Ultraviolet spectrophotometry of comet Giacobini-Zinner during the ICE encounter

ICARUS 69, 32%337 (1987)

Ultraviolet Spectrophotometry of Comet Giacobini-Zinner during the ICE Encounter LUCY A. McFADDEN

AND

MICHAEL F. A'HEARN

Astronomy Program, University o f Maryland, College Park, Maryland 20742

PAUL D. FELDMAN Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218

HERMANN BOHNHARDT Dr. Remeis Sternwarte, Sternwartstrasse 7, D-8600 Bamberg, Federal Republic of Germany

JURGEN RAHE Solar System Exploration Dioision, NASA-Headquarters, Washington, D.C. 20546

MICHEL C. FESTOU Obsematoire de Besanfon, 25044 Besanfon Cedex, France

JOHN C. BRANDT, STEPHEN P. MARAN, MALCOLM B. NIEDNER, AND ANDREW M. SMITH Laboratory for Astronomy and Solar Physics, NASA-Goddard Space Flight Center, Greenbelt, Maryland 20771 AND

DAVID G. SCHLEICHER Lowell Observatory, Flagstaff, Arizona 86001 Received May 13, 1986; revised October 7, 1986 A program of ultraviolet spectrophotometry using the International Ultraviolet Explorer (IUE) Observatory was carried out in support of the International Cometary Explorer (ICE) mission. The H20 production rate was monitored from 1985 June to October. Between 1985 September 9 and 12, the spatial and temporal variation and abundance (or upper limits) of the remotely detectable species, C, CO, CO +, CO~, CS, H, Mg +, O, OH, and S, were obtained. These observations included the time of the ICE encounter (1985 September 11.46) when the H20 production rate was 3 x 1028sec -~ _+ 50%. This rate is consistent with a number of gas production rates derived indirectly from the ICE experiments. The comet was in a nearly steady state around the time of encounter showing no evidence of short-term temporal fluctuations in brightness greater than 6%. A sunward-tailward asymmetry of the OH brightness was observed at 10,000 km from the nucleus. The absence of detected Mg ÷ rules out this species as a possible ion ofM/Q = 24 which was detected by the Ion Composition Instrument, part of the ICE complement of instruments. Comparison of the abundance of CO~ ions with the total electron density measured by the plasma electron and radio science experiments on ICE indicates a deficiency of ions relative to electrons. To satisfy charge balance criteria, a major population of ions not detected by remote sensing must be present. © ~987 AcademicPress, Inc.

329 0019-1035/87 $3.00 Copyright© 1987by AcademicPress, Inc. All rightsof reproductionin any formreserved.

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MC FADDEN ET AL. INTRODUCTION

In support of the International Cometary Explorer's (ICE's) encounter with Comet P/Giacobini-Zinner (1984e), we carried out an extensive program of ultraviolet spectrophotometry of the comet using the International Ultraviolet Explorer (IUE) Observatory. Our complete program included observations from 75 days before perihelion (1985 June 22) through 34 days after perihelion (1985 October 8). The encounter itself took place 6 days after perihelion on 1985 September I 1. During the period 1985 September 9-12, we carried out extensive mapping of the comet in order to determine the spatial and temporal variation of those species which are remotely detectable. We address those aspects of our measurements which are likely to be relevant to the interpretation of the results from the ICE experiments. In particular, from our measurements we are able to determine the total gaseous outflow which determines the extent of the coma, the location of the bow shock, and the overall density from point to point within the coma. Our measurements also enable us to determine the column densities of various species, both ionized and neutral, along a column through the point of closest approach of the spacecraft to the nucleus but nearly orthogonal to the actual track of the spacecraft. These measurements also define some parameters to be measured by various instruments on ICE, particularly the ion composition instrument (Ogilvie et al., 1986), the plasma electron experiment (Bame et al., 1986), and the radio science experiment (Meyer-Vernet et al., 1986). OBSERVATIONS

The IUE spacecraft has two spectrographs roughly covering the spectral ranges 1150-1950 ~ (short wave or SW) and 19003400 ,~, (long wave or LW). Each spectrograph has two SEC Vidicon cameras, a primary and a redundant. The primary cameras were used on each spectrograph

(SWP and LWP). For all measurements reported here they were operated in their low-dispersion mode which provides spectral resolutions of approximately 9 and 18 ,~, respectively. Details of the IUE spacecraft are given by Boggess et al. (1978). The effective slit size used in the analysis was 10 × 15 arcsec. The linear size of the slit projected at the comet varied with the comet's geocentric distance. At the time of the encounter by ICE the slit corresponded to an area roughly 3400 x 5100 km. All spectra were reduced as in our previous work (see Feldman (1983) for a general review of ultraviolet (UV) spectrophotometry of comets and Weaver et al. (1981a) for an example of the details of the analysis). In addition to the two spectrographs, the IUE has an acquisition and tracking camera known as the Fine Error Sensor (FES). In the course of acquiring and tracking the comet, the FES also monitors the brightness of a central-shaped area of 432 arcsec 2 in a broad, visible bandpass with halfpower points at 3800 and 6500 A. This provides a useful check for variability of the comet during the course of the measurements as this bandpass is sensitive to fluctuations in the abundance of CN, C2, and dust. The emission lines and bands in cometary spectra are due primarily to fluorescence of sunlight. A measurement of the flux in an emission band can be reduced directly to the number of molecules in the field of view provided that one knows the fluorescence efficiency, or g factor, of the species and transition in question as well as the basic geometrical circumstances. Conversion of measured abundances to production rates is, however, model dependent. A spectrum of the nuclear region of the comet, combining images from the SWP and LWP cameras taken at slightly different times, is shown in Fig. 1 for purposes of identifying the relevant spectral features. These particular exposures were long ones to detect the fainter spectral features which saturated the images of the strongest fea-

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WAVELENGTH (~) FIG. I. Composite spectrum of P/Giacobini-Zinner from the SWP spectrograph on 1985 September 10.0 (192-min exposure) and the LWP spectrograph on 1985 September 10.4 (90-min exposure). A solar continuum spectrum was subtracted from the LWP spectrum. Lines across the top indicate locations of detectable species. Data were deleted at 2223 A, the location of a cosmic ray hit on the detector. The OH (0-0) and HI Lyman-a bands are saturated in these exposures.

tures due to HI (Lyman-a) and OH. Shorter exposures were used to estimate the fluxes in these features. On the day of encounter, September 11, we took only relatively short exposures because at that time the IUE spacecraft was at perigee and the particle irradiation from the Earth's radiation belts prevented long exposures. Long exposures were taken when the satellite was at apogee on the previous day. RESULTS

In order to estimate the water production rate, we measure the fluxes of the OH (0-0) and/or (1-0) bands in the spectrum from which a solar spectrum has been subtracted. The random uncertainties in any individual OH measurement are less than 10%. In the spectrum shown in Fig. I, the OH (0-0) band is saturated but it is easily measured in shorter exposures. These fluxes, measured with the slit centered on the nucleus, using fluorescence efficiencies from Schleicher (1983), directly yield the number of molecules in a rectangular

column centered on the nucleus. To convert to a production rate we use the vectorial model of Festou (1981a) which assumes that water molecules flow uniformly outward from the nucleus, photodissociating exponentially into H + OH (93%) or H2 + O(~D) (7%) with a mean total lifetime of 8.24 × 104 sec at I AU (Festou, 1981b). Most of the OH is ejected in a random direction with an average excess velocity of 1.15 km sec -1 relative to the original H20 molecule. The OH subsequently photodissociates into O + H with a mean lifetime of 2 x 105 sec at 1 AU. At the time of encounter the water production rate was 3 x 1028 sec -~. For UV observations, the most significant uncertainty in this model is in the velocities of the H20 and OH. Considering the range of plausible values for the parameters in the model, the uncertainty in production rate may be as large as 50%; this is systematic, however, and would shift all results in Fig. 2 up or down together. Note that previously published results on other comets observed with IUE (e.g., A'Hearn

332

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et al., 1984; Weaver et al., 1981b; Feldman,

1982) were obtained using a simpler model first developed by Haser (1957) which assumes collisionless, radial outflow of molecules at constant velocity. For comparison with IUE observations of other comets, the production rate derived from the simple Haser model is 4 × 102s sec -1, using a lifetime of 8.24 × 104 sec and velocity of 1 km sec -~ for the parent water molecules, or 2 × 102~ sec -~, assuming a velocity of 0.5 km sec -I. These velocities were chosen as reasonable extreme cases (cases A and B, respectively) and have been used routinely since the work of Weaver et al. (1981a). Discussions of the applicability of the simple Haser model and the differences between the results so derived and those from a vectorial model can be found in Combi and Delsemme (1980) and Festou (1981a). For observations with a small field of view, such as with IUE, the numerical relationship between the production rates in the vectorial and simple Haser models is approximately: Haser Case A = 1.4 × vectorial and Haser Case B = 0.7 × vectorial.

The results are summarized in Table I where we give the date of our observations, the heliocentric and geocentric distances of the comet, g factor of OH, the surface brightness of OH, and the derived production rate of H20 using the vectorial model. Figure 2 shows these data plotted as a function of time from perihelion. The point of relevance here is that the production of H20 peaked one month before perihelion and decreased by roughly a factor of 2 by the time of the encounter. Cowan and TABLE I PRODUCTION

OF WATER

Date (UT)

r (AU)

A (AU)

g (10 4 photons sec L)

B (kR)

Q (10 TM sec I)

85t06/22.6 85/07/05.9 85108106.5 85/08/12.1 85/08/30.7 85/09/04.8 85/09/09.9 85/09/10.5 85/09/I 1.5 85/10/08.5

1.443 1.329 I.III 1.084 1.031 1.028 1.030 1.031 1.032 1.130

0.931 0.81 I 0.576 0.543 0.472 0.467 0.469 0.470 0.472 0.584

3.10 3.69 3.31 2.64 2.53 2.33 2.65 2.70 2.80 4.91

1.9 4.4 14.8 12.5 9.5 7.8 7.0 7. I 6.8 5.9

2.8 4.0 7.3 7.0 4.5 4.0 3.2 3.2 2.9 2.1

UV SPECTRA OF COMET G-Z AT ICE ENCOUNTER A'Hearn (1979) showed that the observed lightcurves for a number of earlier observed comets could be explained by variations in the angle between the comet's rotation axis and the Sun-comet line, a seasonal variation. Sekanina (1985) developed a model of precession of this comet's nucleus which can explain the orbital motions of the comet. He assumes that precession of the nucleus can be derived from the comet's lightcurve which is a measure of its outgassing profile. The nongravitational forces affecting the comet's orbital motion are presumably due to anisotropic outgassing. Further modeling of the lightcurve of Giacobini-Zinner using these results will be addressed elsewhere. Observations during the several days near encounter showed a smooth decrease in the OH production rate. These data show no indication of short-term fluctuations of the comet and thus indicate that the comet was in a nearly steady-state condition. This is confirmed by the brightness measured by the FES. Those brightnesses showed apparently random fluctuations of up to 6% but nothing else that would indicate transient activity. In this respect the comet was more stable than, e.g., comets P/Halley (e.g., Festou et al., 1986) and IRAS-ArakiAlcock (e.g., Feldman et al., 1984a), both of which have shown large, systematic fluctuations on a time scale of hours. We conclude, then, that at the time of encounter the total gaseous outflow was in a steady state corresponding to 3 x 1028 sec -1 with model parameter uncertainties of roughly 50%. This production rate was derived from measurements centered on the nucleus and is the calculated total gaseous production rate. It is consistent with the production rates derived from magnetic field observations (4 × 1028 sec -1) (Mendis et al., 1986), the plasma electron experiment (3 × 1028) (Fuselier et al., 1986), and from the distribution functions of cometary pick-up ions of the water group using the Ultra-Low Energy Charge Analyzer (2.6 x 1028) (Gloeckler et al., 1986). These latter

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rates are derived indirectly from localized in situ measurements. Their agreement with our results demonstrates the validity of the modeling used to relate the localized measurements to the total gas production rate, at least for a comet in a steady state. The other quantities of particular interest are the column densities of various species that were encountered by the spacecraft. Table II lists the species for which we have measured either column densities or relevant upper limits. The table gives both the directly measured fluxes of the emission features as well as the g factor used to deduce the column densities. We have used g factors calculated from solar fluxes close to minimum solar activity which is closer to the level of solar activity during the comet's apparition than some more recent g factors published in the literature. Spectra were obtained with the slit centered 22 arcsec tailward from the nucleus which corresponds to a point 7500 km tailward, i.e., centered nearly on the actual point of closest approach of ICE to the nucleus (7800 km tailward). Assuming cylindrical symmetry for the tail, the derived column densities correspond to the actual column densities encountered by ICE as it traversed the comet (since the comet's tail lay nearly in the plane of the sky). The column densities of CS, H, and OH were measured essentially at the time of the encounter, although the signal-to-noise ratio was poor for CS. CO~ was measured approximately 12 hr before the encounter when the IUE satellite was relatively free of particle radiation. On the basis of the absence of observable temporal variations discussed above, it seems safe to assume that those column densities were still appropriate at the time of the encounter. There are a variety of other species with spectral features potentially observable in the ultraviolet, but we present in Table l i b upper limits at the encounter point only for those species which have been observed previously or which seem particularly relevant to the ICE experiments and for which meaningful upper lim-

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MC FADDEN ET AL. T A B L E II

Species

Wavelength (,~)

Fluorescence efficiency (photons sec ~ molecule ~)

Date (UT)

Surface brightness (Rayleighs)

CS CO+ H OH

2575 2888 1216 3085

C CO CO + Mg + O S

(B) Derived upper limits on column densities at the e n c o u n t e r point 1657 2.5 x 10 ~" Sep 10.04 <13" 1510 2.2 x 10 7 t Sep 10.56 <86* 2188 2.2 × 10 4~, Sep 10.95 <17" 2800 1.45 x 10 Ih Sep 10.95 < 6* 1302 4.25 x 10 "' Sep 10.04 <92* 1813 7.0 × 10 ~'' Sep 10.04 <63*

(A) O b s e r v e d column densities at the e n c o u n t e r point 7.0 × 10 ~" Sep 10.95 33 2.1 x 10 ~t, Sep 10.95 13 1.4 × 10 3, Sep 11.48 4900 2.80 × 10 4,1 Sep 11.45 4500

C ol umn density (cm 2)

5.0 6.6 3.8 1.7

<4.3 < 2.6 < 8.4 <4.5 <1.6 < 1.4

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10 "~ 10u 10 I2 10 t3

x l0 II x l0 t-` × 10 m x 10v x 1013 x 109

N o t e : * indicates surface brightness values at the nucleus. a J a c k s o n et al. (1982). b FOX and D a l g a r n o (1979). c Opal and C a r r u t h e r s (1977). d S c h l e i c h e r (1983). e F e l d m a n e t al. (1976). Y F e l d m a n and Brune (1976). g M a g n a n i and A ' H e a r n (1986). h C a l c u l a t e d for this paper.

its can be derived. The upper limits of C, O, and S are based on the 3o- detections at the nucleus 36 hr before encounter, when a long exposure of the SWP camera was made. The upper limit for CO is based on the 3o- upper limit at the nucleus during a long exposure when the noise was low on September 10. We calculated the upper limit for these four species at the encounter point using a Haser model. For species with long parent scale lengths (C and O), the difference between the modeled upper limit at the encounter point and the measured detection at the nucleus is small. For S, the parent of which has a short mean lifetime, and CO, which is assumed to be a parent molecule, the calculated upper limit using the Haser model is significantly different than the abundance detected at, or upper limit measured at, the nucleus and is a more accurate measurement of an upper limit at the encounter point. Of the various ions discussed by Ipavich

et al. (1986) and by Ogilvie et al. (1986), only Mg + and CO + have transitions in the spectral region covered by well-calibrated spectra from IUE. Because the CO + emission falls in a noisy region of the LWP spectrum, the upper limit reported for this species is not very useful. Smaller upper limits on the CO + abundance can be derived from ground-based spectra (Wagner et al., 1986; Coplan et al., 1987). Due to the high fluorescence efficiency of Mg ÷ and the low background noise of the IUE detector at 2800 ,~, its presence should be readily detectable. The low upper limit for this ion is therefore significant. Most of the ionic species in the water group [O ÷, H30 +, H20 ÷] are inaccessible to spectroscopy from IUE as is Na + for which M / Q = 23. Features of OH + and CN + are located at the upper limits of the detector's range of spectral sensitivity which precludes quantitative analysis of their emission bands. The absence of a significant detection of Mg ÷ from IUE

UV SPECTRA OF COMET G-Z AT ICE ENCOUNTER spectra rules out this ion as a possible source of the ion with M / Q = 24 detected by the ICI (Ogilvie et al., 1986; Coplan et al., 1987). Measurements of many of these species were also obtained at other projected distances from the nucleus. For the neutral molecule, OH, the spatial profile tailward (from the nucleus to 11, 22, and 30 arcsec) agrees to within a few percent with the densities predicted by the vectorial model as well as by the Haser model. At 2 arcmin from the nucleus, the model predictions are within 10% of the observed column density which is comparable to the random error of the measurements. This agreement confirms our previous assertion based on the temporal variations that the overall gas production was in a steady state. Having made observations out to 41,000 km, which is approximately one-half a scale length of H20, we were observing molecules that were at the center of the comet 11 hr earlier. Had there been an outburst in this time frame while carrying out the spatial mapping, we would not have been able to fit the results to these models. There is a noticeable asymmetry between the sunward and tailward positions that were mapped which is not predicted by the photodissociation models. The OH brightness at an offset of 30" (10,250 km) tailward of the nucleus was 20% brighter than that at 30" sunward on the day before encounter. At the 2' (41,000 km) offset points on September 11, the OH brightness was 12% higher in the tailward relative to the sunward direction. We carried out spatial mapping on these 2 days only and our data are limited by the sensitivity of the IUE detectors. There are no temporal variations observed which correspond to these spatial variations. We point out that the magnitude of this asymmetry is less than that observed in Comet Encke but in the opposite direction (Weaver, 1981; Feldman et al., 1984b). This asymmetry in the OH brightness of P/Giacobini-Zinner is also seen in the FES images taken during our runs and in a

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ground-based, unfiltered CCD image taken the following day, September 12 (Mendillo and Baumgardner, 1986), which contains flux reflected from the dust, as well as emission bands of neutrals and ions. Our aperture contained part of the forked structure in the coma that extends into the tail. This structure was also seen in the 10-/~m image of P/Giacobini-Zinner (Campins et al., 1986). This feature appears to persist over at least a few weeks, the time interval between the acquisition of the 10-/zm image and the ICE encounter. That it appears in the thermal emission image and the FES indicates that the structure is predominantly dust and neutrals and not ions as claimed by Mendillo and Baumgardner (1986). There is no simple, radially symmetric model for the outflow of ions as there is for the neutral molecules. In principle, the observed profiles of column density versus projected distance from the nucleus can be inverted to derive the actual volume density but this would require a series of measurements at various distances perpendicular to the tail axis as well as the assumption of cylindrical symmetry. Our measurements were confined to radial profiles because the column density of ions at positions offset by more than one slit length perpendicular to the axis of the tail was too low for meaningful measurements. We can perform the inversion under the assumption of spherical symmetry, which we have done implicitly for the neutrals by comparing the results with simple, spherically symmetric models, but this symmetry is not likely to be valid for the ions. Bame et al. (1986) and Meyer-Vernet et al. (1986) have measured the electron density along the trajectory of ICE. A very crude integration of the density profile from the plasma electron experiment (Bame et ai., 1986) implies a column density of electrons near 2 x 10I~ cm -2, significantly greater than the observed column density of CO~. As derived from thermal noise spectroscopy (Meyer-Vernet et al., 1986),

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MC FADDEN ET AL.

the corresponding electron plasma column density is 3 x l0 j~. This indicates that other ions must be present in significant amounts or else overall charge neutrality would not be preserved. Even our upper limit for CO ÷ , which is high due to the low sensitivity of the IUE detector, is much lower than the electron density. The contribution from H20 ÷ as measured from groundbased telescopes does not make up the difference (B. L. Lutz and R. M. Wagner, personal communication). It appears that the sum of all the remotely observable ions is inadequate to account for the column density of electrons observed by Bame et al. (1986) and Meyer-Vernet et HI. (1986), indicating that other ions must be present.

the ICE spacecraft. In addition, there appears to be a major population of ions that are not detected with remote sensing techniques as indicated by the large population measured by the plasma electron experiment and by thermal noise spectroscopy using radio antennas. ACKNOWLEDGMENTS We thank the staff of the IUE who were able to restore our observing time shortly after the fourth (of six) gyros failed on the satellite. Data reduction was carded out in part at the University of Maryland Computer Science Center. We thank Paul Butterworth and two anonymous reviewers for helpful comments. This work was funded by NASA Grants NAG5-252 to the University of Maryland and NSG-5393 to the Johns Hopkins University. We thank K. Ogilvie and M. Coplan for helpful discussions.

SUMMARY

From IUE measurements of the OH emission brightness, the model-dependent, total water production rate using the vectorial model is 3 x 1028 sec -1. From these data we established that the comet was in a steady state during and prior to encounter. Because of this condition, the localized results from the ICE mission are consistent with the global results using the IUE spectra. Two hypothesized explanations for the absence of observed temporal variations are that (1) the distribution of ices which would vaporize at the observed distance from the Sun is uniform across the surface of its nucleus (there are no patches of highly volatile ices) and/or (2) the rapid rotation of the comet would make the diurnal thermal wave very shallow; thus it would not reach subsurface volatile pockets which might drive outbursts. There are no observations of the rotation period of Giacobini-Zinner. Sekanina's 0985) precessional model deduces a fast rotation period so further quantitive consideration of these hypotheses is warranted. Based on the absence of a Mg ÷ band at 2800 A, this ion cannot be the source of the M / Q = 24 signature measured by the ICI on

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