2P/Encke

Fernandez, Y. R. et al.   Icarus 147, 145-160, 2000.
"Physical Properties of the Nucleus of Comet 2P/Encke"

• mid-IR photometry and CCD photometry
• Derive a nucleus radius=2.4±0.3 km using a standard thermal model (which may or may not be applicable to comets)
• Optical linear phase coefficient is 0.06 to 0.07 mag/degree.
• Visual albedo 0.049±0.016.
• Determine a rotation period: Best period (assuming double peaked)=15.2±0.3 hr
• Period is synodic NOT sidereal
• Other possible periods: 7.66, 11.46, 22.919, 28.382, 33.652, 38.918 hr.
• These are all harmonics and the first two require single peaked light curve which is inconsistent with light curve resulting from shape
• Determine one axial ratio = 2.6 (0.7mag variation) which points to shape causing lightcurve not albedo
• Nucleus precessing quickly with a precession period less than 81 years (but I found their arguements on this shaky)
• Sub-solar temeperature in mid-July 1997 = 365±14K (around 1au)
• Active surface fraction <2% !!

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• Based on spectra from 0.55 to 1.0 microns (so O (¹D) for H₂O).
• Observed Encke on 9-11 Oct 1990 with R=0.57 Delta=0.87
• Details of fluxes given
• Haser model production rates

Q(H2O)	=	4.6e28	(at 1au this is Q(H2O)=1.2e28)
Q(C2)	=	2.2e26
Q(NH2)	=	2.0e25
Q(CN)	=	1.15e26	--> Q(H2O)/Q(CN) = 400

 

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Hughes, D. W. et al.,    MNRAS 263,247-255 (1993)
"On the variation of cometary coma brightness with comet-Earth distance (the Delta Effect)"

Important conclusion is that assuming that the cometary brightness varies as 1/Delta² is correct. Case of Encke used to prove point.
 

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Sanzovo, G. C. et al.    AJ 106, 1237-1240, 1993
"CS Band Intensity and Column Densities And Production Rates of 15 Comets"

Primarily a discussion of the physical model and the g-factors, this paper reanalyzes IUE observations from the literature. For Encke, observations were obtained in the 1980 and 1984 apparitions.

• 1980: October and November (R=1.01 and 0.81AU; Delta = 0.29 and 0.31AU).
• October: N(CS)=9e10 cm^-2; Q(CS)=3e22 mol/sec.
• November: N(CS)~4e11 cm^-2; Q(CS)~2.7e23 mol/sec.
• 1984: April and May (R=0.75 and 1.06AU; Delta = 0.76 and 0.85AU).
• The May observations are an upper limit.
• For April, N(CS) ~ 3e10 cm^-2; Q(CS)~1.2e23 mol/sec.

NOTE: Paul Feldman informed me that the Q(CS) values of this paper were off by 2 or 3 orders of magnitude since they neglected to account for the finite size of the IUE aperture. Therefore, he advises that this paper should be disregarded.

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• Photometry of Encke in 1977, 1980, 1984, and 1990 with individual
• observations on many dates. Produced Q(OH) and Afrho normalized to 1.5au.
• Q(OH) = 1.0E27
• Afrho = 9
• Then produced production rate ratios:

Q(OH)/Q(CN)	=	510 ±150
Q(CN)/Q(C2)	=	1.27 ±0.3
Q(CN)/Q(C3)	=	21.17±11.0
Q(CN)/Q(NH)	=	1.41 ±0.40

 

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• An evaluation of the historical data on the brightness of Encke.
• Points out that the time of peak brightness, relative to perihelion, has changed from ~19 days preperihelion in the middle of the 19th century to a few days postperihelion today
• When properly account for shift of peak, finds no evidence for a change in the peak brightness of the comet --> the comet is not fading and is not about to die

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Luu, J. and Jewitt, D.    Icarus 86, 69-81, 1990
"The Nucleus of Comet P/Encke"

• CCD observations (photometry and spectra) near aphelion in 1988.
• Photometry (Mould R band):
• Mean apparent magnitude=19.80±0.04
• M_R(1,1,0) = 14.40 assuming a phase coefficient of 0.04 mag/deg
• Delta m = 0.62±0.04 mag
• T = 15.08±0.08 hr (data compatible with 7.54±0.04 and 22.62±0.11)
• T = 15.08 produces a phase plot with two minima and maxima per period
• derive Bowell and Lumme-type phase coefficient = 0.045±0.005 mag/deg
• lower limit of axis ratio 1.8/1
• albedo X cross section = 1.5±0.1 km²
• for P=0.04, R=3.5km; for P=0.02, R=4.9km; for P=0.1, R=2.2km
• Spectra (4200-6800A):
• slightly red
• no emissions

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IR photometry from 2.3 to 18.5 microns during 1974 and 1987 apparitions. IR spectrum from 3-4 microns in 1987

• Detect no superheating of the coma --> coma has "large" grains derive grain size of 5-10 microns
• Upper limit of radius of 5km
• Coma emission dominates nucleus emission near perihelion
• grain density in coma has an upper limit of 3e-8 grains cm^-3
• No feature in the 3-4 micron spectrum so hydrocarbons lacking or underabundant

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• Reanalysis of earlier paper's data
• Observations of Encke 21 Aug - 6 Nov 80 (5 dates)
• Derive production rates (also give column densities) using Haser model
• Values assume v=1km/sec

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Sykes, M. V.    ApJ 334, L55-L58, 1988
"IRAS Observations of Extended Zodiacal Structures"

A discussion of IRAS dust trails, including ones attributed to Encke. The Encke dust trail covers more than 100 degrees of the cometary orbit including 20 degrees in mean anomaly forward of the comet's orbital position. The forward portion implies the trail has a component of particles larger than 1mm.
 

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Sekanina, Z.    AJ 96, 1455-1475 1988
"Outgassing Asymmetry of Periodic Comet Encke. II. Apparitions 1868-1918 And A Study of the Nucleus Evolution"

• Continuation of the analysis in paper I (below)
• Whenever Encke's perihelion occurs in mid-April, it displays no sunward fan.
• The nucleus precesses with a rate averaging 1 degree/revolution
• Sense of the rotation is prograde, with obliquity ranging from 69 to 76 degrees between 1868 and 1984
• Individual vents much evolve appreciably over peiods of time as short as 100 yr. A vent's floow should recede at a rate of ~10m per revolution relative to the ambient regolith. As the vent deepens, the jet becomes more collimated.

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• Examines reports of orientation of the fan in the coma, including the image of Djorgovski and Spinrad, to determine the pole orientation.
• Rotation pole area is an irregular pentagon some 8 degrees across, with RA between 200 and 208 degrees and with dec between 0 and +7 degrees (equinox 1950)
• Rotation pole (in rounded off eulerian units) argument Phi = 230, obliquity i=70
• Two sources of activity:
• Source 1, active on inbound leg until 12 and 6 days preperihelion and again three months after perihelion; latitude +55 degrees
• Source 2, active between 6 days preperihelion and 46 days post-perihelion; latitude -75 degrees
• Dismisses rotation period of Jewitt and Meech as not being possible with the solution here and asserts it is activity

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Using historical visual estimates of the cometary magnitude, derives lifetime and size of the comet.

• extinction date = 2060±15 AD
• absolute magnitude = 14.9±0.3
• diameter = 6.2±2.5 km
• active surface fraction = 0.2%

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• Report thermal infrared (10.6 microns) observations of Encke in 1980 and 1984.
• Encke does not behave like other comets at this wavelength and does not behave like a bare nucleus.
• Derive an UPPER LIMIT for the size of the nucleus of 2.2km. Allowing for CCD lightcurve variations of Jewitt and Meech, radius would be 2.9km - 4.4km with albedo > 4%.
• There is an asymmetry of the thermal IR emission around perihelion similar to the lightcurve, CN and C₂ values.

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IUE high resolution observations from 1980 and 1987 both pre- and post-perihelion were compared with models of the OH fluorescence. A marked Greenstein effect is seen in the pre-perihelion data (but post-perihelion is inconclusive). From this, the non-gravitational forces are determined and a crude mass estimate made: m = 7e15 g

• Q(OH) = 1.02e28      3 Nov 1980 R=0.838 pre-perihelion
• Q(OH) = 1.30e28    15 Aug 1987 R=0.776 post-perihelion

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• Observations obtained with a CCD and a Mould R filter on two runs a year apart. R=3-4AU. No coma apparent.
• 1985 amplitude >0.8mag
• 1986 amplitude ~0.4mag
• Derive a photometric period of 22.43±0.08 hr (with other possible periods at 2/3, 1/2 and 1/3 of this (2min/2max).
• All four periods show up in both data sets.
• Persistence of periodicity --> rotation of a nucleus, not albedo
• equatorial axis ratio ~2/1
• phase coefficient of ~4 to 5 mag per 100 degrees of phase.
• Adopting an albedo of 0.04, they derive a radius of 4.5 km. [ 2.8 < R < 6.4 km for albedos from 0.1 to 0.02]
• Only 1% of the surface active
• Their observations are inconsistent with the Whipple and Sekanina precession model

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A'Hearn, M. F. et al.    Icarus 64, 1-10, 1985
"Comet Encke: Gas Production and Lightcurve"

Ground-based and IUE observations of Encke during the 1984 apparition. C₂ and CN follow the behavior of the visual lightcurve but OH does not "The observations show, however, that the production of trace species becomes high relative to OH inside 0.75 AU preperihelion but then is very low relative to OH postperihelion."

Table IV - production of gas (note uses V=1km/sec) from Haser model 

R	log Q(CS)	log Q(OH)	log Q(CN)	log Q(C3)	log Q(C2)
1.162	--	<27.8	<23.9	--	<22.6
0.902	--	27.49	25.31	23.90	25.13
0.726	--	27.75	25.72	--	25.56
0.634	--	27.60	25.72	--	25.50
0.537	--	--	(25.73)	--	(23.37)
0.554	--	--	25.86	23.61	25.50
0.625	--	28.04	25.50	--	25.28
0.698	--	27.98	25.22	--	25.13
0.718	--	27.95	25.26	23.63	25.06
0.753	24.72	27.94	--	--	--
0.765	24.59	27.92	--	--	--
0.770	--	27.79	25.00	--	--
0.791	--	--	25.10	23.45	24.93
0.808	--	--	25.05	23.44	24.86
0.876	--	27.62	24.87	--	24.84
0.893	--	27.76	24.85	--	24.83
0.913	--	--	24.85	23.51	24.73
0.927	--	27.74	--	--	--
1.054	--	27.39	--	--	--
1.056	<23.7	27.55	--	--	--

 

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Djorgovski, S. and Spinrad, H.    AJ 90, 869-876, 1985
"Surface Photometry of Comet P/Encke"

• Digitized J and F plates obtained during the 1980 apparition.
• R=1.24AU; Delta=0.47AU
• Asymetrical image, brighter on sunward side. Asymmetry decreases with increasing distance from nucleus.
• Strong color gradient with outer coma very blue.
• Front fan reddest
• Fan lag angle is about 23 degrees from solar.
• Flow seems collimated.

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• Continuum measurements and models of area-geometric albedo products and mass loading.
• Observations of Encke 21 Aug - 6 Nov 1980 (8 dates and various positions)
• Data too complex to include here
• Encke mass production 1.13 -- 4.64 Kg/sec
• Mass loading for Encke more than an order of magnitude lower than for the lowest found for other comets.
• Dust production roughly constant with heliocentric distance.

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Feldman, P. D. et al.    Icarus 60, 455-463, 1984
"The Ultraviolet Spectrum of Periodic Comet Encke (1980 XI)"

• IUE observations on 24 Oct and 3 to 5 Nov 1980.
• R between 1.014 and 0.81 AU; delta ~0.3au.
• OH(0,0) 11.6 kR
• values given for H, O, C, S, CS, CO2+ and other OH bands
• derive Q(H₂O) (0.83au) = 1.8e28 and varies as R^-3.3.

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• Combines data from many studies and technigues and from several apparitions to try to compare gas with the visual lightcurve.
• Lightcurve summary: There is a brightness excess of 2-3 mag in the period 3 to 4 weeks prior to perihelion
• Paper includes uniformly reduced production rates.
• OH: from 0.79 < R < 1.23 AU, OH production varies as R^-3.8±0.2
• Sharp drop in OH seen at 0.75 AU
• The observed production of OH drops sharply when the cometocentric latitude of the sub-solar point decreases to approx. 30 degrees.
• OH production follows the visual lightcurve
• C₂ and CN: do not show the sharp decrease exhibited by OH at R=0.75AU; Does follow the lightcurve
• production rate ratios: H₂O/C₂/CN/C₃ are 100/0.43/0.21/0.01 (normal)

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• Radar observations of Encke obtained 2-8 November 1980. Comet was detected.
• Central peak of echo is offset by 7.5Hz from predicted position but could be errors in prediction as well as real
• Derive radar cross-section of 1.1±0.7 km²
• Using various model parameters, derive a radius of R = 1.5 +2.3/-1.0 km
• Dependent on model parameters chosen. Assumes pole of Whipple and Sekanina 1979 which Sekanina (1988 above) show to be wrong. Assumes their rotation period of 6 hrs 20 min±40 minutes which Jewitt and Meech and Fernandez et al. show to be wrong
• Other model parameters will yield R from 0.3 to 4.5 km

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Observations of Encke yielded:

Date	R	delta	B(OH)	Q(H2O)
	(AU)	(AU)	(kR)	(mol/sec)
5 Nov 1980	0.81	0.32	12	2.0 e28
24 Oct 1980	1.01	0.29	2.7	0.96e28

(slit 10X15 arcsec)
NOTE: my running of the vectorial model for the 24 Oct brightness yields Q(H₂O)=2.1e27
 

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Barker, E. S. et al.    in "Modern Observational Techniques for Comets" JPL Publication 81-68, ed. Brandt, J. C., Greenberg, J. M., Donn, B. and Rahe, J. 1981 pp. 150--155
"Observations of Faint Comets at McDonald Observatory: 1978-1980"

Observations with a Digital Area Photometer of Encke near aphelion in 1979. An outburst of 1.1 magnitudes was detected. Encke was bluer in outburst than out of outburst and the amplitude was 0.07 mag and random. However, no significant coma was detected during the outburst (but a coma of 8000km could be hidden in the seeing disk).

Uncalibrated spectra near 1au are also shown. Normal emissions were detected.
 

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A'Hearn, M. F. et al.    AJ 84, 570-579, 1979
"Gas and Dust in Some Recent Periodic Comets"

(perihelion 17 Aug 1977) 

Date	R	delta	log Q(CN)	log Q(C2)	diaphram
					(arcsec)
24 Jul 77	0.850	1.617	25.26	--	58
7 Sep 77	0.632	1.223	25.39	25.79	155
9 Sep 77	0.669	1.226	25.23	25.59	155
11 Sep 77	0.705	1.223	25.25	25.55	155

 

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Newburn, R. L. and Johnson, T. V.    Icarus 35, 360-368, 1978
"Postperihelion Interference Filter Photometry of the 'Annual' Comet P/Encke"

• Photometric observations on 14 June 1974 with delta=0.363 AU and R=1.07 AU.
• Errors could be as large as up to 50% of the quoted values
• Use a Haser model to derive:

Q(CN)	=	4.1e23
Q(C3)	=	5.3e23
Q(C2)	=	4.3e24

• Did not detect continuum so derive an upper limit of the solids production of 4700 g/sec 

Last modified 8 November 1999

Additional references or comments: anita@barolo.as.utexas.edu