From the visual magnitudes of the International Comet Quarterly data base and the OH radio lines measured at the Nancay radio telescope, the law log Q(H2O) = 30.74 (+/-0.02) - 0.240 (+/-.003) m(sub h) is derived from a sample of 13 comets

Crovisier, Jacques

Green, D. W. E.

Jorda, L.

English

NASA Technical Reports Server

o
Asteroids, Comets, Meteors 1991, pp. 285-288
Lunar and Planetary lnstitute, Houston, 1992
THE CORRELATION BETWEEN WATER PRODUCTION RATES
AND VISUAL MAGNITUDES IN COMETS
L. Jorda 1, 2), j. Crovisier 1) and D.W.E. Green 3)
1) Observatoire de Paris, Section de Meudon, F-92195 Meudon, France
2) European Southern Observatory, La Silla, Chile
3) Smithsonian Astrophysical Observatory, Cambridge, MA 02138, USA
Abstract
From the visual magnitudes of the International Comet Quarterly data base and the OH radio lines
measured at the Nanqay radio telescope, the law log Q[H20] = 30.74 (_+0.02) - 0.240 (+.003) mh
is derived from a sample of 13 comets.
INTRODUCTION
A fundamental problem in cometary studies is to relate gas production rates and visual magnitudes.
If such a problem can be solved, it will be possible to use the large historical data set of visual
magnitude measurements to investigate the evolution of gas production along the orbit. It will also
be possible to assess the feasibility of spectrocopic observations (from radio to UV) of new
comets from their brightness behaviour.
DATA SET AND METHOD
We have now two homogeneous data sets in computer-readable form:
1) The compilation of visual magnitudes of the International Comet Quarterly. The base consists of
more than 35 000 entries in the 1990 version (Green, 1990), most of them coming from amateur
astronomers.
2) The data base of 18-cm OH cometary spectra measured with the Nanqay telescope. 35 comets
were observed since 1973, and most of the data are now incorporated in a computer base (GErard
et aI., 1990).
Water production rates can be derived from the OH radio lines following the model of Bockel6e-
Morvan et al. (1990). The model includes pumping by UV fluorescence, collisional quenching,
and a coma expansion velocity deduced from the radio line shapes. The production rates from
radio and UV observations of OH are now in agreement when consistent sets of model parameters
are used. We assume Q[H20] = 1.1 Q[OH].
We have studied the correlation between the water production rate and the visual magnitude in 13
comets. The water production rates were evaluated for a selection of 81 time-averaged OH spectra
which have sufficient signal-to-noise ratios to get reliable results (the corresponding OH data are
published in more detail by Bockel6e-Morvan et al. 1990a, 1990b and 1991). The visual
magnitudes mv were reduced to heliocentric magnitudes mh according to mh = my - 5 log A and
averaged (or interpolated) over the same time intervals as the OH spectra. This averaging process
smoothes the large dispersion of the total visual magnitude measurements. The heliocentric
distances of the resulting sample range from 0.32 to 2.8 AU.
RESULTS AND DISCUSSION
Q[H20] is plotted as a function of mv for the whole sample and for different groups of comets in
the figures. The error bar on Q[H20] is that resulting from the noise in the OH spectrum (it does
https://ntrs.nasa.gov/search.jsp?R=19930009992 2020-03-17T07:31:56+00:00Z
286 Asteroids, Comets, Meteors 1991
not include model uncertainties).The error bar on mh is that resulting from the dispersion on the
measurements. The empirical law deduced from a regression analysis applied to the whole sample
is:
log Q[H20] = 30.74 (_+0.02)- 0.240 (+.003) mh
with a regression coefficient of--0.92. We note _h_ii" _ :_ _: :
- there is no strong deviation to this law for individual comets. From first inspection, the law does
not depend upon gas-to-dust ratio. Aithofigqa -the sample includes short'period, long-period and
new comets, there is no obvious difference between these different classes.
- the slope of the law is steeper for P/Halley (-0.303) than for the set of the 12 other comets
(-0.213).
Similar studies were performed in the past: from OH radio measurements by BockelEe-Morvan et
al. (1981), from UV OH measurements by Festou (1986) and Roettger et al. (i990), for comet
P/Halley alone by Sekanina (1989) and many others. Our regression law is intermediary between
those of BockelEe-Morvan et al. and of Festou; it is in agreement with the one Sekanina proposed
for P/Halley (which may be fortuitous). When comparing these studies, one must have in mind
that they use gas production rates estimated with different models. We believe that our work is an
improvement over the preceding ones, owing to the larger size of our sample.
References: _ :
BockelEe-Morvan D., Colom P., Crovisier J., GErard E., and Bourgois G. (1991) Observations
of comet Levy (1990c) with the Nan_ay radio telescope. In Asteroids, Comets, Meteors 1991
(this volume).
BockelEe-Morvan D., Crovisier J., and GErard E. (1990a). Retrieving the coma gas expansion
velocity in P/Halley, Wilson (1987 VII) and several other comets from the 18-cm OH line
shapes. Astron. Astrophys. 238, 382-400.
BockelEe-Morvan D., Crovisier J., GErard E., and Bourgois G. (1990b) OH radio observations
of comets P/Brorsen-Metcalf (1989o), Okazaki-Levy-Rudenko (1989r), Aarseth-Brewington
(1989al) and AUstin (1989c 1) at the Nanqay radio telescope, in Workshop on Observations of
Recent Comets. Eds W.F. Huebner et al., pp. 75-79.
BockelEe-Morvan D., Crovisier J., GErard E. and Kaz_s I. (1981). Observations of the OH
radical in comets at 18 cm wavelength. Icarus 47, 464-469,
Festou M.C. (1986). The derivation of OH gas production rates from visual magnitudes of
comets. In Asteroids Comets Meteors H (C.-I. Lagerkvist et al. eds.), pp. 299-303. Uppsala
Univ. Press, Uppsa!a, _ :
GErard E., BockelEe-Morvan D., Bourgois G.:I Colom P., and Crovisier, J. (1990). 15 years of
observations of the OH radical in comets with the Nanqay radio telescope. In Asteroids Comets
Meteors III (C.-I. Lagerkvist et al. eds.), pp. 317-320. Uppsala Univ. Press, Uppsala,
Green D.W.E., edt. (1990). Int. Comet Q. Archive of Cometary Photometric Data, magnetic tape,
fourth edition, June 1990_ _ L _ ............ :...... - ..... _ :
Roettger E.E., Feldman P.D., A'Hearn M.F., and Festou M.C. (1990). Comparison of water
production rates from UV spectroscopy and visual magnitudes for some recent comets. Icarus
86, 100-114.
Sekanina Z. (1989). Nuclei of two earth-grazing comets of fan-shaped appearance. Astron. J. 98,
2322-2345.
z
Asteroids, Comets, Meteors 1991 287
Figures: Water production rates derived from the OH 18-cm lines observed at Nan9ay, as a
function of heliocentric visual magnitudes derived from the ICQ archives. The error bars are
+1 c. The full line shows the least-square linear fit to the whole sample and is repeated on each
figure. The dashed line is the fit to P/Halley data alone.
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288 Asteroids, Comets, Meteors 1991
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The correlation between water production rates and visual magnitudes in comets

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