres outward motions have been observed fall into three classes : (1) Pulsating stars. (2) Stars with occasional outbursts. (3) Stars with a continual outflow of atoms. l. PULSATING STARS

The best-known examples of pulsating stars are the cepheid variables which present physical problems of great interest and are useful as distance indicators for galaxies exterior to our own. EDDINGTON showed that the periods, averaging about six days, probably correspond to a volume pulsation in which the star is alternately larger and smaller than its equilibrium size. When it is too large, gravity pulls it back but the acquired motion causes the outer layers to go too far inward where gas pressure takes over and pushes them out, again overshooting the mark. Variables of the I~R Lyrae (or cluster) type are smaller and denser and make the same type of pulsation in about half a d a y . Magnetic variables of the type recently studied by H. W. BABCOCK also may pulsate. The outward velocities in these pulsating stars are far below the velocity of escape, but in certain cepheids they are fairly large in comparison with the kinetic velocities of the atoms. Those variables whose velocity-curves have especially large amplitudes should perhaps receive further study. In the huge and tenuous red variables, EDDINGTON showed that "the dimensions are consistent with the hypothesis that the period of about 300 days is that of the natural pulsation of the star". Spectroscopic measurements show that cyclical motions of the reversing layers, if present, are small. Atoms emitting bright lines, however, appear to ascend through the atmosphere with velocities of 10-20 km per sec. Certain shell stars exhibit a type of oscillation quite different from that of cepheid variables. The term s h e l l is applied to certain B- and A-type stars whose unusually narrow dark spectrum lines bear evidence of having been produced in an atmospheric level high above the normal reversing layer. Typical spectra are striking; the hydrogen lines have conspicuous dark cores with a slow Balmer decrement, the series sometimes being photographed as far shortward as H42; narrow lines of ionized metals, particularly Ca II, Sc II, Ti II, Cr II, Fe IX, and Ni II, are numerous and distinct. In 48 Librae a remarkable eight-year oscillation began about 1934 with an outward thrust of the atmosphere. The atoms were slowly accelerated outward until in 1937 they reached a velocity of about 100 km per sec. ; they then slowed down and in 1939 turned back toward the star. A second cycle has now been observed (MERRILL, 1953). Minimum velocities (in the algebraic sense) occurred about 1937 and 1945,

PAUL W. MERRILL

1377

maximum velocities about 1941 and 1950. The amplitude has diminished as if the oscillation were damped. In H D 33232 a somewhat similar oscillation has a period of ten or eleven years. In both stars some stratification of the atmosphere is observable, certain lines exhibiting a greater range of velocity than others (MERRILL, 1952d). Other stars, not so extensively observed, that probably behave in somewhat the same manner, are fl Monocerotis A, H D 44351, H D 183656, and H D 220300. In Pleione a shell episode began in 1939 marked by the development of numerous narrow dark lines which grew stronger until in 1945 they gave the spectrum a most striking aspect, totally different from its normal appearance (MERRILL, 1952a). At first the motions were too small to be measured but gradually the outward velocity of the upper levels became appreciable. Eventually the supply of atoms failed and the shell blew away. The shell lines are now very faint. Thus in Pleione only the first half of a complete oscillation has been observed. In shell stars there is every indication that atoms are not expelled from the photosphere with high velocities but drift outward very slowly. In certain stars there is little acceleration and the shell appears stable, its measured velocity being sensibly the same as that of the star as a whole. In other stars the atoms are acted on by some outward force shghtly greater than gravity until the outward velocities are appreciable, sometimes reaching 50 km per sec. or more. This outward force, however, may not act indefinitely; after a time, or perhaps beyond a certain distance, it may drop off somewhat, allowing gravity to draw the atoms slowly back. If the force is a steady one, not variable with time, the circumstances indicate that with increasing distance from the star it drops off more rapidly than gravitY. This might be true of a dipolar field or of an electric or magnetic force having its origin near the star's surface. Many facts indicate that stratification plays an important role in the extended atmospheres of shell stars, but the complete interpretation is obscure. The clues we now have could be profitably developed by further observation of the displacements and profiles of the lines at various phases of the velocity-cycle. 2. STARS WITI-[ OCCASIONAL OUTBURSTS

Our Sun is, fortunately, on the whole a pretty well-behaved star. But a little turbulence is always in evidence on its surface; occasionally a real eruption occurs, mighty on a terrestrial scale, but negligible in the Sun's energy balance. Eruptive prominences which last but a few hours may rise as fast as 700 km per see. and m a y attain a height exceeding 1,000,000 km (Sun's radius----700,000 km). Streamers frequently break off from the rising column of gas and turn back toward the Sun (PETTIT, 1943). Other types of prominences which appear to form well above the photosphere exhibit downward motion only, with velocities between 100 and 200 km per sec. Thus the gas in solar prominences as well as that in stellar shells may move either up or down. Are the same physical principles involved in both objects ? Many Be stars have bright lines that exhibit striking changes in intensity from time to time, indicating changes in atmospheric activity. Certain stars have been known to produce temporary shells observable for periods from a few weeks to a few years ; examples are ~ Cassiopeiae, Z Andromedae, and Pleione. These shells are probably formed by atoms that drift slowly out from the usual reversing layers just above the photosphere. The star 17 Leporis has dramatic eruptions covering

137S

Stays with expa,L(ling a t , n o s p h e r e s

practically its whole surface at intervals of approximately 155 days. At these times the spectrum lines are extended shortward by atoms with considerable outward velocities (STR~VE, 1932). The structure of the lines should be studied with high dispersion. The SS Cygni variables exhibit sudden increases in light at irregular" intervals, the physical mechanism being unknown. By far the most sensational stellar phenomena ever observed are the explosions that create the temporary apparitions called, through misapprehension, novae or new stars. A violent internal disturbance sends the outer layers of the star hurtling into space. The velocity may be so great that it is beyond control by gravity, and a gaseous shell expands indefinitely. In the Crab nebula, the explosion took place 900 years ago but the gaseous fragments are still seen flying outwards at a velocity of about 1300 km per see. (MINKOWSKI, 1942). Similarly the curious nebular wisps about R Aquarii are moving outward; they may have originated in a sudden outburst some centuries ago. In planetary nebulae the atoms are moving outward at. moderate veloeities (W~Lso~, 1950), but in none of these objects has expansion of the visible disk yet been detected. 3. S T A R S

WIT]-[ A C O N T I N U A L

OUTFLO~,V

OF

ATOMS

A remarkable fact disclosed by repeated observations is that certain stars are able to maintain, apparently indefinitely, a steady flow of matter outward from their surfaces. These objects are called P Cygni stars after their famous prototype, whose spectral peculiarities were discovered at Harvard about 1890. Most of the available information concerning stars of this kind has been compiled in a recent monograph by Dr. C. S. BEALS (1951). The approximate velocity of outflow from several stars is shown in the accompanying tabulation. Data concerning P Cygni stars MWC

Outward velocity

P Cygni H D 51480 BD + 21 ° 3487

338 161 374

180 k m pe r sec. 160 22O

DM -- 27 ° 11944 •

272

27O

Star

Rc[~rencc (]:~EALS, 1951) (BEALS, 1951 ) (BEALS, 1951 ; MERRILL, 1952(~) (MERRILL, 1948)

In a number of objects the expanding hydrogen layers seem to envelop the entire orbit of a binary star. The famous eclipsing variable fl Lyrae is an example. A B-type and an F-type star are revolving about each other in a period of thirteen days, while enveloping both of them is a gaseous ring expanding with a velocity of 75 km per sec. (STRUVE, 1941; KUIPER, 1941). Somewhat similar is the binary H D 187399; spectroscopic observations indicate that the orbit (period 28 days) is immersed in a cloud of hydrogen whose outer layers are expanding at the rate of 75 km per sec. (MERRILL, 1949). In another binary, v Sagittarii, spectrum Ap, with a period of 138 days, the ejection of matter at a velocity of 300 km per sec. is markedly discontinuous, appearing in some cycles but not in others (BIDELMAN, 1949). Two other objects have much in common with v Sagittarii; both are binaries in which the main component resembles a giant F-type star; both have bright hydrogen lines with displaced dark components indicating large outward velocities. In H D 59771 the outward velocity, possibly variable, is about 130 km per sec.; in

I'AUh W. Mr,;RRHA.

]379

H D 242257 (MWC 490) it was nearly constant at 530 km per sec. from 1942 to 1951 (MERRILL, 1952b). The most mysterious of all stars with expanding atmospheres are, perhaps, those whose spectra exhibit multiple displaced lines indicating outward motion in several discrete velocities. Of these the most extensively studied is H D 190073 (MERRILL, 1951a). Components of the H and K lines of ionized calcium have displacements corresponding to the following outward velocities in kilometres per second: 0, 147, 181, (200), (296), 323, 358, (396). Three pairs of displacements have the ratio 2 : 1. Circumstances would seem to rule out a set of expanding shells; the distances traversed in the observed interval of twenty years would be too great. We must rather consider a succession of atoms passing through fixed or nearly fixed circumstellar zones. But it is hard to imagine what physical circumstances would lead to numerous discrete and narrow ranges of velocity. An explanation alternative to that of velocity might lie in some phenomenon that displaces by small amounts the usual energy levels in the ionized calcium atoms, but I am not aware of any plausible hypothesis of this kind. Peculiarities somewhat similar to those of H D 190073 have been observed in the calcium lines of X X Ophiuchi (MERRILL, 1951b) and of Carinae (THACKERAY, 1951), and in the helium lines of BD ~- l l ° 4673 (MERRILL, 1951C). Stars with expanding atmospheres present challenging physical problems. Except in novae, we do not know what becomes of the outgoing atoms, and we are far from understanding the cause of the motions. Theoretical considerations that would, even by a process of elimination, tend to direct thought toward the correct ideas would be extremely welcome. Additional spectroscopic observations--more numerous and with higher dispersion than the present ones--are needed; they will certainly reveal new and important information.

REFERENCES BEALS, C. S .

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1951

"The Spectra of the P Cygni Stars", Pub.

Dominion Astrophysical Obs. Victoria, BIDELMAN, W . P .

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KUIPER, C~. P .

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MERRILL, P. W .

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1949 1951a 1951b 1951c 1952a 1952b 1952c

9, 1. "Displaced Absorption Lines in the Spectrum of Upsilon Sagittarii", Ap. J., 109, 544. "On the Interpretation of fl Lyrae and Other Close Binaries", Ap. J., 93, 133. "Measurements in the Spectrum of DM -- 27 ° 11944", Pub. Astron. Soc. Pacific, 60, 381. "The Spectrum of HD 187399", Ap. J., 110, 59. "Displaced Calcium Lines in the Spectrum of HD 190073", Ap. J., 118, 55. "The Spectrum of X X Ophiuehi in 1949 and 1950", Ap. J., 114, 37. "Displaced Helium Lines in the Spectrum of BD + 11 ° 4673", Ap. J., 114, 338. "Pleione: the Shell Episode", Ap. J., 115, 145. "Two F-type Stars with Expanding Hydrogen Atmospheres", Ap. J., 115, 154. "Measurements in the Spectra of Ten Shell Stars", Ap. J., 116, 501.

S h e l l s a r o u n d P (?ygni stars

] 380 MERRILL, I'. W .

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1!152d 1953

MI~KOWSKL R . PETTIT, EDISON

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STRUVE, OTTO .

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1942 1943

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1.(132 1941

T~ACKERAY, A. D . WILSON, O. C .

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" O s c i l l a t i o n s in the Shell Star H i ) 3 3 2 3 2 " , Ap. J., 116, 516. " T h e Spectrum of 48 Librae f r o m 1944 to 1952", Ap. J., 117, 7. " T h e C r a b N e b u l a " , Ap. J., 96, 199.

"The Properties of Solar Prominences as Related to Type", Ap. J., 98, 6. "17 L e p o r i s : a N e w Type of S p e c t r u m Variable", Ap. J., 76, 85. " T h e S p e c t r u n l of fl L y r a e " , Ap. J., 93, 104. " T h e Spectrum of E t a Carinae", Observotory, 71, 167. " A Survey of Internal Motions i n t h e Planetary Nebulae", Ap. J., 111, 27,(}.

Shells Around P Cygni Stars A. D. THACKERAY Radcliffe Observatory, Pretoria, S o u t h A f r i c a SUMMARY

The spectra of four stars, previously discovered to be surrounded by nebulous envelopes, are briefly (iiseussed. AG Carinae, H D 138403, and E t a Carinae are truly described as P Cygni stars. Emission and absorption lines are found in C.D. -- 46°11816, but the P Cygni character of this star is not certainly established.

THE spectrum of a normal star, for instance the Sun, consists of a pattern of narrow dark absorption lines crossing a continuous spectrum. Only a relatively few stars show bright lines, and these are mostly found among the hottest or coolest known stars, in which the energy radiated in the photographic or visual region is only a small fraction of the total energy. Among the hotter bright-line stars there are a few whose bright lines are accompanied by dark absorptions on their borders of short wavelength. Typical profiles of such combined emission and absorption lines are illustrated in the accompanying figure, numbered with the types according to BEALS' classification (BEALS, 19511. Any star which shows lines with profiles like these or the other types described by BEALS is classified as P Cygni type after the star P Cygni, which is listed among the novae on account of its brightening to the 3rd magnitude in 1600; for the last 250 years the star's light seems to have remained practically constant at the 5th magnitude. Every known nova which has been studied spectroscopically has temporarily shown bright bands bordered by absorption like Type I. The displacement in wavelength, however is usually considerably larger in the typical nova than in a P Cygni star, and the emission bands of a nova are also usually considerably broader. The accepted interpretation of the nova spectrum is that we witness the radial expulsion of a shell from the surface of the star with a velocity V of the order of 1000 km per sec or more. The broad emission bands correspond to the portions of the shell which do not lie between the observer and the nova; their velocities lie between -- V and + V. The portion of the shell which lies directly in front of the