Herbig-Haro objects (HH) are emission line objects, observed from radio to UV wavelengths, and found in the immediate vicinity of young stellar objects (YSO). Present theoretical understanding interprets these objects as the cooling radiation emanating from shocks formed when out-flowing gas from the vicinity of the YSO interacts with either itself or the ambient medium. HH objects can, based on their spectra, broadly be divided into high excitation- and low excitation objects. Four types of models have been invoked in order to explain the HH phenomenon: The `Shocked cloudlet' model (Schwartz, 1978), where a wind emanating from the YSO impacts and shocks a denser cloudlet in the ambient medium; The `interstellar bullet' scenario (Norman & Silk, 1979), where a small relatively denser cloud is ejected from the vicinity of the YSO and the shock appears as it plows through the ambient medium; The cavity model (Canto, 1980), where shocks are caused by an expanding wind, interacting with the walls of a cavity evacuated by the wind itself. The fourth type is the jet/working surface scenario (Königl, 1982), in which a well collimated atomic jet escapes from the YSO with a very high velocity and shocks at its end (`working surface'), as well as experiencing crossing shocks along the way (e.g. Raga, 1995). Many but not all HH objects appear to be best modeled as the consequence of a jet (e.g. Böhm, 1995), and it could thus be that several of these mechanisms take place in nature.
One case, where this could be true, is in the molecular outflow/HH complex in the L1551 cloud. The originating source of the outflow is designated IRS5, and identified as a low mass YSO by Fridlund et al. (1980). From IRS5 a short (20) well collimated jet emanates (Mundt & Fried, 1983; Fridlund & Liseau, 1994). This jet falls well along the major axis of a large bipolar molecular outflow, which extends for several arc min in opposite directions from IRS5. The molecular source has been well studied. The geometry of the outflow has been modeled as evacuated molecular shells, filled with a faster ( 150 km s-1 versus 50 km s-1 for the molecular component) HI wind (see Staude & Elsässer, 1993 and references theirin). Within the approaching lobe a large number of HH objects is found scattered. Two of these HH objects - HH 28 & HH 29 - have had their proper motion vectors determined (Cudworth & Herbig, 1979). Their tangential velocity is 150 km s-1, and the origin of the vectors when extrapolated, is found near IRS5. The distance to L1551 is only 140 pc (Elias, 1978) allowing the resolution of quite small spatial elements.
Fridlund, Liseau and Perryman (1993, hereafter FLP) performed imaging spectrophotometry of HH 29. They found evidence of high excitation and thus of a strong shock localized at the brightest portion of the object (which was designated HH 29a - 20% of the line flux emanates from this object). The high excitation nature of HH 29a is further substantiated by the IUE results of Cameron & Liseau (1990) and by Liseau et al. (1996 - hereafter LHFC). These authors also find evidence for significant flux variation on time scales of 6 months. Estimates of the extinction (AV 3 - 5 magnitudes) indicate that the HH 29 object is located within the molecular cavity, probably interacting with the cavity walls. HH 29a only makes up 4 % of the total area of HH 29 and the rest of the object also has a decidedly `clumpy' structure, very reminiscent of HH1-2 and HH32 (e.g. Hartigan et al. 1987 - hereafter HRH; Choe, Böhm & Solf, 1985).
In view of the large body of available data, as well as the above mentioned proximity which allows both the resolution of small spatial elements and that of detecting faint spectral features at high velocity resolution, we have decided to attempt the mapping of the radial velocity field of HH 29, at the highest possible spatial and spectral resolutions. We also strived to observe as many shock diagnostic spectral lines as possible. Our ultimate goal is to use the current results together with our previously published data (FLP, LHFC) to characterize physical parameters and to build a 3-D empirical model which can be compared with available (bow-) shock models. We also aim at confirming the size scale and densities of the `mini-clumps' by analyzing the characteristics of the radiation field with respect to the velocities. In this paper we concentrate on the H and the [SII] data which provides information about the electron density, ne, the level of excitation and the `clumpiness' of the object in 3 dimensions through a study of how the flux in H and the above mentioned quantities vary as a function of velocity and of the spatial coordinates.
© European Southern Observatory (ESO) 1998
Online publication: January 8, 1998