[Paper Review] Feedback Processes [in Massive Star Formation]: A Theoretical Perspective

Mordecai‐Mark Mac Low|ArXiv.org|Nov 26, 2007

Astronomy and Astrophysical Research1 references3 citations

TL;DR

This theoretical review examines feedback mechanisms—radiation, winds, jets, and accretion luminosity—in massive star formation, demonstrating that while feedback suppresses fragmentation and limits accretion, it does not fully prevent massive star formation due to radiative focusing, instability-driven bubble collapse, and magnetic effects. The key contribution is explaining how massive stars (up to ~100 M⊙) can form despite strong feedback, though the ultimate mass limit remains unconstrained.

ABSTRACT

I review the evidence for the importance of feedback from massive stars at small and large scales. The feedback mechanisms include accretion luminosity, ionizing radiation, collimated outflows, and stellar winds. The good news is that feedback doesn't entirely prevent the formation of massive stars, while the bad news is that we don't know what does limit their masses. Feedback from massive stars also influences their surroundings. I argue that this does not produce a triggering efficiency above unity, nor does it prevent lots of prompt star formation in GMCs, though it may preserve massive remnants of the clouds for many dynamical times.

Motivation & Objective

• To evaluate the role of feedback mechanisms in regulating massive star formation and their impact on the initial mass function (IMF).
• To investigate why massive stars can form despite strong radiative and mechanical feedback that should halt accretion.
• To assess whether feedback triggers or suppresses star formation in giant molecular clouds (GMCs), and how long-lived GMCs can be.
• To determine the physical mechanisms that allow accretion to proceed in high-mass protostars despite radiation pressure.
• To reconcile conflicting theoretical models of GMC stability and star formation efficiency with observational constraints.

Proposed method

• Numerical simulations using wavelength-dependent dust opacity and nested-grid, 2D models of collapsing molecular cores (Yorke & Sonnhalter 2002).
• Radiative transfer computations to model the 'flashlight effect' in polar cavities, enhancing radiation escape (Krumholz et al. 2005a).
• Analysis of Rayleigh-Taylor and photon bubble instabilities in radiation-pressure-driven bubbles to assess their role in enabling accretion (Krumholz et al. 2005).
• Adaptive mesh refinement (AMR) simulations with temperature-dependent opacity to study fragmentation suppression during collapse (Krumholz et al. 2007).
• Application of turbulent star formation laws (Krumholz & McKee 2005) to spherical, power-law density distributions in GMCs to model cloud stability (Krumholz et al. 2006).
• Comparison of observational timescales from Spitzer and NANTEN surveys (Blitz et al. 2006; Tamburro et al. 2007) with theoretical models of H ii region expansion and cloud dispersal.

Experimental results

Research questions

• RQ1 How do radiative, wind, and jet feedback mechanisms collectively influence the accretion process in massive star formation?
• RQ2 Why does feedback not fully prevent the formation of stars above 30 M⊙, despite radiation pressure on dust?
• RQ3 What physical mechanisms allow accretion to proceed in high-mass protostars despite strong radiation pressure?
• RQ4 To what extent does feedback trigger or suppress star formation in giant molecular clouds (GMCs)?
• RQ5 How long can molecular clouds remain stable against gravitational collapse, and what role does feedback play in this stability?

Key findings

• Radiation pressure does not fully halt accretion in massive stars due to the 'flashlight effect,' where radiation escapes through polar cavities formed by outflows.
• Rayleigh-Taylor instabilities in radiation-driven bubbles promote collapse and allow continued accretion, reducing the effectiveness of radiative feedback.
• Photon bubble instabilities in magnetized cores enhance radiation escape by amplifying compressive MHD waves into low-density channels.
• Radiative heating from accretion suppresses fragmentation in collapsing cores, with simulations showing strong suppression under temperature-dependent opacity.
• Simulations including radiative transfer and AMR reach protostellar masses of 10 M⊙, but no upper mass limit has been confirmed, suggesting feedback may not be the sole mass cap.
• Observational data from Spitzer and NANTEN suggest GMCs can remain stable for up to 27 Myr, with star formation beginning within 2–4 Myr of H i emission peaks, consistent with H ii region expansion supporting cloud stability.