HAL Id: jpa-00221812
https://hal.archives-ouvertes.fr/jpa-00221812
Submitted on 1 Jan 1982
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
HIGH MAGNETIC FIELD SPECTROSCOPY IN ASTROPHYSICS
R. Garstang
To cite this version:
R. Garstang. HIGH MAGNETIC FIELD SPECTROSCOPY IN ASTROPHYSICS. Journal de
Physique Colloques, 1982, 43 (C2), pp.C2-19-C2-28. �10.1051/jphyscol:1982203�. �jpa-00221812�
JOURNAL DE PHYSIQUE
Colloque C2, supplément au n°U, Tome 42, novembre 1982 page C2-19
HIGH MAGNETIC FIELD SPECTROSCOPY IN ASTROPHYSICS
R.H. Garstang
Joint Institute for Laboratory Astro-physios ,University of Colorado and National Bureau of Standards, and Department of Astro-Geophysias and
Department of Physios, University of Colorado,.Boulder, Colorado 80309, U.S.A.
Résumé.- La découverte de champs magnétiques très intenses dans les étoiles naines blanches et dans les étoiles à neutrons a fait naître un grand inté- rêt pour la spectroscopie de l'atome placé dans de telles conditions. Nous rappelons la découverte de la polarisation du spectre continu de certaines naines blanches et discutons l'interprétation qui confirme la présence de champs magnétiques intenses. Des études récentes sur la classe d'étoiles dou- bles naines blanches magnétiques AM Herculis sont ensuite discutées ainsi que la découverte d'une raie spectrale discrête dans le pulsar du Crabe.
Abstract.- The discovery of very large magnetic fields in white dwarf stars and neutron stars has stimulated interest in the spectroscopy of free atoms in high magnetic fields. We review the discovery of continuum polarization in certain white dwarf stars. We discuss the interpretation of the spectra of the magnetic white dwarfs and the confirmation of the presence of large fields. We describe recent investigations on a class of magnetic white dwarf binary stars (AM Herculis stars). Finally we mention the detection of a dis- crete spectrum line in the Crab pulsar.
1. Introduction.- Magnetic fields are of much interest in astrophysics. This in- terest dates from 1908, when Hale discovered magnetic fields in sunspots from the Zeeman splitting of their spectral lines. The fields in large spots are a few thou- sand gauss. In the normal solar photosphere outside sunspots the fields are typi- cally a few gauss. For many years there was no reason to expect that other kinds of stars would have magnetic fields much larger than those of the Sun. This situation changed when Babcock [1] demonstrated the presence of magnetic fields in a number of stars of spectral type A, and in a few other stars. Many of these A stars show spectral peculiarities. Their fields are of order 101* gauss. Many other types of stars have been examined for the presence of large magnetic fields, mostly without success. Success did come in 1970, however, and from an unexpected source. Magnet- ic white dwarf stars were discovered by Kemp, Swedlund, Landstreet and Angel [2] on the basis of circularly polarized continuum radiation and its subsequent interpreta- tion in terms of a magnetic field of order 10^ gauss. Strong evidence has accumu- lated that pulsars have even larger magnetic fields, of order 101 2 gauss. Finally a class of stars has been discovered, the prototype being AM Herculis, which are be- lieved to be binary stars containing a magnetic white dwarf star as one component and a companion star which is shedding matter from its outer layers on to the white dwarf. We shall review these discoveries, starting with single magnetic white dwarf stars.
2. Magnetic White Dwarf Stars.- Ordinary white dwarf stars have masses approximately the same as our Sun, luminosities which range from 1 0- 2 to 10-lt of the solar lumi- nosity, and surface temperatures in the range 4000°K to 80,000°K. Their radii ave- rage 0.011 of the solar radius, slightly larger than the radius of the Earth. Their mean densities are 106 times that of water. They show a variety of spectra. About
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982203
JOURNAL DE PHYSIQUE
two-thirds of t h e white dwarfs show hydrogen l i n e s and a r e c l a s s i f i e d as DA type.
Some show helium l i n e s , a few show l i n e s of n e u t r a l m e t a l s , and a s i g n i f i c a n t number show e s s e n t i a l l y continuous s p e c t r a with a t most only very weak s p e c t r a l f e a t u r e s . White dwarfs a r e b e l i e v e d t o be an end point of s t e l l a r e v o l u t i o n . They a r e com- posed of degenerate m a t t e r , except f o r a t h i n o u t e r atmosphere. S e v e r a l hundred white dwarfs a r e known: G r e e n s t e i n [ 3 ] gave a compilation of 181 of them. A review on white dwarfs was given by L i e b e r t [4]. The white dwarfs a r e believed t o have evolved from normal s t a r s . During t h i s e v o l u t i o n c o n s e r v a t i o n of magnetic f l u x would hold, and i n a s t a r whose r a d i u s d e c r e a s e s by a f a c t o r 100 t h e magnetic f i e l d s t r e n g t h would i n c r e a s e by a f a c t o r of
lo4.
For a t y p i c a l main sequence s t a r with a f i e l d of 1-10 gauss ( i f indeed most main sequence s t a r s have f i e l d s comparable t o t h a t of t h e Sun, which is not known) the white dwarf evolutionary end-product would have a f i e l d i n t h e range lo4-lo5 gauss, too small t o be d e t e c t e d i n white dwarf s p e c t r a a t t h i s time. P u l s a r t h e o r i e s r e q u i r e f i e l d s oflo1*
gauss, and i f a p u l s a r has a r a d i u s oflo6
cm i t s progenitor must have had a f i e l d of lo2 gauss while it w a s on t h e main sequence. There a r e many p u l s a r s , so many former main sequence s t a r s must have had f i e l d s of t h i s magnitude.Many s e a r c h e s have been made f o r magnetic f i e l d s i n white dwarfs. P r e s t o n [ 5 ] looked f o r q u a d r a t i c Zeeman e f f e c t wavelength s h i f t s i n t h e Balmer l i n e s of hy- drogen, concluded t h a t such s h i f t s a r e absent i n t h e DA s t a r s he s t u d i e d , and gave an upper l i m i t of 5 x
lo5
gauss t o t h e magnetic f i e l d s i n t h e s t a r s . E l i a s and Greenstein [6] s e t an upper l i m i t oflo5
gauss f o r seven white dwarfs, and Angel and L a n d s t r e e t [7] obtained a s i m i l a r upper l i m i t f o r nine white dwarfs on t h e b a s i s of s e a r c h e s f o r p o l a r i z a t i o n . Other a u t h o r s have made s e a r c h e s : t h e most e x t e n s i v e survey was by Angel, Borra and Landstreet 181. They observed over 100 white dwarfs.I n s e l e c t i n g s t a r s f o r o b s e r v a t i o n they g e n e r a l l y avoided t h e common DA and DB (he- lium) s p e c t r a , i n which l a r g e magnetic f i e l d s could be d e t e c t e d by examination of t h e s p e c t r a f o r g r o s s p e c u l i a r i t i e s , and c o n c e n t r a t e d on f e a t u r e l e s s and p e c u l i a r s p e c t r a . T h i r t e e n s t a r s i n all show l a r g e f i e l d s , over 5 x
lo6
gauss; t h e s e s t a r s w i l l be d i s c u s s e d below. The remainder of t h e s t a r s show no measurable f i e l d . Per- haps t h e most i n t e r e s t i n g conclusion t o a s p e c t r o s c o p i s t is t h a t when a f i e l d is p r e s e n t it is s t r o n g enough t o s e r i o u s l y d i s t o r t atomic s p e c t r a l f e a t u r e s o r even t o o b l i t e r a t e them. There a r e no examples of more moderate f i e l d s which produce small l i n e a r Zeeman s p l i t t i n g s o r q u a d r a t i c Zeeman s h i f t s . I n t e n s t a r s t h e upper limits on t h e f i e l d a r e 50,000 gauss, and f o r two s t a r s (Wolf 1346 and 40 Eri B) t h e upper l i m i t s a r e 10,000 gauss. The a u t h o r s conclude t h a t of about 350 known w h i t e dwarfs of type DA only f i v e show l a r g e magnetic f i e l d s ( t h e o t h e r e i g h t magnetic white dwarfs a r e of types o t h e r than DA), s o t h a t perhaps 1 percent of white dwarfs may possess high magnetic f i e l d s .We now t u r n t o a d i s c u s s i o n of i n d i v i d u a l magnetic white dwarf s t a r s . Much addi- t i o n a l i n f o r m a t i o n and more r e f e r e n c e s t o o r i g i n a l papers may be found i n t h e re- views of Garstang [ 9 ] , Angel [ l o ] and Angel, Borra and Landstreet 181.
GD 90. C i r c u l a r p o l a r i z a t i o n w a s discovered by Angel, Carswell, S t r i t t m a t t e r , Beaver and Harms [ I l l . They a l s o discovered broad a b s o r p t i o n f e a t u r e s i n t h e spectrum which were recognized t o be resolved Paschen-Back s t r u c t u r e i n t h e Balmer l i n e s I@, Hy and H6. G r e e n s t e i n [12] showed t h a t Ha a l s o has s t r u c t u r e . By com- p a r i s o n w i t h t h e c a l c u l a t i o n s of Kemic [I31 they deduced a f i e l d of 5 x
lo6
gauss.P o l a r i z a t i o n o b s e r v a t i o n s showed s t r u c t u r e c o i n c i d i n g with t h e Paschen-Back compo- nents. L a t e r work on t h i s s t a r i n c l u d e s [14].
BPM 25114. This s t a r was discovered s p e c t r o s c o p i c a l l y by Wickramasinghe and Bessel [ I S ] . The l i g h t v a r i e s w i t h a period of 2.8 days. Subsequent i n v e s t i g a t i o n s have i d e n t i f i e d hydrogen l i n e s with Zeeman s p l i t t i n g corresponding t o a p o l a r magnetic f i e l d s t r e n g t h of 3.6 x
lo7
gauss [16,17].L795-7 = Feige 7. This remarkable s t a r was discovered by L i e b e r t , Angel, Stockman, Spinrad and Beaver [18] t o possess a l a r g e number of s p e c t r a l f e a t u r e s . These were i d e n t i f i e d as hydrogen and He I l i n e s s p l i t i n t o Zeeman components by a f i e l d of 1.8 x
lo7
gauss, determined by comparison of t h e spectrum w i t h t h e c a l c u l a t i o n s ofKemic [13]. The f i e l d is somewhat v a r i a b l e , with a period of 132 minutes and syn- chronously with v a r i a t i o n s i n p o l a r i z a t i o n . Presumably t h e s t a r r o t a t e s , and an o b l i q u e r o t a t o r model is compatible with the observations. The narrowness of t h e s p e c t r a l f e a t u r e s i n d i c a t e s t h a t t h e magnetic f i e l d must be r a t h e r uniform over a s u b s t a n t i a l p a r t of t h e v i s i b l e hemisphere of t h e s t a r . F u r t h e r o b s e r v a t i o n s of t h e s t a r were o b t a i n e d by G r e e n s t e i n and Boksenberg [ 1 9 ] , and by G r e e n s t e i n and Oke [53]
who obtained u l t r a v i o l e t f l u x e s and some weak a b s o r p t i o n f e a t u r e s .
635-26. Discovered by Greenstein [ 2 0 ] , t h i s s t a r shows Zeeman s p l i t t i n g of both H and He I i n i t s spectrum, and t h e r e is a good match with t h e theory of Kemic [13]
f o r a f i e l d of 8 x
lo6
gauss. The i n d i v i d u a l Zeeman components a r e remarkably sharp. The s t a r has a high v e l o c i t y and presumably belongs t o Population 11.P o l a r i z a t i o n o b s e r v a t i o n s were made by L i e b e r t and Stockman [45]; they f a i l e d t o d e t e c t c i r c u l a r p o l a r i z a t i o n , as might be expected f o r a low f i e l d .
G99-47. This s t a r shows an almost f e a t u r e l e s s spectrum 112,21,22]. However, L i e b e r t , Angel and L a n d s t r e e t 1231 found a s i n g l e f e a t u r e a t 6542 A which had a depth of 0.08 of t h e continuum and was 30 A wide. P o l a r i z a t i o n o b s e r v a t i o n s [24]
showed some evidence of s t r u c t u r e i n t h e 6000-7000 A region. The discovery sug- g e s t e d t h a t t h e f e a t u r e i s t h e c e n t r a l Paschen-Back component of Ha s h i f t e d by 21 A due t o q u a d r a t i c Zeeman e f f e c t , corresponding t o a mean s u r f a c e f i e l d of
1.6 x
lo7
gauss. The o u t e r Paschen-Back components would be s h i f t e d and broadened o u t of v i s i b i l i t y by t h e v a r i a b i l i t y of t h e f i e l d over t h e s u r f a c e of t h e s t a r . 699-37. P o l a r i z a t i o n was discovered by L a n d s t r e e t and Angel [25]. The spectrum was found by Greenstein 1261 t o have unusual a b s o r p t i o n f e a t u r e s which have been a t t r i b - u t e d t o CH and Cg. A number of o t h e r s p e c t r a l s t u d i e s were made [21,25,27], followed by a d e t a i l e d a n a l y s i s by Angel and Landstreet 1281. They c a l c u l a t e d i n d i v i d u a l CH l i n e wavelengths as a f u n c t i o n of magnetic f i e l d , and averaged over t h e molecular band s t r u c t u r e . A f i t of t h e p r o f i l e of t h e band suggested a mean p r o j e c t e d compo- nent of t h e magnetic f i e l d of 3.6 x lo6 gauss.LP 790-29. This s t r o n g l y c i r c u l a r l y p o l a r i z e d s t a r shows an extremely s t r o n g ab- s o r p t i o n f e a t u r e , s t r e t c h i n g from 4400 t o 5600 A (half-depth) and with a c e n t r a l d e p t h of 75% of t h e continuum. L i e b e r t , Angel, Stockman and Beaver [291 suggest t h e f e a t u r e is due t o t h e C2 Swan bands i n a f i e l d of
lo8
gauss. This f e a t u r e may w e l l be t h e s t r o n g e s t a b s o r p t i o n f e a t u r e i n any s t e l l a r spectrum.Grw +70°8247. This s t a r w a s t h e f i r s t magnetic w h i t e dwarf t o be d i s c o v e r e d , by Kemp, Swedlund, l a n d s t r e e t and Angel [2]. They found broadband c i r c u l a r p o l a r i z a - t i o n . Other p o l a r i z a t i o n s t u d i e s were made, i n c l u d i n g [301, [ 3 1 ] , [321 and 1331.
Some l i n e a r p o l a r i z a t i o n is a l s o p r e s e n t . The spectrum of t h e s t a r is a long- s t a n d i n g puzzle. As long ago as 1938 Minkowski [34] discovered t h a t t h e spectrum, p r e v i o u s l y considered f e a t u r e l e s s , had wide, shallow a b s o r p t i o n f e a t u r e s a t 4135 A and 4475 A. G r e e n s t e i n [35] found a n o t h e r a b s o r p t i o n f e a t u r e a t 3910 A. The fea- t u r e s a r e known as Minkowski bands. G r e e n s t e i n and Matthews [36] confirmed t h e s e f e a t u r e s and found o t h e r s , a s did Wegner [22,371 and L a n d s t r e e t and Angel 1331.
Numerous d i s c u s s i o n s of t h e o r i g i n of t h e bands have been made. The c i r c u l a r po- l a r i z a t i o n e s t a b l i s h e s t h e presence of a s t r o n g magnetic f i e l d . P o s s i b l e explana- t i o n s of t h e f e a t u r e s i n c l u d e q u a d r a t i c Zeeman-shifted hydrogen l i n e s , n e u t r a l helium, molecular helium and c y c l o t r o n emission. These s u g g e s t i o n s have been re- viewed [9,10]. Recent work i n c l u d e s new p o l a r i z a t i o n o b s e r v a t i o n s (work of Angel, Stockman and L i e b e r t reviewed i n [ l o ] ) showing s t r o n g l i n e a r p o l a r i z a t i o n a t t h e wavelengths of t h e 4135 and 5855 A bands, i n d i c a t i n g a f i e l d of more t h a n
lo8
gauss.I n f u r t h e r d i s c u s s i o n of t h e spectrum by Angel [10,38] he p o i n t s out t h a t many Zeeman components of hydrogen l i n e s show a s t a t i o n a r y c h a r a c t e r a t f i e l d s above
lo8
gauss, s o t h a t t h e wavelengths do not vary g r e a t l y with i n c r e a s i n g f i e l d . Angel s u g g e s t s t h a t t h e f e a t u r e s i n Grw +70°8247 a r e hydrogen l i n e components a t t h e i r ap- proximately s t a t i o n a r y wavelengths. I f t h e wavelengths a r e n e a r l y independent of f i e l d then t h e r e would be r e l a t i v e l y l i t t l e l i n e broadening caused by t h e v a r i a t i o n of f i e l d i n t e n s i t y and d i r e c t i o n over t h e s t e l l a r s u r f a c e . The f e a t u r e s would n o t be broadened t o t h e p o i n t of o b l i t e r a t i o n . Angel's suggested i d e n t i f i c a t i o n seemsC2-22 JOURNAL DE PHYSIQUE
v e r y l i k e l y t o be t h e c o r r e c t one, and i f s o a f i e l d of 2 x
lo8
g a u s s , o r more, is i n d i c a t e d .G r e e n s t e i n and Oke [53] have made u l t r a v i o l e t o b s e r v a t i o n s of t h e spectrum. The f l u x i s f a r from being a smooth f u n c t i o n of wavelength. There a r e l a r g e d e v i a t i o n s from a blackbody and s e v e r a l r e l a t i v e l y s h a r p f e a t u r e s . The l a t t e r might p o s s i b l y be due t o C I l i n e s i n a h i g h magnetic f i e l d , and c y c l o t r o n a b s o r p t i o n is a l s o pos- s i b l e . But t h e a b s o r p t i o n s must s t i l l be considered as u n i d e n t i f i e d .
GD 229. T h i s star shows both c i r c u l a r and l i n e a r p o l a r i z a t i o n [39,40,41]. Its un- u s u a l spectrum shows a very s t r o n g a b s o r p t i o n a t 4185 A and a number of weaker ab- s o r p t i o n s [12,19,22,42,43]. U l t r a v i o l e t s p e c t r o p h o t o m e t r y w a s o b t a i n e d by Green and L i e b e r t [44]. The a b s o r p t i o n f e a t u r e s could be due t o c y c l o t r o n e m i s s i o n , o r they c o u l d be due t o a complex s u p e r p o s i t i o n of many Zeeman components of v a r i o u s ele- ments. The u l t r a v i o l e t d a t a appear t o r u l e out a l a r g e hydrogen abundance i n t h e s t a r , and a Zeeman i n t e r p r e t a t i o n of t h e f e a t u r e s must be i n terms of o t h e r e l e - ments, e s p e c i a l l y He I. I n any c a s e a f i e l d exceeding
lo8
gauss would be r e q u i r e d . PG 1014t01. Discovered by Green ( s e e review i n [ l o ] ) , t h i s i s a h o t s t a r w i t h a r o t a t i o n a l p e r i o d of 1.6 hours. It has s h a l l o w b u t d e f i n i t e a b s o r p t i o n f e a t u r e s and shows v a r i a b l e c i r c u l a r p o l a r i z a t i o n . The f e a t u r e s have n o t been i d e n t i f i e d , G195-19. C i r c u l a r p o l a r i z a t i o n was d i s c o v e r e d by Angel and L a n d s t r e e t 1461; i t was t h e second p o l a r i z e d w h i t e dwarf t o be d i s c o v e r e d . Subsequent work on t h e p o l a r i z a - t i o n 147,481 showed i t was v a r i a b l e w i t h a p e r i o d of 1.33 days, an amplitude 0.24%and a mean -0.232, i n d i c a t i n g an o b l i q u e r o t a t o r . The spectrum is e s s e n t i a l l y con- t i n u o u s [21,22,43,49]. Perhaps a l l s p e c t r a l f e a t u r e s a r e broadened t o i n v i s i b i l i t y by a h i g h f i e l d .
6227-35. C i r c u l a r p o l a r i z a t i o n i n t h i s s t a r ranges from 1% i n t h e r e d t o 3% i n t h e v i o l e t and 8% i n t h e i n f r a r e d [ 5 0 ] . The spectrum i s e s s e n t i a l l y c o n t i n u o u s [12,50]
w i t h s h a l l o w d e p r e s s i o n s of t h e continuum, perhaps i n d i c a t i n g t h a t a l l s p e c t r a l fea- t u r e s are broadened t o i n v i s i b i l i t y by a h i g h magnetic f i e l d .
G240-72. This s t a r shows s t r o n g l i n e a r and c i r c u l a r p o l a r i z a t i o n (511. The spec- trum is e s s e n t i a l l y continuous, w i t h very broad d e p r e s s i o n s of t h e continuum [12,22, 43,511. Perhaps a l l s p e c t r a l f e a t u r e s a r e broadened t o t h e p o i n t of i n v i s i b i l i t y by a h i g h magnetic f i e l d of a t l e a s t lo8 gauss. C y c l o t r o n a b s o r p t i o n is a l s o a p o s s i - b l e e x p l a n a t i o n of t h e f e a t u r e s [52].
It is o u t s i d e t h e scope of t h i s l e c t u r e t o d i s c u s s t h e o r i g i n and e v o l u t i o n of w h i t e dwarf magnetic f i e l d s and t h e e f f e c t s of t h e f i e l d s on a c c r e t i o n phenomena. I n t e r - e s t e d r e a d e r s a r e r e f e r r e d t o t h e review by Angel [ l o ] . The p r e v a i l i n g o p i n i o n is t h a t t h e f i e l d s were f r o z e n i n a t t h e time of f o r m a t i o n of t h e white dwarfs, and t h e r e h a s n o t been time f o r t h e f i e l d s t o decay by ohmic l o s s e s i n t h e age of t h e Galaxy. The w h i t e dwarfs may have been formed from main sequence magnetic A-type s t a r s ; a n o t h e r p o s s i b i l i t y is t h a t t h e y were formed by t h e c o l l a p s e of o r i g i n a l l y massive stars w i t h carbon-burning c o r e s , and convective a m p l i f i c a t i o n of t h e mag- n e t i c f i e l d s .
We mentioned above Angel's s u g g e s t i o n t h a t t h e u n i d e n t i f i e d s p e c t r a l f e a t u r e s i n Grw +70°8247 a r e f e a t u r e s of hydrogen and helium which a r e n e a r l y s t a t i o n a r y i n wavelength a s t h e f i e l d i n c r e a s e s above
lo8
gauss. Whether t h i s same e x p l a n a t i o n a p p l i e s t o t h e u n i d e n t i f i e d f e a t u r e s i n GD 229 and PG 1015i-01 remains t o be seen.Cyclotron e m i s s i o n seems an u n l i k e l y e x p l a n a t i o n : i t would r e q u i r e t o o l a r g e a volume w i t h a uniform f i e l d s t r e n g t h [19]. F u r t h e r c a l c u l a t i o n s of wavelengths of hydrogen and helium l i n e s i n f i e l d s of lo8-lo9 gauss w i l l h e l p t o s o l v e t h e problem.
3. AM H e r c u l i s Stars.- The v a r i a b l e s t a r AM H e r c u l i s was d i s c o v e r e d by Wolf i n 1924.
Subsequent o b s e r v a t i o n s showed t h a t i t resembled t h e c a t a c l y s m i c v a r i a b l e s . I n t e r - e s t i n t h e s t a r became much g r e a t e r f o l l o w i n g t h e s u g g e s t i o n by Berg and Duthie [ 5 4 ]
t h a t AM H e r c u l i s should be i d e n t i f i e d w i t h a high g a l a c t i c l a t i t u d e X-ray source (known a s 3U 1809 +50 and as 4U 1814 +SO). Hearn and Richardson [55] showed t h a t t h e s o f t X-ray f l u x (0.1-0.4 keV) v a r i e s with a period of 3.1 hours, t h e same period which is shown by t h e o p t i c a l l i g h t curves of Szkody and Brownlee [56], t h e r a d i a l v e l o c i t i e s of Cowley and Crampton [57] and Priedhorsky [58] and t h e p o l a r i z a t i o n s t u d i e s of Tapia 1591. This confirmed t h e i d e n t i t y of AM H e r c u l i s and t h e X-ray source. Subsequently a l a r g e number of a d d i t i o n a l o b s e r v a t i o n s have been made by many workers. We r e f e r t h e r e a d e r t o t h e e x c e l l e n t review a r t i c l e by C h i a p p e t t i , Tanzi and Treves [60] where an e x t e n s i v e l i s t of o b s e r v a t i o n s and r e f e r e n c e s may be found. We summarize a few of t h e l e a d i n g c h a r a c t e r i s t i c s of AM H e r c u l i s taken from t h i s review a r t i c l e and from l a t e r papers. This w i l l be followed by b r i e f n o t e s on some similar s t a r s .
AM H e r c u l i s
.
( a ) The s t a r shows a period of 0.128928 days, which is i n t e r p r e t e d as t h e o r b i t a l p e r i o d of a b i n a r y s t a r .
( b ) The l i g h t curve i n t h e v i s u a l r e g i o n shows a broad, deep primary minimum, a shallow secondary minimum, and much f l i c k e r i n g . ('Primary minimum' is defined t o be t h e deeper minimum i n v i s u a l l i g h t . )
( c ) I n t h e i n f r a r e d both l i g h t minima a r e deep, t h e secondary minimum becoming i n c r e a s i n g l y deep a t l o n g e r wavelengths and becoming t h e deeper minimum beyond 1.6 microns wavelength.
( d ) I n t h e b l u e and u l t r a v i o l e t t h e secondary minimum i s absent.
(e) There a r e long-term v a r i a t i o n s i n t h e l u m i n o s i t y of t h e system. I n t h e v i s u a l r e g i o n t h e s t e l l a r magnitude v a r i e s from 12 (high s t a t e ) t o 15 (low s t a t e ) . The s t a r spends most of i t s time i n t h e high s t a t e .
( f ) In t h e high s t a t e t h e spectrum shows s t r o n g emission l i n e s of H, He 11, C N , N V and o t h e r i o n s , superposed on a s t r o n g continuum. The continuum f l u x has ap- proximately a I-* wavelength dependence.
( g ) The components of t h e b i n a r y a r e thought t o r o t a t e synchronously with t h e o r b i t a l r e v o l u t i o n .
( h ) The spectrum l i n e s show r a d i a l v e l o c i t y v a r i a t i o n s with t h e above p e r i o d , &t u n l i k e normal e c l i p s i n g v a r i a b l e s t h e maximum r a d i a l v e l o c i t y of r e c e s s i o n occurs a t t h e time of primary minimum.
( i ) In t h e high s t a t e t h e spectrum l i n e s do not show any Zeeman s p l i t t i n g .
( j ) Absorption l i n e s of Na I a t 8193-8194 A a r e observed, presumably from t h e cool component of t h e binary system.
( k ) The s t a r l i g h t shows both l i n e a r and c i r c u l a r p o l a r i z a t i o n , varying w i t h t h e o r b i t a l p e r i o d . The l i n e a r p o l a r i z a t i o n shows a s h a r p peak of 6%, t h e c i r c u l a r p o l a r i z a t i o n v a r i e s from +3% t o -9%, and t h e maximum l i n e a r p o l a r i z a t i o n occurs a t a phase when t h e c i r c u l a r p o l a r i z a t i o n is zero. The p o l a r i z a t i o n is very small i n t h e n e a r - u l t r a v i o l e t
.
(R) The X-ray f l u x v a r i e s with t h e above period and has a minimum a t t h e time of v i s i b l e secondary minimum.
(m) The s o f t X-ray Flux (<0.5 keV) i s approximately f i t t e d by a blackbody o r by bremsstrahlung
,
with a temperature of 250 ,OOO°K.( n ) Radio emission from AM H e r c u l i s was d e t e c t e d by Chanmugam and Dulk [61] with a f l u x d e n s i t y 6.7 x 10-30 w a t t s m-* H Z - ~ a t a frequency of 4.9 GHz. There w a s no evidence of c i r c u l a r p o l a r i z a t i o n .
JOURNAL DE PHYSIQUE
( 0 ) Spectroscopy of t h e low s t a t e of AM H e r c u l i s has been accomplished by Schmidt, Stockman and Margon [ 6 2 ] , Young, Schneider and Shectman [ 6 3 ] , Latham, L i e b e r t and S t e i n e r [64] and Hutchings, Crampton and Cowley [65]. They f i n d s t r o n g , sharp l i n e s of H and He I. Absorption f e a t u r e s c l o s e t o t h e hydrogen l i n e s a r e i n t e r p r e t e d a s Zeeman components i n f i e l d s of 1.3 x
lo7
gauss. T i 0 bands a r e observed, as w e l l as t h e i n f r a r e d Na I l i n e s . The continuum energy r i s e s longward of 6500 A. The s t a r shows s t r o n g p o l a r i z a t i o n [62,64].( p ) S t u d i e s of t h e X-ray spectrum by Rothschild e t al. [66] and by Tuohy, Mason, Garmire and Lamb [671 covered from 0.1 t o 150 keV. The hard X-rays 0 2 keV) seem t o be thermal bremsstrahlung.
There a r e many o t h e r d e t a i l s , which may be found i n t h e review a r t i c l e and i n t h e papers quoted. We proceed t o d i s c u s s t h e i n t e r p r e t a t i o n of t h e o b s e r v a t i o n s . The l i g h t curves and r a d i a l v e l o c i t y curves c l e a r l y i n d i c a t e t h a t we a r e d e a l i n g with a binary system. One s t a r is a very cool red dwarf, and t h e o t h e r is a small h o t s t a r , t h e hot s t a r being e c l i p s e d a t primary minimum. The s t r o n g p o l a r i z a t i o n s u g g e s t s t h a t a high magnetic f i e l d is present. The d e t e c t i o n of Zeeman s p l i t t i n g d u r i n g t h e low s t a t e s u g g e s t s t h a t t h e l i n e s s e e n a r e formed c l o s e t o t h e hot s t a r , and t h e h o t s t a r is probably a magnetic white dwarf s t a r . The Na I l i n e s and T i 0 bands probably a r i s e from a dwarf s t a t , whose s p e c t r a l type has been e s t i m a t e d as M4.5 V o r M5 V [62,63,64]. The mass of t h i s s t a r can be e s t i m a t e d i n v a r i o u s ways, f o r example by assuming it is on t h e main sequence, o r somewhat above i t . A con- s i d e r e d opinion 1631 i s t h a t t h e mass is 0.26 Mg, t h e magnetic white dwarf s t a r h a s a mass of 0.39 Mg and a r a d i u s of 0.015 Q. The d i s t a n c e of t h e system can be e s t i - mated from a s p e c t r o s c o p i c p a r a l l a x f o r t h e M s t a r , from a t r i g o n o m e t r i c a l p a r a l l a x , and from f i t t i n g of t h e observed continuous spectrum i n t h e b l u e r e g i o n t o a stan- dard DA type spectrum. The r e s u l t s obtained [62] a r e i n t h e range 60-100 parsecs.
Many of t h e f e a t u r e s i n t h e spectrum, p a r t i c u l a r l y when t h e s t a r is i n t h e h i g h s t a t e , do not a r i s e from t h e s t a r s themselves, but from streams of gas between t h e s t a r s . The red dwarf probably f i l l s i t s Roche l o b e , and m a t t e r is s p i l l i n g over and a c c r e t i n g onto t h e white dwarf s t a r . The semimajor a x i s of t h e o r b i t is about 8 x
l 0 l o cm. The r a d i u s of t h e red dwarf is about 3 x 1 0 l 0 cm, t h a t of t h e w h i t e dwarf (mentioned above) about I x
lo9
cm. The r a d i o o b s e r v a t i o n s can be i n t e r p r e t e d as c y c l o t r o n emission by assuming a f i e l d s t r e n g t h of perhaps 50 gauss, and t h i s m i h t e x i s t a t about 8 xlo1'
cm from t h e white dwarf i f t h e s u r f a c e f i e l d is 2 x10 gauss.
9
Emission t a k e s p l a c e i n a range of c y c l o t r o n harmonics c e n t e r e d about harmonic 40, and t h e average energy of t h e e l e c t r o n s is roughly 350 keV. The o r i g i n of t h e e l e c t r o n s is unknown.The a c c r e t i o n of m a t t e r onto white dwarf s t a r s has been s t u d i e d by many a u t h o r s , and r e f e r e n c e s may be found i n [60]. A s t a n d i n g shock wave i s formed above t h e white dwarf s u r f a c e . Hard X-rays a r e emitted by bremsstrahlung from i n f a l l i n g m a t t e r i n t h e hot post-shock region; s o f t X-rays may be emitted thermally from t h e white dwarf s u r f a c e by a b s o r p t i o n and re-emission. I n many cataclysmic v a r i a b l e s t h e white dwarf is a member of a binary system, and t h e a c c r e t i o n w i l l produce an a c c r e t i o n d i s k . A s t r o n g magnetic f i e l d modifies t h e a c c r e t i o n by channeling t h e m a t t e r to- wards one o r both magnetic p o l e s once it has crossed t h e Alfvsn s u r f a c e (where t h e k i n e t i c energy d e n s i t y of t h e i n f a l l i n g m a t t e r is equal t o t h e magnetic energy den- s i t y ) . It seems l i k e l y [631 t h a t a t t h e i n n e r Lagrangian p o i n t of AM H e r c u l i s t h e magnetic energy d e n s i t y would indeed exceed t h e k i n e t i c energy d e n s i t y , s o t h a t t h e r e w i l l be no a c c r e t i o n d i s k 1681. The Alfvgn s u r f a c e w i l l e n c l o s e t h e companion M s t a r . Cyclotron emission may be an important mechanism, and t h e emission may t a k e p l a c e i n h i g h e r harmonics [69] because t h e lowest harmonics a r e self-absorbed.
Models of AM H e r c u l i s have been c o n s t r u c t e d . The e a r l i e s t models 156,571 assumed an a c c r e t i o n d i s k . L a t e r models [68,70,71] assumed magnetic f u n n e l i n g of t h e ac- c r e t i n g m a t e r i a l , and t h e magnetic a x i s is i n c l i n e d t o t h e r o t a t i o n a x i s . There a r e s t i l l f e a t u r e s which a r e unexplained by the models s o f a r c o n s t r u c t e d , and f u r t h e r i n v e s t i g a t i o n s w i l l be needed.
AN Ursae Majoris. This is a magnetic white dwarf i n a binary system, s i m i l a r t o AM H e r c u l i s . C i r c u l a r p o l a r i z a t i o n v a r y i n g between -9% and -35% w a s found by Krzeminski and Serkowski [72]. The period is 1.91 hours. The i n f e r r e d magnetic f i e l d of t h e white dwarf is 3 x lo8 gauss. X-ray emission has been d e t e c t e d by Hearn and Marshall [73]. Five c o l o r photometry w a s reported by Gilmozzi, Messi and N a t a l i 1741.
EF Eridani = 2A 0311-227. This binary has a period of 81 minutes. It was i d e n t i - f i e d by G r i f f i t h s
e.
[ 7 5 ] . P o l a r i z a t i o n was d e t e c t e d by Tapia [ 7 6 ] , photome- t r y was performed by Bond, Chanmugam and Grauer [ 7 7 ] , spectroscopy by Crampton, Hutchings and Cowley (781 and X-ray measurements by White [79]. There is evidence[78] t h a t t h e magnetic f i e l d might have a complex s t r u c t u r e , perhaps quadrupolar.
An a b s o r p t i o n d i p i n t h e X-ray spectrum [79] is thought t o be a s s o c i a t e d with t h e a c c r e t i o n column i n f r o n t of t h e X-ray source.
W Puppis. This nova-like v a r i a b l e was discovered by Herbig [80] t o be a b i n a r y system. I t s p e r i o d is 100 minutes. Its l i g h t curve i s l i k e an e c l i p s i n g v a r i a b l e except t h a t t h e minimum l a s t s more t h a n h a l f t h e period. It has v a r i a b l e l i n e a r and c i r c u l a r p o l a r i z a t i o n , discovered by Tapia [ 8 1 ] , it must have a s u b s t a n t i a l r e g i o n of l i g h t emission where t h e f i e l d exceeds
lo8
gauss [ l o ] , and i t s companion s t a r i s probably a f a i n t red dwarf with a b s o l u t e magnitude f a i n t e r than 15 1821. The o p t i - c a l spectrum, observed by Wickramasinghe and Visvanathan [83] shows f e a t u r e s be- l i e v e d t o be t h e only known case of c y c l o t r o n emission seen a s such. Confirmation of t h e f e a t u r e s was o b t a i n e d by Stockman, L i e b e r t and Bond 1841. The even s p a c i n g of s e v e r a l a b s o r p t i o n f e a t u r e s s t r o n g l y supports t h e c y c l o t r o n theory of t h e i r emis- s i o n . The spacing corresponds t o a f i e l d of about 2.7 x 10' gauss. The a b s o r p t i o n appeared whenever t h e primary emission region was i n view. Models [84,85] assume t h a t t h e emission a r i s e s from a r e g i o n small compared with t h e s i z e of t h e white dwarf, s i t u a t e d near one of t h e magnetic p o l e s , and e c l i p s e d by t h e white dwarf as i t r o t a t e s synchronously w i t h t h e o r b i t a l motion. The o p t i c a l l u m i n o s i t y can be explained i n terms of a c c r e t i o n by t h e white dwarf.DQ H e r c u l i s . This s t a r i s t h e remnant of Nova H e r c u l i s 1934. There i s no evidence f o r magnetic f i e l d s as y e t , but a magnetic f i e l d might be r e s p o n s i b l e f o r t h e ob- served speeding up of t h e r o t a t i o n . The s i t u a t i o n was reviewed by Angel [ l o ] .
4. The Crab Pulsar.- A number of measurements have been made of t h e X-ray spectrum of t h e Crab nebula and i t s p u l s a r . Ling, Mahoney, W i l l e t t and Jacobson [86] re- p o r t e d evidence of a narrow l i n e a t 73 keV with a time-averaged i n t e n s i t y of 3.8 x
photons s e c - l . Strickman, Johnson and Kurfess [87] observed t h e 20- 250 k e V range and analyzed t h e i r r e s u l t s by assuming t h a t a l l t h e unpulsed emission comes from t h e nebula and t h e pulsed emission from t h e p u l s a r . They did not d e t e c t a l i n e a t 73 keV, and gave up e r l i m i t s of 1.3 x photons ~ m ' - ~ sec-l t o a l i n e from t h e nebula and 4.9 x
lo-!
photons cm-2 sec-' t o a l i n e from t h e p u l s a r . Other o b s e r v e r s have claimed t o d e t e c t or not t o d e t e c t t h e l i n e . I f t h e l i n e i s r e a l i t must be v a r i a b l e .Strickman, Kurfess and Johnson I881 have re-analyzed t h e i r e a r l i e r o b s e r v a t i o n s as a f u n c t i o n of p o s i t i o n i n t h e p u l s e c y c l e as well as a f u n c t i o n of energy. They de- t e c t e d a l i n e a t 77 keV with an i n t e n s i t y 4.1 x 10-3 photons cm-2 sec-l and a l i n e width wholly i n s t r u m e n t a l . The f e a t u r e appears only i n t h e 70-89 keV energy b i n ,
and not i n o t h e r energy r a n g e s , and only during p u l s e p e r i o d s , s o t h a t it is pulsed a t t h e p u l s e frequency. A d e f i n i t i v e d e t e c t i o n was a l s o r e p o r t e d by Manchanda, Bazzano, La Padula, Polcaro and U b e r t i n i [89]. Their l i n e i n t e n s i t y is 5 x
photons cm-2 see-l.
Cyclotron emission seems t h e l i k e l y mechanism f o r producing t h e l i n e . I f t h e energy of 77 keV corresponds t o t h e c y c l o t r o n frequency a magnetic f i e l d of 6 x 1012 gauss i s i n d i c a t e d . C o r r e c t i n g f o r a g r a v i t a t i o n a l red s h i f t would i n c r e a s e t h i s v a l u e somewhat. I f t h e e l e c t r o n s a r e streaming o u t from t h e p u l s a r with r e l a t i v i s t i c ve- l o c i t i e s t h e magnetic f i e l d r e q u i r e d could be as low as 1 0 l o gauss, but i n any case
JOURNAL DE PHYSIQUE
this is still quite close to the pulsar. There are many problems associated with the proposed cyclotron mechanism. The observed narrow line requires a large number of electrons to have similar behavior (same Lorentz factors). An energy injection mechanism is required to make up for the cyclotron radiative losses. The magnetic field in the emission region must be nearly constant, implying a small volume of space. Clearly we have much to learn about the Crab pulsar.
Acknowledgment. Preparation of this paper was supported in part by National Science Foundation Grant PHY79-04928 to the University of Colorado.
References
[I] BABCOCK H. W., Astrophys. J. 105 (1947) 105-119.
[2] KEMP J. C., SWEDLUND J. B., -STREET J. D. and ANGEL J. R. P., Astrophys. J.
(Letters) (1970) L77-79.
(31 GREENSTEIN J. L., Astron. J.
81
(1976) 323-338.[4] LIEBERT J., Ann. Rev. Astron. Astrophys.
18
(1980) 363-398.[5] PRESTON G W., Astrophys. J. (Letters)
160
(1970) L143-145.[6] ELIAS J. H. and GREENSTEIN J. L., Publ. Astron. Soc. Pacific
86
(1974) 957- 959.[7] ANGEL J. R. P. and LANDSTREET J. D., Astrophys. J. (Letters)
160
(1970) L147- 152.[81 ANGEL J. R. P., BORRA E. F. and LANDSTREET J. D., Astrophys. J. Suppl.2 (1981) 457-474.
[9] GARSTANG R. H., Rep. Prog. Phys.40 (1977) 105-154.
[lo] ANGEL J. R. P., Ann. Rev. Astron. Astrophys.16 (1978) 487-519.
[ll] ANGEL J. R. P., CARSWELL R. F., STRITTMATTER P. A., BEAVER E. A. and HARMS R., Astrophys. J. (Letters)
194
(1974) L47-50.1121 GREENSTEIN J. L., Astrophys. J. (Letters)
194
(1974) L51-55.[13] KEMIC S. B., Joint Institute for Laboratory Astrophysics, Boulder, Colorado (1974) Reoort No. 113, 1-27.
[I41 BROWN.D. N., RICH A. ,-WILLIAMS W. L. and VAUCLAIR G., Astrophys. J.
218
(1977) 227-231.WICKRAMASINGHE D. T. and BESSEL M. S., Astrophys. J. (Letters) E ( 1 9 7 6 ) W 9 - 41.
WICKRAMASINGHE D. T., WHELAN J. A. J. and BESSEL M. S., Mon. Not. Roy. Astron.
SOC.
180
(1977) 373-378.MARTIN B. and WICKRAMASINGHE D. T., Mon. Not. Roy. Astron. Soc.183 (1978) 533-538.
LIEBERT J., ANGEL J. R. P., STOCKMAN H. S., SPINRAD H. and BEAVER E. A., Astrophys. J.
214
(1977) 457-470.GREENSTEIN J. L. and BOKSENBERG A., Mon. Not. Roy. Astron. Soc.185 (1978) 823-832.
GREENSTEIN J. L., Pub. kstron. Soc. Pacific90 (1978) 303-306.
GREENSTEIN J. L., GUNN J. 2. and KRISTIAN J., Astrophys. J. (Letters)
169
(1971) L63-69.
WEGNER G., Mon. Not. Roy. Astron. Soc. 174 (1976) 191-202.
LIEBERT J., ANGEL J. R. P. and LANDSTRE~J. D., Astrophys. J. (Letters)
202
( 1975) Ll39-143.
ANGEL J. R. P. and LANDSTREET J. D., Astrophys. J. (Letters)
178
(1972) L21- 22.LANDSTREET J. D. and ANGEL J. R. P., Astrophys. J. (Letters)= (1971) L67- 70.
GREENSTEIN J. L., Astrophys. J . z (169) 281-293.
GREENSTEIN J. L., Astrophys. J. (Letters)
162
(1970) L55-59.ANGEL J. R. P. and LANDSTREET J. D., Astrophys. J . a (1974) 457-464.
LIEBERT J., ANGEL J. R. P., STOCKMAN H. S. and BEAVER E. A., Astrophys. J.
225
(1978) 181-190.
ANGEL J. R. P. and LANDSTREET J. D., Astrophys. J. (Letters)
162
(1970) L61- 66.KEMP J. C. and SWEDLUND J. B., Astrophys. J. ( L e t t e r s )
162
(1970) L67-68.ANGE'L J. R. P., LANDSTREJIT J. D. and OKE J. B., Astrophys. J. ( L e t t e r s )
171
( 1972) Lll-15.
LANDSTREET J. D. and ANGEL J. R. P., Astrophys. J.
196
(1975) 819-825.MINKOWSKI R., C a r n e g i e I n s t i t u t e of Washington Yearbook No. 37 (1938) 200.
GREENSTEIN J. L., i n Proc. 3rd Symp. Math. S t a t i s t i c s and P r o b a b i l i t y , Vol. 3 (ed. J. Neyman, Univ. C a l i f o r n i a P r e s s , 1956), 11-30.
GREENSTEIN J. L. and MATTHEWS M. S., Astrophys. J. 126 (1957) 14-18.
WEGNER G., Pub. Astron. Soc. P a c i f i c
83
(1971) 2 0 5 - m .ANGEL J. R. P., i n White Dwarfs and V a r i a b l e Degenerate S t a r s , I n t . Astron.
Union Colloquium No. 53 (eds. H. M. Van Horn and V. Weidemann, Univ.
R o c h e s t e r , I979), 313-316.
SWEDLUND J. B., WOLSTENCROFT R. D., MICHALSKY J. J. and KEMP J. C., Astrophys.
J. ( L e t t e r s )
187
(1974) L121-123.KEMP J. C., COYNE G. V., SWEDLUND J. B. and WOLSTENCROFT R. D., Astrophys. J.
( L e t t e r s )
189
(1974) L79-80.LANDSTREET J. D. and ANGEL J. R. P., Astrophys. J. ( L e t t e r s ) E ( 1 9 7 4 ) L25- 26.
GREENSTEIN J. L., SCHMIDT M. and SEARLE L., Astrophys. J. ( L e t t e r s )
190
(1974) L27-28.LIEBERT J., Pub. Astron. Soc. P a c i f i c
88
(1976) 490-494.GREEN R. F. and LIEBERT J., Pub. Astron. Soc. ~ a c i f i c z ( 1 9 8 1 ) 105-109.
LIEBERT J. and STOCKMAN H. S., Pub. Astron. Soc. P a c i f i c
2
(1980) 657-660.ANGEL J. R. P. and LANDSTREET J. D., Astrophys. J. ( L e t t e r s ) * (1971) L15- 16.
KEMP J. C., SWEDLUND J. B. and WOLSTENCROFT R. D., Astrophys. J. ( L e t t e r s )
164
(1971) L17-18.
ANGEL J. R. P., ILLING R. M. E. and LANDSTREET J. D., Astrophys. J. ( L e t t e r s ) 175 (1972) L85-87.
-
ANGEL J. R. P. and LANDSTREET J. D., Astrophys. J. ( L e t t e r s ) s ( 1 9 7 1 ) L71- 75. - .ANGEL J. R. P., HINTZEN P. and LANDSTREET J. D., Astrophys. J. ( L e t t e r s ) = (1975) L27-29.
ANGEL J. R. P., HINTZEN P., STRITTMATTER P. A. and MARTIN P. G., Astrophys. J.
( ~ e t t e r s ) 190 (1974) L71-72.
MARTIN B.
and
WICKRAMASINGHE D. T., Mon. Not. Roy. Astron. Soc.189
(1979) 69- 77.GREENSTEIN J. L. and OKE J. B., Astrophys. J. Z ( l 9 8 2 ) 285-295.
BERG R. A. and DUTHIE J. G., Astrophys. J. 211 (1977) 859-865.
HEARN D. R. and RICHARDSON J. A., Astrophys.. ( ~ e t t e r s )
213
(1977) L115-117.SZKODY P. and BROWNLEE D. E., Astrophys. J. ( ~ e t t e r s ) a ( 1 9 7 7 ) L113-116.
COWLN A. P. and CRAMPTON D., Astrophys. J. ( L e t t e r s ) 212 (1977) L121-124.
PRIEDHORSKY W. C., Astrophys. J. ( L e t t e r s )
212
(1977) m 7 - 1 1 9 . TAPIA S., Astrophys. J. ( L e t t e r s ) 212 (1977) L125-129.CHIAPPETTI L., TANZI E. G. and T R M ~ A., Space S c i . Rev.
27
(1980) 3-33.CHANMUGAM G. and DULK G. A., Astrophys. J. ( L e t t e r s ) = ( 1 9 8 2 ) L107-110.
SCHMIDT G. D., STOCKMAN H. S. and MARGON B., Astrophys. J. ( L e t t e r s )
243
(1981) L157-161.
YOUNG P., SCHNEIDER D. P. and SHECTMAN S. A., Astrophys. J.
245
(1981) 1043- 1053.LATHAM D. W., LIEBERT J. and STEINER J. E., Astrophys. J.
246
(1981) 919-934.WTCHINGS J. B., CRAMPTON D. and COWLEY, A., Astrophys. J.
247
(1981) 195-201.ROTHSCHILD R. E., GRUBER D. E., KNIGHT F. K., MATTESON J. L., NOLAN P. L., SWANK J. H., HOLT S. S., SERLEMITSOS P. J., MASON K. 0. and TUOHY I. R., Astrophys. J.
250
(1981) 723-732.TUOHY I. R., MASON K. O., GARMIRE G. P. and LAMB F. K., Astrophys. J.
245
( 1981) 183-194.
CHANMUGAM G. and WAGNER R. L., Astrophys. J. ( L e t t e r s )
213
(1977) L13-16.CHANMUGAM G. and WAGNER R. L., Astrophys.
J.232
(1979) 895-902.CHANMUGAM G. and WAGNER R. L., Astrophys. J.
222
(1978) 641-646.PRIEDHORSKY W. C. and RREZMINSKI, W., Astrophys. J . E ( l 9 7 8 ) 597-604.
KRZEMINSKI W. and SERKOWSKI K., Astrophys. J. ( L e t t e r s )
216
(1977) L45-48.C2-28 JOURNAL DE PHYSIQUE
HEARN D. R. and MARSHALL F. J., Astrophys. J. (Letters)= (1979) L21-23.
GILMOZZI R., MESS1 R. and NATAL1 G., Astrophys. J. (Letters)= (1981) L119- 122.
GRIFFITHS R. E., WARD M. J., BLADES J. C., WILSON A. S., CHAISSON L. and JOHNSTON M. D., Astrophys. J. (~etters)= (1979) L27-31.
TAPIA S., Internat. Astron. Union Circ. 3327 (1979).
BOND H. E., CHANMUGAM G. and GRAUER A. D., Astrophys. J. (~etters)
234
(1979)L113-116.
CRAMPTON D., HUTCHINGS J. B. and COWLEY A. P., Astrophys. J . E (1981) 567- 575.
WHITE N. E., Astrophys. J. (Letters)
244
(1981) L85-88.HERBIG G. H., Astrophys. J.
132
(1960) 76-86.TAPIA S., Internat. Astron. Union Circ. 3054 (1977).
LIEBERT J., STOCKMAN H. S., ANGEL J. R. P., WOLFF N. J., HEGE K. and MARGON B., Astrophys.
J.225
(1978) 201-208.WICKRAMASINGHE D. T. and VISVANATHAN N. V., Mon. Not. Roy. Astron. Soc.
191
( 1980) 589-598.
STOCKMAN H. S., LIEBERT J. and BOND H. E., White Dwarfs and Variable Degener- ate Stars, Internat. Astron. Union Colloq. No. 53 (eds. H. M. Van Horn and V.
Wiedemann, Univ. Rochester, 1979).
WICKRAMASINGHE D. T. and MEGGITT S. M. A., Mon. Not. Roy. Astron. Soc.198 ( 1982) 975-983.
LING J. C., MAHONEY W. A., WILLETT J. B. and JACOBSON A. S., Astrophys. J . z (1979) 896-905.
STRICKMAN M. S., JOHNSON W. N. and KURFESS J. D., Astrophys. J. (Letters)
230
(1979) L15-19.
STRICKMAN M. S., KURFESS J. D. and JOHNSON W. N., Astrophys. J. (Letters)
253
( 1982) L23-27.
MANCHANDA 3. K., BAZZANO A., LA PADULA C. D., POLCARO V. F. and UBERTINI P., Astrophys. J.
