Adv. Space Res. Vol. 5, No. 12, pp.317—323, 1985 Printed in Great Britain. All rights reserved.
0273—1177/85 $0.00 + .50 Copyright © COSPAR
GROUND-BASED OBSERVATIONS OF THE DUST EMISSION FROM COMET HALLEY * P. Lamy Max-Planck-lnstitut fur Kernphysik, 6900 Heidelberg, F.R. G. Laboratoire d’Astronomie Spatiale CNRS, 13012 Marseille, France
ABSThACT A preliminary analysts of the dust emIssion from comet Halley is presented based on large scale observat tons of Its dust tail. Selected Images obtained between February 22 and May 10, 1986 are compared to synchrone—syndyne graphs to Infer the hIstory of the dust product ion and the properties of the dust, at least qualitatively. Quantitative modeling of the dust tail has also been initiated and preliminary results are shown for the cases of isotropic and anisotropic (jet) dust production. I WI~RODUCTION The contribution of the analysts of dust tails to the knowledge of cometary dust has long been recognized; in fact, it was the only mean of obtaining the size distribution function before the developments of infra—red observations. Even with the advance of new technics and, in the case of come Halley, with the availibility of in—situ observations, it retains its own advantages and may set useful constraints on dust models. The manifestation of large—scale phenomena of comet Halley began in early December 1985 with the detection of a gaz tail extending over 2°/1, 2/. Spectacular developments occured In the first half of January 1986 but the dust tail remained absent during the pre—perihellon period as expected /3/. Starting February 19.4, 1998, substantial dust developments were observed (ESO, La Silla) by Celnik et a!. /4/ and Pedersen et 81. /5/, including an anti—tail. The analysis presented below is based, to a large extent, on post—perihelion observations with the ESO wide—field C~Dcamera (WFCC). This instrument was set up by H. Pedersen and R. West and operated with the assistance of Fl. Vio and B. Gelly. It consists of a cooled, large—format C~D (640 x 1024 pixels) behind a Canon f/2.B, 100 mm lense /6/. The combination of high sensitivity, wide field (5.5 x 8°), hIgh resolution (1 pixel = 31 arc sec) and fine weather conditions over a long period allowed a continuous monitoring of the tail of comet Halley over 2.5 months of exceptional quality, which is probably unique in the history of cometary observations. Although the instrument was prImarily devised to study the C0 tail, broad—band observations of the dust were regularly obtained (including B, V, H, I with polarizor). The analysis of many large—format images requires considerable time as one can Imagine. Although many tools were prepared in advance (image processing, synchrone—syndyne graphs, photometric modeling), the results obtained so far and presented below should be considered preliminary. As the selected examples will show, the Information which can be extracted from a cometary tail very much depend upon its shape /3/. This shape results from both the level of dust production and the viewing geometric conditions (spreading of the tail and separation from the plasma tail). To these “intrinsec conditions are superimposed observational constraints such as the interferences of the Sun (e.g., twilight), the Moon and of the sky background (e.g., Zodiacal Light, Milky Way). Altogether, the observing conditions for the dust tail of comet Halley were quite favorable, the main problem coming from the interference of the Southern Milky WayCearly April) and the rich stellar bakcground. *
Based on observations made at European Southern Observatory
QUALITATIVE N~ALYSISOF THE DUST TAIL The following analysis is based on the comparison of selected images of comet Halley (Table 1) with calculated synchrones and syndynes under the assumption of zero terminal velocity /7/. The synchrones are labeled in days referred to the date of observation (t; the one corresponding to an emission at perihelion is further noted “P), and the syndynes, with the dimensionless parameter ~ which is the ratio of the radiation pressure force to the solar attraction. February 22.4, 1988 This image (Fig. 1) is a composite of six frames (total exposure time 9 mm) obtained with the ESO—WFCC in the spectral range 500—1100 nm. This is probably the most spectacular view of the multiple dust tails of comet Halley. The various components are all of synchronic nature and therefore illustrate the activity of the nucleus (that is, the time evolution of the dust production rate) as successive outbursts and “quiet” periods, the characteristic time of each component is taken from the synchrone which fits the profile of maximum brightness. This method deserves several remarks which apply as well to the other observations. Although a basic filtering has been applied in order to remove the stars (and increase the signal over noise ratio), this operation has not been totally satisfactory. The brightness distrIbution of the tail may therefore be distorted, introducing errors in the determInation of the time. The broadness of the components, which reflects the duration of the outbursts (as well as the dispersion in terminal velocities), and which tends to increase with their age also limits the accuracy of the method. Finally, the spatial resolution of the synchrones decreases with increasing age (they tend to pile up). In the present case, the accuracy decreases from approximately 0.2 day at x ~ 10 to 0.5 dat at ~ > 20. The times I of major dust emissIons (in days referred to perihelion) are given In Table 2. The broad component centered at T = — 13 days extends over some 12 days and may be considered as an anti—tall; its trailing edge is difficult to date since it is not strictly synchromic (probable influence of the terminal velocity) but a value of at least 100 days before peihelion is likely. The youngest component (1 = + 2.5) has a length of about 1.7°; the corresponding ~ exceeds 2.5, a fact which points to the presence of both absorbing and dielectric materials /8/. February 22.81, 1986 Almost as spectacular is the plate obtained by K.S. Hussel with the 1.2 mU.K. Schmidt telescope near Coonabarabran, Australia. I did not have direct access to this image, but its reproduction on the cover page of Sky and Telescope (May, 1986) is of sufficient quality. The time difference with the previous observation Is only about 10 hours but the gross appearance of the tail is different, probably a consequence of the highly contrasted Schaidt image (and also, its scale and resolution); in particular, the broad, fan—shaped anti—tail is not present. However, the timing of the major outbursts is in excellent agreement with the prevIous observation (Table 2). Two exceptions need further comment. I) The component T = — 0.6 day, conspicuous on the Schmidt plate, cannot be really dstinguished on the WFCC image but, as the latter indicates copious dust production, there is basically no contradiction. ii) The well—defined spike on the Schaidt plate corresponds to an emission time of the order T = — 110 days is only suspected on the inner isophotes of the WFCC image. The different resolution may explain these differences which may also be clarified by further image processing and a more refined analysis. It should be noted that the oldest synchrones become confused with syndynes and the above spike may well be viewed as a syndynic formation corresponding to ~ = 0.005. The dust tail extends to about 1°. Values of ~ in excess of I are reached supporting the above conclusion for the composition of the grains. March 8.4, 1986 This blue plate (Fig. 2) obtained with the 1. a ESO Schmidt telescope at La Silla, Chile (Fig. 2), still show several synchronic components whose times are given in Table 2. Except for a slight discrepancy at I = 0. (to be clarified), there is an excellent agreement with the events of the previous observations. The additional event at I = + 9 days indicates that the emission of dust remained copious after perihelion. A tiny spike (anti—tail) is suspected at T = — 80 days but requires further examination. The maximum extent of the tail is about 3.2°.
Ground—Based Observations of Dust Emission
TABLE I Parameters of the selected observations. The dates (days) in brackets are referred to perihelion. WFCC indIcates the ESO wide—field C~Dcamera used with a f = 100 an or a f = 50 an lenses. V and H are the filters of the Johnson system. Date UT 1986 Feb Feb Mar Apr Apr Apr Nay
22.4(12.95) 22.81 (13.36) 8.4 (26.95) 12.27(61.81) 18.28(67.83) 29.05(79.9) 9.99 (89.54)
0.652 0.656 0.821 1.352 1.443 1.605 1.765
ACAU) 1.394 1.387 1.100 0.420 0.496 0.746 1.082
WFCC—100 UK—Schmidt ESO—Schmidt WFCC—100 WFCC-100 WFCC-100 L.PCC—50
9 2 30 4 6 6 10
500-1100 nm 7 IIaO 4 615385 6 6 V V
1. The tail of comet Halley on February 22.4, 1986. Synchrones in solid lines, syndynes in broken lines. Fig.
1 Fig.2. The tail of comet Halley on March 8.4, 1986
April 12.266, 1986 This image (Fig. 3) obtaIned with the ESD—i.FCC with a H filter (Johnson system) shows a broad, featureless dust tail. At this time, the comet was close to the Milky Way and some contrast may have been lost because of the still large background. The dust emitted between some 20 days before and 55 days after perihelion is displayed in this image, extending over more than 70~ The fairly uniform brightness over a sector angle of approximately 90° indicates a sustained dust production over a period of about two months after perihelion. April
With the comet moving away from the Milky Way, contrasts improve as shown on this red image obtained with the ESO—IFCC (Fig. 4). It is possible to distinguish two broad components, the first one corresponding to the group of emissions which took place near perihelion, the second one roughly centered at I = + 30 days. Note that the projection conditions have evolved as the Earth approaches the orbital plane of the comet. The sector angle decreases as the synchrones are getting compressed (note that they also are getting aligned with the syndynes); this also lead to an improved visibility of the tail which is now detected over 9°. April 29.052, 1986 The above trends are seen to accelerate on this green image (Johnson V filter) obtaIned with the ESO—WFCC (Fig. 5). The two broad components are still visible in the tail which extends over 9°. Nay 9.991, 1986 This is one of the final images obtained with the ES0—WFCC Cf = 50 mm, V filter) 10 days before the Earth crossed the orbital plane of the comet (Fig. 6). Its cometocentric latitude is only 2.5° approximately. The synchrones are highly compressed and almost indistinguishable from the syndynes. The tail is seen almost edge—on, a fact which considerably enhances its visibility in spite of the large distances from both the Sun and the Earth; its length probably exceeds the 13°measured on this image.
TImes (in days referred to perihelion) of major dust outbursts. The column “Periodicity” give multiples of the rotation period of the nucleus
+ — — — —
2.5 4. 6.3 10.2 13
Feb. 22.81 2.4 —0.6 — 4.1 — 6.6 — 10.1 — 13.1 +
+9 + 2.5
0. — — — —
4.2 6.3 10. 13.
0 — — — —
4.3 6.5 10.8 13.
PHI3TOI’ETHIC MODELS OF THE DUST TAIL Preliminary quantitative analysis of the dust tail of Comet Halley has been started in collaboration with K. Jockers (IlPlAe) using the method developped by LIu and Kimura /9/. As it is purely numerical, it Is much more flexible than the Finson and Probstein technIque /7/; in particular, anisotropic dust emissions from the nucleus can be introduced. The examples presented here were calculated for April 11, 1986 (close to the April 12.266 observation), for two different size distributions and for both isotropic and anisotropic dust emissions. The anisotropic case attempts to represent the sunward jets observed by Vega 2 and Giotto. This is probably not completely satisfactory as the dust emission pattern may well be different on the other hemisphere. Nevertheless the trend reflected in the shape of the dust tail is quite interesting as shown in Fig. 7. It Is of course premature to draw any conclusion at this stage, but clearly, investigating the inter—relationship between the dust emission pattern on the nucleus and the shape of the tail looks very exciting.
Ground—Based Observations of Dust Emission
Fig. 3. The tail of comet Halley on April 12.266, 1986
rig. 4. The tail of comet Halley on April 18.278 1986
I 110 .L
rig. 5. The tail of comet Halley on April 29.052, 1986
Fig. 6. The tail of comet Halley on Hay 9.991, 1986
Fig. 7. Photometric models of the dust tail of comet Halley for April 11, 1986 for two different size distributions and for isotropic and aniso— tropic dust emission.
Ground—Based Observations of Dust Emission
CON~LUSION From the early post—perihelion observations of the dust tail of comet Halley, it appears a consistent pattern of major dust outbursts whose chronology is summarized in Table 2. The question naturally arises as to wether the rotation period of 52 hours (~ 2.2 days) determined by Sekanina /10/, is reflected in this chronology. The last column of Table 2 gives the multiples of the period which are close to the recorded times. The agreement is quite impressive except for the events at T ~ — 10 days. However, the behaviour is strictly not periodic since several events are missing Ca more refined analysis may reveal them). Therefore, the dust production rate may be viewed as having an erratic” behaviour (as also indicated by IUE observations /11/) modulated by the rotation of the nucleus. Of course, on a longer time—scale, it is also a function of the heliocentric distance. The onset of substantial dust production as determined from on the Feb. 22.81 observation, may be considered has having perihelion, the comet being then at a heliocentric distance the value assumed by Sekanina for predicting the brightness
the oldest component, the spike taken place some 110 days before of 2 AU. This was, by the way, profiles of the dust tail /3/.
Finally, the values of ~ coming from the syndynes reach 2.5. As already pointed out, this tends to support the presence of an absorbing material in addition to silicates which have been detected by their 10 and 20 ~imspectral signatures /12/. A~19JOWLED6EMENTS I am very much indebted to H. Pedersen (European Southern Observatory) for access to the VWCC images and for reduction programs and to his dedicated collaborators, H. Vio and B. Gelly. The preparation of this work would have been impossible without the help of P. Malburet (Laboratoire d’Astronomie Spatiale). I thank K. Jockers (Max Planck Institut Für Aeronomie) for producing the display of the photometric models In a very short time. Image processing was performed at European Southern Observatory with the assistance of P. Angebault and at Laboratoire d’Astronomie Spatiale with that of A. Llebaria. I thank C. Nitschelm (Institut d’Astrophysique) for his ephemeris program. HEFEREN~ES P. Law, and S. Koutchay, INJ Circular N°4148 (1985) D. Jewitt, K. Mcccli, andG. Flicker, IAU Circular N°4148 (1985) Z. Sekanina, The Comet Halley Handbook, 2nd ed. (1985) W. Celnik, H. Scbulz, and K. Weissbauer, IAU Circulars n°4179 and 4183 (1986) H. Pedersen, B. Gelly, and H. West, IAU Circulars N°4179 and 4183 (1986) H.M. West, H. Pedersen, P. Monderen, H. Vio, and P. Grosbl, Nature 321, 363—365 (1986) M. Finson, and Fl. Probstein, Astrophys. J. 154, 327—352 (1968) J. Burns, P. Lamy, and S. Soter, Icarus 40, 1—48 (1979) C. Liu, and H. Kimura, Chim. Astron. Astrophys. 7, 11—18 (1983) Z. Sekanina, IAU Circular n” 4151 (1985) M. Festou, P. Feldman, N. A’Hearn, C. Arpigny, C. Cosmovici, A.C. flanks, L.A. McFadden, H. Gilmozzi, P. Patriarchi, G. Tozzi, N. Wallis and H. Weaver, Nature 321, 361—363 (1986) 12. 6. Gehrz, IAU Circular N°4179 (1986)
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