arXiv subjects

Mordecai-Mark Mac Low

Publications and source records attributed to Mordecai-Mark Mac Low.

At least 55 records · Page 3Linked to original sources

arXiv2023

Radiation shielding of protoplanetary discs in young star-forming regions

Protoplanetary discs spend their lives in the dense environment of a star forming region. While there, they can be affected by nearby stars through external photoevaporation and dynamic truncations. We present simulations that use the AMUSE framework to couple the Torch model for star cluster formation from a molecular cloud with a model for the evolution of protoplanetary discs under these two environmental processes. We compare simulations with and without extinction of photoevaporation-driving radiation. We find that the majority of discs in our simulations are considerably shielded from photoevaporation-driving radiation for at least 0.5 Myr after the formation of the first massive stars. Radiation shielding increases disc lifetimes by an order of magnitude and can let a disc retain more solid material for planet formation. The reduction in external photoevaporation leaves discs larger and more easily dynamically truncated, although external photoevaporation remains the dominant mass loss process. Finally, we find that the correlation between disc mass and projected distance to the most massive nearby star (often interpreted as a sign of external photoevaporation) can be erased by the presence of less massive stars that dominate their local radiation field. Overall, we find that the presence and dynamics of gas in embedded clusters with massive stars is important for the evolution of protoplanetary discs.

astro-ph.EP↗

arXiv2023

Magnetic fields do not suppress global star formation in low metallicity dwarf galaxies

Many studies concluded that magnetic fields suppress star formation in molecular clouds and Milky Way like galaxies. However, most of these studies are based on fully developed fields that have reached the saturation level, with little work on investigating how an initial weak primordial field affects star formation in low metallicity environments. In this paper, we investigate the impact of a weak initial field on low metallicity dwarf galaxies. We perform high-resolution AREPO simulations of five isolated dwarf galaxies. Two models are hydrodynamical, two start with a primordial magnetic field of 10$^{-6} μ$G and different sub-solar metallicities, and one starts with a saturated field of 10$^{-2} μ$G. All models include a non-equilibrium, time-dependent chemical network that includes the effects of gas shielding from the ambient ultraviolet field. Sink particles form directly from the gravitational collapse of gas and are treated as star-forming clumps that can accrete gas. We vary the ambient uniform far ultraviolet field, and cosmic ray ionization rate between 1\% and 10\% of solar values. We find that the magnetic field has little impact on the global star formation rate, which is in tension with some previously published results. We further find that the initial field strength has little impact on the global star formation rate. We show that an increase in the mass fractions of both molecular hydrogen and cold gas, along with changes in the perpendicular gas velocity dispersion and the magnetic field acting in the weak-field model, overcome the expected suppression in star formation.

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arXiv2023

Fast methods for tracking grain coagulation and ionization. III. Protostellar collapse with non-ideal MHD

Dust grains influence many aspects of star formation, including planet formation, opacities for radiative transfer, chemistry, and the magnetic field via Ohmic, Hall, and ambipolar diffusion. The size distribution of the dust grains is the primary characteristic influencing all these aspects. Grain size increases by coagulation throughout the star formation process. We describe here numerical simulations of protostellar collapse using methods described in earlier papers of this series. We compute the evolution of the grain size distribution from coagulation and the non-ideal magnetohydrodynamics effects self-consistently and at low numerical cost. We find that the coagulation efficiency is mostly affected by the time spent in high-density regions. Starting from sub-micron radii, grain sizes reach more than 100 μm in an inner protoplanetary disk that is only 1000 years old. We also show that the growth of grains significantly affects the resistivities, and indirectly the dynamics and angular momentum of the disk.

astro-ph.SR↗

arXiv2022

The small-scale dynamo in a multiphase supernova-driven medium

Magnetic fields grow quickly, even at early cosmological times, suggesting the action of a small-scale dynamo (SSD) in the interstellar medium (ISM) of galaxies. Many studies have focused on idealized, isotropic, homogeneous, turbulent driving of the SSD. Here we analyze more realistic simulations of supernova-driven turbulence to understand how it drives an SSD. We find that SSD growth rates are intermittently variable as a result of the evolving multiphase ISM structure. Rapid growth in the magnetic field typically occurs in hot gas, with the highest overall growth rates occurring when the fractional volume of hot gas is large. SSD growth rates correlate most strongly with vorticity and fluid Reynolds number, which also both correlate strongly with gas temperature. Rotational energy exceeds irrotational energy in all phases, but particularly in the hot phase while SSD growth is most rapid. Supernova (SN) rate does not significantly affect the ISM average kinetic energy density. Rather, higher temperatures associated with high SN rates tend to increase SSD growth rates. SSD saturates with total magnetic energy density around 5% of equipartition to kinetic energy density, increasing slightly with magnetic Prandtl number. While magnetic energy density in the hot gas can exceed that of the other phases when SSD grows most rapidly, it saturates below 5% of equipartition with kinetic energy in the hot gas, while in the cold gas it attains 100%. Fast, intermittent growth of the magnetic field appears to be a characteristic behavior of SN-driven, multiphase turbulence.

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arXiv2022

Fast methods to track grain coagulation and ionization. II. Extension to thermal ionization

Thermal ionization is a critical process at temperatures T > 10 3 K, particularly during star formation. An increase in ionization leads to a decrease in nonideal magnetohydrodynamics (MHD) resistivities, which has a significant impact on protoplanetary disks and protostar formation. We developed an extension of the fast computational ionization method presented in our recent paper to include thermal ionization. The model can be used to inexpensively calculate the density of ions and electrons and the electric charge of each size of grains for an arbitrary size distribution. This tool should be particularly useful for the self-consistent calculation of nonideal MHD resistivities in multidimensional simulations, especially of protostellar collapse and protoplanetary disks.

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arXiv2022

Anisotropic Infall and Substructure formation in Embedded Disks

The filamentary nature of accretion streams found around embedded sources suggest that protostellar disks experience heterogenous infall from the star-forming environment, consistent with the accretion behavior onto star-forming cores in top-down star-cluster formation simulations. This may produce disk substructures in the form of rings, gaps, and spirals continuing to be identified by high-resolution imaging surveys in both embedded Class 0/I and later Class II sources. We present a parameter study of anisotropic infall, informed by the properties of accretion flows onto protostellar cores in numerical simulations, and varying the relative specific angular momentum of incoming flows as well as their flow geometry. Our results show that anisotropic infall perturbs the disk and readily launches the Rossby wave instability (RWI). It forms vortices at the inner and outer edge of the infall zone where material is deposited. These vortices drive spiral waves and angular momentum transport, with some models able to drive stresses corresponding to a viscosity parameter on the order of $α\sim 10^{-2}$. The resulting azimuthal shear forms robust pressure bumps that act as barriers to radial drift of dust grains, as demonstrated by post-processing calculations of drift-dominated dust evolution. We discuss how a self-consistent model of anisotropic infall can account for the formation of millimeter rings in the outer disk as well as producing compact dust disks, consistent with observations of embedded sources.

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arXiv2021

SIGAME v3: Gas Fragmentation in Post-processing of Cosmological Simulations for More Accurate Infrared Line Emission Modeling

We present an update to the framework called SImulator of GAlaxy Millimeter/submillimeter Emission (SÍGAME). SÍGAME derives line emission in the far-infrared (FIR) for galaxies in particle-based cosmological hydrodynamics simulations by applying radiative transfer and physics recipes via a post-processing step after completion of the simulation. In this version, a new technique is developed to model higher gas densities by parametrizing the gas density probability distribution function (PDF) in higher resolution simulations for use as a look-up table, allowing for more adaptive PDFs than in previous work. SÍGAME v3 is tested on redshift z = 0 galaxies drawn from the SIMBA cosmological simulation for eight FIR emission lines tracing vastly different interstellar medium phases. Including dust radiative transfer with SKIRT and high resolution photo-ionization models with Cloudy, this new method is able to self-consistently reproduce observed relations between line luminosity and star formation rate in all cases, except for [NII]122, [NII]205 and [OI]63, the luminosities of which are overestimated by median factors of 1.6, 1.2 and 1.2 dex, respectively. We attribute the remaining disagreement with observations to the lack of precise attenuation of the interstellar light on subgrid scales (<200 pc).

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arXiv2021

Gravity Versus Magnetic Fields in Forming Molecular Clouds

Magnetic fields are dynamically important in the diffuse interstellar medium. Understanding how gravitationally bound, star-forming clouds form requires modeling of the fields in a self-consistent, supernova-driven, turbulent, magnetized, stratified disk. We employ the FLASH magnetohydrodynamics code to follow the formation and early evolution of clouds with final masses of 3-8 $\times 10^3 M_{\odot}$ within such a simulation. We use the code's adaptive mesh refinement capabilities to concentrate numerical resolution in zoom-in regions covering single clouds, allowing us to investigate the detailed dynamics and field structure of individual self-gravitating clouds in a consistent background medium. Our goal is to test the hypothesis that dense clouds are dynamically evolving objects far from magnetohydrostatic equilibrium. We find that the cloud envelopes are magnetically supported with field lines parallel to density gradients and flow velocity, as indicated by the histogram of relative orientations and other statistical measures. In contrast, the dense cores of the clouds are gravitationally dominated, with gravitational energy exceeding internal, kinetic, or magnetic energy and accelerations due to gravity exceeding those due to magnetic or thermal pressure gradients. In these regions field directions vary strongly, with a slight preference towards being perpendicular to density gradients, as shown by three-dimensional histograms of relative orientation.

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arXiv2021

Symmetry Breaking in Dynamical Encounters in the Disks of Active Galactic Nuclei

Active galactic nucleus (AGN) disks may be important sites of binary black hole (BBH) mergers. Here we show via numerical experiments with the high-accuracy, high precision code {\tt SpaceHub} that broken symmetry in dynamical encounters in AGN disks can lead to an asymmetry between prograde and retrograde BBH mergers. The direction of the hardening asymmetry depends on the initial binary semi-major axis. An asymmetric distribution of mass-weighted projected spin $χ_{\rm eff}$ should therefore be expected in LIGO-Virgo detections of BBH mergers from AGN disks. This channel further predicts that negative $χ_{\rm eff}$ BBH mergers are most likely for massive binaries.

astro-ph.HE↗

arXiv2021

Simulations of the star-forming molecular gas in an interacting M51-like galaxy: cloud population statistics

To investigate how molecular clouds react to different environmental conditions at a galactic scale, we present a catalogue of giant molecular clouds resolved down to masses of $\sim 10$~M$_{\odot}$ from a simulation of the entire disc of an interacting M51-like galaxy and a comparable isolated galaxy. Our model includes time-dependent gas chemistry, sink particles for star formation and supernova feedback, meaning we are not reliant on star formation recipes based on threshold densities and can follow the physics of the cold molecular phase. We extract giant molecular clouds at a given timestep of the simulations and analyse their properties. In the disc of our simulated galaxies, spiral arms seem to act merely as snowplows, gathering gas and clouds without dramatically affecting their properties. In the centre of the galaxy, on the other hand, environmental conditions lead to larger, more massive clouds. While the galaxy interaction has little effect on cloud masses and sizes, it does promote the formation of counter-rotating clouds. We find that the identified clouds seem to be largely gravitationally unbound at first glance, but a closer analysis of the hierarchical structure of the molecular interstellar medium shows that there is a large range of virial parameters with a smooth transition from unbound to mostly bound for the densest structures. The common observation that clouds appear to be virialised entities may therefore be due to CO bright emission highlighting a specific level in this hierarchical binding sequence. The small fraction of gravitationally bound structures found suggests that low galactic star formation efficiencies may be set by the process of cloud formation and initial collapse.

astro-ph.GA↗

arXiv2021

Fast methods for tracking grain coagulation and ionization. I. Analytic derivation

Dust grains play a major role in many astrophysical contexts. They affect the chemical, magnetic, dynamical, and optical properties of their environment, from galaxies down to the interstellar medium, star-forming regions, and protoplanetary disks. Their coagulation leads to shifts in their size distribution and ultimately to the formation of planets. However, although the coagulation process is reasonably uncomplicated to numerically implement by itself, it is difficult to couple it with multidimensional hydrodynamics numerical simulations because of its high computational cost. We propose here a simple method for tracking the coagulation of grains at far lower cost. Given an initial grain size distribution, the state of the distribution at time t is solely determined by the value of a single variable integrated along the trajectory, independently of the specific path taken by the grains. Although this method cannot account for processes other than coagulation, it is mathematically exact, fast, inexpensive, and can be used to evaluate the effect of grain coagulation in most astrophysical contexts. It is applicable to all coagulation kernels in which local physical conditions and grain properties can be separated. We also describe another method for calculating the average electric charge of grains and the density of ions and electrons in environments that are shielded from radiation fields, given the density and temperature of the gas, the cosmic-ray ionization rate, and the average mass of the ions. The equations we provide are fast to integrate numerically and can be used in multidimensional numerical simulations to self-consistently calculate on the fly the local resistivities that are required to model nonideal magnetohydrodynamics.

astro-ph.GA↗

arXiv2021

Small-Scale Dynamo in Supernova-Driven Interstellar Turbulence

Magnetic fields grow quickly even at early cosmological times, suggesting the action of a small-scale dynamo (SSD) in the interstellar medium of galaxies. Many studies have focused on idealized turbulent driving of the SSD. Here we simulate more realistic supernova-driven turbulence to determine whether it can drive an SSD. Magnetic field growth occurring in our models appears inconsistent with simple tangling of magnetic fields, but consistent with SSD action, reproducing and confirming models by Balsara et al. (2004) that did not include physical resistivity $η$. We vary $η$, as well as the numerical resolution and supernova rate, $\dotσ$, to delineate the regime in which an SSD occurs. For a given $\dotσ$ we find convergence for SSD growth rate with resolution of a parsec. For $\dotσ\simeq\dotσ_{\rm sn}$, with $\dotσ_{\rm sn}$ the solar neighbourhood rate, the critical resistivity below which an SSD occurs is $0.005>η_{\rm crit}>0.001\,\rm kpc^{-1}\,\rm km s^{-1}$, and this increases with the supernova rate. Across the modelled range of 0.5--4 pc resolution we find that for $η<η_{\rm crit}$, the SSD saturates at about 5% of kinetic energy equipartition, independent of growth rate. In the range $0.2\dotσ_{\rm sn}\leq \dotσ\leq8\dotσ_{\rm sn}$ growth rate increases with $\dotσ$. SSDs in the supernova-driven interstellar medium commonly exhibit erratic growth.

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arXiv2020

Implementing Primordial Binaries in Simulations of Star Cluster Formation with a Hybrid MHD and Direct N-Body Method

The fraction of stars in binary systems within star clusters is important for their evolution, but what proportion of binaries form by dynamical processes after initial stellar accretion remains unknown. In previous work, we showed that dynamical interactions alone produced too few low-mass binaries compared to observations. We therefore implement an initial population of binaries in the coupled MHD and direct N-body star cluster formation code Torch. We compare simulations with, and without, initial binary populations and follow the dynamical evolution of the binary population in both sets of simulations, finding that both dynamical formation and destruction of binaries take place. Even in the first few million years of star formation, we find that an initial population of binaries is needed at all masses to reproduce observed binary fractions for binaries with mass ratios above the $q \geq 0.1$ detection limit. Our simulations also indicate that dynamical interactions in the presence of gas during cluster formation modify the initial distributions towards binaries with smaller primary masses, larger mass ratios, smaller semi-major axes and larger eccentricities. Systems formed dynamically do not have the same properties as the initial systems, and systems formed dynamically in the presence of an initial population of binaries differ from those formed in simulations with single stars only. Dynamical interactions during the earliest stages of star cluster formation are important for determining the properties of binary star systems.

astro-ph.SR↗

arXiv2020

Modelling of the Effects of Stellar Feedback during Star Cluster Formation Using a Hybrid Gas and N-Body Method

Understanding the formation of stellar clusters requires following the interplay between gas and newly formed stars accurately. We therefore couple the magnetohydrodynamics code FLASH to the N-body code ph4 and the stellar evolution code SeBa using the Astrophysical Multipurpose Software Environment (AMUSE) to model stellar dynamics, evolution, and collisional N-body dynamics and the formation of binary and higher-order multiple systems, while implementing stellar feedback in the form of radiation, stellar winds and supernovae in FLASH. We here describe the algorithms used for each of these processes. We denote this integrated package Torch. We then use this novel numerical method to simulate the formation and early evolution of several examples of open clusters of ~1000 stars formed from clouds with a mass range of 10^3-10^5 M_sun. Analyzing the effects of stellar feedback on the gas and stars of the natal clusters, we find that in these examples, the stellar clusters are resilient to disruption, even in the presence of intense feedback. This can even slightly increase the amount of dense, Jeans unstable gas by sweeping up shells; thus, a stellar wind strong enough to trap its own H II region shows modest triggering of star formation. Our clusters are born moderately mass segregated, an effect enhanced by feedback, and retained after the ejection of their natal gas, in agreement with observations.

astro-ph.SR↗

arXiv2020

Orbital Migration of Interacting Stellar Mass Black Holes in Disks around Supermassive Black Holes II. Spins and Incoming Objects

The masses, rates, and spins of merging stellar-mass binary black holes (BBHs) detected by aLIGO and Virgo provide challenges to traditional BBH formation and merger scenarios. An active galactic nucleus (AGN) disk provides a promising additional merger channel, because of the powerful influence of the gas that drives orbital evolution, makes encounters dissipative, and leads to migration. Previous work showed that stellar mass black holes (sBHs) in an AGN disk migrate to regions of the disk, known as migration traps, where positive and negative gas torques cancel out, leading to frequent BBH formation. Here we build on that work by simulating the evolution of additional sBHs that enter the inner disk by either migration or inclination reduction. We also examine whether the BBHs formed in our models have retrograde or prograde orbits around their centers of mass with respect to the disk, determining the orientation, relative to the disk, of the spin of the merged BBHs. Orbiters entering the inner disk form BBHs with sBHs on resonant orbits near the migration trap. When these sBHs reach ~80 Msun, they form BBHs with sBHs in the migration trap, which over 10 Myr reach ~1000 Msun. We find 68% of the BBHs in our simulation orbit in the retrograde direction, which implies BBHs in our merger channel will have small dimensionless aligned spins, χ_eff. Overall, our models produce BBHs that resemble both the majority of BBH mergers detected thus far (0.66 to 120 Gpc^-3 yr^-1 ) and two recent unusual detections, GW190412 (~0.3 Gpc^-3 yr^-1 ) and GW190521 (~0.1 Gpc^-3 yr^-1 ).

astro-ph.HE↗

arXiv2020

Origin of Weak MgII and Higher Ionization Absorption Lines in an Outflow from an Intermediate-Redshift Dwarf Satellite Galaxy

Observations at intermediate redshifts reveal the presence of numerous, compact, weak MgII absorbers with near to super-solar metallicities, often surrounded by more extended regions that produce CIV and/or OVI absorption in the circumgalactic medium at large impact parameters from luminous galaxies. Their origin and nature remains unclear. We hypothesize that undetected, satellite dwarf galaxies are responsible for producing some of these weak MgII absorbers. We test our hypothesis using gas dynamical simulations of galactic outflows from a dwarf satellite galaxy with a halo mass of $5\times10^{9}$ M$_{\odot}$, which could form in a larger $L^{*}$ halo at z=2, to study the gas interaction in the halo. We find that thin, filamentary, weak MgII absorbers are produced in two stages: 1) when shocked core collapse supernova (SNII) enriched gas descending in a galactic fountain gets shock compressed by upward flows driven by subsequent SNIIs and cools (phase 1), and later, 2) during an outflow driven by Type Ia supernovae that shocks and sweeps up pervasive SNII enriched gas, which then cools (phase 2). The width of the filaments and fragments are $\lesssim~100$ pc, and the smallest ones cannot be resolved at 12.8 pc resolution. The MgII absorbers in our simulations are continuously generated for >150 Myr by shocks and cooling, though each cloud survives for only ~60 Myr. Their metallicity is 10-20% solar metallicity and column density is $<10^{12}$ cm$^{-2}$. They are also surrounded by larger (0.5-1 kpc) CIV absorbers that seem to survive longer. In addition, larger-scale (>1 kpc) CIV and OVI clouds are produced in both expanding and shocked SNII enriched gas which is photoionized by the UV metagalactic radiation at intermediate redshift. Our simulation highlights the possibility of dwarf galactic outflows producing highly enriched multiphase gas.

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arXiv2020

The Dynamics, Destruction, and Survival of Supernova-Formed Dust Grains

Observations have demonstrated that supernovae efficiently produce dust. This is consistent with the hypothesis that supernovae and asymptotic giant branch stars are the primary producers of dust in the Universe. However, there has been a longstanding question of how much of the dust detected in the interiors of young supernova remnants can escape into the interstellar medium. We present new hydrodynamical calculations of the evolution of dust grains that were formed in dense ejecta clumps within a Cas A-like remnant. We follow the dynamics of the grains as they decouple from the gas after their clump is hit by the reverse shock. They are subsequently subject to destruction by thermal and kinetic sputtering as they traverse the remnant. Grains that are large enough ($\sim 0.25\,μ$m for silicates and $\sim 0.1\,μ$m for carbonaceous grains) escape into the interstellar medium while smaller grains get trapped and destroyed. However, grains that reach the interstellar medium still have high velocities, and are subject to further destruction as they are slowed down. We find that for initial grain size distributions that include large ($\sim 0.25 - 0.5\,μ$m) grains, 10--20\% of silicate grains can survive, while 30--50\% of carbonaceous grains survive even when the initial size distribution cuts off at smaller ($0.25\,μ$m) sizes. For a 19 M$_{\odot}$ star similar to the progenitor of Cas A, up to 0.1 M$_{\odot}$ of dust can survive if the dust grains formed are large. Thus we show that supernovae under the right conditions can be significant sources of interstellar dust.

astro-ph.HE↗

arXiv2020

The Role of Stellar Feedback in the Chemical Evolution of a Low Mass Dwarf Galaxy

We investigate how each aspect of a multi-channel stellar feedback model drives the chemodynamical evolution of a low-mass, isolated dwarf galaxy using a suite of high-resolution simulations. Our model follows individual star particles sampled randomly from an adopted initial mass function, considering independently feedback from: supernovae; stellar radiation causing photoelectric heating of dust grains, ionization and associated heating, Lyman-Werner (LW) dissociation of H$_2$, and radiation pressure; and winds from massive main sequence (neglecting their energy input) and asymptotic giant branch (AGB) stars. Radiative transfer is done by ray tracing. We consider the effects each of these processes have on regulating the star formation rate, global properties, multi-phase interstellar medium (ISM), and driving of galactic winds. We follow individual metal species from distinct nucleosynthetic enrichment channels (AGB winds, massive star stellar winds, core collapse and Type Ia supernovae) and pay particular attention to how these feedback processes regulate metal mixing in the ISM, the metal content of outflows, and the stellar abundance patterns in our galaxy. We find that---for a low-metallicity, low-mass dwarf galaxy ---stellar radiation, particularly ionizing radiation and LW radiation, are important sources of stellar feedback whose effects dominate over photoelectric heating and HI radiation pressure. However, feedback is coupled non-linearly, and the inclusion or exclusion of each process produces non-negligible effects. We find strong variations with: the star formation history; the ejection fractions of metals, mass, and energy; and the distribution of elements from different nucleosynthetic sources in both the gas and stars.

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