We describe the first three-dimensional simulation of the gravitational collapse of a massive, rotating molecular cloud that includes heating by both non-ionizing and ionizing radiation. These models were performed with the FLASH code, incorporating a hybrid, long characteristic, ray tracing technique. We find that as the first protostars gain sufficient mass to ionize the accretion flow, their H ii regions are initially gravitationally trapped, but soon begin to rapidly fluctuate between trapped and extended states, in agreement with observations. Over time, the same ultracompact H ii region can expand anisotropically, contract again, and take on any of the observed morphological classes. In their extended phases, expanding H ii regions drive bipolar neutral outflows characteristic of high-mass star formation. The total lifetime of H ii regions is given by the global accretion timescale, rather than their short internal sound-crossing time. This explains the observed number statistics. The pressure of the hot, ionized gas does not terminate accretion. Instead the final stellar mass is set by fragmentationinduced starvation. Local gravitational instabilities in the accretion flow lead to the build-up of a small cluster of stars, all with relatively high masses due to heating from accretion radiation. These companions subsequently compete with the initial high-mass star for the same common gas reservoir and limit its mass growth. This is contrary to the classical competitive accretion model, where the massive stars are never hindered in growth by the low-mass stars in the cluster.Our findings show that the most significant differences between the formation of low-mass and high-mass stars are all explained as the result of rapid accretion within a dense, gravitationally unstable, ionized flow.

In previous studies, we identified two classes of starless cores, thermally subcritical and supercritical, distinguished by different dynamical behaviour and internal structure. Here, we study the evolution of the dynamically unstable, thermally supercritical cores by means of a numerical hydrodynamic simulation that includes radiative equilibrium and simple molecular chemistry. From an initial state as an unstable Bonnor–Ebert (BE) sphere, a contracting core evolves towards the configuration of a singular isothermal sphere by inside–out collapse. We follow the gas temperature and abundance of CO during the contraction. The temperature is predominantly determined by radiative equilibrium, but in the rapidly contracting centre of the core compressive heating raises the gas temperature by a few degrees over its value in static equilibrium. The time‐scale for the equilibration of CO depends on the gas density and is everywhere shorter than the dynamical time‐scale. The result is that the dynamics do not much affect the abundance of CO which is always close to that of a static sphere of the same density profile, and CO cannot be used as a chemical clock in starless cores. We use our non‐local thermodynamic equilibrium (non‐LTE) radiative transfer code mollie to predict observable CO and N2H+ line spectra, including the non‐LTE hyperfine ratios of N2H+, during the contraction. These are compared against observations of the starless core L1544. The comparison indicates that the dust in L1544 has an opacity consistent with ice‐covered rather than bare grains, the cosmic ray ionization rate is about 1 × 10−17 s−1 and the density structure of L1544 is approximately that of a BE sphere with a maximum central density of 2 × 107 cm−3, equivalent to an average density of 3 × 106 cm−3 within a radius of 500 au. The observed CO linewidths and intensities are reproduced if the CO desorption rate is about 30 times higher than the rate expected from cosmic ray strikes alone, indicating that other desorption processes are also active.

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Water is a crucial molecule in molecular astrophysics as it controls much of the gas/grain chemistry, including the formation and evolution of more complex organic molecules in ices. Pre-stellar cores provide the original reservoir of material from which future planetary systems are built, but few observational constraints exist on the formation of water and its partitioning between gas and ice in the densest cores. Thanks to the high sensitivity of the Herschel Space Observatory,

New observations of CO (J = 1 → 0) line emission from M33, using the 25 element BEARS focal plane array at the Nobeyama Radio Observatory 45-m telescope, in conjunction with existing maps from the BIMA interferometer and the FCRAO 14-m telescope, give the highest resolution (13 ′′ ) and most sensitive (σ rms ∼ 60 mK) maps to date of the distribution of molecular gas in the central 5.5 kpc of the galaxy. A new catalog of giant molecular clouds (GMCs) has a completeness limit of 1.3×10 5 M ⊙ . The fraction of molecular gas found in GMCs is a strong function of radius in the galaxy, declining from 60% in the center to 20% at galactocentric radius R gal ≈ 4 kpc. Beyond that radius, GMCs are nearly absent, although molecular gas exists. Most (90%) of the emission from low mass clouds is found within 100 pc projected separation of a GMC. In an annulus 2.1 < R gal < 4.1 kpc, GMC masses follow a power law distribution with index −2.1. Inside that radius, the mass distribution is truncated, and clouds more massive than 8 × 10 5 M ⊙ are absent. The cloud mass distribution shows no significant difference in the grand design spiral arms versus the interarm region. The CO surface brightness ratio for the arm to interarm regions is 1.5, typical of other flocculent galaxies.