The Milky Way Project: What are Yellowballs?

TL;DR

Yellowballs are compact infrared sources identified by citizen scientists, representing various stages of star formation, including ultra-compact HII regions and less massive B-type star regions, providing insights into stellar evolution.

Contribution

This study characterizes yellowballs as a diverse set of star-forming regions using infrared analysis, expanding the understanding of Galactic star formation processes.

Findings

Yellowballs are a mix of compact star-forming regions.

They share infrared signatures across different luminosities.

The catalog complements existing surveys like RMS.

Abstract

Yellowballs are a collection of approximately 900 compact, infrared sources identified and named by volunteers participating in the Milky Way Project (MWP), a citizen-science project that uses GLIMPSE/MIPSGAL images from Spitzer to explore topics related to Galactic star formation. In this paper, through a combination of catalog cross-matching and infrared color analysis, we show that yellowballs are a mix of compact star-forming regions, including ultra-compact and compact HII regions, as well as analogous regions for less massive B-type stars. The resulting MWP yellowball catalog provides a useful complement to the Red MSX Source (RMS) survey. It similarly highlights regions of massive star formation, but the selection of objects purely on the basis of their infrared morphology and color in Spitzer images identifies a signature of compact star-forming regions shared across a broad range of luminosities, and by inference, masses. We discuss the origin of their striking mid-infrared appearance, and suggest that future studies of the yellowball sample will improve our understanding of how massive and intermediate-mass star-forming regions transition from compact to more extended bubble-like structures.