Abundances and production rates of CN, C3, and C2 for fifteen comets were determined and the variation of these parameters with helicentric distance for two comets were monitored. While these measurements provide much information about the similarities and differences among comets, only a small fraction of the total material in any of the comets observed was sampled

Ahearn, M. F.

Willis, R. L.

English

NASA Technical Reports Server

GROUND-BASED PHOTOMETRY OF COMETS
IN THE SPECTRAL INTERVAL 3000 TO 3500A
R. L. Millis*
Planetary Research Center
Lowell Observatory
Flagstaff, AZ 86002
M. F. A'Hearn
Astronomy Program
University of Maryland
College Park, MD 20742
INTRODUCTION
During the past four years we have conducted an extensive program of narrow-band photometry of
comets in the spectral region between 3800 and 5300 A (A'Hearn and Millis, 1980). In the course
of this program we have determined abundances and production rates of CN, C3, and C£ for
fifteen comets and have monitored the variation of these parameters with heliocentric distance for
two comets. While these measurements have provided much new information about the similarities
and differences among comets, we have really sampled only a small fraction of the total material
in any of the comets observed.
There are a number of cometary emission bands, including those of the presumably most abundant
species, which have not previously been measured quantitatively and reliably from the ground.
Among these are the 0-0 band of OH at 3085 A and the 0-0 band of NH at 3360 A. The OH band is the
strongest feature in cometary spectra and is of particular interest because OH is derived from
water, which is believed to be the dominant constituent of cometary nuclei. Quantitative
measurements of the NH band would also be very useful because the interpretation of observations
of this diatomic radical in terms of a molecular abundance is relatively straightforward compared
to that of NH2, the only related species observed in comets.
A few measurements of the OH band have been made from high-flying aircraft (Blamont and
Festou, 1974) and spacecraft (e.g., Feldman and Brune, 1976; Keller and Lillie, 1978; and Feldman
et al., 1980). So far as we are aware, there have been no measurements of the NH band published.
Clearly it would be very desirable to measure these bands with ground-based telescopes. In
that way observations could be extended to fainter comets and more complete temporal coverage
could be achieved than is generally the case with presently available spacecraft. Such
ground-based measurements have not been attempted or have not been successful in the past because
of the large atmospheric extinction in the ultraviolet. This paper summarizes our efforts to
measure the strength of the 0-0 band of OH in the spectrum of Comet Bradfield (19791) using the
0.6-meter Planetary Patrol telescope at Mauna Kea Observatory.
OBSERVATIONS
At 3085 A there are three significant contributors to atmospheric extinction: Rayleigh
scattering by air molecules, aerosol scattering by particles, and molecular absorption by ozone.
Most of the ozone resides between 10 and 35 km altitudes; therefore, except for small seasonal and
latitudinal variation, its contribution to extinction is more or less the same for all
observatories. Extinction due to both Rayleigh and aerosol scattering, on the other hand, vanes
with altitude (h) as eh/c, where c is a constant (Hayes and Latham, 1975). High-altitude
observatories for this reason do have an advantage when ultraviolet measurements are being
attempted.
*Guest Observer, Mauna Kea Observatory, Institute for Astronomy, University
of Hawaii.
57
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We obtained observations of Comet Bradfield (19791) from Mauna Kea Observatory on seven nights
in January and February of 1980 through a series of narrow-band filters between 3135 and 5240 A.
On five of these nights, extinction coefficients were carefully measured by repeated observation
of a standard star as it rose or set. The resulting extinction coefficients are listed at the top
of Table I. The extinction coefficients at the three shorter wavelengths (3135 A, 3300 A, and
3365 A) were well determined and quite stable from night to night. They are comparable to values
found for the U passband near sea level. The Mauna Kea extinction coefficients are compared in
Figure 1 with those obtained using the same filters at Lowell Observatory, a site whose altitude
is about half that of Mauna Kea. The advantage of the higher altitude site for photometry in the
near ultraviolet is apparent, but the difference is not so great as to preclude accurate
photometry in this spectral region from more typical mountain sites such as Flagstaff.
The steep increase in extinction by ozone across the passband of our OH filter causes its
effective wavelength to be shifted from the central wavelength at 3135 A to near 3165 A. The OH
bands, however, are centered at approximately 3085 A. Consequently, the measured extinction
coefficient for this filter, while appropriate for reduction of the standard star observations, is
significantly smaller than the value to be applied to the comet observations, we have computed an
extinction coefficient of 1.38 mag/air mass at 3085 A for Mauna Kea using formulae given by Hayes
and Latham (1975). The value for the total ozone at 20°N latitude in January was taken from
Allen (1963) and the absorption coefficient for ozone from Toolin (1965). At this wavelength,
extinction by aerosols is expected to be negligible compared to the contributions of ozone and
Rayleigh scattering and has been ignored. Extinction coefficients computed in this way for longer
wavelengths are shown as the solid curve in Figure 1. The agreement between the computed and
observed extinction coefficients is excellent, giving confidence that the result for 3085 A cannot
be far off the mark. This assertion is supported by the fact that, on nights when several OH
measurements of Comet Bradfield were made over a range of air masses, the r.m.s. scatter in the
comet's brightness when reduced to the top of the atmosphere was less than 0.02 mag.
The OH production rates for Comet Bradfield derived from our Mauna Kea observations are
compared in Figure 2 with the nearly contemporaneous results from I.U.E. (Weaver et al., 1980).
Also plotted are the results from two nights' photometry at Lowell Observatory. WHiTe the
production rates of OH from the Mauna Kea and Lowell Observatories agree reasonably well, they are
about 45 percent larger than the I.U.E. results. The source of this discrepancy is not readily
apparent nor is it clear which data set is to be preferred. The same lifetimes and scale lengths
for OH were used in reducing the ground-based and spacecraft observations. However, the
ground-based measurements were made through an aperture about 2 arcmin in diameter, while the
I.U.E. observations were made with a small rectangular aperture about 10 x 15 arcsec on a side.
The large difference in the fraction of the coma sampled may contribute to the difference in
derived production rates, especially if the coma is not axially symmetric. The ground-based and
I.U.E. observations were reduced relative to different standard stars. Errors in the absolute
calibrations of these stars will be reflected in the derived OH production rates. Furthermore,
uncertainties in the OH filter transmission curve due to variation with temperature or tilt would
impact the production rates.
CONCLUSIONS
While the agreement between our results and those of I.U.E. is not as close as we would like,
we are convinced that accurate photometry of the OH bands in the spectra of comets is possible
from the ground. The extinction coefficients in the near ultraviolet both at Mauna Kea and at
Flagstaff were found to be sufficiently small and stable as to present no serious obstacle to
precise photometric measurement.
We thank M. C. Festou and L. Dunkelman for very helpful discussions of atmospheric extinction
by ozone. D. T. Thompson and C. Aurand assisted with the observations and data reduction,
respectively. This research was suported at Lowell Observatory by NASA grant NGR-03-003-001 and
at the University of Maryland by NASA grant NSG-7322.
59
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Figure 1. Extinction coefficients measured at Mauna Kea Observatory
(filled circles) compared with those determined at Lowell
Observatory (crosses). The solid curve represents calculated
extinction at Mauna Kea due to ozone and Rayleigh scattering.
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NOTE ADDED IN PROOF
Further calculations of theoretical extinction coefficients show that the attenuation of the
cometary OH band should be nearly linear with air mass. However, the attenuation of a flat
continuum source such as an early-type star, is very non-linear with air mass due to the shifting
of the effective wavelength by the atmosphere. Thus stellar extinction coefficients observed
between zero and three air masses significantly underestimate the attenuation between zero and one
air mass. Allowance for this effect eliminates a large part of the discrepancy in production
rates of OH as determined from IUE and from the ground.
References
A'Hearn, M. F. and Millis, R. L. 1980, Abundance correlations among comets. Astron. ^j. 85, 1528.
Allen, C. W. 1963, Astrophysical Quantities (London: Athlone Press).
j
Blamont, J. E. and Festou, M. 1974, Observations of the Comet Kohoutek (1973f) in the resonance
light (A2 +-X2ir) of the OH radical. Icarus 23, 538-544.
Feldman, P. D. and Brune, W. H. 1976, Carbon production in Comet West 1975n. Astrophys. ±. 209,
L45-L48. ~
Feldman, P. D., Weaver, H. A., Festou, M. C., A'Hearn, M. F., Jackson, W. M., Donn, B., Rahe, J.,
Smith, A. M. and Benvenuti, P. 1980, IDE observations of the UV spectrum of Comet Bradfield.
Nature 286, 132-135.
Hayes, D. S. and Latham, D. W. 1975, A rediscussion of the atmospheric extinction and the
absolute spectral-energy distribution of Vega. Astrophys. J_. 197, 593-601.
Keller, H. U. and Lillie, C. F. 1978, Hydrogen and hydroxyl production rates of Comet
Tago-Sato-Kosaka (1969IX). Astron. Astrophys. 62, 143-147.
Toolin, R. B. 1965, in Handbook of Geophysics and Space Environments, ed. S. L. Valley (Air Force
Cambridge Research Laboratories).
Weaver, H. A., Feldman, P. D., Festou, M. C. and A'Hearn, M. F. 1980 (In preparation).
62


Ground-based photometry of comets in the spectral interval 3000 to 3500 angstrom

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