arXiv subjects

Mark R. Krumholz

Publications and source records attributed to Mark R. Krumholz.

At least 217 records · Page 12Linked to original sources

arXiv2013

Line Emission from Radiation-Pressurized HII Regions I: Internal Structure and Line Ratios

The emission line ratios [OIII]5007/H-beta and [NII]6584/H-alpha have been adopted as an empirical way to distinguish between the fundamentally different mechanisms of ionization in emission-line galaxies. However, detailed interpretation of these diagnostics requires calculations of the internal structure of the emitting HII regions, and these calculations depend on the assumptions one makes about the relative importance of radiation pressure and stellar winds. In this paper we construct a grid of quasi-static HII region models to explore how choices about these parameters alter HII regions' emission line ratios. We find that, when radiation pressure is included in our models, HII regions reach a saturation point beyond which further increases in the luminosity of the driving stars does not produce any further increase in effective ionization parameter, and thus does not yield any further alteration in an HII region's line ratio. We also show that, if stellar winds are assumed to be strong, the maximum possible ionization parameter is quite low. As a result of this effect, it is inconsistent to simultaneously assume that HII regions are wind-blown bubbles and that they have high ionization parameters; some popular HII region models suffer from this inconsistency. Our work in this paper provides a foundation for a companion paper in which we embed the model grids we compute here within a population synthesis code that enables us to compute the integrated line emission from galactic populations of HII regions.

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arXiv2013

Line Emission from Radiation-Pressurized HII Region II: Dynamics and Population Synthesis

Optical and infrared emission lines from HII regions are an important diagnostic used to study galaxies, but interpretation of these lines requires significant modeling of both the internal structure and dynamical evolution of the emitting regions. Most of the models in common use today assume that HII region dynamics are dominated by the expansion of stellar wind bubbles, and have neglected the contribution of radiation pressure to the dynamics, and in some cases also to the internal structure. However, recent observations of nearby galaxies suggest that neither assumption is justified, motivating us to revisit the question of how HII region line emission depends on the physics of winds and radiation pressure. In a companion paper we construct models of single HII regions including and excluding radiation pressure and winds, and in this paper we describe a population synthesis code that uses these models to simulate galactic collections of HII regions with varying physical parameters. We show that the choice of physical parameters has significant effects on galactic emission line ratios, and that in some cases the line ratios can exceed previously claimed theoretical limits. Our results suggest that the recently-reported offset in line ratio values between high-redshift star-forming galaxies and those in the local universe may be partially explained by the presence of large numbers of radiation pressured-dominated HII regions within them.

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arXiv2013

Steady Outflows in Giant Clumps of High-z Disk Galaxies During Migration and Growth by Accretion

We predict the evolution of giant clumps undergoing star-driven outflows in high-z gravitationally unstable disk galaxies. We find that the mass loss is expected to occur through a steady wind over many tens of free-fall times (t_ff ~ 10 Myr) rather than by an explosive disruption in one or a few t_ff. Our analysis is based on the finding from simulations that radiation trapping is negligible because it destabilizes the wind (Krumholz & Thompson 2012, 2013). Each photon can therefore contribute to the wind momentum only once, so the radiative force is limited to L/c. When combining radiation, protostellar and main-sequence winds, and supernovae, we estimate the total direct injection rate of momentum into the outflow to be 2.5 L/c. The adiabatic phase of supernovae and main-sequence winds can double this rate. The resulting outflow mass-loading factor is of order unity, and if the clumps were to deplete their gas the timescale would have been a few disk orbital times, to end with half the original clump mass in stars. However, the clump migration time to the disk center is on the order of an orbital time, about 250 Myr, so the clumps are expected to complete their migration prior to depletion. Furthermore, the clumps are expected to double their mass in a disk orbital time by accretion from the disk and clump-clump mergers, so their mass actually grows in time and with decreasing radius. From the 6-7 giant clumps with observed outflows, 5 are consistent with these predictions, and one has a much higher mass-loading factor and momentum injection rate. The latter either indicates that the estimated outflow is an overestimate (within the 1-sigma error), that the SFR has dropped since the time when the outflow was launched, or that the driving mechanism is different, e.g. supernova feedback in a cavity generated by the other feedbacks.

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arXiv2013

Very Massive Stars (VMS) in the Local Universe

Recent studies have claimed the existence of very massive stars (VMS) up to 300 solar masses in the local Universe. As this finding may represent a paradigm shift for the canonical stellar upper-mass limit of 150 Msun, it is timely to discuss the status of the data, as well as the far-reaching implications of such objects. We held a Joint Discussion at the General Assembly in Beijing to discuss (i) the determination of the current masses of the most massive stars, (ii) the formation of VMS, (iii) their mass loss, and (iv) their evolution and final fate. The prime aim was to reach broad consensus between observers and theorists on how to identify and quantify the dominant physical processes.

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arXiv2013

An IMF Study of the Dwarf Starburst Galaxy NGC 4214

The production rate of ionizing photons in young < 8 Myr, unresolved stellar clusters in the nearby irregular galaxy NGC 4214 is probed using multi-wavelength Hubble Space Telescope WFC3 data. We normalize the ionizing photon rate by the cluster mass to investigate the upper end of the stellar initial mass function (IMF). We have found that within the uncertainties the upper end of the stellar IMF appears to be universal in this galaxy, and that deviations from a universal IMF can be attributed to stochastic sampling of stars in clusters with masses < 10^3 M_sun. Furthermore, we have found that there does not seem to be a dependence of the maximum stellar mass on the cluster mass. We have also found that for massive clusters, feedback may cause an underrepresentation in Ha luminosities, which needs to be taken into account when conducting this type of analysis.

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arXiv2013

The Fragmentation of Magnetized, Massive Star-Forming Cores with Radiative Feedback

We present a set of 3-dimensional, radiation-magnetohydrodynamic calculations of the gravitational collapse of massive (300 Msun), star-forming molecular cloud cores. We show that the combined effects of magnetic fields and radiative feedback strongly suppress core fragmentation, leading to the production of single star systems rather than small clusters. We find that the two processes are efficient at suppressing fragmentation in different regimes, with the feedback most effective in the dense, central region and the magnetic field most effective in more diffuse, outer regions. Thus, the combination of the two is much more effective at suppressing fragmentation than either one considered in isolation. Our work suggests that typical massive cores, which have mass-to-flux ratios of about 2 relative to critical, likely form a single star system, but that cores with weaker fields may form a small star cluster. This result helps us understand why the observed relationship between the core mass function and the stellar initial mass function holds even for ~100 Msun cores with many thermal Jeans masses of material. We also demonstrate that a ~40 AU Keplerian disk is able to form in our simulations, despite the braking effect caused by the strong magnetic field.

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arXiv2013

Protostellar Disk Formation Enabled by Weak, Misaligned Magnetic Fields

The gas from which stars form is magnetized, and strong magnetic fields can efficiently transport angular momentum. Most theoretical models of this phenomenon find that it should prevent formation of large (>100 AU), rotationally-supported disks around most protostars, even when non-ideal magnetohydrodynamic (MHD) effects that allow the field and gas to decouple are taken into account. Using recent observations of magnetic field strengths and orientations in protostellar cores, we show that this conclusion is incorrect. The distribution of magnetic field strengths is very broad, and alignments between fields and angular momentum vectors within protostellar cores are essentially random. By combining the field strength and misalignment data with MHD simulations showing that disk formation is expected for both weak and misaligned fields, we show that these observations imply that we should expect disk fractions of ~10 - 50% even when protostars are still deeply embedded in their parent cores, and even if the gas is governed by ideal MHD.

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arXiv2012

Direct Numerical Simulation of Radiation Pressure-Driven Turbulence and Winds in Star Clusters and Galactic Disks

[abridged] The pressure exerted by the radiation of young stars may be an important feedback mechanism in forming star clusters and the disks of starburst galaxies. However, there is great uncertainty in how efficiently radiation couples to matter in these high optical depth environments. In particular, it is unclear what levels of turbulence the radiation can produce, and whether the infrared radiation trapped by the dust opacity can give rise to heavily mass-loaded winds. 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. We consider both systems where the radiation flux is sub-Eddington throughout the gas column, and where it is super-Eddington at the midplane but sub-Eddington in the atmosphere. In the latter, we find that the radiation-matter interaction gives rise to radiation-driven Rayleigh-Taylor instability, which drives supersonic turbulence at a level sufficient to fully explain the turbulence seen in Galactic protocluster gas clouds, and to make a non-trivial contribution to the turbulence observed in starburst galaxy disks. However, the instability also produces a channel structure in which the radiation-matter interaction is reduced because the radiation field is not fully trapped. For astrophysical parameters relevant to forming star clusters and starburst galaxies, we find that this effect reduces the net momentum deposition rate in the dusty gas by a factor of ~2-6 compared to simple analytic estimates, and that in steady state the Eddington ratio reaches unity and there are no strong winds. 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.

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arXiv2012

Star Formation in Atomic Gas

Observations of nearby galaxies have firmly established, over a broad range of galactic environments and metallicities, that star formation occurs exclusively in the molecular phase of the interstellar medium (ISM). Theoretical models show that this association results from the correlation between chemical phase, shielding, and temperature. Interstellar gas converts from atomic to molecular only in regions that are well shielded from interstellar ultraviolet (UV) photons, and since UV photons are also the dominant source of interstellar heating, only in these shielded regions does the gas become cold enough to be subject to Jeans instability. However, while the equilibrium temperature and chemical state of interstellar gas are well-correlated, the time scale required to reach chemical equilibrium is much longer than that required to reach thermal equilibrium, and both timescales are metallicity-dependent. Here I show that the difference in time scales implies that, at metallicities below a few percent of the Solar value, well-shielded gas will reach low temperatures and proceed to star formation before the bulk of it is able to convert from atomic to molecular. As a result, at extremely low metallicities, star formation will occur in a cold atomic phase of the ISM rather than a molecular phase. I calculate the observable consequences of this result for star formation in low metallicity galaxies, and I discuss how some current numerical models for H2-regulated star-formation may need to be modified.

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arXiv2012

Radiation-Hydrodynamic Simulations of the Formation of Orion-Like Star Clusters II. The Initial Mass Function from Winds, Turbulence, and Radiation

[abridged] We report a series of simulations of the formation of a star cluster similar to the Orion Nebula Cluster (ONC), including both radiative transfer and protostellar outflows, and starting from both smooth and self-consistently turbulent initial conditions. Each simulation forms >150 stars and brown dwarfs, yielding a stellar mass distribution from < 0.1 to > 10 Msun. We show that a simulation that begins with self-consistently turbulence embedded in a larger turbulent volume, and that includes protostellar outflows, produces an initial mass function (IMF) consistent both with that of the ONC and the Galactic field. This is the first simulation published to date that reproduces the observed IMF in a cluster large enough to contain massive stars, and where the result is determined by a fully self-consistent calculation of gas thermodynamics. This simulation also produces a star formation rate that, while still somewhat too high, is much closer to observed values than if we omit either the larger turbulent volume or the outflows. Moreover, we show that the combination of outflows, self-consistently turbulent initial conditions, and turbulence continually fed by motions on scales larger than that of the protocluster yields an IMF that is in agreement with observations and invariant with time, resolving the "overheating" problem in which simulations without these features have an IMF peak that shifts to progressively higher masses over time. The simulation that matches the observed IMF also reproduces the observed trend of stellar multiplicity strongly increasing with mass. We show that this simulation produces massive stars from distinct massive cores whose properties are consistent with those of observed massive cores. However, the stars formed in these cores also undergo dynamical interactions that naturally produce Trapezium-like hierarchical multiple systems.

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arXiv2012

Was The Sun Born In A Massive Cluster?

A number of authors have argued that the Sun must have been born in a cluster of no more than about 1000 stars, on the basis that, in a larger cluster, close encounters between the Sun and other stars would have truncated the outer Solar System or excited the outer planets into eccentric orbits. However, this dynamical limit is in tension with meteoritic evidence that the Solar System was exposed to a nearby supernova during or shortly after its formation; a 1000-star cluster is much too small for supernova contamination to be likely. In this paper we revisit the dynamical limit in the light of improved observations of the properties of young clusters. We use a series of scattering simulations to measure the velocity-dependent cross-section for disruption of the outer Solar System by stellar encounters, and use this cross-section to compute the probability of a disruptive encounter as a function of birth cluster properties. We find that, contrary to prior work, the probability of disruption is small regardless of the cluster mass, and that it actually decreases rather than increases with cluster mass. Our results differ from prior work for three main reasons: (1) unlike in most previous work, we compute a velocity-dependent cross section and properly integrate over the cluster mass-dependent velocity distribution of incoming stars; (2) we adopt realistically-short cluster lifetimes of a few crossing times, rather than assuming lifetimes of 10 to 100 Myr; and (3) following recent observations, we adopt a mass-independent surface density for embedded clusters, rather than a mass-independent radius as assumed many earlier papers. Our results remove the tension between the dynamical limit and the meteoritic evidence, and suggest that the Sun was born in a massive cluster.

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arXiv2012

Evolving Gravitationally Unstable Disks Over Cosmic Time: Implications For Thick Disk Formation

Observations of disk galaxies at z~2 have demonstrated that turbulence driven by gravitational instability can dominate the energetics of the disk. We present a 1D simulation code, which we have made publicly available, that economically evolves these galaxies from z~2 to z~0 on a single CPU in a matter of minutes, tracking column density, metallicity, and velocity dispersions of gaseous and multiple stellar components. We include an H$_2$ regulated star formation law and the effects of stellar heating by transient spiral structure. We use this code to demonstrate a possible explanation for the existence of a thin and thick disk stellar population and the age-velocity dispersion correlation of stars in the solar neighborhood: the high velocity dispersion of gas in disks at z~2 decreases along with the cosmological accretion rate, while at lower redshift, the dynamically colder gas forms the low velocity dispersion stars of the thin disk.

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arXiv2012

Metallicity-Dependent quenching of Star Formation at High Redshift in Small Galaxies

[abridged] The star formation rates (SFR) of low-metallicity galaxies depend sensitively on the gas metallicity, because metals are crucial to mediating the transition from intermediate-temperature atomic gas to cold molecular gas, a necessary precursor to star formation. We study the impact of this effect on the star formation history of galaxies. We incorporate metallicity-dependent star formation and metal enrichment in a simple model that follows the evolution of a halo main progenitor. Our model shows that including the effect of metallicity leads to suppression of star formation at redshift z>2 in dark halos with masses <~ 10^11 Msun, with the suppression becoming near total for halos below ~10^9.5-10 Msun. We find that at high redshift the SFR cannot catch up with the gas inflow rate (IR), because the SFR is limited by the free-fall time, and because it is suppressed further by a lack of metals. As a result, in each galaxy the SFR is growing in time faster than the IR, and the integrated cosmic SFR density is rising with time. The suppressed in situ SFR at high z makes the growth of stellar mass dominated by ex situ SFR which implies that the specific SFR (sSFR) remains constant with time. The intensely accreted gas at high z is accumulating as an atomic gas reservoir. This provides additional fuel for star formation in 10^10 - 10^12 Msun halos at z ~ 1-3, which allows the SFR to exceed the instantaneous IR, and may enable an even higher outflow rate. At z<1, following the natural decline in IR with time due to the universal expansion, the SFR and sSFR are expected to drop. We specify the expected dependence of sSFR and metallicity on stellar mass and redshift. At a given z, and below a critical mass, these relations are predicted to be flat and rising respectively. Our model predictions qualitatively match some of the puzzling features in the observed star formation history.

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arXiv2012

What Sets the Initial Rotation Rates of Massive Stars?

The physical mechanisms that set the initial rotation rates in massive stars are a crucial unknown in current star formation theory. Observations of young, massive stars provide evidence that they form in a similar fashion to their low-mass counterparts. The magnetic coupling between a star and its accretion disk may be sufficient to spin down low-mass pre-main sequence (PMS) stars to well below breakup at the end stage of their formation when the accretion rate is low. However, we show that these magnetic torques are insufficient to spin down massive PMS stars due to their short formation times and high accretion rates. We develop a model for the angular momentum evolution of stars over a wide range in mass, considering both magnetic and gravitational torques. We find that magnetic torques are unable to spin down either low or high mass stars during the main accretion phase, and that massive stars cannot be spun down significantly by magnetic torques during the end stage of their formation either. Spin-down occurs only if massive stars' disk lifetimes are substantially longer or their magnetic fields are much stronger than current observations suggest.

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arXiv2012

A General Model for the CO-H2 Conversion Factor in Galaxies with Applications to the Star Formation Law

The most common means of converting an observed CO line intensity into a molecular gas mass requires the use of a conversion factor (Xco). While in the Milky Way this quantity does not appear to vary significantly, there is good reason to believe that Xco will depend on the larger-scale galactic environment. Utilising numerical models, we investigate how varying metallicities, gas temperatures and velocity dispersions in galaxies impact the way CO line emission traces the underlying H2 gas mass, and under what circumstances Xco may differ from the Galactic mean value. We find that, due to the combined effects of increased gas temperature and velocity dispersion, Xco is depressed below the Galactic mean in high surface density environments such as ULIRGs. In contrast, in low metallicity environments, Xco tends to be higher than in the Milky Way, due to photodissociation of CO in metal-poor clouds. At higher redshifts, gas-rich discs may have gravitationally unstable clumps which are warm (due to increased star formation) and have elevated velocity dispersions. These discs tend to have Xco values ranging between present-epoch gas-rich mergers and quiescent discs at low-z. This model shows that on average, mergers do have lower Xco values than disc galaxies, though there is significant overlap. Xco varies smoothly with the local conditions within a galaxy, and is not a function of global galaxy morphology. We combine our results to provide a general fitting formula for Xco as a function of CO line intensity and metallicity. We show that replacing the traditional approach of using one constant Xco for starbursts and another for discs with our best-fit function produces star formation laws that are continuous rather than bimodal, and that have significantly reduced scatter.

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arXiv2012

Radiatively Efficient Magnetized Bondi Accretion

We have carried out a numerical study of the effect of large scale magnetic fields on the rate of accretion from a uniform, isothermal gas onto a resistive, stationary point mass. Only mass, not magnetic flux, accretes onto the point mass. The simulations for this study avoid complications arising from boundary conditions by keeping the boundaries far from the accreting object. Our simulations leverage adaptive refinement methodology to attain high spatial fidelity close to the accreting object. Our results are particularly relevant to the problem of star formation from a magnetized molecular cloud in which thermal energy is radiated away on time scales much shorter than the dynamical time scale. Contrary to the adiabatic case, our simulations show convergence toward a finite accretion rate in the limit in which the radius of the accreting object vanishes, regardless of magnetic field strength. For very weak magnetic fields, the accretion rate first approaches the Bondi value and then drops by a factor ~ 2 as magnetic flux builds up near the point mass. For strong magnetic fields, the steady-state accretion rate is reduced by a factor ~ 0.2 β^{1/2} compared to the Bondi value, where βis the ratio of the gas pressure to the magnetic pressure. We give a simple expression for the accretion rate as a function of the magnetic field strength. Approximate analytic results are given in the Appendixes for both time-dependent accretion in the limit of weak magnetic fields and steady-state accretion for the case of strong magnetic fields.

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arXiv2011

A Universal, Local Star Formation Law in Galactic Clouds, Nearby Galaxies, High-Redshift Disks, and Starbursts

[abridged] While observations of Local Group galaxies show a very simple, local star formation law in which the star formation rate per unit area in each patch of a galaxy scales linearly with the molecular gas surface density, recent observations of both Milky Way molecular clouds and high redshift galaxies apparently show a more complicated relationship, in which regions of equal surface density can form stars at quite different rates. These data have been interpreted as implying either that different star formation laws apply in different circumstances, that the star formation law is sensitive to large-scale galaxy properties rather than local properties, or that there are high density thresholds for star formation. Here we collate resolved observations of Milky Way molecular clouds, kpc-scale observations of Local Group galaxies, and unresolved observations of both disk and starburst galaxies in the local universe and at high redshift. We show that all of these data are in fact consistent with a simple, local, volumetric star formation law. The apparent variations stem from the fact that the observed objects have a wide variety of 3D size scales and degrees of internal clumping, so even at fixed gas column density the regions being observed can have wildly varying volume densities. We provide a simple theoretical framework to remove this projection effect, and we show that all the data, from small Solar neighborhood clouds with masses ~10^3 Msun to sub-mm galaxies with masses ~10^11 Msun, fall on a single star formation law in which the SFR is simply ~1% of the molecular gas mass per local free-fall time. In contrast, proposed star formation laws in which the star formation timescale is set by the galactic rotation period or the SFR is linearly proportional to the gas mass above some density threshold fail to match at least some of the data.

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arXiv2011

Evolution of Blister-Type HII Regions in a Magnetized Medium

We use the three-dimensional Athena ionizing radiation-magnetohydrodynamics (IRMHD) code to simulate blister-type HII regions driven by stars on the edge of magnetized gas clouds. We compare these to simulations of spherical HII regions where the star is embedded deep within a cloud, and to non-magnetized simulations of both types, in order to compare their ability to drive turbulence and influence star formation. We find that magnetized blister HII regions can be very efficient at injecting energy into clouds. This is partly a magnetic effect: the magnetic energy added to a cloud by an HII region is comparable to or larger than the kinetic energy, and magnetic fields can also help collimate the ejected gas, increasing its energy yield. As a result of these effects, a blister HII region expanding into a cloud with a magnetic field perpendicular to its edge injects twice as much energy by 5 Myr as a non-magnetized blister HII region driven by a star of the same luminosity. Blister HII regions are also more efficient at injecting kinetic energy than spherical HII regions, due to the recoil provided by escaping gas, but not by as much as predicted by some analytic approximations.

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