Large Aperture [O I] 6300 A Photometry ofComet Hale-Bopp: Implications for the

Photochemistry of OH

Jeffrey P. Morgenthaler,���������

Walter M. Harris,�

FrankScherb,

����Christopher M. Anderson,

Ronald J.

Oliversen,��� �

Nathaniel E. Doane,�����

Michael R. Combi,�

Maximus L. Marconi,�

William H. Smyth���

jpmorgen@alum.mit.edu

1Currently a National Research Council fellow at NASA/GSFC, Code 6812Department of Physics, University of Wisconsin–Madison3Space Astronomy Laboratory, University of Wisconsin–Madison4Visiting Astronomer at the National Solar Observatory, operated by the Association for Research in

Astronomy, under contract to the National Science Foundation5Department of Astronomy, University of Wisconsin–Madison6NASA/GSFC, Code 6817ITSS Lanham, MD, currently at Department of Astronomy, University of Wisconsin–Madison8Department of Atmospheric, Oceanic and Space Sciences, The University of Michigan9Fresh Pond Research Institute

10Atmospheric and Environmental Research, Inc.

Introduction

� What: The 6300 A line of neutral oxygen ([O I] 6300 A)� Where: From the coma (expanding neutral gas cloud) of comets� When: During cometary perihelion passage (Hale-Bopp 1997 Feb-Apr)� Why [O I] 6300 A?

– Transition between metastable ��������� and ground state (lifetime �130 s)

– Not excited by solar radiation

– Tracer of multi-body interactions such as collisional excitation, colli-sional dissociation or photodissociation

– In cometary comæ, densities imply photodissociation with collisionalde-excitation active only in inner coma� How: large-aperture narrow-band photometry with multi-object spectro-

graphs and Fabry-Perot spectrometers/imagers� Wow: Found a factor of 3–4 times more [O I] 6300 A emission thatexpected coming from comet Hale-Bopp� Excess emission appears to be limited to outer coma� Implies one or both:

– a source of ��������� in the outer coma that is unknown or has been pre-viously ignored (unlikely)

– there is an error in the standard model of OH photochemistry unde-tected by previous, narrower FOV measurements

The Big Picture: optical image

Fig. 1.— Wide-field ( � 10 ) optical photograph of comet Hale-Bopp from1997 March 9, courtesy of H. Mikuz & B. Kambic (http://www.amtsgym-sdbg.dk/as). The ion tail is visible in the blue (the ion tail is mostly !#"��%$ ,but the blue lines are from carbon-bearing molecules such as CO and CN), thedust tail is white (scattered sunlight).

Cometary Spectroscopy� Gas densities range from near terrestrial atmospheric to interplanetary� Solar photon field� Solar wind� Solar gravity� Outer regions are collisionless� Ideal environment for study of photochemistry

Hale-Bopp 40" ring 1996 Oct. 22 WIYN/Hydra/MOS

Surf

ace

Bri

ghtn

ess

( er

g s

cm

A a

rcse

c )

-1-1

-2-2

H O2+

[O I]

Most unmarked emission lines are OH

NH2

WAVELENGTH ( A )

NH2

+ Comet

Fig. 2.— Sample spectrum recorded by the WIYN Hydra MOS. The extractionand reduction of these MOS data are discussed in detail by Anderson (1999)and Glinski et al. (2001).

Narrow-band Fabry-Perot imaging:[O I] 6300 A coma

Fig. 3.— Wisconsin !'& Mapper (WHAM) Hale-Bopp image (1997 March 5)with [O I] emission shown in color, dust in contours. The tailward extensionin the [O I] emission has not been seen in other comets but is seen in narrow-band observations of other lines (Harris et al. 2002; Oliversen et al. 2002). Theasymmetry contributes � 13% of the total [O I] emission.

Consistency between instruments: Annular view

Fig. 4.— Comparison of the WHAM and Hydra [O I] data. The triangles showthe azimuthal distribution of the [O I] surface brightness, the asterisks showthe dust surface brightness. The Hydra surface brightness values are plotteddirectly, with the greyscale and legend indicating which ring in fig. 3 the datacorrespond to. The values for the points in the top right plot were derived fromthe WHAM image by dividing it into 5-pixel wide rings (1 pixel = 0. ( 8) centeredon the Hydra rings and finding the average pixel value in 20 azimuthal bins.

Consistency between telescopes: Radial Profiles

� Upper profile is OH 3080 A data (Harris et al. 2002) fit with a two-componentHaser model (Haser 1957; Krishna-Swamy & Brandt 1986) to constrainshape� Using standard ! " � and OH photochemistry Haser model [O I] 6300 Aresults fit outer coma [O I] 6300 A emission only if production rate isscaled by 3

Fig. 5.— Measured and modeled radial profiles of [O I] 6300 and OH 3080emission in comet Hale-Bopp on March 2, March 5 ([O I]) and March 28 (OH).The WHAM profile indicated with the plus symbols is created from the threequadrants of fig. 3 away from the tailward direction; Hydra point are averagedexcluding this quadrant. The good agreement between the Hydra and WHAMradial profiles at greater than )+* )-,/. km is our strongest evidence of the cor-roboration between these datasets.

Consistency between instruments: Radial Profiles

� Densepak data (asterisks) and contemporaneous Hydra data (diamonds)show Haser model is not consistent with data.� Take your pick: fit inner coma ( ! " � dominated) or outer coma (OH domi-nated)

Fig. 6.— Measured and modeled radial profiles of [O I] 6300 emission in cometHale-Bopp on March 16, March 18, (models indicated in solid and dashed lines)and April 21. Note the good agreement between the March 16 (Hydra) andMarch 18 (Densepak) data. We infer from this and fig. 5 good agreement be-tween WHAM and Densepak.

Wide-field spectroscopic photometry–WHAM

Fig. 7.— WHAM spectrum of Comet Hale-Bopp, from 1997 March 5. In this30 s exposure, the WHAM emission line sensitivity is less than 0.1 R. Solid lineis a model with three Voigt profiles in emission plus seven Voigts in absorptionrepresenting the scattered solar spectrum. The dotted line is the same minusthe cometary [O I] emission line. The wavelength of airglow [O I] line is6300.304 A.

Wide-field spectroscopic photometry–50 mmFabry-Perot

Fig. 8.— Sample spectrum recorded 1997 April 14 by a 50 mm Fabry-Perotspectrometer operated at the main telescope of the McMath-Pierce solar tele-scope facility. The field of view is 200,000 km in radius, centered on the comethead.

Converting photometry to production rates ( 0 132#4 )5 6 �7� � ����8:9 ;=<>@? � <BADC "FEHGJILKNMLM �FOQP (1)

where the factor of R K corrects for the emission in the 6364 A decay path of��� � ��� , which is outside of our bandpass, C is the distance between the earthand the comet, and OQP is the aperture correction.

5 �L!S"T�U�V9 5 W ���X�Y����Z[]\ )_^ � []\a` �-� []\b> �dc (2)

where the BR e are the branching ratios of the ! " � photolysis reactions.

Table 1. Photodissociation Branching Ratios

Reaction BR e Quiet Sun Active Sun Ref. f! " � ^ gihkj ! " ^ ��� � ��� BR1 0.050 0.067 H! " � ^ gihkj ! ^ �l! . . . BR2 0.855 0.801 H�U! ^ gihkj ! ^ ���X����� . BR3 0.094 mnmom M�U! ^ gihkj ! ^ ���X����� . BR> ( 0.357 momnm M�U! ^ gihkj ! ^ ��� KTp � . BR4 0.662 0.513 V�U! ^ gihkj ! ^ ��� K p � . BR < ( 0.472 momnm M

f H, Huebner et al. (1992); M, Morgenthaler et al. (2001); V, vanDishoeck & Dalgarno (1984). The van Dishoeck & Dalgarno OHcross sections have been calculated for a heliocentric velocity of -14 qsr t�uv� , appropriate for 1997 early March.

OH branching ratio calculations

� Molecular cross section (some disagreement)� Solar spectrum (old calculations need updating)� Relevant parameters:

– Total lifetime against photodestruction

– Fraction of each product (branching ratio)

Table 2. Quiet-Sun OH Photodissociation Calculations

Reference f BR3 BR4 wyx{z}|VD . . . . . . . . . . . . . 0.048 0.718 120H/VD . . . . . . . . . . 0.094 0.662 134VD + S88 (BR3) 0.066 0.686 123VD + S88II . . . . . 0.300 0.415 123NL + VD . . . . . . . 0.183 0.600 107NL + VDII (BR

> ( ) 0.357 0.472 85H/NL . . . . . . . . . . . 0.390 0.453 50

f VD, using van Dishoeck & Dalgarno 1984 theoretical OH cross sections;H/VD, treatment of VD cross sections by Huebner et al. 1992; VD + S88, VDupdated for OH predissociation calculations of Schleicher & A’Hearn 1988 as-suming ~����������:9 ,Uqsr t uv� (these values are used as BR3 and BR4 in Table 1);VD + S88II, VD + S88 with BR4 at its 1 � minimum and the resulting extraphotons shifted to BR3; NL + VD, Nee & Lee 1984 experimental cross sectionsdivided by 2.5 to match VD total cross section in the ) < ,�,�� )-��,/, A region.NL + VDII, same as NL + VD but NL cross section at Ly & is not scaled (thesevalues are used as BR

> ( and BR < ( in Table 1); H/NL, treatment of NL crosssections in Huebner et al. 1992.| OH lifetime in kiloseconds.

Predicted water production rates

� Radial profiles are weak evidence because of known difficulties with theHaser model� Counting photons (photometry) is much easier� Convert photometric observations to water production rates (

5 �L!�"��U� )

Fig. 9.—5

( !S"�� ) values from various works. Open symbols denote productionrates derived with standard ! " � and OH photochemistry (denoted “VD + S88”in the figure). Filled symbols are the same but with a modified �U! ^ h j���X������^ ! branching ratio proposed by Morgenthaler et al. (2001).

Implications

� Large source of �7������ in the outer coma that does not come from ! " �(otherwise OH distribution would show it)� Problem with OH photochemistry

OH photochemistry

� Two major OH cross section works: van Dishoeck & Dalgarno (1984) (the-oretical) and Nee & Lee (1984) (laboratory).� van Dishoeck & Dalgarno total lifetime results in sensible radial profiles(radial outflow velocities are constrained by radio line profiles)� Nee & Lee total cross section clearly too high (Nee & Lee 1984)� Nee & Lee �U!k^�hkj ��� � ���v^�! branching ratio gives correct answerin [O I] 6300 A case� Provisional solution proposed by Morgenthaler et al. (2001): use the vanDishoeck & Dalgarno cross section from ) < ,�,�� )-��,/, A to scale the Nee& Lee cross section down, except at ����& .� Clearly a more satisfying solution is a recalculation of the OH pho-todissociation cross section

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This preprint was prepared with the AAS L� TEX macros v5.0.