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

Abstract

We investigate the physical properties of the HH 30 jet by applying the spectroscopic diagnostic technique described in Bacciotti & Eislöffel (\cite{be99}) to ground-based spectra and Hubble Space Telescope (HST) calibrated emission-line images. We derive the variation along the beam of the ionization fraction 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 the dataset used) near the source of the flow and of 1.8" further out. In the jet x_e rapidly rises from 0.065 at 0.2" to 0.1 at 0.4", and then slowly 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(deg) . At the beginning of the jet n_H is at least ~ 10(5) cm(-3) , but it decreases to 5 10(4) cm(-3) within the first arcsecond and then slowly falls to 10(4) cm(-3) at large distance from the source. On average T_e decreases from ~ 2 10(4) K to 10(4) K within the first arcsecond 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 rises from 0.07 up to 0.35 at 2-3 arcsec from the source, n_H 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 filling factor is of order unity in this flow. The emission-weighted jet width calculated with the parameters that we derive is in good agreement with the observed FWHM; we find, however, that the jet radius apparently goes to zero at the source location, defining an initial full opening angle of about 10(deg) . The intensity peaks, i.e. the knots, are clearly correlated with local temperature maxima. The ionization fraction and the electron and total densities do not show any evident increase at the same positions, although we cannot exclude the presence of small-scale variations, because of the lower spatial resolution with which these quantities have been derived. Alternatively, the lack of large density enhancements at the locations corresponding to the knots may be due to the presence of a substantial magnetic field in the body of the jet. Anyway, the absence of evident bow-shaped features suggests that in this jet it is more likely that the chain of bright spots traces travelling plasma instabilities, rather than a series of internal working surfaces. Along the jet the mass-loss rate is quite moderate: assuming an average flow speed of 200 km s(-1) , and adopting as our jet diameter the emission-weighted jet width, we find dot {M} ~ 1.7 10(-9) M_sun yr(-1) and correspondingly dot {P} ~ 3.5 10(-7) M_sun yr(-1) km s(-1) . In the counter-jet, in contrast, dot {M} (dot {P}) decreases from about 1.8 10(-9) M_sun yr(-1) (3.6 10(-7) M_sun yr(-1) km s(-1) ) at 0.6" from the source to about 9.3 10(-10) M_sun yr(-1) (1.9 10(-7) M_sun yr(-1) km s(-1) ) further out.