Photoelectric spectrophotometric- and image tube equipped spectrographic-studies on comets are used to formulate a new model for the cometary nucleus that is physically similar to the icy-conglomerate. Absolute flux distributions of the principal spectral features for five comets of different compositions and heliocentric distances were evaluated. Comparison of visible and infrared brightnesses was used to calculate the optical albedos of observed particles

Odell, C. R.

English

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FINAL REPORT
covering the period June 1965 - August 1974
NASA GRANT NGL 14-001-060
entitled
SPECTROPHOTOMETRY OF COMETS *
to
The University of Chicago
5801 Ellis Avenue
Chicago, Illinois 60637 2 3
Original Principal Investigator:
Dr. C. R. O'Dell
Principal Investigator at the time of termination:
Dr. Thomas F. Adams
* NASA Technical Officer for this grant is
Maurice Dubin, NASA Heaaquarters.
(NASA-CR-140058) SPECTROPHOTONETRy OF N74-33291COMETS Final Report, Jun. 1965 - Aug.
1974 (Chicago Univ.) 12 p HC $4.00
CSCL 03A Unclas
https://ntrs.nasa.gov/search.jsp?R=19740025178 2020-03-23T05:14:31+00:00Z
Nine years ago (March 1965), the proposal was submitted
that led to this grant, which was extended through additional
funds and no cost extensions to the present. The goals of
the program were rather broad, as suited an area where little
quantitative observation had been made. The original abstract
reads:
"It is proposed that the National Aeronautics
Space Administration financially support in
part a systematic program of photoelectric
spectrophotometry of comet tails and nuclei.
Such a study should enable us to greatly in-
crease our knowledge of the nature of the
solid particles and origin of the gases and to
re-evaluate the theories of formation and de-
cay. A major feature of this grant will be
the construction of a high performance telescope-
photoelectric spectrophotometer system capable
of measuring the faint comets. It is shown that
the design presented is optimum for such studies
and should enable a new degree of understanding
to be derived for the conditions in comets.
Applications of this instrument to problems in
the detection of faint lines in stellar spectra
and nature of the interstellar medium are also
described."
-2-
Examining these original goals and comparing them with the
achievements during that time, as evidenced by published
research papers, one can justifiably claim success for this
grant.
This final report will first give a chronological account
of the activities, list the papers that resulted, give their
abstracts, and finally summarize what were the achievements
of this activity.
The intent was to provide accurate quantitative data on
the flux distribution in cometary spectra. To this end, at
the beginning of the grant (June 1965) all effort was con-
centrated on construction of a unique research tool, the
Photoelectric Nebular Spectrometer (PNS). Initially working
alone, the original principal investigator was joined by Dr.
Pavel Mayer of Charles University in Prague. Together the
system was completed and the first successful comet observa-
tions were made in December, 1966, of Comet Rudnicki - 1966e(l).
More extensive observations were then made by Mayer's successor,
Wm. L. Gebel, of Comets 1967n, 1968b, and 1968c(2). Bright
comets appear sporadically and have no regard for the scheduling
needs of manpower or weather. Therefore, the success in ob-
serving came in spurts. Some unrelated scientific programs
were conducted between apparitions. The most successful of
these was conducted by Robert Brucato (3) who used the PNS
for obtaining highly accurate line profiles of hot stars, a
-3-
subject-that later became his Ph.D. thesis topic.
By the end of 1969 it was felt that PNS had identified
and established the principal problems that it could address.
Although continued observations were worthwhile and attempted,
it was felt that the main effort should evolve to the use of
higher spectral resolution and extending the wavelength range.
Therefore, by the winter of 1969-70, when two bright comets
appeared, our efforts were concentrated in these areas. Comet
Tago-Sato-Kosaka - 1969g was observed with an image tube equipped
spectrograph on the Yerkes Observatory 41-inch reflector (4).
This comet and Comet Bennett 1969i were both observed (with
only upper limits) in the far infra-red from a Lear Jet research
aircraft operated by the Ames Research Center - through the
cooperation of Dr. Frank J. Low (5). This infra-red work was
then brought together with the results of other studies for
interpretation (6). Spectrographic observations of Comet 1969i
were also made for determination of the scattered light character-
istics (7). The results of these Comet 1969i programs were then
subjected to a discussion in terms.of particle size and composi-
tion (8).
The results of these investigations, both the molecular
studies and those of the particles, were brought together in
a new model for the cometary nucleus (9). Tied closely to this
quantitative data, it produced a model physically similar to
the now classic icy-conglomerate;.but, with a very different
-4-
origin.
In September, 1973, Dr. Thomas F. Adams assumed the role
of principal investigator and a no-cost extension was granted
to facilitate observations of Comet Kohoutek 1973f. Spectra
of this comet were taken at Yerkes on eleven nights in January
and February, 1974. The spectra generally covered the range
4700 - 8400A at a dispersion of 180A mm-1. The first usable
spectrogram was taken on January 15 UT. It shows marginal
evidence for the H20+ bands reported by other observers, al-
though it is somewhat underexposed. The H20 + bands are not
readily apparent on later spectra, consistent with the reported
rapid drop in the strength of these bands with increasing helio-
centric distance. Because of the fainter than expected scattered
light continuum in this comet and certain mechanical difficulties
with the observing equipment, it was not possible to derive use-
ful spectrophotometric data from these plates.
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Publications Resulting From This Grant
1. Title: Emission-Band Ratios in Comet Rudnicki (1966e)
Author: P. Mayer and C. R. O'Dell
Reference: Ap. J. 153, 951-962, 1968.
Abstract:
"A photoelectric spectrophotometric investigation
of Comet Rudnicki (1966e) is described, with atten-
tion given to the specialized instrument developed
for this program. Total band-sequence fluxes are
determined for three sequences of C2 , two sequences
of CN, and the X4050 features of C3. Although the
vibrational Boltzmann temperature describing the
relative population of the lower state of C2 should
be close to the color temperature of the Sun if de-
populating transitions do not exist, these observa-
tions indicate a temperature of about 40000 K. The
source of this discrepancy remains enigmatic after
examination of the processes of spontaneous emission
and collisional de-excitation, although the latter
processes are very unlikely. It is inferred from
these observations that depopulating transitions of
some type do occur. The X4050 feature is seen to
vary more slowly in brightness than either the C2
or CN emission, and the total absolute brightness
of the comet is lower than that of previously in-
vestigated objects."
2. Title: Spectrophotometry of Comets 1967n, 1968b, and 1968c
Author: William L. Gebel
Reference: Ap. J. 161, 765-777, 1970.
Abstract:
"Photoelectric spectrum scans of comets Ikeya-Seki
(1967n), Thomas (1968'), and Honda (1968c) covering
the range 3500 - 6300A are presented. A previously
reported intensity anomaly in the C2 Swan Bands is
shown to be independent of heliocentric distance,
which thereby rules out depopulation by forbidden
-6-
magnetic-dipole vibrational transitions as an
explanation of the anomaly. For future study
of this problem, mean profiles of the principal
band sequences have been determined. All of the
comets exhibited a moderately strong continuum.
The reflected or scattered light is found to be
gray, in contrast to the reddening previously
reported for dust tails."
3. Title: Hydrogen Line Profiles in Early-Type Stars
Author: R. J. Brucato
Reference: Pub. A.S.P., 80, 263-268, 1968.
Abstract:
"The line profiles of H were photoelectrically
measured for a Leo, n UMa, YoPeg, a Del, and
6 Cas at a resolution of 0.5A. These observed
profiles were then compared with profiles pre-
dicted by model-atmosphere calculations for ro-
tating and nonrotating early-type stars."
4. Title: Spectrophotometry of Comet 1969g (Tago-Sato-Kosaka)
Author: C. R. O'Dell
Reference: Ap. J., 164, 511-519, 1971.
Abstract:
"The spectrum of Comet 1969g (Tago-Sato-Kosaka)
was §tudied over the wavelength interval 3800 -
8500A. Composite spectra were formed, and the
results are shown as relative absolute energy-
distribution plots. Earlier (photoelectric data)
conclusions of a disagreement of theory and obser-
vations of Swan Band intensities are confirmed,
and it is suggested to be due to noninclusion of
the Ballik-Ramsay bands in the calculations. The
Phillips bands are seen in emission and enable a
C2 singlet/triplet population ratio to be derived.
Both the red and the violet CN band sequences are
observed and quantitatively compared with theory.
NH112 is quite strong and dominates the red spectral
-7-
region. An upper limit to the Ht surface bright-
ness is consistent with the OAO observations and
Biermann's chromospheric resonance fluorescence
model, but does not allow discrimination between
detailed models. A tentative identification of
the X5015 line of He I is made, possibly produced
by the Biermann mechanism."
S. Title: Infrared Observations of Comets 1969g and 1969i
Author: D. E. Kleinmann, T. Lee, F. J. Low, and
C. R. O'Dell
Reference: Ap. J. 165, 633-636, 1971.
Abstract:
"Ground-based and airborne infrared observations
have been made over the wavelength range 1.25 -
70p of Comets Tago-Sato-Kosaka (1969b) and
Bennett (1969i). The flux distributions in
wavelength and across the coma and tail at fixed
wavelengths were determined. The observations
are similar to previous work in that they show
color temperatures higher than those of a black-
body. An outburst of infrared luminosity was
seen in Comet Tago-Sato-Kosaka."
6. Title: Nature of Particulate Matter in Comets as
Determined from Infrared Observations
Author: C. R. O'Dell
Reference: Ap. J. 166, 675-681, 1971.
Abstract:
"Infrared and optical wavelength photometry are
combined to determine the albedo (0.3 ± 0.15)
of particles in three bright comets. The infra-
red data also indicate that the 10-) absorptivity
is only about one-fourth that in optical wave-
lengths. Surface-brightness distributions give
particle radii of about 0.1p. The resulting
particle models are.similar in these aspects to
the interstellar particles."
-8-
7. Title: The Scattered-Light Continuum of Comet
Bennett 1969i
Author: Gerald M. Stokes
Reference: Ap. J.) 177, 829-834, 1972.
Abstract:
"The scattered-light continuum of Comet Bennett
has been observed in the region between 4500 and
8000A. This continuum is found to be distinctly
redder than sunlight, and analysis leads to a low-
er limit on mean particle size of 0.1p. The con-
tribution to the reddening due to the C2 Phillips
bands and blackbody thermal radiation is considered
and found to be negligible."
8. Title: Particle Sizes in Comet Bennett (1970 II)
Author: C. R. O'Dell
Reference: Icarus 21, 96-99, 1974
Abstract:
"The particle size distribution in the coma and
tail of Comet Bennett has been determined by
several methods, each sensitive to a particular
size range. It is confirmed that a minimum value
of the particle density (p), size (d), and radiation
pressure efficiency (Qr)2 function (pd/Qrp) exists
at about 3 - 10x10-sgcm . The existence of such a
cutoff is probably due to the decreasing radiation
pressure efficiency for particles smaller than the
wavelength of the light being scattered. An exact
determination of this cutoff may allow identifica-
tion of the particle type."
9. Title: A New Model for Cometary Nuclei
Author: C. R. O'Dell
Reference: Icarus 19, 137-146, 1973.
Abstract:
"A new model for the nucleus of comets is presented,
hypothesizing formation at large heliocentric dis-
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tances from many independent solid bodies. It is
shown that such a configuration would collapse to
a single assemblage if it is to survive into the
inner solar system. Prior to collapse, the bodies
would be subject to coating by interstellar gas and
particles, which would form the material lost into
the coma at subsequent inner solar system perihelia.
Quantitative estimates place an upper limit to the
body sizes of 2.3m and a lower limit of the number
as 3x1010 with sizes of a few tenths of a micron
and numbers of 1033 most probable. The major
structural and evolutionary features of such comet
nuclei are consistent with the Whipple icy-conglomer-
ate model."
-10-
Achievements of this Research Activity
The principal achievement of papers 1, 3, and 4 is to
provide absolute flux distributions of the principal spectral
features for five comets of different composition and helio-
centric distance. The interpretation was centered about
diatomic carbon (C2). From study of the Swan Bands it was
concluded that previous studies of the population distribution
were incorrect. The breadth of the observational material
allowed the search for the source of the discrepancy to be
narrowed down to a radiative mechanism. It was then shown
that the presence of the infra-red Ballik-Ramsey bands could
account for the problem. The detection of the Phillips bands
was an important first-for it allows one to do diagnostics of
both the singlet and triplet sequences. The greatest long-
range impact lies ~in the fact that C2 is the best molecule
for determination of isotopic ratios - a powerful diagnostic
tool for comet cosmogony. We believe that this work has sig-
nificantly altered and improved our understanding of this, the
most visible, molecule in comets.
The study of particles has not only provided basic observa-
tions but has developed fundamental diagnostic tools. It was
shown that the particles have a characteristic elevation
above black-bodies under the same illumination, thereby
establishing their low infra-red emissivities. Comparison of
visible (scattered light) and infra-red (thermal emission)
-11-
brightnesses can allow calculation of the optical albedoes
of the particles - an important first. This technique has
now been adopted and applied to several other comets. A
final major contribution was to show that in Comet Bennett -
1969i, there was a minimum value of pd/Qrp " 3-10 x 10-sgcm -2
(p density, d diameter, Qrp E radiation pressure scatter-
ing efficiency). This means that either there is a minimum
particle size in that comet or (more likely) that the effects
of diminishing Qrp at small sizes causes pd/Qrp to be constant,
causing an apparent cut-off. The point at which this cut-off
occurs is different for various particle types and this can
be used for diagnostics. Of the presently evaluated particles,
only small iron and graphite spheres match the observations.


Spectrophotometry of comets

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