Due to extremely favorable excitation conditions, comet Levy (1990c) exhibited in August-September 1990 the strongest OH 18-cm signal ever recorded in a comet at the Nancay radio telescope. This unique opportunity was used to measure the OH satellite lines at 1612 and 1721 MHz, to perform extensive mapping of the OH radio emission and to make a sensitive evaluation of the cometary magnetic field, of the H2O outflow velocity and of the OH production rate

Bockelee-Morvan, Dominique

Bourgois, G.

Colom, P.

Crovisier, Jacques

Gerard, E.

English

NASA Technical Reports Server

Asteroids, Comets, Meteors 1991, pp. 73-76
Lunar and Planetary Institute, Houston, 1992
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OBSERVATIONS OF OH IN COMET LEVY WITH THE NANCAY RADIO TELESCOPE
D. Bockel6e-Morvan, P. Colom, J. Crovisier, E. G6rard, G. Bourgois
Observatoire de Paris, Section de Meudon, F-92195 Meudon, France
73
Abstract. Due to extremely favourable excitation conditions, comet Levy (1990c) exhibited in August-
September 1990 the strongest OH 18-cm signal ever recorded in a comet at the Nanqay radio telescope.
This unique opportunity was used to measure the OH satellite lines at 1612 and 1721 MHz, to perform
extensive mapping of the OH radio emission and to make a sensitive evaluation of the cometary magnetic
field, of the H20 outflow velocity and of the OH production rate.
OBSERVATIONS
Comet Levy (1990c) was monitored almost daily, one hour per day, from June 16 to September 30 1990,
simultaneously in the1667 and 1665 MHz main lines of the OH ground state A-doublet (Fig. 2a), both in
right and left circular polarizations. The general observing procedure is detailed in G6rard et al (1989). As
shown in Fig. 1, the signal was most of the time notably larger than that recorded in comet Halley at
Nanqay, and exhibited a strong enhancement at the beginning of September. This can be explained from
the combination of favourable geometry and excellent excitation conditions (Table 1), in particular the
crossing of the galactic plane by the comet on September 5. Indeed, this resulted in the amplification by
the OH maser lines of a continuum background notably larger than the 2.7 K cosmic background. We
took advantage of the strong signal to measure, on Sept. 3 and 4, the satellite OH lines at 1612 and 1721
MHz. Both satellite lines, which were predicted to be 9 times weaker than the 1667 MHz main line, were
detected with a signal-to-noise ratio of about 8 (Fig. 2b, c). To our knowledge, the only reported attempt
to observe the satellite lines in a comet was made at Nanqay in comet Meier 1978 XXI (Despois et al
1979): the lines were detected at a 5-sigma level after nearly one month of observations. From Aug. 21 to
Sept. 1 and Sept. 7 to 18, we spent part of the daily integration time with the beam aimed at offset
positions with respect to the comet centre. The offsets were +7, +10.5 and +14' in right ascension,
corresponding to 2, 3 and 4 beams. The signal was more intense in the anti-Sun side.
4000
Figure 1 • Line area of
the 1667 MHz OH line
in comet Levy (1990c)
as a function of aooo
heliocentric distance. '_'
Daily values are
displayed. The arrow
shows the maximum "_ zooo
intensity recorded at _,'_
Nanqay for P/Halley.
tO00
I !
!
P/Halley
I t 1 , . i I I I t
1.5 2
rh [AU]
https://ntrs.nasa.gov/search.jsp?R=19930009940 2020-03-17T07:34:23+00:00Z
74 Asteroids, Comets, Meteors 1991
Table 1 • Parameters of the OH spectra of comet Levy (1990c) and inferred OH production rates.
date A rh Vh a inv b line area c Bs d Q[OH] e Q[OH] f
AU AU km s -1 mJy.km s"1 km s "1 1028s "1 1028s -1
00616-900627 2.20 2.20 -21.5 0.50 202+11 1.62+0.09 9.5+0.7 12.0+0.9
00701-900705 1.85 2.05 -21.7 0.49 322+__08 1.65+_0.04 14.0+0.6 16.1-t-0.7
00712-900717 1.48 1.91 -21.7 0.49 4195:11 1.565:0.04 14.55:0.5 16.8+0.5
00726-900729 1.08 1.75 -21.7 0.49 571+13 1.58+_0.04 15.4+0.4 16.75:0.4
00731-900803 0.92 1.68 -21.6 0.49 597+12 1.665:0.04 14.7+_0.3 15.0+0.3
00809-900814 0.65 1.56 -21.3 0.50 893+t3 1.755:0.03 21.2+0.3 16.0+0.2
00815-900819 0.51 1.48 -20.9 0.50 961+12 1.74+0.02 16.1:t:0.2 13.6+_0.1
00820-900824 0.44 1.41 °20.6 0.50 10465:17 1.805:0.04 17.25:0.2 13.1+0.2
00825-900901 0.45 1.34 -19.9 0.50 1355:t:21 1.775:0.03 21.5+_0.3 17.05:0.2
00902-900904 0.51 1.29 - 19.3 0.48 2860+38 1.82+0.03
00905-900909 0.58 1.25 -18.8 0.46 2558+29 1.82+0.03
00910-900913 0.70 1.19 -17.8 0.42 860+32 2.09-1-0.09
00914-900918 0.80 1.15 -16.8 0.38 569+20 1.83+0.08 22.3+0.7 16.6+0.5
00919-900923 0.92 1.10 -15.5 0.32 329+14 1.795:0.09 16.2+0.8 13.5+0.6
00925-900929 1.07 1.05 -13.6 0.22 232+15 2.05+0.14 24.2+1.6 17.55:1.1
a : comet heliocentric velocity ; b : OH maser inversion from Despois et al (1981) ; c and d : area and half larger base of
the best trapezia ; e : OH production rate computed according to Bockel6e-Morvan et al (1990) (see text) ; f : OH
production rate computed using the OH density model 1986a, i.e. OH and OH-parent scalelengths of 1.4 105 and 6 104 km
respectively, OH radial velocity of 1.4 km s-1, collisional quenching not being included.
H20 OUTFLOW VELOCITY
Following Bockel6e-Morvan et al (1990a), we applied trapezoidal fitting in order to remove the
contribution of the OH ejection velocity vd in the OH line shapes and determine the H20 outflow velocity
Vp. The results of the fits, in particular the half larger base Bs of the best trapezia which should be close to
Vp+Vd, are displayed in Table 1. Assuming Vd = 0.9 km s-l, the inferred velocities range from 0.6 km s-1
to 1.1 km s -1 and confirm the global increase of the outflow velocity with decreasing heliocentric distance
observed in other comets (Bockel6e-Morvan et al 1990a, b).
OH PRODUCTION RATES AND TIME VARIABILITY
OH production rates were computed with the model of Bockelde-Morvan et al (1990a), which uses the
inferred outflow velocities and takes into account the quenching of the maser by collisions. The results
obtained for selected average periods are given in Table 1. The evaluation of the production rates during
the crossing of the galactic plane requires a careful evaluation of the continuum background brightness
temperature at 18-cm wavelength, which is not discussed here. In Fig. 1, the OH emission exhibits short
term variati0ns, which might be related to the periodicities of i7 andl9 hours seen in UV and visible light
curves (Feldman et al 1991; Schleicher et al 1991).
HYPERFINE ANOMALIES
The 1721:1667:1665:1612 line intensities were expected to be in the statistical weight ratios 1:9:5:1 (i.e.
0.111:1:0.555:0.111). Significant departures from these theoretical ratios are present in comet Levy: the
1721 MHz line is weaker ([1721]/[1667] = 0.069 + 0.012 on Sept. 4); the 1612 MHz line is stronger
([1612]/[1667] = 0.137 + 0.015 on Sept. 3); the [1665]/[1667] ratio is below the normal value before
Sept. 3 (0.506 _0.01 from July 26 to Aug.16; 0.527 + 0.008 from Aug. 24 to Sept. 2) and above it after
(Asteroids, Comets, Meteors 1991 75
(0.582 5:0.008 for Sept. 5-6). [1665]/[1667] anomalous ratios are also present in other comets, with a
systematic trend for enhanced ratios at negative inversions of the OH A-doublet. UV pumping by the Sun,
which is the main excitation mechanism of the OH cometary radio lines, tends to establish normal
hyperfine ratios (Despois et al 1981). Departures from normal ratios should occur if the optical depths are
not negligible. The line intensity is _: (exp(-'c)-1), the optical depth 'c having the opposite sign of the
inversion (Despois et aI 1981). Therefore, for positive inversions, the 1667 MHz line should be enhanced
with respect to the 1665 MHz line, and the main lines should be enhanced with respect to the satellite
lines. This is in contradiction with what is observed in comet Levy. On the other hand, the relative line
intensities are well explained by a net population transfer from F=2 to F=I, occuring within both the upper
and lower levels of the A-doublet (Fig. 2a). Sub-normal values of the [1665]/[ 1667] ratio suggest a more
efficient transfer inside the lower level than in the upper level, while values above the theoretical value
indicate the opposite. A possible explanation for this redistribution of the hyperfine populations might be
inelastic collisions.
Ip
+ 2
, I -,
/ 11665.402 I 1720.530 MHz
a=5/2-_ I I I I
]667t5591 2
F
_']"Iz,,2 a.
' ' ' I ' ' ' ' I ' ' ' ''1 ' ' '
o.,0 2:_ tbi
", , , 1 .... l .... I , , ,
-0.!
Figure 2 : a. The A-doublet of the OH ground
state. Hyperfine splitting has been exaggerated for
clarity; b. The 1721 MHz line detected in comet
Levy on Sept. 4, 1990; c. The 1612 MHz line
detected on Sept. 3. The OH Doppler velocities are
referred to the nucleus.
0,2
vlulvu
, I
u,l,i.o_
C.
' ' ' I J l I I
-5 0 5
Velocity [km s -t]
ZEEMAN EFFECT AND COMETARY MAGNETIC FIELD
The Zeeman effect (i.e. the frequency difference between the left and fight circular polarization
components) of the OH 18-cm lines is proportional to the projection of the magnetic field on the line of
sight, averaged over the line-of-sight and over the beam. GErard (1985) measured the Zeeman effect in the
OH lines of comet Austin 1982 VI and obtained a tentative detection of the projected magnetic field <Bp>
= 50 + 21 nT for Aug. 6-12, 1982. In P/Halley, outside the magnetic cavity where the field was nil,
Giotto measured total field values of several 10 nT (Neubauer et al 1987). As shown in Fig. 3, the
Zeeman effect was clearly detected in comet Levy at the end of August. From Fig. 4, which shows the
evolution of <Bp> for individual days, one can see that <Bp> was nearly constant over the period Aug.
31 to Sept. 6, with a mean value of -22 5:3 nT from Aug. 31 to Sept. 2, the negative sign indicating a
magnetic field directed towards the observer. The averages made over several days before August 31
show that <Bp> was, in this period, less intense. It is interesting to note that the viewing conditions were
rapidly varying at the end of August.
76 Asteroids, Comets, Meteors 1991
Figure 3 : Average 1667-i665
MHz spectrum of Levy on Aug.
31 to Sept. 3 in right-handed
(thick line) and left-handed (dotted
line) circular polarization. The
frequency shift from left to right-
handed polarization corresponds
to a line of sight magnetic field of
22+3 nT towards the observer.
H
0.5
!
-- right
...... left
i . , =| . i . . . , 1 |
-5 0 5
velocity [km s -I]
Figure 4 "The line of sight
projected magnetic field inferred in
comet Levy (1990c) as a function
of time.
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?
-6Q
2O _ 3O
Awlu_ _tombor
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Observations of OH in comet Levy with the Nancay radio telescope

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