A,,,,o,dC,.o,,,,',,,'.,,,o,',", PP,.77- S0 ,'II --B Lunar andPlanetary Institute, Houston, 1992 477 OBSERVATIONS OF COMET LEVY 1990c IN THE [O1] 6300-A LINE / - WITH AN IMAGING FABRY-PEROT C. Debi Prasad, K. Jockers, H. l_auer, Max-Planck-Institut fiir Aeronomie, D-W-3411 Katlenburg-Lindau, Germany E.H. Geyer, Observatorium Hoher List, D-W-5568 Daun, Germany ABSTRACT We have observed the comet Levy 1990c during 16-25 August 1990 usingthe MPAE focal reducer systembased Fabry-Perot etalon coupledwith the 1 meter telescopeof Observatory of Hoher List. The free spectral range and resolution limit of the interferometer was,-_2.18 ,_and ,-,0.171 A respectively. ClassicalFabry-Perot fringes were recorded on a CCD inthe cometary [01] 6300 A line. They are well resolvedfrom telluric air glow and cometary NH2 emission. Our observations indicate that the [01] is distributed asymmetrically with respect to the center of the comet. Inthis paper we report the spatial distribution of [01] emission and its line width in the coma of comet Levy. INTRODUCTION The mapping of [OIl 6300/_ emission in the cometary coma provides information about the distribution of its source OlD, which is a major dissociation product of H_O. The earlier studies of the cometary spectrum in 6300/_ region mostly used high resolution grating spectrometers and central aperture scanning Fabry-Perot (FP) spectrometers (Arpigny et al., 1987, Combi and McCrosky 1991, Magee-Sauer et al., 1989, Debi Prasad et al., 1988). Whereas the high resolution grating spectrometers provide the one-dimensional distribution along the slit, the central aperture scanning FP averages across the field of view. The FP in imaging mode has been used by Magee- Sauer et al. (1988) to map the OlD distribution in the inner coma region and, with limited signal to noise ratio, Debi Prasad and Desal (1989) have derived the velocity information. We have used the Fabry-Perot interferometer in classical mode and recorded the fringe system in wide field of view with a focal reducer and CCD. This paper reports the 6300/_ line width and intensity distribution in the coma of comet Levy. OBSERVATIONS AND DATA ANALYSIS Comet Levy was observed with MPAE focal reducer based FP interferometer and CCD camera (Jockers et al, 1987) which can be rapidly switched between the imaging and interferometric mode. The free spectral range, resolution limit and optical gap of the FP interferometer derived from the spectral lamp calibration frames at the time of observation axe 2.2/_, 0.12/_ and 905.45 micron respectively. The field of view and spatial resolution of the focal reducer axe 29×20 arcmin and 3.0 axcsec pixe1-1 respectively. The observations of August 1990, 17.95 UT axe reported here. During the observations the cometary heliocentric (rh). and geocentric distances axe 1.48 and 0.51 AU respectively. The geocentric velocity was -30.7 km/s, which corresponds to 0.64/_ Doppler shift of cometaxy line with respect to the terrestrial line at 6300/_. The focal reducer images of comet Levy in 6300/_ filter (IL+c) were obtained with the FP etalon in the collimated part of the optical path. The FP etalon was removed while exposing for the continuum images (Ic) with 6420/_ pre-filter. The images of a diffuser screen illuminated by a continuum source were taken with and without FP etalon. The ratio of these two bias subtracted images normalized to unity with the average counts in the central region of the frame, (IR) serves for correcting distortion in intensity distribution that might be caused by introducing the FP etalon. The position of the comet in CCD pixel coordinate was the same during interferogram and imaging exposures. The images are bias subtracted and flat-field corrected. In IL+C the airglow, cometary lines and continuum light is convolved with the periodic Airy function with the instrumental finesse 19 478 Asteroids, Comets, Meteors 1991 Figure 1: Fabry-Perot Interferogram of Comet Levy in 6300/_. The bright points not marked by arrow are due to instrumental reflections. Sun is to the left. which is determined from the spectral lamp calibrations during the observations. The airglow and cometary profiles are broader than the instrumental profile. We have computed the Airy function with degraded finesse in order to fit the tel]uric and cometary OlD emission. The best fit of Airy functi0n with the observations was obtained for _he effective finesse of 16 and 13 in the case of airglow and cometary e_ssion. This indicates that the cometary lines are broader than the airglow. The FP interferograzn due to the emission lines IL obtained by using the relation, [L = ([L+C) -- (IR X lc) (1) is shown in Figure 1. The full circles are due to the airglow and partial circles are due to the cometary emission. The cometary [Oil 6300A [s readffy identified as the fringes axe Doppler shifted from the airglow corresponding to its geocentric velocity. With respect to the cometary [Oil line z the other cometary ]_ne_isted by Combi and _M_C_r-osky(19§]_)are identified by takinglnto account E of the filter transmission and free spectral range of the FP. Further the inter-order overlap of the cometary lines transmitted by the pre-filter was also taken into account for the identification of NH2 lines. Figure 2 shows an example of the radial scan from a part of the interferogram within one free spectra] range corresponding to the two inner circles of the interferogram, The observed FP Z profile was divided by the computed instrumental transmission function for airglow and cometary [OIl emission in order to take into account the FP modulation. The absolute calibratlon is made by observing the the spectrophot0metric standard star (29 Vul) with the pre-filter and FP etalon. The effect of FP eta]on on the observation of the standard star have been taken into account. The calibrated cometary [OIl flux is extracted from four sectors A, B, C and D with respect to the center of the comet as shown in Figure 3. The sector A is in the tailward hemisphere and sector B is in the sunward hemisphere of the Comet, Theeriorin estlmating the absolute: values in [Oil intensity is ,,_30_- The relative error in intensit[e-s is- much:sma_er and does not exceed ,,_10%. Asteroids, Comets, Meteors 1991 479 1 Comet Levy 1990e Airglow [0118300.31 1990 Aug. 17.95 .6 Airglo w [0118300.31 i - h-, .+J .4 ,.Q .+J Oq (D .2 .K,.J (0 Cr_ 20 40 60 80 100 120 Radius (pixel number) Figure 2: Radial scan within two inner circles of the interferogram RESULTS AND DISCUSSION The line profile analysis show that, in general the cometary [OI] emission line is wider than the airglow profile. In a selected area of 3×24" (3x29 103 km) with the long side perpendicular to the fringe, at cometocentric distance of 2.2x 10_ km in the sunward side, we find the half width at half maximum to be 2.25 km/s. This refers to the outflow speed of oxygen atom within the field of view of the cometary atmosphere, since the natural width of OlD line is small. Our values are slightly higher than similar measurements in the case of comet Kohoutek and Halley (Huppler et al, 1975, Magee-Sauer et al, 1988, Debi Prasad et al, 1988). However, the earlier measurements have used a large field of view, as a result, the contribution from the low velocity component at the near nucleus region is dominant in these signal. On the other hand, our data corresponds to a narrow field of view, isolating the signal from from the near nucleus region. The dusty-gas-dynamic Monte- Carlo model by Combi (1989) in case of comet Halley predicts aa outflow speed of 0.91 km/s at a heliocentric distance of 1.5 AU in the post-perihelion epoch within the collision zone. According to this model the expected velocity at the distance of our measurements is _ 1.5 km/s. We have traced the cometary [Oil emission up 3x 10s km from the nucleus of comet Levy as shown in Figure 3. Our data show an approximate rh 1 distribution of OlD within a radial distance of 1.2×10 s km from the nucleus which is close to the photo-dissociation scale length of water at the heliocentric distance of our observation, beyond which the distribution is flat. Earlier the [OI] distribution have been reported up to ,,_10skm from the nucleus (Magee-Sauer et al, 1988), with which our results are in agreement. The fiat distribution beyond the photo-dissociation scale length of water could be due to the parent molecules with longer life time such as CO a_d CO2. A sunward to antisunward asymmetry in the distribution of [OI] emission is noticeable up to a cometocentric distance of 10_ km beyond which the signal is not strong enough. As in case of comet Halley (Combi and McCrosky, 1991) the observed asymmetry cart be understood as the 480 Asteroids, Comets, Meteors 1991 2.5 I l I ' I ' ' ' ' I t Comet Levy 1990c I I 1990 August 17.95 I.lT 2 .= T., g 1.5 w • ÷ 2 a c a ow +÷ ao I m+ E) • i÷+÷ 0 • ÷ tad ÷ o o log projected distance from the nucleus (kin) Figure 3: Distribution of cometary [OI] 6300_emission from four sectors of the interferogram (A:A; +:B; D:C, .:D). See text for the explanation of different sectors. result of preferential sunward ejection of material from the cometary nucleus. The inner coma imaging of comet Levy with the Hubble Space Telescope, indeedrevea] a fan-shaped inner coma in which the sunward facing hemisphere is significantly brighter (by a factor of _2.5) than the tailward hemisphere (Weaver et al, 1991), consistent with our observations. REFERENCES Arpigny, C., Manfroid, J., Magain, P., and Haefner, R., 1987, On the forbidden red lines of oxygen in comets. In Proceedings of the symposium on the diversity and similarity of comets, ESA SP 278, 571-576. _ : : _: iii_ i ...... -C. Deb_r_i-_=. Cl_andrasekhar(J: N. Des_ _d:_: M. Ash0k, 1988, High-resolution studies B of [OI] and NH2 line emission at 6300 Ain Halley's Comet, Pub. Astron. Soc. Paci 100, 702. C. Debi Prasad and Desai, J. N., 1989, Imaging Fabry-Perot Observations of [OI] 6300 A emission in the coma of Halley's comet 1982i, Earth, Moon and Planets 44, 191-195. Combi, M: R. and McCrosky, R. E., 1991, High-Resolution Spectra of the 6300-A region of comet P/Halley, Icarus 91,270-279. ]_uppier, H., Reyn0ld, 1%.3, Roesler, F. L:: Scherb, F. arm Trauger, J., i975, Observation ) ±........ :: ...... Of comet Ko]aoutek-_-9-73f) with a ground-based Fabry-Perot spectrometer, Astrophy. J. 202, 276-282. Jockers, K., Geyer, E. H., Rosenbauer, R. and H£nel, A., Observations of ions incomet P/Halley wlth_ focaIreaucer_:_stron, Astrophy: __187, _6_- 260.= ...... Magee-Sauer, K., Roesler, F. L., Scherb, F. and Harlander, J., 1988, Spatial distribution of &D from Comet Halley, Icarus 76, 88-99. Weaver, H. A. et al, 1991, This conference.