line in cometary comæ that results primarily from the photodissociation of water

Transcription

1 10000 which for conventional grating spectrographs implies a slit width of order one arcsecond. Maximum slit length limited by practical considerations is therefore of order a few arcminutes. For a typical geocentric distance of 1 AU [O I] emission in this FOV is dominated by water photodissociation so with only knowledge of water photochemistry and an estimate of the aperture correction from the distribution The 6300 Å component of the oxygen ( D) doublet is a bright easily observed line in cometary comæ that results primarily from the photodissociation of water and its daughter OH. If the cometary emission can be separated from foreground airglow either by foreground subtraction or spectral resolution comparable to the geocentric velocity of the comet [O I] photometry should in principle make an excellent proxy for Q(H than 60 km s spectral separation requires instruments with resolving powers of O). With cometary geocentric velocities frequently less ABSTRACT J. P. Morgenthaler W. M. Harris (U. Washington Dept. of Earth & Space Sciences) F. Scherb (U. Wisconsin Dept of Physics). M. R. Combi (U. Michigan Dept. of Atm. Ocean and Space Sciences) Large aperture [O I] 6300 Å photometry of comets Hyakutake Halley and Austin: implications for the photochemistry of OH

2 than the accepted values suggesting a revision to the OH photochemistry is needed (Morgenthaler et al. 2001). In this work we will revisit our large aperture [O I] measurements of comets 1P/Halley C/1989X1 Austin and C/1996 B2 Hyakutake and show that revision of the OH photochemistry is necessary to bring these results into agreement with accepted Q(H O) values. images and spectra of comet HaleBopp that encompassed the entire [O I] coma. In this case the emission was dominated by OH photodissociation. Using conventional OH photochemistry our derived Q(H O) values were a factor of 3 4 higher 2 along the slit water production rates can be derived (e.g. Fink & Di Santi 1990). Interferometric techniques such as FabryPérot Michelson and Spatial Heterodyne Spectroscopy (SHS) achieve very high spectral resolution over FOVs of arcminutes to degrees. Using the 1 FOV Wisconsin Halpha Mapper (WHAM) we recorded

3 (1) Q( ) depends only on aperture photometry and branching ratios: Much less quenching than radio transitions No complex factor The forbidden to O( P) transition occurs in 1983) via the bright forbidden Å doublet s (Froese Fischer & Saha decays promptly to the metastable state interpretation is complicated by a velocity dependent A Hearn 1988) factor (e.g. Schleicher & OH resonant fluorescence is observed in the near UV at 3080 Å these transitions are easily quenched (e.g. Schloerb 1988) OH rotational transitions are observable in the radio at 18 cm Table 1 shows the photodissociation pathways of 3 Background

4 % " " M Morgenthaler et al. (2001).The van Dishoeck & Dalgarno OH cross sections have been calculated for a heliocentric velocity of 14 km s appropriate for HaleBopp 1997 early March. H Huebner et al. (1992); V van Dishoeck & Dalgarno (1984); $ $! BR... BR H. BR H #M. BR V. BR #M # # # # BR H Reaction BR Quiet Sun Active Sun Ref. Table 1. Photodissociation Branching Ratios 4

5 * ' ( Conventional narrowband filters don t work if narrow enough lose too much to cometary redshift variation Spectral separation (need + resolution) Background subtraction works if airglow stable Minimum spectral resolving power is & ' % or 80 km s Must separate cometary [O I] from cometary NH (geocentric velocity dependent) (& ' )Å) and airglow [O I] Observational difficulties: If [O I] 6300 Å observations are so easy why do we get it wrong? (1998) found: Results derived from observations of the forbidden oxygen lines are particularly discrepant nearly always yielding water production rates larger than those derived from other datasets. Using radio optical and UV derived Q( ) values from comet Halley Schleicher et al. 5 Problem

6 ... WHAM observations corroborated by [O I] observations of three other instruments (Morgenthaler et al. 2001) WHAM spectroscopic observations were sensitive enough (0.1 R sensitivity limit in 30 s exposures) to detect all the [O I] emission coming from HaleBopp Widefield [O I] 6300 Å observations of HaleBopp yielded Q( 3 4 times higher than other techniques (Morgenthaler et al see reprint) ) values a factor of Problem intensifies and suggests a solution Bonus: high sensitivity due to large FOV and ability to match FOV to coma size even at high resolving power Fourier transform spectroscopy (e.g. Michelson or Spatial Heterodyne Spectroscopy) FabryPérot imaging and spectroscopy figs. 1 2 Medium to highresolution longslit or multiobject spectroscopy (e.g. Fink & Hicks 1996; Combi et al fig. 1) 6 Observational difficulties have been solved

7 7 No model dependent aperture correction needed Only error could be in the branching ratios (Equation 1) Morgenthaler et al. use this as evidence to propose a modification to the widely used theoretical OH photodissociative cross section of van Dishoeck & Dalgarno (1984) used to derive the branching ratios BR3 and BR4 in Table 1 Modified OH cross section results in a shorter OH lifetime (85 ks vs. 120 ks) Models constrained by radial profile data would need higher outflow velocities Models constrained by highresolution line profile data would need to be adjusted to reflect smaller sampled volume Does modification of BR3 and BR4 improve Q( all comets? ) values from [O I] observations of

8 8 Comet C/1995 O1 HaleBopp Haser (1957) models are not sophisticated enough to handle the details of the complex changes in outflow velocity observed in comets (e.g. Combi et al ) Haser models provide adequate fits for determining aperture corrections Good fits to [O I] 6300 Å and OH profiles are obtained with Morgenthaler et al. (2001) OH cross section and reasonable outflow velocities (see figures) Comet C/1996 B2 Hyakutake Hyakutake was observed using the Wisconsin Halpha Mapper (WHAM) telescope during the WHAM commissioning phase at Pine Bluff Observatory Wisconsin (Tufte 1997) Preliminary reduction of data performed by high school interns Michelle Krok (spectral fitting to determine production rates) and Kyle Ripp (image reduction for radial profiles) are well fit by Haser models using the Morgenthaler et al. (2001) OH cross section and outflow velocities from (Combi et al. 1999)

9 9 C/1989 X1 Austin Schultz et al. (1993) had too small a projected FOV to detect [O I] from OH photodissociation Comet 1P/Halley (MageeSauer et al. 1988) measured two points beyond the scale length which are now fit with the Morgenthaler et al. (2001) OH cross section (fig. 6) MageeSauer et al. (1990) Q( for aperture correction!) rates are low because of very long OH lifetime used

10 10 Fig. 1. Wisconsin Halpha Mapper (WHAM) spectrum (left) and image (right) of Comet HaleBopp from 1997 March 5 showing relative ease of [O I] 6300 Å detection even over an airglowdominated 1 FOV. Resolving power of the WHAM FabryPérot is30000 or 10 km s (Tufte 1997; Morgenthaler et al. 2001; Haffner et al. 2003).

11 Voigt fit to bi.txt; chi^2 = 328 fit data resid (Plot generated Tue Jul 25 09:30:50 CDT 2000) Fig. 2. WHAM spectrum (left) and image (right) of comet Hyakutake on 1996 March 23 preliminary analysis.

12 2 / / ( Fig. 3. Comet HaleBopp ( Q( ) = 34.

13 2 / / Fig. 4. Comet Hyakutake ( Q( ) = 7.

14 2 / / 0 12 Fig. 5. Comet Hyakutake ( ( +. Poor conditions.

15 2 / 4 / ( 6+ Fig. 6. Comet Halley extended [O I] observations (MageeSauer et al. 1988) Q( 0 ( + ) =

16 ( 4 5 Careful evaluation of expansion velocity data is needed to verify the lowering of the OH lifetime to 85 ks since lifetime and outflow velocity are intimately linked More sophisticated coma models are needed for precise fitting of radial distributions An unexpeted result of using the Morgenthaler et al. OH cross section is the ability to return of the quenching rate to the value of found in Streit et al. (1976) 8 $ 9 [O I] 6300 Å radial profiles of comets HaleBopp Hyakutake and Halley are reasonably well fit by simple twocomponent Haser models (KrishnaSwamy & Brandt 1986) using the OH phtodisosciative branching ratio and lifetime of Morgenthaler et al. (2001) 16 Conclusions

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