2 Observations and results

A full-range spectral survey (43-196 $\mu$m) of the IRAS 4 region was performed using LWS. The observations were obtained on three positions in LWS grating mode, with a spectral resolution of about 200 (AOT LO1). The first position was centered in between IRAS 4A and IRAS 4B ( $\alpha_{2000}~=~03^{\rm h}29^{\rm m}11.9^{\rm s}$, $\delta_{2000}~=~31 \hbox{$^\circ$ }13 \hbox{$^\prime$ }20.3 \hbox{$^{\prime\prime}$ }$), and hence LWS80 $\hbox{$^{\prime\prime}$ }$ beam includes both sources. The two other positions aimed to the lobe peaks of the outflow powered by IRAS 4: NE-red, $\alpha_{2000}~=~03^{\rm h}29^{\rm m}15.6^{\rm s}$, $\delta_{2000}~=~31 \hbox{$^\circ$ }14 \hbox{$^\prime$ }40.1 \hbox{$^{\prime\prime}$ }$), and SW-blue, $\alpha_{2000}~=~03^{\rm h}29^{\rm m}06.6^{\rm s}$, $\delta_{2000}~=~31 \hbox{$^\circ$ }12\hbox{$^\prime$ }08.7 \hbox{$^{\prime\prime}$ }$). These observations, performed during revolution 847, are made of 30 scans on the central position and 10 scans on the two other positions. The sampling rate was 1/4 of the grating resolution element (0.29 $\mu$m in the 43-92 $\mu$m range, and 0.15 $\mu$m in the 84-196 $\mu$m range). The integration time for each sampled point was 12 s on the central position, and 4 s on the outflow positions.

Figure 1: ISO-LWS spectra observed towards IRAS 4 on source (top line), NE-red (middle line) and SW-blue (bottom line).

The same data set has been previously analyzed by Ceccarelli et al. (1999) and Giannini et al. (2001, hereafter GNL01). We here report again the analysis of this dataset as we used an improved data processing module, allowing the correction of the transient effects that affect the LWS detectors (Caux 2001). As for standard pipeline products, the data are calibrated against Uranus, and the calibration uncertainty is estimated to be better than 30% (Swinyard et al. 1998). On the three observed positions, the spectra were then defringed for all ten detectors, and the continuum was removed fitting a polynomial baseline outside the lines. The data were then averaged over the ten detectors and binned at a single resolution to produce a single spectrum for each observed position. The line flux measurements were finally performed with the ISAP package, using gaussian fits. A particular attention was given to the determination of uncertainties associated with the line fluxes measurement. These are due to the statistical uncertainties, the absolute calibration and the baseline determination uncertainties that affect low resolution LWS observations. In grating mode, on a spectrum rich in lines as it is the case here, the line confusion may be important, making the baseline determination difficult. This uncertainty, often neglected in the literature, should be taken into account, as it can lead to important errors, especially for faint lines.

Figure 1 shows the observed 60-200 $\mu$m spectra in the three observed positions, and Table 1 reports the measured line fluxes. The errors quoted in this table include statistical errors, errors due to the uncertainty of the baseline removal, and an absolute calibration error of 30%. The first striking result of these observations is the dramatic difference within the three spectra: while that including IRAS 4A and 4B is very rich in CO and water lines, molecular emission is barely detected towards the outflow peaks (where the millimeter CO emission is the brightest). On the contrary, the fine structure [OI] 63 $\mu$m and [CII] 157 $\mu$m lines have comparable fluxes in the three observed positions. Finally, we wish to comment our results with respect to previous published data reductions. The present line flux determination agrees with that quoted by Ceccarelli et al. (1999) when considering the uncertainties. However, we note several differences with respect to the values quoted in GNL01. While there is a relatively good agreement between their fluxes of the strongest lines and ours (see Table 1), there is a noticeable discrepancy between our respective reductions regarding the weakest lines. We think that this is probably due to a too optimistic evaluation of the noise in GNL01. For example, in the NE-red position we find a statistical error around 175 $\mu$m of $1.5 \times 10^{-13}$ erg s^-1 cm^-2 $\mu$m^-1, while Giannini et al. quote $2 \times 10^{-14}$ erg s^-1 cm^-2 $\mu$m^-1 in their Table 3. We do not confirm neither the detection of CO lines with $J_{\rm up} \geq 21$, nor the 125.4, 83.3, 66.4 and 58.7 $\mu$m water lines on-source. As shown in our Fig. 1, we only detected the 179.5 $\mu$m 174.6 $\mu$m and 108.0 $\mu$m lines in the outflow peak position NE-red, and the CO lines between $J_{\rm up} = 14$ and 17. We also do not confirm their detections of CO $J_{\rm up} \geq 18$ lines in the NE-red outflow peak position. Finally, in the SW-blue position we only detected C⁺ and OI 63 $\mu$m emission and very marginally the H₂O 179 $\mu$m lines.

 

 

Table 1: Measured line fluxes in units of 10^-12 ergs s^-1 cm^-2. Upper limits are given as 2 $\sigma$.

Specie	Transition	Wavelength	${E_{\rm up}}$	Fluxes
		($\mu$m)	(cm-1)	On source	NE-red	SW-blue
o-H2O	221-221	180.49	134.9	$1.1 \pm 0.5$	<0.5	<0.5
	212-101	179.53	79.5	$2.7 \pm 1.0$	$0.5 \pm 0.4$	0.6 $\pm$ 0.4
	303-212	174.63	136.7	$1.6 \pm 0.7$	$0.5 \pm 0.4$	< 0.5
	414-303	113.54	224.5	$3.0 \pm 1.0$	<0.5	<0.5
	221-110	108.07	134.9	$2.0 \pm 0.8$	$0.9 \pm 0.5$	<0.5
	505-414	99.48	325.3	$1.4 \pm 0.6$	<0.5	<0.5
	616-505	82.03	447.3	$1.2 \pm 0.6$	<0.5	<0.5
	423-312	78.74	300.5	$1.4 \pm 0.6$	<0.5	<0.5
	321-212	75.38	212.1	$1.7 \pm 0.7$	<0.5	<0.5
p-H2O	322-313	156.19	206.3	$0.5 \pm 0.4$	<0.5	<0.5
	313-202	138.53	142.3	$1.2 \pm 0.6$	<0.5	<0.5
	220-111	100.98	136.2	$1.0 \pm 0.5$	<0.5	<0.5
	322-211	89.99	206.3	$1.4 \pm 0.6$	<0.5	<0.5
	331-220	67.09	285.1	$0.6 \pm 0.4$	<0.5	<0.5
CO	14-13	186.00	403.5	$1.3\pm 0.6$	$1.2 \pm 0.4$	<0.5
	15-14	173.63	461.1	$1.5 \pm 0.7$	$0.8 \pm 0.4$	<0.5
	16-15	162.81	522.5	$1.7 \pm 0.7$	$0.7 \pm 0.4$	<0.5
	17-16	153.26	587.7	$1.7 \pm 0.7$	$0.6 \pm 0.5$	<0.5
	18-17	144.78	656.8	$1.3\pm 0.6$	<0.5	<0.5
	19-18	137.20	729.7	$1.3\pm 0.6$	<0.5	<0.5
	20-19	130.37	806.4	$1.2 \pm 0.6$	<0.5	<0.5
OH	$^{2}\Pi_{3/2,5/2}$- $^{2}\Pi_{3/2,3/2}$	119.33	83.7	$1.0 \pm 0.5$	<0.5	<0.5
	$^{2}\Pi_{3/2,7/2}$- $^{2}\Pi_{3/2,5/2}$	84.51	201.9	$1.1 \pm 0.5$	<0.5	<0.5
	$^{2}\Pi_{1/2,1/2}$- $^{2}\Pi_{3/2,3/2}$	79.15	126.3	$1.1 \pm 0.5$	<0.5	<0.5
[OI]	3P1-3P0	145.48	227.7	<0.5	<0.5	<0.5
	3P2-3P2	63.17	158.7	$2.2 \pm 0.9$	$4.3 \pm 1.5$	$2.2 \pm 0.9$
[CII]	2P3/2-2P1/2	157.74	63.7	$2.2 \pm 0.9$	$1.8 \pm 0.7$	$1.1 \pm 0.5$