Comet P/Brorsen-Metcalf (1989o) has been extensively observed in the visible and in the ultraviolet during its latest apparition of summer 1989. In this paper we report a preliminary determination of the C2 production rates and lifetimes and we compare those rates to the H2O production rates obtained from UV data

Barbieri, C.

Festou, Michel C.

Lazzarin, M.

Tozzi, Giau Paolo

English

NASA Technical Reports Server

Asteroids, Comets, Meteors 1991, pp. 345-348
Lunar and Planetary Institute, Houston, 1992 3459o
.7
HELIOCENTRIC DISTANCE DEPENDENCIES OF THE C2
LIFETIME AND C2 PARENT PRODUCTION RATE IN COMET
P/BRORSEN-METCALF (19890)
Lazzarin M. 1, Tozzi G.P. 2, Barbieri C. 1'3 and Festou M.C. 4
x Dipartimento di Astronomia dell'Universitk - Vicolo dell'Osservatorio - I 35122 Padova
20sservatorio Astrofisico di Arcetri - Largo E. Fermi 5 - I 50125 Firenze
30sservatorio Astronomieo di Padova - Vicolo dell'Osservatorio - I 35122 Padova
40bservatoire Midi-Pyr_n_es - 14, Av. Edouard-Belin - F 31400 Toulouse
1 Summary
Comet P/Brorsen-Metcalf (1989o) has been extensively observed in the visible and in
the ultraviolet during its latest apparition of summer 1989. In this paper we report a
preliminary determination of the C2 production rates and lifetimes and we compare those
rates to the H_O production rates obtained from UV data.
2 O bst'e_'_at ions.
Visible spectra of P/Brorsen-Metcalf (19890) were recorded from 26 July until 25 August
1989 while the heliocentric and the geocentric distances changed from 1.1 to 0.6 AU and
0.6 to 0.7 AU, respectively. This set of 14 spectrophotometric optical observations (7 of
them analyzed; see table I) overlaps a set of 6 ultraviolet observations obtained with the
International Ultraviolet Explorer (IUE) collected around 1 August 1989.
The long slit visible spectra were collected with the 1.5 m f/8 Cassegrain telescope of the
Loiano Observatory (I) equipped with a Boller& Chivens spectrograph and a CCD as de-
tector. As shown on table I, the spectra cover usually the range 4500-5800/_ or 4000-7000
/_, with an instrumental FWHM of 5 or 10 _,, depending on the diffraction grating used.
The projected slit length was 4.8 arc min on the sky (=t, 120-150 103 km at the comet
distance) with a pixel size of 1.6 arc sec (=_ 700-800 kin). The effective spatial resolution
was dominated by the seeing conditions and by the guiding errors of the telescope. In a
typical night of observation two cometary spectra were recorded, with different slit position
angles: one in the sun-comet direction and the other perpendicular to that line. Bias, flat
field, calibration lamp, sky and standard star spectra were recorded each night in order to
calibrate the cometary spectra. These data allow the study of the spatial profiles of the
C2, Av = +1,0,-1 Swan band emissions and the determination of C2 production rates as
a function of heliocentric distance.
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346 Asteroida, Comets, Meteors 1991
Low resolution IUE spectra recorded between 27 July and 5 August 1989 are used to de-
termine the water production_rate. _ i_ : , i:
Table I - Observing log of the analyzed visible spectra
Date Rh A /3
AU AU deg.
29 Jul. 1.09 0.67 65.2
31 Jul. 1.06 0.65 66.5
7 Aug. 0.94 0.62 77.9
10 Aug. 0.89 0.63 82.0
13 Aug. 0.84 0.64 85.6
18 Aug. 0.75 0.68 89.9
21 Aug. 0.70 0.71 91.1
Spectral
Range (_)
3800-6800
3800-6800
3800-6800
5800-7100
4500-5800
4500-5800
4500-5800
Dispersion
/_/pixel
5.9
5.9
5.9
2.6
2.6
2.6
2.6
=il : :7: :?
3 Data reduction and analysis.
Since the comet and sky spectra were not recorded at the same time and in similar con-
ditions, sky spectra were only used to determine the position of the strongest undesirable
lines in the comet spectra. Those latter were used to perform the sky subtraction in the
manner described by Festou et al. (1990), i. e. by using information contained near the
edge of each frame, away from the emission lines of the comet.
The dust component in this comet was generally weak, so that no particular care was
necessary to subtract it from the cometary spectra. When it was stronger, at the end of
August, a synthetic dust spectrum has been created and subtracted.
The radial intensity profiles for each observed species were constructed using the largest
possible bandpass. The actual extension of these profiles were almost the same, of the
order of 105 km during the entire observing period, which allowed us to perform an inves-
tigation of the creation and destruction processes of the observed species.
After discarding the non photometric data (standard stars were observed before and after
the comet observations), it was found that the absolute calibration errors were always be-
low the +15% level.
Absolute calibration, reduction procedure, seeing, tracking qua!!ty and S/N affect the rel-
ative uncertaint:gof the individual data points.in a complex manner. The sky subtraction
procedure can a priori introduce a significant systematic error:_ our_technique, _ha_'_use
the comet..... frames themselves in regions where n0comet l!nes ar e Present , a!10ws_to reduce
this source of uncertainty to a very 10w l_vel. Fat ` from thecenter of thecoma-, where the
signal is weak, the scattering:0f'the data points gives an idea 9 f the internal consistency of
the data. Near the center of the coma, Where relative photometric inaccuracies are small,
the shape of the profile is mostly determined by the tracking quality. The quality of this
parameter Can be determined by examining the shape of the continuum near the center of
Asteroids, Comets, Meteors 1991 347
the coma: in the worse case, the combined effects of tracking and seeing are equivalent to
a drift of 5 arc seconds along the sun-comet line. That effect was taken into account in
the data analysis by appropriately enlarging the simulated instrumental slit.
4 Interpretation.
The entire data set was interpreted with the vectorial model (Festou, 1981a,b). Parent
molecule velocities were assumed to vary with the heliocentric distance according to the
law 0.85Rh °'s km/s (Cochran and Barker, 1986). Water and OH lifetimes were computed
for the solar flux conditions of mid 1989.
The water production rate has been computed from the mean intensity, averaged over the
10 by 20 arc sec IUE slit, of the OH(0-0) and OH(l-0) bands.
The nucleus activity was assumed to be steady, an assumption found a posteriori to be
valid over periods of time of the order of one day. In our model-data fitting attempts,
once the production channel for the coma species is selected (one or two photodissociation
steps), only the parent lifetime, the daughter species velocity and lifetime are adjustable
parameters. In what follows, we will restrict ourselves to the results concerning the C2
radicals.
Guided by preceding analysis of similar nature, we first tested a single photodissociation
as the production channel of the radicals. When the comet is far from the sun, only the
parent lifetime significantly affects the computed profile and this parameter can be evalu-
ated (the lifetime of the radicals is then taken as 105 s at 1 AU and the velocity is set to 1
km/s). A value close to 20 103 s was found using our complete set of profiles. This value is
not compatible with the C2 radicals being produced by the dissociation of C2H2 parents.
When the comet is closer to the sun, since our profiles still extend out to about 105 km,
the radical lifetime and velocity become measurable quantities. The two parameters have
non independent values: as an e×ample, if the C2 velocities are assumed to be 1 and 1.4
km/s, C2 lifetimes of 0.9 to 1.5 l0 s and 1.2 to 2. l0 s seconds are found, respectively.
It is important to note here that none of the inner coma theoretical profiles fits the obser-
vations: the observed profiles are too flat (see Fig.l), a strong indication that either the
C2 radicals are produced via a two step process or that two parents at least contributed
to their production. An extended source of the nature of that found in comet P/Halley
(1986 III) does not seems likely (or it would not contribute much) since the comet was not
very dusty.
We evaluated the C2 production rates assuming the canonical value of 1 km/s for the
radial velocity (other parameters are thus derived quantities). Fig. 2 shows our results:
i) short term variations of no more than 15-30% are possibly present (each point in this
figure represents generally an average of about 6 individual measurements), ii) the long
term variation of the water and the C2 production rates vary according to Rh 3"s+°'3 law,
iii) the water to C2 production rate ratio is of the order of 500, which indicates that comet
348 Asteroids, Comets, Meteors 1991
19890 was "normal", iv) the variation of the C2 lifetime is compatible with a R2h variation
and photodissociation of parent molecules appears as the most likely production process.
C, /_,--0 Aug. 13.08, 1989
' ' I ' ' ' ' I * ' ' ' I ' '
=
5.0 Vp=0.93 krn/s T_=20000. s
V.,=l.O krn/s T==120000. s
4.0
3.0
.}
, , J _ , , • I , , , I , ,
3.0 4.0 5.0
Logto COMETOCENTRIC DISTANCE (lun)
- Fig. I - Intensity profile of Cs _=0 band.
26.5
o
v
<7 26.0
u
25.5
P/Brorsen-Metcalf (1989o)
! J i , i I ! I , | , = , l
,,q(c,)
t = Q(H,O)/500
"'I'%.=,.]..°
t
, , , , I , , , , I .L ,.. , ,
-0.2 -0.I 0.0 O.l
Log,0 HELIOCENTRIC DISTANCE (AU)
Fig. 2 - Gas production rate vs 1_
5 References
Cochran, A. L. and Barker E. S. (1986) Spectrophotometric observations of comet Halley
in Exploration of Italley's Comet pp 439-444 - ESA SP 250
Festou, M. C. (1981a) The density of neutral compounds in cometary atmosphere I.
Astronom. Astrophys., 95, 69-79
Festou, M. C. (1981b) The density of neutral compounds in cometary atmosphere II.
Astronom. Astrophys., 96, 52-57
Festou, M. C., Tozzi, G. P., Smaldone, L. A., Felembok, P., Falciani R. and Zucconi, J.-M.
(1990) Did-an 0utburst occur on 4 December 1985 in Halley's comet? Astronom Astrophys.,
227, 609-618


Heliocentric distance dependencies of the C2 lifetime and C2 parent production rate in comet P/Brorsen-Metcalf (1989o)

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