The value of coordination among different observers particularly, those working in different wavelength regions is highlighted. This coordination is more important among observers of comets than among observers of almost any other class of astronomical object. The basic reason for this is that comets are highly time-variable, often erratically so, and observations of a particular comet usually cannot be repeated. As a result, some uncoordinated observations cannot be interpreted at all while others are susceptible to misinterpretation. Several specific examples of coordinated observations are discussed

Ahearn, M. F.

English

NASA Technical Reports Server

CORRELATED GROUND-BASED AND IUE .OBSERVATIONS
Michael F. A'Hearn
University of Maryland
College Park, MD 20742
In this talk I will not really be discussing a new observational technique but rather
something which deals primarily with the psychology of cometary observers and of those who
schedule telescope time for cometary observers. My goal is to point out the great value of
coordination among different observers, particularly those working in different wavelength
regions. This coordination, which has already been mentioned yesterday, is more important among
observers of comets than among observers of almost any other class of astronomical object. -The
basic reason for this is that comets are highly time-variable, often erratically so, and
observations of a particular comet usually cannot be repeated. As a result, some uncoordinated
observations cannot be interpreted at all while others are susceptible to misinterpretation. I
w i l l discuss several specific examples of coordinated observations based on recent experience. In
some of these the coordination was deliberate while in others the coordination was a fortunate,
accidental circumstance. Although I was involved to some extent in many of these observations, it
should be obvious from the nature of the talk that many other people were also involved, some of
them to an even greater degree than I was.
OH Production Rates in Comets
A first example was discussed yesterday by M i l l i s when he compared the OH production rates
derived from ground-based, filter photometry with those derived from IUE Spectrophotometry. The
ground-based results could easily have been criticized either on the grounds of inadequate
treatment of atmospheric extinction or on the grounds of inadequate absolute calibration. The IUE
results, on the other hand, could have been criticized because they are oased on observations of
only a very small fraction of the comet and are thus quite model sensitive. The fact that the two
methods are subject to unrelated sources of error and still yield results in reasonably good
agreement gives much greater confidence that both methods are reliable.
In one sense, this is a relatively trivial example since one can carry out the relevant
science using either technique alone. The second technique merely corroborates the first. In
other cases, the science simply cannot be done without the coordination of observations.
HCN and CN Production Rates
A much more critical need for coordinated observations exists in the area ot radio
spectroscopy of comets. The only universally accepted observations of radio lines are those of OH
and even here there appear to be discrepancies between the results derived from radio observations
and those derived from optical and ultraviolet observations. More important, however, are the
many negative searches for other spectral lines and the tentative detections of a tew species
(HCN, CH3CN, H20). How should one interpret the many negative results? It seems that tne
only sensible approach is to compare the upper limits to abundances obtained from optical
observations of chemically related species.
When Comet Bradfield (19791) was first discovered, we were able to use the experience of
previous ground-based photometry of comets and the apparent magnitude to estimate the production
rate of CN that we would observe for this comet (c.f., A'Hearn and M i l l i s , 1980). Since the comet
would make an unusually close approach to Earth, it would also significantly alleviate the problem
of beam dilution that plagues most radio observations. We then estimated that if the comet
behaved normally and if HCN were really the parent of CN, then the HCN should be observable
because of the favorable geometry. Zuckerman obtained observing time with the 36-foot telescope
at Kitt Peak and searched for HCN as well as several other species. All searches were negative.
Using the inner coma rotational temperature (kinetic temperature) determined for CS by Jackson
ejt a_l_. (1980) and using a Gaussian to approximate the spatial distribution of HCN, we were able to
"se~t an upper limit (5a) on the production rate of HCN at a few times 10^6 sec-1 and on the
138
https://ntrs.nasa.gov/search.jsp?R=19820006135 2020-03-21T10:11:23+00:00Z
peak column density at 8 x 10^ cm-2. These numbers are an order of magnitude lower than
those found for Comet Kohoutek (Huebnar, Snyder, and Buhl, 1974) and the molecular production rate
is less than the expected CN production rate. In the absence of any other information we would
have concluded that HCN could not be the parent of CN. Fortunately, we did have other information
because Mi 11 is was carrying out filter photometry of the comet from Hawaii within 6 hours of the
radio observations as well as on numerous other nights. It subsequently turned out that the CN
production rate in Comet Bradfield had varied approximately as r-4 rather than the r-2 that we
had observed in some previous comets (A'Hearn, Millis, and Birch, 1980). The upper limit for HCN
production then turned out to be somewhat larger than the observed CN production rate ratner than
smaller. Although we might ultimately have recognized this variation in CN production from the
visual magnitude light curve of the comet, it is quite possible that we would have drawn a totally
invalid conclusion if we had not had nearly simultaneous radio and optical observations.
Although this was a relatively straightforward case, because there are several pieces of
indirect evidence suggesting that HCN might be the parent of CN, it was also a particularly
significant one because of the rather low upper limit achieved. There have been numerous other
searches for radio spectral lines both of species observed in other spectral ranges and of species
not previously observed in comets. Even negative results in some of these searches might have
significance if properly related to optical results.
Triplet/Singlet Ratio of C2
A third example will show that purely serendipitous results can be achieved it different
observers happen to make relevant observations nearly simultaneously. When ananlyzing the C0+
bands in the IDE spectra of Comet Bradfield, we discovered that the bands were not due to C0+ at
all and that the strongest of these bands was the AV = 0 sequence of the Mulliken bands of C£
(A'Hearn and Feldman, 1980). Although C2 triplets have long been observed in the optical (the
well known Swan bands), this was the first unambiguous observation of C2 singlets from which
convincing fluxes could be derived. Fortunately Birch in Australia and later Millis in Hawaii had
been observing the Swan bands and, by coincidence, some of their observations were on precisely
the same dates as the IUE observations. This enabled us to derive the triplet-to-singlet ratio
for C2 and> using the theory of Krishna-Swamy and O'Dell (1979). to derive the absolute
transition moment for the forbidden transition cL3lfT — X !£"*".
Observations of "Bare" Nuclei.
Recently there has been much interest in attempting to observe the nuclei of comets,
particularly to determine whether they bear any similarity to the asteroids. Comet Arend-Rigaux,
at two previous apparitions, had appeared to exhibit a purely asteroidal behavior and at the 1977
apparition Paul Weissman stimulated a number of observers to attempt observations with the filter
systems used successfully on asteroids. Fortunately, other observers were also stimulated to
carry out the more usual cometary observations in the same time period. Degewij (1978) obtained a
spectrum showing the typical cometary emission bands of CN agd C3 while I (1978) obtained
Schmidt photographs indicating the presence of both a gaseous coma and a short, dusty tail. These
observations showed quite clearly that the photometry of the asteroid filter sets could not be
related in any simple way to the true nuclear properties.
More recently Ray Newburn has stimulated efforts to observe tne nucleus of Comet Tempel 2 when
it is relatively far from the sun. In this case a number of different observers carried out
filter photometry (much of it not yet published) which should have led to a determination of the
color of the nucleus. Other observers obtained spectral scans which should have described the
nucleus. Most of these observers had carried out their observations on a simple, one-shot basis.
When the observations of the photometrists were compared with each other (Zellner et al. 1979;
Millis, private communication), it was clear that this comet had undergone unexpecTecToutbursts.
This makes it rather difficult to be sure, for example, that the spectrophotometry carried out by
Spinrad e_t aj_. (1979) refers to the nucleus rather than to a halo of grains.
Although these various observers were not explicitly coordinated in the usual sense of the
word, they were all stimulated to obtain data in the same general time period, if there had not
been several observers active at this time, the outburst phenomena of Tempel 2 might have been
suspected depending on which observers happened to get data, but might also have entirely escaped
detection. It seems clear that somewhat greater coordination, perhaps involving UBV photometry in
139
\
\
', \
the week preceding and the week following the spectrophotometry, would have gone far to
determining whether the spectrophotometry was relevant to the nucleus itself.
Infrared and Optical Albedo
We have already heard in this meeting about the attempts to determine the albedo of the grains
in the cometary coma during the discussions by Gradie and Campins. Determination of the albedo
involves comparison of the reflected solar continuum with the thermal emission, both of these
quantities being measured by broad-band photometry. Although there is some evidence that the
broad-band photometry in the infrared is dominated by true thermal, continuum radiation, it is
certain that the broad-band photometry in the optical includes a large contribution from
fluorescent emission lines. It appears to me that a significant improvement in these albedo
estimates might result if the infrared photometrists were in closer coordination with the optical
photometrists who use narrow-band filters specifically to isolate the reflected continuum.
Interpretation of Continuum/Emission Ratios
As a final example in which coordination might lead to improvements in understanding, I would
mention the case of continuum/emission ratios measured to estimate the dust/gas ratio. The
standard programs of narrow-band filter photometry of comets, such as that discussed yesterday by
W i l l i s but also including programs by a number of other observers, usually include a method for
determining the continuum/emission ratio in whatever diaphragm is being used for observation. In
some cases these measurements are made in a variety of different diaphragms but in general this is
not the case. Direct interpretations of the continuum/emission ratio as a measure of the dust/gas
ratio can be very misleading. For some comets, the continuum might be predominantly from a strong,
nuclear condensation while for other comets the continuum might be spread out over the entire
diaphragm (as is usually observed for new, dusty comets). It seems clear that the usual
photometric results could be very profitably combined with contemporaneous spectra of the type
described just a few minutes ago by Larson. The^  photometry would provide the absolute calibration
while the long-slit spectra would provide the information on spatial variation of the
continuum/emission ratio. This should considerably enhance our ability to interpret both sets of
data.
In summary, i think we have a large number of different examples in which coordinated
observing has led or could lead to significant, scientific gains. In the past, such coordination
has been largely hit-or-miss although a few observers have made specific attempts for special
projects. Coordination of this type has been discussed in the context of the International Hal ley
Watch, which will be described tomorrow by Newburn, but the coordination is not a widely accepted
procedure for cometary observing. I strongly urge that this coordination be more widespread than
it is at present.
References
»
A'Hearn, M. p. 1978, Sky and Telescope 55, 293.
A'Hearn, M. F. and Feldman, P. D. 1980, Ap. ^J. Letters, in press.
A'Hearn, M. F. and M i l l i s , R. L. 1980, Ap_. _J. 85, 1528.
A'Hearn, M. F., M i l l i s , R. L., and Birch, p. V. 1980, BAAS 12, 730.
Degewij, J. 1978, Sky and Telescope 55, 14. ,
Huebner, w. F., Snyder, L. E., and Buhl, D. 1974, Icarus 23, 580.
Jackson, W. M., Halpern, W., Feldman, P. D., and Rahe, J. 1980, in "The Universe at Ultraviolet
Wavelengths - The First Two Years of IDE", in press.
Knshna-Swamy, K. S. and O'Dell, C. R. 1979, Ap_. 3_. 231, 624.
Spinrad, H., Stauffer, J. and Newburn, R. L. 1979, PASP 91. 707.
Zellner, B., Tedesco, E., and Degewij, J. 1979, IAU Circular No. 3326.
140


Correlated Ground-based and IUE Observations

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server