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Astron. Astrophys. 327, 1206-1214 (1997)

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4. The OMC-1 ortho-para distribution

In Fig. 2 we present the OMC-1 1-0 S(1) image. We see the extended bullets very clearly in this image since the line/continuum ratio is enhanced over that measured in normal narrow-band filters (the FP bandpass measures [FORMULA]). In Figs. 3 & 4 we show the ortho-para ratio for several east-west cuts through the source as well as the relative intensity of the 1-0 S(0) line. The rotational temperature corresponding to the E-W cuts is shown in Figs. 5 & 6. Note that no values are derived for pixels in which any individual line flux could not be accurately determined.

[FIGURE] Fig. 2. The 1-0 S(1) map of OMC-1 at [FORMULA] resolution with east-west cuts 1-8 running from south (1) to north (8).

[FIGURE] Fig. 3. The ortho-para ratio in the north of OMC-1 for the top four slit positions in Fig. 2. Pixels here are [FORMULA] The broken line is the relative 1-0 S(0) flux.

[FIGURE] Fig. 4. The ortho-para ratio in the south of OMC-1 for the bottom four slit positions in Fig. 2. Pixels here are [FORMULA] The broken line is the relative 1-0 S(0) flux.

[FIGURE] Fig. 5. The rotational temperature in the north of OMC-1 for the top four slit positions in Fig. 2

[FIGURE] Fig. 6. The rotational temperature in the south of OMC-1 at [FORMULA] resolution for the bottom four slit positions in Fig. 2

We find:

  • The ortho-para ratio is consistent with 3 in all regions where the 1-0 S(0) line is strong.
  • The rotational temperature increases quite monotonicly from east to west in the northern lobe. The 1-0 S(1) flux does not show any related variation. Hence a differential extinction effect, in which extinction would dominate the intensity distribution, can be excluded. Temperature variations were found in the southern lobe by Schild et al (1997) from the S(1) and S(2) transitions (with the same instrument), having assumed an ortho-para ratio equal to 3. In the south (see Cuts 1 & 2), we see that the rotational temperature rises steadily from 880K to over 1300K from east to west.
  • The increase in the ortho-para ratio above 3 on the western side of the outflow is a sensitivity effect; the 1-0 S(0) emission becomes too weak (as the temperature rises) to be reliably measured. Similarly, the rotational temperature across Cut 3 is highly non-uniform, and the derived [FORMULA] unreliable.

If we were to assume, on theoretical grounds, that [FORMULA] throughout the lobes, then the measured value of [FORMULA] provides a check on the flux levels. If we find [FORMULA] in large areas, then we can be assured that any associated temperature variations are not due to badly-measured line fluxes but actually require a physical explanation. In the above findings, it thus seems unlikely that [FORMULA] really exceeds 3, supporting the interpretation that the 1-0 S(0) flux is not fully measured when the temperature is high.

We present the vibrational temperature [FORMULA] in Figs. 7 & 8. The 2-1 S(1) and the 2-1 S(3) (see Fig. 9), have not been so accurately measured. Nevertheless, it is remarkable how constant [FORMULA] remains through some of the cuts (e.g. 7 & 8) even though the rotational temperature rises steadily in the same region.

[FIGURE] Fig. 7. The vibrational temperature derived from the 1-0 S(1) and 2-1 S(1) fluxes in the north of OMC-1 for the top four slit positions in Fig. 2

[FIGURE] Fig. 8. The vibrational temperature derived from the 1-0 S(1) and 2-1 S(1) fluxes in the south of OMC-1 for the bottom four slit positions in Fig. 2

[FIGURE] Fig. 9. The vibrational temperature derived from the 1-0 S(1) and 2-1 S(3) fluxes in the north of OMC-1 for the top four slit positions in Fig. 2. The data for the south cuts, of low quality, are not shown.

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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998
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