Astron. Astrophys. 353, 1101-1114 (2000)

2. Observations

The observations presented here were carried out in February, March, and April 1997 with the 10.4-m Leighton telescope at the Caltech Submillimeter Observatory (CSO), located on Mauna Kea (Hawaii), and the two telescopes of the Institut de Radioastronomie Millimétrique (IRAM): the 30-m in the Sierra Nevada (Spain) and the five 15-m antennas of the Plateau-de-Bure interferometer (PdB) in the Alps (France), used in single-dish mode. Part of these observations were reported in Lis et al. (1999), Bockelée-Morvan et al. (1997, 1999) and Paper I.

From February to April 1997, comet Hale-Bopp spanned a heliocentric distance of 0.91 to 1.2 AU and the geocentric distance was between 1.32 and 1.7 AU. The comet was tracked using ephemeris or orbital elements kindly provided by Don Yeomans. For the observations made at IRAM PdB from March 6 to 22, we used the orbital elements referred to as solution DE403-55. The ephemeris was however corrected by 6" (March 11-13) and 5" (March 16-21) North in declination, since the first interferometric maps obtained on March 6 at IRAM PdB have revealed that the maximum intensity of the molecular emissions was slightly offset from the presumed nucleus position (Bockelée-Morvan et al. 1998a; Wink et al. 1999). Observations at the IRAM 30-m from April 3 to 10 were performed using Don Yeoman's orbit DE403-57. With respect to more definite orbital elements published by Brian Marsden and based on astrometric positions until August 1997 (MPC 30428), the position offsets in (RA, Dec) were about (1", - 4") on March 6, (1", 2") on March 11-13, (1", 0.5") on March 16-22 and (0", - 4") on April 1-10. These pointing offsets during IRAM PdB and 30-m observations, admittedly small when compared to the beams of the antennas, have been taken into account in deriving molecular production rates. At the CSO, the pointing was adjusted on the comet itself, by performing five-point maps of the strong (265.7 GHz) and (354.5 GHz) HCN transitions.

In the standard observing mode of the IRAM Plateau-de-Bure interferometer, the signals received by the antennas are cross-correlated to produce maps. Several molecular lines were mapped in comet Hale-Bopp using this interferometer, as reported elsewhere (Bockelée-Morvan et al. 1998a; Wink et al. 1999). The observations reported here were made in the autocorrelation mode (i.e., single dish mode). We used position switching with 5´ offset to cancel the sky background. The spectra from the five antennas were then co-added. Most observations at CSO were also made using position switching, while beam switching (by wobbling the secondary mirror) was used at the IRAM 30-m. The full width at half maximum (FWHM) of the 15-m PdB antennas' beam is 22" at 230 GHz. FWHM are 31" and 11" at 230 GHz, for the CSO and the IRAM 30-m, respectively. The beam efficiencies of the antennas used to convert antenna temperatures into main beam brightness temperatures are 72% and 65 % for the CSO (230 and 345 GHz receivers respectively), 52 % for IRAM PdB (230 GHz receiver) and 39 % for the IRAM 30-m (230 GHz receiver).

The observations were made using several receivers giving access to the 80-115, 130-180 and 203-370 GHz spectral regions. Observations at IRAM were focussed on frequencies below 245 GHz. Most CSO observations were made at frequencies below 300 GHz, because of the low elevation of comet Hale-Bopp at the time when the telescope could be operated (just after sunset or before sunrise). In order to optimize the search for molecular lines, most available spectrometers were used. The CSO backends consist in acousto-optical spectrometers with 50, 500 MHz, and 1.5 GHz bandwidths and spectral resolutions of 0.1, 2.4, and 2.6 MHz, respectively. The IRAM interferometer is equipped with six independent correlator units with adjustable bandwidths (8 to 160 MHz) and spectral resolutions (0.05 to 3.25 MHz); most observations reported here were made with 0.1 MHz spectral resolution. At the IRAM 30-m, available backends are two banks of 512 1 MHz filters, one bank of 256 100 kHz filters and a correlator splittable in several sub-bands of adjustable resolution. High-resolution spectrometers (corresponding to a velocity resolution of 0.1 km s^-1) with small bandwidths were used for dedicated searches of cometary lines. In parallel, low-resolution spectrometers allowed to survey a larger spectral range. All molecular species reported here, except HCOOCH₃, were observed with high-resolution spectrometers. We selected the most promising transitions on the basis of predictive models, using the JPL molecular line catalogue as an input (Pickett et al. 1998). Frequency settings were chosen so that several strong molecular lines of potential cometary molecules fell within the spectrometers' bandwidths. At CSO and IRAM PdB, we took benefit of the fact that the receivers operate in double sideband (that is, actually observe two frequency bands separated by 3 GHz) to optimize the molecular survey. For the detection of new species to be unambiguous, we observed, when possible, several of their rotational transitions.

In this molecular survey, we identified six new cometary molecular species: HC₃N and SO for the first time at the CSO in February 1997, HCOOH and SO₂ at IRAM PdB in March, NH₂CHO on April 5 at CSO and IRAM 30-m and HCOOCH₃ at the 30-m on April 5. Two species (HNCO and OCS), marginally detected in comet Hyakutake (Lis et al. 1997; Woodney et al. 1997), were confirmed. Table 1 lists the rotational transitions detected. A small selection of spectra is shown in Fig. 1. All species reported here, except HCOOCH₃, were detected through several lines (Table 1), making their identifications completely secure. We note that the line widths (typically 2 km s^-1, Fig. 1) and line asymmetries with respect to rest frequencies (believed to be associated with the outgassing pattern) are consistent with those observed for strong lines of other molecules (e.g., HCN, Paper I). On the other hand, the line at 227.562 GHz, that we attribute to HCOOCH₃, is significantly broader than the other lines. The HCOOCH₃ line at this frequency is a blend of eight rotational transitions whose rest frequencies spread over 227.5599 to 227.5637 GHz (Oesterling et al. 1999; Pickett et al. 1998), corresponding to a velocity separation of 5 km s^-1. Most of the intensity (86 %) is equally distributed between four stronger lines, the rest being equally distributed between the remaining four weaker lines (i.e., the stronger transitions are 6 times stronger than the weaker ones). Fig. 1 shows the respective positions of these eight lines. A synthetic spectrum of the blend has been calculated using the line profile of HC₃N at 227.4189 GHz detected with a high signal-to-noise ratio at the same time (Fig. 1). It has a shape in close agreement with that of the detected line at 227.562 GHz, supporting its identification to HCOOCH₃.

Fig. 1. Spectra of SO (CSO, February 21), SO2 (IRAM PdB, March 18, 20, 21), OCS (CSO, March 26), HC3N (CSO, February 20), HNCO (CSO, February 19), NH2CHO (IRAM 30-m, April 5), HCOOH (IRAM, PdB, March 20-21) and HCOOCH3 (IRAM 30-m, April 5) in comet Hale-Bopp (see Table 1). The velocity frame is with respect to the comet nucleus velocity. The dashed line superimposed on the observed spectrum of HCOOCH3 is a synthetic profile calculated using the HC3N line at 227.419 GHz observed at the same time. It takes into account that the HCOOCH3 line at 225.562 GHz is a blend of eight transitions whose positions are shown

Table 1. Observed rotational transitions in comet Hale-Bopp and inferred molecular production rates Q.
Notes:
a) Only transitions detected over the February-April 1997 period are listed. Several additional measurements at low spectral resolution or with weak signal-to-noise ratios are not presented.
b) Frequencies from Pickett et al. (1998).
c) Line area () with statistical uncertainties in units of main beam brightness temperature. Additional systematic errors due to the calibration are estimated to 10-20%.
d) HCN production rates from Paper I. Their uncertainties are dominated by calibration and model uncertainties.
e) Blend of several rotational transitions.
f) Line intensity possibly affected by instrumental effects. The corresponding production rate has been multiplied by a factor of 1.8 (see text).
g) From low resolution spectra, possibly spurious. Not considered for the abundance given in Table 4.

As part of our long-term monitoring program at radio wavelengths (Biver et al. 1997; Paper I), we also observed, on several occasions between February and May 1997, rotational lines of HCN, HNC, CH₃CN, CH₃OH, H₂CO, CO, H₂S, CS, whose first detections in comet Hale-Bopp were already obtained far from the Sun. This allows us to provide a consistent set of molecular abundances for 16 species in comet Hale-Bopp near perihelion. We also observed several ions (CO⁺, HCO⁺, H₃O⁺), a radical (CN) and isotopic varieties (H¹³CN, HDO) (Lis et al. 1999; Bockelée-Morvan et al. 1999), which are not discussed here.

© European Southern Observatory (ESO) 2000

Online publication: January 18, 2000
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