Photographic light curve of Eta Carinae

D. ,L K . O'CoNNELL, S.J.

1165

1953 1953

Bull. Astron. Inst. Netherlands, X I I , 31. Z. Astrophys., 32, 69.

LEDOUX, P . . . . . . . . . . .

1954 1922 1936 1938 1947 1950

LURJE, M. A .

1951 1950

B u d a p e s t Obs. ( u n p u b l i s h e d data). Bull. Astron. Inst. Netherlands, I, 139. Bull. Astron. Inst. Netherlands, V I I , 313. Bull. Astron. Inst. Netherlands, V I I I , 211. Bull. Astron. Inst. Netherlands, X , 251. Inst. d'Astrophys. Univ. Liege Coll., 4 °, 5 pp., No. 25. Ap. J., 114, 373.

L. . . . . . . . . . G R A T T O N , L. and L A V A G N I N O . . . GRATTON,

GUMAN, I .

.

.

.

.

.

.

.

.

.

.

.

~IERTZSPRUNG,E . . . . . . . . . KLUYVER, H. A .

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

Peremennye Zvezdy, Akad. Nauk USSR, Moscow (Variable S t a r Bull.), 7, 182.

MARTINOFF, D. J . . . . MULLER, A. B . . . . . MtiNCH, G . . . . . . OOSTERHOFF, TH . . . .

. . . . . . . . . . . .

. . . . . . . .

SANFORD, It. F .

.

.

.

.

.

.

.

.

SCHWA~ZSCRILD, M . . . . . . . . SHAPLEY, H . . . . . . . . . . STRUVE, O .

.

.

.

.

.

.

.

.

.

STRUVS, O. a n d BLAUUW, A . . . . . STRUVE, O. a n d VAN HOOF, A . . . . . WALRAVEN, TH . . . . . . . . . WOLTJER, J .

.

.

.

.

.

.

.

.

.

1940 1953 1951 1946a 1946b 1949 1952 1941 1915 1916 1947 1949 1952 1948 1949 1949 1952 1953 1937 1946

Bull. Engelhardt Obs. Kazan, No. 17. Bull. Astron. Inst. Netherlands, X I I , 11. Ap. J., 114, 546. Bull. Astron. Inst. Netherlands, X , 101. Bull. Astron. Inst. Netherlands, X , 123. Ap. J., 109, 208. Ap. J., 116, 331. Ap. J., 94, 245. Ap. J., 42, 148. Ap. J., 43, 217. Publ. Astron. Soc. Pacific, 59, 192. Astron. J., 54, 50. Ann. Astrophys., 15, 157. Ap. J., 108, 60. Ap. J., 109, 214. Bull. Astron. Inst. Netherlands, X I , 17. Bull. Astron. Inst. Netherlands, X I , 421. Bull. Astron. Inst. Netherlands, X I I , 57. Bull. Astroa. Inst. Netherlands, V I I I , 193. Bull. Astron. inst. Netherlands, X , 125.

Photographic Light Curve of Eta Carinae D. J. K. O'CoNNELL, S.J. Specola V a t i c a n a , Castel Gandolfo, V a t i c a n City S t a t e

SUMMa_BY T h e h i s t o r y of t h e u n u s u a l s t a r E t a Carinae is reviewed. P h o t o g r a p h i c m a g n i t u d e s o b t a i n e d f r o m plates t a k e n a t R i v e r v i e w O b s e r v a t o r y , 1935-52, a n d f r o m s o m e early S y d n e y A s t r o g r a p h i c plates, are listed. I t is s h o w n t h a t t h e s t a r h a d c o m m e n c e d to b r i g h t e n b y April, 1941. T h e b r i g h t e n i n g h a s c o n t i n u e d since t h e n , w i t h fluctuations. T h e colour of t h e s t a r is discussed.

1. INTRODUCTION

IN 1837 Sir JoHn H E R S C H E L , during his stay at the Cape of Good Hope, noticed that Carinae had increased greatly in brightness (HERSCHEL, 1847). HALLEY had observed it in 1677 as a fourth magnitude star, and LACAILLEin 1751 classed it as of the second magnitude. HERSCHEL found it to be nearly zero magnitude. Since that time, ~ Carinae has attracted attention as one of the most remarkable stars in the sky. IN~ES (1903) gives a history of the star and the visual light curve to 1902. By 1843 it had reached --1 m and was second only to Sirius in brightness. B y 1886 it had faded to about 8m. I t then brightened again and reached 6m in 1889. By 1897 it had again faded to about 8m and remained at about that level for m a n y years.

1166

1)hotographic light curve of Eta Cat'inae

At the end of January, 1952, DE VAUCOULEURS, who had just arrived for the first time in the Southern hemisphere, observed the star visually at the Commonwealth Observatory, Mount Stromlo. He noticed that it was about one magnitude brighter than he expected. During the next few months visual observations by DE VAUCOULEURS and photoelectric observations by EGGEN and HoGc at Mount Stromlo showed that the star was still brightening. At the request of Dr. DE VAUCOULEURS, the writer examined the Riverview Observatory plates of the region and found that the brightening had begun several years previously. Preliminary results of the observations were communicated to DE VAUCOULEURS who gave a summary of them in his paper on ~ Carinae (DE VAUCOULEURS, 1952). The magnitudes of the comparison stars used for these preliminary results, and also the estimates, were only provisional. The magnitudes have since been determined more precisely. More plates have been used and the earlier estimates revised. The observations, reduced with the revised magnitudes, are given in this paper. 2.

COMPARISON

STARS

The comparison stars are listed in Table 1. Successive columns give: (1) the writer's designation; (2) DE VAUCOULEURS' designation; (3) HD number; (4) Table 1. Comparison stars Designation O'Connell

de Vaucouleurs

A a b d f

C O J K

HD number

Spectrum

Photographic magnitude

C.P.D. number i

k

i

__

l

I

__

93206 92741 93403 93469 93500

--

B0 B3 B0 A5 B9

-----

59?2572 59 2 4 4 7 58 2 6 8 0 58 2 6 9 2 58 2 6 9 5 59 2600 59 2603

6~28 7.03 7.24 7.90 8.46 8.55 8.83

spectrum, from the HD except for stars k and 1, for which the spectra were taken from BOK'S Doctorate Thesis (1932); (5) C.P.D. number; (6) photographic magnitude. The magnitudes were determined from comparison with six stars of DE VAUCOULEURS' sequence, for which EGGEN (1950) and HOGG at Mount Stromlo had determined magnitudes on their Pgp scale (privately communicated), and with the sequence for the ~ Carinae field determined by BOK and VAN WIJK, also on the Pgp scale (1952). The measures were made with a graduated-image scale on six plates taken with the G camera at Riverview. The six stars of EGGEN and HOGG'S sequence range from 6¢1 to 8m0 pg. Ten stars of BOK and VA~ WIJK'S sequence were used, ranging from 7m0 to 9.m9pg. NO difference in scale or zero point was detected between these two sequences, although of course the material was not adequate for a thorough comparison. 3.

PHOTOGRAPHIC

OBSERVATIONS

The estimates were made with the aid of a low-power microscope. About 700 plates of the region, taken at Riverview from 1935 to 1952, were examined. About 600 of these plates were taken with the P and R cameras (Zeiss Astrotriplets, f : 4 . 5 , f.1.27 cm). With the normal exposure time of 10 min the nebulosity around ~7Carinae

°.

mmmmm~NNN~NN~N~N~N~NN~N~N~N~

~6

1168

Photographic

light curve of Eta

Carinae

comes out strongly. On such plates it was not possible to make reliable estimates of ~l Carinae. There were, however, forty-seven P and R plates on which useful estimates were made. Another plate series, taken with the G camera (Ross-GrubbParsons lens, f : 4.5, ill. 45 cm), was started in 1939. On these plates the nebulosity is less troublesome and, owing to the larger scale, ~ Carinae is more widely separated from the neighbouring stars, so that estimates were more satisfactory than on the P and R plates. Estimates were made on eighty-two G plates. The estimates for 1952 were made on sh0rt-exposure plates (! or 2 rain), taken for this purpose. In previous studies no appreciable difference in colour equation has been found by the writer between plates of the P, R, and G series. The region is in the Sydney Astrographic Zone and estimates were made on six plates taken with the Sydney Astrograph, kindly put at my disposal by Mr. H. W. WOOD, Government Astronomer for New South Wales. On these plates the comparison stars are widely separated and estimates correspondingly more difficult. The variable was also estimated on one of the Union Observatory Charts (special chart of ~] Carinae region, plate of 1917). The estimates are given in Table 2. Successive columns give the year, the plate series, the Julian Day, and the photographic magnitude. The magnitude reductions were made originally to two places of decimals, but the magnitudes in Table II are rounded off to the nearest tenth of a magnitude. It should be noted that on these small-scale plates the magnitudes are integrated magnitudes of the nucleus of ~ Carinae and the surrounding halo, referred to later in this paper. The images are sufficiently round to be comparable with other star images. On the other hand, on the latest Sydney Astrographie plate examined by the writer (that of 10th June, 1952), with exposures of 30, 60, and 120 sec., the images of ~ Carinae are roughly oval in shape, corresponding to the outline of the halo, so that estimates are difficult. 4. LIGHT CURVE It will be seen from Table 2 that V Carinae had brightened considerably by the time of the first observation in 1941 (20th April).* Unfortunately, there were no plates in 1940 on which reliable estimates could be made, so that it is not certain exactly when the brightening commenced. In order to show the general trend of the light curve from 1935-52, the observations were grouped into annual means. In computing the means, the original magnitudes with two decimal places were used. Double weight was given to G plates, weight one to P and R plates, and half weight to estimates of lower quality (indicated by a colon in Table 2). The annual means are given in Table 3, with the number of estimates included in each mean. There were only two estimates for each of the years 1936 and 1947, and only one each for 1938, 1943, and 1949. No means are given for these years. The mean points are shown in the figure, the area of the circles being proportional to the weight. The Sydney plate of 1895 shows ~ Carinae on the descent from the maximum of 1889. On Sydney plates from 1896 to 1909 the star is about 8~.5 pg, agreeing well with Miss HOFFLEIT'S photographic light curve (1933). From 1935-39 the star is * I n a preliminary account c o m m u n i c a t e d to ])~ VAUCOULEURS by letter a n d referred to b y h i m in his l~aper (1952), tile writer stated that ~ Cartnae r'emalned at about 8m5 pg until May, 1941. These results were provisional, and based on only part of the plate material available. W h e n more plates were e x a m i n e d , and the provisional estimates revised ( m a d e necessary because some of the original comparison stars were found to vary), it was found t h a t the brightening had already taken place in April, although there were s u b ~ q u e n t appreciable fluctuations of brightness.

D. J. K. O'CoNNELL, S.J.

1169

Table 3. A n n u a l means Year

1935 1937 1939 1941 1942 1944 1945 1946 1948 1951 1952

Number of

Julian Day 2400000 +

Photographic magnitude

estimates

27970 28681 29373 30135 30491 31246 31631 31920 32688 33763 34127

8T61 8.44 8.41 7.74 7.44 7.63 7-73 7.26 7.34 7"23 7-11

9 13 3 l0 16 6 3 11 20 14 18

still about 8m5 pg. B y 1941 the mean brightness had increased to about 7.~8 pg. With some fluctuations the mean brightness increased to 7.ml pg in 1952. The extreme range of the estimates in each of the years 1935, 1937, and 1939 is less than 0.m4, which is about the range of accidental errors for photographic estimates made b y the writer. In 1941, however, the range of the estimates is about 1.~0, which shows that there were real fluctuations after the brightening began. During the previous brightening, which culminated in the maximum of 1889, Carinae took about three years to rise from 7.~9 to 6.ml (on DE VAUCOULEURS' visual scale). The present brightening has been much slower, the rise from 8.~5 pg to 7.ml pg having taken about twelve years. o 7.0

|re_m! ~8"(3 ~T

9"0

I 28000

3(9( XgO

32000

J.D. ~ 4 0 0 0

Fig. 1. Photographic light curve of Eta Carinae, 1935-1952. Dots denote annual means, the area of the circles being proportional to the weight 5. HALO ROUND ETA CARINAE T~ACKERAY, i n a n i n t e r e s t i n g n o t e (1953), s h o w s r e a s o n f o r c o n c l u d i n g t h a t t h e r e c e n t b r i g h t e n i n g o f ~ C a r i n a e is d u e m a i n l y t o b r i g h t e n i n g o f t h e h a l o s u r r o u n d i n g t h e s t a r , a n d t h a t t h e n u c l e u s i t s e l f h a s b r i g h t e n e d l i t t l e , i f a t all. T h e h a l o w a s first n o t i c e d a b o u t 1933 b y VAN DE~ B o s (1938). H e d e s c r i b e s i t a s r e d . GAV*OLA (1946, 1950) a n d T*{ACKERAY (1949, 1950) h a v e p u b l i s h e d s t u d i e s o f t h e h a l o . O n a s h o r t - e x p o s u r e p l a t e t a k e n w i t h t h e 74-in. R a d c l i f f e r e f l e c t o r i n A p r i l , 1949,

P h o t o g r a p h i c light curve of E t a Carinae

,1170

THACKERAY (1953) estimates that the nucleus is 8.m0, or perhaps even fainter. At that time the integrated photographic magnitude of nucleus and halo was about 7"72. GAVIOLA suggests that the halo has been moving outwards from the nucleus at the rate of about 5" per century since the 1843 outburst. 6. COLOUR AND SPECTRUM

From a comparison of DE VAUCOULEURS' visual light curve with the writer's photographic one, it appears that the colour index at present is about + 0m5. DE VAUCOULEURS (1952) finds a colour index of q- 0m42 on the Cp scale. One would expect a larger colour index for a star as red as ~ Carinae appears visually. Most of the colour is apparently due to the halo. The halo is red, with strong H~ emission. The nucleus was described by VAN DEN Bos as orange in colour. It is at any rate less red than the halo. If the halo continues to brighten, without a corresponding brightening of the nucleus, one would expect the combined light to become redder. The first published observation of the colour of ~ Carinae is that of Sir J. HERSCHSL, who found it ruddy or yellow. The colour has since been noted at various times as red, yellow, reddish-yellow, orange. THOME (1899) found that the colour changed during the brightening of 1886-89 from red to bright orange. The first spectrum observation of ~ Carinae was made visually by Miss CLERKS (1888) during a visit to the Cape Observatory. It was then an absorption spectrum. Miss CANNON (1897) studied the spectrum on Harvard plates of 1892 and 1893. BOK (1930) later classified this spectrum as cF5. By 1895 the absorption spectrum had disappeared and many bright lines were visible (CAN~O~, 1916). This emission spectrum has persisted, with various changes, since that time. These changes have been studied by Miss HOFFLEIT (1933) and by WHITNEY (1952), who give references to earlier work. GILL (1901) and Miss CANNON (1916) noted the resemblance of the spectrum in 1895 to that of Nova T Aurigae. PICKERING, in his 2nd Catalogue of Variable Stars, listed the star as Nova Carinae 1. LUDENDORFF (1928) classed it as a nova-like variable. 7. CONCLUSION

Carinae is certainly not an ordinary nova. Its behaviour is indeed not paralleled by any other known star. Further observations are badly needed--photometric measures of the nucleus and of the integrated brightness of nucleus and halo in different colours, further studies of the spectrum, and attempts to measure the motion of the halo. The behaviour of the star is unpredictable. The possibility is not excluded that it may once again become one of the brightest stars in the sky.

REFERENCES BOK,

BART

J. . . . . . . . . . . .

1930

BOK, BART J . a n d WIJK, U. VAN Bos, W. VAN DEN . . . . . . CANNON, ANNIE J . . . . . .

. . . . . . . . . . . . .

CLERKS, AGNES EOOE~r, O H w J .

. .

. .

. .

. .

. . . .

. .

. .

. .

. .

1932 1952 1938 1897 1916 1888 1950

Popul. Astroa., 38, 399. Harvard Coll. Obs. Repr. No. 77, 23. Astron. J . , 57, 220. Union Obs. Circ., No. 100, 522. Harvard Coll. Obs. Ann., 28, 175. Harvard Coll. Obs. Ann., 7{}, 36. Observatory, 11, 430. Ap. J., 111, 333.

FRANK BRADSHAWWOOD

1171

.

1946

Revista Astronomica, 18, 252 ( = Obs. de A p . J., 111, 408.

GILL, Sir D.4.YID . . . . . . . . . . . . . . . . .

1950 1901 1847 1933 1903 1928 1949 195O 1953 1899 1952 1952

Harvard Coll. Obs. Bull., No. 893, 12. Cape A n n . IX, 75B. Handbuch d. Astrophys., vol. VI, 66. Observatory, 1}9, 31.

GAVIOLA, E .

.

.

.

.

.

.

.

.

.

.

HERSCHEL, Sir JoHN F. W .

Cordoba, Tirada Aparte No. 12). M.N., 81, Appendix, 66. Results of Astronomical Observations . . . at the Cape of Good Hope. (London),

p. 32. HOI~FLEIT,DORRIT. INN~.S, R. T. A . . LUDEI~DORFF,H . . THACKERAY,A. D .

. . . . . . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

THEME, J. M. . . . . . . . . . . . VAUCOULEURS,G. DE . . . . . . . . . WnITSEY, C. A . . . . . . . . . . .

Eclipsing Variable Stars:

M.N., 110, 524. M . N . , 118, 237 (also M . N . , 118, 211,526). Astron. 1Vachr., 112, 305. Publ. Astron. Soc. Pacific, 64, 185. Harvard Coll. 058. Bull., No. 921, 8.

a Survey of the Field

FRANK BRADSHAW WOOD Flower and Cook Observatories, University of Pennsylvania, U.S.A. SUMMARY

This paper first presents an extremely brief general survey of the history of the study of eclipsing stars from the time of JOHn GOODRICKEto the present. The next section discusses international co-operation in the field. It includes a short discussion of existing publications and active observatories which deal with the systematization and distribution of information concerning eclipsing systems, and emphasizes cases in which active co-operation and frequent interchange of information between observers is of fundamental importance. The final section indicates certain areas in which future research may yield advances in our knowledge of these systems which will be important both for our understanding of the systems themselves and also for a greater insight into many of the general astrophysical problems which can be thus attacked.

1. HISTORY

systematic study of eclipsing variables, in both its observational and theoretical aspects, goes back to 1782, when GOODRICKEobserved the variability in the brightness of Algol, and suggested that this could be explained by a large dark body revolving around Algol and periodically passing in front of it. More than a century later, VOOEL (1890) confirmed this suggestion by observing that the radial velocity of the star varied in the same period as did the light, and that the relation in phase between light and velocity curves was that which would be predicted by the eclipse hypothesis. Following this discovery, the field expanded rapidly. The development of visual, photo-electric, and photographic methods of making measurements of precision, and their application with vigour to the observation of eclipsing systems by such men as DUGAN, STEBBINS, a n d BAKER were c o m p l e m e n t e d b y t h e t h e o r e t i c a l i n v e s t i g a t i o n s THE

o f RVSSELL a n d b y SHAPLEY'S a p p l i c a t i o n o f t h e s e to m a n y s y s t e m s . I t is n o w well k n o w n t h a t a c o m b i n a t i o n of p h o t o m e t r i c a n d radial velocity observations permits t h e c o m p u t a t i o n o f t h e sizes, masses, d e n s i t i e s , a n d s h a p e s of t h e c o m p o n e n t s of t h e s y s t e m , t h e size a n d s h a p e o f t h e o r b i t a n d t h e i n c l i n a t i o n o f its p l a n e t o t h e l i n e of sight, q u a n t i t i e s s u c h as t h e degree o f l i m b d a r k e n i n g a n d a m o u n t of r e r a d i a t i o n (or " r e f l e c t i o n " ) effect w h i c h are i n t i m a t e l y r e l a t e d t o t h e s t r u c t u r e o f s t e l l a r a t m o s p h e r e s , a n d e v e n i n a few f a v o u r a b l e cases t h e degree o f c e n t r a l c o n d e n s a t i o n of a t