Study Of the Cometary Globules in the Gum Nebula

Abstract

From our analysis of the measured velocities of cometary globules (CGs), we draw the following conclusions: Kinematic Interpretation The motion of the CGs cannot be explained by a model dominated by galactic rotation, contrary to the suggestion made by Z83 based on a smaller sample. Instead, our data supports two key interpretations: System Expansion After accounting for galactic differential rotation, the radial velocity distribution of the CG heads is best explained by an expansion of the system from a common center. The data fits more accurately with a model where the globules are distributed throughout the interior of a sphere, rather than confined to a shell. The expansion velocity of the outermost globules is approximately 12 km/s, implying an expansion age of around 6 million years (Myr). Tail Velocity Gradients Some cometary tails exhibit systematic velocity gradients. Interestingly, the estimated age for the formation of these tails-based on the velocity difference between the heads and tail-ends-is about 3 Myr, which is comparable to the expansion age. Young Stellar Object (YSO) Distribution Figure 4.12 presents a histogram of the apparent tail stretching ages, defined as the time required for the velocity difference between the head and tail-end of a CG to stretch it to its observed length. There appears to be a real tendency for YSOs to be located on the front side of the globules rather than the back. This trend is most confidently observed in clouds with angular sizes between 0? and 10?. In larger clouds, the effects of external mechanisms may be overshadowed by normal star formation processes. Even among smaller clouds, external triggering does not seem to be the sole mechanism of star formation. It’s important to note that not all IRAS sources identified as YSOs are necessarily true YSOs. Due to the larger search area, some sources may not be physically associated with the clouds, which introduces uncertainty. Without this confusion, the confidence level of the observed trend would likely be even higher.