1 Introduction

A new era in the study of comets began in 1973 with the observations of the 18-cm lines of the OH radical, first detected in comet Kohoutek 1973 XII (Biraud et al. 1974; Turner 1974). This discovery initiated a long-term programme of observation at the Nançay radio telescope (Table 1).

   

Table 1: The comets observed at Nançay up to 1999.

comet			perihelion	q	range of	r range
a)	b)	c)			observations	d)	e)	f)	g)	h)
			[yymmdd]	[AU]	[yymmdd]	[AU]
1973 XII	1973f	C/1973 E1 Kohoutek	731228.43	0.142	731129-740215			N	D
1975 IX	1975h	C/1975 N1 Kobayashi-Berger-Milon	750905.33	0.426	750826-750904			L	D
1976 VI	1975n	C/1975 V1 West	760225.22	0.197	760324-760512			L	D
1977 XI		2P/Encke	770817.01	0.341	770808-770911			J	-
1977 XIV	1977m	C/1977 R1 Kohler	771110.57	0.991	771021-771130			L	D
1978 VII	1978c	C/1978 C1 Bradfield	780317.69	0.437	780304-780316			L	D
1978 XXI	1978f	C/1978 H1 Meier	781111.41	1.137	780531-781029			N	D
1979 X	1979l	C/1979 Y1 Bradfield	791221.60	0.545	800127-800203			L	M
1980 XI		2P/Encke	801206.58	0.340	800919-801229			J	M
1980 XII	1980q	C/1980 V1 Meier	801209.65	1.520	801116-810218			L	D
1980 XV	1980t	C/1980 Y1 Bradfield	801229.54	0.260	810103-810303			L	M
1981 XIX	1981j	64P/Swift-Gehrels	811127.27	1.361	811031-811111			J	-
1982 I	1980b	C/1980 E1 Bowell	820312.29	3.364	821127-821219	4.17-4.29	15	N	-	1
1982 IV	1982a	26P/Grigg-Skjellerup	820515.00	0.989	820507-820625	0.99-1.15	15	J	-	2
1982 VI	1982g	C/1982 M1 Austin	820824.73	0.648	820626-820905	0.65-1.33	38	L	D	3
1982 VII	1982e	6P/d'Arrest	820914.31	1.291	820708-820823	1.32-1.51	34	J	M	4
1982 VIII	1982f	67P/Churyumov-Gerasimenko	821112.10	1.306	821009-821025	1.32-1.37	7	J	M	5
1984 IV	1983n	27P/Crommelin	840220.17	0.735	840121-840320	0.73-0.92	35	H	M	6
1984 XIII	1984i	C/1984 N1 Austin	840912.14	0.291	840727-840811	0.29-0.55	14	L	D	7
1985 XIII	1984e	21P/Giacobini-Zinner	850905.21	1.028	850331-851120	1.03-2.20	117	J	D	8
1985 XVII	1985l	C/1985 R1 Hartley-Good	851209.12	0.695	850924-860113	0.70-1.54	93	L	D	9
1985 XIX	1985m	C/1985 T1 Thiele	851219.21	1.317	851027-851216	1.32-1.53	35	L	D	10
1986 III	1982i	1P/1982 U1 Halley	860209.45	0.587	850126-860729	0.59-5.04	408	H	D	11
1987 II	1986n	C/1986 V1 Sorrells	870309.65	1.721	870101-870214	1.62-2.44	10	L	D	12
1987 III	1987c	C/1987 B1 Nishikawa-Takamizawa-Tago	870317.29	0.872	870303-870312	0.88-0.91	10	L	D	13
1987 VII	1986l	C/1986 P1 Wilson	870420.78	1.200	860825-870622	1.24-3.42	116	L	D	14
1987 XXIX	1987s	C/1987 P1 Bradfield	871107.26	0.869	871001-871128	0.89-1.09	34	L	D	15
1988 I	1987d1	C/1987 W1 Ichimura	880110.10	0.200	880103-880109	0.78-0.99	7	L	M	16
1988 V	1988a	C/1988 A1 Liller	880331.11	0.841	880317-880324	0.85-0.88	6	L	D	17
1988 XV	1988j	C/1988 P1 Machholz	880917.57	0.165	880912-881019	0.26-0.97	18	L	-	18
1989 X	1989o	23P/1989 N1 Brorsen-Metcalf	890911.94	0.479	890804-891031	0.48-1.17	61	H	D	19
1989 XIX	1989r	C/1989 Q1 Okazaki-Levy-Rudenko	891111.92	0.642	891003-891202	0.64-1.03	49	N	D	20
1989 XXII	1989a1	C/1989 W1 Aarseth-Brewington	891227.89	0.301	891208-891230	0.30-0.63	19	L	D	21
1990 V	1989c1	C/1989 X1 Austin	900409.97	0.350	900215-900615	0.35-1.52	100	N	D	22
1990 XX	1990c	C/1990 K1 Levy	901024.63	0.939	900616-900930	1.03-2.26	77	L	D	23
1992 III	1991g1	C/1991 Y1 Zanotta-Brewington	920131.99	0.644	920109-920129	0.72-0.92	17	N	D	24
1992 VII	1992b	C/1992 B1 Bradfield	920319.54	0.500	920301-920312	1.02-1.14	10	L	-	25
1992 VIII	1991h1	C/1991 X2 Mueller	920321.20	0.199	920305-920318	0.22-0.56	12	N	-	26
1992 XIX	1991a1	C/1991 T2 Shoemaker-Levy	920723.75	0.829	920610-920722	0.84-1.16	38	L	D	27
1992 XXVIII	1992t	109P/1992 S2 Swift-Tuttle	921212.33	0.958	921015-930112	0.96-1.36	54	H	D	28
1993 III	1992x	24P/Schaumasse	930303.96	1.202	930121-930220	1.21-1.32	30	J	D	29
1994 V		2P/Encke	940209.47	0.331	940111-940303	0.34-0.77	36	J	M	30
1994 IX	1993p	C/1993 Q1 Mueller	940326.31	0.967	940120-940320	0.97-1.45	40	N	D	31
1994 XI	1993v	C/1993 Y1 McNaught-Russell	940331.09	0.868	940301-940420	0.96-1.02	25	L	D	32
1994 XV	1992r	8P/Tuttle	940625.29	0.998	940604-940616	1.01-1.05	12	H	M	33
1994 XXVI	1994o	141P/1994 P1 Machholz 2	940918.63	0.753	940908-940914	0.76-0.77	7	J	M	34
1994 XXX	1994l	19P/Borrelly	941101.49	1.365	940905-950118	1.39-1.62	51	J	D	35
		15P/Finlay	950505.04	1.036	950405-950422	1.04-1.12	17	J	-	36
		C/1995 Q1 Bradfield	950831.42	0.436	950824-950926	0.47-0.83	18	L	D	37
		73P/Schwassmann-Wachmann 3	950922.75	0.933	950830-951101	0.94-1.10	36	J	D	38
		122P/1995 S1 de Vico	951006.02	0.659	951001-951020	0.67-0.72	10	H	D	39
		45P/Honda-Mrkos-Pajdusáková	951225.93	0.532	951125-960120	0.53-0.81	37	J	M	40
		C/1996 B2 Hyakutake	960501.40	0.230	960301-960518	0.23-1.51	49	L	D	41
		22P/Kopff	960702.19	1.580	960409-960523	1.63-1.78	40	J	D	42
		C/1996 Q1 Tabur	961103.53	0.840	961002-961027	0.85-1.03	17	L	D	43
		C/1995 O1 Hale-Bopp	970401.13	0.914	950804-970921	0.91-7.04	329	L	D	44
		46P/Wirtanen	970314.14	1.064	970204-970509	1.07-1.30	40	J	-	45
		81P/Wild 2	970506.64	1.583	970121-970323	1.64-1.88	37	J	M	46
		2P/Encke	970523.60	0.331	970513-970522	0.33-0.43	9	J	-	47
		C/1998 J1 SOHO	980508.62	0.153	980601-980608	0.79-0.96	8	L	D	48
		C/1998 P1 Williams	981017.84	1.147	980918-981004	1.17-1.24	12	L	M	49
		21P/Giacobini-Zinner	981121.32	1.034	981020-981115	1.04-1.13	22	J	D	50
		C/1998 U5 LINEAR	981221.89	1.236	981124-981210	1.25-1.30	15	L	M	51
		C/1999 H1 Lee	990711.17	0.708	990508-990814	0.71-1.39	66	L	D	52
		C/1999 N2 Lynn	990723.05	0.761	990716-990719	0.76-0.77	4	L	D	53

a) Old-style definitive designation; b) old-style provisional designation; c) new-style designation; d) lowest and highest heliocentric distances of the observations; e) number of observations in the data base; f) comet type; N: dynamically new, long-period comet; L: other long-period comet; H: Halley-type comet; J: Jupiter-family comet; g) -: no detection; M: marginal detection; D: clear detection; h) corresponding order number of the electronic tables and figures.

The OH radical in cometary atmospheres comes from the photodissociation of water, the major constituent of cometary ices. The direct observation of cometary water from the ground is difficult and one usually has to rely on secondary products such as OH to assess the outgassing of comets and its evolution. The OH radical can be observed through its A-X electronic system in the near UV, which can be done from the ground under good observing conditions (e.g., A'Hearn et al. 1995), or from space with dedicated instruments (e.g., Feldman 1999). The vibrational bands of OH can also be observed in the infrared (either from space or from the ground), but this has only been possible in a few cases. Radio observations of the 18-cm lines of OH are quite different from UV or infrared spectroscopic observations. Radio lines are fully resolved and their Doppler profiles allow us to investigate the kinematic properties of the cometary atmosphere. This is hardly possible at other wavelengths. The radio telescope beam samples a large part of the coma, so that minimal extrapolations to the whole coma are required (in contrast to typical visible or UV spectroscopic observations, which sample only a very small fraction of the coma). Radio observations at 18-cm wavelength do not have visibility constraints such as solar elongation and are almost insensitive to weather conditions. However, because of the particularities of the excitation mechanism of the OH radical, which depends upon the heliocentric radial velocity (see Sect. 3), the observation of the 18-cm OH lines is only possible part of the time.

The purpose of the present paper is to describe the set of cometary observations made at Nançay in the 1982-1999 period, that began with the implementation of an efficient autocorrelator spectrometer and ended with a major upgrading of the telescope.

The resulting spectra are now organized in a data base where they can be accessed interactively. Dedicated software (XCOM) allows us to display and process the spectra, and to retrieve physical parameters such as the OH production rates using model parameters specified by the user.

The scientific analysis of these data is beyond the scope of the present paper and is the subject of separate publications, present or future (Table 2):

• study of the excitation mechanisms of the OH radio lines (Biraud et al. 1974; Despois et al. 1981; Gérard 1990; Gérard et al. 1998; Colom et al. 1999);

• statistical analysis of the correlation between OH production rates and visual magnitudes (Despois et al. 1981; Bockelée-Morvan et al. 1981; Jorda 1995; Jorda et al. 1992, 1996);

• study of the long-term and short-term variability of the OH production rate in 1P/Halley (Gérard et al. 1987; Colom & Gérard 1988);

• analysis of the 18-cm line widths in relation with the expansion velocity of the atmosphere (Bockelée-Morvan & Gérard 1984; Bockelée-Morvan et al. 1990a);

• analysis of the 18-cm line profile in relation with anisotropic outgassing and non-gravitational forces (Bockelée-Morvan & Gérard 1984; Colom et al. 1990 and in preparation);

• study of the Zeeman effect and of its relation to the cometary magnetic field (Gérard 1985; Bockelée-Morvan et al. 1992; Gérard et al. 1993).

Section 2 describes the characteristics of the Nançay radio telescope and the observing procedures. Section 3 explains how OH production rates can be derived from the radio observations. Section 4 briefly recalls the 1973-1981 observations. Section 5 is a detailed description of the 1982-1999 observations while notes on individual comets are given in an Appendix. Section 6 discusses some statistical aspects of the data base. Section 7 concludes on the future of cometary observations at Nançay.

Most of the tables and figures of the present paper are only available in electronic form. The Nançay spectra are public domain and may be requested from the CDS. The data base itself and its companion software XCOM are not transportable and cannot be remotely accessed. They could be used, however, in collaboration with the authors.