The Astronomical Journal, 126:444–451, 2003 July # 2003. The American Astronomical Society. All rights reserved. Printed in U.S.A.
HUBBLE SPACE TELESCOPE STIS OBSERVATIONS OF COMET 19P/BORRELLY DURING THE DEEP SPACE 1 ENCOUNTER H. A. Weaver,1 S. A. Stern,2 and J. Wm. Parker2 Received 2002 July 25; accepted 2003 April 4
ABSTRACT In support of the NASA Deep Space 1 (DS1) mission to comet 19P/Borrelly, we obtained Hubble Space Telescope (HST) images and ultraviolet (UV) spectra of the comet near the time of the DS1 flyby in 2001 September. The HST data provide context information on 19P/Borrelly’s circumnuclear dust environment, the rotational period and rotational phase of its nucleus, the H2O and CS2 production rates, the dust production rate, the dust reflectivity in the visible and mid-UV, and the time variability of these quantities around the time of the DS1 encounter. We derive average values of QH2 O ¼ ð3:0 0:6Þ 1028 molecules s1, ½CS2 =H2 O ¼ ð1:0 0:3Þ 103 , and Qdust 240 kg s1. The corresponding dust-to-gas mass ratio is 0.24, but this is only a rough estimate because the dust production rate is uncertain by about an order of magni˚ , and the Af value of 745 15 cm tude. The dust continuum was strongly reddened between 2400 and 3200 A ˚ ˚ near 6500 A was 2.5 times larger than the value near 2900 A. The observed coma morphology consisted of a strong jet centered 6 from the projected solar vector, one broad fan centered 23 from the sunward direction, and another broad fan centered 18 from the antisunward direction. The light curve of the optical continuum, as measured in target acquisition images, has an amplitude of 40% in a square aperture that subtends 160 km at the comet; the rotational period could not be independently derived from the HST images but is consistent with the value of 26 hr derived from HST observations in 1994 and ground-based images in 2000. The new HST data reveal a prominent offset in the emission peak of neutral gas molecules, and therefore in the peak column densities of gas in the coma, relative to the position of the cometary nucleus, which may be related to the offset in ion densities observed in situ by the DS1 Plasma Experiment for Planetary Exploration (PEPE) plasma spectrometer. Key words: comets: general — comets: individual (19P/Borrelly) — solar system: general — ultraviolet emission 1. INTRODUCTION
2. OBSERVATIONS AND DISCUSSION
The NASA Deep Space 1 (DS1) spacecraft passed within 2200 km of comet 19P/Borrelly on 2001 September 22.938 UT, thereby achieving the fourth close encounter of a spacecraft with a comet and the first-ever flyby imagery of a Jupiter family (Kuiper belt) comet (Soderblom et al. 2002). The DS1 spacecraft carried three scientific packages: (1) the Miniature Camera and Spectrometer (MICAS), a remote sensing package that includes a visible-band imager, a 1.3– 1.9 lm imaging infrared spectrometer, and a UV spectrometer; (2) the Plasma Experiment for Planetary Exploration (PEPE) ion/electron spectrometer; and (3) the Ion Diagnostics System (IDS) package, which includes a magnetometer, Langmuir probes, and a potential analyzer. Because the UV spectrometer aboard MICAS was inoperable, we felt that it was especially important to seek observing time on the Hubble Space Telescope (HST) to obtain ultraviolet spectroscopic observations of 19P/Borrelly in support of DS1. The objectives of our HST observations, all performed with the Space Telescope Imaging Spectrograph (STIS), were to measure gas and dust production rates and to obtain context imagery of the inner coma, bracketing the time of the flyby.
The HST observations were split into two groups, made 24 hours before and after DS1’s closest approach to 19P/ Borrelly. The HST STIS observing plan for each epoch was an identical four-orbit sequence. In each sequence we obtained: (1) a 30 s acquisition image (500 500 field of view) of the comet with the STIS CCD using the F28X50LP longpass filter at the beginning of each orbit, (2) 4200 s of integration time with the far-UV MAMA detector and G140L ˚ , 0>2 2800 slit, and dispersion = grating (1150–1740 A ˚ pixel1), (3) 3240 s of integration time with the near0.60 A ˚, UV MAMA detector and the G230L grating (1570–3180 A 1 00 ˚ 0>5 28 slit, and dispersion = 1.58 A pixel ), and (4) 36 s CCD F28X50LP images (5200 2800 field of view) at the end of each of the two G230L orbits. Additional details of the observations are given in Table 1. Figure 1 depicts sample imagery and spectra obtained during the HST observations of 19P/Borrelly on 2001 September 21.8 UT. At that time, the comet was 1.361 AU from the Sun, 1.478 AU from Earth, had a heliocentric radial velocity of +1.22 km s1, and a solar phase angle of 41 . For the observations on 2001 September 23.8 UT, 19P/Borrelly was 1.362 AU from the Sun, 1.469 AU from Earth, had a heliocentric radial velocity of +1.56 km s1, and a solar phase angle of 41 .
1 Space Department, Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723-6099;
[email protected]. 2 Department of Space Studies, Southwest Research Institute, 1050 Walnut Street, Suite 426, Boulder, CO 80302;
[email protected],
[email protected].
2.1. Optical Imaging Results 2.1.1. Spatial Brightness Distribution
All the optical images were taken with the STIS CCD using the F28X50LP filter, which has a central wavelength 444
HST OBSERVATIONS OF 19P/BORRELLY TABLE 1 HST STIS 19P/Borrelly Observing Log
Exposure Type
Start Time (UT)
Integration Time (s)
Spectral Element
30 2 2100 30 2 1620 36 30 2 2100 30 2 1620 36
F28X50LP Grating 140L F28X50LP Grating 230L F28X50LP F28X50LP Grating 140L F28X50LP Grating 230L F28X50LP
30 2 2100 30 2 1620 36 30 2 2100 30 2 1620 36
F28X50LP Grating 140L F28X50LP Grating 230L F28X50LP F28X50LP Grating 140L F28X50LP Grating 230L F28X50LP
2001 Sep 21 Acquisition image ...... Spectrum ................... Acquisition image ...... Spectrum ................... Full image .................. Acquisition image ...... Spectrum ................... Acquisition image ...... Spectrum ................... Full image ..................
15:40:32 15:51:35 17:16:51 17:28:52 18:01:04 18:53:09 19:04:12 20:29:25 20:41:26 21:13:38 2001 Sep 23
Acquisition image ...... Spectrum ................... Acquisition image ...... Spectrum ................... Full image .................. Acquisition image ...... Spectrum ................... Acquisition image ...... Spectrum ................... Full image ..................
11:00:28 11:13:31 14:13:03 14:25:04 14:57:16 15:49:22 16:00:25 20:38:13 20:50:14 21:22:26
˚ and a FWHM of 2720 A ˚ . These images sample of 7230 A primarily visible-wavelength sunlight scattered from dust in the coma and from the surface of the nucleus. The nucleus is assumed to lie within the pixel having the greatest intensity and contributes 20%–40% of the signal in that pixel, assuming that the nucleus has an effective radius of 2.5 km, a geometric albedo of 4%, and a phase variation of 0.04 mag deg1, values that are consistent with the measurements from HST (Lamy, Toth, & Weaver 1998) and DS1 (Soderblom et al. 2002). The images displayed in Figure 1 (left) show that the coma was elongated toward the east, with a morphology very similar to that observed during the HST observations of 19P/Borrelly during its previous apparition (Lamy et al. 1998). To analyze the image morphology, we constructed radial and azimuthal spatial brightness profiles for each observing date. Examples of the average image obtained on 2001 September 21.8 UT are shown in Figure 1. A least-squares routine was used to fit a function of the form Ck, where C is the amplitude, k is the power-law index, and is the distance to the peak pixel, to the observed radial brightness profile. Two different cases were considered in constructing the radial brightness profiles: using all pixels that lie within 10 in azimuth of the central axis of the strong jet (see below) and using pixels at all azimuths, except those in the quadrant containing the strong jet. The results are displayed in the top panel of Figure 2. In fitting the observed profile to a power law, we used only pixels satisfying 0>30 3>0 (320 km 3200 km), where is the projected distance from the nucleus. Pixels closer to the nucleus were excluded because the convolution of the intensity with the instrumental point-spread function severely flattens the spatial profile in that region. Intensities for
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pixels greater than 300 from the nucleus are progressively more sensitive to the assumed value of the sky brightness. We adopted a sky brightness value of 2 analog-to-digital units (ADU) based on measurements farthest from the nucleus but still within the optically active region of the CCD; this is less than 10% of the measured intensities within 300 of the nucleus.3 The fit to the azimuthally averaged radial profiles yields power-law indices of 1:012 0:008 and 0:995 0:008 for September 21.8 and September 23.8, respectively. The corresponding values for the jet profiles are 0:996 0:006 for September 21.8 and 0:979 0:008 for September 23.8. All of these indices are very close to 1, as expected for material expanding unimpeded into a vacuum at constant velocity. However, the amplitude of the jet profile is more than twice as large as the amplitude of the azimuthally averaged profile, which demonstrates that the dust flow pattern is highly anisotropic. We discuss the azimuthal asymmetry of the brightness distribution next. To analyze the azimuthal variation in the spatial brightness distribution, we first constructed an azimuthally symmetric image by averaging all the values within 1 pixel wide annuli centered on the nucleus, but excluding the quadrant containing the bright jet. Then the average image for each day was divided by this azimuthally averaged image to enhance the azimuthal variations while at the same time essentially removing the radial brightness variation. Using this ratio image, we constructed azimuthal profiles using pixels located in an annulus between 10 and 20 pixels from the nucleus (i.e., 0>51 10>2, 550 km 5500 km). This choice ensures that only pixels with a relatively strong signal, but outside the region strongly affected by the instrumental point-spread function, are included but is otherwise arbitrary. The azimuthal profiles (one from September 21.8 is displayed in the bottom panel of Fig. 2) were fitted using a least-squares routine that included a linear function and three Gaussians, which seemed to be the minimum number of components needed to give a good match to the data. The centers of the three Gaussians have position angles, measured relative to the projected solar vector4 and with negative values corresponding to clockwise rotation in Figure 1, of 162=2, 22=9, and 5=8. The FWHMs of the Gaussians are 106=5, 105=5, and 27=3, respectively. The areas under each component are, respectively, 11%, 40%, and 9% of the total, which can be compared to 40% of the total for the linear background component. Thus, there are two broad, low surface brightness, fanlike emissions whose centers lie within 23 of the sunward and tailward directions on both observing dates. The most prominent spatial feature is the narrow, linear dust jet, which is nearly aligned with the projected solar vector. The jet has a peak intensity nearly 3 times larger than the background coma level (Fig. 2) and is clearly seen in the enhanced images (Fig. 1). Since the position angle of the jet appears fixed relative to the Sun on both dates, the jet’s source may originate near the rotational pole of the nucleus, although we cannot rule out the possibility that we just happened to observe the nucleus at similar points on a repeatable phase curve. Our data are too sparse to investigate this issue in detail.
3
The peak intensity is 1452 ADU for the image displayed in Fig. 1. The celestial position angle of the projected solar vector was 100=4 on September 21.8 and 101=3 on September 23.8. 4
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Fig. 1.—Top left: A 300 300 pixel portion of an STIS F28X50LP image of 19P/Borrelly produced by combining the two images taken at 2001 September 21.751 and 21.884 UT and displaying them on a logarithmic intensity scale (1 pixel ¼ 0>0508 ¼ 54 km). These images sample primarily the visible-wavelength sunlight scattered from dust in the coma and from the surface of the nucleus. The overlaid circle has a radius of 2>1, which corresponds to a projected distance of 2200 km, the closest approach distance of the DS1 spacecraft to the nucleus. The two white vertical lines show the position of the outside edges of the 0>5 spectrographic slit, which was used for the STIS near-UV (G230L) observations (right). Bottom left: This image was produced by dividing the image in the top left-hand panel by an azimuthally averaged version of itself, excluding the lower right quadrant, which contains the jet, in order to enhance asymmetrical structures. The intensity scale is linear and spans values from 0.8 to 3.0. The cross marks the location of the nucleus. A sunward jet is easily seen, while sunward and tailward dust fans are faintly visible. Right: A portion of the average, calibrated spectral image of 19P/Borrelly taken with the STIS G230L grating between 2001 September 21.728 and 21.881 UT. The spectral dimension runs along the horizontal axis, and the spatial dimension runs along the vertical axis. The continuum is centered at a distance of 000 , and the dashed horizontal lines define the spatial extent of the extracted spectrum shown in Fig. 5 (0>5 ¼ 535 km).
2.1.2. A Search for Fragments
Given the proclivity for comets to split, or to eject substantial fragments, we searched the optical images carefully for evidence of objects in the vicinity of the comet. No such objects were detected. We then created model point sources of varying intensities and placed them throughout the observed image to test our ability to detect faint objects in the presence of the coma. On the basis of this analysis, we conclude that there are no fragments within 0>5 (530 km in projected distance) of the principal nucleus of 19P/ Borrelly brighter than 1%–2% of the peak intensity. 2.1.3. Temporal Variations
The eight STIS CCD acquisition images of 19P/Borrelly were used to construct light curves of the near-nucleus
region (Fig. 3). For a 3 3 pixel synthetic aperture, which subtends 160 160 km at the comet, the amplitude of the signal varied by 40% during our observations. For a 9 9 pixel synthetic aperture, which corresponds roughly to the size of the aperture used for the spectral extractions discussed later, the amplitude variation was 15%. In principle, the observed temporal variation could be caused by a changing cross-sectional area presented by the nucleus, albedo variation on a rotating nucleus, variation in the dust production rate, or some combination of all of these. Smaller temporal variations are expected for larger apertures owing to the longer aperture crossing times. In addition, the contribution of scattered light from the highly elongated nucleus becomes smaller as the size of the aperture is increased. In any case, both visual light curves
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Fig. 3.—Visual light curves have been constructed from the HST acquisition images. All times are corrected for light travel, i.e., as seen at Earth. The flux collected in a 3 3 pixel (0>152 0>152) box (diamonds), centered on the brightest pixel, and the flux collected in a 9 9 pixel (0>457 0>457) box (crosses) both show what appear to be a systematic variation in time, probably associated with rotational motion of the nucleus with a period of 26 hr. The dashed curve is a pure sinusoid with a period of 13 hr; the light curve should be double-peaked for a highly elongated body, like the nucleus of 19P/Borrelly.
Fig. 2.—Top: Radial surface brightness profiles derived from the average image taken on 2001 September 21.8 UT are displayed for a strong jet (crosses) and for an azimuthally averaged profile (squares). The statistical error for each point is approximately the size of the symbol. The solid straight lines are power-law fits to the two observed profiles, as described in the text. Bottom: Azimuthal variation in the intensity for the ratio image displayed in Fig. 1 for all distances between 10 and 20 pixels from the nucleus (i.e., 0>51 10>2, 550 km 5500 km; crosses). All angles are given relative to the direction of the projected solar direction with negative values corresponding to clockwise rotation in Fig. 1. The heavy solid curve is a least-squares fit to the observed azimuthal profile, assuming that it is composed of a linear function and three Gaussians, with these four individual components plotted separately as dashed lines. The centers of the three Gaussians are indicated by the arrows. See the text for further details.
dust albedo, the dust filling factor, and the radius of the effective circular aperture used during the observations (Fig. 4). This quantity is directly proportional to the observed continuum flux and was defined to provide an aperture-independent measure of the dust production rate (A’Hearn et al. 1984). Using the plateau values for Af from Figure 4 (i.e., using the region of the coma in which Af is not affected by the point-spread function of the telescope and the flux contributed by the nucleus is negligible), we
displayed in Figure 3 are consistent with the light curve obtained during the HST observations in 1994, from which the rotational period of the nucleus was estimated to be 25:02 0:5 hr (Lamy et al. 1998), and with the groundbased light curve obtained in 2000, from which a period of 26 1 hr was derived (Mueller & Samarasinha 2002). Owing to its small size (8 4 4 km; Lamy et al. 1998; Soderblom et al. 2002), the nucleus of comet 19P/Borrelly could not be resolved by HST or ground-based telescopes. However, the light curves shown in Figure 3 indicate that DS1 encountered the comet near the time when the smallest projected area of its nucleus was facing Earth. 2.1.4. Af and the Dust Production Rate
Using the full-frame HST images obtained at visible wavelengths, we converted the observed signal to R-band magnitudes5 and then calculated Af, the product of the 5 We used m ¼ 2:5 log S þ 23:58, where m is the R-band magnitude R R in the Landolt-Kron-Cousins photometric system and S is the observed count rate in ADU s1.
Fig. 4.—Af, the product of the dust albedo, the dust filling factor, and the radius of the effective circular aperture used, is plotted as a function of aperture radius. Data derived from average STIS CCD images on 2001 September 21.8 UT (crosses) and September 23.8 UT (squares) are plotted separately. The sharp decrease for small aperture sizes is an artifact caused by the convolution of the spatial brightness distribution with the instrumental point-spread function. Both curves are roughly constant with aperture radius, as expected when cometary grains expand into space with uniform speed under steady state conditions. The statistical error in the individual points is approximately the size of the plotting symbols.
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derive Af ¼ 745 15 cm using the average images on both observing dates.6 This is comparable to the value derived from the HST observations of 19P/Borrelly during its 1994 ˚ and r = 1.401 AU; perihelion (610 10 cm at = 6500 A Lamy et al. 1998) and to the value quoted by A’Hearn et al. ˚ and r = 1.38 AU. A some(1995), 646 cm at = 5240 A ˚ ) was what smaller value for Af (350 cm at = 5260 A measured on 2001 September 19.4 by Schleicher, Woodney, & Millis (2003). We note that if we apply the reddening of 19P/Borrelly’s optical spectrum as measured by Schleicher et al. (2003) to the value of Af quoted by A’Hearn ˚ would be et al. (1995), the estimated Af near = 6500 A 775 cm. Using a power-law model for the dust size distribution, Lamy et al. (1998) derived Qdust 200 kg s1 during the 1994 apparition when Af 610 cm. Scaling this latter result to our Af value yields a formal dust production rate of 240 kg s1. However, we caution that uncertainties in the size distribution, albedo, density, and speed of the dust particles make all estimates of Qdust uncertain by about an order of magnitude.
Vol. 126 TABLE 2 Derived Emission Brightnesses Brightness (R)
Species
2001 Sep 21.8 UT
2001 Sep 23.7 UT
OH (0, 0).............. OH (1, 0).............. CS (0, 0)...............
4600 143 535 36 680 29
4572 134 594 33 667 30
Note.—The listed errors are from the 1 counting statistics. There is an additional uncertainty of 15% from the continuum subtraction, as well as an uncertainty of 10% in the absolute calibration of STIS.
September 21. The similarities in the G230L spectra from 2001 September 21 and 23 UT, and the consistency in the emission brightness measurements (Table 2), suggest that these spectra make reasonable proxies for 19P/Borrelly’s mid-UV spectrum at the time of the DS1 encounter, which occurred midway between our HST observations.
2.2. UV Spectral Imaging Results Spectra of 19P/Borrelly were obtained over the entire ˚ , using separate expowavelength range from 1150 to 3180 A sures with two different gratings, as described above and in Table 1. From the STIS spectra taken with the G140L grating, we detected strong emission from the comet in the ˚ . However, no other cometary hydrogen Ly line at 1216 A emissions were clearly detected using that grating, and the hydrogen Ly emission was contaminated by terrestrial airglow. Thus, we do not discuss those far-UV spectra further here. Instead we focus on the spectra taken at longer wavelengths with the G230L grating, in which we detected molecular emissions from the OH (0, 0), OH (1, 0), and CS (0, 0) ˚ , respectively (Fig. 5). bands at 3090, 2826, and 2576 A We divided the average G230L spectrum by a solar spectrum of the same resolution (Woods et al. 1996), and then fit this ratio by a spline function that varied smoothly with wavelength. Multiplying this spline function by the solar spectrum provides an essentially noiseless estimate of the cometary continuum. This model continuum was subtracted from the raw spectrum to produce an emission spectrum. The extracted emission band brightnesses are given in Table 2. In addition to the OH and CS emissions given in Table 2, there appears to be residual emission between 2650 and ˚ , which may be due to the CS (Dv ¼ 1) bands, and 2700 A ˚ , which may be from COþ . residual emission near 2890 A 2 However, the solar spectrum is very complex in this latter region, making accurate measurements of weak cometary emissions difficult. The UV spectra were obtained in time-tagged mode, but the low signal levels allow only coarse descriptions of the temporal behavior. We detected an increase of 15%–20% in the UV emissions (continuum, OH, and CS) on September 23 that mimicked the rise in the optical continuum on the same day, but we did not detect any significant ( 10%) temporal variations in the UV emissions on 6 The Af values quoted in this paper have not been corrected to zero phase angle, even though Af is expected to vary with phase angle, because the raw values are commonly used.
Fig. 5.—Mid-ultraviolet (UV) spectrum of 19P/Borrelly, as observed by HST STIS on 2001 September 21.8 UT (histogram, top). This spectrum is an average of two STIS G230L exposures, each having an integration time of 1620 s and an aperture size of 0>5 0>5, taken on different HST orbits. The solid curve is the estimated cometary continuum, which is subtracted from the data to produce the emission spectrum shown in the bottom panel. Emissions from OH and CS are identified, and their extracted brightnesses are given in Table 2. The average spectrum measured on 2001 September 23.8 UT is virtually identical to the spectrum displayed here, and the extracted brightnesses are also given in Table 2. The noise level can be gauged by looking at the point-to-point fluctuations in the spectrum in the region outside the region containing the emission bands.
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Fig. 6.—Model for Af, the product of the dust albedo, the dust-filling factor, and the radius of the effective circular aperture used (0>282) was derived from the average G230L spectrum and plotted as a function of wavelength. The undulations are caused by small mismatches in spectral resolution and/or spectral registration between the observed cometary spectrum and the solar spectrum used to calculate Af. The strong reddening observed for the dust in 19P/Borrelly is typical of the behavior exhibited by other comets in this wavelength region. See the text for further discussion.
2.2.1. UV Af and UV versus Optical Color
Using our estimate for the cometary continuum in the average G230L spectrum (see earlier discussion), we calculated Af as a function of wavelength (Fig. 6). We derive ˚ in a 0>5 0>5 aperture, Af ¼ 280 60 cm near 2900 A which can be compared with a value of 690 70 cm near ˚ for the same size aperture. Thus, the optical/UV 6500 A Af ratio is 2:5 0:6, which is comparable to the value (1.7–2) observed in C/1995 O1 (Hale-Bopp; Weaver et al. 1999). The strong reddening of 19P/Borrelly’s dust between ˚ is similar to that observed for other comets 2400 and 3200 A at these wavelengths (Feldman & A’Hearn 1985). At optical wavelengths, the dust continuum typically has a reddening ˚ (Jewitt 1991). Our results on of 10%-15% per 1000 A 19P/Borrelly indicate that this reddening extends to UV ˚. wavelengths, down to at least 2400 A 2.2.2. Gas Production Rates
OH molecules in cometary comae are produced mainly by the photodissociation of H2O that is sublimating from the nucleus. We used a vectorial model (Combi & Delsemme 1980; Festou 1981a, 1981b) to relate the observed OH (0, 0) band spatial brightness profile to the water production rate (QH2 O ; Fig. 7). In the vectorial model calculation, we used H2 O ¼ 6:5 104 r2 s and OH ¼ 1:0 105 r2 s ( x is the lifetime, in seconds, of species ‘‘ x ’’ at r ¼ 1 AU), and we assumed that 79% of the total H2O destruction produces OH, as recommended by Budzien, Festou, & Feldman (1994) for conditions of high solar activity.7 We also adopted outflow velocities (km s1) of vH2 O ¼ 0:8r0:5 and vOH ¼ 1:05 (Weaver et al. 1999). We assumed optically thin conditions and used an OH (0, 0) band fluorescence 7 During the time bracketing the HST observations of 19P/Borrelly, the average daily solar 10.7 cm flux varied between 241 and 281 sfu (1 sfu ¼ 1022 W m2 Hz1). For reference, the 10.7 cm flux is 70 and 200 sfu under solar minimum and solar maximum conditions, respectively.
Fig. 7.—Top: The spatial brightness profile of the OH (0, 0) band derived from the average G230L spectral image taken on 2001 September 21.8 UT, after subtraction of the continuum. Also plotted are two vectorial model spatial profiles for two different water production rates (QH2 O ): the solid curve has QH2 O ¼ 3:5 1028 molecules s1 and passes through the observed profile in the sunward direction (to the left), while the dashed curve has QH2 O ¼ 2:4 1028 molecules s1 and passes through the observed profile in the tailward direction. Bottom: The spatial brightness profile of the CS (0, 0) band derived from the same spectral image, after subtraction of the continuum. Also plotted are two different Haser model spatial profiles: one for a CS2 lifetime of 500r2 s (solid line) and the other for a CS2 lifetime of 1000r2 s (dashed line). The corresponding CS2 production rates are (2.2 0.5) 1025 molecules s1 and (3.7 0.7) 1025 molecules s1, respectively. For both OH and CS, the spatial brightness profiles are offset by 0>5 (535 km) into the sunward hemisphere from the position of the peak in continuum spatial brightness profile (dashed vertical line). The point-to-point variations in the observed profiles are a good indicator of the noise level for the data in each panel.
efficiency factor (g-factor) of 2:56 104 r2 photons s1 molecule1 (Schleicher & A’Hearn 1988) to convert between column density, N, and column brightness, B, using B ¼ gNr2 . Note that the ‘‘ quenched ’’ and ‘‘ unquenched ’’8 OH g-factors are essentially identical at this heliocentric radial velocity (+1.22 km s1). Applying the above formalism to the observed OH spatial brightness profiles, we derive QH2 O ¼ ð3:0 0:6Þ 1028 molecules s1 ð900 180 kg s1) as an average value for the entire period from September 21 to September 24. Schleicher, Woodney, & Millis (2003) derived a similar value (QH2 O 2:5 1028 ) from ground-based observations of 19P/Borrelly during 2001 September 18–19 UT using a larger aperture and filter photometry, but we do not know if their model parameters are consistent with ours. 8 The OH molecule is called ‘‘ quenched ’’ if the two -doubled levels of the ground state have equal populations, presumably produced by collisions; ‘‘ unquenched ’’ refers to the case in which the populations of the two -doubled levels are determined solely by radiative effects.
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Using a water ice sublimation model (Cowan & A’hearn 1979), we estimate that the sublimation rate at the subsolar point on the surface of the nucleus is 7:8 1017 molecules cm2 s1. Thus, our derived water production rate requires an icy area of 3.8 km2. Assuming that the nucleus of 19P/ Borrelly is a prolate spheroid having dimensions of 8 4 4 km, which is consistent with both the HST (Lamy et al. 1998) and the DS1 (Soderblom et al. 2002) results, the total surface area of the nucleus is 86 km and the fractional area covered by ice is 4%. The latter is significantly smaller than the value (10%) estimated by Lamy et al. (1998), but most of the difference can be attributed to different assumptions for the sublimation rate (i.e., we adopt a subsolar rate here, whereas Lamy et al. adopted a sublimation rate averaged over an entire nucleus whose rotational axis pointed at the Sun). Assuming a total gas production rate 20% higher than the H2O production rate alone (e.g., Krankowsky 1991) and using the dust production rate of 240 kg s1 derived earlier, we estimate that the dust/gas mass ratio is on the order of 0.2. However, we caution again that the large uncertainty in Qdust means that our nominal estimate for the dust/gas mass ratio could be incorrect by a large factor. CS molecules in cometary comae are thought to be produced by the photodissociation of CS2 molecules that are sublimating from the nucleus. We used a Haser model (Haser 1957) to relate the observed CS spatial brightness profile to the CS2 production rate (QCS2 ). Huebner, Keady, & Lyon (1992) calculate that the CS2 lifetime is (315–345)r2 s, depending on solar activity, but this range seems inconsistent with the observed CS spatial brightness profile for 19P/Borrelly (Fig. 7). Perhaps the recent theoretical calculation for the CS2 lifetime is wrong; if not, the observed CS spatial brightness profile may indicate either that CS2 is not the primary parent of CS, or that CS2 is not sublimating directly from the nucleus. We simply note this discrepancy and adopt somewhat larger values (500–1000 s) for the lifetime of the parent of CS, which are actually similar to the values adopted in some earlier IUE and HST work (Weaver et al. 1999). For the Haser model calculation, we assumed that one CS molecule is produced for every CS2 molecule that is destroyed. We used vCS2 ¼ vCS ¼ 0:8r0:5 km s1 and CS ¼ 1:0 105 r2 s (Weaver et al. 1999). We assumed optically thin conditions and used a CS (0, 0) band fluorescence efficiency factor (g-factor) of 7:0 104 r2 photons s1 molecule1 (Jackson et al. 1982) to convert between column brightness and column density. Applying the above formalism to the observed CS spatial brightness profiles, we derive QCS2 ¼ ð2:23 0:5Þ 1025 molecule s1 for an assumed CS2 lifetime of 500 and QCS2 ¼ ð3:7 0:7Þ 1025 molecule s1 for an assumed CS2 lifetime of 1000 s, at r ¼ 1 AU, as an average value for the entire period from September 21 to September 24. Combining the H2O and CS2 results, we find that the estimated CS2/H2O production rate ratio of 19P/Borrelly during our observations was ð1:0 0:3Þ 103 , which is well within the range of values observed in other comets near 1 AU (Meier & A’Hearn 1997). 2.2.3. Spatial Brightness Profiles
We now turn to the asymmetry in the spatial profiles of the molecular emissions in the inner coma. This asymmetry is obvious in the spatial profiles shown in Figure 7: the surface brightness is larger in the sunward-facing hemisphere
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(by 45% for OH) and the peak intensities for both OH and CS are offset by a projected distance of 0>5 (535 km) into the sunward-facing hemisphere, relative to the position of the peak in the UV continuum spatial brightness profile. A similar emission pattern is also seen in our 2001 September 23 UT spectra. The sunward-tailward brightness asymmetry is presumably related to the anisotropic production of gas and dust, with most material being ejected from the surface of the nucleus into the sunward-facing hemisphere. Offsets between the continuum and gas peak intensities have been detected in several other comets (Delsemme & Combi 1983; Spinrad 1991), including during HST observations of C/Hale-Bopp (Weaver et al. 1999), but no satisfactory, quantitative explanation for this effect has yet been found. Intriguingly, the DS1 PEPE plasma spectrometer detected an ion density peak some 3000 km north of Borrelly’s nucleus (Reisenfeld et al. 2002). If the PEPE ion peak offset is due to an ion production mechanism such as charge exchange, which is directly proportional to the local neutral gas density, then the two coma gas offsets may be intimately related. Further modeling of these effects may be interesting to pursue.
3. SUMMARY
Near the time of the DS1 flyby of 19P/Borrelly, the comet was moderately active with a water production rate of (3.0 0.6) 1028 molecules s1 and a CS2/H2O ratio of (1.0 0.3) 103, which is typical of other comets. The peak brightnesses of the OH and CS emissions were offset from the peak continuum brightness, which is assumed to coincide approximately with the position of the nucleus, by 535 km in projection. This offset may be related to the offset in the peak ion density measured by the DS1 PEPE plasma spectrometer. The coma morphology was very similar to that observed during previous apparitions, with a strong jet, whose central axis was offset by 6 from the projected solar vector, and two faint fans, one in the sunward hemisphere and one in the tailward hemisphere. The stationary nature of the jet indicates that it may originate near the rotational pole of the nucleus. The average value for Af, which is proportional to the dust production rate, ˚ , and this is similar to that was 745 15 cm near 6500 A observed at comparable heliocentric distances during other ˚ is 2.5 times apparitions. The dust albedo near 2900 A ˚ . Finally, the HST optical smaller than the value near 6500 A light curve is consistent with a rotational period for the nucleus of 26 hr, with the smallest cross section facing Earth at the time of the DS1 encounter. We thank the HST ground system personnel, particularly A. Lubenow and C. Proffitt, for their excellent support of our investigation. We thank our colleagues L. Young, R. Gladstone, and M. A’Hearn for useful comments on an early draft of this manuscript. We also thank the anonymous referee, whose comments led to substantial revisions in the paper. We thank D. Young and D. Crary of the DS1 PEPE team for helpful discussions concerning their data on 19P/Borrelly. Support for this project was provided by NASA through a grant (GO 09062) from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.
No. 1, 2003
HST OBSERVATIONS OF 19P/BORRELLY
451
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