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>. <jpa00221812>
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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 intérêt 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 doubles 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 discrete spectrum line in the Crab pulsar.
1. Introduction.- Magnetic fields are of much interest in astrophysics. This interest 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 thousand gauss. In the normal solar photosphere outside sunspots the fields are typically 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. Magnetic white dwarf stars were discovered by Kemp, Swedlund, Landstreet and Angel [2] on
the basis of circularly polarized continuum radiation and its subsequent interpretation in terms of a magnetic field of order 10^ gauss. Strong evidence has accumulated that pulsars have even larger magnetic fields, of order 10 1 2 gauss. Finally
a class of stars has been discovered, the prototype being AM Herculis, which are believed 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 10 - 2 to 10-lt of the solar luminosity, and surface temperatures in the range 4000°K to 80,000°K. Their radii average 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 metals, 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 believed t o be an end point of s t e l l a r evolution. They a r e composed of degenerate m a t t e r , except f o r a t h i n o u t e r atmosphere. Several hundred
white dwarfs a r e known: Greenstein [3] gave a compilation of 181 of them. A review
on white dwarfs was given by Liebert [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 conservation of magnetic f l u x
would hold, and i n a s t a r whose r a d i u s decreases 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 detected i n white dwarf
s p e c t r a a t t h i s time. Pulsar t h e o r i e s r e q u i r e f i e l d s of lo1* gauss, and i f a p u l s a r
has a r a d i u s of lo6 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 searches have been made f o r magnetic f i e l d s i n white dwarfs. Preston [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 hydrogen, 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 of lo5 gauss f o r seven white dwarfs, and Angel and
Landstreet [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
searches f o r p o l a r i z a t i o n . Other authors have made searches: 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 observation they g e n e r a l l y avoided t h e common DA and DB (helium) 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 concentrated 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 discussed below. The remainder of t h e s t a r s show no measurable f i e l d . Perhaps 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
present 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 or q u a d r a t i c Zeeman s h i f t s . In 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 authors conclude t h a t of about 350 known white 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), so 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 addit i o n a l information 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 the reviews 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
Hy and H6. Greenstein [12] showed t h a t Ha a l s o has s t r u c t u r e . By coml i n e s I@,
parison with 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 observations showed s t r u c t u r e coinciding with t h e Paschen-Back components. L a t e r work on t h i s s t a r includes [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
[IS]. The l i g h t v a r i e s with 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 of
Kemic [13]. The f i e l d is somewhat v a r i a b l e , with a period of 132 minutes and synchronously 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
oblique 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 . Further observations of t h e
s t a r were obtained by Greenstein and Boksenberg [19], and by Greenstein 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 absorption f e a t u r e s .
635-26.
Discovered by Greenstein [20], 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 observations 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 Landstreet 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 observations [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 suggested t h a t t h e f e a t u r e is 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
out 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 the 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 Landstreet and Angel [25]. The spectrum was
found by Greenstein 1261 t o have unusual absorption f e a t u r e s which have been a t t r i b uted 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 the band suggested a mean projected component of the magnetic f i e l d of 3.6 x lo6 gauss.
This s t r o n g l y c i r c u l a r l y polarized s t a r shows an extremely s t r o n g abLP 790-29.
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
depth of 75% of the 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 absorption 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 the f i r s t magnetic white dwarf t o be discovered, 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 polarizat 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, [31], [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 present. The spectrum of t h e s t a r is a longstanding puzzle. As long ago as 1938 Minkowski [34] discovered t h a t t h e spectrum,
previously considered f e a t u r e l e s s , had wide, shallow absorption f e a t u r e s a t 4135 A
and 4475 A. Greenstein [35] found another a b s o r p t i o n f e a t u r e a t 3910 A.
The feat u r e s a r e known as Minkowski bands. Greenstein and Matthews [36] confirmed these
f e a t u r e s and found o t h e r s , a s did Wegner [22,371 and Landstreet 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 pol 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 explanat i o n s of t h e f e a t u r e s include 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 cyclotron emission. These suggestions have been reviewed [9,10].
Recent work includes new p o l a r i z a t i o n observations (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 than 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
suggests 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 approximately 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 surface. The f e a t u r e s would not
be broadened t o the point 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 seems
C2-22
JOURNAL DE PHYSIQUE
very l i k e l y t o be t h e c o r r e c t one, and i f so a f i e l d of 2 x
indicated.
lo8
g a u s s , or more, is
Greenstein 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 poss 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. This 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 unu 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 abs o r p t i o n s [12,19,22,42,43].
U l t r a v i o l e t spectrophotometry w a s obtained 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 emission, or they
could 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 elements. 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 is a h o t s t a r w i t h a
r o t a t i o n a l period of 1.6 hours. It has shallow but 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 discovered by Angel and Landstreet 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 discovered. 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 with a period of 1.33 days, an amplitude 0.24%
and a mean -0.232, i n d i c a t i n g an oblique r o t a t o r . The spectrum is e s s e n t i a l l y continuous [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 high 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 red 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 [50]. The spectrum is e s s e n t i a l l y continuous [12,50]
with shallow 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 feat 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 spectrum 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 point 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. Cyclotron a b s o r p t i o n is a l s o a possib 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 white
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.
Intere 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 opinion 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 formation of t h e white dwarfs, and
t h e r e has 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 white dwarfs may have been formed from main sequence magnetic A-type
s t a r s ; another p o s s i b i l i t y is t h a t they 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 magnetic fields.
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 emission seems an u n l i k e l y explanation: i t would r e q u i r e too 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 discovered by Wolf i n 1924.
InterSubsequent 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 .
e s t i n t h e s t a r became much g r e a t e r following t h e s u g g e s t i o n by Berg and Duthie [54]
t h a t AM Herculis should be i d e n t i f i e d with 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 observations have been made by
many workers. We r e f e r t h e reader 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 extensive list of observations and references may be
found. We summarize a few of t h e leading c h a r a c t e r i s t i c s of AM Herculis 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 notes on
some similar s t a r s .
.
AM Herculis
( 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
period of a binary s t a r .
( b ) The l i g h t curve i n t h e v i s u a l region 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 ) In 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 longer wavelengths and becoming t h e deeper minimum beyond
1.6 microns wavelength.
( d ) In t h e blue and u l t r a v i o l e t t h e secondary minimum is absent.
(e) There a r e long-term v a r i a t i o n s i n t h e luminosity of t h e system.
In the visual
region 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 strong 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 approximately a I-* wavelength dependence.
(g) The components of t h e binary a r e thought t o r o t a t e synchronously with t h e
o r b i t a l revolution.
( 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 period, &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 the 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 with t h e
o r b i t a l period. The l i n e a r p o l a r i z a t i o n shows a sharp peak of 6%, the 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 the 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 the near-ultraviolet
.
(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) is 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 Herculis was detected 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 Herculis 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. Ti0 bands a r e observed, as well 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 ) Studies of the 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 observations.
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
hot 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
suggests 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 suggests t h a t t h e l i n e s seen a r e formed c l o s e t o t h e hot s t a r ,
and t h e hot s t a r is probably a magnetic white dwarf s t a r . The Na I l i n e s and Ti0
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 estimated as
M4.5 V o r M5 V [62,63,64].
The mass of t h i s s t a r can be estimated i n various ways,
f o r example by assuming it is on t h e main sequence, o r somewhat above it. A cons i d e r e d opinion 1631 is t h a t t h e mass is 0.26 Mg, t h e magnetic white dwarf s t a r has
a mass of 0.39 Mg and a radius 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 spectroscopic p a r a l l a x f o r t h e M s t a r , from a trigonometrical 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 blue region t o a standard 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 its Roche lobe, and matter 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 white dwarf
(mentioned above) about I x lo9 cm. The r a d i o observations can be i n t e r p r e t e d as
cyclotron 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 x lo1' 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 x
10 gauss. Emission takes p l a c e i n a range of cyclotron harmonics centered 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.
9
The a c c r e t i o n of matter onto white dwarf s t a r s has been studied by many authors, and
r e f e r e n c e s may be found i n [60]. A standing shock wave i s formed above t h e white
dwarf surface. Hard X-rays a r e emitted by bremsstrahlung from i n f a l l i n g matter 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 absorption 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
disk. 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 matter towards one o r both magnetic poles 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 matter is equal t o t h e magnetic energy dens 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 point 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 , so 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 enclose 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
place i n higher harmonics [69] because t h e lowest harmonics a r e self-absorbed.
Models of AM Herculis have been constructed. 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 funneling of t h e acc 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 constructed, 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 Herculis. C i r c u l a r p o l a r i z a t i o n varying 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 detected by
Hearn and Marshall [73]. Five color 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 [75]. P o l a r i z a t i o n was d e t e c t e d by Tapia [76], photomef i e d by G r i f f i t h s
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 absorption 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.
e.
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
Its l i g h t curve is l i k e an e c l i p s i n g v a r i a b l e
system. Its period is 100 minutes.
It has v a r i a b l e l i n e a r and
except t h a t t h e minimum l a s t s more than h a l f t h e period.
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 region
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 bel i e v e d t o be t h e only known case of cyclotron emission seen a s such. Confirmation
of t h e f e a t u r e s was obtained by Stockman, Liebert and Bond 1841. The even spacing
of s e v e r a l absorption f e a t u r e s s t r o n g l y supports t h e cyclotron theory of t h e i r emiss 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 region 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 poles, and e c l i p s e d by t h e white dwarf as
it r o t a t e s synchronously with t h e o r b i t a l motion.
The o p t i c a l luminosity can be
explained i n terms of a c c r e t i o n by t h e white dwarf.
DQ Herculis.
This s t a r is t h e remnant of Nova H e r c u l i s 1934. There is 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 observed 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] reported 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
sec-l.
Strickman, Johnson and Kurfess [87] observed t h e 20250 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
photons ~ m ' - sec-l
~
to a line
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 cm-2 sec-' t o a l i n e from t h e p u l s a r . Other
from t h e nebula and 4.9 x lo-!
observers 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 is r e a l it
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 observations as
a f u n c t i o n of p o s i t i o n i n t h e pulse c y c l e as well as a f u n c t i o n of energy. They det 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 instrumental. 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 ranges, and only during pulse p e r i o d s , so t h a t it is pulsed
a t t h e pulse 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 reported by Manchanda,
Bazzano, La Padula, Polcaro and Ubertini [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 the 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 cyclotron frequency a magnetic f i e l d of 6 x 1012 gauss
i s i n d i c a t e d . Correcting 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 value
somewhat. I f t h e e l e c t r o n s a r e streaming out from t h e p u l s a r with r e l a t i v i s t i c vel o c i t i e s the magnetic f i e l d required could be as low as 10lo 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.
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