Millimeter - Wavelength Spectroscopy

Millimeter-wavelength spectra were obtained of CO, HCN, H2CO, and CH3OH in Comet C/1996 B2 (Hyakutake) with the NRAO 12-m telescope on 21 Mar 1996 UT. The emission of CO, H2CO and HCN was mapped in the coma when the comet was 0.2 AU from the Earth, providing a detailed look at the behavior of these species in the inner coma. The mapping data indicate the presence of multiple velocity components in CO, HCN and H2CO emission near the center of brightness and a possible velocity gradient for CO emission across the inner coma. We also present relative abundances for these species near perigee. Introduction Carbon monoxide, HCN, H2CO, and CH3OH together contain a large fraction of the gaseous carbon in comae, and may be chemically inter-related (Huebner 1985; Boice et al. 1991; Colom et al. 1992; Fegley 1993; Samarasinha & Belton 1994). Some cometary models hypothesize that these are \parent molecules", or species directly incorporated into the comet nucleus from the surrounding protosolar nebula. On the other hand, recent studies indicate that some CO and H2CO may partly derive from extended grain sources in comae (cf. Mumma et al. 1993). Understanding the chemical composition of nuclei has signi cant implications for gas and grain chemistry of the solar system formation. These molecules all have transitions at millimeter-wavelengths which are favorably excited in comets. The brightness of comet Hyakutake (C/1996 B2) provided the rst opportunity to observe all four molecules at approximately the same time with similar aperture sizes. This made it possible to derive more accurate relative abundances of these species in comae. The recent close approach to the Earth of the bright comet Hyakutake also presented a rare opportunity to study molecular emission in comae with high spatial resolution normally not attainable at millimeter wavelengths. In order to probe the near-nucleus behavior of CO, HCN, H2CO and CH3OH, we mapped of the emission of these volatiles in the inner coma of Hyakutake four days before perigee. We used the National Radio Astronomy Observatory (NRAO) 12-m telescope on Kitt Peak, Arizona. [1]. [1] The National Radio Astronomy Observatory is operated by Associated Universityies, Inc. under cooperative agreement with the National Science Foundation. 2 Observations Spectra were obtained of the CO J=2-1 transition at 230.538 GHz, HCN J=3-2 at 265.886 GHz, H2CO JKaKc = 312-211 at 225.698 GHz, and the CH3OH J = 3 2 K = 0 group at 145.103 GHz. The spectra presented here were obtained on Mar 21, 1996 UT when the comet was 1.12 AU from the Sun and 0.16 AU from the Earth. The telescope's 1-mm and 2-mm receivers consist of dual-channel, cooled SIS mixers, which were tuned to reject the image sideband. Temperature scales were established by the chopper-wheel method. The line radiation temperatures, TR, were determined from the corrected line radiation temperatures, T * R, and the beam e ciency, , where TR=T * R/ . The half-power beamwidth (HPBW) and beam e ciency of the telescope was =2700 and =0.51 at 230 GHz, =4800 and =0.72 at 145 GHz, and =2400 and = 0.36 at 266 GHz (Je Mangum, personal communication). The spectrometer backends consisted of two 128channel lterbanks with 250 kHz and 500 kHz resolutions, providing velocity resolutions in the range of 0.3 0.7 km s 1 at 1-mm, and 0.5 1.0 km s 1 at 2-mm. A hybrid correlator spectrometer was also used to provide higher spectral resolutions of 98 kHz for the CO J=2-1 and H2CO data (0.13 km s 1) and HCN J=3-2 transition (0.10 km s 1), and 195 kHz (0.30 km s 1) for the methanol. The data were obtained in position-switching mode with an o set of 150 to the east in right ascension. Ephemerides were calculated using current elements from D. Yeomans and B. Marsden (personal communication). These data are summarized in Table 1. Maps of the various emissions were obtained with 1000 and 2000 spacings. Maps for CO, HCN and H2CO are shown in Figures 1 3. The methanol emission was not strong enough to map the features in the coma with a high signal-to-noise ratio, and so a composite spectrum was created by co-adding the data obtained within a 3000 radius of the center of brightness. The composite methanol spectrum is shown in Figure 4. Results and Discussion Spectra of the carbon monoxide J=2-1 transition are presented in Figure 1. The emission was mapped at 1000 ( 1200 km at the comet) intervals along the north-south and east-west directions about the center of brightness. The CO 2-1 line pro les often exhibit complicated structure which we interpret as multiple velocity components within the inner coma. The CO FWHM linewidth varied from 0.84 0.12 km s 1 at the 1000 and 2000 positions toward the east and 1.2 0.12 km s 1 toward the North and West. Thus for the the central 2000 radius of the coma, the CO emission has an averaged FWHM 3 linewidth of 1.1 0.2 km s 1. The velocity of the CO emission was measured by ttinga gaussian pro le to the individual lines. The CO line pro le appears to be symmetricwith respect to the topocentric velocity of the comet along the north-south direction andis blue-shifted by -0.15 0.08 km s 1. However, along the east-west direction, the COspectra appear to exhibit a velocity gradient. The CO emission is redshifted by +0.240.08 km s 1 at 2000 east of the center of brightness and changes to a blueshift of -0.300.08 km s 1 at 2000 west. We note that there appears to be a blue-shifted shoulder featurefor the CO emission at the 1000 and 2000 east positions. This apparent \velocity gradient"is consistent with a sunward ejection of molecules which are then turned back by solarradiation pressure. We also note that there may be multiple velocity components in theCO emission, especially at the 1000 east and 2000 east and south positions. CO, therefore,appears to be a good tracer of velocity structure within the inner coma of Hyakutake.Figures 2 and 3 show spectra of HCN and H2CO, respectively, across the innercoma. The maps were constructed with spacings of 1000 both molecules. Unlike CO, nosigni cant velocity gradients are present in either the HCN nor the H2CO data in theinner coma. As determined by gaussian ts to the line pro les, the HCN and H2COemissions are approximately centered on the comet's topocentric velocity indicating aprimarily symmetric out ow of molecules. Upon closer examination the pro les appearto have a slight asymmetry which is consistent with a blueshift of -0.20 0.08 km s 1 forHCN and -0.15 0.08 km s 1; however, this may be also consistent with the presence ofdi erent velocity components. The emission pro les have average linewidths (FWHM) of1.3 0.2 km s 1 for HCN, and 1.2 0.2 km s 1 for H2CO, similar to what was observedfor CO.The CH3OH co-added spectrum shown in Figure 4 exhibits no measurable shift fromthe topocentric velocity of the comet. The co-added spectrum has a broad linewidth of1.9 0.4 km s 1, although individual spectra have linewidths of 1.0 km s 1. Thus, themethanol emission may also exhibit multiple velocity components or a gradient, either ofwhich would result in a broader line pro le of the co-added spectrum.Production rates were derived for CO, HCN, H2CO and CH3OH using spectra ob-tained at the center of brightness position and are listed in Table 1. A simple pho-todissociative decay model was used assuming parent molecule distributions. Rotationaltemperatures were assumed to be Trot = 30 K for CO, HCN and H2CO and were measuredto be Trot = 30 K for CH3OH. Also listed in Table 1 are estimated relative abundances of4 these species with respect to water, which were derived using an average value for waterof Q(H2O) = 2.5 1029 mol s 1 for 23 Mar (Schleicher et al. 1996, Festou et al. 1996).The high relative abundance of CO/H2O 0.30 that we have found in Hyakutake wasalso reported for other millimeter-waveCOmeasurements (Matthews et al. 1996; Womacket al. 1996). However, measurements of the CO relative abundance at infrared and ultra-violet wavelengths which were also obtained near perigee indicate a much lower ratio ofCO/H2O 0.05 0.06 (Mumma et al. 1996; Weaver et al. 1996, Feldman et al. 1996).This di erence between the IR and UV abundances and the mm-wave abundances maybe due to the fact that the beamsizes for the IR and UV observations were small enoughso that optical e ects caused an underestimation of the production rates. However, wedoubt this would be substantial enough to account for all of the discrepancy. Instead,however, it may be that with the larger beamsizes the mm-wavelength spectra measurea signi cant amount of CO released from a distributed source as well as CO directlyfrom the nucleus. Thus, the CO data at all wavelengths present strong evidence for bothnuclear and distributed sources of CO in comet Hyakutake.AcknowledgementsThe authors would like to thank B. Marsden, and D. Yeomans for providing updatedorbital elements and the NRAO 12-m sta for scheduling extensive observations on shortnotice. M.W. acknowledges support from NSF grants AST-9520797 and AST-9625360from the Research Planning Grant Program and Faculty Early Career Development Pro-gram (CAREER). S.A.S. acknowledges support from a SWRI Internal Research grant.ReferencesBoice, D.C., Huebner, W.F., Konno, I. 1991, BAAS, 23, 160Colom, P., Crovisier, J., Bockelee-Morvan, D., Despois, D., & Pauber, G. 1992, A&A;,264, 270Feldman, P.D., Festou, M., Rodriguez, P.M., and Gonzalez, R. 1996, IAU Circular 6370Fegley, B. 1993, in Chemistry of Life's Origins, eds. M. Greenberg and V. Pirronello,(Kluwer: Dordrecht)Festou, M., Feldman, P., Gonzalez, R., and Rodriguez, P.M. 1996, IAU Circular 6355Huebner, W.F. 1985, in The Photochemistry of Atmospheres, ed. J.S. Levine (Orlando:Academic Press), 438Matthews, H.E., et al. 1996, IAU Circular 63535 Mumma,M.J., DiSanti, M.A., Dello Russo, M., Fomenkova,M., Magee-Sauer, K., Kamin-ski, C.D., and Xie, D.X. 1996, Science, 272, 1310Samarasinha, N.H., & Belton, M.J.S., 1994, Icarus, 108, 103Schleicher, D.G., Osip, D., and Lederer, S. 1996, IAU Circular 6372Weaver, H.A., Feldman, P.D., A'Hearn, M.H., Arpigny, C., Brandt, J.C., and Randall,C.E., 1996, IAU Circular 6374Womack, M., Festou, M.C., and Stern, S.A. 1996, IAU Circular 63456 Figure CaptionsFig. 1. Spectra of the CO J=2-1 line at 230 GHz in comet Hyakutake obtained withthe NRAO 12-m telescope on 1996 Mar 21 UT, at 98 kHz (0.12 km s 1) resolution.Spectra were obtained with 1000 spacings across the inner 5000 km. Lines are drawnin Figure 1 to denote the position of topocentric velocity. The CO emission appearsto be symmetric about the ephemeris velocity along the N-S direction in the coma,but exhibits a red-shift in the east which changes to a blue-shift in the west.Fig. 2. Spectra of the HCN J=3-2 transition at 266 GHz on 1996 Mar 21 UT with 1000spacings. In contrast with the CO, no signi cant frequency shift is observed acrossthe inner coma for HCN, although multiple velocity components may be present.Fig. 3. Spectra of the H2CO JK = 312 211 transition at 226 GHz on 1996 Mar 21UT with 1000 spacings. The formaldehyde emission pro les may contain multiplevelocity components; however, no signi cant frequency shift is observed across theinner coma.Fig. 4. Composite spectrum of the CH3OH J = 3-2, K = 0 group at 145 GHz on 1996Mar 21 UT within a 3000 radius of the center of brightness.7