Strongly magnetized, rapidly rotating neutron stars are contenders for the central engines of both long gamma-ray bursts (LGRBs) and hydrogen-poor superluminous supernovae (SLSNe-I). Models for ...typical (minute long) LGRBs invoke magnetars with high dipole magnetic fields (B
d ≳ 1015 G) and short spin-down times, SLSNe-I require neutron stars with weaker fields and longer spin-down times of weeks. Here, we identify a transition region in the space of B
d and birth period for which a magnetar can power both a LGRB and a luminous supernova. In particular, a 2 ms period magnetar with a spin-down time of ∼104 s can explain both the ultralong GRB 111209 and its associated luminous SN2011kl. For magnetars with longer spin-down times, we predict even longer duration (∼105 − 6 s) GRBs and brighter supernovae, a correlation that extends to Swift J2058+05 (commonly interpreted as a tidal disruption event). We further show that previous estimates of the maximum rotational energy of a protomagnetar were too conservative and energies up to E
max ∼ 1–2 × 1053 ergs are possible. A magnetar can therefore comfortably accommodate the extreme energy requirements recently posed by the most luminous supernova ASASSN-15lh. The luminous pulsar wind nebula powering ASASSN-15lh may lead to an ‘ionization breakout’ X-ray burst over the coming months, accompanied by a change in the optical spectrum.
We present analytic calculations of angular momentum transport and gas inflow in galaxies, from scales of ∼ kpc to deep inside the potential of a central massive black hole (BH). We compare these ...analytic calculations to numerical simulations and use them to develop a sub-grid model of BH growth that can be incorporated into semi-analytic calculations or cosmological simulations. Motivated by both analytic calculations and simulations of gas inflow in galactic nuclei, we argue that the strongest torque on gas arises when non-axisymmetric perturbations to the stellar gravitational potential produce orbit crossings and shocks in the gas. This is true both at large radii ∼0.01-1 kpc, where bar-like stellar modes dominate the non-axisymmetric potential, and at smaller radii ≲10 pc, where a lopsided/eccentric stellar disc dominates. The traditional orbit-crossing criterion is not always adequate to predict the locations of, and inflow due to, shocks in gas+stellar discs with finite sound speeds. We derive a modified criterion that predicts the presence of shocks in stellar-dominated systems even absent formal orbit crossing. We then derive analytic expressions for the loss of angular momentum and the resulting gas inflow rates in the presence of such shocks. We test our analytic predictions using hydrodynamic simulations at a range of galactic scales, and show that they successfully predict the mass inflow rates and quasi-steady gas surface densities with a small scatter ≃0.3 dex. We use our analytic results to construct a new estimate of the BH accretion rate given galaxy properties at larger radii, for use in galaxy and cosmological simulations and semi-analytic models. While highly simplified, this accretion rate predictor captures the key scalings in the numerical simulations. By contrast, alternate estimates such as the local viscous accretion rate or the spherical Bondi rate fail systematically to reproduce the simulations and have significantly larger scatter.
Feedback from massive stars is believed to play a critical role in shaping the galaxy mass function, the structure of the interstellar medium (ISM) and the low efficiency of star formation, but the ...exact form of the feedback is uncertain. In this paper, the first in a series, we present and test a novel numerical implementation of stellar feedback resulting from momentum imparted to the ISM by radiation, supernovae and stellar winds. We employ a realistic cooling function, and find that a large fraction of the gas cools to ≲100 K, so that the ISM becomes highly inhomogeneous. Despite this, our simulated galaxies reach an approximate steady state, in which gas gravitationally collapses to form giant 'molecular' clouds (GMCs), dense clumps and stars; subsequently, stellar feedback disperses the GMCs, repopulating the diffuse ISM. This collapse and dispersal cycle is seen in models of Small Magellanic Cloud (SMC)-like dwarfs, the Milky Way and z∼ 2 clumpy disc analogues. The simulated global star formation efficiencies are consistent with the observed Kennicutt-Schmidt relation. Moreover, the star formation rates are nearly independent of the numerically imposed high-density star formation efficiency, density threshold and density scaling. This is a consequence of the fact that, in our simulations, star formation is regulated by stellar feedback limiting the amount of very dense gas available for forming stars. In contrast, in simulations without stellar feedback, i.e. under the action of only gravity and gravitationally induced turbulence, the ISM experiences runaway collapse to very high densities. In these simulations without feedback, the global star formation rates exceed observed galactic star formation rates by 1-2 orders of magnitude, demonstrating that stellar feedback is crucial to the regulation of star formation in galaxies.
We develop and implement numerical methods for including stellar feedback in galaxy-scale numerical simulations. Our models include simplified treatments of heating by Type I and Type II supernovae, ...gas recycling from young stars and asymptotic giant branch winds, heating from the shocked stellar winds, H ii photoionization heating and radiation pressure from stellar photons. The energetics and time dependence associated with the feedback are taken directly from stellar evolution models. We implement these stellar feedback models in smoothed particle hydrodynamic simulations with pc-scale resolution, modelling galaxies from Small Magellanic Cloud (SMC) like dwarfs and Milky Way (MW) analogues to massive z∼ 2 star-forming discs. In the absence of stellar feedback, gas cools rapidly and collapses without limit into dense sub-units, inconsistent with observations. By contrast, in all cases with feedback, the interstellar medium (ISM) quickly approaches a statistical steady state in which giant molecular clouds (GMCs) continuously form, disperse and re-form, leading to a multiphase ISM. In this paper, we quantify the properties of the ISM and GMCs in this self-regulated state. In a companion paper we study the galactic winds driven by stellar feedback.
Our primary results on the structure of the ISM in star-forming galaxies include the following.
1
Star-forming galaxies generically self-regulate so that the cool, dense gas maintains Toomre's Q∼ 1. Most of the volume is occupied by relatively diffuse hot gas, while most of the mass is in dense GMC complexes created by self-gravity. The phase structure of the gas and the gas mass fraction at high densities are much more sensitive probes of the physics of stellar feedback than integrated quantities such as the Toomre Q or gas velocity dispersion.
2
Different stellar feedback mechanisms act on different spatial (and density) scales. Radiation pressure and H ii gas pressure are critical for preventing runaway collapse of dense gas in GMCs. Shocked supernova ejecta and stellar winds dominate the dynamics of the volume-filling hot gas. However, this gas primarily vents out of the star-forming disc and contributes only modestly to the mid-plane ISM pressure.
3
The galaxy-averaged star formation rate is determined by feedback, with different mechanisms dominating in different galaxy types. For a given feedback efficiency, restricting star formation to molecular gas or modifying the cooling function has little effect on the star formation rate in the galaxies we model (including an SMC-mass dwarf). By contrast, changing the feedback mechanisms or assumed feedback efficiencies directly translates to shifts off of the observed Kennicutt-Schmidt relation.
4
Self-gravity leads to GMCs with an approximately self-similar mass function ∝M
−2, with a high-mass cut-off determined by the characteristic Jeans/Toomre mass of the system. In all of our galaxy models, GMCs live for a few dynamical times before they are disrupted by stellar feedback. The net star formation efficiency in GMCs ranges from ∼1 per cent in dwarfs and MW-like spirals to nearly ∼10 per cent in gas-rich rapidly star-forming galaxies. GMCs are approximately virialized, but there is a large dispersion in the virial parameter for a given GMC mass, and lower mass GMCs tend to be preferentially unbound.
We present three-dimensional magnetohydrodynamic simulations of magnetized gas clouds accelerated by hot winds. We initialize gas clouds with tangled internal magnetic fields and show that this field ...suppresses the disruption of the cloud: rather than mixing into the hot wind as found in hydrodynamic simulations, cloud fragments end up comoving with the external medium and in pressure equilibrium with their surroundings. We also show that a magnetic field in the hot wind enhances the drag force on the cloud by a factor
${\sim } (1+v_{{\rm A}}^2/v_{{\rm wind}}^2)$
, where v
A is the Alfvén speed in the wind and v
wind measures the relative speed between the cloud and the wind. We apply this result to gas clouds in several astrophysical contexts, including galaxy clusters, galactic winds, the Galactic Centre, and the outskirts of the Galactic halo. Our results can help explain the prevalence of cool gas in galactic winds and galactic haloes, and how this cool gas survives in spite of its interaction with hot wind/halo gas. We also predict that drag forces can lead to a deviation from Keplerian orbits for gas clouds in the galactic center.
Abstract
Active galactic nuclei (AGN) drive fast winds in the interstellar medium of their host galaxies. It is commonly assumed that the high ambient densities and intense radiation fields in ...galactic nuclei imply short cooling times, thus making the outflows momentum conserving. We show that cooling of high-velocity shocked winds in AGN is in fact inefficient in a wide range of circumstances, including conditions relevant to ultraluminous infrared galaxies (ULIRGs), resulting in energy-conserving outflows. We further show that fast energy-conserving outflows can tolerate a large amount of mixing with cooler gas before radiative losses become important. For winds with initial velocity v
in ≳ 10 000 km s−1, as observed in ultraviolet and X-ray absorption, the shocked wind develops a two-temperature structure. While most of the thermal pressure support is provided by the protons, the cooling processes operate directly only on the electrons. This significantly slows down inverse Compton cooling, while free-free cooling is negligible. Slower winds with v
in ∼ 1000 km s−1, such as may be driven by radiation pressure on dust, can also experience energy-conserving phases but under more restrictive conditions. During the energy-conserving phase, the momentum flux of an outflow is boosted by a factor ∼v
in/2v
s by work done by the hot post-shock gas, where v
s is the velocity of the swept-up material. Energy-conserving outflows driven by fast AGN winds (v
in ∼ 0.1c) may therefore explain the momentum fluxes of galaxy-scale outflows recently measured in luminous quasars and ULIRGs. Shocked wind bubbles expanding normal to galactic discs may also explain the large-scale bipolar structures observed in some systems, including around the Galactic Centre, and can produce significant radio, X-ray and γ-ray emission. The analytic solutions presented here will inform implementations of AGN feedback in numerical simulations, which typically do not include all the important physics.
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.
A new class of faint, spectroscopically peculiar transients has emerged in the last decade. We term these events "calcium-strong transients" (CaSTs) because of their atypically high calcium-to-oxygen ...nebular line ratios. Previous studies have struggled to deduce the identity of their progenitors, due to a combination of their extremely extended radial distributions with respect to their host galaxies and their relatively high rate of occurrence. In this work, we find that the CaST radial distribution is consistent with the radial distribution of two populations of stars: old (ages >5 Gyr), low-metallicity (Z/Z < 0.3) stars, and globular clusters. While no obvious progenitor scenario arises from considering old, metal-poor stars, the alternative production site of globular clusters leads us to narrow down the list of possible candidates to three binary scenarios: mergers of helium and oxygen/neon white dwarfs; tidal disruptions of helium white dwarfs by neutron stars; and stable accretion from low-mass helium-burning stars onto white dwarfs. While rare in the field, these binary systems can be formed dynamically at much higher rates in globular clusters. Subsequent binary hardening both increases their interaction rate and ejects them from their parent globular clusters prior to mass transfer contact. Their production in, and ejection from, globular clusters may explain their radial distribution and the absence of globular clusters at their explosion site. This model predicts a currently undiscovered high rate of CaSTs in nuclear star clusters. Alternatively, an undetermined progenitor scenario involving old, low-metallicity stars may instead hold the key to understanding CaSTs.
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.
Abstract
We study the evolution of accreting oxygen–neon (ONe) white dwarfs (WDs), with a particular emphasis on the effects of the presence of the carbon-burning products 23Na and 25Mg. These ...isotopes lead to substantial cooling of the WD via the 25Mg–25Na, 23Na–23Ne and 25Na–25Ne Urca pairs. We derive an analytic formula for the peak Urca-process cooling rate and use it to obtain a simple expression for the temperature to which the Urca process cools the WD. Our estimates are equally applicable to accreting carbon–oxygen WDs. We use the Modules for Experiments in Stellar Astrophysics (MESA) stellar evolution code to evolve a suite of models that confirm these analytic results and demonstrate that Urca-process cooling substantially modifies the thermal evolution of accreting ONe WDs. Most importantly, we show that MESA models with lower temperatures at the onset of the 24Mg and 24Na electron captures develop convectively unstable regions, even when using the Ledoux criterion. We discuss the difficulties that we encounter in modelling these convective regions and outline the potential effects of this convection on the subsequent WD evolution. For models in which we do not allow convection to operate, we find that oxygen ignites around a density of log(ρc/g cm−3) ≈ 9.95, very similar to the value without Urca cooling. Nonetheless, the inclusion of the effects of Urca-process cooling is an important step in producing progenitor models with more realistic temperature and composition profiles which are needed for the evolution of the subsequent oxygen deflagration and hence for studies of the signature of accretion-induced collapse.
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