High-velocity string of knots in the outburst of the planetary nebula Hb4

The bipolar collimated outflows of the Hb4 planetary nebula (PN) exhibit an evident decrease in their expansion velocity with respect to the distance from the central star. So far, similar velocity law has also been found in Herbig–Haro objects. The interpretation of this peculiar velocity law and the classification of the outflows is the main focal point of this paper. High-dispersion long-slit e...