If the neutrino luminosity from the proto-neutron star formed during a massive star core collapse exceeds a critical threshold, a supernova (SN) results. Using spherical quasi-static evolutionary ...sequences for hundreds of progenitors over a range of metallicities, we study how the explosion threshold maps onto observables, including the fraction of successful explosions, the neutron star (NS) and black hole (BH) mass functions, the explosion energies (E sub(SN)) and nickel yields (M sub(Ni)), and their mutual correlations. Successful explosions are intertwined with failures in a complex pattern that is not simply related to initial progenitor mass or compactness. We predict that progenitors with initial masses of 15 + or - 1, 19 + or - 1, and ~21-26 M sub(middot in circle) are most likely to form BHs, that the BH formation probability is non-zero at solar-metallicity and increases significantly at low metallicity, and that low luminosity, low Ni-yield SNe come from progenitors close to success/failure interfaces. We qualitatively reproduce the observed E sub(SN)-M sub(Ni) correlation, we predict a correlation between the mean and width of the NS mass and E sub(SN) distributions, and that the means of the NS and BH mass distributions are correlated. We show that the observed mean NS mass of Asymptotically = to 1.33 M sub(middot in circle) implies that the successful explosion fraction is higher than 0.35. Overall, we show that the neutrino mechanism can in principle explain the observed properties of SNe and their compact objects. We argue that the rugged landscape of progenitors and outcomes mandates that SN theory should focus on reproducing the wide ranging distributions of observed SN properties.
White dwarf-white dwarf (WD-WD) and neutron star-neutron star (NS-NS) mergers may produce Type Ia supernovae and gamma-ray bursts (GRBs), respectively. A general problem is how to produce binaries ...with semi-major axes small enough to merge in significantly less than the Hubble time (t H), and thus accommodate the observation that these events closely follow episodes of star formation. I explore the possibility that such systems are not binaries at all, but actually coeval, or dynamical formed, triple systems. The tertiary induces Kozai oscillations in the inner binary, driving it to high eccentricity, and reducing its gravitational wave (GW) merger timescale. This effect significantly increases the allowed range of binary period P such that the merger time is t merge < t H. In principle, Chandrasekhar-mass binaries with P ~ 300 days can merge in t H if they contain a prograde solar-mass tertiary at high enough inclination. For retrograde tertiaries, the maximum P such that t merge t H is yet larger. In contrast, P 0.3 days is required in the absence of a tertiary. I discuss implications of these findings for the production of transients formed via compact object binary mergers. Based on the statistics of solar-type binaries, I argue that many such binaries should be in triple systems affected by the Kozai resonance. If true, expectations for the mHz GW signal from individual sources, the diffuse background, and the foreground for GW experiments like LISA are modified. This work motivates future studies of triples systems of A, B, and O stars, and new types of searches for WD-WD binaries in triple systems.
The neutron star (NS) merger GW170817 was followed over several days by optical-wavelength ("blue") kilonova (KN) emission likely powered by the radioactive decay of light r-process nuclei ...synthesized by ejecta with a low neutron abundance (electron fraction Ye 0.25-0.35). While the composition and high velocities of the blue KN ejecta are consistent with shock-heated dynamical material, the large quantity is in tension with the results of numerical simulations. We propose an alternative ejecta source: the neutrino-heated, magnetically accelerated wind from the strongly magnetized hypermassive NS (HMNS) remnant. A rapidly spinning HMNS with an ordered surface magnetic field of strength B (1-3) × 1014 G and lifetime trem ∼ 0.1-1 s can simultaneously explain the velocity, total mass, and electron fraction of the blue KN ejecta. The inferred HMNS lifetime is close to its Alfvén crossing time, suggesting that global magnetic torques could be responsible for bringing the HMNS into solid-body rotation and instigating its gravitational collapse. Different origins for the KN ejecta may be distinguished by their predictions for the emission in the first hours after the merger, when the luminosity is enhanced by heating from internal shocks; the latter are likely generic to any temporally extended ejecta source (e.g., magnetar or accretion disk wind) and are not unique to the emergence of a relativistic jet. The same shocks could mix and homogenize the composition to a low but nonzero lanthanide mass fraction, , as advocated by some authors, but only if the mixing occurs after neutrons are consumed in the r-process on a timescale 1 s.
Galactic outflows commonly contain multiphase gas, and its physical origin requires explanation. Using the Cholla Galactic OutfLow Simulations suite of high-resolution isolated galaxy models, we ...demonstrate the viability of rapid radiative cooling as a source of fast-moving (v ∼ 1000 km s−1), cool (104 K) gas observed in absorption-line studies of outflows around some star-forming galaxies. By varying the mass loading and geometry of the simulated winds, we identify a region of parameter space that leads to cool gas in outflows. In particular, when using an analytically motivated central feedback model, we find that cooling flows can be produced with reasonable mass-loading rates ( ), provided that the star formation rate surface density is high. When a more realistic clustered feedback model is applied, destruction of high-density clouds near the disk and interactions between different outflow regions indicate that lower mass-loading rates of the hot gas within the feedback region may still produce multiphase outflows. These results suggest an origin for fast-moving cool gas in outflows that does not rely on directly accelerating cool gas from the interstellar medium. These cooling flows may additionally provide an explanation for the multiphase gas ubiquitously observed in the halos of star-forming galaxies at low redshift.
It is typically assumed that radiation-pressure-driven winds are accelerated to an asymptotic velocity of v
∞ ≃ v
esc, where v
esc is the escape velocity from the central source. We note that this is ...not the case for dusty shells and clouds. Instead, if the shell or cloud is initially optically thick to the UV emission from the source of luminosity L, then there is a significant boost in v
∞ that reflects the integral of the momentum absorbed as it is accelerated. For shells reaching a generalized Eddington limit, we show that v
∞ ≃ (4R
UV
L/M
sh
c)1/2, in both point-mass and isothermal-sphere potentials, where R
UV is the radius where the shell becomes optically thin to UV photons, and M
sh is the mass of the shell. The asymptotic velocity significantly exceeds v
esc for typical parameters, and can explain the ∼1000–2000 km s−1 outflows observed from rapidly star-forming galaxies and active galactic nuclei (AGN) if the surrounding halo has low gas density. Similarly fast outflows from massive stars can be accelerated on ∼few–103 yr time-scales. These results carry over to clouds that subtend only a small fraction of the solid angle from the source of radiation and that expand as a consequence of their internal sound speed. We further consider the dynamics of shells that sweep up a dense circumstellar or circumgalactic medium. We calculate the ‘momentum ratio’
$\dot{M} v/(L/c)$
in the shell limit and show that it can only significantly exceed ∼2 if the effective optical depth of the shell to re-radiated far-infrared photons is much larger than unity. We discuss simple prescriptions for the properties of galactic outflows for use in large-scale cosmological simulations. We also briefly discuss applications to the dusty ejection episodes of massive stars, the disruption of giant molecular clouds, and AGN.
The pressure exerted by the radiation of young stars may be an important feedback mechanism that drives turbulence and winds in forming star clusters and the disks of starburst galaxies. In this ...paper, we report a series of two-dimensional flux-limited diffusion radiation-hydrodynamics calculations performed with the code orion in which we drive strong radiation fluxes through columns of dusty matter confined by gravity in order to answer these questions. We consider both systems where the radiation flux is sub-Eddington throughout the gas column, and those where it is super-Eddington at the midplane but sub-Eddington in the atmosphere. However, the instability also produces a channel structure in which the radiation-matter interaction is reduced compared to time-steady analytic models because the radiation field is not fully trapped. We provide an approximation formula, appropriate for implementation in analytic models and non-radiative simulations, for the force exerted by the infrared radiation field in this regime.
We present the fourth simulation of the Cholla Galactic OutfLow Simulations suite. Using a physically motivated prescription for clustered supernova feedback, we successfully drive a multiphase ...outflow from a disk galaxy. The high resolution (<5 pc) across a relatively large domain (20 kpc) allows us to capture the hydrodynamic mixing and dynamical interactions between the hot and cool (T ∼ 104 K) phases in the outflow, which in turn leads to direct evidence of a qualitatively new mechanism for cool gas acceleration in galactic winds. We show that mixing of momentum from the hot phase to the cool phase accelerates the cool gas to 800 km s−1 on kiloparsec scales, with properties inconsistent with the physical models of ram pressure acceleration or bulk cooling from the hot phase. The mixing process also affects the hot phase, modifying its radial profiles of temperature, density, and velocity from the expectations of radial supersonic flow. This mechanism provides a physical explanation for the high-velocity, blueshifted, low-ionization absorption lines often observed in the spectra of starburst and high-redshift galaxies.
Star formation is slow in the sense that the gas consumption time is much longer than the dynamical time. It is also inefficient; star formation in local galaxies takes place in giant molecular ...clouds (GMCs), but the fraction of a GMC converted to stars is very small, epsilon{sub GMC} approx 5%. In luminous starbursts, the GMC lifetime is shorter than the main-sequence lifetime of even the most massive stars, so that supernovae can play no role in GMC disruption. We investigate the disruption of GMCs across a wide range of galaxies from normal spirals to the densest starbursts; we take into account the effects of H II gas pressure, shocked stellar winds, protostellar jets, and radiation pressure produced by the absorption and scattering of starlight on dust grains. In the Milky Way, a combination of three mechanisms-jets, H II gas pressure, and radiation pressure-disrupts the clouds. In more rapidly star-forming galaxies such as 'clump' galaxies at high-redshift, ultra-luminous infrared galaxies (ULIRGs), and submillimeter galaxies, radiation pressure dominates natal cloud disruption. We predict the presence of approx10-20 clusters with masses approx10{sup 7} M{sub sun} in local ULIRGs such as Arp 220 and a similar number of clusters with M{sub *} approx 10{sup 8} M{sub sun} in high redshift clump galaxies; submillimeter galaxies will have even more massive clusters. We find that epsilon{sub GMC} = piGSIGMA{sub GMC} c/(2(L/M{sub *})) for GMCs that are optically thin to far-infrared radiation, where SIGMA{sub GMC} is the GMC gas surface density. The efficiency in optically thick systems continues to increase with SIGMA{sub GMC}, but more slowly, reaching approx35% in the most luminous starbursts. The disruption of bubbles by radiation pressure stirs the interstellar medium (ISM) to velocities of approx10 km s{sup -1} in normal galaxies and to approx100 km s{sup -1} in ULIRGs like Arp 220, consistent with observations. Thus, radiation pressure may play a dominant dynamical role in the ISM of star-forming galaxies.
Galactic outflows of cool (~104 K) gas are ubiquitous in local starburst galaxies and in most high-redshift galaxies. Hot gas from supernovae has long been suspected as the primary driver, but this ...mechanism suffers from its tendency to destroy the cool gas. We propose a modification of the supernova scenario that overcomes this difficulty. Star formation is observed to take place in clusters. We show that, for L galaxies, the radiation pressure from clusters with M cl 106 M is able to expel the surrounding gas at velocities in excess of the circular velocity vc of the disk galaxy. This cool gas travels above the galactic disk before supernovae erupt in the driving cluster. Once above the disk, the cool outflowing gas is exposed to radiation and hot gas outflows from the galactic disk, which in combination drive it to distances of ~50 kpc. Because the radiatively driven clouds grow in size as they travel, and because the hot gas is more dilute at large distance, the clouds are less subject to destruction. Therefore, unlike wind-driven clouds, radiatively driven clouds can give rise to the metal absorbers seen in quasar spectra. We identify these cluster-driven winds with large-scale galactic outflows. The maximum cluster mass in a galaxy is an increasing function of the galaxy's gas surface density, so only starburst galaxies are able to drive cold outflows. We find the critical star formation rate for launching large-scale cool outflows to be , in good agreement with observations.
The physical origin of high-velocity cool gas seen in galactic winds remains unknown. Following work by B. Wang, we argue that radiative cooling in initially hot thermally-driven outflows can produce ...fast neutral atomic and photoionized cool gas. The inevitability of adiabatic cooling from the flow's initial 107–108 K temperature and the shape of the cooling function for T ≲ 107 K imply that outflows with hot gas mass-loss rate relative to star formation rate of
$\beta =\dot{M}_{\rm hot}/\dot{M}_\star \gtrsim 0.5$
cool radiatively on scales ranging from the size of the energy injection region to tens of kpc. We highlight the β and star formation rate surface density dependence of the column density, emission measure, radiative efficiency, and velocity. At r
cool, the gas produces X-ray and then UV/optical line emission with a total power bounded by ∼10−2 L
⋆ if the flow is powered by steady-state star formation with luminosity L
⋆. The wind is thermally unstable at r
cool, potentially leading to a multiphase medium. Cooled winds decelerate significantly in the extended gravitational potential of galaxies. The cool gas precipitated from hot outflows may explain its prevalence in galactic haloes. We forward a picture of winds whereby cool clouds are initially accelerated by the ram pressure of the hot flow, but are rapidly shredded by hydrodynamical instabilities, thereby increasing β, seeding radiative and thermal instability, and cool gas rebirth. If the cooled wind shocks as it sweeps up the circumgalactic medium, its cooling time is short, thus depositing cool gas far out into the halo. Finally, conduction can dominate energy transport in low-β hot winds, leading to flatter temperature profiles than otherwise expected, potentially consistent with X-ray observations of some starbursts.
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