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Astron. Astrophys. 343, 571-584 (1999)

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2. Observations

HH 211 was observed with the IRAM Plateau de Bure millimeter array (Guilloteau et al. 1992) in November and December 1995 and March 1996. Four 15-m antennas equipped with dual-frequency receivers were used in three different configurations (D, C1, and C2) with (unprojected) baseline lengths ranging from 24 m to 180 m. The receivers were tuned to 115.27 GHz USB with a sideband rejection of about 5 dB, and 230.54 GHz LSB in double sideband mode to cover the CO [FORMULA] and [FORMULA] lines simultaneously. Excellent weather conditions resulted in typical SSB system temperatures around 300 K at both frequencies, and phase noise lower than [FORMULA]. Two correlator units were set up to provide a channel spacing of 0.4 km  s -1 and 0.8 km  s -1 for the CO [FORMULA] and [FORMULA] lines respectively, in both cases in a [FORMULA] km  s -1 wide velocity interval. The four remaining units of the correlator were used in broad-band mode, to allow continuum measurements over 320 MHz at 115 GHz and [FORMULA] MHz (summing upper and lower sidebands) at 230 GHz. Amplitude and phase were calibrated with frequent observations of the quasars 3C 84 and 3C 111, which were also used to derive a flux density scale (the flux densities of 3C 84 were 4 Jy at 115 GHz and 2.1 Jy at 230 GHz; 3C 111 underwent a strong outburst and its flux densities at the three dates of observations were respectively 6.7, 7.7 and 12.3 Jy at 115 GHz, and 6.6, 7.8 and 8.5 Jy at 230 GHz). We estimate the final flux density accuracy to be [FORMULA]. A 9-field mosaic was observed to cover the whole CO outflow (Fig. 1). Antenna pointing was carefully monitored during the observations (the accuracy was [FORMULA]), to ensure that no distortion of the images, especially at 230 GHz, could result from pointing errors. The data was processed using the GILDAS software package. A non-linear joint deconvolution using a CLEAN-based algorithm was performed (see Gueth et al. 1995 for a description of the method). The images were constructed using natural weighting. The resulting clean beams are [FORMULA] at a position angle of [FORMULA] for the CO [FORMULA] line and [FORMULA] at a position angle of [FORMULA] for the CO [FORMULA] line.

In addition, we obtained CO [FORMULA] single-dish spectra throughout the outflow at the IRAM 30-m telescope in April 1994. An SIS mixer receiver with sideband rejection of about 7 dB provided system temperatures ranging from 400 to 700 K. Classical on-off observing mode was used. At this frequency, the antenna half-power beamwidth and main-beam efficiency are [FORMULA] and 0.39 respectively. Note that the limited sampling of these observations did not allow us to combine them with the interferometer data.

We also present H13CO+ [FORMULA] observations obtained with the Plateau de Bure interferometer during the winter 1994-1995. Five configurations of the four antenna array were used, with baselines extending up to 288 m. Typical SSB system temperatures were [FORMULA] K. One correlator unit was used to observe the H13CO+ transition with a channel spacing of 0.27 km  s -1, while two units were tuned in broad-band mode to measure the 86 GHz continuum emission over a bandwidth of 320 MHz. Phase and amplitude were calibrated with frequent observations of the quasar 3C 84, whose flux density at 86 GHz was 6.1 Jy at this epoch. A mosaic of five fields was observed (Fig. 1) and deconvolved using the CLEAN-based method available in the GILDAS package. The final clean beam is [FORMULA] at a position angle of [FORMULA].

Channel maps of the CO [FORMULA] and [FORMULA] emission observed with the Plateau de Bure interferometer are presented in Figs. 2 and 3 (coordinates in all the figures are in the J2000.0 system). At the center of the field of view, the noise level is [FORMULA] mJy/beam and [FORMULA] mJy/beam for the [FORMULA] and [FORMULA] lines respectively. Note that the mosaics are corrected for primary-beam attenuation and the noise thus strongly increases at the edges of the field of view. Channels near the systemic velocity ([FORMULA] km  s -1) show negative contours which are artifacts caused by extended emission. In the following, we shall mainly discuss the CO [FORMULA] observations, which provide the highest angular resolution. An overlay of the CO [FORMULA] images superimposed on the H2 v=1-0 S(1) line emission from McCaughrean et al. (1994) is presented in Fig. 4 and the position-velocity plot is shown in Fig. 5. Note that the positional accuracy of the H2 image ([FORMULA]) is good enough to allow reliable comparison with the interferometric maps (whose positional uncertainties are below [FORMULA]).

[FIGURE] Fig. 2. Channel maps of the CO [FORMULA] emission, averaged over 4 km  s -1 wide velocity intervals, whose central velocity is indicated in the upper left corner of each panel (the systemic velocity is 9.2 km  s -1). The cross denotes the position of the exciting source. The clean beam is [FORMULA] at PA [FORMULA] and is indicated in the lower left box. Contour step is 50 mJy/beam ([FORMULA] K), except in the three panels drawn with thick boxes, where it is 100 mJy/beam. The last panel shows the integrated emission (contours are 0.9 Jy km  s -1/beam); the thin line represents the jet axis.

[FIGURE] Fig. 3. Channel maps of the CO [FORMULA] emission, averaged over 4 km  s -1 wide velocity intervals, whose central velocity is indicated in the upper left corner of each panel (the systemic velocity is 9.2 km  s -1). The cross denotes the position of the exciting source. The clean beam is [FORMULA] at PA [FORMULA] and is indicated in the lower left box. Contour step is 200 mJy/beam ([FORMULA] K). The arrows in the 8.2 km  s -1 channel indicate structures discussed in Sect. 5.2. The last panel shows the integrated emission (contours are 2 Jy km  s -1/beam); the thin line represents the jet axis (PA [FORMULA]) used to compute the position-velocity plot (Fig. 5).

[FIGURE] Fig. 4. CO [FORMULA] emission (thin contours) integrated in two different velocity intervals and superimposed on the H2 v=1-0 S(1) emission (greyscale; from McCaughrean et al. 1994) and the 230 GHz continuum emission (thick contours; contours are 10, 30, 50 and 70 mJy/beam). The angular resolutions are [FORMULA] for the H2, [FORMULA] at PA [FORMULA] for the CO, and [FORMULA] at PA [FORMULA] for the continuum observations. For clarity, the noise at the edges of the mosaic has been masked. Upper panel: CO [FORMULA] emission integrated between LSR velocities 2.2 and 18.2 km  s -1 (the systemic velocity is 9.2 km  s -1); contours are 1.6 Jy km  s -1/beam. Lower panel: CO [FORMULA] emission integrated for velocities lower than 2.2 km  s -1 and larger than 18.2 km  s -1; first contour is 1 Jy km  s -1/beam and contour step is 1.5 Jy km  s -1/beam. The crosses denote the positions of the main features identified within the jet (see text).

[FIGURE] Fig. 5. Position-velocity plot of the CO [FORMULA] emission, computed along the jet axis and integrated in the direction perpendicular to the jet. Contour step is 10% of the maximum. The cross indicates the position of the protostar (systemic velocity is 9.2 km  s -1). The velocity and angular resolutions (4 km  s -1 and [FORMULA]) are represented in the lower right corner. The horizontal dashed lines separate the velocity intervals used to compute the two images presented in Fig. 4. BI, BII, RI, and RII are the main features identified within the jet (see text).

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© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999
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