Shock-wave models

From planar to bow shocks

Prior to the IUE launching, the optical emission line spectra of the HHOs was attributed to the radiation from the cooling regions of planar shock waves (Dopita 1978, Schwartz 1978, Raymond 1979). Some features of the visual spectra were well accounted for by these models although the agreement was less satisfactory than that obtained for supernova remnants. The first IUE observations of HH1 (Ortolani & D’Odorico 1980, Bohm et al 1981) pointed out that planar shock waves models cannot simultaneously explain the optical and the UV spectrum of the HHOs. For instance, the small ratio between the [OIII]5007 and the [OII]3728 optical lines implies relatively small shock Mach numbers, but for these small values the flux of th CIV1550 lines is predicted to be ~ 2 orders of magnitude smaller than observed. Bohm et al (1981) suggested that UV lines could be formed in a denser region than the optical “probably due to a different, higher density shock wave”; later on this suggestion was shown no to be very plausible since the optical and UV lines of HH1 and HH2 are formed in the same region (Bohm-Vitense et al 1982). Brugel et al (1982) were the first to interprete these characteristics as a consequence of having a range of velocities in the shock wave; they pointed out that this velocity range arises naturally in bow-shocks which are expected to be produced if the HHOs are shocked cloudlets (Schwartz & Dopita 1980).

In 1983, the first low excitation HHOs (HH43 and HH47) were observed (Schwartz, 1983). No high excitation lines of CIV or CIII] were detected and the far UV spectrum was found to be dominated by the H2 Lyman band emission lines. The UV (as well as the IR) data could be explained by low-velocity (≤ 35 km/s) planar shocks (Dopita et al 1982).

In 1984, the first bow-shock models were applied successfully to the UV and the optical spectra of an HHO (HH1 and HH2) (Hartmann & Raymond 1984, Brugel & Shull, 1984). In these models the bow-shock is approximated by a series of oblique plane shocks. The first theoretical predictions for the spatial distribution of emission in bow shocks became available soon after (Raga & Bohm, 1985, 1986, Hartigan et al 1987) and then, detailed studies on the spatial structure of the UV spectrum began. Bohm et al (1987) carried out the first analysis about the spatial distribution of the line emission (CIV, CIII], SiIII]) in HH1 and HH2. They found that, in general, the size of the CIII] and SiIII] emitting regions is slightly larger than the size of the CIV as predicted by the theoretical models. A more complete study making use of data from 6 HHOs confirmed this behaviour (Lee et al 1988). Also a detailed comparison between optical (Hα and [SII]) and UV (CIV, CIII], CII] and Mg II) lines showed that the size of the UV lines emitting region is comparable to the optical size of the object (for instance in Hα) and that the UV emission lines region is centered in the optical object.

The fluorescent H2 emission lines of low excitation HHOs (HH43 and HH47) were also found to be formed in narrow (unresolved) regions with characteristic sizes of at least a half of the optical ([SII]) emission region (Bohm et al 1991).

By the end of the IUE mission, Moro-Martin et al (1996) showed that, in general, the parameters obtained from optical spectra provide a self-consistent picture when used to explain the UV data. The remaining differences between models and observations are attributed to the effect of complex preionization structures. Accurate calculations of the ionization state of the material entering the bow-shock had already shown the significance of preionization for shock speeds of ~ 150 km/s and below (Raymond et al 1988).

Clumpiness and non-stationarity

In the mid-time, UV variability studies showed up rapid variations in the UV emission-line spectrum of HH1 which were not accompanied with analogous optical changes (Brugel et al 1985). The relevance of the medium clumpiness was put forward. This was confirmed some years later from the detailed analysis of HH29 variability (Cameron & Liseau 1990;Liseau et al 1996). In particular, Liseau et al (1996) showed that the flux variations of the UV forbidden lines from low ionization species are anticorrelated with the variations of the short wavelength continuum and the variations of the high excitation species. Such behaviour is consistent with HH29 changing is degree of excitation with time. The combination of optical and UV data drove them to suggest a 2 phase model with a component at T=104 K and Ne = 103 cm-3 and a hot and dense component with T=105 K and Ne = 106 cm-3 with a very small filling factor (~ 0.1 - 1 %). They suggested a variability time scale of some weeks and a non-stationarity and non-homogeneity of the flow.

 

The continuum problem