Herbig-Haro Objects and T Tauri Nebulae

Abstract

The visible parts of Herbig-Haro objects (Herbig, 1951; Haro, 1950,1952) appear as small nebulae with characteristic but rather unusual spectra. The fact that they occur only in regions in which T Tauri stars are present (cf. Herbig, 1969,1974) leads to the belief that the appearance of Herbig-Haro objects is somehow connected with the very early phases of stellar evolution (cf. Herbig, 1957; Strom, Grasdalen and Strom, 1974). The additional fact that Herbig-Haro objects have a spectrum almost identical to the spectrum of the small nebulae surrounding T Tauri (Herbig, 1950; Schwartz, 1973, 1974a,b) makes it even more probable that they are related to very young stars (or protostars). From this point of view it seems justified to include a discussion of Herbig-Haro objects and T Tauri nebulae in a volume on stellar envelopes. In fact, Strom, Grasdalen and Strom (1974) have suggested than the line and continuous spectrum of Herbig-Haro objects is formed in envelopes of pre-T Tauri stars. However, we have to admit that in certain respects the Herbig-Haro (HH) objects are different from other known stellar envelopes: 1) It is very improbable that they are excited by the ultraviolet radiation of a normal star (Bohm, 1956; Osterbrock, 1958; Bohm, Perry and Schwartz, 1973). 2) They are variable (cf. Herbig, 1957,1969) by several magnitudes with a typical time scale of the order of 5 to 10 years (or less?). This typical time is almost comparable to the characteristic recombination time in these nebulae. 3) Though they are sometimes associated with a star (e.g. Burnham’s nebula surrounding T Tauri) the objects which we usually call HH-objects do not show such a close association with a T Tauri star. 4) Their masses (∼10-3 M⊙) and apparent sizes (diameters ∼2000 — 5000 a.u.) are fairly large for a stellar envelope.