[Paper Review] A gallery of bubbles - The nature of the bubbles observed by Spitzer and what ATLASGAL tells us about the surrounding neutral material

A. Zavagno, F. Schüller+8 more|arXiv (Cornell University)|Aug 5, 2010

Astrophysics and Star Formation StudiesPhysics and Astronomy99 references7 citations

TL;DR

This study analyzes 102 Spitzer-identified infrared bubbles using multi-wavelength data, finding that 86% host ionized H ii regions powered by O-B2 stars, with 40% showing dense cold dust shells at their borders—strong evidence for triggered massive star formation via the collect-and-collapse mechanism. The ATLASGAL survey reveals that 28% of bubbles have interacting condensations, and 13% host ultracompact H ii regions in adjacent dust clumps, indicating widespread feedback-driven star formation in the Galactic plane.

ABSTRACT

We attempt to determine the nature of the bubbles observed by Spitzer in the Galactic plane, mainly to establish if possible their association with massive stars. We take advantage of the very simple morphology of these objects to search for star formation triggered by HII regions, and to estimate the importance of this mode of star formation. We consider a sample of 102 bubbles detected by Spitzer-GLIMPSE, and catalogued by Churchwell et al.(2006). We use mid-infrared and radio-continuum public data to discuss their nature. We use the ATLASGAL survey at 870 micron to search for dense neutral material collected on their borders. Results: We find that 86% of the bubbles contain ionized gas detected by means of its radio-continuum emission at 20-cm. Thus, most of the bubbles observed at 8.0 micron enclose HII regions ionized by O-B2 stars. Ninety-eight percent of the bubbles exhibit 24 micron emission in their central regions. The ionized regions at the center of the 8.0 micron bubbles seem to be devoid of PAHs but contain hot dust. Among the 65 regions for which the angular resolution of the observations is high enough to resolve the spatial distribution of cold dust at 870 micron, we find that 40% are surrounded by cold dust, and that another 28% contain interacting condensations. The former are good candidates for the collect and collapse process, as they display an accumulation of dense material at their borders. The latter are good candidates for the compression of pre-existing condensations by the ionized gas. Eighteen bubbles exhibit associated ultracompact HII regions and/or methanol masers in the direction of dust condensations adjacent to their ionization fronts. Our results suggest that more than a quarter of the bubbles may have triggered the formation of massive objects.

Motivation & Objective

• To determine the nature of Spitzer-identified infrared bubbles in the Galactic plane, particularly their association with massive stars.
• To assess the role of H ii region feedback in triggering massive star formation by analyzing dense neutral material at bubble borders.
• To resolve discrepancies in earlier studies (e.g., Churchwell et al. 2006) that underestimated H ii region associations due to limited angular resolution.
• To use high-resolution 870 µm dust emission from ATLASGAL to identify sites of potential triggered star formation.

Proposed method

• Used a sample of 102 bubbles from the CH06 catalog detected in Spitzer-GLIMPSE 8.0 µm images.
• Cross-identified bubbles with radio-continuum emission at 20 cm from the MAGPIS survey to detect ionized H ii regions.
• Analyzed 24 µm emission from Spitzer-MIPSGAL to study hot dust and PAHs around ionized regions.
• Utilized high-resolution 870 µm dust emission from the ATLASGAL survey to map cold, dense neutral material at bubble peripherie
• Identified dust condensations and ultracompact H ii regions at bubble edges to assess star formation triggering.
• Correlated methanol masers and compact H ii regions with dust clumps to trace early-stage massive star formation.

Experimental results

Research questions

• RQ1 What fraction of Spitzer-identified infrared bubbles actually enclose ionized H ii regions, and what type of stars power them?
• RQ2 To what extent do the dense neutral material distributions around bubbles support the collect-and-collapse model of triggered star formation?
• RQ3 How common are ultracompact H ii regions and methanol masers in dust condensations adjacent to bubble ionization fronts?
• RQ4 What is the spatial relationship between PAH emission, hot dust, and ionized gas in the central regions of the bubbles?
• RQ5 How does the morphology of the bubbles reflect the interaction with surrounding neutral material?

Key findings

• 86% of the 102 bubbles exhibit radio-continuum emission at 20 cm, indicating they enclose ionized H ii regions, primarily powered by O-B2 stars.
• 98% of the bubbles show 24 µm emission in their central regions, indicating the presence of hot dust, while PAHs are absent in the ionized cores but detected in surrounding photodissociation regions.
• Among 65 bubbles with sufficient angular resolution, 40% display cold dust shells at their borders, consistent with the collect-and-collapse mechanism for triggered star formation.
• An additional 28% show interacting dust condensations at their edges, suggesting compression of pre-existing dense cores by expanding H ii regions.
• Thirteen bubbles host ultracompact H ii regions in adjacent dust condensations, and five show methanol masers in similar locations, indicating active massive star formation triggered by H ii region feedback.
• The study concludes that more than 25% of the bubbles may have triggered massive star formation, highlighting the importance of H ii region feedback in the star formation process.