Triggered Star Formation in a Double Shell near W51A

TL;DR

This study uses CO observations to confirm that star formation near W51A is triggered by the expansion of an HII region, with evidence of molecular material collection, young stellar objects, and shell expansion dynamics.

Contribution

It provides observational evidence linking HII region expansion to triggered star formation through molecular shell analysis near W51A.

Findings

Dense molecular material collected along the shell

Two young stellar objects identified in dense condensations

Shell expansion age estimated at approximately 1 million years

Abstract

We present Heinrich Hertz Telescope CO observations of the shell structure near the active star-forming complex W51A to investigate the process of star formation triggered by the expansion of an HII region. The CO observations confirm that dense molecular material has been collected along the shell detected in Spitzer IRAC images. The CO distribution shows that the shell is blown out toward a lower density region to the northwest. Total hydrogen column density around the shell is high enough to form new stars. We find two CO condensations with the same central velocity of 59 km/s to the east and north along the edge of the IRAC shell. We identify two YSOs in early evolutionary stages (Stage 0/I) within the densest molecular condensation. From the CO kinematics, we find that the HII region is currently expanding with a velocity of 3.4 km/s, implying that the shell's expansion age is ~1 Myr. This timescale is in good agreement with numerical simulations of the expansion of the HII region (Hosokawa et al. 2006). We conclude that the star formation on the border of the shell is triggered by the expansion of the HII region.