A&A 430, 1129-1132 (2005)
DOI: 10.1051/0004-6361:20041766
L. Pansecchi - M. Scardia
INAF - Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate LC, Italy
Received 1 August 2004 / Accepted 4 September 2004
Abstract
Three CCD images of Comet C/2004 F4 (Bradfield), taken on three different dates close to the time of the Earth crossing
through the plane of the comet orbit (2004 May 2.9280 UT), show a ray-shaped structure (RSS) in the dust tail, accompanied by a short,
sunward spike (SWS). This morphology and behaviour strongly recall a Neck-Line Structure (NLS; Kimura & Liu 1977; Pansecchi et al. 1987). Astrometric measurements of the images show that, on all the three dates, the position on the sky of the observed RSS is consistent with several sky-projected points of a theoretical Neck-Line, computed for the precise date of each observation. Furthermore, the SWS turns out to be a real (not caused by perspective) antitail, which is predicted by NLS models.
Key words: comets: general - comets: individual: C/2004 F4 (Bradfield)
A Neck-Line Structure (NLS) is a special dusty feature
that was found by Kimura & Liu (1977) in the development of their theoretical
model of cometary dust tails. In such a model, each dust particle is taken as
an infinitely small planetary body, whose orbital elements, and then its position
in space at any time, can be rigorously computed using Keplerian orbit mechanics.
Consequently, because of the Keplerian motion of the individual particles, the
initial spherical (or quasi-spherical, depending on the ejection geometry) shells,
ejected from the nucleus before perihelion, remain spherical for a while after emission.
Soon they become ellipsoidal in shape by collapsing after perihelion onto the orbital
plane of the parent comet at the second node of the particle orbits, that is, at the
point
away in true anomaly from the ejection point (the first node). At the
second node, the initial three-dimensional shells are completely flattened on the plane
of the comet orbit. The ideal line connecting the comet nucleus with all the second
nodes of the particle orbits (in practice with the centers of the flattened ellipsoids)
is called the Neck-Line (NL), and the resulting flat structure of dust particles of
different sizes and different emission times that cross the comet orbital plane
(each in its second node) at a given instant of time is called the NLS for that instant.
As shown by Pansecchi et al. (1987), such a structure has a short, sunward extension.
In their NLS model, the dust ellipsoids composed of the largest dust grains, on which
the radiation pressure force has negligible effects,
after having collapsed onto the plane of the comet orbit,
form in such a plane a flat ellipse. This ellipse is approximately centered
on the comet nucleus, and its major axis is nearly parallel to the sunward
prolongation of the NL. Therefore half of such an ellipse
lies in the inner part of the comet orbit, thus resulting in a real antitail
(see Fig. 4 in the paper by Pansecchi et al. 1987). In
general, two conditions should be satisfied for a NLS to be observed in a well
developed dust tail: (i) the comet must have passed its
perihelion point; (ii) the Earth must be close to the plane of the comet orbit.
The first condition is imposed by the spatial
location of the second node of the particle orbits. For the second one,
the resulting edgewise (or quasi-edgewise) perspective
allows better visibility of the NLS, because of the piling up of the dust
particles along the line of sight. Under such conditions, a
NLS is expected to look like a ray-shaped structure (RSS), stretching from
the nucleus across a regular dust tail, extending somewhat
sunward in a short, narrow antitail (i.e., in a sunward spike; SWS).
It is surrounded by a fainter envelope representing the edgewise
projection on the sky of the three-dimensional part of the tail, that is, of
the collapsing ellipsoids composed of dust ejected
continuously by the comet nucleus. NLSs were identified in the dust tails
of at least seven comets: Arend-Roland 1957III (Kimura & Liu 1977),
Bennett 1970II (Pansecchi et al. 1987),
Halley 1986III (Cremonese & Fulle 1989), Great January Comet 1910I
(Pansecchi & Fulle 1990), Austin 1990V (Fulle et al. 1993),
Hale-Bopp 1995 O1 (observed at the beginning of 1998; Fulle 2004),
and 67P/Churyumov-Gerasimenko (Fulle et al. 2004).
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Figure 1:
Observations and related reductions: in ![]() ![]() ![]() ![]() ![]() |
Open with DEXTER |
In this paper we present and analyse three photographs of Comet C/2004 F4 (Bradfield), taken after perihelion, around the time of the Earth crossing through the plane of the comet orbit, and therefore under the ideal conditions described above. All the photographs show a RSS in the dust tail, accompanied by a short SWS, which, on the whole, strongly resemble a NLS. Our analysis will consist in comparing the position on the sky of the observed structures with some sky-projected points of a theoretical NL, by means of astrometric measurements of the images.
Table 1:
Observational data and geometrical circumstances. No: Serial number
of the observation. Obs: Observer (DT - Diego Tirelli,
Italian Group of Comet Observers; MJ & GR - Michael Jäger & Gerald Rhemann,
Astrostudio G. Rhemann Ges. m. b. h). Date UT (2004): Time
at midexposure. RA, Dec: right ascension and declination of the comet nucleus,
respectively, measured directly on the CCD images through
astrometric calibration. ,
r: Earth-comet and Sun-comet distances.
:
Phase angle.
:
True anomaly of the comet nucleus.
:
Cometocentric latitude of the Earth; it is positive when
the Earth is in the comet north hemisphere.
According to the orbital elements we used in this paper (Marsden 2004),
Comet C/2004 F4 passed its perihelion point on 2004 April
17.0899 UT. Nearly sixteen days later, on May 2.9280 UT,
the Earth crossed the plane of the comet orbit. The observations analysed here
consist of three CCD images, taken on three different dates, at -2.9,
-0.8,
and +4.2 days from the date of the Earth crossing. The
original photographs, labelled a, are shown in the left-hand column
of Fig. 1. The quantities describing the relevant geometrical
circumstances and projection conditions are summarized in Table 1.
The first observation (Fig. 1.1a) is a wide-field, unfiltered
photograph, taken with a 20/30 cm Schmidt camera, whereas the others
(Figs. 1.2a, and 1.3a) are images of much smaller field,
taken with a 25-cm, f/6 reflector, through a R and I-Cousins filter,
centered at 647.2 nm and 786.9 nm, with a total WHM of 261.3 nm
(Moro & Munari 2000). The original images have been astrometrically
calibrated by means of the field stars and then processed in order
to isolate and emphasize the axis (i.e., the line of maximum density
or maximum brightness) of the RSS for a comparison with a theoretical NL.
These processed images are assembled in Col. b of Fig. 1. A
series of isophotes of the area near the coma were also obtained to measure
the orientation of the SWS (Col. c of Fig. 1). Here the dashed line connecting the position of
the comet nucleus (marked with a small cross) with the vertices of the
isophotes was taken as denoting the position angle on the sky of the SWS
(PA
in Table 2). We also tried to measure the length of the SWS,
but, after some attempts, we abandoned this idea, the results
being too uncertain because of three factors: the general faintness of such a
feature; the small scale and star pollution in observation 1; the fact that,
in observations 2 and 3, the SWS seems to extend outside the field. The
measured PA
was then projected on the plane of the comet orbit,
into the diagrams in Col. d, where the orientation of the SWS
is represented by a short, dashed arrow. All the photographs (and the derived
images) in Fig. 1 have the same orientation,
and arrows N and E in Fig. 1.1a indicate North and East through the nucleus.
For the precise date of each observation, we then computed several points
along the theoretical NL in the plane of the comet orbit, following the
procedure described by Pansecchi at al. (1987, 1988). The diagrams with
the orbit plane views are shown in the last column of Fig. 1. The black dots,
sequentially numbered from the nucleus, are the computed centers of the dust
shells, collapsed (flattened) onto the orbit plane of the parent comet at
the second node of the particle orbits. Each point is characterized by zero
ejection velocity from the comet nucleus and by a given value of (1
),
the solar radiation pressure force acting on the dust grain expressed in solar
gravitational force units, and of
,
the time (in days) between dust
ejection and observation. All the parameters concerning the computed NL
points are summarized in the right hand part of Table 2. The ideal (dotted)
line connecting the black dots with the comet nucleus (unfilled dot) is the
NL for the instant of time of the observation. Finally, the computed NL
points have been projected on the plane of the sky, into the processed images
in Col. b of Fig. 1, as white dots with the same sequence number. Figure 1
clearly shows that, on the three dates, the sky-projected points of the
theoretical NL closely fit the axis of the observed RSS. In Table 2,
the average position angle of the RSS (PA
), measured directly
on the photographs, is also given. The precise orientation on the sky of
the RSS, at various distances from the nucleus, is represented by the
position angles of the single, sky-projected, NL points (PA
in Table 2).
Table 2:
RSS, SWS and NL parameters. No.: serial number of the observation.
PA
:
measured position angle on the sky of the average orientation
of the RSS. PA
:
measured position angle on the sky of the SWS.
PA
:
computed position angle on the sky of the Sun-comet radius vector.
PA
:
computed position angle on the sky of the positive velocity vector.
NLp: NL point sequential number (Fig. 1). (1
): radiation pressure acting on
the dust grain.
:
days elapsed between ejection of the dust grain from the comet
nucleus and observation. d
,
d [106 km]: angular (in the plane of the
sky) and linear (in the plane of the comet orbit) distance from the nucleus of the
computed NL point, respectively. PA
:
position angle of the sky-projected
NL point. oPA
:
orbital position angle of
the NL point. Position angles in the plane of the sky are measured from north to east,
as usual, whereas the ones in
the plane of the comet orbit (prefix o) are measured counterclockwise from the
Sun-comet prolonged radius vector
(r').
The results of the astrometric measurements described in the previous section show that,
throughout the relevant time, the position on the sky of the RSS is consistent with several
sky-projected points of a theoretical Neck-Line. We could conclude that the RSS in
the dust tail of Comet C/2004 F4 (Bradfield) is really a NLS. However, two
objections might be raised: (i) the RSS might be a plasma feature; or (ii) a
layer of dust (DL) widespread on the plane of the comet orbit, that we see as a
RSS only because of the particular projection conditions. The filter used
in observations 2 and 3 appears to be insufficient to suppress all plasma features.
Indeed, a faint
trace of the plasma tail is still visible in the image of May 7 (Figs. 1.3a, and
1.3b),
south of the RSS, up to a distance from the nucleus of 8'. But, fortunately,
such an image also shows that the plasma tail lies nearly along the prolonged Sun-comet
radius vector (r'), in a position angle quite different from that of the RSS.
Therefore, the two structures cannot be confused. The same thing can be said for
the unfiltered image of Apr. 30 (Figs. 1.1a and 1.1b), where the two
structures appear to be clearly separated. On the other hand, the fact that the
sky-projected NL points coincide with the RSS implies, of course, that, if
reduced to the plane of the comet orbit, such a structure would coincide with
the NL points in that plane. Now, it is easy to observe from the orbit
plane views in Fig. 1 or from the orbital position angles (oPA
)
in Table 2
that the aberration angle of such points (difference with respect to the antisolar
direction = 2
- oPA
)
would be too large (between 72
and 82
,
depending on the date and on the distance from the nucleus) to
be consistent with a plasma tail, unless the RSS is outside the plane of the comet orbit.
Furthermore, the general shape of the RSS does not change substantially over the
relevant time (seven days), and its PA
clearly changes only with the
changing projection conditions. Both such behaviours are characteristic of a dusty feature.
As to objection (ii), we acknowledge that for the image of May 2, taken when the Earth
was almost exactly in the plane of the comet's orbit (see parameter
in Table 1),
it is impossible to discriminate between the NLS model and the DL model, because of
the too severe projection conditions. The situation is however more favourable for
the observations of April 30 and May 7. We consider it unlikely (although not impossible)
that the consistency of the sky-projected NL points with the RSS on the three dates
may be a coincidence. But, as already pointed out by Pansecchi et al. (1987), the
discriminating criterion may be the presence of the SWS. Astrometric measurements
show that such a feature lies on the sky nearly
along the sunward prolongation of the RSS, in the sector delimited by the Sun-comet
radius vector (r) and by the comet velocity vector +V (see PA
and
PA
in Table 2). Figure 1.3c offers the best evidence of such a geometry.
On the (reasonable) assumption that such a structure is confined to the plane of
the comet orbit, this means that the SWS lies inside the path of the comet orbit
with respect to the Sun, and therefore should be considered as a real (not-perspective)
antitail in the Bredikhin sense (Jaegermann 1903). As explained in Sect. 1, such a
real antitail is predicted by the NLS model of Pansecchi et al. (1987) as a natural, sunward extension of the NLS. The orbit plane diagrams in Fig. 1 show that the projected
orientation of the SWS is not parallel to the NL, as required by the model, but more or
less tilted toward the Sun. This, however, might be explained by the measurement uncertainties.
In conclusion, the presence of the SWS as a real antitail should be a strong
argument in favour of the NLS interpretation of the RSS.
Acknowledgements
We are indebted to Michael Jäger & Gerald Rhemann (Astrostudio G. Rhemann Ges. m. b. h.), and Diego Tirelli (Italian Group of Comet Observers; GOC), who kindly made available the photographs analysed here. Thanks are due also to Rolando Ligustri, and Giovanni Sostero (they too members of gOC) who, alerted by the first author, worked hard to secure images during the critical period, although unsuccessfully because of bad weather.