We present millimeter-wave observations of HNCO, HC3N, SO, NH2CHO, H(13)CN, and H3O(+) in comet C/1995 O1 (Hale-Bopp) obtained in February-April, 1997 with the Caltech Submillimeter Observatory (CSO). HNCO, first detected at the CSO in comet C/1996 B2 (Hyakutake), is securely confirmed in comet Hale-Bopp via observations of three rotational transitions. The derived abundance with respect to H2O is (4-13) x 10(exp -4). HC3N, SO, and NH2CHO are detected for the first time in a comet. The fractional abundance of HC3N based on observations of three rotational lines is (1.9 +/- 0.2) x 10(exp -4). Four transitions of SO are detected and the derived fractional abundance, (2-8) x 10(exp -3), is higher than the upper limits derived from UV observations of previous comets. Observations of NH2CHO imply a fractional abundance of (1-8) x 10(exp -4). H3O(+) is detected for the first time from the ground. The H(13)CN (3-2) transition is also detected and the derived HCN/H(13)CN abundance ratio is 90 +/- 15, consistent with the terrestrial C-13/C-12 ratio. in addition, a number of other molecular species are detected, including HNC, OCS, HCO(+), CO(+), and CN (the last two are first detections in a comet at radio wavelengths)

Benford, D.

Biver, N.

Bockelee-Morvan, D.

Colom, P.

Crovisier, J.

Despois, D.

Gardner, M.

Lis, D. C.

Mehringer, D. M.

Phillips, T. G.

Rauer, H.

English

NASA Technical Reports Server

NASA/CR- /_ _ff f_ 207519
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New Molecular Species In Comet C/1995 (Hale-Bopp)
Observed with the Caltech Submillimeter Observatory
D.C. Lis, D.M. Mehringer, D. Benford, M. Gardner, T.G. Phillips,
D. Bockel6e-Morvan, N. Biver, P. Colom, J. Crovisier, D. Despois &
H. Rauer.
1998-4
Accepted for publication in
Earth, Moon, and Planets
https://ntrs.nasa.gov/search.jsp?R=19980055133 2020-06-15T23:53:41+00:00Z
NEW MOLECULAR SPECIES IN COMET
C/1995 O1 (HALE-BOPP) OBSERVED WITH THE
CALTECH SUBMILLIMETER OBSERVATORY
D.C. Lis, D.M. Mehringer, D. Benford, M. Gardner and T.G. Phillips
California Institute of Technology, MS 320-,_7, Pasadena, CA 91125, USA
D. Bockelde-Morvan, N. Biver,* P. Colom and J. Crovisier
Observatoire de Paris, F-92195 Meudon Principal Cede, France
D. Despois
Observatoire de Bordeaux, B.P. 89, F-33270, Floirac, France
H. Rauer
DLR, Institut fiir Planetenerkundung, Berlin, Germany
(Received February 4, 1998 ; Accepted in final form March 11, 1998)
Abstract.
We present millimeter-wave observations of HNCO, HC3N, SO, NI-I2CHO,
H13CN, and H30 + in comet C/1995 O1 (Hale-Bopp) obtained in February-April,
1997 with the Caltech Submillimeter Observatory (CSO). HNCO, first detected at
the CSO in comet C/1996 B2 (Hyakutake), is securely confirmed in comet Hale-
Bopp via observations of three rotational transitions. The derived abundance with
respect to H20 is (4 - 13) × 10 -4. HC3N, SO, and NH2CHO are detected for the
first time in a comet. The fractional abundance of HC3N based on observations of
three rotational lines is (1.9 4- 0.2) x 10 -4. Four transitions of SO are detected and
the derived fractional abundancel (2 - 8) × 10 -3, is higher than the upper limits
derived from UV observations of previous comets. Observations of NH2CHO imply
a fractional abundance of (1 - 8) x 10 -4 . H30 + is detected for the first time from the
ground. The H13CN (3 - 2) transition is also detected and the derived HCN/H13CN
abundance ratio is 90 4- 15, consistent with the terrestrial 1_C/13C ratio. In addi-
tion, a number of other molecular species are detected, including HNC, OCS, HCO +,
CO +, and CN (the last two are first detections in a comet at radio wavelengths).
Key words: comets: composition; radio observations; spectroscopy
1. Introduction
The appearance of comet C/1995 O1 (Hale-Bopp) offered a rare oppor-
tunity to probe with unprecedented sensitivity the inner atmosphere of
a comet and to investigate its chemical composition. With the recent
improvements in the quality of telescopes and receivers operating at
millimeter and submillimeter wavelengths, this wavelength range has
been playing an increasingly important role in studies of the volatile
composition of comets (e.g. Lis et al. 1997). In the present paper we
" Currently at the Institute for Astronomy, University of Hawaii, USA.
2 D.C.LISETAL.
summarize results of the observational campaign of comet Hale-Bopp
organized at the CSO concerning new molecular species, ions, and iso-
topic ratios. Preliminary estimates of molecular abundances are pre-
sented. CSO observations of "classical" species are included in the long
term monitoring of the gas production rates (Biver et al., 1998).
2. Observations
Observations of Comet C/1996 O1 (Hale-Bopp) presented here were
carried out between Feb. 17 and Apr. 21, 1997 using the 10.4-m
Leighton telescope of the CSO at the summit of Manna Kea, Hawaii.
CSO facility receivers, spectrometers, and calibration procedures are
described in Lis et al. (1997). A list of the most interesting transitions
detected at the CSO in comet Hale-Bopp is given in Table I along with
the derived production rates for some of the new species. All fraction-
al abundances with respect to H20 given in § 3 have been calculated
using HCN production rates of Biver et al. (1998) and assuming an
HCN fractional abundance of 3 x 10 -3 with respect to H20.
3. Results
3.1. ISOCYANIC ACID (HNCO) AND FORMAMIDE (NH2CHO)
Isocyanic acid, an important constituent of interstellar grains, was first
detected in comet Hyakutake (Lis et al., 1997). A single rotational
transition was detected implying an abundance with respect to H20 of
-,- 7 x 10-4. In comet Hale-Bopp three rotational transitions of HNCO
are detected (Table I). High-resolution spectra of the (110,11 - 100,10)
and (120,12 - 110,11) lines are shown in Fig. 1 (left and middle panels,
respectively). The HNCO abundance is (4- 13) x 10-4 and appears
to be decreasing monotonically over the period of two days. The HCN
production rate increases by a factor of ,,, 1.7 over the same period.
Two transitions of formamide are detected with the CSO and IRAM
30-m telescopes (Bockel_e-Morvan et al., 1998). The derived fractional
abundance, (1 - 8) × 10-4, depends strongly on the assumed photodis-
sociation rate, D. The low value, corresponding to fl = 5 x 10-5 s -I,
gives consistent production rates for the two transitions observed.
3.2. CYANOACETYLENE (HC3N)
A 3a upper limit for the fractional abundance of cyanoacetylene in
comet Hyakutake was 3.5 x 10 -4 (Lis et al., 1997). Three rotational
CSO OBSERVATIONS OF COMET HALE-BOPP
Table I. Selected CSO Observations of Comet Hale-Bopp
Molecule Transition Frequency UT Date f T_tdv a Q/10 _6
HNCO 11o,ll - 100,1o 241.774 Feb 18.7 0.27 0.04 33
12o,12 - 110,11 263.749 Feb 19.7 0.18 0.02 20
16o,16 - 150,15 351.633 Feb 17.7 0.46 0.08 41
HC3N 24-23 218.325 Feb 20.7 0.24 0.03 10
28-27 254.700 Feb 21.7 0.26 0.03 11
29-28 263.792 Feb 19.7 0.19 0.03 8
SO 55 - 44 215.221 Feb 20.7 0.11 0.02 136
56 - 45 251.826 Feb 21.7 0.33 0.04 268
7s - 65 261.843 Apt 21.3 0.56 0.07 345
87 - 76 304.078 Feb 23.7 0.50 0.05 158
OCS 22-21 267.530 Mar 26.7 0.50 0.06 250
25-24 303.993 Feb 23.7 0.15 0.03 470
HsO + 32 - 22 364.797 Apr 6-10 0.38 0.07
HCO + 3-2 267.558 Mar 26.7 6.0 0.09
CO + 25/2 - ls/2 236.063 Feb 22.7 0.43 0.04
NH2CHO 125,7 - 115,6 254.878 Apr 5.8 0.070 0.009
HNC 4-3 362.630 Feb 16.7 5.8 0.10
HCN 4-3 354.505 Feb 16.7 18.8 0.10
H13CN 3-2 259.012 Feb 19.7 0.27 0.02
HCN 3-2 265.886 Feb 19.7 23.0 0.08
CN 2s/2 - 13/2 226.875 Feb 23.7 0.21 0.04
100
NaOH 10- 9 251.226 Apr 20-21 <0.07 0.024 <0.3
NaCI 20- 19 260.223 Apr 16-17 <0.17 0.057 <0.8
Notes: Line frequenciesare in GHz. Integrated lineintensitiesand theiruncertainties
(in K kms -l) are corrected for main beam efficienciesof 72% and 65% (230 and
345 GHz receivers,respectively).Production rates (in s-I) are computed assuming
LTE at 80 K kinetic temperature (based on CHsOH observations) and a parent
density distribution.The duration of the observations was typically--_1- 2h. NaOH
and NaCl integrated intensitiesand production rates are 3a upper limits.
transitions of HC3N detected in comet Hale-Bopp (Table I) provide
the first identification of this species in a comet. The high-resolution
spectrum of the HC3N (24-23) line is shown in Fig. 1 (right panel). The
HC3N (28-27) spectrum taken with the wideband (500 MHz) spectrom-
eter is shown in Fig. 2. The fractional abundance of HC3N relative to
H20 based on our observations is (1.9+0.2) × 10 -4, similar to that mea-
sured for CH3CN (Biver et al., 1998). The three transitions observed
give consistent abundance values.
D.C. LIS ET AL.
0.1
o.o5
o
-0.05
i HNCO(11o,11- 100.10)
'.
-10 0
.i/, ;2.o.,_:,,i. , .
10 -10 0 10
Velocity (kin S-I)
' 0 ' ' ' ' | ' ' ' ' I
HC3N(24-23 )
r
-10 0 10
Figure I. High-resolution spectra of HNCO (llo,u - 100,10),HNCO (12o,12- llo,z,),
and HCsN (24 - 23) in comet Hale-Bopp. The intensityscale is in antenna tem-
perature units (T_). The velocity scale is with respect to the comet restvelocity.
The lineshave characteristicdouble-peaked lineshapes expected for opticallythin
isotropicoutgassing. The strong linein the leftpanel isthe 241.767 GHz transition
of methanol.
v
v
T,_=77+/- 1OK SO(5e-4s) HC.,,N(28-27)
0.1 _
0.05
-0.05 : --CH3OH--- ----..----------,,_
251.6 251.8 252
Frequency (GHz)
Figure 2. Wideband spectrum of the HC3N (28 - 27) linein comet Hale-Bopp (in
the image sideband). A band of 12 methanol linescan also be seen, as well as the
(56 - 45) lineof SO. The rotationaltemperature based on the observed intensities
of the 252 GHz methanol linesis 77 + 10 K.
3.3. SULFUR MONOXIDE (SO)
Four rotational transitions of sulfur monoxide detected at the CSO in
comet Hale-Bopp (Table I) provide the first identification of this species
in a comet. Low- and high-resolution spectra of the (56 - 45) line are
shown in Fig. 2 and 3 (left panel), respectively. Interferometric obser-
vations of the (65 - 54) line performed at IRAM show that SO does
not have a parent molecule distribution and is produced by a short-
lived species (Wink et al. 1998; see also Despois 1998), presumably
SO2 (Bockelde-Morvan et al., 1998). The fractional abundance of SO
derived from our measurements is (2 -8) x 10-3. The abundance is
strongly model dependent, primarily due to the poorly constrained SO
photodissociation rate,/_. For a given value of/3, the four measurements
CSO OBSERVATIONS OF COMET HALE-BOPP
give consistent abundance values within the observational uncertain-
ties. The production rates given in Table I correspond to/3 = 4 x 10 -4
s -1 and a parent density distribution.
Our derived SO abundance is much larger than the upper limits
derived in comets C/IRAS-Araki-Alcock and P/Halley from UV obser-
vations (Kim and A'Hearn, 1991). As discussed by Kim et al. (1998),
the SO abundances based on the UV upper limits were underestimated
due to the wrong published SO line strengths in the UV.
0.15
0.1
"" 0.05
-0.05 : '"' '.I!","-
.... I , --, , , , ,
-10 0 10
I " = , i I =.
! ....
-10 0 10
Velocity (krn s-I)
iol,I,,
", I , , , I , , , ,
-10 0 10
Figure 3. High-resolution spectra of SO (56-45), Hz3CN (3-2), and H30 + (32-2_)
in comet Hale-Bopp.
3.4. 12C/13C ISOTOPIC RATIO (HI3CN)
The HZ3CN (4 - 3) line was first detected in comet Hyakutake (Lis et
al., 1997). The derived HCN/HZ3CN abundance ratio was 34 -4-12, sig-
nificantly lower than the terrestrial 12C/13C ratio of 89. This low ratio
may be in part due to contamination by an SO2 line blended with the
H_3CN (4 - 3) line. To avoid possible contamination, we observed the
H13CN (3 - 2) transition (Fig. 3, middle panel) for determination of
the HCN/H13CN ratio in comet Hale-Bopp. The HCN/H13CN abun-
dance ratio derived from our observations is 90 4- 15, consistent with
the value measured by Jewitt et al. (1997), as well as the terrestrial
12C/13C abundance ratio.
3.5. IONS
Based on in situ measurements (Altwegg et al., 1993), H30 + was the
most abundant ion in the inner coma of comet P/Halley. Our observa-
tions of the H30 + (32 - 22) line (Fig. 3, right panel) provide the first
ground based detection of this species in a comet.
The HCO + emission (Fig. 4) is found to be strong and extended
along the tail on scales larger than 300" (290,000 km). The maximum
6 D.C. LIS ET At.
=_6" ......... ' ................................................... ' ................ ' .... ' .........
+.. __ L.A.. • ..It .k+. .n- .
o _...,,.........2 "...::.:........! '?..,Z:....._'w_.;_.......: "..Y.'.7.Z. . ...' T.,-?.... ...:_
-_o _ -50 5 -50 _ -5o 5 -5o s -so 5
Velocity (krn -'s )
Figure 4. A strip map of HCO + emission along the tail. Offsets from the nucleus
(in arcsec) are given in the upper-left corner of each panel.
intensity is not at the nucleus position, but _100,000 km toward the
tail direction. The observed variations in the line shape and line center
velocity are caused by the acceleration of the ions by interactions with
the solar wind magnetic field. These results are consistent with the
observations of Lovell et al. (1998), Womack et al. (1998), and Bockel6e-
Morvan et al. (1998).
The CO + emission is detected for the first time at radio wavelengths
(Fig. 5, left panel).
0.05
0
I " " g
/
[,,,,IT
I
t
JLJl.,,J
' CN(2s/2-13/2)
° _.
I II ,- I
.c°:(zv'-!3,") , CH;C_(_.3-12). .,.... ,....,.... , ....,..
236.05 236.1 236.15 -20-10 0 10 20
Frequency (GHz) Velocity (kin s-1)
Figure 5. Spectra of CO + (25/2 - ls/2) and CN (25/2 - 13/2) in comet Hale-Bopp
(left and right panels, respectively). Five lines of CI-IsCN from the image sideband
can also be seen in the CO + spectrum.
3.6. OTHER MOLECULES
Numerous other molecular species are detected, including CN (Fig. 5,
right panel; first detection at radio wavelengths). The HCN/HNC abun-
dance ratio is found to be highly variable between Dec. 96 and Apr. 97
(this ratio varied between 4 and 17), suggesting that a significant frac-
tion of HNC is produced in the coma (see Irvine et al. 1998). Another
sulfur-bearing species, carbonyl sulfide (OCS), first detected in comet
Hyakutake (Woodney et al., 1997), is securely confirmed via observa-
tions of two rotational transitions (Table I). The derived OCS abun-
CSO OBSERVATIONS OF COMET HALE-BOPP
dance in comet Hale-Bopp is ,-_ 6 x 10 -3, a factor of 6 higher than that
in comet Hyakutake (Woodney et al., 1997).
In order to elucidate the origin of the release of sodium in comets,
we searched for Na-bearing molecules (Table I). A comparison of our
3a upper limit for the NaOH production rate derived on Apr. 20-21
(assuming a lifetime of 104 s) with the Na production rate derived
by Cremonese et al. (1997) for the sodium tail of comet Hale-Bopp
suggests that NaOH is unlikely to be a parent of atomic sodium.
We thank D. Yeomans of the Jet Propulsion Laboratory for provid-
ing us with cometary ephemerides. Research at the CSO is supported
by NSF under grant AST 96-15025.
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Address for correspondence:
D.C. Lis, Downs Laboratory of Physics 320-47, California Institute of Technology,
Pasadena, CA 91125, USA


New Molecular Species In Comet C/1995 (Hale-Bopp) Observed with the Caltech Submillimeter Observatory

New Molecular Species In Comet C/1995 (Hale-Bopp) Observed with the Caltech Submillimeter Observatory

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