The physical properties of the HH 30 jet from HST and ground-based data

We investigate the physical properties of the HH 30 jet by applying the spe ctroscopic diagnostic technique described in Bacciotti & Eisloffel (1999) t o ground-based spectra and Hubble Space Telescope (HST) calibrated emission -line images. We derive the variation along the beam of the ionization frac tion x(e), of the total hydrogen density n(H) and of the average excitation temperature T-e, with a spatial sampling of 0." 1 to 0." 6 (depending on t he dataset used) near the source of the flow and of 1." 8 further out. In t he jet x(e) rapidly rises from 0.065 at 0." 2 to 0.1 at 0." 4, and then slo wly increases up to 0.140 within 2" from the source. From 2." 4 to 12." 5, x(e) decreases very slowly down to a value of 0.04. The slow recombination in the outermost collimated part is consistent with a flow opening angle of about 2". At the beginning of the jet n(H) is at least similar to 10(5) cm (-3), but it decreases to 5 10(4) cm(-3) within the first arcsecond and the n slowly falls to 10(4) cm(-3) at large distance from the source. On averag e T-e decreases from similar to 2 10(4) K to 10(4) K within the first arcse cond of the jet, then it slowly decays to 6000-7000 K. In the faint counter -jet, which appears to be substantially more excited than the jet, x(e) ris es from 0.07 up to 0.35 at 2-3 " from the source, nH decreases from about 8 10(4) cm(-3) to a few 10(3) cm(-3), while T-e is scattered around 1.2-1.3 10(4) K. A comparison between the observed and calculated line fluxes shows that the filing factor is of order unity in this flow. The emission-weight ed jet width calculated with the parameters that we derive is in good agree ment with the observed FWHM; we find, however, that the jet radius apparent ly goes to zero at the source location, defining an initial full opening an gle of about 10 degrees. The intensity peaks, i.e. the knots, are clearly c orrelated with local temperature maxima. The ionization fraction and the el ectron and total densities do not show any evident increase at the same pos itions, although we cannot exclude the presence of small-scale variations, because of the lower spatial resolution with which these quantities have be en derived. Alternatively, the lack of large density enhancements at the lo cations corresponding to the knots may be due to the presence of a substant ial magnetic field in the body of the jet. Anyway, the absence of evident b ow-shaped features suggests that in this jet it is more likely that the cha in of bright spots traces travelling plasma instabilities, rather than a se ries of internal working surfaces. Along the jet the mass-loss rate is quit e moderate: assuming an average flow speed of 200 km s(-1), and adopting as our jet diameter the emission-weighted jet width, we find M similar to 1.7 10(-9) M. yr(-1) and correspondingly P similar to 3.5 10(-7) M. yr(-1) km s(-1). In the counter-jet, in contrast, M (P) decreases from about 1.8 10(- 9) M. yr(-1) (3.6 10(-7) M. yr(-1) km s(-1)) at 0." 6 from the source to ab out 9.3 10(-10) M. yr(-1) (1.9 10(-7) M. yr(-1) km s(-1)) further out.