Herbig–Haro Object - Physical Characteristics

Physical Characteristics

Emission from HH objects is caused by shock waves when they collide with the interstellar medium, but their motions are complicated. Spectroscopic observations of their doppler shifts indicate velocities of several hundred kilometres per second, but the emission lines in the spectra of HH objects are too weak to have been formed in such high speed collisions. This probably means some of the material they are colliding with is also moving outwards, although at a slower speed.

The total mass being ejected to form typical HH objects is estimated to be of the order of 1–20 Earth-masses, a very small amount of material compared to the mass of the stars themselves. The temperatures observed in HH objects are typically about 8000–12,000 K, similar to those found in other ionized nebulae such as H II regions and planetary nebulae. They tend to be quite dense, ranging from a few thousand to a few tens of thousands of particles per cm3, compared to generally less than 1000/cm3 in H II regions and planetary nebulae. HH objects consist mostly of hydrogen and helium, which account for about 75% and 25% respectively of their mass. Less than 1% of the mass of HH objects is made up of heavier chemical elements, and the abundances of these are generally similar to those measured in nearby young stars.

Near to the source star, about 20–30% of the gas in HH objects is ionised, but this proportion decreases at increasing distances. This implies the material is ionised in the polar jet, and recombines as it moves away from the star, rather than being ionised by later collisions. Shocking at the end of the jet can re-ionise some material, however, giving rise to bright "caps" at the ends of the jets.