The two-photon continuum in Herbig-Haro objets.

The blue continuum seen in many Herbig-Haro (HH) objects is shown to be intrinsic to the source, and is due to a collisionally excited two-photon continuum of hydrogen. A strong inverse correlation is found between its strength and the excitation class of the HH spectrum. The most likely explanation of the data, based on theoretical models, is that both the lines and continuum arise in a shock of finite age running into a partially preionized medium, so that if HH shocks are excited by a stellar wind from a pre-main-sequence star, this wind is likely to be episodic rather than continuous.