Stellar Pulsation: Proceedings of a Conference Held as a Memorial to John P. Cox at the Los Alamos National Laboratory Los Alamos, New Mexico, USA, August 11–15,1986

398 Pages·1987·20.88 MB·English

by  Charles A. Whitney (auth.), A. N. Cox, W. M. Sparks, S. G. Starrfield (eds.)

EARLY SCIENTIFIC ACTIVITIES OF JOHN PAUL COX
Charles A. Whitney
Harvard-Smithsonian Center for Astrophysics
Cambridge, MA 02138
l, Introduction
I eagerly accepted the invitation to speak about John Cox's
scientific research, because I felt close,t o John after our early
collaboration, and because I knew that the broader task of
assessing his work and putting it into context must wait for a
more objective historian. And the task of broadly summarizing
the fields that interested him has already been undertaken by
the organizing committee -- the program of this meeting is a list
of the numerous topics John's research enriched.
On the other hand, I doubt that I am in a position to
discuss this remarkable body of work in a way that might be
useful to this audience, which contains many who collaborated
with John and who know the work more intimately than I. So I
have chosen a more personal approach and I will confine myself
to John's early pioneering papers on the cause of pulsational
ins t ab i l i t y .
2. A Brief Vita
John would have been 60 years old this autumn. He ~ras
born on November 4, 1926, and he died on August 19, 1984,
survived by his wife, Jane.
He did his undergraduate and graduate work at Indiana
U n i v e r s i t y , w h e r e he r e c e i v e d his Ph. D. in a s t r o n o m y in 1954
u n d e r t h e g u i d a n c e of Marsha l l Wrube l , The n e x t 8 y e a r s w e r e
spent t e a c h i n g a t Cornell U n i v e r s i t y - - w i t h t i m e out for s u m m e r
r e s e a r c h jobs. In t h e s u m m e r of 1957, he c a m e to Cambr idge .
We w r o t e a j o in t p a p e r a n d I b e c a m e his f irst c o - a u t h o r . In
1963, he m o v e d to Boulder , Colorado, w h e r e he b e c a m e a Fellow
of JILA a n d a professor a t t h e U n i v e r s i t y of Colorado. For n e a r l y
25 y e a r s he s e r v e d as a c o n s u l t a n t to t h e Los Alamos group t h a t
is hos t ing this c o n f e r e n c e .
In 1981, I w a s w o r k i n g on a h i s t o r y of t h e pu l sa t ion
t h e o r y a n d I w r o t e J o h n asking abou t his e a r l y i n t e r e s t in s te l lar
pulsa t ion . He sen t m e some l e t t e r s ( l a rge ly b e t w e e n t h e two of
us) t h a t he h a d p r e s e r v e d in his files. These l e t t e r s p rov ide a n
u n u s u a l gl impse a t J o h n ' s e a r l y c a r e e r , because he of ten w r o t e
l e t t e r s to c l ea r his m i n d a n d l a y ou t t h e possible d i rec t ions for his
work . Reading t h e m again has been an in t r igu ing lesson in
h i s t o r y for m e . In fac t , t h r e e lessons e m e r g e . First , we h a d
u n d e r e s t i m a t e d t h e d i f f i cu l ty of a p rope r n o n - a d i a b a t i c
t r e a t m e n t ' , second, we w e r e a t f i rs t mis led by a too l i tera l
a c c e p t a n c e of Eddington ' s idea t h a t t he pu lsa t iona l in s t ab i l i ty a n d
t h e s u r f a c e phase lag of t he flux w e r e i n t i m a t e l y c o n n e c t e d ;
th i rd , our e a r l y p e r i o d - l u m i n o s i t y re la t ion was r igh t for t h e
w r o n g reason .
5, J o h n ' s Doctoral Thesis
His Ph. D. thes is (1954) was a s t u d y of t he pu lsa t iona l
d r iv ing force p r o d u c e d b y n u c l e a r sources in g ian t s ta r s . The
resu l t w a s u n a m b i g u o u s a n d nega t ive . Using Epstein 's (1950)
adiabatic pulsation solutions for a new, highly condensed
red-giant star model, John was able to show that no nuclear
processes, either at the center or in a she11, could account for
the pulsation unless they had a temperature exponent of at least
one hundred million. This was clearly impossible, and at the end
of the paper, John pointed to the next region for the search --
the outer layers of the star. He said,
In order to have sustained pulsations ... it appears to be
a necessary condition that the contribution to [dissipation] from
the outlying "non-adiabatic" region rrn~st be sufficiently negative
to balance exactly the positive contributions from the adiabatic
region. This implies that whatever is the cause of the pulsation
phenomenon, it must be in the regions occupying, roughly, the
outer 15 per cent of the stellar radius.
It remains to be seen whether models with extensive
hydrogen convection zones or different boundary conditions will
remove these diff icul t ies .
The mention of hydrogen convection zones is an allusion to
Eddington's hypothesis that such a zone might behave as a heat
valve producing a phase lag in the emitted flux and causing the
star to act as a heat engine. John's thesis provided a proof
that such a valve mechanism was needed. Looking back in
1981, John wrote, "I became interested in the basic problem of
the cause of cepheid pulsations, I think, only when I realized
how inadequate nuclear sources were. " And in the abstract of
his 1955 paper, we find the statement that the "cause of the
pulsations must be sought ... where many of the usual
approximations are not va}id. " This turned out to be prophetic of
John's later work, which often involved careful formulations at
the analytical boundary of current pulsation theory.
~4. A o D r o x i m a t e T r e a t m e n t of L i n e a r n o n - a d i a b a t i c P u l s a t i o n
At Cornell during 1955 and 1956, John started looking for
the Eddington heat valve. He wrote in retrospect,
I remember during the early days (mostly while I was at
Cornell) I was quite struck by the very small amount of work
(in fact, essentially none) that had been done, or that was then
being done, on the basic problem of the cause of the pulsations.
It seems that Eddington, in his 1941-42 papers [pointing to the
hydrogen ionization zone as the direct cause of heat-valve effect]
was about the only person who had addressed that question. Yet
i remember seeing quite a few papers on details of the shapes of
light and velocity curves, etc. I found this quite an amazing
fact.
John adopted an iterative approach to the linear
non-adiabatic equation for radial pulsation starting from the
quasi-adiabatic approximation. Progress was slow because of his
teaching load, but he began obtaining results in mid-1956.
That year, I returned from a Post-Doc with Ledoux and
%vrote John summarizing our work. We did not plan to publish
because it seemed so tentative and we knew that Evry
Schatzman was in the process of publishing independent work
that was quite similar.
None of us in the United States were aware of the seminal
work then being done by Zhevakin in the Soviet Union, and we
all still focussed on the ionization of hydrogen as the critical
process, and | told John that | felt that the best way to attack
the non-adiabatic pulsations would be "by setting up a
discrete-shell model for the star and using electronic computers. "
But John preferred an analytical and more general approach, and
he soon outstripped us all in his understanding of the essential
process. During the summer and fall of 1956, I received a
series of letters describing his steady progress with an iterative
approach to non-adiabaticity based on the formulations of Woltjer
and of Schwarzschild, and John analytically developed the
relationship between the run of £~amma in a stellar envelope and
the phase lag of the emitted flux during pulsation.
• . . J -
5. The Theoretical Penod-Lummosltv Relation
Joh n ' s f ocus a t th i s t i m e w a s on t h e p h a s e lag of t h e
o b s e r v e d f lux t h a t w e all t h o u g h t w a s d i r e c t l y r e s p o n s i b l e for
i n s t a b i l i t y . In a l e t t e r w r i t t e n to m e on A u g u s t 22, ! 9 5 6 , he
said:
My own work seems to be ~oin~ rather well now, I've
succeeded in generalizin£~ the treatment somewhat, and it now
appears possible to prove that a phase lag in the emitted flux is a
necessary condition for instability, without making any
assumptions regarding the non-adiabaticity in the region under
consideration.
But it soon became clear that the relationship between the
envelope structure and the instability was more complex than he
had first judged. On November 2, 1956, he wrote that the
phase lag for maximum instability depended on the detailed run
of the wave function in the outer layers, so the purely schematic
models he had been using would not be adequate to answer the
question of cepheid instability. Then he added an exciting
development:
If it should turn out to be possible to say that sustained
pulsation may in general exist only if the phase lag is near a
quarter period, then a basis for a period luminosity relation
seems to exist somewhere in the present theory, but I haven't
yet been able to pin it down precisely.
Eight d a y s l a t e r , h e w r o t e w i t h his u s u a l c a u t i o n , "I s e e m
to h a v e f o u n d a p e r i o d - l u m i n o s i t y r e l a t i o n w h i c h a p p e a r s to
a g r e e r e a s o n a b l y we l l w i t h o b s e r v a t i o n . " In e f f ec t , h e s h o w e d
t h a t if, a long t h e c e p h e i d s e q u e n c e , t h e l a y e r of p a r t i a l i o n i z a t i o n
of hydrogen occurred at a depth corresponding to a constant
phase lag, then he cou ld derive a relation among period,
luminosity, and mass. He had estimated the phase lag as the
ratio of heat capacity to flux emitted in a full cycle -- essentially
the thermal time to the surface.
I suggested that he come to Cambridge the following
summer so we could work out the numerical details on a "larI{e
machine" that was being installed.
John presented a brief description of his work at the
Christmas, 1956, meeting of the American Astronomical Society.
Although limited to first-order non-adiabatic terms, it
constituted a mathematical statement of the hypothesis that had
been rather intuitively expressed by Eddington.
During the spring of 1957 John wrote a detailed discussion
of his iterative treatment of non-adiabaticity in the schematic
models. The paper (Cox 1958) was received by the Astrophysical
Journal on April 29. The gist of that paper was that no
particular phase lag was a necessary condition for instability,
although an abrupt drop in the radiative flux probably was. He
also broadened the search for the Eddington valve beyond the
hydrogen zone and stressed the weakness of the first order
theory.
During John's visit to Cambridge in the summer of 1957,
we performed some homology calculations based on his recent
paper, and we made his theory of the period-luminosity relation
quantitative, This was done without being able to calculate the
actual net dissipation. John had merely considered the condition
for minimum dissipation, and we were still unable to show the
net dissipation was negative, because we had not done the full
pulsation calculation. According to our formulation, the hydrogen
and neutral helium ionizations occurred too close to the surface to
explain the phase lag of classical cepheids, but the second
ionization of helium appeared to occur at the correct depth. At
the time, we still felt that Eddington's description was correct,
and the instability ought to be related to the phase lag of the
observed flux.
At the end of the summer, l received a translation of
Zhevakin's work on the non-adiabatic oscillation of discrete zone
stars and I sent a copy to John. It, too, pointed to the second
ionization of helium and it contained many important results on
pulsational instability. We emended our manuscript (Cox and
Whitney 1958) and added the references.
Zhevakin had insisted that the phase lag of the surface
flux was not related in a simple way to the pulsational
instability. What counted was the degree of non-adiabaticity at
the level of partial ionization. As it turned out, he was correct in
this. Ironically, the heat-capacity function that John and I used
to estimate the phase lag was actually a measure of the degree
of non-adiabaticity in the critical region, so we got the right
p e r i o d - l u m i n o s i t y re la t ion , bu t looking back, it s e e m s t h a t
Z h e v a k i n w a s m o r e n e a r l y co r r ec t t h a n w e w e r e a t t h e t ime ,
because he h a d a l r e a d y r e j e c t e d Eddington 's s imple r e l a t i on
b e t w e e n ins t ab i l i ty and s u r f a c e phase - l ag .
5_. Stems Toward The Exact Linear Non-adiabatic Treatment
This was my last substantial collaboration with John,
although we corresponded regularly for the next few years, and
he visited Cambridge several times to use our computational
facilities.
Dur ing t h e w i n t e r of 1957-58 , J o h n felt doubts abou t t h e
a d e q u a c y of his f i r s t - o r d e r t r e a t m e n t a n d he s t a r t e d w o r k i n g
w i t h t he Wol t j e r v - e q u a t i o n - - a m o r e complex bu t , he hoped ,
also m o r e a c c u r a t e p r o c e d u r e t h a n t h e S c h w a r z s c h i l d technique~
(In t h e course of this work , he also publ ished a pape r ex t end ing
his ana lys i s of t h e p e r i o d - l u m i n o s i t y r e l a t i on to r a d i a t i v e
envelopes.) Finally, on January 29, 1960, after a year and a
half, his paper on the approximate analysis was received at the
Astrophysical Journal. It was titled '~A Preliminary Analysis of
the Effectiveness of Second Helium Ionization in Inducing Cepheid
Instability in Stars" (Cox 1960).
Even the title of that paper reveals that John was not
Convinced by this approximate treatment either, and in a letter
dated Jan. 12, 1960 -- before he had submitted the paper -- he
~¢rote that he was already at work on the "exact linear
treatment but progress is temporarily slow because of a rush of
other things to do and because the algebra is setting to be a real
mess," The work went so well that he started an extensive
numerical study that spring, and when the approximate paper
came out, John had added a footnote in the proofs announcing
the successful numerical integration of the eighth-order system of
linearized pulsation equation.
The work on the full set of linearized equations was finally
submitted in July 1962. It was titled "On Second Helium
Ionization as a Cause of Pulsational Instability in Stars, " and it
ran 49 pages (Cox 1963). Here, at last, was a treatment that
could evaluate the net dissipation quantitatively. The paper has
a tone of authority that was lacking in the earlier "preliminary"
papers.
John concluded that helium second ionization "probably
accounts for the instability in classical cepheids and 111% Lyrae
variables and also (but less certainly) in W Virginis variables and
dwarfs cepheids of the 6 Scuti type. " But the phase lag came
out wrong. It was clear that his linear theory, which ignored
the ionizations of hydrogen and neutral helium, was not giving
the observed phase lag of the surface flux. This paper, with its
mathematical rigor, ~vas a key to our understanding that the
phase lag problem was quite distinct from the instability itself.
Writing with D. S. King (King and Cox 1968), John later said,
This study isolated the driving at small amplitudes, due to
second helium ionization alone. It was possible, therefore, to
obtain a clear picture of how this mechanism works and how it
can lead to an instability strip which has the essential features of
the observed strip.
But I~etting the proper phase lag was another matter, and
the clue came from independent, concurrent work of Baker and
Kippenhahn (1962), who had included hydrogen and neutral
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h e l i u m a n d t r e a t e d t h e l i nea r . n o n ' a d i a b a t i c p u l s a t i o n s of a
c a r e f u l l y c o n s t r u c t e d c e p h e i d enve lope . T h e y f o u n d a la rge
pos i t ive p h a s e lag p r o d u c e d b y n e u t r a l h e l i u m a n d h y d r o g e n . I
t h i n k t h e s e p a p e r s w e r e t h e f i rs t c l ea r s igns c o n f i r m i n g
Z h e v a k i n ' s conc lu s ion t h a t t h e i n s t a b i l i t y a n d t h e p h a s e lag w e r e
two s e p a r a t e p r o b l e m s .
In his classic r e v i e w p a p e r of i 9 7 4 , J o h n c lar i f ied t h e
d i s t i nc t ion as follows:
It is a r e m a r k a b l e fac t t h a t t h e c o n d i t i o n for t h e
a p p e a r a n c e of t h e p h a s e lag a n d t h e n e c e s s a r y c o n d i t i o n for
i n s t a b i l i t y a r e b o t h sa t is f ied . . . w h e r e m a n y c o m m o n t y p e s of
p u l s a t i n g s t a r s a r e f o u n d . Because t h e s e t w o p h e n o m e n a
( i n s t ab i l i t y a n d p h a s e lag) a r e c a u s e d b y t h e ac t i on of t w o
different ionization zones, it appears that the occurrence of the
phase lag in pulsating stars is more or less an accident of nature;
attributing both phenomena to a single physical mechanism,
which was Eddington's view, is evidently not entirely correct.
In 1960, John began a series of fruitful collaborations with
the Los A l a m o s g roup , a t t h e i n s t i ga t ion of Ar t Cox, w h o h a d
b e g u n n o n - l i n e a r n o n - a d i a b a t i c c o m p u t a t i o n s . J o h n ' s e a r l y
a t t i t u d e t o w a r d t h e r e l a t i o n s h i p of l i nea r a n d n o n - l i n e a r
m o d e l i n g w a s desc r ibed in s e v e r a l l e t t e r s :
I feel t h a t th i s [ l inear] a p p r o a c h is still v a l u a b l e in v i e w of
ou r p r e s e n t s t a t e of i g n o r a n c e r e g a r d i n g t h e c a u s e of t h e
pu l sa t i ons . H o w e v e r , it will u l t i m a t e l y be n e c e s s a r y , of cour se , to
go in to a n o n - l i n e a r t h e o r y before a r e a s o n a b l y c o m p l e t e
U n d e r s t a n d i n g is possible. [To A . N . C . , Nov. 2, 1959]
I ' m s o m e w h a t inc l ined to t h e v i e w p o i n t t h a t one shou ld
a t t e m p t to e x h a u s t t h e possibili t ies of a l i n e a r n o n - a d i a b a t i c
t h e o r y f i rs t . This shou ld , if n o t h i n g else, r e v e a l ~vhat f a c t o r s a r e
l ikely to be i m p o r t a n t in a n o n - l i n e a r t r e a t m e n t . [To A . N . C . ,
Dec. 16, 19593
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