Astron. Astrophys. 350, 529-540 (1999)

2. Observations and results

L1448-mm ( = , = 30^o 33 35) was observed with the LWS (Long Wavelength Spectrometer, Clegg et al. 1996) and SWS (Short Wavelength Spectrometer, de Graauw et al. 1996) on the ISO satellite. In Fig. 1 the apertures of the two instruments are superimposed on a 2.12 µm map of L1448 to show the region covered by the observations.

Fig. 1. The LWS and SWS beams of 75" and 14"20" respectively are superimposed on a H2(1-0)S(1) + continuum emission from Davis and Smith (1995). The contours delineate the EHV CO emission in the blue (solid line) and red (dotted line) lobes of the outflow (adapted from Guilloteau et al. 1992). R1 and B1 indicates the EHV molecular bullets closest to the central source.

2.1. LWS observations

A low resolution (R 300) spectrum of the source extending from 45 to 197 µm was obtained with the LWS during revolution 653 (August 30, 1997). The spectrum was oversampled by a factor of four and each spectral sample had 10 s integration, for a total integration time of 4265 s. The raw data were reduced and calibrated using version 7 of the LWS pipeline, which resulted in an absolute intensity calibration within 30% (Swinyard et al. 1996). Post-pipeline processing included the removal of spurious signals resulting from cosmic ray impacts, and averaging of the various grating scans from each detector. The line fluxes, obtained from gaussian fitting to the spectral profiles, are listed in Table 1, where the associated errors refer to the rms noise of the local baseline. We show in Fig. 2 the global spectrum obtained by merging together the sub-spectra obtained from the ten LWS detectors just to give an idea of the relative contribution of the line emission with respect to the underline continuum, while a detailed account for the FIR continuum emission of the mm source will be given elsewhere (Nisini et al. 1999, Paper II).

Fig. 2. The LWS spectrum of L1448-mm obtained by matching each other the ten LWS detectors and rebinning the data at the instrumental resolution

Table 1. Emission lines measured with LWS toward L1448-mm

Fig. 3 shows the continuum subtracted (by second order polynomial fitting) spectrum from 50 to 190 µm while in Fig. 4 the [OI ] 63µm line is plotted. The spectrum appears extremely rich in rotational lines of both carbon monoxide and water vapour. All the CO transitions with J from 14 (the lowest transition observable with the LWS, at 186 µm) to 21 were detected. The J=22 line is not seen to a 3 level of 7 10^-20 W cm^-2, but the J=25 line at 104.5 µm is clearly detected with a S/N of about 5. The J=23, 24 and 26 lines are blended with strong H₂O transitions which have a FWHM slightly broader than the instrumental resolution element of 0.6 µm. An attempt to deblend with a two gaussian fit was made for the J=26 and J=24 lines, which are separated from the adjacent p-H₂O 2₂₀-1₁₁ and o-H₂O 2₂₁-1₁₀ lines by about 0.5 and 0.6 µm. The CO J=23 at 113.46 µm is however too close to the o-H₂O 4₁₄-3₀₃ line (at 113.54 µm) to permit a meaningful deblending. The fact that the observed line peaks at 113.65 µm suggests however that most of the flux can be attributed to the water line.

Fig. 3. The continuum subtracted LWS spectrum of L1448-mm in the wavelength range where molecular emission lines have been detected. This part of the spectrum also includes the [CII ] 158 µm line.

Fig. 4. The continuum subtracted portion of the LWS spectrum showing the [OI ] 63 µm line.

Water lines are observed in both the ortho and para form; almost all the backbone lines up to excitation temperatures of about 500 K are present, plus most of the strongest transitions which fall in the LWS spectral range (see Fig. 6). In addition to CO and water, the OH fundamental at 119 µm (a blend of the four low-lying transitions of the 3/2 ladder) is also observed. An upper limit to the 99 µm line of OH 5/2-3/2 was set by attempting to deblend it from the nearby o-H₂O 5₀₅-4₁₄ line at 99.5 µm. The [CII ] 158µm and [OI ] 63µm lines are the only atomic emission lines present in the spectrum.

2.2. SWS observations

The SWS was used in the AOT SWS02 mode to scan the H₂ pure rotational lines from S(1) to S(7) and the [SiII ] 35µm line with a resolution ranging from 1000 to 2000. The beam sizes of these observations (obtained during revolution 814, on February 6, 1998) are 14"20" for the lines from S(3) to S(7), 14"27" for the S(1), S(2) and 20"27" for [SiII ].

Each line was integrated for a total time of 200 s. The data were reprocessed through the SWS pipeline version 6; systematic flux errors due to the SWS calibration uncertainties are estimated to be about 30% (Schaeidt et al. 1996). Of the scanned lines, only the H₂ S(3), S(4) and S(5) were detected and their spectrum is shown in Fig. 5. The measured and upper limits fluxes are reported in Table 2.

Fig. 5. Continuum subtracted SWS spectra of the H2 pure rotational lines detected towards L1448-mm. Gaussian fits through the data are indicated by dashed lines.

Table 2. Emission lines measured with SWS towards L1448-mm

© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999
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