THE EFFECT OF SILICON AND CARBON OPACITY
ULTRAVIOLET STELLAR
O W E N G I N G E R I C H and D A V I D
Smithsonian
Astrophysical
Observatory
and Harvard
Cambridge, Mass.,
U.S.A.
ON
SPECTRA
LATHAM
College
Observatory,
Abstract. Silicon a n d especially c a r b o n b o u n d - f r e e a b s o r p t i o n s c o n s i d e r a b l y r e d u c e t h e e m e r g e n t
flux in t h e ultraviolet for s t a r s n e a r spectral t y p e AO (T tt = 1 0 0 0 0 K ) . A n o v e r a b u n d a n c e of silicon
a n d / o r a n u n d e r a b u n d a n c e of c a r b o n c a n affect t h e B a l m e r d i s c o n t i n u i t y a n d t h e P a s c h e n c o n t i n u u m
by a few per cent at m o s t . H o w e v e r , t h e a b u n d a n c e s of these u l t r a v i o l e t a b s o r b e r s will h a v e little
effect o n t h e t e m p e r a t u r e d i s t r i b u t i o n c a l c u l a t e d for a s t a r if t h e m o d e l is c h o s e n t o m a t c h t h e visual
s p e c t r u m . A n e x a m i n a t i o n of t h e u l t r a v i o l e t s p e c t r u m of Sirius s h o w s t h a t still m o r e opacity is
n e e d e d ; part of this a b s o r p t i o n c a n be s u p p l i e d by line b l a n k e t i n g .
e
The importance of bound-free silicon opacity in the ultraviolet spectra of solar-type
stars has been appreciated for at least half a dozen years. After the identification in
the solar spectrum of the ground-state emission edge by Tousey (1963) and the ab­
sorption edge from the first excited level by Gingerich and Rich (1966), computations
made at the Smithsonian Astrophysical Observatory predicted that these edges should
be visible in hotter stars in spite of the fact that over 99% of the silicon was ionized.
Because a negligible amount of the solar flux emerges in the ultraviolet, the silicon
absorption has comparatively little effect on the radiation balance. For hotter stars,
however, the silicon can play an important role, especially when its abundance is
enhanced.
The upper portion of Figure 1 shows the part of the continuous spectrum that can be
observed from the ground for a main-sequence AO star. Two observational parameters
are shown: the Balmer discontinuity D and the slope of the Paschen continuum
S . In spite of the different names attached to these parameters, they are both
essentially color indices. In effect, they give us a means of comparing temperatures at
different depths in the stellar atmosphere. This can be seen more readily from the
lower part of the graph, where we have plotted nonobservable information available
from the model-atmosphere calculation. On this graph we have plotted, as a function of
wavelength, the relative location of optical depth unity. Where the atmosphere is
comparatively transparent (for example, just redward of the Balmer discontinuity),
we can see to deep, hot layers. In the more opaque regions (such as just to the violet of
the Balmer discontinuity), the radiation arises from the higher, cooler layers. Hence,
the Balmer decrement gives information about the temperature difference between a
deep layer and a shallow layer in the atmosphere, while the slope of the Paschen con­
tinuum measures the relative temperature of an intermediate layer.
B
P
In Figure 2, this same graph has been extended into the ultraviolet. When the silicon
is increased 10 times over the solar abundance, the augmented ultraviolet opacity
raises the depth of formation of the continuum below 1527 A. Now, in this ultraviolet
region the tail of the Planqk curve is rising increasingly rapidly with increasing temperHouziaux
and Butler
All Rights
Reserved.
(eds.),
Ultraviolet
Stellar
Copyright
© 1 9 7 0 by the
Spectra
IAU.
and
Ground-Based
Observations,
64-70.
Downloaded from https:/www.cambridge.org/core. IP address: 88.99.165.207, on 18 Jun 2017 at 05:51:57, subject to the Cambridge Core terms of use,
available at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S007418090010186X
THE EFFECT OF SILICON A N D C A R B O N OPACITY
65
ature, and therefore with increasing depth, so that an appreciable fraction of the
energy flowing into the outer layers of the atmosphere actually comes from much
deeper layers than the optical-depth-unity curve suggests. Increasing the silicon opac­
ity in this part of the ultraviolet effectively blocks this source of energy, and conse­
quently the temperature of the overlying layers must increase in order to maintain the
same total flux. With the outermost layers about 200° warmer, the temperature differ­
ence on the two sides of the Balmer discontinuity becomes less, and the discontinuity,
as measured in magnitudes, becomes less negative. This effect from the silicon opacity
alone has been discussed by Strom and Strom (1969).
However, when we include bound-free carbon opacity, which is far more important
than silicon at these temperatures, the situation is no longer so straightforward.
Figure 2 shows how a normal carbon abundance raises the depth of formation of the
ultraviolet spectrum below 1239 A. The opacity from a normal carbon abundance is so
severe that in this wavelength region any given atmospheric layer is not much affected
by radiation from adjacent layers. For the same reasons as before, the temperature in
the outer layers must increase, but this time the effect reaches deeper and the Paschen
continuum as well as the Balmer discontinuity will register a change. Even with the
carbon abundance reduced to ^ the solar value, it still dominates the ultraviolet
opacity, but the effect on the Balmer discontinuity and Paschen continuum is compa­
rable to enhancing the silicon opacity by a factor of 10.
Downloaded from https:/www.cambridge.org/core. IP address: 88.99.165.207, on 18 Jun 2017 at 05:51:57, subject to the Cambridge Core terms of use,
available at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S007418090010186X
66
O W E N GINGERICH A N D DAVID LATHAM
1
i
i
O P T I C A L
T
D E P T H
= 1 0 0 0 0 °K
e f f
+ + +++++
C
log g = 4 . 0
lOxSil
-2.0
|
1
T„ = I.O
x
S
i
1/10
]
x
k
„
l
N
0
„
—
c
C
N O R M A L
\
1
C
x
\
\l\
0.0
•
Xsi
\ i
8205
: ++
+ +
:
+
t
l
1
3647
1527
+
-
r
1
1239
1100
911
v ( W A V E L E N G T H S IN A )
Fig. 2.
The observational effects of including carbon as well as silicon opacity are shown in
Figure 3, where the Balmer discontinuity is plotted vs. the slope of the Paschen con­
tinuum. In the absence of the carbon opacity, we find the effect reported by the Stroms.
When normal carbon is added, the effect of increasing the silicon abundance is halved,
so that it becomes only marginally observable. On the basis of these results, we believe
that it would be premature to attempt to deduce carbon or silicon abundances from
ground-based observations of the continuous spectrum. An additional uncertainty
exists on account of the line blocking due to the many strong ultraviolet resonance
lines, not included in these models.
On the other hand, the ultraviolet continuum of late B- and A-type stars should be
comparatively sensitive to enhanced abundances of silicon or reduced abundances of
carbon. Predicted ultraviolet continuum fluxes for several cases are shown in Figure 4.
The size of the discontinuity at 1527 A should provide some indication of any over­
abundance of silicon, while those at 1100 and 1239 A could indicate any underabundance of carbon. Notice the enormous change in the flux below 1200 A in the various
models.
We wish to make two separate but intimately related points: (1) If the opacity from
a normal carbon abundance is omitted and the model is fitted to the visual continuum,
then the total predicted flux would be about 10% too high, which is 250° in effective
temperature. (2) Nevertheless, the temperature distribution established by fitting the
Downloaded from https:/www.cambridge.org/core. IP address: 88.99.165.207, on 18 Jun 2017 at 05:51:57, subject to the Cambridge Core terms of use,
available at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S007418090010186X
67
THE EFFECT OF SILICON A N D C A R B O N O P A C I T Y
visual continuum will have only a weak dependence on the detailed choice of the
ultraviolet opacities, as we have just shown, and hence abundance analyses based on
visual lines will not be critically affected by the amount of carbon or silicon absorption.
In other words, a correct fit of the models in the visual does not necessarily yield the true
bolometric correction', conversely, a change in the ultraviolet opacities (and hence in the
bolometric correction) will not vitiate the fit in the visual if the effective temperature is
adjusted appropriately.
1.10
1
T
e f f
21
=
10
000
10 x S i
NO
o
log g = 4.0
^
•
•
1.20 —
1 x Si
1/10 x C
ro
-I*
1.30
10 x S i
1 x C
• l
1 x Si
x C
1 x Si
NO C
i
-0.75
S
p
i
-0.80
= m ,
(4019) - m
X
,(8205)
T
F i g . 3.
Fig. 4.
Downloaded from https:/www.cambridge.org/core. IP address: 88.99.165.207, on 18 Jun 2017 at 05:51:57, subject to the Cambridge Core terms of use,
available at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S007418090010186X
68
O W E N GINGERICH A N D DAVID LATHAM
We must now inquire to what extent actual observations can be compared against
these models. As yet, we have access to comparatively few data, either from direct
ultraviolet spectrum scans or from the Celescope experiment. The Celescope filters
overlap in just such a way as to make the color index U4-U3 a measure of the flux
blocked by the carbon opacity, and U3-U2 a measure of the silicon. However, for the
models shown here, the maximum effect on U4-U3 would be about 10%, and on
U3-U2, about 30%. Whether the final reductions of the Celescope data will allow us
to deduce silicon or carbon abundance differences remains to be seen.
In Figure 5 we compare two models with some recent ultraviolet observations of
Sirius. We wish to thank Arthur Code and Theodore Stecher for permission to use
some of their data in advance of publication. The slightly enhanced silicon and magne­
sium abundances of these models have been deduced in Latham's thesis research on
the visible spectrum of Sirius. The effective temperatures of the two models differ by
only 290°, but at the shortest wavelength this results in a rather dramatic difference in
the predicted continuous spectrum. However, the 10000° model is at the cooler limit
of matching the visual observations; that is, it just barely matches both the mono­
chromatic surface flux in the visual and the shape of the spectral-energy distribution
from 11000 to 3000 A. A cooler model would require the error of both these obser­
vations to exceed 5%. The absolute calibrations of the ground-based observations and
Stecher's rocket measurements are independent (Stecher, 1969); but if Stecher's cal­
ibration in the middle ultraviolet turns out to be 10 to 20% too low, then the rocket
observations would match the ground-based data better at 3000 A, where the two
Downloaded from https:/www.cambridge.org/core. IP address: 88.99.165.207, on 18 Jun 2017 at 05:51:57, subject to the Cambridge Core terms of use,
available at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S007418090010186X
THE EFFECT OF SILICON A N D C A R B O N O P A C I T Y
69
sets of measurements join. However, the observations would still lie well below the
predicted curves throughout the ultraviolet regions. We have adjusted the relative
photometry of the Wisconsin Orbiting Astronomical Observatory to match Stecher's
curve, but have adjusted his wavelength scale to match the Wisconsin features.
Just longward of the carbon edge at 1239 A, there are numerous strong resonance
lines of C i , Sin, Si, and Sn. We have calculated profiles for about 30 of them; we
find that many of the equivalent widths exceed 1 A. In the first 70 A longward of the
carbon edge, our incomplete set of calculations shows that about 50% of the flux
would be blocked by resonance lines. Thus, we conclude that the strong bound-free
absorption by carbon below 1239 A is present, but that the sharp absorption edge is
entirely smoothed out by lines. On the other hand, we have no explanation for the
apparent absence of the silicon edge at 1527 A. Until many more detailed calculations
taking lines into account are available, it will probably be rather difficult to deduce
carbon or silicon abundances solely from the approximate level of the far-ultraviolet
spectrum.
References
G i n g e r i c h , O . a n d R i c h , J. C : 1966, Astron. J. 7 1 , 161.
Stecher, T. P . : 1969, Astron. J. 74, 9 8 .
S t r o m , S. E. a n d S t r o m , K . M . : 1969, Astrophys. J. 155, 17.
T o u s e y , R . : 1963, Space Sci. Rev. 2, 3.
Discussion
Stecher:
1 w o u l d believe t h a t h y d r o g e n L y m a n - a a n d Lyman-/? w o u l d c o m p l e t e l y o b s c u r e the
effects of C i since t h e L y m a n lines s h o u l d be t h e strongest lines in t h e s p e c t r u m of a n A star.
Gingerich: D . P e t e r s o n ' s c a l c u l a t i o n of t h e L y m a n - a profile, based o n t h e G r i e m T h e o r y , indicated
t h a t c a r b o n w o u l d be a m u c h m o r e i m p o r t a n t c a u s e of o p a c i t y since it covers a wider wavelength
region. C o n s e q u e n t l y we identified t h e L y m a n - a core with a s m a l l e r feature o n y o u r s p e c t r u m , a n d
L a t h a m f o u n d t h a t a g o o d a g r e e m e n t w i t h individual features of t h e W i s c o n s i n O A O d a t a could be
o b t a i n e d by sliding y o u r s p e c t r u m by 30 A.
Underhill: It is q u i t e possible t h a t if y o u included, in a s c h e m a t i c m a n n e r , t h e o p a c i t y d u e to t h e
series of r e s o n a n c e lines a p p r o a c h i n g t h e C i a n d Sii limits y o u w o u l d get a m o d e l with t h e s a m e
s t r u c t u r e ( T , P ) as for o n e w i t h o u t lines. T h i s is because t h e gf value of t h e lines t o g e t h e r is 3 o r so
t i m e s t h a t of the c o n t i n u u m , at least. Y o u r t o t a l opacity d e p e n d s o n Ngf a n d if y o u increase gf'xi is
n o t necessary t o increase N.
Gingerich: I agree t h a t line b l o c k i n g c a n be r e p r e s e n t e d fairly well in a s c h e m a t i c way, as we a r e
d o i n g in t h e n e w S m i t h s o n i a n grid of m o d e l s . H o w e v e r , if I u n d e r s t a n d t h e r e a s o n for y o u r r e m a r k s ,
y o u a r e suggesting t h a t w e use line b l o c k i n g t o r e p l a c e t h e e x t r a o p a c i t y f r o m t h e e n h a n c e d silicon.
T h i s a b u n d a n c e is based o n L a t h a m ' s line analysis in t h e visual s p e c t r u m , n o t o n a n a t t e m p t t o fit
t h e U V . I n a n y e v e n t , w e n e e d more U V o p a c i t y , w h i c h c o u l d b e s u p p l i e d by r e s o n a n c e line b l o c k i n g .
Praderie: O n e m u s t stress t h e u r g e n t n e e d for space ultraviolet o b s e r v a t i o n s for late A, F , a n d
G t y p e stars. A p r o p o s a l for t h e o b s e r v a t i o n of metallic d i s c o n t i n u i t i e s i n s u c h stars h a s b e e n m a d e
by D r . B o n n e t a n d myself as guest o b s e r v e r s o n t h e O A O 2. T h e basis for such a p r o p o s a l is t w o f o l d :
firstly t o s t u d y t h e v a r i a t i o n s w i t h t e m p e r a t u r e a n d e l e c t r o n pressure of t h e 2076 A d i s c o n t i n u i t y
o b s e r v e d by B o n n e t (1968, Ann. Astrophys.
31) in t h e s o l a r s p e c t r u m , w h i c h is sensitive t o
t e m p e r a t u r e as s h o w n from t h e c e n t e r t o l i m b v a r i a t i o n ; s e c o n d l y t o s t u d y t h e v a r i a t i o n of t h e
silicon edges at 1520 A a n d 1680 A with spectral type. T h e m a g n i t u d e of these p r e d i c t e d discontinu­
ities varies s t r o n g l y with Teff: at 8 0 0 0 K , \ogg =•- 3.9 a n d with a n o r m a l silicon a b u n d a n c e , AXogFv =
1.26 at 1520 A a n d J l o g F , - 0.94 at 1680 A. These values s h o u l d be c o m p a r e d w i t h t h e values
e
Downloaded from https:/www.cambridge.org/core. IP address: 88.99.165.207, on 18 Jun 2017 at 05:51:57, subject to the Cambridge Core terms of use,
available at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S007418090010186X
70
O W E N GINGERICH A N D DAVID LATHAM
c o m p u t e d by Gingerich for a 1 0 0 0 0 K star. M o r e o v e r , t h e r a t i o of t h e predicted silicon steps d e p e n d s
o n t h e silicon a b u n d a n c e ( P r a d e r i e , 1968, T h i r d H a r v a r d Conference).
T h e aspect of t h e solar silicon edges c a n n o t be interpreted using a classical r a d i a t i v e e q u i l i b r i u m
s o l a r m o d e l a t m o s p h e r e ; e v e n with a realistic low c h r o m o s p h e r e m o d e l , a n e x t r a a b s o r b e r m u s t be
i n v o k e d t o a c c o u n t for t h e 1680 A d i s c o n t i n u i t y . C o m i n g b a c k t o t h e o b s e r v e d ultraviolet s p e c t r u m
of Sirius, o n e m a y w o n d e r if such a n a b s o r b e r is still present, d u e t o t h e a b s e n c e of t h e 1520 A
discontinuity. C o n s i d e r i n g a l s o t h a t t h e theoretical flux c o m p u t e d by G i n g e r i c h is higher t h a n t h e
o n e observed, t h e c h o s e n m o d e l for Sirius m a y b e q u e s t i o n e d as far as t h e o u t e r layers a r e c o n c e r n e d ;
t h e log (T5000) VS. X c u r v e for T;. = 1 seems t o exclude this possibility for a n AO star. B u t for late A type
s t a r s , t h e U V s p e c t r u m e m e r g e s f r o m h i g h e r layers t h a n in AO s t a r s a n d c h r o m o s p h e r e s a r e n o t
i m p r o b a b l e . A recent a n d p r e l i m i n a r y result o n t h e K line o f y B o o ( A 7 I I I ) , o b s e r v e d at 2.5 A m m
b y L e C o n t e l a n d myself, favours a c h r o m o s p h e r i c t e m p e r a t u r e rise: t h e K line s h o w s a small
reversal.
Bonnet: I w o u l d like t o stress t h a t t w o features o b s e r v e d in t h e Sirius s p e c t r u m d o a p p e a r a l s o in
the solar spectrum:
(1) F o r wavelength l o n g e r t h a n 2000 A a difference a p p e a r s b e t w e e n c o m p u t e d a n d m e a s u r e d value
of t h e intensities t h e latter lying b e l o w t h e former. T h i s c o u l d be d u e either t o a n e w s o u r c e of o p a c i t y
o r t o c r o w d e d a b s o r p t i o n lines. T h i s last possibility might be settled easily since m a n y gf values are
n o w available a n d t h e spectra c a n be c o m p u t e d t a k i n g this effect i n t o a c c o u n t .
(2) T h e m a i n discontinuities a r e a l m o s t c o m p l e t e l y absent in t h e s p e c t r a of t h e s u n a n d Sirius.
T h i s m i g h t be d u e either t o L T E d e p a r t u r e s o r t o t h e existence of a lower t e m p e r a t u r e gradient t h a n
of t h e m o d e l s used in t h e c o m p u t a t i o n s .
Gingerich a n s w e r s t o b o t h P r a d e r i e a n d B o n n e t :
M r s P r a d e r i e ' s c a l c u l a t i o n s r e m i n d us of t h e fact t h a t for F stars t h e ultraviolet o p a c i t y (and hence
t h e d e p t h of f o r m a t i o n ) varies over a m u c h greater r a n g e t h a n for AO, a n d t h e r e f o r e we have the
possibility of c o n s t r u c t i n g e m p i r i c a l t e m p e r a t u r e d i s t r i b u t i o n s for F stars f r o m o b s e r v e d U V inten­
sities, j u s t as we c a n d o for t h e S u n . O n t h e o t h e r h a n d , at 10000° t h e 1520 A Si discontinuity is
f o r m e d at a b o u t t h e s a m e d e p t h as t h e B a l m e r discontinuity, so we c a n n o t i n t r o d u c e a lower t e m p e r ­
a t u r e gradient t o e x p l a i n t h e a b s e n c e of t h e Si edge in Sirius. N o n - L T E will t e n d t o wash o u t t h e
discontinuities, b u t I believe t h a t n u m e r o u s a b s o r p t i o n lines m a y b e e v e n m o r e effective.
1
Downloaded from https:/www.cambridge.org/core. IP address: 88.99.165.207, on 18 Jun 2017 at 05:51:57, subject to the Cambridge Core terms of use,
available at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S007418090010186X