2 Observations

2.1 Pluto

Pluto observations were performed with the IRAM 30-m telescope on April 20, 21, 24, and 25, 2000, approximately one month before the opposition (June 1, 2000) when Pluto was at $\Delta$ = 29.5 AU from the Earth and at $r_{\rm h}=30.26$ AU from the Sun. They benefitted from the new generation receivers available at the telescope, and of the versatility of the system, which now allows the use of 4 receivers at the same time. The A100, B100 receivers were tuned to the frequency of the CO J(1-0) line (115.271 GHz), and the A230 and B230 receivers to that of CO J(2-1) (230.538 GHz). In order to provide the best sensitivity, the observations were made in the frequency-switch mode. The frequency throw was 6.5 or 7.5 MHz at 230 GHz, and 6.5 or 14 MHz at 115 GHz, depending on the date. The frequency throw for the 230 GHz receivers was optimized in order to minimize baseline ripples and to avoid, as far as possible, the contamination of the Pluto CO J(2-1) line by the CO emission from an extended, uncatalogued galactic source (RA(J2000) = 16:47:48; Dec(J2000) = -12:05:00), close to Pluto. The spectra were acquired at a high spectral resolution (23 and 47 kHz), using the 2048-channel autocorrelator as backend. The total integration time on source was $2 \times 1045$ min for CO J(2-1), taking into account the two receivers. Technical problems on April 20 made the CO J(1-0) observations obtained on this date unusable. Data were acquired under good weather conditions. SSB system temperatures at transit ($\sim$40$^\circ$ elevation) were $\sim$260 K and from 290 to 360 K in the $T_{\rm A}^*$ scale, for the J(1-0) and J(2-1) lines, respectively.

   

Table 1: Pluto observations of CO at the IRAM 30-m telescope.

Date	Line	$t_{\rm int}^a$	Res.	rmsb	$T_{\rm B}{\rm d}\nu^c$
		[min]	[kHz]	[mK]	[mK MHz]
00/04/24-25	J(1-0)	1230	79	8	<6.6d
00/04/20-25	J(2-1)	2090	79	10	18 $\pm$ 4

^a Total integration time on source.
^b 1-$\sigma$ rms noise in units of main beam brightness temperature at 79 kHz spectral resolution.
^c Line area between -1 and +1 MHz from line center, in units of main beam brightness temperature.
^d 3-$\sigma$ upper limit.

A summary of the observations is given in Table 1. Noise levels and line intensities are expressed in the main beam brightness temperature scale ($T_{\rm B}$), using beam efficiencies derived from continuum observations of Uranus (Table 2) and forward efficiencies of 0.95 and 0.90 for the 115 and 230 GHz lines, respectively.

The obtained spectra are quite satisfactory in terms of baselines; a second-order polynomial was used for their subtraction. On the other hand, the spectra are contaminated by several strong lines, namely the CO emission line from the above-mentioned galactic source, and their negative counterparts which are 2 times weaker and are artifacts resulting from the frequency-switch procedure.

In the CO J(2-1) spectrum, a negative ghost feature, due to the galactic CO emission, is present between +1 and 3.4 MHz with respect to the expected position of the Pluto's CO line (Fig. 1). Since Pluto's CO J(2-1) line is expected to be at most $\sim$1 MHz broad, for most atmospheric models, it should be free of contamination (Fig. 1). This was checked in more detail by looking at the velocity extent of the galactic emission line for which the signal-to-noise ratio is higher. We do not see any hint of galactic emission or absorption above the 1-$\sigma$ level in the velocity range which would correspond to -1 to 1 MHz with respect to Pluto, and the red wing of the galactic line extends at most to 0.6 MHz. The spectrum shown in Fig. 1 and the line area integrated from -1 to 1 MHz with respect to line center (Table 1) suggest a marginal detection of the CO J(2-1) line at the 4.5-$\sigma$ level in the Pluto-Charon system. As discussed above, galactic spectral contamination is excluded. However, this marginal detection requires further confirmation and we regard this tentatively measured line area of 18 mK MHz as an upper limit.

The CO J(1-0) spectra acquired on April 24 and 25 are not affected by ghost lines, in contrast to those obtained on April 21 with a different frequency throw. Therefore, we only consider these 24-25 April J(1-0) spectra in Table 1 and the following discussion. There is no hint of detection of the CO J(1-0) line.

Figure 1: Observed and modelled CO J(2-1) Pluto spectra. Observed spectra are smoothed to 79 kHz resolution. Top figure: synthetic spectra with the atmospheric thermal structure of Strobel et al. (1996) (solid lines in Fig. 2). Bottom figure: synthetic spectra with the atmospheric model of Stansberry et al. (1994) (dashed lines in Fig. 2). For each atmospheric model, synthetic spectra (dashed lines) are shown for CO mixing ratios of 0.01, 0.1, 1, 10%, from bottom to top.

The 3-$\sigma$ sensitivity limits in flux units, after smoothing to the expected 2 MHz Pluto CO linewidths, are 13 and 23 mJy for the J(1-0) and J(2-1) lines, respectively. This is more than one order of magnitude below the previously published upper limits based on observations with the Haystack 37-m and the National Radio Astronomy Observatory (NRAO) 12-m telescopes (Barnes 1993, 1996).

 

 

Table 2: Characteristics of the telescopes: half-power beam widths (HPBW) and beam efficiencies ( $B_{\rm eff}$).

Telescope	Frequency	HPBW	$B_{\rm eff}$
	[GHz]	[arcsec]
IRAM 30-m	115	21.5	0.75
	230	10.6	0.52
JCMT	230	21.0	0.69
	345	13.6	0.62
CSO	230	31.0	0.65
	345	21.0	0.60

2.2 Centaurs and KBOs

   

Table 3: Centaurs and KBOs observations.

Date	$r_{\rm h}$	$\Delta$	Tel.	Line	$t_{\rm int}^a$	offsetb	$T_{\rm B}{\rm d}v^c$	Q (10 K)d	Q (50 K)d
	[AU]	[AU]			[min]	[arcsec]	[mK kms-1]	[1028 mols-1]
2060 Chiron
98/03/25.6	8.9	8.2	JCMT	CO(3-2)	82	1.5	<21	<0.88	<0.39
99/02/28.6	9.3	9.2	CSO	CO(3-2)	64	2.5	<50	<3.3	<1.5
99/06/09.5-11.5	9.5	8.5	JCMT	CO(2-1)	314	2.0	<13	<0.55	<0.57
99/06/24.4-27.4	9.5	8.7	CSO	CO(2-1)	53	2.0	<48  <34	<2.2	<2.2
00/07/26.4	10.2	9.5	CSO	CO(2-1)	43	2.5	<49
5145 Pholus
99/02/28.5	13.9	13.3	CSO	CO(3-2)	85	3.0	<36	<4.6	<1.7
99/06/24.3-27.3	14.2	13.9	CSO	CO(2-1)	109	4.0	<33	<3.4	<2.8
00/02/16.5-17.6	14.8	14.5	JCMT	CO(3-2)	117	2.0	<44	<4.4	<1.4
7066 Nessus
99/06/24.4-25.5	15.0	14.0	CSO	CO(2-1)	53	4.0	<53	<5.6	<4.6
8405 Asbolus
99/02/27.7-28.6	9.2	8.8	CSO	CO(3-2)	43	4.5	<46	<3.0	<1.4
99/06/26.3-28.3	8.8	8.2	CSO	CO(2-1)	53	3.0	<42	<2.5	<2.5
10199 Chariklo
98/11/14.6-15.7	13.6	13.5	CSO	CO(2-1)	184	7.0	<21	<2.2	<1.8
98/11/26.7	13.5	13.3	JCMT	CO(3-2)	143	1.5	<21	<1.8	<0.62
			JCMT	HCN(4-3)	143	1.5	<18	<0.78	<0.04
99/02/28.4	13.5	12.6	CSO	CO(3-2)	75	3.5	<40  <31	<3.6	<1.3
00/01/02.5	13.4	12.7	CSO	CO(3-2)	149	1.5	<58
00/01/03.5-04.6	13.4	12.7	CSO	CO(2-1)	192	2.5	<22	<2.0	<1.7
1994 TB
98/11/09.3-13.4	30.1	29.4	CSO	CO(2-1)	205	2.0	<30	<8.3	<5.1
1996 TL66
98/07/8.8-9.8	35.1	35.5	JCMT	CO(3-2)	66	3.0	<27  <15	<6.8	<1.4
98/10/31.44-4.5	35.1	34.1	JCMT	CO(3-2)	153	2.0	<16
99/08/22.5	35.1	34.7	JCMT	CO(3-2)	29	2.0	<85
98/11/09.4-10.5	35.1	34.1	CSO	CO(2-1)	107	2.0	<47  <16	<5.7	<3.3
00/07/26.6-31.6	35.0	35.2	CSO	CO(2-1)	203	2.5	<17
1996 TO66
99/08/22.6	45.9	45.1	JCMT	CO(3-2)	30	1.5	<78  <13	<9.9	<1.7
99/09/06.4-07.5	46.0	45.0	JCMT	CO(3-2)	175	3.5	<28
99/10/30.2-11/01.3	46.0	45.2	JCMT	CO(3-2)	150	2.0	<21
99/10/03.3-04.3	46.0	45.0	JCMT	CO(2-1)	165	3.0	<21	<7.0	<3.9
00/07/26.6	46.0	45.5	CSO	CO(2-1)	43	2.0	<36	<19	<10
1996 TP66
98/11/01.4-04.4	26.4	25.4	JCMT	CO(3-2)	40	2.0	<17	<4.5	<1.0
1998 WH24
00/01/3.4-4.5	42.4	41.7	CSO	CO(2-1)	59	2.5	<40	<19	<10
1998 SG35
98/10/31.4-11/01.4	10.6	9.9	JCMT	CO(3-2)	68	7.5	<25	<1.8	<0.77

^a Total integration time: on+off source, in case of beam switching; on source, in case of frequency switching.
^b Beam offsets due to ephemeris and pointing errors.
^c Line area between -0.6 and +0.6 kms^-1 in units of main beam brightness temperature. 3-$\sigma$ upper limits are quoted.
Values on the right are averages over several periods, as indicated by the vertical bars.
^d Production rates upper limits computed with a kinetic temperature of either T = 10 K or T = 50 K. Calculations take into account beam offsets.

Observations of Centaurs and Kuiper Belt objects were performed with the CSO 10.4-m dish and with the JCMT 15-m antenna, both in Hawaii. They cover the March 1998 to July 2000 period. These observations were often performed as backup observations to other programmes. As a result, they did not always have the benefit of excellent weather conditions. Opacities at 230 GHz during the observations varied from 0.03 to 0.4, with an average around 0.12. At JCMT, the observational mode was, most of the time, frequency-switch with throws of 8.1 or 16.2 MHz. Beam-switching with a beam-throw of 120'' was used on a few occasions. CSO observations were performed in beam-switching mode. We used the high spectral resolution spectrometers offered at these telescopes: 100 kHz at CSO, and 94 to 188 kHz at JCMT. The beam efficiencies were measured several times, by observing Mars, Jupiter, Saturn, and Uranus (Table 2). A log of the observations is given in Table 3. Depending on the weather, or on the availability of the receivers, the CO J(2-1)or J(3-2) lines were observed. At JCMT, the B3 345 GHz receiver is equipped with two mixers, which permits us to observe two orthogonal polarizations. For one object (10199 Chariklo), this receiver was used in double side band mode with HCN J(4-3) at 354.505 GHz in the upper side band and CO J(3-2) at 345.796 GHz in the lower side band.

Six Centaurs and five KBOs were observed (Table 3), whose optical characteristics and diameters are summarized in Table 5. They were tracked using the most recent orbital elements available in the Minor Planet Circulars (MPC) or Minor Planet Electronic Circulars (MPEC). In the case of 1998 SG35 and 10199 Chariklo on 14-15 November 1998, improved elements showed that the ephemeris used for the observations was off by $\sim$7''. Total beam offsets due to ephemeris and pointing uncertainties are given in Table 3. None of the objects were detected in CO. The 3-$\sigma$ upper limits obtained for the integrated line intensity are typically $\sim$20 mK kms^-1 in the main beam brightness temperature scale, $T_{\rm B}$ (Table 3).