The contribution of the IUE to the understanding of the HHOs
Since the very first UV observation of HH1 two intriguing characteristics of the HHOs became evident: (1) the UV lines spectrum shows a higher degree of ionization than the optical spectra, and (2) the UV continuum rises to short wavelengths (at least up to 1300 Å). Most of the IUE observations were intended to find a physical explanation for them. The evolution from planar shock-wave models to bow-shock models in order to explain the spectrum of the HHOs occurred during the IUE lifetime. The physical mechanism which generates the UV continuum of HHOs remains today uncertain. The following is just a brief accounting of the IUE major contributions.
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
Prior to the IUE launching it was known that the continuum of the HHOs rises towards short wavelengths (Bohm et al 1974). Two possible explanations were suggested: (1) scattered light from nearby stars and (2) two photon emission from a hot shocked gas. The UV observations showed that the continuum rises steeply towards the shortest wavelengths observable with the IUE (~ 1300 Å). Therefore the UV continuum energy distribution cannot be explained by a T Tauri stellar continuum scattered by the dust. Mundt and Witt (1983) suggested that, at least for HH1 and HH2 (which are in the Orion region), there could be a contamination of the UV spectra by the Orion Reflection Nebula which could account by as much as a 30 -50 % of the continuum flux at λ ≤ 1500Å. However detailed analysis, where the nebular background contribution was properly substracted out, pointed out a similar structural shape of the continuum rising towards short wavelengths (Lee et al 1988). Therefore, it became clear that the peculiar spectral distribution of the UV continuum is not caused by scattered light from nearby stars.
The possible influence of the extinction curve on the continuum was discussed at length. The best correction was shown to be a θ-Ori extinction curve for HH43 (Schwartz et al 1985) and also for HH1 and HH2 (Bohm et al 1987). Therefore Cameron & Liseau (1990) introduced a UV colour index which is extinction free for dust constituents giving rise to the θ-Ori extinction curves. This index was used to compare the observed continuum values with those calculated for a recombining hydrogenic plasma. With the possible exception of HH11, they found large deviations from the 2-photon decay continuum of HI. They also found that H2+ free-bound emission does not offer a solution to the continuum problem. In fact, previous studies had already shown that the short wavelength UV continuum has a the maximum around 1580 Å but not near 1410 Å where the two photon continuum of hydrogen peaks (Bohm et al 1987).
A new key was provided by the discovery of the differences between the spatial distribution of the UV continuum in the long (2000-3200 Å) and in the short (1300-1950 Å) wavelength ranges (Lee et al 1988). The long wavelength continuum is quite narrow and comparable to the optical; both can be basically produced by the two photon decay mechanism. However, the short wavelength continuum is broader than any emission line (UV or optical) pointing out that an additional continuum formation mechanism is present.
At the end of the IUE mission, the most likely mechanism for the formation of this short wavelength continuum was believed to be H2 continuum emission (Dalgarno et al 1970). Bohm et al (1987) realized that the observed UV continuum enhancement occurs only in the wavelength interval in which H2 emission should occur and proposed this mechanism to explain the HH1 and HH2 spectra. In fact, this mechanism is also suitable to explain the close similarity between the structural shape of continuum in low and high excitation HHOs (Schwartz 1983, Bohm et al 1991); the H2continuum is formed in the destruction process of H2 molecules which can be caused either by photodissociation by radiation shortwards of 912 Å or by collisions with low energy thermal particles. Unfortunately, there is not a complete agreement between the detailed spectral shape of the H2continuum and the UV data (even qualitative). Therefore, the physical mechanism which generates the short wavelength UV continuum of HHOs remains today uncertain.