[Paper Review] Spitzer/IRAC view of Sh 2-284: Searching for evidence of triggered star formation in an isolated region in the outer Milky Way

E. Puga, S. Hony+6 more|UvA-DARE (University of Amsterdam)|Jul 7, 2009

Astrophysics and Star Formation Studies43 references18 citations

TL;DR

This study uses Spitzer/IRAC and Hα imaging to investigate Sh 2-284, an isolated H ii region in the outer Milky Way, revealing a complex, symmetric multiple-bubble morphology with young stellar objects (YSOs) concentrated at bubble rims. The findings indicate triggered star formation via radiation-driven implosion, with younger Class I YSOs located at cometary globule vortices and shells, while central clusters show a narrow age spread, suggesting the bubbles were not triggered by the central clusters but by localized dense clumps.

ABSTRACT

Using Spitzer/IRAC observations of a region to be observed by the CoRoT satellite, we have unraveled a new complex star-forming region at low metallicity in the outer Galaxy. We perform a study of S284 in order to outline the chain of events in this star-forming region. We used four-band Spitzer/IRAC photometry as well as Halpha imaging obtained with INT/WFC. Combining these data with the optical photometry obtained in the frame of CoRoTs preparation and the 2MASS catalog we analysed the properties and distribution of young stellar objects (YSOs) associated with point-like sources. We also studied the SEDs of regions of extended emission, complementing our dataset with IRAS and MSX data. We find that S284 is unique in several ways: it is very isolated at the end of a spiral arm and both the diffuse dust and ionized emission are remarkably symmetric. We have partially resolved the central clusters of the three bubbles present in this region. Despite the different scales present in its multiple-bubble morphology, our study points to a very narrow spread of ages among the powering high-mass clusters. In contrast, the particular sawtooth structure of the extended emission at the rim of each ionized bubble harbours either small lower-mass clusters with a younger stellar population or individual young reddened protostars. In particular, triggered star formation is considered to be at work in these regions.

Motivation & Objective

• To investigate the nature and triggering mechanisms of high-mass star formation in Sh 2-284, an isolated H ii region in the outer Galaxy.
• To determine whether the multiple-bubble morphology results from triggered star formation due to expanding H ii regions.
• To characterize the distribution, ages, and evolutionary stages of young stellar objects (YSOs) in the region using multi-wavelength photometry.
• To assess the role of environmental conditions such as density variations and ionization fronts in shaping the observed morphology.

Proposed method

• Acquired four-band Spitzer/IRAC photometry (3.6, 5.8, 8.0, and 24 μm) to detect and classify YSOs based on infrared colors.
• Combined IRAC data with Hα imaging from the Isaac Newton Telescope's WFC to map ionized gas structures and identify ionization fronts.
• Integrated optical photometry from CoRoT preparation and 2MASS catalog to improve source classification and distance estimates.
• Used color-color diagrams to distinguish Class I (embedded) and Class II (classical) YSOs based on infrared excess.
• Analyzed spectral energy distributions (SEDs) of extended emission regions using IRAS and MSX data to infer dust and protostellar content.
• Compared dynamical ages from H ii region expansion with isochrone-fitting ages from optical photometry to assess age gradients.

Experimental results

Research questions

• RQ1 Is the multiple-bubble morphology in Sh 2-284 consistent with triggered star formation via the collect-and-collapse mechanism?
• RQ2 What is the age distribution of YSOs in relation to the ionization fronts and bubble shells?
• RQ3 Are the younger Class I YSOs spatially correlated with cometary globules or dense clumps at the rims of ionized bubbles?
• RQ4 Do the central high-mass clusters in Sh 2-284 trigger the formation of surrounding lower-mass clusters?
• RQ5 What role do local density variations and ionization front instabilities play in shaping the observed structures?

Key findings

• The region exhibits a remarkably symmetric multiple-bubble morphology with three distinct H ii regions, each centered on a high-mass cluster, and the bubbles show minimal asymmetry.
• A total of 155 Class I and 183 Class II YSO candidates were identified, with the majority concentrated at the shells of the ionized bubbles.
• Younger Class I YSOs are preferentially located at the vortices of cometary globules and dense clumps along the bubble rims, indicating localized triggering.
• The central clusters (Dolidze 25, Cl2, Cl3) show a narrow age spread consistent with coeval formation, suggesting they did not trigger the surrounding clusters.
• SED analysis of extended emission regions reveals six additional embedded clusters located within the dust shells, supporting the presence of triggered star formation.
• The dynamical ages inferred from H ii region expansion align with isochrone-fitting ages, reinforcing the conclusion that the central clusters are not the trigger for the outer clusters.