TIME VARIABLE SHOCKS IN THE UV - LONG-TERM IUE MONITORING OF HH-29

We have used the IUE to the limit of its capability (similar to 10(-15 ) erg cm(-2) s(-1) Angstrom(-1)) and present results based on long ter m monitoring in the UV of the Herbig-Haro object HH 29, which is dynam ically coupled to the outflow activity from the deeply embedded low ma ss stellar object IRS 5 in the L 1551 dark cloud. The eight years of I UE observations confirm the degree of variability of the object, origi nally discovered by Cameron & Liseau (1990; A&A 240, 409), both what c oncerns the amplitudes (factor of two) and short time scales (less tha n 0.5 yr). We have now also found declines in brightness with these sh ort time scales, implying local particle densities clearly in excess o f 10(4) cm(-3). The variations of the shortwave continuum (similar to 1200-1950 Angstrom) and of the high ionization species follow a simila r pattern, whereas the intensity variations of forbidden lines from lo w ionization species appear anti-correlated. Such behaviour is consist ent with HH 29 changing its degree of excitation with time, probably b ecause of multiple shocks in the object. We argue that the Mg II h&k l ines are optically thick which would explain why they are observed not to vary. From this we estimate that the true variability time scale o f HH 29 is of the order of weeks (10(6) s) rather than that determined by the observing frequency, which is several months (10(7) s). The sl ope of the very blue shortwave continuum varies in time as well, which we interpret to be caused by changing temperatures as a consequence o f the different shock waves passing through the object. Combining the IUE data with simultaneous ground based observations leads us to const ruct a two-phase model for HH 29 from which we derive the physical par ameters of the object. In addition to a conventional component (10(4) K and 10(3) cm(-3)), a hot component (several times 10(4) up to more t han 10(5) K) having average densities at least as high as 10(6) cm(-3) is required to reconcile with the observations. The volume filling fa ctor of this gas is consequently small (on the order of 0.1-1%). From comparisons with the high excitation objects HH 112 we infer that the multi-phase conditions characterizing HH 29 probably also apply to oth er, less systematically observed HH objects. From the UV luminosity ge nerated by the shocks in HH 29 (0.5 L.) we infer a lower limit to the rate at which the central source IRS 5 loses mass (> 2 10(-5) M. yr(-1 )). This limit on the mass loss rate is considerably larger than any c onceivable mass accretion rate for the stellar object (probably signif icantly less than 10(-5) M. yr(-1)). Given also the observed variabili ty of HH 29 we argue that the flow from IRS 5 is probably neither homo geneous nor steady in time. This could potentially reduce the mass los s rate otherwise needed to account for the observed radiative losses i n the L 1551 flow.