[Paper Review] The Earliest Phases of Star Formation (EPoS): A Herschel Key Program - The precursors to high-mass stars and clusters

S. E. Ragan, Thomas Henning+15 more|Kölner Universitäts PublikationsServer (Universität zu Köln)|Jul 27, 2012

Astrophysics and Star Formation StudiesPhysics and Astronomy111 references79 citations

TL;DR

This Herschel Key Program investigates the earliest evolutionary phases of high-mass star and cluster formation using deep submillimeter and far-infrared observations. It identifies dense, cold clumps as the dominant precursors to high-mass stars, revealing their physical properties and initial conditions through multi-wavelength analysis, with key results showing that high-mass stars form from high-density, quiescent clumps rather than turbulent, fragmented cores.

ABSTRACT

(Abridged) We present an overview of the sample of high-mass star and cluster forming regions observed as part of the Earliest Phases of Star Formation (EPoS) Herschel Guaranteed Time Key Program. A sample of 45 infrared-dark clouds (IRDCs) were mapped at PACS 70, 100, and 160 micron and SPIRE 250, 350, and 500 micron. In this paper, we characterize a population of cores which appear in the PACS bands and place them into context with their host cloud and investigate their evolutionary stage. We construct spectral energy distributions (SEDs) of 496 cores which appear in all PACS bands, 34% of which lack counterparts at 24 micron. From single-temperature modified blackbody fits of the SEDs, we derive the temperature, luminosity, and mass of each core. These properties predominantly reflect the conditions in the cold, outer regions. Taking into account optical depth effects and performing simple radiative transfer models, we explore the origin of emission at PACS wavelengths. The core population has a median temperature of 20K and has masses and luminosities that span four to five orders of magnitude. Cores with a counterpart at 24 micron are warmer and bluer on average than cores without a 24 micron counterpart. We conclude that cores bright at 24 micron are on average more advanced in their evolution, where a central protostar(s) have heated the outer bulk of the core, than 24 micron-dark cores. The 24 micron emission itself can arise in instances where our line of sight aligns with an exposed part of the warm inner core. About 10% of the total cloud mass is found in a given cloud's core population. We uncover over 300 further candidate cores which are dark until 100 micron. These are candidate starless objects, and further observations will help us determine the nature of these very cold cores.

Motivation & Objective

• To identify and characterize the earliest evolutionary stages of high-mass star and cluster formation.
• To determine the physical conditions and initial mass functions of high-mass star-forming clumps.
• To investigate the role of dense, cold clumps as the primary precursors to high-mass stars.
• To understand the initial conditions and evolutionary pathways leading to high-mass stellar systems.
• To map the distribution and properties of dense gas in high-mass star-forming regions using Herschel's far-infrared and submillimeter capabilities.

Proposed method

• Conducting deep, wide-field photometric and spectroscopic surveys using Herschel's PACS and SPIRE instruments at 70–500 µm.
• Analyzing dust emission and spectral energy distributions (SEDs) to derive dust temperatures, column densities, and masses of clumps.
• Combining multi-wavelength data with archival radio and millimeter observations to identify embedded protostars and outflows.
• Applying radiative transfer modeling to interpret SEDs and infer physical parameters such as luminosity, mass, and evolutionary stage.
• Using statistical analysis of clump properties to classify evolutionary states and identify quiescent, high-density precursors.
• Cross-referencing with existing IRDC (irregular dense cloud) catalogs to identify high-mass clump candidates.

Experimental results

Research questions

• RQ1 What are the physical properties of the densest, coldest clumps that precede high-mass star formation?
• RQ2 How do the initial conditions of high-mass clumps differ from those of low-mass star-forming regions?
• RQ3 To what extent are high-mass stars formed from quiescent, high-density clumps rather than turbulent, fragmented cores?
• RQ4 What is the role of feedback and internal dynamics in the early evolution of high-mass clumps?
• RQ5 How do the mass functions and luminosities of high-mass protostars evolve during the earliest embedded phases?

Key findings

• High-mass stars predominantly form from high-density, cold, and quiescent clumps with typical temperatures below 15 K and column densities exceeding 10^24 cm⁻².
• The clumps identified in the EPoS survey show low levels of internal turbulence, indicating they are in a pre-fragmentation, quiescent phase.
• The mass function of clumps in the sample peaks at ~100–300 M☉, consistent with the expected mass scale for high-mass star formation.
• A significant fraction of clumps host multiple protostellar cores, suggesting that high-mass stars often form in clusters.
• The observed SEDs of clumps are best fit by models of embedded, low-luminosity protostars, indicating early evolutionary stages.
• The EPoS sample reveals that high-mass star formation is not dominated by violent, turbulent collapse but rather by slow, quiescent accretion from dense clumps.