Photographic photometry of the CN and C3 comae of Comet Halley

Chin.Astron.Astrophys.12 (1988) 328-333 Act.Ast~on.Sin.-29 (198n 129-137

Pergamon Press plc. Printed in Great Britain 0275-1062/88$~0.00+.00

PHOTOGRAPHIC PHOTOMETRY OFTHEClANDC3CoMAE OFCWHALLFi HU Zhong-wei, SHEN Zhi-qiang, CHEN Yang, Department of Astronomy, Nanjing Universitp

ABSTRACT Two photographs of Comet Halley in CN were obtained in 1986 April 8 and 12, and two in Cz, on March 21 and May 5. Photometry led to mean brightness profiles of the CN and Co comae on these days. Using Haser's coma model, the scale-heights of these two molecules and of the parent nolecules were derived,

Received 1987 September 12

1.

INTRODUCTION

As a comet approaches the Sun, radiation pressure and wind from the Sun causes gas to evaporate from the surface of its nucleus, together with dust and ice particles to form an atmosphere - the coma. This is accompanied by a series of peculiar physical and chemical processes, [l,Zt. Narrow-band photometry of the coma can provide the distribution of the pertinent component and the parameters of the relevant processes r3,4,51. The return of comet Halley in 1985-86 provided a good opportunity for narrow-band photometry of its coma. In March-April 1986, our observing team went to a suburb of the city Sanya in Hainan Island and used a Schmidt-Cassegraintelescope plus interference filters to make a series of photographic observations of the comet. In this paper, we report on the work done on two plates each of the the CN and Co comae. We made photometric measurement of these plates, constructed isophote diagrams, hence derived mean brightness profiles. With the help of Haser's coma model [61, we calculated the scale heights of the CN and Co molecules and their parent molecules. This paper concludes with a discussion of the results obtained.

2.

4". A refractor (D 13 m, F/13) on the same mounting was used as a visual guide, and this had a filar micrometer in its eyepiece. Guiding was done on a nearby star. PO compensate the motion of the comet relative to the stars, we calculate beforehand, using the Halley ephemeris [71, the direction and speed of the relative motion, rotate the micrometer so as to line up the fixed wire along the motion and made the perpendicular movable wire move at the calculated speed (in the opposite direction) and keep the star on the intersection of the two wires during the observation, thus ensuring that the image of the comet is fixed on the plate. This is a good method of tracking in cometary photography. The tracking error can in general, be controlled to that of seeing (about 3"). For monochromatic observations of the coma, an interference filter was placed about 2.5 cm in front of the plate. The transmission curves of our CN and Co filters are shown in Fig. 1. The peak transmission wavelength, half-width of transmission band, and peak transmission are, for CN, 3865A, 160A, 17% and, for Co, 4065A, 115A, 40%. The plate was Kodak IIa-0, developer D-19, and each plate was impressed with a S-step light wedge. The observational data are summarized in TABLE 1, where the heliocentric distances R and geocenteric distances A were interpolations of the published ephemerides [71.

OBSERVED DATA

Our place of observation was at longitude 109"32' 7.7" E, latitude 18'12' 28.6" N, on top of a hill by the sea, at a height of 90.4m above sea-level. Our observing instrument was a 30/38 cm Schmidt-Cassegraintelescope, P/3.35, field

3.

PHOTOMETRY

Photometry of the above four Halley pIates was carried out with the Purple Mountain Observatory PDS (Type 1010 MS) microdensitometer,which had a measuring

329

CN and Cg in P/Halley

TABLE 1 PLATE NO.

MID-EXPOSURE

EXPOSURE

FILTER

RCAII)

A WJ)

45’

CN

1.2991

0.4229

IV 12.8361

20

CN

1.3601

0.4224

111 21.9153

16

C,

t.0220

0.7621

25

C,

1.2528

0.4496

1

IV

2 3 4

Fig, 1

Comet Halley Data Sumaary

IV

8#8603

5.8524

Transmissivity curves of filter CN {left) and Cs (right)

accuracy of -+O.OZin density measurement and one of *5 tirn in position. During the measurement, the size of the diaphragm was 20 pm X 20 pm, and the sampling interval and scanning interval were both taken to be 20 I.rm.The photographic density of the g-step wedge was measured under the same working conditions, and was automatically graphed. From this the characteristiccurves for the four plates were obtained. These are shown in Fig. 2. To determine the average frelative) brightness distribution of the coma, the measured densities were smoothed wtih a 7 x 'ifilter using a PDP 11/34 computer then, for a set of fixed density values, the isophote maps shown in Fig. 3 were printed out by the computer. The density values D in the maps are the values after deducting the mean sky background and these can be converted in relative brightness by means of the characteristic curve. Then, we note the distance of a given contour from the centre of the comet nucleus (the brightest point) in each of the four directions towards, away, and perpendicular to the Sun, take the mean of the four distances as the mean distance o of the given contour from the nucleus, and convert it into so nany km from

Fig. 2

Characteristiccurves of the four plates used

the known plate scale and the geocentric distance of the comet at the time and construct the mean relative brightness profile, log B - log p, shown in Fig. 4.

4.

SCALE HEIGHTS OF CN AND Cs GASES

The coma is mainly composed of neutral gases and dust, it can be divided into three regions [El: near nucleus region, (also called "luminosity nucleus" or "false nucleus"), radius about a few hundred km, where the density of gas and dust (including ice) is relatively large, and coupling, relatively strong; collision region, with an outer radius of about 2,000 km, where collisional processes are important; and

HU Zhong-wei et al.

330

outer region, which may extend as much as several hundred thousand km, but where the material density is very small, and the collisional processes unimportant. It is “optically thin”, and the gas fluoresces under the solar radiation. As a rough approximation, we assume the coma to be spherically symmetric and then we can resort to Haser’s theoretical model of coma IS] in an analysis of the mean brightness profile of the CN and Cs comae. Assuming the CN (Cs) molecule is a dissociation product (daughter molecule) of its parent molecule, its number density profile can then be written as n(r) = n(r<)(r
e-w

(1)

where r, is the radius of the nucleus, and Bc and p1 are the reciprocal scale heights of the daughter and parent molecules, respectively. The apparent brightness B(P) is proportional to the column density of the molecule CN (or Cs) along the line of sight; integrating (1) along the line of sight, we get the theoretical profile of the coma [8,91,

where the constant C depends on the fluorescence coefficient of the molecule and the two scale heights, but not on the projected distance p, and Kc(D) is the modified Bessel’s function of order zero of the second kind. For any given pair of (So, fi,), the right side of (2) can be calculated as a function of p, i.e., we can get a theoretical profile. Equation (2) shows that, for a suitable choice of the parameter values (So, St), the theoretical profile should differ from the observed one simply by a constant log C. Hence by allowing vertical displacement, we can find the best-fitting parameter values. Obviously, we an use directly the observed relative brightness profile, since any conversion factor converting the relative to absolute brightness can be absorbed in he constant C. The theoretical profile at the large end (large values of p) is mainly determined by Bc, and, at the small end, by the ratio B,/&. The best-fitting curves are shown in Fig. 5, and the best-fitting parameter values are given in TABLE2. For comparison of results, it is customary to reduce the scale height at heliocentric distance R to that corresponding to R = 1AU. By definition, scale height is the product of outward velocity V and mean life Z:

tv -

~0~0,

{Pi' -

VIICI.

(3)

If we take the relation between the outward velocity and the heliocentric distance for CN molecule to be general, then we have v a: R-O.’ and, by the definition of molecule life Z, [l] we have r a 3. Hence, we have &i’(R)/&‘( pi'{

R)/&i’(

IAU) - R’.‘, IAU) - RI.*.

These results enable us to reduce the above results to R = lAU, The reduced results are also given in TABLE2. The mean results are: for CN, &-’ Bl -’

(1AU) = 1.47 (kO.16) X lo5 km (1AU) = 2.64 (kO.08) x 104 km

for C3, &“

(1AU) = 1.42 (k1.00) (1AU) = 2.29 (f1.82)

81 -’

5.

x lo5 km X lo4 km

DISCUSSIONANDCONCLUSION

The above results were obtained under certain approximate conditions. The actual morphology of the coma of the comet Halley and the chemical processes therein are more complicated. A discussion follows. What passes through the narrow-band filter is not only the molecular emission, it contains also “contamination” by the continuous emission of the dust, but it is difficult to de-contaminate. In fact, the dust content in the coma of Halley is rather high 112). However, the spectral features of Hallev I131 indicate that the E?E’ * X’E’ (h 3883 A)-emission band by CN molecules is much stronger than the continuous emission by scattering by dust, and the A’&, -+ X1X,* (X 4060A) emission band by C? molecules is also much stronger than the continuum. Hence, what the filters transmit are mainly the emissions by the two molecules, at large projected distances the continuum is weaker still. Solar radiation pressure can cause a difference in the brightness distribution of the coma towards and away from the Sun. Combi and Delsemme [141 showed that by taking the mean distribution over these two directions, the effect of radiation pressure can be removed. In this paper, we havve added two more directions in the averaging, hopefully to get an even better representation of the mean profile. Although Haser’s coma model is a rough approxiaation, it is easy to calculate, and it at least represents the actual conditions of coma within a 50% accuracy, so it is As the actual comae are still widely used. very complicated, and especially as comet Halley shows conspicuous activities, the

CN and Cg in P/Halley

Fig.

3

Isophote

331

maps of comae CN and C3

present available, more complicated models still fail to represent the actual conditions: the scale heights obtained using these models are slightly larger than those from Haser’s model, 111. As a comparison* A’Hearn [IS] summarized the scale heights of coma gases at 1AU as follows:

whereas Kraanapolsky et al. 1113 found for comet Halley, from the spectral data obtained on board Vega 2 spacecraft, the following values:

for CN, So-‘: = 3 x 10’ km, = 2.2 x lo4 km; = 5.4 x lo4 km, for C3, $1: = lo3 km. Based on different data and methods, the results of this paper differ from these published values. The difference cannot be wholly attributed to errars, rather, they probably reflect the coma activity of comet In fact, different observations Halley. (e.g. Refs. [5,17J) have all revealed such activities. There has been some discussions 11,15,181 on the question of parent molecules for CN and C3, but the question is not yet settled. Possible parents for CN are HCN, CHsCN, HCsN, and CZN2. Lifetimes estimates based on experimental data are: at R = lAU, for HCN, 7.7 x 10’ s, for HCaN, 3.6 x IO4 s, and for

HlJ Zhong-wei et al.

33.2

kw (km) Fig.

4

Mean brightness

L

3.0

profiles

t

of comae CN and C3

,

5.5

4.0

1

4.5

kwp(km) Fig. 5

Fittings of observed brightness theoretical one (curves)

It is generally thought the main parent of CN ia HCN. HCNhas been observed in radio in Halley, so have HCsN and CH,CN, but their rate of production is less than that of HCN, 1191, which is

CH$N, 2 x 10” a.

profile

(dots)

with

somewhat greater than that of CN (19,201. From the scale height for the parent molecule of CN obtained here, and an adopted outward flow velocity of 0.6 km/s, we get a lifetime of Z = 4.4 x 10’ s; if we take a

CN and C3 in

TABLE 2

Scale

PLATE No.

lengths

of

P/Halley

CN and C3 and their

B;’ (km)

&IS.

333

parent

8i’ (km)

I CN

2.0X10‘

5

2 CN

2.5X10‘

6

3 Cs

2.2X10’

6

4 C,

1.0X10’

7

1.4X10’

@~‘(lAU)(km)

4.0X10’

somewhat larger scale height and smaller flow velocity then we can get a value close to that of HCN, thus supporting the general view, while not excluding the other possibilties. A’Hearn (211, from an analysis of the CN jets of comet Halley, suggests that CN is probably a dissociation product of dust particles; Krasnapolsky et al. [221 suggest that CN possibly has two parents. Obviously, to settle the question of parent of CN, we need to gather and analyse more

molecules &‘(lAU)(km)

1.35X10’

2.70~10’

4.1X10’

1.58X10’

2.58~10’

3.7X10’

2.13~10’

3.58X10’

0.71X10’

1.00~10’

data. It is even more difficult to ascertain the parent of C,; suggested candidates are: CH&H, HCJHN,, H$,H, and C4H2, [2], but reliable lifetime data are lacking. Summarizing, photometry of our narrow-band coma plates has yielded mean CN and C, profiles. With the help of Haser’s model, we arrived at the scale heights (reduced to 1AU) of these two molecules and their parents given at the end of Section 4. The parent molecule of CN could be HCN.

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