We report a search for emissions from the OH Meinel system in high-resolution near-infrared spectra of comet P/Swift-Tuttle. Because of the large cometary heliocentric velocity and high resolution of the spectrograph, the cometary lines should be well separated from the bright OH sky lines. Contrary to the findings of Tozzi et al. (1994) - who report seeing cometary OH at intensities comparable to the sky emissions in their low-resolution spectra - we find no OH in these spectra with an upper limit of 5% the value of the night sky lines. The non-detection of these cometary lines is consistent with theoretical calculations of expected emission strengths from prompt and fluorescent emission from cometary OH

Bouchez, A. H.

Brown, M. E.

Johns-Krull, C. M.

Spinrad, H.

English

Caltech Authors - Main

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Astron. Astrophys. 301, Ll-L4 (1995) 
Letter to the Editor 
ASTRONOMY 
AND 
ASTROPHYSICS 
Non-detection of the OH Meinel system 
in comet P /Swift-Tuttle 
M.E. Brown1•2·, A.H. Bouchez2, H. Spinrad2, and C.M. Johns-Krull2•3 
1 Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA 
2 Department of Astronomy, University of California, Berkeley, CA 94720, USA 
3 McDonald Observatory, University of Texas at Austin, Austin, TX 78712, USA 
Received 9 March 1995 I Accepted 2 May 1995 
Abstract. We report a search for emissions from the 
OH Meine! system in high-resolution near-infrared spec-
tra of comet P /Swift-Tuttle. Because of the the large 
cometary heliocentric velocity and high resolution of the 
spectrograph, the cometary lines should be well separated 
from the bright OH sky lines. Contrary to the findings 
of Tozzi et al. (1994) - who report seeing cometary OH 
at intensities comparable to the sky emissions in their 
low-resolution spectra - we find no OH in these spec-
tra, with an upper limit of 5% the value of the night 
sky lines. The non-detection of these cometary lines is 
consistent with theoretical calculations of expected emis-
sion strengths from prompt and fluorescent emission from 
cometary OH. 
Key words: Comet - OH - Spectra 
1. Introduction 
OH is a primary dissociation product of water, the most 
abundant cometary constituent. Measurements of OH in-
tensities at UV and radio wavelengths are often used to 
infer the production rate of cometary water. Thus, the 
report of detection of the near-infrared OH Meine! sys-
tem using low-resolution visible spectroscopy in comet 
P /Swift-Tuttle gave hope that more readily-available op-
tical telescopes could be used to measure OH intensities 
and thus monitor the water production rates in comets 
(Tozzi et al., 1994). P/Swift-Tuttle was a good test case, 
because the bright comet was closely monitored at many 
wavelengths, and the OH production rate was well know 
(Feldman et al. 1993, Bockelee-Morvan et al. 1994). How-
ever, the line strengths reported by Tozzi et al. are be-
tween 20 and 1000 times stronger than those predicted 
from calculations of the prompt or fluorescent emission 
Send offprint requests to: M.E. Brown 
strengths, requiring the e.'<istence of an unknown excita-
tion mechanism for the OH Meine} system. 
In an attempt to confirm the observations of Tozzi et 
al., we examined high-resolution spectra of comet P /Swift-
Tuttle obtained just a few days after their observations. 
Because of the high resolution of the spectra and the 
large geocentric velocity of the comet, the positions of the 
cometary OH emission lines are well separated from the 
night sky lines, so we do not need to perform the difficult 
task of subtracting the sky lines to see the comet lines. A 
careful examination of the spectrum reveals 55 night-sky 
OH lines, but no cometary lines. 
2. Observations 
Observations of comet P /Swift-Tuttle were made us-
ing the 0.6-m coude auxiliary telescope attached to the 
Hamilton echelle spectrograph at Lick Observatory (Vogt, 
1988). Orders 58-149 of the cross-dispersed spectrum were 
recorded using a 20482 Ford CCD (binned 2 x 2 to de-
crease readout noise), giving essentially continuous spec-
tral coverage from 3800 - 9800A with a resolution of 
>.j A>. "' 32,000. The spectral aperture for the observa-
tions was 6.8 by 12.5 arc-seconds, which corresponds to 
an area of 6000 by 10000 km at the cometary position. 
The comet spectrum was obtained at 2:02 UT on 18 
November 1992, from a single 3000 secoqd exposure with 
a median air mass of 1.65. Flat-fielding was performed by 
dividing the comet spectrum by the spectrum of an inter-
nal quartz lamp. The spectral orders are curved on the 
CCD chip, and we made a 3 pixel wide extraction by fol-
lowing the curvature on a template quartz spectrum. The 
grating blaze function was removed by fitting a smooth 
continuum to each order of the template spectrum and 
dividing the comet orders by that continuum. 
Wavelength calibration was performed by comparison 
with a thorium-argon lamp with a large number of lines 
© European Southern Observatory • Provided by the NASA Astrophysics Data System 
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L2 
of known wavelength. A two-dimensional fit is made to 
simultaneously obtain a wavelength solution for all spec-
tral orders. Based on comparisons with OH night emis-
sion lines and cometary CN lines of known wavelength, 
the wavelength scale is accurate to better than 0.05A in 
the region of interest. The wavelength scale is then shifted 
to account for the 15.5 km s- 1 geocentric comet velocity. 
No absolute calibrations were performed, so we com-
pare all intensities to the intensities of the OH night sky 
lines. From the experience of obtaining many calibrated 
lower resolution spectra, we believe that these intensities 
vary by no more than a factor of 2 to 3. 
3. Results 
A search of the visible and near-infrared regions of the 
spectrum reveals no OH emissions from the comet, but 
55 OH sky emission lines from 11 different bands between 
6200 and 9800A, all shifted by the expected 15.5 km s- 1 
from the cometary positions. As an example, we show in 
Figure 1 a portion of the (7-3) band with the sky and 
expected comet OH positions marked. 
en 
c 
v 
-c 
v 
> 
-0 
v 
.... 
60 :1_ :1_ =·-,,. :.• ;,o 
=·& :•g :,E 
:lu u :.8 
:• I ,, 40 :, I :, :, I ;, 
:1 :. 
,, ,, 
,, ,, 
20 ., ,, I ., 
-20~~~--~--~----~~~ 
8900 8910 8920 8930 8940 
wavelength (A) 
Fig. 1. A section of the OH (7-3) band showing the P2(3), the 
P1 (2), and the P2 ( 4) night-sky lines. These lines are typical 
of the signal-to-noise in the other 52 detected sky lines. The 
expected cometary emission positions are labeled by the dashed 
line. i-To OH emission from the comet are seen. 
To search for faint emissions from the comet, we took 
all detected sky emission lines, shifted them to a common 
center, and combined them (either weighted by signal-to-
noise of the sky line, or medianed). Because the spectrum 
is at constant dispersion, the expected cometary line po-
sition is always shifted by 3.14 pixels (15.5 km s- 1) from 
the sky line, so we look for faint emission at that location, 
as shown in Figure 2 (a) and (b). We fit a gaussian line 
profile to the composite sky line and show the residuals in 
Figure 2 (c) and (d). Based on these residuals, we place a 
1.0 
0.8 
0.6 
0.00 14.---+ffl-++-t---1 
-0.02 
-0.04 
-O.OSL----.i...----..r-
-100 -50 0 50 100 
relative velocity (km/s) 
1.0 me<:uon 
0.8 
0.6 
0.4 
0.2 
0.0 
-100 -50 0 50 100 
relative velocity (km/s) 
Fig. 2. A combination of all of the 0 H sky lines, shifted to 
a common center to increase signal-to-noise. In (a) the lines 
a.re weighted by the signal-to-noise and then averaged, in (b) a 
median of all of the lines is taken. The solid vertical line at a po-
sition of 15.5 km s-1 shows the expected position of cometary 
OH. Plots (c) and (d) show the residuals remaining after sub-
tracting a gaussian profile fit to the sky line. From these resid-
uals, we place a 30' upper-limit of 5% to the comet/sky OH 
emission intensity ratio. 
3-u upper limit of 5% on the ratio between cometary and 
sky emission. 
4. Discussion 
Based on the IUE production rate obtained on 16 Novem-
ber 1992 (Feldman et al. 1993) and using the upper limits 
for prompt emission calculated by Tozzi et al. (1994), we 
estimate a theoretical upper limit to emission by water-
photodissociation-excited OH of about 1 kR in our 6000 x 
10000 km projected aperture. Assuming that the OH sky 
brightness for these 55 lines is approximately equal to the 
brightness Krassovsky et al. (1962) measured (a total of 
14 kR for the 55 observed lines), we then predict an upper 
limit of 6 x 10-2 for the ratio of cometary to sky emission 
line intensities. As noted by Tozzi et al. (1994), intensi-
ties from pure fluorescent emission of OH are at least an 
order of magnitude lower. This theoretical upper limit is 
consistent with our measured upper limit of 5 x 10- 2 for 
this ratio. 
In contrast, Tozzi et al. (1994), using long-slit low-
resolution spectra, reported detection of OH emission from 
P /Swift-Tuttle at an average level of about 75% of the sky. 
Because of dilution in our larger aperture, this level would 
correspond to an average ratio of about 40% in our obser-
vations, or about an order of magnitude above our 3u 
upper-limit. Several convincing factors led Tozzi et al. to 
believe that their detections of 0 H emission lines were not 
spurious, including the reproducibility of the lines with 
© European Southern Observatory • Provided by the NASA Astrophysics Data System 
19
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different exposures and conditions, the structural differ-
ences between the cometary and sky lines, and the ap-
parent (but non-Doppler) shift between the comet and 
sky lines. No explanation for these characteristics seemed 
plausible unless the OH lines had a cometary origin. Be-
cause the water production rate and therefore the OH den-
sity were known, an unknown excitation mechanism had 
to be invoked to explain the apparently high emission rate 
ofOH. 
The Tozzi et al. observations took place 12 and 14 
November 1992 (6 and 4 days before these), and covered 
the (3-0) and (9-5) bands. The only direct spectral over-
lap with our observations is in 3 lines of the (3-0) band. 
A comparison between our spectrum and the measured 
values of Tozzi et al. (corrected for aperture dilution) for 
that band is shown in Figure 3. In addition, we observe 10 
other bands which show no cometary emission, including 
the (9-4) and (9-3) which should respond to any unusual 
excitation in a manner similar to the (9-5) band. 
~ 
(/) 
c: 
<1.) 
..... 
c:: 
<1.) 
-~ 
...... 
30 0,( 1) R1(1) P1(2) 
20 
10 
~ 0~~~~~~~--~~~~~~~+*~~ 
'-
-10~~~~~~~~~~~~~~~~~ 
86 88 90 92 94 
9700+ 
16 18 20 22 2468 70 72 74 76 
9700+ 9800+ 
wavelength (A) 
Fig. 3. A plot of the three lines common to these observations 
a.nd. those of Tozzi et al. (1994). The dotted line shows the 
posi,ion of OH night-sky emission, a.nd the dashed line shows 
the expected position of cometary OH emission. The height of 
the dashed line corresponds to the relative intensity of the line 
measured by Tozzi et al., compared to the sky line. 
The discrepancy between these observations is difficult 
to reconcile. While the observed variability of P /Swift-
Tuttle might be invoked to try to explain the differences 
between these two observations, this explanation seems 
unlikely. Visible imaging of dust and gas jets shows that 
the nucleus was rotating with a period of about 2.8 days 
(Boehnhardt & Birkle, 1994). The two observations of 
Tozzi et al. were taken 2 days apart, so were about 100 
degrees out of phase, and the apparent OH emission in-
tensity was apparently similar for both observations. Our 
observation occured approximately 81 hours after their 
L3 
second observation, so the nucleus had rotated once and 
was approximately midway in position between the two 
Tozzi et al. observations, but showed no emission. If the 
enhanced OH emission was periodic with the nuclear ro-
tation and was smoothly varying, we would expect our 
observations to show similar emissions. While it is pos-
sible that Tozzi et al. caught the nucleus at two special 
locations, this fortuitous arrangement seems unlikely. 
Along with its spin-periodic variabilities, P /Swift-
'futtle could have exhibited more random variabilities be-
tween the times of the observations which could have po-
tentially placed the OH in an unusual state. Because it 
was a bright well-placed comet, it was observed exten-
sively during this time. Observations of Bockelee-Morvan 
et al. (1994) of the OH radio lines during this time show 
an OH production rate that is smoothly varying with he-
liocentric distance (though because of the large apera-
ture these observations are insensitive to short-term vari-
ations). Boehnhardt & Birkle (1994) imaged the coma 
nightly between 12 and 27 November and saw no unusual 
non-rotational effects. However, in the same spectra in 
which they reported an OH detection, Tozzi et al. (1993) 
showed that the CN spatial profiles implied the presence 
of intense activity on the comet. While none of these ob-
servations is directly applicable to the near-infrared OH 
bands or the unknown excitation mechanism, they do give 
a general indication that the comet was active, but per-
haps not unusually so, during the observations. 
Finally, we consider observational effects. Tozzi et al. 
carefully discuss all reasonable effects that might have in-
fluenced their observations and conclude that the apparent 
OH emission lines are real. We can think of no additional 
mechanism which would cause spurious emission lines but 
point out that attempting to extract cometary lines in 
the presence of strong sky lines using a low-resololution 
spectrum is a difficult task in which errors could occur. 
Based on the non-detection of the OH Meinel band in 
our observations and the difficulty of explaining the inten-
sities and electron populations observed by Tozzi et al, we 
conclude that current estimates of fluorescent and prompt 
emission efficiencies are consistent with the observations 
and that no new excitation mechanism is required or re-
vision of the OH physics is required. The prospect that 
cometary OH might be observable with readily available 
ground-based equipment remains intriguing, and efforts to 
observe these lines should continue. 
Acknowledgements. This research was supported by NASA 
grant# NAGW-2883. We thank G.P. Tozzi for enlighten-
ing comments on the manuscript. The staff at Lick Ob-
servatory, and Tony Misch in particular, continue to give 
invaluable assistance with cometary observations. 
References 
© European Southern Observatory • Provided by the NASA Astrophysics Data System 
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lA 
Bockelee-Morvan, D., Bourgois, G., Colom, P., Crovisier, 
J ., Gerard, E., Jorda, 1., 1994, Planet. Spac. Sci., 42, 
193. 
Boehnhardt, H., Birckle, K. 1994, A&A Suppl., 107, 101. 
Crovisier, J. 1989, A&A 213, 459. 
Feldman, P.D., McPhate, J.B., A'Hearn, M.F., et al. 1993, 
IAU Symposium 160: Asteroids, Comets, Meteors 1993 
- Beligrate, Novara (Italy)- LPI contr., N 810, p. 106. 
Krassovsky, V.I., Shefov, N.N., Yarin, V.I., 1962, Planet. 
Space Sci., 9, 883. 
Tozzi, G.P., Feldman, P.O., Weaver, H.A. 1993, BAAS, 
23, 1050. 
Tozzi, G.P., Feldman, P.O., Weaver, H.A., 1994, A&A, 
285, 19. 
Turball, D.N., Lowe, R.P., 1989, Planet. Space Sci., 37, 
723. 
Vogt, S. 1988, Pub. Astron. Soc. Pacific, 99, 1214. 
This article was processed by the author using Springer-Verlag 
TEX A&A macro package 1992. 
© European Southern Observatory • Provided by the NASA Astrophysics Data System 


Non-detection of the OH Meinel system in comet P/Swift-Tuttle

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Astron. Astrophys. 301, Ll-L4 (1995) 
Letter to the Editor 
ASTRONOMY 
AND 
ASTROPHYSICS 
Non-detection of the OH Meinel system 
in comet P /Swift-Tuttle 
M.E. Brown1•2·, A.H. Bouchez2, H. Spinrad2, and C.M. Johns-Krull2•3 
1 Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA 
2 Department of Astronomy, University of California, Berkeley, CA 94720, USA 
3 McDonald Observatory, University of Texas at Austin, Austin, TX 78712, USA 
Received 9 March 1995 I Accepted 2 May 1995 
Abstract. We report a search for emissions from the 
OH Meine! system in high-resolution near-infrared spec-
tra of comet P /Swift-Tuttle. Because of the the large 
cometary heliocentric velocity and high resolution of the 
spectrograph, the cometary lines should be well separated 
from the bright OH sky lines. Contrary to the findings 
of Tozzi et al. (1994) - who report seeing cometary OH 
at intensities comparable to the sky emissions in their 
low-resolution spectra - we find no OH in these spec-
tra, with an upper limit of 5% the value of the night 
sky lines. The non-detection of these cometary lines is 
consistent with theoretical calculations of expected emis-
sion strengths from prompt and fluorescent emission from 
cometary OH. 
Key words: Comet - OH - Spectra 
1. Introduction 
OH is a primary dissociation product of water, the most 
abundant cometary constituent. Measurements of OH in-
tensities at UV and radio wavelengths are often used to 
infer the production rate of cometary water. Thus, the 
report of detection of the near-infrared OH Meine! sys-
tem using low-resolution visible spectroscopy in comet 
P /Swift-Tuttle gave hope that more readily-available op-
tical telescopes could be used to measure OH intensities 
and thus monitor the water production rates in comets 
(Tozzi et al., 1994). P/Swift-Tuttle was a good test case, 
because the bright comet was closely monitored at many 
wavelengths, and the OH production rate was well know 
(Feldman et al. 1993, Bockelee-Morvan et al. 1994). How-
ever, the line strengths reported by Tozzi et al. are be-
tween 20 and 1000 times stronger than those predicted 
from calculations of the prompt or fluorescent emission 
Send offprint requests to: M.E. Brown 
strengths, requiring the e.'<istence of an unknown excita-
tion mechanism for the OH Meine} system. 
In an attempt to confirm the observations of Tozzi et 
al., we examined high-resolution spectra of comet P /Swift-
Tuttle obtained just a few days after their observations. 
Because of the high resolution of the spectra and the 
large geocentric velocity of the comet, the positions of the 
cometary OH emission lines are well separated from the 
night sky lines, so we do not need to perform the difficult 
task of subtracting the sky lines to see the comet lines. A 
careful examination of the spectrum reveals 55 night-sky 
OH lines, but no cometary lines. 
2. Observations 
Observations of comet P /Swift-Tuttle were made us-
ing the 0.6-m coude auxiliary telescope attached to the 
Hamilton echelle spectrograph at Lick Observatory (Vogt, 
1988). Orders 58-149 of the cross-dispersed spectrum were 
recorded using a 20482 Ford CCD (binned 2 x 2 to de-
crease readout noise), giving essentially continuous spec-
tral coverage from 3800 - 9800A with a resolution of 
>.j A>. "' 32,000. The spectral aperture for the observa-
tions was 6.8 by 12.5 arc-seconds, which corresponds to 
an area of 6000 by 10000 km at the cometary position. 
The comet spectrum was obtained at 2:02 UT on 18 
November 1992, from a single 3000 secoqd exposure with 
a median air mass of 1.65. Flat-fielding was performed by 
dividing the comet spectrum by the spectrum of an inter-
nal quartz lamp. The spectral orders are curved on the 
CCD chip, and we made a 3 pixel wide extraction by fol-
lowing the curvature on a template quartz spectrum. The 
grating blaze function was removed by fitting a smooth 
continuum to each order of the template spectrum and 
dividing the comet orders by that continuum. 
Wavelength calibration was performed by comparison 
with a thorium-argon lamp with a large number of lines 
© European Southern Observatory • Provided by the NASA Astrophysics Data System 
1
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of known wavelength. A two-dimensional fit is made to 
simultaneously obtain a wavelength solution for all spec-
tral orders. Based on comparisons with OH night emis-
sion lines and cometary CN lines of known wavelength, 
the wavelength scale is accurate to better than 0.05A in 
the region of interest. The wavelength scale is then shifted 
to account for the 15.5 km s- 1 geocentric comet velocity. 
No absolute calibrations were performed, so we com-
pare all intensities to the intensities of the OH night sky 
lines. From the experience of obtaining many calibrated 
lower resolution spectra, we believe that these intensities 
vary by no more than a factor of 2 to 3. 
3. Results 
A search of the visible and near-infrared regions of the 
spectrum reveals no OH emissions from the comet, but 
55 OH sky emission lines from 11 different bands between 
6200 and 9800A, all shifted by the expected 15.5 km s- 1 
from the cometary positions. As an example, we show in 
Figure 1 a portion of the (7-3) band with the sky and 
expected comet OH positions marked. 
en 
c 
v -c 
v 
> -0 
v .... 
60 :1_ :1_ =·-,,. :.• ;,o 
=·& :•g :,E :lu u :.8 
:• I ,, 40 :, I :, :, I ;, 
:1 :. ,, ,, ,, ,, 
20 
., ,, 
I 
., 
-20~~~--~--~----~~~ 
8900 8910 8920 8930 8940 
wavelength (A) 
Fig. 1. A section of the OH (7-3) band showing the P2(3), the 
P1 (2), and the P2 ( 4) night-sky lines. These lines are typical 
of the signal-to-noise in the other 52 detected sky lines. The 
expected cometary emission positions are labeled by the dashed 
line. i-To OH emission from the comet are seen. 
To search for faint emissions from the comet, we took 
all detected sky emission lines, shifted them to a common 
center, and combined them (either weighted by signal-to-
noise of the sky line, or medianed). Because the spectrum 
is at constant dispersion, the expected cometary line po-
sition is always shifted by 3.14 pixels (15.5 km s- 1) from 
the sky line, so we look for faint emission at that location, 
as shown in Figure 2 (a) and (b). We fit a gaussian line 
profile to the composite sky line and show the residuals in 
Figure 2 (c) and (d). Based on these residuals, we place a 
1.0 
0.8 
0.6 
0.00 14.---+ffl-++-t---1 
-0.02 
-0.04 
-O.OSL----.i...----..r-
-100 -50 0 50 100 
relative velocity (km/s) 
1.0 me<:uon 
0.8 
0.6 
0.4 
0.2 
0.0 
-100 -50 0 50 100 
relative velocity (km/s) 
Fig. 2. A combination of all of the 0 H sky lines, shifted to 
a common center to increase signal-to-noise. In (a) the lines 
a.re weighted by the signal-to-noise and then averaged, in (b) a 
median of all of the lines is taken. The solid vertical line at a po-
sition of 15.5 km s-1 shows the expected position of cometary 
OH. Plots (c) and (d) show the residuals remaining after sub-
tracting a gaussian profile fit to the sky line. From these resid-
uals, we place a 30' upper-limit of 5% to the comet/sky OH 
emission intensity ratio. 
3-u upper limit of 5% on the ratio between cometary and 
sky emission. 
4. Discussion 
Based on the IUE production rate obtained on 16 Novem-
ber 1992 (Feldman et al. 1993) and using the upper limits 
for prompt emission calculated by Tozzi et al. (1994), we 
estimate a theoretical upper limit to emission by water-
photodissociation-excited OH of about 1 kR in our 6000 x 
10000 km projected aperture. Assuming that the OH sky 
brightness for these 55 lines is approximately equal to the 
brightness Krassovsky et al. (1962) measured (a total of 
14 kR for the 55 observed lines), we then predict an upper 
limit of 6 x 10-2 for the ratio of cometary to sky emission 
line intensities. As noted by Tozzi et al. (1994), intensi-
ties from pure fluorescent emission of OH are at least an 
order of magnitude lower. This theoretical upper limit is 
consistent with our measured upper limit of 5 x 10- 2 for 
this ratio. 
In contrast, Tozzi et al. (1994), using long-slit low-
resolution spectra, reported detection of OH emission from 
P /Swift-Tuttle at an average level of about 75% of the sky. 
Because of dilution in our larger aperture, this level would 
correspond to an average ratio of about 40% in our obser-
vations, or about an order of magnitude above our 3u 
upper-limit. Several convincing factors led Tozzi et al. to 
believe that their detections of 0 H emission lines were not 
spurious, including the reproducibility of the lines with 
© European Southern Observatory • Provided by the NASA Astrophysics Data System 
1
9
9
5
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&
A
.
.
.
3
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L
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.
.
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different exposures and conditions, the structural differ-
ences between the cometary and sky lines, and the ap-
parent (but non-Doppler) shift between the comet and 
sky lines. No explanation for these characteristics seemed 
plausible unless the OH lines had a cometary origin. Be-
cause the water production rate and therefore the OH den-
sity were known, an unknown excitation mechanism had 
to be invoked to explain the apparently high emission rate 
ofOH. 
The Tozzi et al. observations took place 12 and 14 
November 1992 (6 and 4 days before these), and covered 
the (3-0) and (9-5) bands. The only direct spectral over-
lap with our observations is in 3 lines of the (3-0) band. 
A comparison between our spectrum and the measured 
values of Tozzi et al. (corrected for aperture dilution) for 
that band is shown in Figure 3. In addition, we observe 10 
other bands which show no cometary emission, including 
the (9-4) and (9-3) which should respond to any unusual 
excitation in a manner similar to the (9-5) band. 
~ 
(/) 
c: 
<1.) ..... 
c:: 
<1.) 
-~ ...... 
30 0,( 1) R1(1) P1(2) 
20 
10 
~ 0~~~~~~~--~~~~~~~+*~~ 
'-
-10~~~~~~~~~~~~~~~~~ 
86 88 90 92 94 
9700+ 
16 18 20 22 2468 70 72 74 76 
9700+ 9800+ 
wavelength (A) 
Fig. 3. A plot of the three lines common to these observations 
a.nd. those of Tozzi et al. (1994). The dotted line shows the 
posi,ion of OH night-sky emission, a.nd the dashed line shows 
the expected position of cometary OH emission. The height of 
the dashed line corresponds to the relative intensity of the line 
measured by Tozzi et al., compared to the sky line. 
The discrepancy between these observations is difficult 
to reconcile. While the observed variability of P /Swift-
Tuttle might be invoked to try to explain the differences 
between these two observations, this explanation seems 
unlikely. Visible imaging of dust and gas jets shows that 
the nucleus was rotating with a period of about 2.8 days 
(Boehnhardt & Birkle, 1994). The two observations of 
Tozzi et al. were taken 2 days apart, so were about 100 
degrees out of phase, and the apparent OH emission in-
tensity was apparently similar for both observations. Our 
observation occured approximately 81 hours after their 
L3 
second observation, so the nucleus had rotated once and 
was approximately midway in position between the two 
Tozzi et al. observations, but showed no emission. If the 
enhanced OH emission was periodic with the nuclear ro-
tation and was smoothly varying, we would expect our 
observations to show similar emissions. While it is pos-
sible that Tozzi et al. caught the nucleus at two special 
locations, this fortuitous arrangement seems unlikely. 
Along with its spin-periodic variabilities, P /Swift-
'futtle could have exhibited more random variabilities be-
tween the times of the observations which could have po-
tentially placed the OH in an unusual state. Because it 
was a bright well-placed comet, it was observed exten-
sively during this time. Observations of Bockelee-Morvan 
et al. (1994) of the OH radio lines during this time show 
an OH production rate that is smoothly varying with he-
liocentric distance (though because of the large apera-
ture these observations are insensitive to short-term vari-
ations). Boehnhardt & Birkle (1994) imaged the coma 
nightly between 12 and 27 November and saw no unusual 
non-rotational effects. However, in the same spectra in 
which they reported an OH detection, Tozzi et al. (1993) 
showed that the CN spatial profiles implied the presence 
of intense activity on the comet. While none of these ob-
servations is directly applicable to the near-infrared OH 
bands or the unknown excitation mechanism, they do give 
a general indication that the comet was active, but per-
haps not unusually so, during the observations. 
Finally, we consider observational effects. Tozzi et al. 
carefully discuss all reasonable effects that might have in-
fluenced their observations and conclude that the apparent 
OH emission lines are real. We can think of no additional 
mechanism which would cause spurious emission lines but 
point out that attempting to extract cometary lines in 
the presence of strong sky lines using a low-resololution 
spectrum is a difficult task in which errors could occur. 
Based on the non-detection of the OH Meinel band in 
our observations and the difficulty of explaining the inten-
sities and electron populations observed by Tozzi et al, we 
conclude that current estimates of fluorescent and prompt 
emission efficiencies are consistent with the observations 
and that no new excitation mechanism is required or re-
vision of the OH physics is required. The prospect that 
cometary OH might be observable with readily available 
ground-based equipment remains intriguing, and efforts to 
observe these lines should continue. 
Acknowledgements. This research was supported by NASA 
grant# NAGW-2883. We thank G.P. Tozzi for enlighten-
ing comments on the manuscript. The staff at Lick Ob-
servatory, and Tony Misch in particular, continue to give 
invaluable assistance with cometary observations. 
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Non-detection of the OH Meinel system in comet P/Swift-Tuttle

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