THE ASTROPHYSICAL JOURNAL, 461:L119[–]L122, 1996 April 20

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  Evolution of a Spiral Jet in the Inner Coma of Comet Hale-Bopp (1995 O1)



Mark R. Kidger, Miquel Serra-Ricart, Luis R. Bellot-Rubio, and Ricard Casas



    Instituto de Astrofísica de Canarias, Via Lactea E-38200 La Laguna,

Tenerife, Canary Islands, Spain; mrk@iac.es; mserra@iac.es; lbellot@iac.es;

                                 rcr@iac.es



            Received 1995 September 7; accepted 1995 December 6



                                  ABSTRACT



     We present observations of the evolution of a prominent spiral jet in

the inner coma of comet Hale-Bopp (1995 O1). The observations, taken with

the 82 cm IAC-80 telescope at the Teide Observatory, were made on 1995

August 25, 27, 28, and 31, and on September 4[–]7, as part of an

ongoing program of monitoring the comet in Tenerife. The jet is observed to

show a nearly, but not completely, constant position angle over the two

weeks of observation. Although it is generally assumed that the jet is a

dust event, some aspects of the morphology and behavior mean that the

hypothesis that it is a gas jet cannot be ruled out. No single hypothesis

is thought to be completely satisfactory. Between our first detection of

the jet on August 25 and its disappearance on September 7, we see the point

of inflection within the jet expand away from the nucleus at a highly

constant velocity. At the same time, the jet fades considerably. This jet

event seems different from others that have been observed later because the

collimation of the beam is very tight, rather than the highly wound spiral

structure shown by some later jets.



Subject headings: comets: individual (Hale-Bopp 1995 O1)



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CONTENTS



   * 1. INTRODUCTION

   * 2. OBSERVATIONS

   * 3. RESULTS

   * 4. DISCUSSION

   * 5. CONCLUSIONS

   * ACKNOWLEDGMENTS

   * REFERENCES

   * FIGURES

   * TABLES

   * REFERENCES TO THIS ARTICLE



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                              §1. INTRODUCTION



     Comet Hale-Bopp was discovered visually by A. Hale and T. Bopp (Hale &

Bopp 1995) on 1995 July 23 at the unprecedented distance of 7.3 AU from the

Sun, by far the greatest distance for a visual comet discovery, and unusual

even for photographic or CCD discoveries. Since the comet exhibits a very

bright total visual magnitude at discovery, it is evident that it is either

particularly large and/or active or is suffering an exceptional outburst.

Despite the announcement of various prediscovery images of the comet, the

very sparse coverage that they offer and the doubts expressed about some of

these images mean that it is still not obvious whether comet Hale-Bopp is a

giant object showing its [“]normal[”] activity or a rather

smaller object showing an outburst. As a consequence, there is a range of

at least 10 mag between the best and the worst cases in the extrapolation

of its light curve to perihelion (Kidger 1995). One way of distinguishing

between scenarios is to establish the comet's degree and pattern of

activity. A high and stable degree of observed activity, combined with a

consistently bright total magnitude, would indicate that the more

optimistic predictions about the light-curve evolution may be correct. In

contrast, single-vent activity (from a lone active zone) would be a warning

sign that the comet may not fulfill the more optimistic predictions, even

fading out before perihelion.



     Indirect evidence, such as the multiple similarities to comet 1811 I

(the orbit, very bright absolute magnitude, and activity at high

heliocentric distance), has been used (Marsden 1995) to suggest that comet

Hale-Bopp may be similarly spectacular near perihelion, although there is

little strong physical evidence that exists to support either of the

extreme scenarios (very bright or fizzle). Jet activity at high

heliocentric distance, though, is potentially a good indicator of the

intrinsic activity of the comet. No really bright object has been observed

since comet West in 1976, hence, the apparition of a potentially magnitude

zero (or brighter) comet, which will be well positioned to observe from the

northern hemisphere for several months around perihelion, is of great

interest for cometary physics. The fact that the comet is still 18 months

from perihelion allows detailed observing plans to be made. The advances in

astronomical instrumentation since 1986 will allow detailed spectroscopic

and morphological studies to be made that have never previously been

possible, especially if the comet is particularly bright.



     Reports were made soon after the discovery of unusual activity (Offut

1995) with a spiral coma developing and decaying. This has been interpreted

as outburst activity similar to comet P/Schwassmann-Wachmann 1 (Sekanina

1995a). Such activity allows, in principle, the rotation period of the

nucleus to be estimated from the change in position angle of the jet (for

gaseous events), or from the synchrone trajectory (for dusty events). To

date, very few comets have a really well-determined rotation curve, and,

even in the case of P/Halley (the best observed object), the presence of

both 50 hr and 7 day periods means that there is no real consensus as to

the exact mode of rotation and precession around the long and short axes of

the nucleus.



                              §2. OBSERVATIONS



     Regular observations of comet Hale-Bopp were started on August 10

using the CCD camera of the 82 cm IAC-80 telescope sited in Instituto de

Astrofísica de Canarias's Teide Observatory, Tenerife, Canary Islands,

Spain. A Thomson 1024 × 1024 chip was used, offering a field of nearly 7

[&farcm;]5. Standard BVRI broadband filters were used.



     Because of the movement of the comet and the inability of the

telescope at present to track differentially, comparatively short exposures

are taken (each of 300[–]400 s), which are then recentered on the

cometary nucleus and summed to give any desired total exposure. The

position of the nuclear condensation was measured using the imexamine

routine, and images were combined using the imcombine routine, both

included in the Image Reduction and Analysis Facility (IRAF) 1 environment.

Images were previously flat fielded using very high S/N master dome flat

fields obtained by combining many individual exposures.



     On some nights exposures were taken in all four filters to give color

information, but, on the eight nights to be discussed here, many exposures

were taken in a single filter, with the aim of combining them into a very

deep image in a single band. On discovering the jet, our observing program

switched to intensive monitoring in a single band on each night, to follow

the jet evolution with time. The observing log for the eight nights in

question is given in Table 1.



FOOTNOTES



     1 IRAF is distributed by the National Optical Astronomy Observatories,

which is operated by the Association of Universities for Research in

Astronomy, Inc. (AURA), under cooperative agreement with the National

Science Foundation.



                                §3. RESULTS



     On-line visual inspection of the images from August 28 revealed an

unusual jet emanating from the nucleus in P.A. = 280°. This jet wrapped

around the nucleus to P.A. = 030° approximately. The jet was also detected

by Jewitt & Chen (1995) some 9 hr after the start of observations from

Teide Observatory. On inspection of images from previous nights, the jet

was found to be very obviously present when the images were scaled

logarithmically to show the central condensation, rather than being scaled

to show the extended coma.



     On August 25, the jet was significantly less extended in position

angle than on August 28, being clearly detected only to P.A. = 000°

approximately. The observations on August 27 were taken through an

occasionally dense cirrus cloud, which much reduced their quality. Even so,

the jet can be clearly traced from the nuclear condensation to P.A. = 020°

approximately, rather less than the observed extension on August 28, but

consistent with the poorer conditions.



     To investigate the possible rotation of the jet, the images from each

night were grouped to give high S/N master frames. The images from August

25 and 28 were split initially into three sets of seven or eight frames,

recentered and combined. All the usable images from August 27 were combined

into a single frame. This frame was first smoothed slightly with a low-pass

Gaussian filter, and then a Laplacian filter was applied, leaving just the

jet and inner part of the central condensation visible. The combined and

recentered frames were then converted to MPEG format and animated (not

shown here, but available at http://www.ll.iac.es/general/index.html) to

show the evolution of the jet visually and dynamically. For the purposes of

this paper, though, all images from a single night have been combined,

given that we can rule out the existence of a significant rotation within a

single observing run. Figures 1(a)[–]1(e) (Plates L21[–]L25)

show the final reduced images for the nights when the jet is most clearly

seen. From September 4, the visibility was greatly reduced, partly by the

reduction in surface brightness and partly by the proximity of the Moon.

The jet is seen to have a three-part structure: there is an initial narrow

straight jet of material [∼]7[&arcsec;] long and gradually increasing

with date, leaving the nucleus in P.A. [∼] 280°. This straight section

appears to be highly collimated and has negligible curvature. This we refer

to as [“]the collimated jet.[”] This section abruptly changes

direction by 90° and opens out at a comparatively narrow opening angle

before starting to sweep round to the east and opening out further. Similar

behavior was reported by West (1995), who also observed the jet on several

nights, confirming the position angle of the collimated jet and its

constancy.



[Image] [Image] [Image] [Image] [Image] Fig. 1



     Considerable differences are seen in the structure of the jet between

August 25 and 31. Apart from the extent of the jet in position angle, it is

seen to be wider and much brighter on the former date. The initial

collimated section of the jet increases slowly in length as it fades. We

cannot rule out, though, that there is a small oscillation in position

angle, although this appears to be less than [∼]15° and of indefinite

period. There is, though, no significant rotation of the jet on timescales

of either a few hours or a few days. We also note that the position angle

given by Jewitt & Chen (1995), observing from 7[–]9 hr after us on

August 26, was also 280°.



     The end of the jet increases its distance considerably from the

nucleus, giving the false impression of rotation because it is

[“]unwinding.[”] On August 31 and September 4, the trend of a

gradual fade and pronounced increase in distance of the [“]spiral arm

[”] from the nucleus continues. After September 4, precise

measurements of the jet are extremely difficult due to its faintness. On

various nights, the data taken were nonphotometric or of dubious

photometric quality. This makes it difficult for us to quantify the rate of

fade of the jet, a potentially powerful diagnostic tool of its composition.



     We are unable to say exactly when the jet appeared. There is no sign

of it in the images from August 15, which we include for comparison to show

that there are no important artifacts created by our reduction procedure.

From the rate of growth of the jet, we estimate that it took several days

before our first detection. Various reports on the Internet from reliable

visual observers speak of a sharp brightening of the nuclear condensation

of the comet around August 20, consistent with the initiation of an

outburst. Figure 2 shows the growth of the linear section of the jet during

the observations. A highly linear expansion is seen, with a projected

velocity of 32 m s-1, which cuts the x-axis at -7.69 days (August 17.31);

although there is no strong reason why this should be the actual date of

initiation of the structure.



[Image] Fig. 2



                               §4. DISCUSSION



     The most popular explanation presented to date is that the jet is a

pure dust event, caused probably by CO sublimation, and that the curvature

reflects synchrone trajectories of grains of very different sizes. If the

jet is caused primarily by dust (or ice) and neutral gas ejection, no

rotation in position angle would be seen, although the morphology of the

jet would reveal the rotation period and axial inclination. This

explanation is favored by various authors (e.g., Sekanina 1995b, c).



     To obtain a good fit to the jet morphology, some very tight

constraints are made on models. It is necessary to suppose that the event

was caused by the combination of synchronized venting or two independent

orifices. A small time delay between the initiation of venting from the

first and the second orifice, combined with perspective effects, can

reproduce both the highly collimated beam and the spiral structure at the

end of it. In this model, one orifice causes the collimated beam and the

second the spiral structure. Support for a dust model is given by the fact

that the velocity of expansion is very much lower than the gas velocity for

CO expulsion ([∼]30 m s-1 against [∼]1000 m s-1), although the true

velocity may be significantly higher if we are looking along the jet.



     Given the observed timescale of jet events (approximately one per

month), it is statistically implausible that two independent venting

episodes would be triggered nearly simultaneously. The fact that a later

jet has produced a somewhat similar morphology with a position angle close

to 000° makes us reluctant to accept this model at present, despite its

obvious attractions. A further problem that has yet to be fully addressed

is whether the venting is a single instantaneous event (see below) or a

continuous emission over a number of days; significant difficulties with

the fit are found if a long duration of emission is assumed. A

long-duration event, though, is more in accord with the thermal triggering

mechanism and long rotation period that have been proposed to explain the

venting (Sekanina 1995d).



     An alternative method (Shulman 1995, private communication) proposes

to explain the jet in terms of an invisible gas beam carrying visible dust

within it. The jet is seen as a two-dimensional projection of an Archimedes

spiral. This method does not require synchrone trajectories, thus removing

one potential difficulty, although it is similar in some respects to the

model proposed by Sekanina. An important difference is that this model

assumes a single emission event of very short duration, thus avoiding some

of the morphological difficulties. No specific triggering mechanism is

assumed, although thermal triggering is felt to be unlikely. The model is

proving to be promising in its results and, contrary to the synchrone

model, suggests that the different jets originate from different points on

the nucleus. However, it requires further development, given that some

aspects of the jets' development are still problematic at present,

particularly the derived ages of different parts of the jet.



     We have been struck by the peculiar morphology of the August jet

event, some aspects of which appear more consistent with a plasma event

than with pure dust emission. The jet shows a very narrow, highly

collimated section that expands away from the nucleus. This shows a 90°

break at a projected distance initially of 23,000 km, at which point the

material directs itself very precisely in the antisolar direction. This

could be due to a chance alignment, and it is also consistent with a

plasma-jet model. The ejected material proceeds outward until it reaches

the contact surface and is open to the influence of the solar wind. At this

point, solar wind pickup occurs and the position angle is abruptly changed

as it sweeps round the contact surface until it reaches a position angle

corresponding to the antisolar direction. The fact that the end of the jet

was very closely aligned with the antisolar direction 2 favors a plasma

model.



     Our data limit any possible position angle change in the jet to a

maximum of [∼]15°, which implies that, if the jet is caused by plasma,

it is located close to, but not at, the pole of the nucleus. This is

consistent with the slight jitter that is seen in the position angle

between the grouped integrations. This jitter is less easy to explain given

a dust-jet model.



     We find that the point of inflection, where the jet suddenly comes

under the influence of the solar wind, is at a projected distance

[∼]23,000[–]39,000 km from the nucleus, according to the date of

observation. This gives us an estimate of the projected distance of the

contact surface, where a local equilibrium exists between the pressure of

the solar wind and the gas pressure within the inner coma. The angular

distance of the point of inflection from the nucleus is seen to increase

with time. This is not a perspective effect, since the geocentric distance

was increasing slowly during the observations, but rather reflects what

would be a genuine increase in the radius of the contact surface (Table 2).

This we can understand if there was a significant increase in gas

production corresponding to the jet event, and if the contact surface

expands until reaching a new pressure equilibrium.



     Figure 2 shows the variation of the linear extent of the collimated

jet with time. Note that these are projected distances, and that the true

distances, and hence the derived velocity of expansion, may be much greater

if the viewing angle of the jet is not close to 90°. To make the plasma jet

model more plausible, we have to suppose that there is a significant

projection effect and that the length of the collimated jet is actually

significantly greater than 23,000 km; this would permit a significant

fraction of the molecules in the jet to become ionized, even if the density

of ions in the inner coma as a whole is rather low.



     An obvious difficulty with this model is the lack of visible ions in

the spectrum. The most likely species to be detected at high heliocentric

distance, because of its abundance and very strong lines, is CO+.

Observations in the submillimetric range have shown significant neutral CO

emission, with a production rate when no jet was active of [∼]1 tonne

s-1 (Matthews, Jewitt, & Senay 1995; Rauer et al. 1995), but no reports

have been made of the presence of CO+ lines in the spectrum. Other species,

though, may exist that do not have easily detectable lines. IUE

observations have established an upper limit to H2O production, although

this corresponds to 3 tonnes s-1. Since H2O is a high-temperature volatile,

it is unlikely to be more active than CO anyway. An alternative

low-temperature volatile is NH3; the NH[$\mathstrut{^{+}_{2}}$] line is a

well-known line in cometary spectra but is very weak and difficult to

detect, except in very high S/N spectra.



     Assuming that the jet is well described by a gaseous emission, which

is later photoionized, and that the emission is slow enough to permit the

coma to be in a quasi[–]steady state, the distance between the

nucleus and the point of inflection can, in theory, be used to make an

estimate of the total gas production rate. This assumes a model suggested

by Schmidt & Wegmann (1982). Various difficulties are found that obviate

the possibility of obtaining a firm numerical estimate, in particular, the

fact that only the projected distance of the point of inflection is known.



     This very simple model would give a rather high total production rate

compared with the measured production rate of CO or the upper limit to H2O.

Since the CO production rate was measured with a quiescent nucleus, it is

not impossible that at the peak of outburst the production rate could be 2

orders of magnitude higher than this quiescent level ([∼]100 tonnes

s-1). The observation of large variations in the total brightness of the

comet and morphological changes reported by visual and CCD observers (e.g.,

formation of an intense starlike nucleus) lends support to the idea of a

highly variable production rate.



     None of the three models that have been proposed are at this juncture

wholely satisfactory, and further work is needed on all of them. This means

that the plasma-jet model cannot be rejected simply because there is a

proved alternative explanation that renders it unnecessary.



FOOTNOTES



     2 Something similar is observed with the September jet-event and

probably with the October event, suggesting that this is not simply

coincidence, given the rather different morphologies, position angles, and

evolution that have been seen.



                              §5. CONCLUSIONS



     This paper presents a small subset of our data that covers 46 nights

of imaging to the last week of October. Work is progressing on detailed

modeling of the observations taken so far, including photometric

calibration and, where available, color information. A more detailed report

on our monitoring is being prepared (Kidger et al. 1996). We hope that

further analysis will allow us to differentiate more exactly between

models.



     We find that the jet observed in comet Hale-Bopp (1995 O1) between

1995 August 25 and September 7 shows a highly characteristic morphology and

evolution. Some aspects of this morphology and evolution are challenging to

dust-ejection models and may be more consistent with a plasma model. No

single model, though, is totally satisfactory, and we hope that the

observations reported here will open a debate on the various possible

models and their limitations. We stress that the observed morphology,

distances, and velocities reported are projected values only and may bear

no relation to the true situation. Observations of the comet are

continuing, and a detailed examination of the different events observed to

date may shed more light on their causes and the validity of the different

models.



                              ACKNOWLEDGMENTS



     The authors would like to thank the telescope operators at Teide

Observatory (Luis Chinarro, Angel Gómez, Luis Manadé, and Santiago López)

for their work in taking images of the comet during the extended monitoring

campaign, and Jesús Jiménez for making the telescope readily available to

us.



                                 REFERENCES



   * Hale, A., & Bopp, T. 1995, IAU Circ., No. 6187

   * Jewitt, D. C., & Chen, J. 1995, IAU Circ., No. 6216 First citation in

     article

   * Kidger, M. R. 1995, Earth Moon Planets, in press First citation in

     article

   * Kidger, M. R., et al. 1996, in preparation First citation in article

   * Marsden, B. G. 1995, IAU Circ., No. 6202 First citation in article

   * Matthews, H. E., Jewitt, D., & Senay, M. C. 1995, IAU Circ., No. 6234

     First citation in article

   * Offut, W. 1995, IAU Circ., No. 6194 First citation in article

   * Rauer, H., Despois, D., Moreno, R., & Paubert, G. 1995, IAU Circ., No.

     6236 First citation in article

   * Schmidt, H. U., & Wegmann, R. 1982, in Comets, ed. L. L. Wilkening

     (Tucson: Univ. Arizona Press), 538 First citation in article

   * Sekanina, Z. 1995a, IAU Circ., No. 6194 First citation in article

   * [—][—][—]. 1995b, IAU Circ., No. 6223 First citation

     in article

   * [—][—][—]. 1995c, IAU Circ., No. 6240 First citation

     in article

   * [—][—][—]. 1995d, IAU Circ., No. 6248 First citation

     in article

   * West, R. 1995, IAU Circ., No. 6226 First citation in article



                                  FIGURES



[Image] Full image (117kb) [Image] Full image (140kb) [Image] Full image

(140kb) [Image] Full image (138kb) [Image] Full image (130kb) | Discussion

in text



     Fig. 1.[—]Processed images of the near-nucleus region of comet

Hale-Bopp (1995 O1) from (a) August 15, (b) August 25, (c) August 27, (d)

August 28, and (e) August 31. Contours of the coma brightness have been

drawn only at distances well beyond the jet, to show that the outer coma

was very nearly circular at this time, despite the near-nucleus activity.

Contours are drawn at intervals from 1 [σ] to 5 [σ] of the sky

brightness. The direction of the projected cometary velocity vector (

[$\mathstrut{{\bmi v}}$]) and the antisolar direction (

[$\mathstrut{{\bmi r}}$]) are marked, along with the scale and orientation

of the figures.



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[Image] Full image (5kb) | Discussion in text



     Fig. 2.[—]Evolution of the length of the jet from the nucleus to

the point of inflection over the period covered by these observations. A

steady increase in length can be seen.



                                   TABLES



                                TABLE 1

              Observing Log for the Observations of the Jet



                                                       Total

                    Number of                         Exposure

   UT Date   Band     Images          UT Range           (s)      Notes



  Aug 15...    R        6       22:33[–]23:12     2400



  Aug 25...    R        21      21:19[–]23:47     7400



  Aug 27...    R        6       20:35[–]22:33     1620     Cirrus



  Aug 28...    B        23      21:06[–]23:21     6800     Cirrus



  Aug 31...    R        15      20:12[–]21:45     4500     Cirrus



  Sep 4...     R        42      20:51[–]23:00     5040



  Sep 5...     R        23      20:35[–]22:49     6900     Cirrus



  Sep 6...     R        24      20:24[–]22:50     7200



  Sep 7...     R        14      21:23[–]22:42     7200



  Sep 8...     R        23      20:31[–]22:44     6900



Image of typeset table (28kb) | Discussion in text

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                       TABLE 2

       Details of the Observations of the Jet a



                                       Linear Extent

    Date     UT Time   Angular Extent       (km)



  Aug 25...   21:19     5[&farcs;]4        24,500



  Aug 25...   22:13     5[&farcs;]0        22,500



  Aug 25...   23:11     5[&farcs;]4        24,500



  Aug 27...   20:34     5[&farcs;]8        26,500



  Aug 28...   20:56     7[&farcs;]3        33,400



  Aug 28...   21:48     7[&farcs;]1        32,400



  Aug 28...   22:44     7[&farcs;]3        33,400



  Aug 31...   21:01     8[&farcs;]6        39,300



  Sep 4...    21:56     11[&farcs;]2       51,400



     a Note.[—]The angular and linear extent refer to the distance

between the nucleus and the point of inflection where the jet is swept back

by the solar wind.



Image of typeset table (27kb) | Discussion in text

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