Eleven high-dispersion spectra of Venus, taken with blue Doppler shifts have permitted the unmasking of the 7323.88A forbidden line of Ca II from terrestrial absorption. An equivalent width is obtained of 7.4 + or - 0.4mA for this line in integrated sunlight. Our value of W sub lambda is smaller than previous values and much more accurate. The HSRA solar model gives a solar calcium abundance of A sub Ca = 6.21

Barker, E. S.

Schorn, R. A.

Young, A. T.

English

NASA Technical Reports Server

Forbidden Ca II in the Sun
Unmasked by Way of Venus
by
Ronald A. Schorn
(0 oo Andrew T. Young
-I Physics Department
Texas A&Mi University
College Station, Texas 77843
and
m Edwin S. Barker
a University of Texas
) - McDonald Observatory
SEnFort Davis, Texas 79734
Report No 14
The research described in this report was
I E-4 funded by the
rPrLI National Aeronautics and Space Administration
Contract No. NGR 44-001-117
I-, 39Ln
U) a 4 Department of Physics z /
z Texas A & M University
SCollege Station, Texas 77843
March 14, 1975
https://ntrs.nasa.gov/search.jsp?R=19750011041 2020-03-22T22:28:42+00:00Z
-1-
Forbidden Ca II in the Sun
Unmasked by way of Venus
by
Ronald A. Schorn
Andrew T. Young
Physics Department
Texas A&M University
College Station, Texas 77843
and
Edwin S. Barker
The University of Texas
McDonald Observatory
Fort Davis, Texas 79734
-- 2 -
Abstract
Eleven high-dispersion spectra of Venus, taken with blue .Doppler
shifts have enabled us to unmask the 7323.88R forbidden line of Ca II
from terrestrial absorption. We obtain an equivalent width of 7.4 ± 0.4mR
for this line in integrated sunlight. Our value of W is smaller than
previous values and much more accurate. The HSRA solar model gives a
solar calcium abundance of ACa= 6.21.
- 3 -
I. Introduction
The forbidden line of Ca II at 7323.88R has been the subject of
interest in the last few years, as it is the most accessible line of
[Ca II] in the solar spectrum. The subject has been discussed by
Grevesse and Swings (1968), Lambert, Mallia, and Warner (1969), and
Griffin and Griffin (1969). This line is difficult to measure because of
a terrestrial H20 line at 7323.972R, which clobbers the [Ca II] line.
Observers have exercised considerable ingenuity to get through, or
around, this barrier. Grevesse and Swings used solar spectra obtained
on the Jungfraujoch on a very dry day, to reduce the strength of the
telluric line and bring out [Ca III as a hump on its blue side. Lambert,
Mallia, and Warner looked at the approaching (East) limb of the Sun and
used the Doppler shift provided by solar rotation to expose the [Ca II]
line as a hump in the blue wing of the telluric line (which was, of
course, stronger at Oxford). Griffin and Griffin observed Arcturus,
and, using the Doppler shift due to the Earth's orbital motion to avoid
the H20 line, detected and measured the 7323.88R [Ca II] line. Then,
working back through model atmospheres of Arcturus and the Sun, they
inferred a value for the equivalent width of the solar [Ca II] line.
These results all were in only fair agreement.
We ourselves have long used the Doppler shift due to orbital motion
of the planets to uncover weak planetary H20 lines normally masked by
telluric lines. This shift can reach 1/3R at 7200X for Venus. On a dry
day, at a mountain observatory, we felt that solar [Ca II] should be
clearly visible in the spectrum of Venus. Such, indeed, was the case
(see Fig. 1) Our results have been communicated privately to others
before, but are published in full here.
-4-
II. Observations
Our spectra are listed in Table I. Plate numbers prefixed with
"T" in the first column were taken with the coude spectrograph of the
24-inch reflector at the Table Mountain Observatory of the Jet Propulsion
Laboratory. The plate number prefixed with a "C" was obtained with the
coude spectrograph at the 82-inch Struve telescope at McDonald Observatory
while plate numbers prefixed with an "N" were obtained with the McDonald .
107-inch coude spectrograph. The "EQ" entry is a spectrum scan taken
with the 107-inch telescope coude rapid scanner. All these spectra were
taken as a part of our.regular spectroscopic patrol of Venus CO2 and
H 20 regions. Young, et al. (1973), Young, et al. (1971), and Barker
(1973) describe the 24-inch, the 82-inch and 107-inch photographic,
and the 107-inch scanner set-ups respectively. All of the plates were
taken at approximately 2 X/mm. A 10-micron projected slit width was
used for the 24- and 107-inch plates, while a 20-micron width was
used for the 82-inch plate. These correspond to the resolution of the
ammoniated IV-N plates used. The spectral scan was taken, in part, to
see if there was any noticable difference between results obtained by
two different methods. The slit was always oriented pole-to-pole on
Venus through the area of maximum brightness.
Column 2 gives the dates of the observations, which were all made
when Venus had an appreciable blue Doppler shift. Furthermore, we
have used only specially selected plates taken on very dry days, when
no more than a few mm of terrestrial water vapor were above the
telescope. A useful rule of thumb was that the spectral region should
at first glance be unidentifiable when compared to the Mount Wilson
infrared solar spectrum. The rainy season at McDonald is summer, while
that for .California is winter, and the dates illustrate this fact.
- 5-
Finally, spectra were selected which were taken close to the meridian
transit of Venus. Figs. 1 and 2 show a representative spectrum with
the [Ca II] line at 7323.88 well removed from the 7329.972 telluric line.
The third column of the Table gives the Doppler shift of Venus, in
%ngstroms, from the tables of Niehaus and Petrie (1962). *The last three
columns give the equivalent widths of the [Ca II] line measured by the
three authors.
III. Determination of the Equivalent Width
The plates listed in Table I were traced in the density mode with
the recording microdensitometer at the Jet Propulsion Laboratory. The
microdensitometer slit covered the central three quarters of the width of
the spectrum in all cases. Projected slit widths of 10.9 and 21.8 microns
were used. We did not prepare intensity tracings, for Young (1972) has
shown that, for properly exposed, hypersensitized IV-N plates, a density
tracing is equivalent to an intensity tracing, apart from a zero-point
shift.
Equivalent width calibrations were based on the equivalent widths
of four solar lines found in Moore, Minnaert, and Houtgast (1966):
7320.689 (72mR), 7326.160 (13m), 7330.150 (17mR), and 7343.226 (20mR).
The principal source of error was the measurement of the weak [Ca II] line.
For example, when fourteen solar comparison lines were used to calibrate
plate T 53, the resultant equivalent width was only 4.5 per cent lower
than that obtained using only four solar lines.
Moore, Minnaert, and Houtgast give line intensities for the center
of the solar disk, while our spectra of Venus show solar lines integrated
over the disk. The difference can be up to 50 per cent for a given line.
The scanner run was taken to check this point. It indicates that this
source of error (again) is less than that introduced by the estimation
of the area of the [Ca II] line itself.
-6-
The scanner data were processed as described by Barker (1973). In
this case, the largest source of uncertainty was in estimating where the
edges of the [Ca II] line reached the true solar continuum. At this
resolution (about 275,000) the continuum is seldom flat for long, due to
innumerable weak solar and terrestrial lines.
The last three columns of Table I list the estimates of the equiva-
lent width (Wx) by the three authors. For plates, the subscript "y"
denotes estimates by one measurer for the best tracing of each plate,
while the "s" column contains averages of two or more of the better
,tracings for each plate by another measurer. We give (formal) standard
deviations for each plate measurer's average and for the average of the
scan measures.
IV. As Figs. 1 and 2 show, 7323.88X [Ca II] has been clearly observed.
N487 is one of five "fair" spectra, in which the 7320.846 H20 line is
about as strong as the 7320.689 Fe I + Fe II line. On our six "good"
spectra, all H20 lines are weaker, and the [Ca II] line is well clear
of the 7323.972 H20 line. Because of the small number of measurements,
and because of possible systematic errors involved (varying effects of
the H 20 line wing, location of the continuum, etc.) it seems footless
to do any elaborate statistical analysis. By weighing each scanner
estimate three times as much as each plate estimate, we find
W = 7.4±0.4mR for the equivalent width of the 7323.88 [Ca II] line in
integrated sunlight. The ±0.4 is not a statistical error estimate, but
is our opinion of the uncertainty in our results which we feel is about
5%, because the systematic differences between plate and scanner values
are of that size.
- 7 -
How does our result compare with earlier work? Grevesse and-Swings
estimated W = 7.0±0.4m for light from the center of the disk. Lambert,
Mallia, and Warner predicted W = 8.3mR at Cos 0 = 1 (center of the solar
disk) and W. = 12.0mX at Cos 0 = 0.3 (near the limb), and estimated
WA = 15±5mR at Cos 0 = 0.3 from their observations. Griffin and Griffin
obtained an estimate of W = llm from their indirect "Arcturus" method.
In sum then, our value of W is lower than previous values, and should
be considerably more accurate.
In general, this lower value of W lowers the derived solar calcium
abundance. The particular value of ACa obtained depends on the solar
model adopted. For example, Fig. 3 is a plot of WX vs ACa supplied to
us by N. Grevesse, which uses the Harvard-Smithsonian Reference Atmosphere
(Gingerich, et al., 1971); for WA =7.4mX, we obtain A = 6.21.
V. Acknowledgement
This research was supported by NASA Grants NGR 44-001-117,
NGR 44-012-152, and a grant from the Texas A&M University Office of
Scientific Research, administered by the College of Science.
-8-
REFERENCES
E. S. Barker: 1973, in IAU Symposium No, 65,
"Exploration of the Planetary System," Torun, Poland, September 5-9,
1973. A. Woszczyk and C. Iwaniszewska, Eds. (D. Reidel Publishing Company,
1974) p. 203
0. Gingerich, R. W. Noyes, W. Kalkofen and Y. Cuny: 1971, Solar Phys.
18, 347.
N. Grevesse and J. P. Swings: 1968, Compt. Rend., 226, 110.
N. Grevessejand J. P. Swings: 1972, Astrophys. J., 171, 179.
R. Griffin and R. Griffin: 1969, M. N., 89, 9.
D. L. Lambert, E. A. Mallia, and B. Warner: 1969, Solar Phys., 7, 11.
C. E. Moore, M. G. J. Minnaert, and J. Houtgast: 1966, "The Solar
Spectrum 2935R to 8770R. Second Revision of Rowland's Preliminary
Table of Solar Spectrum Wavelengths". U. S. National Bureau of
Standards Monograph 61, Washington, D.C.
W. C. Niehaus and T. W. Petrie: 1961, "Tables of Stellar and Planetary
Doppler Shifts from 1962 to 1982. Standard Oil Co. of Ohio.
A. T. Young: 1972, J. Opt. Soc. Amer., 62, 1385.
L. G. Young, A. T. Young, J. W. Young, and J. T. Bergstralh: 1973,
Astrophys. J. Letters, 181, L5.
L. D. G. Young, R. A. J. Schorn, E. S. Barker, and A. Woszczyk: 1971,
Acta Astron., 21, 329.
-9-
Table I
Observations of solar [Ca II]
Plate Date AA () W (mR)y W (m)S -W (m)B
T 32 Apr. 14, 1970 -0.202 7.3 7.1 --
T .36 May 5, 1970 -0.193 12.9 9.0
C 6804 May 24, 1970 -0.233 10.1 6.9
T 53 Aug. .19, 1970 -0.331 6.2 6.6
T 56 Aug. 20, 1970 -0.331 7.0 6.7
T 57 Aug. 21, 1970 -0.331 7.7 7.5
T 62 Aug. 25, 1970 -0.331 5.3 6.8
N 487 Oct. 6, 1970 -0.283 7.7 7.3
N 489 Oct. 7, 1970 -0.280 6.6 7.3
Averages of plate measures 7.7 ±0.7 7.4 ± 0.3
EQ 4977 Dec. 11, 1973 -0.294 6.7 7.1 7.1
Average of scan measures 7.0 ± 0.1
- 10 -
List of Figures
Figure 1 Section of a spectrum of Venus showing the solar
[Ca II] line at 7323.880R.
Figure 2 Microdensitometer tracing of the spectrum shown
in Figure 1. The intensity scale is very nearly
linear.
Figure 3 Equivalent width of [Ca II] 7323.880k for inte-
grated sunlight (W1 ) versus the logarithm of
solar calcium abundance (ACa).
[Ca II] N487
7320. 689 7327 370
Fel I (72) 7327
7323. 972 )
7320.846 7323. 880• [Ca 1]
7323.354
Fe, CN (9)
Ca I] N487
7323.354
Fe, CN (9)
7320. 846
7 7323. 880
7320.689 [Ca ll]
Fe , II (72)
7327. 3707323. 972
E E
w (MA)
15
H.S.R.A. SOLAR MODEL
12
MICRO TURBULENCE: 1.5 km/sec;
11 NO CONVECTION
10
9
8
.7-
6
6.0 6.1 6.2 6.3 6.4 6.5 ACa
OF loopa


Forbidden Ca 2 in the sun unmasked by way of Venus

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