5 IRAM interferometer observations

5.1 Observing conditions

A total of 52 objects was mapped at the frequency of the ¹²CO($J=1\rightarrow 0$) line with the IRAM interferometer (PdBI) placed at Plateau de Bure, France (Guilloteau et al. 1992). The observations consisted in multi-configuration snapshots, each object being typically observed in four different configuration at various times over a period of two years (see Table 1 - December 1990 to April 1992). The objects were observed typically at two hour angles in each configuration, each hour angle consisting of 4-minute integrations on primary calibrators interspersed with two 20-minute periods of on-source integration time.

The observing parameters were consistent from one run to the next. Two antennas were equipped with SIS mixers, while the third had a Schottky mixer. All receivers were operating in double side band (DSB) mode. The cross-correlator was set to a bandwidth of either 40MHz or 80MHz with a nominal channel spacing of 0.625MHz. Because of channel apodization, the effective frequency resolution was 1MHz, or equivalently 2.6kms^-1 at the frequency of the ($J=1\rightarrow 0$) transition. We used a redundant correlator setting with 64 channels per baseline, instead of 128. The on-source integration time equivalent to single-baseline observing was 4.5 hr on average. The point source sensitivity, derived from emission-free channels, was 50 mJy on average, and was consistent with system temperatures of 500K and atmospheric phase decorrelations of 10 to 30 degrees. The IRAM continuum correlator, which was operating simultaneously over a bandwidth of 500MHz consisting of 10 contiguous 50MHz channels, was used mainly for the purpose of calibration. However, as a valuable by-product, we were able to measure with the central 400MHz the continuum flux at 112GHz of CIT 6, $\mu\,$Cep (tentatively detected) and 19480+2504, and to give $5\sigma$ sensitivity upper limits for all other sources.

 

Table 1: Flux densities of the interferometric calibrators at the frequency of the ¹²CO($J=1\rightarrow 0$) transition. The fluxes are in units of Jy. Observational periods are labeled A-J and are compiled in Table 4 for every star in the sample (e.g. period C observations were carried out between February and April 1991)

5.2 Data reduction

Data calibration was effected in the baseline-based manner using the CLIC (Lucas 1991) interactive software package. We used whenever available either 3C 273 or 3C 345 as bandpass calibrator. Phase and amplitude calibrations were carried out with the sources listed in Table 1.

Conventional image restoration algorithms like CLEAN produce maps without sidelobes and without features that a synthesized antenna pattern can show as a consequence of missing information. Accordingly, these methods imply interpolation or extrapolation of information that can be retrieved under ``reasonable'' assumptions, only when the objects are of small extent, compared to the size of the primary field of view. However, when the emission structures are large with respect to the beamsize of an individual dish (spacings smaller than the dish diameter are for obvious physical reasons fundamentally inaccessible to measurements by an interferometer), short spacing information needs to be supplied. We circumvented this difficulty by using the 30 m telescope to measure the missing information.

While the IRAM interferometer is ideal for achieving the required high resolution, emission structures larger than 17$^{\prime\prime}$ are essentially resolved out: 17$^{\prime\prime}$^-1 down to about 4$^{\prime\prime}$^-1 is the typical range of spatial frequencies to which our observations were sensitive. This results in interferometric maps in which regions of negative intensity surrounding small-scale structures can hardly be removed. For envelopes larger than 20$^{\prime\prime}$, including the 30 m data recovered typically 50% of the flux missing from the maps obtained from interferometer data alone.

The 30 m telescope was used to obtain maps sampled at twice or more the telescope beamwidth, those maps that completely cover the region of the interferometer primary beam and that greatly account for the missing flux. Short spacing visibilities were created from the single-dish data and combined with the interferometer samples using the data reduction scheme described below. Depending on the signal-to-noise ratio, however, only short-spacing visibilities up to 20-25 m were kept to avoid spurious effects in the merging process.

The following steps were taken with the GILDAS (Guilloteau & Forveille 1989) software package in combining data from the IRAM 30 m telescope and the IRAM interferometer, once the initial data processing of single-dish and interferometer data was carried out:

 

Table 2: ¹²CO($J=1\rightarrow 0$) observational summary for the 7 objects for which the combination of single-dish data with interferometric data was not carried out. Noise estimates (1$\sigma$) are for a spectral resolution of 2.6kms^-1

• resample the interferometer data to a frequency resolution of 1MHz to be consistent with the single-dish observations,
• correct the antenna temperatures for the forward gain to obtain main beam temperatures, $T_{\rm MB}$, and apply the standard conversion factor to convert the single-dish data from $T_{\rm MB}$ to the interferometer units Jybeam^-1,
• interpolate irregularities in the spatial sampling of the single-dish data (missing or bad sampling points) using a CLEAN based algorithm. The purpose of this step is to avoid an all too poor representation of the large emission structure when the sampling was too irregular,
• deconvolve the single-dish maps by dividing their Fourier transforms by the Fourier transform of the 22$^{\prime\prime}$ beam of the 30 m telescope (assumed to be gaussian), multiply the inverse Fourier transform of the result by the 42$^{\prime\prime}$ primary beam of the interferometer, and Fourier transform back to the uv-plane to obtain single-dish visibility tables,
• recenter the single-dish and the interferometric emission centroids, determined from averaging blue ($V \le -V_{\rm exp}/3$) and red ($V \ge V_{\rm exp}/3$) velocity maps, to the phase tracking center of the interferometer. Errors in the pointing (2''-4'' are reported for the 30 m telescope) need to be removed as accurately as possible. Since the 30 m velocity maps contribute predominantly to the large scale structure, residual uncertainties shoud not significantly affect the results.
• generate visibility tables that combine both single-dish and interferometer data from which visibility plots along circular uv trajectories are created. The single-dish weight is scaled according to the mean weight of the short spacings interferometer visibilities in order to provide single-dish and interferometer uv cells of similar weight,
• check the compatibility of single-dish and interferometer data in the common uv regions (spacings in the 15-25m range) and, depending on the signal-to-noise, account for possible flux calibration errors, $\eta_{\rm C}$, by scaling the single-dish visibilities to the interferometer visibilities. $\eta_{\rm C}$ was found to vary between 0.5 and 2. In general, however, the two data sets agreed to about 20%.

The 7 objects listed in Table 2 were discarded from the original sample.