The dust environment of comet Levy 1990XX

Planet. Space Sci., Vol. 42, No. 4, pp. 263-268,

1994 Copyright 0 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0032-0633/94 $7.00+0.00

Pergamon 0032_0633(93)E0045-E

The dust environment of comet Levy 1990Xx G. Cremonese’ and M. Fulle’ ’ Osservatorio ’ Osservatorio

Astronomico, Astronomico,

Received for publication

Vicolo Osservatorio 5, I-35122 Padova, Via Tiepolo 11, I-341 3 1 Trieste, Italy

17 December

Italy

1993

Abstract. Two wide-field plates taken with the U.K.

Schmidt Telescope Unit of the perspective dust antitail of comet Levy 1990Xx are analysed by means of the inverse Monte-Carlo model of dust tails (Fulle M.. Astron. Astrophys. 217, 283, 1989). The model considers dust grains of diameters between 2 pm and 2 cm ejected from 5 months before perihelion to 3 months after. The observation geometry allows a very accurate estimate of the dust ejection velocity for the pre-perihelion times, which results to be 30110 m s-’ for grains of 0.1 mm diameter and strongly constrains coma models interpreting pre-perihelion observations. At perihelion the velocity increases to about 120 m s-‘, and then decreases to pre-perihelion values. The mass loss rate reaches a wide maximum of 3 x 10’ g s ’ 10 days before perihelion. Only a negligible fraction (lower than 2%) of the observed produced dust mass (4+ 1 lOI g) appears to be injected into bound orbits; however, such an estimate for the meteoroid contribution to the zodiacal cloud should be a crude lower limit, since the 1990Xx antitail mainly concerns quite small grains. Nevertheless, as for other already analysed new comets (in the Oort sense, C/1987VII. Cremonese. G. and Fulle, M., Astron. J. 100, 1285, 1990; CjlSUOV, Fulle, M. et al., Astron. Astruphys., in press. 1994), the time-averaged size distribution is dominated by large grains. with a power index of - 3.2 + 0.1.

Introduction

Comet Levy 1990Xx appeared as a bright and large object in the 1990 summer sky during its approach to both Earth and Sun. Such a passage was very favourable for extended observations of its well developed dust coma and tail. Several observations were collected during this period from the northern hemisphere. However, in order to study the dust environment of a comet by means of dust tail analysis, post-perihelion observations are far more useful, allowing one to study the dust production of a comet during both pre- and post-perihelion times and, overall, during the perihelion passage, when the comet has its largest activity. Therefore, we obtained observation time at the U.K. Schmidt Telescope Unit (Siding Spring) at the beginning of February 1991, about 4 months after the perihelion passage, when the Earth was very close to the comet orbital plane and the apparition of a bright NeckLine was predicted. The observations we collected are listed in Table 1, together with useful geometric parameters. The former two plates were devoted to the analysis of the Neck-Line (which is in progress), the latter two to the whole dust tail analysis, because the Earth cometocentric latitude was too high (I in Table 1) to allow a proper Neck-Line photometry (Fulle and Sedmak, 1988). In this paper we discuss the results of the inverse Monte-Carlo dust tail analysis, by means of the well tested

Table 1. Log of observations* Plate

Time UT

r

A

t(

n

Exp.

Size

h

14123 14146 14159 14158

9.625 13.650 18.625 22.656

1.983 2.033 2.095 2.146

1.149 1.155 1.181 1.217

20.1 16.9 13.7 12.1

0.7 3.4 6.5 8.8

60 60 60 60

3.4 3.4 3.4 3.4

1.2 1.0

*Plate, serial number February 1991. r, A, cometocentric latitude sampling step adopted images.

of the photographic plates (IIIa-F + RG630). Time Sun-Comet and Earth-Comet distances (AU). a, on the comet orbital plane (degrees). Exp., exposure for the plate digitalization (arcsec). h, normalization

UT, time of midexposure, phase angle (“). ?,, Earth time (min). Size, size of the factor of the reconstructed

264

G. Cremonese

and M. Fulle : Dust environment

of comet Levy 1990Xx

Table 2. Plate calibration* Plate

AB

Star

V

B-V

V-R

Is

1,

s li,H

14123

4565 4430 3750 5815 6000 6000 6000 7870 7680 6810 6090 5810 3750

s5ooc S500D S500E S500F S5OlC S50lD S50lE S566C S566D S566E S636C S636D S636E

10.68 12.24 13.16 14.26 1 I .52

I .23 0.65 0.75 0.60 0.24 0.49 0.93 I .oo 0.82 I .32 1.07 0.48 0.91

1.07 0.54 0.59 0.51 0.24 0.44 0.75 0.81 0.64 1.17 0.91 0.44 0.73

4358 649 257 116 369 329 229 x99 351 217 687 392 222

734 716 631 927 342 343 344 488 467 421 452 427 275

685 675 796 562 627 655 772 991 710 785 1233 1077 744

14146

14159

14168

II.80 12.33 10.70 Il.89 12.85 11.05 Il.32 12.63

RB 20.7 F0.2

20.7 * 0. I

20.5 10.2

20.3 * 0.2

*Plate, Serial number of the plate. As, sky area covering the star trails and the sky background (arcscc’). Star, calibration star of V magnitude and B-V colour index (GSPC-I ; Lasker et N/., 1988). V-R, colour index corresponding to B- V (Johnson, 1966). I, and IB, light intensity (integrated over A,,) of the star and of the background (arbitrary units). SIOR, background surface light intensity expressed in IO R-mag stars per square degree. R,, background light intensity expressed in R-mag arcsec ‘.

model developed by Fulle (I 989) [for a detailed discussion see also Fulle et al. (1992)]. A common source of concern about inverse dust tail models is their dependence on many free parameters, which allow easy fitting of the data, but create uncertainty on the significance of the best fitting parameters. However, a real comet is surely a multi-parameter object, and inverse tail models consider a subset of such parameters. Several applications of the model have shown that all the free parameters considered have significant influence on the shape and brightness of the tail (they are all first order parameters). Therefore, simpler models, which consider less free parameters, surely provide less significant solu-

Table 3. Parameters

tions, because it is impossible to know if other free first order parameters, not considered by the model, would change those considered. This fact implies that we should search for more complicated models which consider all the first order parameters. In such a complicated (but realistic) situation, we can only search for the most probable parameters best fitting the data. Therefore, the parameters provided by the inverse tail model have a probabilistic significance : if the comet activity has a smooth behaviour over a long period, then the solutions we propose are the most probable ones, i. e. the smoothest ones best fitting the data. The emulsion-filter combination provides a passband

of the models of comet Levy 1990Xx*

. i/h

1 ‘,,

/ f ‘,

N,

N,,

N,

IV,,

N,\

T

x2

. //

(%)

s

284

100

200

20

IO

2

30

30

15

3091

5.1

3

A

45” 90 180 90 45

382 143 284 143 382

100 100 100 100

200 200 200 200

20 20 20 20

IO IO IO IO

2 2 2 2

30 30 30 30

30 30 30 30

IX 31 I4 I9 I3

2864 3357 3657 4153 4402

3.2 4.8 4.2 4.2 3.1

02 2 2 3

t

I80 90 45‘

2X4 143 382

100 100 100

200 200 200

20 20 20

IO IO IO

2 2 2

30 30 30

30 30 30

13 26 15

4824 3029 3849

5.2 4.1 2.6

; 0 0

u

b1

./I ‘\

-l/6

I80

- l/6 ~ - l/4 - l/4 -l/4 - I,/2 -l/2 -l/2

_I

X

0 *

*U = (? log ~(t, d)/Z log d). W, half width of the dust ejection cone : hemispherical ejection (half width of n/,2) and strongly anisotropic ejection (half width of n/4). ..Z‘,, .,1 ‘,<, 1‘,, dust samples on the dust shells, in the modified size and in time. N,, N,,, samples of the solution in time and in the modified size. N,&,, N,, samples of the N, source images in the M and N directions. T. number of test function t’(r). x2, quality of the image fit expressed in S,,) units. .N, total ejected dust mass (IO” g) for Ap(r) = 0.05. N,,. total mass of meteoroids injected into bound orbits (percentage of the total mass N). S. symbol in Figs 4 and 5.

G. Cremonese

and M. Fulle : Dust environment

of comet Levy 1990Xx

(60 nm centred at 660 nm) very close to the R-photometric system [95 nm centred on 680 nm (Thuan and Gunn,

1976)]. All the plates were digitized by means of the PDS machine of the Padova Astronomical Observatory with a square sampling window of 50 pm size. With the same procedure we digitized the calibration spots available on the plates and standard fields of the first edition of the Guide Star Photometric Catalogue (GSPC-I; Lasker et uf.. 1988). Then we linearized the plates by means of cubic polynomials best fitting the intensity-densrty relation, and calibrated the plates absolutely (Table 2) following the procedure described by Cremonese and Fulle (1990). Due to the length of the star trails, the photometry accuracy was lower than 0.2 msg. Systematic errors of the plate catibration should not exceed such a value, as pointed out by the normalization factor h (Table 2) of the recon-

265

strutted dust tails from the model results : the systematic error is close to 20%, corresponding to 0.2 mag. Such an error is lower than the intrinsic error affecting the solution of the inversion of the dust tail functional. which is always close to 50%.

Conclusions

Table 3 lists the adopted parameters of the dust tail model best fitting the C/1990Xx dust tail. Figures 1 and 2 show the observed tail isophotes, dominated by a well developed perspectical antitail, the photometric axis of which is the Neck-Line locus. Figure 3 shows the synchrony-.syndyne

Fig. 1. Observed (continuous lines) and computed (dashed lines) isophotes of the dust tail of comet Levy 1990Xx from plate 14159 for the intensity levels S,,, = 80, 190, 350, 680 and 2000 (number of 10 R-mag stars per square degree). The distances along the axes are expressed in 10’ km units ; u = (3 log c(t. cl)/? log fi), )I‘is the anisotropy parameter (Table 3)

266

G. Cremonese

and M. Fuk

: Dust enviranment

of comet Levy 1990Xx

0

-1

-2

0

Fig. 2. Observed (continuous linep) and computed (dashed lines} isophotes of the dust tail of comet Levy 1990xX from plate 14168 for the intensity kvels S,,, = 80, 190, 350, 680 and 2000 (number of 10 R-mag star:4 per square degree). The distances along the axes WC expressed in IV km units: u = (a fog I.(,&&‘$ Fog if), w is the anisotropy garamet~r (Table 3)

G. Cremonese

and M. Fulle : Dust environment

-150

261

of comet Levy 1990Xx

-1.00

-0.50

0.00

N

Fig. 3. Synchrones (dashed lines) and syndynes (continuous lines) related to plate 14168. The distances along the axes are expressed in 1Ohkm units. Each synchrone is labelled with the ejection time (days related to perihelion), whereas each syndyne is labelled with the diameter of the dust grain expressed in mm (for an assumed dust bulk density of I g cm ~’ the sizes are inversely proportional to the

density)

related to plate 14168. It allows us to conclude that the antitail of C/1990Xx concerns quite small grains [much smaller than, for instance, in the case of Cj199OV (Fulle et al., in press, 1994)]. The fits of the dust tail do not show any dependence on the size dependence power index of the dust ejection velocity II = (L7log ~(t, L/)/I? log d), or on the anisotrophy of dust ejection ~7. This fact is confirmed by the x2 values listed in Table 3. which are not sensitive to particular u or II values. In any case, for the sake of simplicity, we selected from the nine fits those with the best x’ values, and for these we show the results in Figs 4 and 5. As shown in Fig. 3, the antitail of C/1990Xx was built up by the dust ejected during pre-perihelion times : since the Earth was very close to the comet orbital plane. the width of such an antitail strongly constrains the dust ejection values, the estimate of which is therefore very accurate for pre-perihelion times. Figure 4 shows that this velocity was 30+ 10 m SK’ for dust sizes of about 0.1 mm: this result strongly constrains all the coma models interpreting pre-perihelion observations. Around peridiagram

helion, C/1990Xx ejected dust at much higher velocities, up to 120 m SK’. The mass loss rate was always high. reaching a wide maximum of 3 x 10’ g s’, 10 days before perihelion. The mass loss rates were computed with an albedo times the phase function of 0.05, following the measures of Hanner et al. (1990) for other new comets (in the Oort sense). Following other new comets already analysed with the same model [C,i1987Vll (Cremonese and Fulle, 1990), 1990V (Fulle rt ul., in press, 1994)], C/1990Xx is also characterized by a time-averaged size distribution dominated by large grains, with a power index of -3.2fO.l (Fig. 5). However, since the observations do not allow us to directly analyse the large grains, C/1990Xx does not seem to have produced an appreciable quantity of meteoroids in bound orbits (Table 3). This fact may confirm that an accurate estimate of the dust contribution to the zodiacal cloud from comets in near parabolic orbits requires the analysis of several samples, the meteoroid contribution being strongly dependent on the observed dust sizes and on the dust ejection velocities from the inner coma.

268

G. Cremonese

and M. Fulle : Dust environment

of comet Levy 1990Xx

0

-o_oooooo & 2 ’ k+oooo”ooooo

C-

ooooo

Tim. E ’ 8W .^ 2

1

- iz’P-J_ 000

0,

u

“.L_

oooooooooOO OO

0

000 1.2

‘I‘ *

-152 -68 -32 -9.9 time (days related

9.9 to

32

68

ir , -152

time

perihelion)

-68

(hays

-32

-9.9

related

9.9

32

60

to periheiion)

Fig. 4. Dust environment of comet Levy 1990Xx. The diagram shows the dust loss rates [for an assumed albedo times the phase function, Ap(r) = 0.05, r = 13 , the loss rates are inversely proportional to the albedo], the dust ejection velocity and the power index of the time-dependent size distribution. The symbols are related to Table 3. The diameter interval to which all the solutions are related (for an assumed dust bulk density of 1 g cm..“, the sizes are inversely proportional to the dust density) is also shown

References

nn

m

t

5 yt, -5

-4

log dust

,

,

-3

-2

diameter

-I

,

,j

-1

0

(cm)

Fig. 5. Time-averaged size distribution assuming a dust bulk density of 1 g cm-’ (the sizes are inversely proportional to the assumed dust bulk density). The power index of the distribution of comet Levy 1990Xx is -3.2kO.l. The symbols are related to Table 3

Cremonese, G. and F&e, M., Astron. J. 100, 1285, 1990. Fulle, M., Asrron. Astrophys. 217, 283, 1989. Fulle, M. and Sedmak, G., Icarus 74, 383, 1988. Fulle, M., Cremonese, G., Jockers, K. and Rauer, H., Astron. Astrophys. 253, 615, 1992. Fulle, M., Bosio, S., Cremonese, G., Cristaldi, S., Lifter, W. and Pansecchi, L., /Istron. Astrophys. (in press), 1994. Hanner, M. S., Newburn, R. L. Jr., Gehrz R. D., Harrison T., Ney E. P. and Hayward T. L., Ap. J. 348, 3 12, 1990. Johnson, H. L., ARAA 4, 193, 1966. Lasker, B. M., Sturch, C. R., Lopez, C., Mallama, A. D., McLaughlin, S. F., Russell, J. L., Wisniewsky, W. Z., Giitespie, B. A., Jenkner, H., Siciliano, E. If., Kenny, II., Baumert, J. H., Goldberg, A. M., Henry, G. W., Kemper, E. and Siegel, M. J., ApJSS 68, 1, 1988. Thuan, T. X. and Gunn, J. E., PASP 88, 543, 1976.