Large Aperture [O I] 6300˚A Photometry of Comet Hale

Large Aperture [O I] 6300 Å Photometry of
Comet Hale-Bopp: Implications for the
Photochemistry of OH
Jeffrey P. Morgenthaler,
Walter M.
Harris, Frank
Scherb, Christopher M. Anderson,
Ronald J. Oliversen, Nathaniel E. Doane,
Michael R. Combi,
Maximus L. Marconi, William H. Smyth
jpmorgen@alum.mit.edu
1
Currently a National Research Council fellow at NASA/GSFC, Code 681
2
Department of Physics, University of Wisconsin–Madison
3
Space Astronomy Laboratory, University of Wisconsin–Madison
4
Visiting Astronomer at the National Solar Observatory, operated by the Association for Research in
Astronomy, under contract to the National Science Foundation
5
Department of Astronomy, University of Wisconsin–Madison
6
NASA/GSFC, Code 681
7
ITSS Lanham, MD, currently at Department of Astronomy, University of Wisconsin–Madison
8
Department of Atmospheric, Oceanic and Space Sciences, The University of Michigan
9
Fresh Pond Research Institute
10
Atmospheric and Environmental Research, Inc.
Introduction
What: The 6300 Å line of neutral oxygen ([O I] 6300 Å)
Where: From the coma (expanding neutral gas cloud) of comets
When: During cometary perihelion passage (Hale-Bopp 1997 Feb-Apr)
Why [O I] 6300 Å?
– Transition between metastable and ground state (lifetime 130 s)
– Not excited by solar radiation
– Tracer of multi-body interactions such as collisional excitation, collisional dissociation or photodissociation
– In cometary comæ, densities imply photodissociation with collisional
de-excitation active only in inner coma
How: large-aperture narrow-band photometry with multi-object spectrographs and Fabry-Pérot spectrometers/imagers
Wow: Found a factor of 3–4 times more [O I] 6300 Å emission that
expected 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 previously ignored (unlikely)
– there is an error in the standard model of OH photochemistry undetected by previous, narrower FOV measurements
The Big Picture: optical image
Fig. 1.— Wide-field ( 10 ) optical photograph of comet Hale-Bopp from
1997 March 9, courtesy of H. Mikuz & B. Kambic (http://www.amtsgymsdbg.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), the
dust 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
+ Comet
NH 2
NH 2
-2 -1
Surface Brightness ( erg s-1cm A arcsec -2 )
[O I]
H2O+
Most unmarked emission lines are
OH
WAVELENGTH ( A )
Fig. 2.— Sample spectrum recorded by the WIYN Hydra MOS. The extraction
and reduction of these MOS data are discussed in detail by Anderson (1999)
and Glinski et al. (2001).
Narrow-band Fabry-Pérot imaging:
[O I] 6300 Å coma
Fig. 3.— Wisconsin !'& Mapper (WHAM) Hale-Bopp image (1997 March 5)
with [O I] emission shown in color, dust in contours. The tailward extension
in the [O I] emission has not been seen in other comets but is seen in narrowband observations of other lines (Harris et al. 2002; Oliversen et al. 2002). The
asymmetry 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 show
the azimuthal distribution of the [O I] surface brightness, the asterisks show
the dust surface brightness. The Hydra surface brightness values are plotted
directly, with the greyscale and legend indicating which ring in fig. 3 the data
correspond to. The values for the points in the top right plot were derived from
the WHAM image by dividing it into 5-pixel wide rings (1 pixel = 0. ( 8) centered
on the Hydra rings and finding the average pixel value in 20 azimuthal bins.
Consistency between telescopes: Radial Profiles
Upper profile is OH 3080 Å data (Harris et al. 2002) fit with a two-component
Haser model (Haser 1957; Krishna-Swamy & Brandt 1986) to constrain
shape
Using standard ! " and OH photochemistry Haser model [O I] 6300 Å
results fit outer coma [O I] 6300 Å emission only if production rate is
scaled by 3
Fig. 5.— Measured and modeled radial profiles of [O I] 6300 and OH 3080
emission 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 three
quadrants of fig. 3 away from the tailward direction; Hydra point are averaged
excluding this quadrant. The good agreement between the Hydra and WHAM
radial profiles at greater than )+* )-,/. km is our strongest evidence of the corroboration 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 dominated)
Fig. 6.— Measured and modeled radial profiles of [O I] 6300 emission in comet
Hale-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) and
March 18 (Densepak) data. We infer from this and fig. 5 good agreement between WHAM and Densepak.
Wide-field spectroscopic photometry–WHAM
Fig. 7.— WHAM spectrum of Comet Hale-Bopp, from 1997 March 5. In this
30 s exposure, the WHAM emission line sensitivity is less than 0.1 R. Solid line
is a model with three Voigt profiles in emission plus seven Voigts in absorption
representing the scattered solar spectrum. The dotted line is the same minus
the cometary [O I] emission line. The wavelength of airglow [O I] line is
6300.304 Å.
Wide-field spectroscopic photometry–50 mm
Fabry-Pérot
Fig. 8.— Sample spectrum recorded 1997 April 14 by a 50 mm Fabry-Pérot
spectrometer operated at the main telescope of the McMath-Pierce solar telescope facility. The field of view is 200,000 km in radius, centered on the comet
head.
Converting photometry to production rates ( 0
5
6
7 8:9
;=<>@?
"FEHGJILKNMLM
<BADC
132#4
FOQP
)
(1)
where the factor of RK corrects for the emission in the 6364 Å decay path of
, which is outside of our bandpass,
is the distance between the earth
C
and the comet, and OQP is the aperture correction.
5
5
L!S"TUV9
[]\
)_^
W
XYZ
[]\a` []\b>
-
dc
(2)
where the BRe are the branching ratios of the ! " photolysis reactions.
Table 1.
Photodissociation Branching Ratios
BRe
Reaction
! " ! " U!
U!
U!
U!
^
gihkj
^ gihkj
^ gihkj
^ gihkj
^ gihkj
^ gihkj
!
!
!
!
! " ^
! ^
^
^
^
^
l! . . .
X .
XKT
p .
K p .
.
BR1
BR2
BR3>
BR (
BR4
BR (
<
Quiet Sun Active Sun Ref. f
0.050
0.855
0.094
0.357
0.662
0.472
0.067
0.801
mnmom
momnm
0.513
momnm
H
H
M
M
V
M
f H, Huebner et al. (1992); M, Morgenthaler et al. (2001); V, van
Dishoeck & Dalgarno (1984). The van Dishoeck & Dalgarno OH
cross sections have been calculated for a heliocentric velocity of 14 qsr tuv , 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
Referencef
BR3
BR4 wyx{z}|
VD . . . . . . . . . . . . .
H/VD . . . . . . . . . .
VD + S88 (BR3)
VD + S88II . . . . .
NL + VD . . . . . > . .
NL + VDII (BR ( )
H/NL . . . . . . . . . . .
0.048
0.094
0.066
0.300
0.183
0.357
0.390
0.718
0.662
0.686
0.415
0.600
0.472
0.453
120
134
123
123
107
85
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, VD
updated for OH predissociation calculations of Schleicher & A’Hearn 1988 assuming ~€ƒ‚„†…: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 extra
photons shifted to BR3; NL + VD, Nee & Lee 1984 experimental cross sections
divided by 2.5 to match VD total cross section in the ) ,,‰ˆ )-Š,/, Å region.
<
NL + VDII, same as NL
+
VD
but
NL
cross
section
at
Ly
& is not scaled (these
>
values are used as BR ( and BR ( in Table 1); H/NL, treatment of NL cross
<
sections in Huebner et al. 1992.
| OH lifetime in kiloseconds.
Predicted water production rates
Radial profiles are weak evidence because of known difficulties with the
Haser model
Counting photons (photometry) is much easier
5
Convert photometric observations to water production rates ( L!‹"U )
5
Fig. 9.— ( !S" ) values from various works. Open symbols denote production
rates derived with standard ! " and OH photochemistry (denoted “VD + S88”
in the figure). Filled symbols are the same but with a modified U! ^ h j
XŒ^ ! 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) (theoretical) 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
in [O I] 6300 Å case
v^!
branching ratio gives correct answer
Provisional solution proposed by Morgenthaler et al. (2001): use the van
Dishoeck & Dalgarno cross section from ) ,,Žˆ -) Š,/, Å to scale the Nee
& Lee cross section down, except at ‘’& . <
Clearly a more satisfying solution is a recalculation of the OH photodissociation cross section
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This preprint was prepared with the AAS L“ TEX macros v5.0.