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Astron. Astrophys. 346, 260-266 (1999)

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4. The emitted spectrum

We have computed the emission line spectrum for the above grid of mixing layer models. Table 1 lists the parameters for the two sequences (constant [FORMULA] and constant [FORMULA]) with [FORMULA] cm-3, as well as the H[FORMULA] flux (emitted per unit area perpendicular to the surface of the mixing layer) and the ratios of several emission lines with respect to H[FORMULA]. Tables 2 and 3 list the same quantities for models with [FORMULA] cm-3 and [FORMULA] cm-3, respectively.


[TABLE]

Table 1. Line intensities[FORMULA] predicted from mixing layer models with [FORMULA] cm-3.
Notes:
a) Line intensities such that H[FORMULA].



[TABLE]

Table 2. Line intensities[FORMULA] predicted from mixing layer models with [FORMULA] cm-3.
Notes:
a) Line intensities such that H[FORMULA].



[TABLE]

Table 3. Line intensities[FORMULA] predicted from mixing layer models with [FORMULA] cm-3.
Notes:
a) Line intensities such that H[FORMULA].


The line ratios listed in Tables 1, 2, and 3 show that for all of the models that we have computed, the spectrum has very low excitation characteristics. The model sequence with varying [FORMULA] (and constant [FORMULA]) clearly shows that for increasing widths of the mixing layer (i.e., for increasing values of [FORMULA]), the strength (relative to H[FORMULA]) of all of the tabulated forbidden lines increases. Interestingly, higher excitation lines (e.g., [O III] 5007) are absent in all of the models.

The constant [FORMULA] model sequence shows that for jet velocities above [FORMULA] km s-1, the strongest forbidden lines become insensitive to further increases in [FORMULA]. Therefore, [FORMULA] is the important parameter for determining the spectrum emitted from a mixing layer, as we expect that the [FORMULA] km s-1 condition should be satisfied by most outflows from young stars.

In order to carry out a comparison with observations, we have taken the data compiled by Raga et al. (1996) for low excitation HH objects, and plotted in Fig. 4 some of the observed lines in the form of two-line ratio graphs. On the same graphs, we have plotted the predictions from mixing layer [FORMULA] sequences (which all have [FORMULA] km s-1 and [FORMULA] K) of Tables 1, 2 and 3 which correspond to [FORMULA], [FORMULA] and [FORMULA] cm-3, respectively. We have also plotted the two-line ratio curves obtained from the plane-parallel shock wave models of Hartigan et al. (1994, 1995) with preshock density [FORMULA] cm-3.

[FIGURE] Fig. 4. Two-line ratio plots, showing the characteristics of the theoretical and observed spectra. The solid squares correspond to observations of low excitation HH objects. The dashed line represent the line ratios obtained from plane-parallel steady-state shock models (Hartigan et al. 1994) of preshock density [FORMULA] cm-3 and shock velocities ranging from 15 to 40 km s-1 (the lower velocity models having higher [S II]/H[FORMULA]). The solid lines correspond to three sequences in [FORMULA] of mixing layer models with [FORMULA] km s-1 and different densities: [FORMULA] (thin line), [FORMULA] (medium line) and [FORMULA] cm-3 (thick line). The details of the models are discussed in the text.

From Fig. 4, it is clear that the [N II] 6583/H[FORMULA] and [S II] 6731/6716 vs. [S II]/H[FORMULA] plots are reproduced with similar success both by the shock wave and the mixing layer models. However, the [O I] 6300/H[FORMULA] line ratio of the mixing layer models is too high by a factor of 2 or more with respect to the observed ratios. The observed [O I] 6300/H[FORMULA] line ratios are better reproduced by the shock wave models.

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© European Southern Observatory (ESO) 1999

Online publication: May 6, 1999
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