Gas and dust in Comet 2P/Encke observed in the visual and submillimeter wavelength ranges
In November 2003 Comet 2P/Encke was observed simultaneously with the 10-m Heinrich–Hertz Submillimeter Telescope on Mount Graham, Arizona, USA, and the 2-m optical telescope on Mount Rozhen, Bulgaria. Simultaneous radio observations of the 4–3 and 3–2 rotational transitions of HCN and the 0–0 transi...
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| Cite this: | Gas and dust in Comet 2P/Encke observed in the visual and submillimeter wavelength ranges / K. Jockers, S. Szutowicz, G. Villanueva, N. Kiselev, T. Bonev, P. Hartogh // Кинематика и физика небесных тел. — 2005. — Т. 21, № 5-додаток. — С. 458-464. — Бібліогр.: 18 назв. — англ. |
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| author | Jockers, K. Szutowicz, S. Villanueva, G. Kiselev, N. Bonev, T. Hartogh, P. |
| author_facet | Jockers, K. Szutowicz, S. Villanueva, G. Kiselev, N. Bonev, T. Hartogh, P. |
| citation_txt | Gas and dust in Comet 2P/Encke observed in the visual and submillimeter wavelength ranges / K. Jockers, S. Szutowicz, G. Villanueva, N. Kiselev, T. Bonev, P. Hartogh // Кинематика и физика небесных тел. — 2005. — Т. 21, № 5-додаток. — С. 458-464. — Бібліогр.: 18 назв. — англ. |
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| description | In November 2003 Comet 2P/Encke was observed simultaneously with the 10-m Heinrich–Hertz Submillimeter Telescope on Mount Graham, Arizona, USA, and the 2-m optical telescope on Mount Rozhen, Bulgaria. Simultaneous radio observations of the 4–3 and 3–2 rotational transitions of HCN and the 0–0 transition of the CN violet band system provide a three-dimensional view on the comet. The observations are consistent with outgassing from the source region I with location and pole position of Comet Encke taken from [14]. The outflow speed is 1.2 km. There is some evidence for another possible parent for CN besides HCN. The visual dust coma of Comet Encke is nearly spherical with a diameter of about 1000 km and a slight extension into Comet Encke’s fan. The polarization of the observed NH₂ transition at 662 nm is 7% at a phase angle of 94.5°, close to the value for two-atomic molecules. At this phase angle and a wavelength of 642 nm the polarization of Comet Encke’s dust is greater than 30%, i.e., exceeds the value for so-called dusty comets.
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GAS AND DUST IN COMET 2P/ENCKE OBSERVED IN THE VISUAL
AND SUBMILLIMETER WAVELENGTH RANGES
K. Jockers1, S. Szutowicz2, G. Villanueva1, N. Kiselev3, T. Bonev4, P. Hartogh1
1Max–Planck–Institute for Solar System Research
2 Max–Planck–Str., 37191 Katlenburg-Lindau, Germany
e-mail: jockers@linmpi.mpg.de
2Space Research Centre of Polish Acad. Sci.
18A Bartycka, 00-716 Warsaw, Poland
3Institute of Astronomy, V.N. Karazin Kharkiv National University
35 Sumskaya Str., 61022 Kharkiv, Ukraine
4Institute of Astronomy, Bulgarian Acad. Sci.
72 Tsarigradsko Chaussee, 1784 Sofia, Bulgaria
In November 2003 Comet 2P/Encke was observed simultaneously with the 10-m Heinrich–Hertz
Submillimeter Telescope on Mount Graham, Arizona, USA, and the 2-m optical telescope on Mount
Rozhen, Bulgaria. Simultaneous radio observations of the 4–3 and 3–2 rotational transitions of
HCN and the 0–0 transition of the CN violet band system provide a three-dimensional view on
the comet. The observations are consistent with outgassing from the source region I with location
and pole position of Comet Encke taken from [14]. The outflow speed is 1.2 km. There is some
evidence for another possible parent for CN besides HCN. The visual dust coma of Comet Encke
is nearly spherical with a diameter of about 1000 km and a slight extension into Comet Encke’s
fan. The polarization of the observed NH2 transition at 662 nm is 7% at a phase angle of 94.5◦,
close to the value for two-atomic molecules. At this phase angle and a wavelength of 642 nm
the polarization of Comet Encke’s dust is greater than 30%, i.e., exceeds the value for so-called
dusty comets.
INTRODUCTION
Comet 2P/Encke is the second comet (after Comet 1P/Halley) of which a periodic orbit was determined.
In contrast to the orbit of Comet Halley the orbit of Comet Encke turned out to have a period of 3.4 years
(the smallest one known up to now) and a perihelion distance of 0.33 AU. Because Comet Encke is always
orbiting in the inner solar system it has lost a large amount of its volatile substances and is perhaps the most
evolved short-period comet. In its visual appearance Comet Encke does not display a tail, i.e., lacks the most
well-known attribute of a comet. Instead, a so-called “fan” is observed, a broad feature visible at an angle to
the Sun-comet line. It is generally assumed that most of the surface of aged short-period comets is an inactive
crust left behind after sublimation of the volatile material. The activity of such comets is restricted to small
localized active vents (holes in the inactive crust) deep enough to reach down to depths which are less depleted
from volatile substances. Z. Sekanina studied optical images of the inner coma region of comets (which do not
show the nucleus itself but only jets or fans emanating from it) in order to infer the existence and location of
active areas on the surfaces of cometary nuclei and to derive the position of their rotation axis and the rotation
rate. Three papers refer to Comet Encke [14–16]. According to this work the north rotation pole of Comet Encke
is located at right ascension 205◦ and declination 2◦. The obliquity of the orbit plane relative to the nucleus
equator is 70◦ and the solar longitude at perihelion is 230◦. The Sun transits the equator of the cometary
nucleus from north to south 8 days before perihelion (at the heliocentric distance rH = 0.40 AU) and from
south to north 65 days after perihelion at rH = 1.35 AU. Two vents on the nucleus surface were identified,
one at latitude +55◦ (source I) and another one at latitude –75◦ (source II). While source I is in sunlight 94%
of the time of revolution of the comet, source II is illuminated only 6% of the time, but this time includes
the perihelion.
Z. Sekanina attributes the fan to being caused mostly by scattered light from dust grains. The dust coma
of Comet Encke has been imaged by the ISOCAM device on the ISO satellite of ESA [13] at a wavelength of
11 μm. No dust tail in the usual sense was observed and there is agreement with other authors that the size of
c© K. Jockers, S. Szutowicz, G. Villanueva, N. Kiselev, T. Bonev, P. Hartogh, 2004
458
the dust grains emitted by Comet Encke is about 15 μm. The dust fan was not observed, but the reason may be
the visibility restrictions. In the visual wavelength range Encke’s dust has not been convincingly observed, as
it is difficult to separate it from molecular emissions. In particular there is disagreement about the polarization
of cometary dust from optical measurements.
In this paper we report about observations in the visual and millimeter wavelength range conducted in
November 2003. The simultaneous microwave and visual observations provide a three-dimensional view on
the source region I. In addition, photometric and polarimetric observations of Comet Encke’s dust and NH2
emission at 0.642 and 0.662 μm will be presented.
Figure 1. The sub-millimeter spectra observed with the Heinrich–Hertz telescope (histograms). Superimposed as heavy
lines are the line profiles calculated from our active region emission model. The vectors (resembling the spokes of a wheel)
are the projected emission directions during a full nucleus rotation (north is up, the sight direction is from left to right).
The absolute values of the (unprojected) vectors vary with cos1/4 of solar elevation angle
OBSERVATIONS
Comet 2P/Encke had a favourable apparition in fall 2003 when it approached the Earth to a minimum distance of
0.260 AU. We observed the comet on November 18–30, 2003. During this period the comet was at the distances
rh = 0.97–0.77 AU from the Sun and Δ = 0.26–0.32 AU from the Earth. The phase angle was large and
increased from 87◦ to 124◦. As the comet went through perihelion on December 29, it was still observed during
activity of source I.
The radio observations took place at the 10-m Heinrich–Hertz telescope of the Steward Observatory on
Mount Graham, Arizona, on November 18–30, 2003. The heterodyne spectrometer of MPS [17] was used to
observe the 4–3 (354.505 GHz, beam diameter 21 arcsec) and 3–2 (265.886 GHz, beam diameter 28 arcsec)
rotational transitions of HCN. The integration times per day were usually about 4 hours. The observations
were performed with beam switching and position switching (t/2= ON, t/2=OFF). The aim of the observations
was to derive the HCN production rate and the velocity profile of the HCN lines in order to draw conclusions
about the nucleus source from the velocity distribution along the line of sight. The spectral scans averaged over
one observing day are shown in Fig. 1.
459
The visual observations were conducted with the Two-Channel Focal Reducer of MPS [4] at the 2-m telescope
of the Bulgarian Institute of Astronomy (Rozhen, Bulgaria) on November 18–25, 2003. Comet Encke was imaged
at 388 nm (CN 0–0 band of the Violet System) to get information on the spatial distribution of CN, the daughter
molecule of HCN. Imaging polarimetry and photometry were performed at 642 nm (continuum) and 662 nm
(NH2 0–7–0 α ammonia band), in order to derive a dust image from comparison of images in the “continuum”
(central wavelength 642 nm, FWHM 2.6 nm) filter (which also transmits faint lines of NH2) with images in
the “NH2” (central wavelength 662 nm, FWHM 5.9 nm) filter (which also transmits the dust continuum, but
to a much smaller extent), and to measure the polarization of the dust. One pixel of the detector corresponds
to 0.89 arcsec and the imaged area was at least 7.5 arcmin (1.5 arcmin for polarimetry). Each clear night
observations were conducted for about 3 hours with exposure times of 5 to 10 minutes. The inner area of
the CN images, corresponding in a linear extent to about two times the beamwidth of the radio telescope, is
shown in isocontours in Fig. 2. Adjacent contours differ by a factor of 0.9.
Figure 2. Isophotes of Comet 2P/Encke observed at 388 nm (CN 0–0 band of Violet System). The vectors indicate
the changing emission direction of source I proposed by Z. Sekanina during one nucleus rotation. The absolute values of
the (unprojected) vectors vary with cos1/4 of solar elevation angle. North is up, east to the left
ROTATION OF THE NUCLEUS AND POSSIBLE STRUCTURE OF THE SOURCE REGION
The gas coma of the CN radical is observed to be asymmetric. This contradicts the usual understanding that
a gas coma is isotropic and spherical. It also contradicts the idea expressed in [14–16] that the fan consists
of dust grains. An asymmetric gas coma of Comet Encke has already been inferred in [6]. Evidently, the gas
is mostly released from a single active vent on Comet Encke’s nucleus. This fact opens the possibility to
investigate the properties of this active region and of the nucleus itself. The rotation period of Comet 2P/Encke
is 15.2± 0.3 hr [5] or 15.08± 0.08 hr [12]. Such a rotation rate should be immediately apparent in the radio as
well as in the optical data. Figure 2 shows optical data from four consecutive days. At least two of the four
images should show the rotating source at upper or lower elongation. Some tendency of the innermost ellipsoidal
contour to have its long axis inclined with respect to the horizontal can be noticed but this by far does not go
up to the full opening angle of the cone. In [14] the rotation axis was determined from the projected center
axis of the fan (some visibility criteria were checked a posteriori and data rejected if they were not satisfied)
and the latitude of the active region was determined from the width of the fan. From this work no conclusion
460
can be drawn if the source region is localized at a certain nuclear longitude or if it forms a ring at its latitude.
Even more, the source region could extend all the way from the pole to the determined latitude, i.e., the source
region could comprise the polar cap down to the derived latitude. These possibilities will be studied in more
detail in the future. In the following we will assume a ring-shaped source region extending at latitude 55◦ all
the way around the nucleus. This model describes an active polar cap almost as well.
MODELLING THE HCN LINE PROFILES
A Monte Carlo model of the outflow from a ring-type source region was constructed. The rotation axis was
assumed at right ascension 205◦ and declination 2◦ as proposed in [14]. Precession of the pole was not taken into
account. The source region was assumed at a cometocentric latitude of 55◦ extending through all longitudes.
HCN particles are released with a thermal speed of 0.176 km s−1 (temperature of 100 K) superimposed on
a prescribed outflow speed vout into a (three-dimensional) cone with opening half angle of 30◦. The number of
particles released was modulated ∼ cos1/4(ε) (ε solar zenith angle) corresponding to radiative energy balance.
After a stationary state has been reached in the Monte Carlo model, the particles in the telescope beam
are counted and the velocity profile is determined. An HCN lifetime τHCN = 1.5 · 104 s was determined from
the CN observations (see next section). The telescope beam halfwidth at the comet is ≈ 2300 km at the frequency
of the HCN (4–3) line and ≈ 3100 km at the frequency of the HCN (3–2) line. As both sizes are small as compared
to the scale length of HCN = τHCN × vout the line profile is not affected by the selection of the HCN life
time. The crucial input parameter is the outflow speed. A good fit to the observed profiles is obtained with
vout = 1.2 km s−1 (see Fig. 1). As in course of the observations the rotation axis of the nucleus gradually tilts
away from the observer the line becomes gradually more redshifted. Figures 1 and 2 do not support any evidence
for a rotation of the active area. The source could be a ring of constant latitude or, more probable, could be
centered at the pole and extend down to 55 degree latitude. A more quantitative check still needs to be done.
PRODUCTION RATES: HCN SOLE PARENT OF CN?
Figure 3. Production rates of parent molecule HCN and its daughter CN. Full lines: HCN isotropic model [1]. Crosses:
(4–3) transition. Diagonal crosses: (3–2) transition. Dashed line and diamonds: HCN jet model. Full line and triangles:
CN isotropic model. Lifetimes and velocities are indicated
It is interesting to compare HCN and CN production rates in Comet Encke to find out, if HCN can be the sole
source of CN. In order to derive CN production rates, the CN images of each night were combined into single
images with the star trails removed. These combined images were azimuthally averaged and fitted with Monte
461
Carlo calculations [2] which assume isotropic HCN and CN distributions. The daughter speed was 1.02 km s−1
based on molecular data [3]. The field of view of the ground-based CN images is large enough to allow the de-
termination of the parent life time. The outflow speed vout is already known from the radio observations.
The fitted parent life time τHCN is consistent with a heliocentric variation in proportion to the inverse square
of heliocentric distance rH . Its derived value at 1 AU of 1.5 · 104 s is, however, small as compared to the theore-
tical values of the HCN life time of 79 400 s for low solar activity and 31 900 s for high solar activity [8]. It is
also small as compared to the value adopted by [3]. The reason for this must be investigated. In any case, if
the same values of parent lifetime are assumed for the production rate determination of parent and daughter,
the production rate ratio is rather insensitive to the actual values [7].
The HCN production rate was determined using the standard method for isotropic outflow [1]. The integrated
line areas
∫
TBdv varied from 0.067 ± 0.009 K km s−1 (HCN(3–2), November 21) to 0.194 ± 0.036 K km s−1
(HCN(4–3); November 28). They were converted to column densities. For this purpose, thermal equilibrium was
assumed. A rotational temperature of 43 K scaled with heliocentric distance ∼ r−1.5
H gave the best agreement
between the observations of the 4–3 and 3–2 transitions. The evolution of the rotational population with
increasing distance from the nucleus from LTE to fluorescence equilibrium was not taken into account. From
the column densities we calculated production rates as described in [1]. The isotropic model underestimates
the real HCN production rate. The telescope beam of the Heinrich–Hertz telescope is small as compared to
the extent of the HCN coma (see above). In case of an isotropic coma a significant amount of flux detected by
the telescope comes from particles moving close to the line of sight either toward or away from the observer (this
is why line profiles of comets observed in the millimeter wavelength range frequently show a dip at zero velocity
shift). In our case such particles are underabundant. Therefore, the outflow model with a ring-type source
as described in the preceding section has also been used to calculate the HCN production rate. As expected,
the values of this model are higher but remain smaller than the CN production rates by about a factor of two.
In view of the low signal/noise ratio of the HCN observations we cannot decide if the difference necessitates
an additional parent of CN. An additional parent seems, however, to be required to explain the large difference
between the life time of the parent of CN derived from the CN profiles and the theoretical HCN life time. For
a more detailed discussion see [18].
COMET ENCKE’S DUST AND ITS POLARIZATION
Figure 4. East-west cuts (along fan) through images of Comet Encke. Dotted: filter 662 nm. Dashed: filter 642 nm.
Full: pure NH2. Long-dash: pure dust. Images of a series of 300 s exposure frames were median filtered to obtain
the displayed profiles. Three rows of the image passing closest to the nucleus have been averaged (see text)
As mentioned above, polarimetric images of Comet Encke in the filters “642” and “662” (the names stand for
the central wavelength in nanometers) were acquired during the observations. Both filters transmit continuum
and NH2 lines, but the NH2 contribution is much weaker in the “642” filter. The continuum contribution in
both filters is proportional to their transmission integral multiplied with the solar intensity in both wavelengths.
As the two wavelengths are not far from each other a reddening of the cometary dust can possibly be neglected.
462
Subtraction of the normalized images cancels out the dust contribution and produces a clean NH2 image.
To derive a dust image we have subtracted from the “642” filter image the true NH2 image multiplied with
the largest possible factor not producing negative counts in the resulting dust image. As can be seen in Fig. 4
the true dust image is nearly symmetric with respect to the nucleus with a slight extension into the fan. As small
particles should form a dust tail we confirm the large size of Comet Encke’s dust grains [13]. The resulting NH2
image is shifted with respect to the dust image in the direction of the fan.
The photometric profiles have been derived by adding the four polarimetric subimages of the imaging po-
larimeter [4]. Applying the above discussion to the polarimetric subimages themselves we have determined
the polarization of NH2 and of the dust corrected for the influence of the NH2 coma. The results are shown
in Fig. 5. The left panel shows the polarization in the uncorrected “662” filter. The polarization rises to-
ward the nucleus because of the increased contribution of the Comet Encke’s dust. Farther from the nucleus
the polarization becomes constant and equals ≈ 7%. The dust polarization is expected to be higher than that.
Therefore, if there would be a significant but decreasing amount of dust at larger distances from the nucleus in
the fan the polarization would be higher and continue to decrease. Therefore, we interpret the polarization of
7% as polarization of the NH2 molecule in the observed 0–7–0 α ammonia band (we do not know of a determi-
nation of this value in the laboratory) and conclude that there is no dust at larger distances from the nucleus
in the fan. In this way our scaled subtraction procedure to derive the true dust image is a posteriori justified.
The right panel of Fig. 5 shows the polarization of the Comet Encke’s dust. In contrast to [11] the polarization
is comparable to or even larger than the polarization of so-called dust-rich comets. This result was predicted
in [10] and agrees with observations in [9].
Figure 5. E–W cuts (along fan) of polarization of Comet Encke (November 21, 2003). Left panel: uncorrected image
through “662” filter. Right panel: corrected dust image
CONCLUSIONS
HCN and CN was observed in November 2003 with the submillimeter Heinrich–Hertz telescope on Mount
Graham, Arizona, USA, and the 2-m telescope on Mount Rozhen, Bulgaria. The work on data reduction and
interpretation is continuing but the following results are already apparent:
1. The outgassing from the nucleus of Comet 2P/Encke is not isotropic but occurs from a limited active
area.
2. The observed HCN line profiles can be explained by our Monte Carlo model of a ring-shaped active source,
with the position of Comet Encke’s north pole at right ascension 205◦ and declination 2◦ [14], and the ring-
shaped activity at a latitude of 55◦ (source I of [14]). Instead of a ring-shaped active region an active
polar cap extending from the north pole to 55◦ latitude could possibly also explain the observations.
3. The observed radio lines of HCN reveal a gas expansion velocity of the coma equal to 1.2 km s−1.
4. The location of the optical CN jet is also consistent with the pole and source position of [14].
5. The CN production rate is about twice as high as the HCN production rate, even if the outgassing from
the local source (our Monte Carlo model) is taken into account. Within the systematic errors present in
the production rate determinations HCN can possibly still be the sole parent of CN. But the life time of
463
the CN parent derived from our data is 15 000 s at 1 AU, that is significantly less than theoretical values
of 31 900 s for high solar activity and 79 400 s for low solar activity [8]. This provides a strong argument
in favour of another CN parent.
6. The visual dust coma is only a few 1000 km wide and almost spherically symmetric with only a very slight
extension in the fan direction. This contradicts the idea of the fan consisting mostly of dust particles [14].
7. The polarization of NH2 at 662 nm (0–7–0 vibrational transition of α ammonia band) at a phase angle of
94.6◦ is ≈ 7%, close to the value for two-atomic molecules.
8. The polarization of the dust of Comet Encke at a phase angle of 94.6◦ exceeds 30%, i.e., the dust of
Comet Encke has a polarization even higher than so-called dusty comets. This contradicts the idea that
all gas-rich comets have lower polarization than the dust-rich ones [11].
Acknowledgements. T. Bonev, N. Kiselev, and S. Szutowicz gratefully acknowledge support by the Max–
Planck–Society.
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| id | nasplib_isofts_kiev_ua-123456789-79699 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 0233-7665 |
| language | English |
| last_indexed | 2025-12-07T17:11:37Z |
| publishDate | 2005 |
| publisher | Головна астрономічна обсерваторія НАН України |
| record_format | dspace |
| spelling | Jockers, K. Szutowicz, S. Villanueva, G. Kiselev, N. Bonev, T. Hartogh, P. 2015-04-03T19:43:40Z 2015-04-03T19:43:40Z 2005 Gas and dust in Comet 2P/Encke observed in the visual and submillimeter wavelength ranges / K. Jockers, S. Szutowicz, G. Villanueva, N. Kiselev, T. Bonev, P. Hartogh // Кинематика и физика небесных тел. — 2005. — Т. 21, № 5-додаток. — С. 458-464. — Бібліогр.: 18 назв. — англ. 0233-7665 https://nasplib.isofts.kiev.ua/handle/123456789/79699 In November 2003 Comet 2P/Encke was observed simultaneously with the 10-m Heinrich–Hertz Submillimeter Telescope on Mount Graham, Arizona, USA, and the 2-m optical telescope on Mount Rozhen, Bulgaria. Simultaneous radio observations of the 4–3 and 3–2 rotational transitions of HCN and the 0–0 transition of the CN violet band system provide a three-dimensional view on the comet. The observations are consistent with outgassing from the source region I with location and pole position of Comet Encke taken from [14]. The outflow speed is 1.2 km. There is some evidence for another possible parent for CN besides HCN. The visual dust coma of Comet Encke is nearly spherical with a diameter of about 1000 km and a slight extension into Comet Encke’s fan. The polarization of the observed NH₂ transition at 662 nm is 7% at a phase angle of 94.5°, close to the value for two-atomic molecules. At this phase angle and a wavelength of 642 nm the polarization of Comet Encke’s dust is greater than 30%, i.e., exceeds the value for so-called dusty comets. T. Bonev, N. Kiselev, and S. Szutowicz gratefully acknowledge support by the Max–Planck–Society. en Головна астрономічна обсерваторія НАН України Кинематика и физика небесных тел MS5: Dynamics and Physics of Solar System Bodies Gas and dust in Comet 2P/Encke observed in the visual and submillimeter wavelength ranges Article published earlier |
| spellingShingle | Gas and dust in Comet 2P/Encke observed in the visual and submillimeter wavelength ranges Jockers, K. Szutowicz, S. Villanueva, G. Kiselev, N. Bonev, T. Hartogh, P. MS5: Dynamics and Physics of Solar System Bodies |
| title | Gas and dust in Comet 2P/Encke observed in the visual and submillimeter wavelength ranges |
| title_full | Gas and dust in Comet 2P/Encke observed in the visual and submillimeter wavelength ranges |
| title_fullStr | Gas and dust in Comet 2P/Encke observed in the visual and submillimeter wavelength ranges |
| title_full_unstemmed | Gas and dust in Comet 2P/Encke observed in the visual and submillimeter wavelength ranges |
| title_short | Gas and dust in Comet 2P/Encke observed in the visual and submillimeter wavelength ranges |
| title_sort | gas and dust in comet 2p/encke observed in the visual and submillimeter wavelength ranges |
| topic | MS5: Dynamics and Physics of Solar System Bodies |
| topic_facet | MS5: Dynamics and Physics of Solar System Bodies |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/79699 |
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