Properties of the UCHII region G25.4NW and its associated molecular cloud

TL;DR

This study investigates the properties of the ultra-compact HII region G25.4NW and its surrounding molecular cloud, revealing its distance, luminosity, and its role within a larger star-formation complex in the inner Galaxy.

Contribution

It provides new HI and CO observations that determine the distance, physical properties, and star-formation environment of G25.4NW, enhancing understanding of its nature and context.

Findings

G25.4NW is at a near-side distance of 5.7 kpc.

Its bolometric luminosity is approximately 10^5.6 L_sun.

It is part of a larger molecular star-formation complex with about 10^4 M_sun of gas.

Abstract

UCHII G25.4NW is a bright IR source in the inner Galaxy region. New HI images from the VLA Galactic Plane Survey (VGPS) show clear absorption features associated with the UCHII region up to 95 km s$^{-1}$, and there is not any other absorptions up to the tangential velocity. It reveals that G25.4NW is at a near-side distance of 5.7 kpc, and it is located in the inner Galactic molecular ring region. Using the new distance, the bolometric luminosity of G25.4NW is estimated as $10^{5.6} L_{\odot}$, which corresponds to an O6 star. It contains 460 $M_{\odot}$ of ionized gas. High-resolution $^{13}$CO image from the Galactic Ring Survey (GRS) reveals that G25.4NW is part of a more extended star-formation complex with about $10^{4} M_{\odot}$ molecular gas.