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Feedback from supernovae is often invoked as an important process in limiting star formation, removing gas from galaxies and hence as a determining process in galaxy formation. Here we report on numerical simulations investigating the interaction between supernova explosions and the natal molecular cloud. We also consider the cases with and without previous feedback from the high-mass star in the form of ionising radiation and stellar winds. The supernova is able to find weak points in the cloud and create channels through which it can escape, leaving much of the well shielded cloud largely unaffected. This effect is increased when the channels are pre-existing due to the effects of previous stellar feedback. The expanding supernova deposits its energy in the gas that is in these exposed channels, and hence sweeps up less mass when feedback has already occurred, resulting in faster outflows with less radiative losses. The full impact of the supernova explosion is then able to impact the larger scale of the galaxy in which it abides. We conclude that supernova explosions only have moderate effects on their dense natal environments but that with pre-existing feedback, the energetic effects of the supernova are able to escape and affect the wider scale medium of the galaxy.
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In interstellar clouds the deposition of water ice onto grains only occurs at visual extinctions above some threshold value A_th. At extinctions greater than A_th there is a (near-linear) correlation between the inferred column density of the water ice and A_V. For individual cloud complexes such as Taurus, Serpens and Rho-Ophiuchi, A_th and the gradients of the correlation are very similar along all lines of sight. We have investigated the origin of this phenomenon, with careful consideration of the various possible mechanisms that may be involved and have applied a full chemical model to analyse the behaviours and sensitivities in quiescent molecular clouds. Our key results are: (i) the ubiquity of the phenomenon points to a common cause, so that the lines of sight probe regions with similar, advanced, chemical and dynamical evolution, (ii) for Taurus and Serpens; A_th and the slope of the correlation can be explained as resulting from the balance of freeze-out of oxygen atoms and photodesorption of H2O molecules. No other mechanism can satisfactorily explain the phenomenon, (iii) A_th depends on the local density, suggesting that there is a correlation between local volume density and column density, (iv) the different values of A_th for Taurus and Serpens are probably due to variations in the local mean radiation field strength, (v) most ice is accreted onto grains that are initially very small (<0.01 microns), and (vi) the very high value of A_th observed in Rho-Ophiuchi cannot be explained in the same way, unless there is complex microstructure and/or a modification to the extinction characteristics.
We present simulations of a 500 pc$^2$ region, containing gas of mass 4 $times$ 10$^6$ M$_odot$, extracted from an entire spiral galaxy simulation, scaled up in resolution, including photoionising feedback from stars of mass > 18 M$_odot$. Our region is evolved for 10 Myr and shows clustered star formation along the arm generating $approx$ 5000 cluster sink particles $approx$ 5% of which contain at least one of the $approx$ 4000 stars of mass > 18 M$_odot$. Photoionisation has a noticeable effect on the gas in the region, producing ionised cavities and leading to dense features at the edge of the HII regions. Compared to the no-feedback case, photoionisation produces a larger total mass of clouds and clumps, with around twice as many such objects, which are individually smaller and more broken up. After this we see a rapid decrease in the total mass in clouds and the number of clouds. Unlike studies of isolated clouds, our simulations follow the long range effects of ionisation, with some already-dense gas becoming compressed from multiple sides by neighbouring HII regions. This causes star formation that is both accelerated and partially displaced throughout the spiral arm with up to 30% of our cluster sink particle mass forming at distances > 5 pc from sites of sink formation in the absence of feedback. At later times, the star formation rate decreases to below that of the no-feedback case.
It is a major open question which physical processes stop the accretion of gas onto giant molecular clouds (GMCs) and limit the efficiency at which gas is converted into stars within these GMCs. While feedback from supernova explosions has been the popular feedback mechanism included in simulations of galaxy formation and evolution, `early feedback mechanisms such as stellar winds, photoionisation and radiation pressure are expected to play an important role in dispersing the gas after the onset of star formation. These feedback processes typically take place on small scales ($sim 10-100$ pc) and their effects have therefore been difficult to constrain in environments other than the Milky Way. We apply a novel statistical method to $sim 1$ resolution maps of CO and Ha emission across a sample of nine nearby disc galaxies, in order to measure the time over which GMCs are dispersed by feedback from young, high-mass stars, as a function of the galactic environment. We find that GMCs are typically dispersed within $sim$ 3 Myr after the emergence of unembedded high-mass stars, showing no significant trend with galactocentric radius. Comparison with analytical predictions demonstrates that, independently of the environment, early feedback mechanisms (particularly photoionisation and stellar winds) play a crucial role in dispersing GMCs and limiting their star formation efficiency in nearby galaxies. Finally, we show that the efficiency at which the energy injected by these early feedback mechanisms couples with the parent GMC is relatively low (a few tens of per cent), such that the vast majority of momentum and energy emitted by the young stellar populations escapes the parent GMC.
Supernovae from core-collapse of massive stars drive shocks into the molecular clouds from which the stars formed. Such shocks affect future star formation from the molecular clouds, and the fast-moving, dense gas with compressed magnetic fields is associated with enhanced cosmic rays. This paper presents new theoretical modeling, using the Paris-Durham shock model, and new observations, using the Stratospheric Observatory for Infrared Astronomy (SOFIA), of the H$_2$ S(5) pure rotational line from molecular shocks in the supernova remnant IC443. We generate MHD models for non-steady-state shocks driven by the pressure of the IC443 blast wave into gas of densities $10^3$ to $10^5$ cm$^{-3}$. We present the first detailed derivation of the shape of the velocity profile for emission from H$_2$ lines behind such shocks, taking into account the shock age, preshock density, and magnetic field. For preshock densities $10^3$-$10^5$ cm$^{-3}$, the the predicted shifts of line centers, and the line widths, of the H$_2$ lines range from 20-2, and 30-4 km/s, respectively. The a priori models are compared to the observed line profiles, showing that clumps C and G can be explained by shocks into gas with density 10$^3$ to $2times 10^4$ cm$^{-3}$ and strong magnetic fields. For clump B2 (a fainter region near clump B), the H$_2$ spectrum requires a J-type shock into moderate density (~100 cm$^{-3}$) with the gas accelerated to 100 km/s from its pre-shock location. Clump B1 requires both a magnetic-dominated C-type shock (like for clumps C and G) and a J-type shock (like for clump B1) to explain the highest observed velocities. The J-type shocks that produce high-velocity molecules may be locations where the magnetic field is nearly parallel to the shock velocity, which makes it impossible for a C-type shock (with ions and neutrals separated) to form.
Giant Molecular Clouds (GMCs) are observed to be turbulent, but theory shows that without a driving mechanism turbulence should quickly decay. The question arises by which mechanisms turbulence is driven or sustained. It has been shown that photoionising feedback from massive stars has an impact on the surrounding GMC and can for example create vast HII bubbles. We therefore address the question of whether turbulence is a consequence of this effect of feedback on the cloud. To investigate this, we analyse the velocity field of simulations of high mass star forming regions by studying velocity structure functions and power spectra. We find that clouds whose morphology is strongly affected by photoionising feedback also show evidence of driving of turbulence by preserving or recovering a Kolmogorov-type velocity field. On the contrary, control run simulations without photoionising feedback have a velocity distribution that bears the signature of gravitational collapse and of the dissipation of energy, where the initial Kolmogorov-type structure function is erased.
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