We have used the IUE to the limit of its capability (~10^-15^erg/cm^2^/s/A) and present results based on long term monitoring in the UV of the Herbig-Haro object HH 29, which is dynamically coupled to the outflow activity from the deeply embedded low mass stellar object IRS5 in the L1551 dark cloud. The eight years of IUE observations confirm the degree of variability of the object, originally discovered by Cameron & Liseau (1990; A&A 240, 409), both what concerns the amplitudes (factor of two) and short time scales (less than 0.5yr). We have now also found declines in brightness with these short time scales, implying local particle densities clearly in excess of 10^4^cm^-3^. The variations of the shortwave continuum (~1200-1950A) and of the high ionization species follow a similar pattern, whereas the intensity variations of forbidden lines from low ionization species appear anti-correlated. Such behaviour is consistent with HH 29 changing its degree of excitation with time, probably because of multiple shocks in the object. We argue that the Mg IIh & k lines are optically thick which would explain why they are observed not to vary. From this we estimate that the true variability time scale of 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 slope of the very blue shortwave continuum varies in time as well, which we interpret to be caused by changing temperatures as a consequence of the different shock waves passing through the object. Combining the IUE data with simultaneous ground based observations leads us to construct a two-phase model for HH 29 from which we derive the physical parameters 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 than 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 factor of this gas is consequently small (on the order of 0.1-1%). From comparisons with the high excitation objects HH 1/2 we infer that the multi-phase conditions characterizing HH 29 probably also apply to other, less systematically observed HH objects. From the UV luminosity generated by the shocks in HH 29 (0.5Lsun_) we infer a lower limit to the rate at which the central source IRS5 loses mass (>210^-5^Msun_/yr). This limit on the mass loss rate is considerably larger than any conceivable mass accretion rate for the stellar object (probably significantly less than 10^-5^Msun_/yr). Given also the observed variability of HH 29 we argue that the flow from IRS5 is probably neither homogeneous nor steady in time. This could potentially reduce the mass loss rate otherwise needed to account for the observed radiative losses in the L1551 flow.
Astronomy and Astrophysics
- Pub Date:
- February 1996
- ISM: CLOUDS ISM: JETS AND OUTFLOWS ISM: INDIVIDUAL OBJECTS: L1551 HH 29 STARS: FORMATION