CO observation of the Galactic bubble N4

TL;DR

This study investigates the molecular gas and star formation activity in the Galactic bubble N4 using CO line observations, revealing its structure, potential energy sources, and signs of triggered star formation.

Contribution

The paper provides detailed CO observations of N4, suggesting it is an inclined expanding ring and identifying a candidate massive star and infall motion, indicating possible triggered star formation.

Findings

N4 is likely an inclined expanding ring, not a spherical bubble.

A massive star candidate (~15 M_sun) may be driving the expansion.

Signs of infall motion suggest ongoing star formation.

Abstract

We presented a study on the Galactic bubble N4 using the 13.7 m millimeter telescope of Purple Mountain Observatory at the Qinghai Station. N4 is one of the science demonstration regions for the Milky Way Imaging Scroll Painting (WMISP). Simultaneous observations of $^{12}$CO (J = 1$-$0), $^{13}$CO (J = 1$-$0) and C$^{18}$O (J = 1$-$0) line emission towards N4 were carried out. We analyzed the spectral profile and the distribution of the molecular gas. Morphologically, the CO emissions correlate well with Spitzer IRAC 8.0 $\mu$m emission. The channel map and velocity-position diagram shows that N4 is more likely an inclined expanding ring than a spherical bubble. We calculated the physical parameters of N4 including the mass, size, column density and optical depth. Some massive star candidates were discovered in the region of N4 using (J, J$-$H) color-magnitude diagram. We found an energy source candidate for the expansion of N4, a massive star with a mass of ${\sim} 15\,M_{\odot}$ and an age of $\sim$ 1 Myr. There exists infall motion signature in N4, which can be a good candidate of infall area. Combined mm and infrared data, we think there may exists triggered star formation in N4.