GASEOUS JETS IN COMET HALE BOPP (1995 O1) 1. Introduction

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1 GASEOUS JETS IN COMET HALE BOPP (1995 O1) SUSAN M. LEDERER, HUMBERTO CAMPINS and DAVID J. OSIP Department of Astronomy, University of Florida, 211 Bryant Space Sciences Center, Gainesville, FL 32611, USA ( lederer@astro.ufl.edu) DAVID G. SCHLEICHER Lowell Observatory, 1400 Mars Hill Road, Flagstaff, AZ 86001, USA (Received 16 February 1998; Accepted 13 April 1998) Abstract. We report the identification of gas jets in comet Hale Bopp in OH, NH, CN, C 2 and C 3. This is the first time OH and NH jets without an obvious optical dust jet counterpart have been identified in narrowband comet images. We also confirm the existence of CN jets as reported by Larson et al. (1997) and Mueller et al. (1998). Jet features can be seen in the March and April 1997 datasets, approximately a month before and after perihelion. Our results contribute to the understanding of both the chemical properties of the comet as well as the physical mechanisms necessary to produce these features. Keywords: Comets, Hale Bopp, coma morphology, narrowband imaging, jets 1. Introduction The first report of collimated gas jets without an associated dust jet was presented by A Hearn et al. (1986a) for Comet Halley. Since then, CN, C 2,andC 3 jets have been reported in Comet Halley by several authors (A Hearn et al., 1986a,b; Cosmovici et al., 1988; Hoban et al., 1988). A Hearn et al. (1986a,b) noted that one of these jets appears to correlate with a similar feature in the dust images while two others do not. Their examination of the former jet indicates that it originates from the same region of the nucleus as the dust jet, but follows a different path after ejection. These observations suggest that the gases in these jets are not emitted by the dust responsible for the scattered continuum. In fact, A Hearn et al. (1986a,b) attribute these to a sub micron CHON particle extended source which is not apparent at visible wavelengths. In contrast, two jets reported by Clairemidi et al. (1990a,b, 1992) were detected not only in CN, C 2,C 3, and dust, but also in OH and NH spectro-imaging (using the Vega 2 spectrometer) of Comet Halley. However, the same two jets identified by Clairemidi et al. in all the gas species have a dust counterpart. Jets have been detected in Comet Hale Bopp data as well. Sunward dust and C 2 jets were reported by Laffont et al. (1997). In addition, both Larson et al. (1997) Earth, Moon and Planets 78: , Kluwer Academic Publishers. Printed in the Netherlands.

2 132 SUSAN M. LEDERER ET AL. and Mueller et al. (1998) have identified CN and dust jets but indicate that certain CN jets (in the anti-sunward direction) do not overlap spatially with the dust jets. 2. Observations Data were obtained at Lowell Observatory with the 42 (1.1 m) Hall Telescope using a CCD camera on February 26, March 1 6, and April 23, 25 28, North is down and east is left. The position angle of the sun was during March and in April. The phase angle was 45 and 32 35, respectively. The scale of the resultant images is 1.14 /pixel (2 2 binned). The new Hale Bopp set of narrowband comet filters was used to isolate the continuum (at 3448Å, 4450 Å, 5260 Å, and 7128 Å) as well as emission from OH (3090 Å), NH (3362 Å), CN (3870 Å), C 3 (4062 Å) and C 2 (5141 Å). We estimate that the underlying continuum contributes 20 35%, 80 95%, 20 35%, 80 90%, and 60 75% of the total signal in these gas filters, respectively. These are estimates from multiaperture photometry (aperture sizes ) of Comet Hale Bopp obtained in March and April, While the ranges listed are representative of the measurement uncertainties, in general these values decrease with increasing aperture size. Because of the underlying continuum in the NH, C 2,andC 3 filters, the exposure times of each image were chosen to prevent saturation of the photo-center (where the signal from the dust is greatest). As a result, the signal to noise (S/N) of these gas jets is lower than the S/N of the OH and CN jets. This may partially explain why the gas jets are more evident in CN and OH. The images were bias subtracted, flat fielded and sky subtracted. However, no absolute calibration or continuum subtraction was applied because the new filters have not yet been calibrated; a proper calibration of the standard stars in the new filter set is expected by mid The images were then centered on the comet s photo-center and trimmed to assist in the comparison of each species with the others. The final field of view of the images presented is 171 (176,000 km) per side for March 6 and 285 (349,000 km) per side for April 26, The morphology of the gas and dust jets can be discerned in the raw images. We applied several processing techniques to increase the contrast of the jets. To insure that the weakest features were real and not artifacts of the processing, we compared unsharp masked images with azimuthally renormalized images (Larson et al., 1992). Both methods yielded the same conclusion: jets indeed exist in the anti-sunward direction of the gas images, but not in this direction in the dust images. (In rare instances, there is some indication of a very low signal, <1% of the surrounding signal in azimuthally renormalized images, anti-sunward dust jet within of the central condensation). The unsharp mask accentuates the jets most clearly and thus was applied to the images presented in Figure 1.

3 GASEOUS JETS IN COMET HALE BOPP (1995 O1) 133 Figure 1. CCD narrow band filter images of comet Hale Bopp. North is down, east is left. The position angle of the sun is 159 in March and 247 in April. The phase angle is 45 in March and 34 in April. After flat-fielding and bias subtraction, each image was processed with an unsharp masking technique to accentuate the jets. These images demonstrate that gas jets without an obvious dust jet counterpart are apparent in OH and (in March) NH for the first time, as well as in CN, C 2, and C 3. The single smooth spiral evident in the April OH and CN images suggests one active source on the nucleus is responsible for the gas jets.

4 134 SUSAN M. LEDERER ET AL. 3. Results We have identified both dust and gas jets in comet Hale Bopp (Figure 1). Further, we provide the first identification of OH or NH jets without an optical dust jet counterpart for any comet. OH is particularly important since it is often used as a tracer of the comet s water component. In addition, we have found C 2,C 3, and dust jets as well as confirmed the existence of CN jets in comet Hale Bopp (Larson et al., 1997; Mueller et al., 1998). In all cases, the extent of jet activity is limited to within 125,000 km or 125 of the central condensation in the March data, and within 125,000 km or 90 in April. This is consistent with the extent of the CN jets reported in comet Halley (A Hearn et al., 1986a). The continuum (dust) images show varying jet structure, but it is generally restricted to the projected sunward direction. This refers to approximately the upper half of the March images and the right half of the April images. The brightest of these jets is about twice as bright as the nearby background (in azimuthally renormalized images). In comparison, the brightest sunward structures in the CN and OH images (before continuum subtraction) are on the order of 40% and 15% above background, respectively. The greatest contrasts created by the antisunward gaseous jets are only approximately 20% and 7%, respectively. We also detected jets with contrasts as small as a few percent above the nearby background. While antisunward jet structure is evident due to gaseous fluorescent emission, if dust was entrained in these features, not enough light was scattered by it at visible wavelengths to be detected above the isotropically scattered light. An examination of the anti-sunward jets shows that the gas species have features that appear to connect smoothly with the sunward jets. These jets can be seen most easily in CN and OH fluorescence due in part to a proportionally smaller contribution from the underlying continuum. For example, in the March OH data (Figure 1) one s eye can connect the second loop in the sunward direction (that runs through the position (+30, +5) in both the OH and UV continuum images) counterclockwise through the lower half of the image. This suggests both that the sunward jets have a significant gas component in addition to the dust, and that these dust and gas jets may originate from the same location on the nucleus. The smooth spiral apparent in the OH April data (Figure 1) suggests that one primary active area is responsible for the majority of the spiral structure. A comparison of the April OH gas and dust morphologies shows that the width of a single OH shell is approximately equivalent to two dust shells. An additional comparison with the CN data suggests that the narrow sunward shells evident in the remaining gas species (NH, CN, C 3,andC 2 ) are primarily due to dust, and that the morphology due to the gas is much broader. Two interpretations of physical mechanisms capable of producing gas jets have been modeled by Combi (1987) and by Klavetter and A Hearn (1994). Combi s model suggested that an initially confined parent jet expanding radially away from the nucleus could be sustained by the daughter radicals. Alternatively, photosput-

5 GASEOUS JETS IN COMET HALE BOPP (1995 O1) 135 tering of sub-micron CHON grains has been proposed as an extended source for the CN and C 2 jets in Comet Halley (A Hearn et al., 1986; Schulz and A Hearn, 1995). At this time, we are unable to determine which model is more consistent with our comet Hale Bopp data. However, we will continue to analyze and model these data to determine the source(s) for the gaseous jets. 4. Conclusions We have identified jets in the OH, NH, CN, C 2,C 3, and continuum (dust) narrowband images of comet Hale Bopp. This is the first time OH and NH jets without an obvious optical dust counterpart have been identified in any comet. Specifically, while all the gas species display jets in both the sunward and anti-sunward directions, dust jets are only evident in approximately the sunward direction. This implies that while both the gas and dust jets may originate from the same location on the nucleus, optically scattering dust is not the source of the gases. An extended source may be able to maintain the collimation apparent in each of the species. This source must be capable of creating jets in all gas species while remaining transparent at optical wavelengths. CHON particles have been proposed as a candidate for similar gas jets identified in Comet Halley and may likewise be responsible for gas jets in comet Hale Bopp. Alternatively, the extensive gas-dust coupling in the coma may act to retain the enhancement created by a nuclear jet(s) which emits both the dust and the parent molecules of the observed gases. Further analysis of these features, including a comparison with models, will be used to constrain both formation mechanisms of the jets as well as the composition of the proposed parent particles. Acknowledgements We wish to thank M. F. A Hearn for helpful suggestions. This work was supported by NASA and a Grant-in-Aid of Research from the National Academy of Sciences, through Sigma Xi. References A Hearn, M. F., Hoban, S., Birch, P. V., Bowers, C., Martin, R., and Klinglesmith, D. A.: 1986a, Cyanogen Jets in Comet Halley, Nature 324, A Hearn, M. F., Birch, P. V., and Klinglesmith, D. A.: 1986b, Gaseous Jets in Comet P/Halley, Proc. of the 20th ESLAB Symposium on the Exploration of Halley s Comet, Heidelberg, West Germany 1, Clairemidi, J. and Moreels, G.: 1990a, Gaseous CN, C 2 and C 3 Jets in the Inner Coma of Comet P/Halley Observed from the Vega 2 Spacecraft, Icarus 86,

6 136 SUSAN M. LEDERER ET AL. Clairemidi, J., Moreels, G., and Krasnopolsky, V. A.: 1990b, Spectro-Imagery of P/Halley s Inner Coma in the OH and NH Ultraviolet Bands, Astron. Astrophys. 231(1), Clairemidi, J., Rousselot, P., Vernotte, F., and Moreels, G.: 1992, Dust and Gas Jets: Evidence for a Diffuse Source in Halley s Coma, in Lunar and Planetary Inst., Asteroids, Comets, Meteors 1991, pp Combi, M. R.: 1987, Sources of Cometary Radicals and their Jets Gases or Grains?, Icarus 71, Cosmovici, C. B., Schwarz, G., Ip, W. H., and Mack, P.: 1988, Gas and Dust Jets in the Inner Coma of Comet Halley, Nature 332, Hoban, S., Samarasinha, N. H., A Hearn, M. F., and Klinglesmith, D. A.: 1988, An Investigation into Periodicities in the Morphology of CN Jets in Comet P/Halley, Astron. Astrophys. 195, Klavetter, J. J. and A Hearn, M. F.: 1994, An Extended Source for CN Jets in Comet P/Halley, Icarus 107, Laffont, C., Rousselot, P., Clairemidi, J., Moreels, G., and Boice, D. C.: 1997, Jets and Arcs in the Coma of Comet Hale Bopp from August 1996 to April 1997, Earth, Moon, and Planets 78, Larson, S. M. and Slaughter, C. D.: 1992, Evaluating Some Computer Enhancement Algorithms that Improve the Visibility of Cometary Morphology, Asteroids, Comets, Meteors 1991, Larson, S. M., Hergenrother, C. W., and Brandt, J. C.: 1997, The Spatial and Temporal Distribution of CO+ and CN in C/1995 O1 (Hale Bopp), Bull. Am. Astron. Soc. 29, Mueller, B. E. A., Samarasinha, N. H., and Belton, M. J. S.: , Imaging of the Structure and Evolution of the Coma Morphology of Comet Hale Bopp (C/1995 O1), Earth, Moon, and Planets 77, Schulz, R. and A Hearn, M. F.: 1995, Shells in the C 2 Coma of Comet P/Halley, Icarus 115,