Mechanical and radiative energy input by massive stars stir up the environment, heat the gas, produce cloud & intercloud phases in the interstellar medium and disrupt molecular clouds, the birthsites of new stars 1,2 . Ionization by UV photons, stellar wind action and supernova explosions control molecular clouds lifetimes 3,4,5,6,7 . Theoretical studies predict that momentum injection by radiation dominates by far over momentum injected by a stellar wind 8 , but this has hitherto been difficult to assess observationally. Velocity-resolved large-scale images in the fine structure line of ionized carbon ([CII]) provide an observational diagnostic of the radiative energetics and the dynamics of the ISM in the immediate vicinity of massive stars. Here, we present the [CII] 1.9 THz (158 µm) study of ~1 square degree region (~7pc in diameter) at a resolution of 16" (0.03pc) of the nearest region of massive star formation, Orion. The results reveal that the stellar wind originating from the star, θ 1 Ori C, has created a ~2pc sized bubble by sweeping up a 2600 M ! shell expanding at 13 km/s. This shows that the stellar wind mechanical energy is coupled very efficiently to the molecular core and its action dominates over photoionization/evaporation or future supernova explosions.We have surveyed one square degree of the Orion Molecular cloud, centered on the Trapezium cluster and the Orion Molecular Core 1 (OMC-1) behind it, in the 1.9 THz (158µm) [CII] fine-structure line with the 14 pixel upGREAT heterodyne highspectral resolution spectrometer 9 on board of the Stratospheric Observatory For Infrared Astronomy (SOFIA) (see method). Figure 1 compares the [CII] integrated intensity map with the mid-IR and far-IR maps due to UV-pumped fluorescence by polycyclic aromatic hydrocarbon (PAHs) molecules and thermal dust continuum emission, respectively. Each map clearly shows the direct interaction of the Trapezium cluster with the dense molecular core (center), the large, wind-blown bubble, associated with the Orion Veil (South), and the bubble created by the B stars illuminating the reflection nebulae, NGC 1973, 1975. Here, we focus on the prominent Veil bubble associated with the stellar wind from θ 1 Ori C. This shell consists of neutral atomic (H) gas and is very prominent in the [CII] map but there is no detectable counterpart in carbon monoxide, H 2 , or other molecular tracers as the shell is too tenuous for these species to persist; e.g., H 2 /H fraction <2x10 -4 and C/C + =10 -4 10,11 . Likewise, the complex pattern of absorption and emission features and the presence of multiple (foreground) components preclude recognition of the large scale structure of the shell in 21cm HI studies 12 . X-ray observations 13 have shown that this bubble is filled with tenuous (~1cm -3 ) hot (2x10 6 K) gas created by the strong stellar wind (mechanical luminosity, L w =8x10 35 erg/s 14,15 ) from the most massive star in the region, θ 1 Ori C (see Extended Data Figure 5).While each IR image ( Fig. 1) traces the Veil morphology, only [...

Intermediate-scale spurs are common in spiral galaxies, but perhaps most distinctively evident in a recent HST image of M51 (Scoville & Rector 2001). We investigate, using time-dependent numerical MHD simulations, how such spurs could form (and subsequently fragment) from the interaction of a gaseous ISM with a stellar spiral arm. We model the gaseous medium as a self-gravitating, magnetized, differentially-rotating, razor-thin disk. The basic flow shocks and compresses as it passes through a local segment of a tightly-wound, trailing stellar spiral arm, modeled as a rigidly-rotating gravitational potential. We first construct 1D profiles for flows with spiral shocks. When the post-shock Toomre parameter Q_sp is sufficiently small, self-gravity is too large for one-dimensional steady solutions to exist. The critical values of Q_sp are 0.8, 0.5, and 0.4 for our models with zero, sub-equipartition, and equipartition magnetic fields, respectively. We then study the growth of self-gravitating perturbations in fully 2D flows, and find that spur-like structures rapidly emerge in our magnetized models. We associate this gravitational instability with the magneto-Jeans mechanism, in which magnetic tension forces oppose the Coriolis forces. The shearing and expanding velocity field shapes the condensed material into spurs as it flows downstream from the arms. Although we find swing amplification can help form spurs when the arm-interarm contrast is moderate, unmagnetized systems that are quasi-axisymmetrically stable are generally also stable to nonaxisymmetric perturbations, suggesting that magnetic effects are essential. In nonlinear stages, the spurs in our models undergo fragmentation to form 4\times 10^6 solar mass clumps, which we suggest could evolve into bright arm/interarm HII regions as seen in spiral galaxies.Comment: 32 pages, 14 figures, Accepted for publication in ApJ; better postscript figures available from http://www.astro.umd.edu/~kimwt/FIGURE2/ ; for associated Animated GIF movies, see http://www.astro.umd.edu/~kimwt/MOVIES

UV radiation feedback from young massive stars plays a key role in the evolution of giant molecular clouds (GMCs) by photoevaporating and ejecting the surrounding gas. We conduct a suite of radiation hydrodynamic simulations of star cluster formation in marginally-bound, turbulent GMCs, focusing on the effects of photoionization and radiation pressure on regulating the net star formation efficiency (SFE) and cloud lifetime. We find that the net SFE depends primarily on the initial gas surface density, Σ 0 , such that the SFE increases from 4% to 51% as Σ 0 increases from 13 M pc −2 to 1300 M pc −2 . Cloud destruction occurs within 2-10 Myr after the onset of radiation feedback, or within 0.6-4.1 freefall times (increasing with Σ 0 ). Photoevaporation dominates the mass loss in massive, low surfacedensity clouds, but because most photons are absorbed in an ionization-bounded Strömgren volume the photoevaporated gas fraction is proportional to the square root of the SFE. The measured momentum injection due to thermal and radiation pressure forces is proportional to Σ −0.74 0 , and the ejection of neutrals substantially contributes to the disruption of low-mass and/or high-surface density clouds. We present semi-analytic models for cloud dispersal mediated by photoevaporation and by dynamical mass ejection, and show that the predicted net SFE and mass loss efficiencies are consistent with the results of our numerical simulations.

We use vertically-resolved numerical hydrodynamic simulations to study star formation and the interstellar medium (ISM) in galactic disks. We focus on outer disk regions where diffuse H I dominates, with gas surface densities Σ = 3 − 20 M pc −2 and starplus-dark matter volume densities ρ sd = 0.003 − 0.5 M pc −3 . Star formation occurs in very dense, self-gravitating clouds that form by mergers of smaller cold cloudlets. Turbulence, driven by momentum feedback from supernova events, destroys bound clouds and puffs up the disk vertically. Time-dependent radiative heating (FUV from recent star formation) offsets gas cooling. We use our simulations to test a new theory for self-regulated star formation. Consistent with this theory, the disks evolve to a state of vertical dynamical equilibrium and thermal equilibrium with both warm and cold phases. The range of star formation surface densities and midplane thermal pressures is Σ SFR ∼ 10 −4 − 10 −2 M kpc −2 yr −1 and P th /k B ∼ 10 2 − 10 4 cm −3 K. In agreement with observations, turbulent velocity dispersions are ∼ 7 km s −1 and the ratio of the total (effective) to thermal pressure is P tot /P th ∼ 4 − 5, across this whole range (provided shielding is similar to the Solar neighborhood). We show that Σ SFR is not well correlated with Σ alone, but rather with Σ √ ρ sd , because the vertical gravity from stars and dark matter dominates in outer disks. We also find that Σ SFR has a strong, nearly linear correlation with P tot , which itself is within ∼ 13% of the dynamical-equilibrium estimate P tot,DE . The quantitative relationships we find between Σ SFR and the turbulent and thermal pressures show that star formation is highly efficient for energy and momentum production, in contrast to the low efficiency of mass consumption. Star arXiv:1109.0028v1 [astro-ph.GA] 31 Aug 2011 formation rates adjust until the ISM's energy and momentum losses are replenished by feedback within a dynamical time.

Dynamical expansion of H II regions around star clusters plays a key role in dispersing the surrounding dense gas and therefore in limiting the efficiency of star formation in molecular clouds. We use a semianalytic method and numerical simulations to explore expansion of spherical dusty H II regions and surrounding neutral shells and the resulting cloud disruption. Our model for shell expansion adopts the static solutions of Draine (2011) for dusty H II regions and considers the contact outward forces on the shell due to radiation and thermal pressures as well as the inward gravity from the central star and the shell itself. We show that the internal structure we adopt and the shell evolution from the semi-analytic approach are in good agreement with the results of numerical simulations. Strong radiation pressure in the interior controls the shell expansion indirectly by enhancing the density and pressure at the ionization front. We calculate the minimum star formation efficiency ε min required for cloud disruption as a function of the cloud's total mass and mean surface density. Within the adopted spherical geometry, we find that typical giant molecular clouds in normal disk galaxies have ε min 10%, with comparable gas and radiation pressure effects on shell expansion. Massive clusterforming clumps require a significantly higher efficiency of ε min 50% for disruption, produced mainly by radiation-driven expansion. The disruption time is typically of the order of a free-fall timescale, suggesting that the cloud disruption occurs rapidly once a sufficiently luminous H II region is formed. We also discuss limitations of the spherical idealization.

We investigate the gravitational wake due to, and dynamical friction on, a perturber moving on a circular orbit in a uniform gaseous medium using a semianalytic method. This work is a straightforward extension of Ostriker (1999) who studied the case of a straight-line trajectory. The circular orbit causes the bending of the wake in the background medium along the orbit, forming a long trailing tail. The wake distribution is thus asymmetric, giving rise to the drag forces in both opposite (azimuthal) and lateral (radial) directions to the motion of the perturber, although the latter does not contribute to orbital decay much. For subsonic motion, the density wake with a weak tail is simply a curved version of that in Ostriker and does not exhibit the front-back symmetry. The resulting drag force in the opposite direction is remarkably similar to the finitetime, linear-trajectory counterpart. On the other hand, a supersonic perturber is able to overtake its own wake, possibly multiple times, and develops a very pronounced tail. The supersonic tail surrounds the perturber in a trailing spiral fashion, enhancing the perturbed density at the back as well as far front of the perturber. We provide the fitting formulae for the drag forces as functions of the Mach number, whose azimuthal part is surprisingly in good agreement with the Ostriker's formula, provided V p t = 2R p , where V p and R p are the velocity and orbital radius of the perturber, respectively.

We investigate the susceptibility of gaseous, magnetized galactic disks to the formation of selfgravitating condensations using two-dimensional, local models. We focus on two issues : (1) determining the threshold condition for gravitational runaway, taking into account nonlinear e †ects ; and (2) distinguishing the magneto-Jeans instability (MJI) that arises under inner galaxy rotation curves from the modiÐed swing ampliÐcation (MSA) that arises under outer galaxy rotation curves. For axisymmetric density Ñuctuations, instability is known to require a Toomre parameter Q \ 1. For nonaxisymmetric Ñuctuations, any nonzero shear q 4 [d ln )/d ln R winds up wave fronts such that in linear theory ampliÐcation saturates. Any Q threshold for nonaxisymmetric gravitational runaway must originate from nonlinear e †ects. We use numerical magnetohydrodynamic simulations to demonstrate the anticipated threshold phenomenon, to analyze the nonlinear processes involved, and to evaluate the critical value Q c for stabilization. We Ðnd for a wide variety of conditions, with the largest values corre-Q c D 1.2È1.4 sponding to nonzero but subthermal mean magnetic Ðelds. Our Ðndings for are similar to those Q c inferred from thresholds for active star formation in the outer regions of spiral galaxies. MJI is distinct from MSA in that opposition to Coriolis forces by magnetic tension, rather than cooperation of epicyclic motion with kinematic shear, enables nonaxisymmetric density perturbations to grow. We suggest that under low-shear inner disk conditions, will be larger than that in outer disks by a factor Q c D(v A /qc s )1@2, where and are the respective and sound speeds. v A c s Alfven

Various instabilities have been proposed as candidates to prompt the condensation of giant, star-forming cloud complexes from the diffuse interstellar medium. Here, we use three-dimensional ideal MHD simulations to investigate nonlinear development of the Parker, magneto-Jeans (MJI), and swing mechanisms in galactic disk models. The disk models are local, isothermal, and begin from a vertically-stratified magnetohydrostatic equilibrium state with both gaseous and stellar gravity. We allow for a range of surface densities and rotational shear profiles, as well as unmagnetized control models. We first construct axisymmetric equilibria and examine their stability. Finite disk thickness reduces the critical Toomre stability parameter below unity; we find Q c ∼ 0.75, 0.72, and 0.57 for zero, sub-equipartition, and equipartition magnetic field cases, respectively. We then pursue fully three-dimensional models. In non-self-gravitating cases, the peak mid-disk density enhancement from the "pure" Parker instability is less a factor of two. The dominant growing modes have radial wavelengths λ x comparable to the disk scale height H, much shorter than the azimuthal wavelength (λ y ∼ 10−20H). Shearing disks, being more favorable to midplane-symmetric modes, have somewhat different late-time magnetic field profiles from nonshearing disks, but otherwise saturated states are similar. Late-time velocity fluctuations at 10% of the sound speed persist, but no characteristic structural signatures of Parker modes remain in the new quasi-static equilibria. In self-gravitating cases, the development of density structure is qualitatively similar to our previous results from thin-disk simulations. The Parker instability, although it may help seed structure or tip the balance under marginal conditions, appears to play a secondary role -not affecting, for example, the sizes or spacings of the bound structures that form. In shearing disks with Q less than a threshold level ≈ 1, swing amplification can produce bound clouds of a few times the local Jeans mass. The most powerful cloud-condensing mechanism, requiring low-shear conditions as occur in spiral arms or galactic centers, appears to be the MJI. In thick disks, the MJI occurs for λ y > ∼ 2πH. Our simulations show that condensations of a local Jeans mass ( < ∼ 3 × 10 7 M ⊙ ) grow very rapidly, supporting the idea that MJI is at least partly responsible for the formation of bound cloud complexes in spiral galaxies.