[Paper Review] Probing the formation of intermediate- to high-mass stars in protoclusters: A detailed millimeter study of the NGC 2264 clumps

N. Peretto, Ph. André+1 more|ORCA Online Research @Cardiff (Cardiff University)|Aug 29, 2005

Astrophysics and Star Formation StudiesPhysics and Astronomy55 references100 citations

TL;DR

This study investigates massive star formation in the NGC 2264 protocluster using millimeter continuum and molecular line observations, revealing a large-scale, near-free-fall collapse of a prolate, unstable clump in NGC 2264-C. Radiative transfer modeling shows a high mass inflow rate (~3×10⁻³ M⊙ yr⁻¹) toward the central protostar C-MM3, supporting the formation of a high-mass star (~10–20 M⊙) via gravitational merger of multiple Class 0 cores in a turbulent, gas-dominated environment, intermediate between merger and turbulent core models.

ABSTRACT

We present the results of dust continuum and molecular line observations of two massive cluster-forming clumps, NGC 2264-C and NGC 2264-D, including extensive mapping performed with the MAMBO bolometer array and the HERA heterodyne array on the IRAM 30m telescope. Both NGC 2264 clumps are located in the Mon OB1 giant molecular cloud complex, adjacent to one another. Twelve and fifteen compact millimeter continuum sources (i.e. MMSs) are identified in clumps C and D, respectively. Evidence for widespread infall motions is found in, e.g., HCO+(3-2) or CS(3-2) in both NGC 2264-C and NGC 2264-D. A sharp velocity discontinuity ~ 2 km/s in amplitude is observed in N_2H+(1-0) and H^{13}CO+(1-0) in the central, innermost part of NGC 2264-C, which we interpret as the signature of a strong dynamical interaction between two MMSs and their possible merging with the central MMS C-MM3. Radiative transfer modelling supports the idea that NGC 2264-C is a highly unstable prolate clump in the process of collapsing along its long axis on a near free-fall dynamical timescale ~ 1.7x10^5 yr. Our model fit of this large-scale collapse suggests a maximum mass inflow rate ~ 3x10^{-3} Msun/yr toward the central protostellar object C-MM3. Such infall rates are sufficiently high to overcome radiation pressure and allow the formation of ~ 20 Msun stars by accretion in ~ 1.7x10^5 yr, i.e., a time similar to the global dynamical timescale of the central part of NGC 2264-C. We conclude that we are likely witnessing the formation of a high-mass (> 10 Msun) protostar in the central part of NGC 2264-C. Our results suggest a picture of massive star formation intermediate between the scenario of stellar mergers of Bonnell et al. (1998) and the massive turbulent core model of McKee & Tan (2003).

Motivation & Objective

• To understand the formation mechanisms of intermediate- to high-mass stars in clustered environments, particularly in protoclusters.
• To test competing theoretical models of massive star formation—specifically, the turbulent core model versus the stellar merger scenario—using detailed kinematic and dynamical observations.
• To determine whether high accretion rates can overcome radiation pressure to form stars above 8 M⊙ in dense, collapsing clumps.
• To investigate the role of large-scale infall, turbulence, and dynamical interactions in shaping the initial conditions for massive star birth.
• To characterize the physical properties of millimeter continuum sources (MMSs) and their association with protostars in NGC 2264-C and NGC 2264-D.

Proposed method

• Conducted deep millimeter dust continuum and molecular line observations using the MAMBO bolometer array and HERA heterodyne receiver on the IRAM 30m telescope.
• Mapped two massive cluster-forming clumps, NGC 2264-C and NGC 2264-D, with high angular resolution to identify compact millimeter sources (MMSs).
• Analyzed line profiles of HCO⁺(3–2), CS(3–2), N₂H⁺(1–0), and H¹³CO⁺(1–0) to detect infall motions and velocity discontinuities.
• Identified a sharp velocity discontinuity (~2 km s⁻¹) in N₂H⁺(1–0) and H¹³CO⁺(1–0) in NGC 2264-C, interpreted as a dynamical interaction between MMSs.
• Applied radiative transfer modeling to HCO⁺ and CS line profiles to derive infall velocities and mass inflow rates toward central protostars C-MM3 and D-MM1.
• Compared observed kinematics and mass inflow rates with theoretical models of massive star formation, including McKee & Tan’s turbulent core and Bonnell et al.’s merger scenario.

Experimental results

Research questions

• RQ1 Can high accretion rates sufficient to overcome radiation pressure be sustained in massive protocluster clumps?
• RQ2 What is the role of large-scale collapse and dynamical interactions in the formation of high-mass stars?
• RQ3 How do the kinematic properties of MMSs in NGC 2264 compare to those in other well-known protostellar regions like ρ Ophiuchi?
• RQ4 Is the observed velocity discontinuity in NGC 2264-C indicative of a gravitational merger of protostellar cores?
• RQ5 To what extent does the observed infall and turbulence support a hybrid formation scenario between turbulent core and stellar merger models?

Key findings

• Twelve compact millimeter sources (MMSs) were identified in NGC 2264-C, with masses ranging from ~2 to 41 M⊙, and a median mass of ~10 M⊙, indicating a more massive clump than NGC 2264-D.
• Fifteen MMSs were detected in NGC 2264-D, with a median mass of ~6 M⊙, and a total gas mass of ~1310 M⊙, consistent with a less massive but still active cluster-forming environment.
• The MMSs in NGC 2264 exhibit supersonic line-of-sight velocity dispersions (~0.7 km s⁻¹), which are twice those in ρ Ophiuchi and five times those in Taurus, indicating significant turbulence.
• Seventy percent of MMSs in NGC 2264-C are associated with shocked H₂ jets, indicating they host candidate Class 0 protostars, suggesting a recent, efficient burst of star formation.
• A sharp velocity discontinuity (~2 km s⁻¹) in N₂H⁺(1–0) and H¹³CO⁺(1–0) in NGC 2264-C’s central region is interpreted as a dynamical interaction between MMSs merging into the central source C-MM3.
• Radiative transfer modeling of NGC 2264-C indicates a large-scale, near-free-fall collapse along the long axis of the prolate clump with a dynamical timescale of ~1.7×10⁵ yr and a maximum mass inflow rate of ~3×10⁻³ M⊙ yr⁻¹ toward C-MM3.