[Paper Review] Triggered massive-star formation on the borders of Galactic HII regions. II. Evidence for the collect and collapse process around RCW 79

A. Zavagno, Lise Deharveng+5 more|ArXiv.org|Sep 12, 2005

Astrophysics and Star Formation StudiesPhysics and Astronomy41 references91 citations

TL;DR

This study presents multi-wavelength evidence that massive-star formation at the border of the Galactic H ii region RCW 79 was triggered by the collect and collapse process, where expanding ionization fronts compress and fragment a surrounding dust layer into massive cores. Observations reveal diametrically opposed, high-mass millimeter condensations and associated young, luminous Class I sources, confirming gravitational collapse of the compressed layer as the dominant mechanism for triggering massive star formation.

ABSTRACT

We present SEST-SIMBA 1.2-mm continuum maps and ESO-NTT SOFI JHK images of the Galactic HII region RCW 79. The millimetre continuum data reveal the presence of massive fragments located in a dust emission ring surrounding the ionized gas. The two most massive fragments are diametrically opposite each other in the ring. The near-IR data, centred on the compact HII region located at the south-eastern border of RCW 79, show the presence of an IR-bright cluster containing massive stars along with young stellar objects with near-IR excesses. A bright near- and mid-IR source is detected towards maser emissions, 1.2 pc north-east of the compact HII region centre. Additional information, extracted from the Spitzer GLIMPSE survey, are used to discuss the nature of the bright IR sources observed towards RCW 79. Twelve luminous Class I sources are identified towards the most massive millimetre fragments. All these facts strongly indicate that the massive-star formation observed at the border of the HII region RCW 79 has been triggered by its expansion, most probably by the collect and collapse process.

Motivation & Objective

• To investigate the physical mechanism triggering massive star formation at the periphery of Galactic H ii regions, focusing on RCW 79 as a key case study.
• To test the collect and collapse hypothesis by identifying massive, gravitationally unstable fragments in the compressed shell formed by the expanding H ii region.
• To determine the nature and age of young stellar objects (YSOs) associated with these fragments using near- and mid-infrared data.
• To assess whether the observed morphology and kinematics support the theoretical model of Whitworth et al. (1994) for shell fragmentation and core collapse.

Proposed method

• Acquired 1.2-mm continuum maps using the SEST-SIMBA instrument to trace cold dust emission and identify massive, dense fragments in the periphery of RCW 79.
• Obtained near-IR JHKs imaging with the ESO-NTT SOFI instrument to detect embedded massive stars and YSOs with infrared excesses.
• Utilized Spitzer GLIMPSE survey data at 8 μm to identify polycyclic aromatic hydrocarbon (PAH)-emitting photodissociation regions (PDRs) and locate luminous IR sources.
• Analyzed Hα emission from the SuperCOSMOS survey to assess the kinematic state of the ionized gas and detect signs of a champagne flow.
• Correlated millimeter condensations with infrared sources to identify sites of ongoing or recent massive star formation.
• Applied age estimates based on the luminosity and evolutionary stage of Class I YSOs and the compact H ii region to constrain the timing of fragmentation.

Experimental results

Research questions

• RQ1 Are massive, dense fragments present in the dust ring surrounding RCW 79, as predicted by the collect and collapse model?
• RQ2 Do the locations of these fragments coincide with young, massive stellar objects or luminous IR sources indicative of recent star formation?
• RQ3 Is the fragmentation of the compressed shell consistent with gravitational instability and collapse, as modeled by Whitworth et al. (1994)?
• RQ4 What is the age of the triggered massive star formation episode, and how does it relate to the age of the H ii region and the compact H ii region at its border?
• RQ5 Are additional triggering mechanisms, such as radiation-driven implosion or bright-rimmed cloud collapse, also contributing to star formation in the region?

Key findings

• Five massive millimeter condensations were detected along the dust ring surrounding RCW 79, with the two most massive fragments located diametrically opposite each other.
• Twelve luminous Class I YSOs were identified toward the most massive millimeter fragments, indicating ongoing or recent massive star formation.
• A bright IR source associated with maser emission was found 1.2 pc northeast of the compact H ii region, marking a site of active, high-mass protostar formation.
• The age of the compact H ii region and its exciting cluster, as well as the Class I YSOs, is estimated at ~1.7 Myr, consistent with fragmentation of the collected shell occurring ~10^5 years ago.
• The presence of a champagne flow, inferred from Hα kinematics, suggests that the ionized gas is escaping through low-density zones, likely formed after fragmentation of the shell.
• The alignment of fragments and YSOs in a preferred plane contradicts the spherical symmetry of the standard collect and collapse model, indicating the need for non-spherical, inhomogeneous models.