A photometric investigation of Comet P/Encke was undertaken using the 2.2 m telescope and the 0.6 m telescope. The two primary scientific objectives were: (1) measurement of the production rates of OH, CN, C3, and C2 as a function of heliocentric distance; and (2) determination of the comet's rotational period by searching for periodic brightness variations in the inner coma. Synoptic observation in the Canary Islands; Cambridge, Massachusetts; Flagstaff, Arizona; and Mauna Kea were planned for the study of the comet's rotation

Millis, R. L.

English

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(NASA-CR-164580) A 21WTOMMUC	 N81-28026
INVESTIGATION OF COMET P/ENCKE Final Report
(Lowell Observatory) 12 p HC A02/MF AU1
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A PHOTDMETRIC INVESTIGATION OF COMET P/ENCKE
V
Report No. 1
FINAL REPORT under JPL Contract BP-724891
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Robert L. Millis*
Lowell Observatory
Flagstaff, Arizona 86002
i
June 30, 1981
This work was performed for the Jet Propulsion Laboratory,
California Institute of Technology sponsored b y the National
Aeronautics and Space Administration.
*Guest Observer, Mauna Kea Observatory, Institute for Astronomy,
University if Hawaii.
- 2 -
Abstr,4ct
Photometric observations of Comet P/Encke were obtained on
October 9 and 10, 1980, with the 2.2-meter telescope at Mauna Kea
Observatory. Additional measurements were made using the 0.6-
meter reflector at Lowell Observatory on November 6 and 8, 1980.
Production rates of OH, CN, C,, and C. were determined.
Introduction
A photome-tric investigation of Comet P/Encke was undertaken
using the 2.2-meter telescope at Mauna Kea Observatory and the 0.6-
meter telescope at Lowell Observatory. The program had two primary
scientific objectives: (1) measurement of the production rates of
OH, CN, C,, and C. as a function of heliocentric distance and (2)
determination of the comet's rotational period by searching for
periodic brig;-itness variations in the inner coma, as had previously
been attempted by Fay and Wisniewski (1978) for Comet d'Arrest.
The first objective was to be accomplished using equipment and
techniques developed for the purpose over the last few years (e.g..
A'Hearn and Millis, 1980; Millis and A'Hearn. 1981). Synoptic
observation by observers in the Canary Islands; Cambridge, Massa-
chusetts; Flagstaff, Arizona; and Mauna Kea were planned for the
study of the comet's rotation.
- 3 -
Technical Discussion
In Figure 1 we have plotted the elongation, predicted total
magnitude, and number of hours per night with sec z < 2 for Comet
Encke as a function of date. Note that during October 1980 the
comet was expected to brighten rapidly from 11 mag to 7.5 mag,
while the number of hours per night that the comet could be observed
decreased rapidly. Towards the end of the month, the phase of the
Noon also became unfavorable.
Observing time early in October was requested in order to
maximize the number of hours per night that Encke was observable.
Continuous observational coverage for a period of several hours was
essential for the study of rotation. Experience with other comets
led us to believe that at its predicted brightness, Encke would be
easily within reach of both the 2.2-meter and 0.6-meter telescopes
at Mauna Kea Observatory. Four nights on the 2.2-meter telescope
(October 7 through 10) were allocated for our program, along with
an additional 13 nights on the 0 . 6-meter Planetary Patrol telescope.
By late September, it became evident that Comet Encke was not
going to be nearly as easy an object as expected. Several efforts
to visually locate it with the 1.8-meter and 1.0-meter telescopes
in Flagstaff were unsuccessful. Reports of similar difficulties
were received from other observatories.
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Figure 1. Parameters affecting planned
observations of Comet P/Encke during
October 1980.
- 5 -or
The problem apparently was due primarily to Encke's large
angular extent and to the very diffuse character of its coma. By
mid-October, visual observers were reporting coma diameters of
10 arcmin (Marsden, 1980), yet long-exposure photographs showed only
a weak coma with an unresolved nuclear condensation (e.g., Shao and
Schwartz, 1980). Figure 2 shows a 5-minute exposure on Comet Encke
obtained by Alan Stockton nn 6 October 1980 with an image tube on
the Mauna Kea 2.2-meter ;.i:,escope. The fan-shaped coma could not
be seen when viewing directly through the telescope, while the nucleus
was distinguishable from a star only by its rapid m tion. Encke was
barely visible in the 0.6-meter telescope on 5 October 1980, but it
was so faint and diffuse that photometry was not possible.
The first two nights of the 2.2-meter run were cloudy. On the
third night, the brightness of Comet Encke was monitored continuously
for four hours-through three filters: V of the UBV system, a 250 -A-
wide (FWHM) interference filter centered at 4850 A, and a 260- A-wide
(FWHM) filter at 5120 A. The V light curve is shown in Figure 3.
Each point represents the mean of six 10-second integrations through
a 10-aresec-diameter aperture centered on the nucleus. The signals
from the comet and the sky background were approximately equal.
based on photon statistics alone, an uncertainty of ;0.03 mag would
be expected in the plotted points. Judging from the repeatability
of consecutive measurements, the actual error bars are estimated to
Figure 2. Direct, image tube photograph
of Comet P/Encke taken with the 2.2-meter
telescope at Mauna Kea. The scale of the
print is approximately 5 aresec/mm.
••
•
•
•
0
-
•
0
^e
0
W	 UT	 Is N
W
	 1 1"	 1 p"	 13"
Figure 3. V magnitude of Comet P/Encke
through a 10-arc-sec-diameter aperture
centered on the nucleus plotted against
UT on 9 October 1980. Error bars are
typically ±0.06 mag.
jNG
 PACE 3^ ,
- 8-
closer to twice that amount. Possible causes of the increased error
are inclusion of unseen stars in the entrance aperture and centering
errors. Two comparison stars, BD+51 0 1040 (V - 10.34 Haag) and
BD+51°1038 (V - 9.95 mag), were used. These stars were constant
during the four-hour period of observation to within better than
±0.003 mag.
While the observations in Figure 3 perhaps show some evidence
of intrinsic brig l:'iess variation, no believable period can be derived
without independent confirmation from other ciics. Unfortunately,
efforts to do coordinated photometry of this crmet from Tenerife,
Agassiz, and Flagstaff were unsuccessful. Hence, the only result
that can be stated with complete confidence is that the mean V mag-
nitude of the nuclear region of Comet Encke on 9 October 1980 was
16.40 ± 0.05 mag. For comparison, Yeomans (1980) predicted a nuclear
magnitude of 16.8 mag.
October 10, the final night of the 2.2-meter run, was devoted
to narrowband photometry. Measurements were made through a standard
set of interference filters isolating emission bands of OH, CN, C,,
and C. along with selected regions of continuum. The resulting fluxes
are listed in Table I together with measurements from the Lowell
Observatory 0.6-meter telescope on November 6 and 8. On the later
two dates, Comet Encke was much brighter but was visible only briefly
in the predawn sky. Production rates of the various molecular species
- 9 -
Table I.
Comet Encke
Date:
Telescope:
Diaphragm Diameter
(aresec):
10 October 1980 6 November 1980 8 November 1980
W 2.2-meter Lowell 0.6-meter Lowell 0.6-meter
10	 ill
	
79
r	 ^::u.
16
Log Fu(OH)
Log F? 3300)
Log Fa 3615)
Log F (CN)
Log F (C,)
Log Fx (4120)
Log F (Ca)
Log Fa (5240)
1.230
0.451
-12.246 s 0.051
-15.143 3 0.016
-15.003 t 0.031
-12.453 ± 0.009
-12.643
-14.460
-12.539 t 0.015
-15.123 ± 0.032
0.789
0.330
-9.027 t 0.036
-13.050 ± 0.124
-12.624 ± 0.060
-9.263 t 0.003
-10.040
-9.212
0.752
0.354
-9.981 *- 0.090
-12.970 3 0.010
-13.878
-9.494 ± 0.010
-10.153 ± 0.120
-12.682
-9.366
are listed in Table II. Observing and reduction techniques, pass-
bands, and standard stars were the same as described previously by
A'Hearn, Millis, and Birch (1979, 1981), A'Hearn and Millis ( 1980),
and Millis and A ' Hearn ( 1981).
Table II.
Comet Encke Production Fates
UT Date r o Log Q
1 980) A.U.1 (a.u.) OH _ CN C3 Cs
October 10.598 1.230 0.451 27.199 24.551 24.746 24.645
November 6.515 0.789 0.330 27.983 25.406 25.689 25.781
November 8.522 0.752 0.354 27.213 25.353 25.703 25.814
I
- 10 -
Several points concerning the data in Table II are of interest.
The production rate of OH at r = 0.79 a.u. reported here agrees well
with thit observed from IUE at r • 0.81 a.u. (A'Hearn. 1961). How-
ever, the production rate of this radical apparently fell quite
steeply between November 6 and November 8. The observations on both
dates were repeatable with no evidence of significant observational
error. The OH observations listed in Tables I and II are the first
to be made for Comet Encke using ground-based photometric techniques.
In Figure 4 the production rate of CN is plotted against helio-
centric distance. The cross represents a preperihelion measurement
by A'Hearn, Millis, and Birch (1979) during the 1977 apparition of
Comet Encke. The variation of production rate with heliocentric
distance is very steep with an r'" •lta•S dependence giving the best-
fitting line shown in the figure. Similarly steep power laws are
observed over this range in heliocentric distance for the other
species and for the total magnitude (Mumma, 1976).
New Technology
The research program described here involved no reportable items
of new technology.
Acknowledgemen ts
The author wishes to thank the staff of Mauna Kea Observatory
i
25.1
24.
aW
QOJ
25.
-0.2
	
-0.1	 0	 0.1	 Lop r	 0.2
Figure 4. Production rate of CN in Cw.t
P/Encke as a function of heliocentric
distance. Filled circles represent the
current observations. The cross is from
A'Hearn. Millis. and Birch (1919). The
straight line was fitted to the data by
least squares and corresponds to an
r'4•1 dependence of production rate on
distance.
ORIGINAL PAGL
OF P00,, QVALITY
- 12 -
for providing telescope time and observing assistance in suoport
of this research. Dr. Alan Stockton kindly provided the image of
Comet P/Encke shown in Figure 2. Don Tho-ipson and Mary Lou Kantz
assisted with certain aspects of the research.
References
A'Hearn, M. F. (1981). Private communication.
A'Hearn, M. F., and Millis, R. L. (1980). Abundance correlations
among comets. Astron. J. ,U, 1528-1537.
A'Hearn, M. F., Millis, R. L., and Birch, P. V. (1979). Gas and
dust in some recent periodic comets. Astron. J. 84, 570-579.
A'Hearn, M. F., Millis, R. l., and Birch, P. V. (198)). Comet
Bradfield (1979X): 'the gassiest comet? Astron. J., in press.
Fay. R. D.. and Wisniewski, W. (1978). The lightcurve of Lhe nucleus
of Comet d'Arrest. Icarus a4, 1.
Marsden, B. G. (1980). IAU Circular No. 3531.
Millis, R. L., and A'Hearn, M. F. (1981). Ground-based photometry
of comets in the spectral interval 3000 to 3500 R. In Proceedings
of the Workshy on Modern Observational Techniques for Comets.
NASA SP, in press.
Mumma, M. J. (1976). Expected scientific results on ballist'c
spacecraft missions to Comet Encke during the 1980 apparition.
In The Study of Comets. NASA SP-393. page 997.
Shoo. C.-Y., and Schwartz. G. (IMO). IAU Circular No. 3526.
Yeomans. D. (1980). Private communication.


A photometric investigation of Comet P/Encke

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