Emission Mechanism of "Green Fuzzies" in High-mass Star Forming Regions

TL;DR

This study analyzes the emission mechanisms behind 'Green Fuzzy' infrared features in high-mass star-forming regions, suggesting scattered continuum as the primary cause and exploring other possible contributions through color analysis.

Contribution

It provides the first detailed color analysis of 'Green Fuzzy' emission in high-mass protostars, proposing scattered continuum as the main mechanism and highlighting the need for spectroscopic follow-up.

Findings

Scattered continuum explains the linear color correlation.

No clear PAH emission detected near protostars.

High extinction levels indicate heavy embedding of protostars.

Abstract

The Infrared Array Camera (IRAC) on the Spitzer Space Telescope has revealed that a number of high-mass protostars are associated with extended mid-infrared emission, particularly prominent at 4.5-micron. These are called "Green Fuzzy" emission or "Extended Green Objects". We present color analysis of this emission toward six nearby (d=2-3 kpc) well-studied high-mass protostars and three candidate high-mass protostars identified with the Spitzer GLIMPSE survey. In our color-color diagrams most of the sources show a positive correlation between the [3.6]-[4.5] and [3.5]-[5.8] colors along the extinction vector in all or part of the region. We compare the colors with those of scattered continuum associated with the low-mass protostar L 1527, modeled scattered continuum in cavities, shocked emission associated with low-mass protostars, modeled H2 emission for thermal and fluorescent cases, and modeled PAH emission. Of the emission mechanisms discussed above, scattered continuum provides the simplest explanation for the observed linear correlation. In this case, the color variation within each object is attributed to different foreground extinctions at different positions. Alternative possible emission mechanisms to explain this correlation may be a combination of thermal and fluorescent H2 emission in shocks, and a combination of scattered continuum and thermal H2 emission, but detailed models or spectroscopic follow-up are required to further investigate this possibility. Our color-color diagrams also show possible contributions from PAHs in two objects. However, none of our sample show clear evidence for PAH emission directly associated with the high-mass protostars, several of which should be associated with ionizing radiation. This suggests that those protostars are heavily embedded even at mid-infrared wavelengths.