arXiv subjects

Harold W. Yorke

Publications and source records attributed to Harold W. Yorke.

At least 19 recordsLinked to original sources

arXiv2020

Makemake + Sedna: A Continuum Radiation Transport and Photoionization Framework for Astrophysical Newtonian Fluid Dynamics

Astrophysical fluid flow studies often encompass a wide range of physical processes to account for the complexity of the system under consideration. In addition to gravity, a proper treatment of thermodynamic processes via continuum radiation transport and/or photoionization is becoming the state of the art. We present a major update of our continuum radiation transport module, MAKEMAKE, and a newly developed module for photoionization, SEDNA, coupled to the magnetohydrodynamics code PLUTO. These extensions are currently not publicly available; access can be granted on a case-by-case basis. We explain the theoretical background of the equations solved, elaborate on the numerical layout, and present a comprehensive test suite for radiation-ionization hydrodynamics. The grid-based radiation and ionization modules support static one-dimensional, two-dimensional, and three-dimensional grids in Cartesian, cylindrical, and spherical coordinates. Each module splits the radiation field into two components, one originating directly from a point source - solved using a ray-tracing scheme - and a diffuse component - solved with a three-dimensional flux-limited diffusion (FLD) solver. The FLD solver for the continuum radiation transport makes use of either the equilibrium one-temperature approach or the linearization two-temperature approach. The FLD solver for the photoionization module enables accounting for the temporal evolution of the radiation field from direct recombination of free electrons into hydrogen's ground state as an alternative to the on-the-spot approximation. A brief overview of completed and ongoing scientific studies is given to explicitly illustrate the multipurpose nature of the numerical framework presented.

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arXiv2018

Science Impacts of the SPHEREx All-Sky Optical to Near-Infrared Spectral Survey II: Report of a Community Workshop on the Scientific Synergies Between the SPHEREx Survey and Other Astronomy Observatories

SPHEREx is a proposed NASA MIDEX mission selected for Phase A study. SPHEREx would carry out the first all-sky spectral survey in the near infrared. At the end of its two-year mission, SPHEREx would obtain 0.75-to-5$μ$m spectra of every 6.2 arcsec pixel on the sky, with spectral resolution R>35 and a 5-$σ$ sensitivity AB$>$19 per spectral/spatial resolution element. More details concerning SPHEREx are available at http://spherex.caltech.edu. The SPHEREx team has proposed three specific science investigations to be carried out with this unique data set: cosmic inflation, interstellar and circumstellar ices, and the extra-galactic background light. Though these three themes are undoubtedly compelling, they are far from exhausting the scientific output of SPHEREx. Indeed, SPHEREx would create a unique all-sky spectral database including spectra of very large numbers of astronomical and solar system targets, including both extended and diffuse sources. These spectra would enable a wide variety of investigations, and the SPHEREx team is dedicated to making the data available to the community to enable these investigations, which we refer to as Legacy Science. To that end, we have sponsored two workshops for the general scientific community to identify the most interesting Legacy Science themes and to ensure that the SPHEREx data products are responsive to their needs. In February of 2016, some 50 scientists from all fields met in Pasadena to develop these themes and to understand their implications for the SPHEREx mission. The 2016 workshop highlighted many synergies between SPHEREx and other contemporaneous astronomical missions, facilities, and databases. Consequently, in January 2018 we convened a second workshop at the Center for Astrophysics in Cambridge to focus specifically on these synergies. This white paper reports on the results of the 2018 SPHEREx workshop.

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arXiv2018

Python Radiative Transfer Emission code (PyRaTE): non-LTE spectral lines simulations

We describe PyRaTE, a new, non-local thermodynamic equilibrium (non-LTE) line radiative transfer code developed specifically for post-processing astrochemical simulations. Population densities are estimated using the escape probability method. When computing the escape probability, the optical depth is calculated towards all directions with density, molecular abundance, temperature and velocity variations all taken into account. A very easy-to-use interface, capable of importing data from simulations outputs performed with all major astrophysical codes, is also developed. The code is written in Python using an `embarrassingly parallel' strategy and can handle all geometries and projection angles. We benchmark the code by comparing our results with those from RADEX (van der Tak et al. 2007) and against analytical solutions and present case studies using hydrochemical simulations. The code is available on GitHub (https://github.com/ArisTr/PyRaTE).

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arXiv2016

Protostellar Outflows and Radiative Feedback from Massive Stars. II. Feedback, Star Formation Efficiency, and Outflow Broadening

We perform two-dimensional axially symmetric radiation-hydrodynamic simulations to assess the impact of outflows and radiative force feedback from massive protostars by varying when the protostellar outflow starts, the ratio of ejection to accretion rates, and the strength of the wide angle disk wind component. The star formation efficiency, i.e. the ratio of final stellar mass to initial core mass, is dominated by radiative forces and the ratio of outflow to accretion rates. Increasing this ratio has three effects: First, the protostar grows slower with a lower luminosity at any given time, lowering radiative feedback. Second, bipolar cavities cleared by the outflow are larger, further diminishing radiative feedback on disk and core scales. Third, the higher momentum outflow sweeps up more material from the collapsing envelope, decreasing the protostar's potential mass reservoir via entrainment. The star formation efficiency varies with the ratio of ejection to accretion rates from 50% in the case of very weak outflows to as low as 20% for very strong outflows. At latitudes between the low density bipolar cavity and the high density accretion disk, wide angle disk winds remove some of the gas, which otherwise would be part of the accretion flow onto the disk; varying the strength of these wide angle disk winds, however, alters the final star formation efficiency by only +/-6%. For all cases, the opening angle of the bipolar outflow cavity remains below 20 degree during early protostellar accretion phases, increasing rapidly up to 65 degree at the onset of radiation pressure feedback.

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arXiv2016

Formation of Massive Primordial Stars: Intermittent UV Feedback with Episodic Mass Accretion

We present coupled stellar evolution (SE) and 3D radiation-hydrodynamic (RHD) simulations of the evolution of primordial protostars, their immediate environment, and the dynamic accretion history under the influence of stellar ionizing and dissociating UV feedback. Our coupled SE-RHD calculations result in a wide diversity of final stellar masses covering 10 Msun $\lesssim M_* \lesssim$ 1000 Msun. The formation of very massive ($\gtrsim$ 250 Msun) stars is possible under weak UV feedback, whereas ordinary massive (a few x 10 Msun) stars form when UV feedback can efficiently halt the accretion. This may explain the peculiar abundance pattern of a Galactic metal-poor star recently reported by Aoki et al. (2014), possibly the observational signature of very massive precursor primordial stars. Weak UV feedback occurs in cases of variable accretion, in particular when repeated short accretion bursts temporarily exceed 0.01 Msun/yr, causing the protostar to inflate. In the bloated state, the protostar has low surface temperature and UV feedback is suppressed until the star eventually contracts, on a thermal adjustment timescale, to create an HII region. If the delay time between successive accretion bursts is sufficiently short, the protostar remains bloated for extended periods, initiating at most only short periods of UV feedback. Disk fragmentation does not necessarily reduce the final stellar mass. Quite the contrary, we find that disk fragmentation enhances episodic accretion as many fragments migrate inward and are accreted onto the star, thus allowing continued stellar mass growth under conditions of intermittent UV feedback. This trend becomes more prominent as we improve the resolution of our simulations. We argue that simulations with significantly higher resolution than reported previously are needed to derive accurate gas mass accretion rates onto primordial protostars.

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arXiv2015

Supermassive star formation via episodic accretion: protostellar disc instability and radiative feedback efficiency

The formation of SMSs is a potential pathway to seed SMBHs in the early universe. A critical issue for forming SMSs is stellar UV feedback, which may limit the stellar mass growth via accretion. In this paper we study the evolution of an accreting SMS and its UV emissivity under conditions of realistic variable accretion from a self-gravitating circumstellar disc. First we conduct a 2D hydrodynamical simulation to follow the long-term protostellar accretion until the stellar mass exceeds $10^4~M_\odot$. The disc fragments due to gravitational instability, creating a number of small clumps that rapidly migrate inward to fall onto the star. The resulting accretion history is thus highly time-dependent: short episodic accretion bursts are followed by longer, relative quiescent phases. We show that the circumstellar disc for the so-called direct collapse model is more unstable and generates greater variability over shorter timescales than normal Pop III cases. We conduct a post-process stellar evolution calculation using the obtained accretion history. Our results show that, regardless of the strong variability of the accretion rates, the stellar radius monotonically increases with almost constant effective temperature at $T_{\rm eff} \simeq 5000$ K as the stellar mass increases. The resulting UV feedback is too weak to hinder mass accretion due to the low flux of stellar UV photons, thus verifying our implicit assumption of no stellar feedback during the hydrodynamic simulations. The insensitivity of stellar evolution to variable accretion is attributed to the fact that typical timescales of variability, $\lesssim 10^3$ years, are too short to affect the stellar structure. We argue that this evolution will continue until the SMS eventually collapses to produce a massive black hole by the general relativistic instability after the stellar mass reaches $\gtrsim 10^5~M_\odot$.

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arXiv2015

Early structure formation from primordial density fluctuations with a blue-tilted power spectrum

While observations of large-scale structure and the cosmic microwave background (CMB) provide strong constraints on the amplitude of the primordial power spectrum (PPS) on scales larger than 10~Mpc, the amplitude of the power spectrum on sub-galactic length scales is much more poorly constrained. We study early structure formation in a cosmological model with a blue-tilted PPS. We assume that the standard scale-invariant PPS is modified at small length scales as $P(k) \sim k^{m_{\rm s}}$ with $m_{\rm s} > 1$. We run a series of cosmological hydrodynamic simulations to examine the dependence of the formation epoch and the characteristic mass of primordial stars on the tilt of the PPS. In models with $m_{\rm s} > 1$, star-forming gas clouds are formed at $z > 100$, when formation of hydrogen molecules is inefficient because the intense CMB radiation destroys chemical intermediates. Without efficient coolant, the gas clouds gravitationally contract while keeping a high temperature. The protostars formed in such "hot" clouds grow very rapidly by accretion to become extremely massive stars that may leave massive black holes with a few hundred solar-masses at $z > 100$. The shape of the PPS critically affects the properties and the formation epoch of the first generation of stars. Future experiments of the CMB polarization and the spectrum distortion may provide important information on the nature of the first stars and their formation epoch, and hence on the shape of the small-scale power spectrum.

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arXiv2015

Primordial Star Formation under the Influence of Far Ultraviolet Radiation: 1540 Cosmological Halos and the Stellar Mass Distribution

We perform a large set of cosmological simulations of early structure formation and follow the formation and evolution of 1540 star-forming gas clouds to derive the mass distribution of primordial stars. The star formation in our cosmological simulations is characterized by two distinct populations, the so-called Population III.1 stars and primordial stars formed under the influence of far ultraviolet (FUV) radiation (Population III.2D stars). In this work, we determine the stellar masses by using the dependences on the physical properties of star-forming cloud and/or the external photodissociating intensity from nearby primordial stars, which are derived from the results of two-dimensional radiation hydrodynamic simulations of protostellar feedback. The characteristic mass of the Pop III stars is found to be a few hundred solar masses at z ~ 25, and it gradually shifts to lower masses with decreasing redshift. At high redshifts z > 20, about half of the star-forming gas clouds are exposed to intense FUV radiation and thus give birth to massive Pop III.2D stars. However, the local FUV radiation by nearby Pop III stars becomes weaker at lower redshifts, when typical Pop III stars have smaller masses and the mean physical separation between the stars becomes large owing to cosmic expansion. Therefore, at z < 20, a large fraction of the primordial gas clouds host Pop III.1 stars. At z =< 15, the Pop III.1 stars are formed in relatively cool gas clouds due to efficient radiative cooling by H_2 and HD molecules; such stars have masses of a few x 10 Msun. Since the stellar evolution and the final fate are determined by the stellar mass, Pop III stars formed at different epochs play different roles in the early universe.

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arXiv2014

Protostellar Outflows and Radiative Feedback from Massive Stars

We carry out radiation hydrodynamical simulations of the formation of massive stars in the super-Eddington regime including both their radiative feedback and protostellar outflows. The calculations start from a prestellar core of dusty gas and continue until the star stops growing. The accretion ends when the remnants of the core are ejected, mostly by the force of the direct stellar radiation in the polar direction and elsewhere by the reradiated thermal infrared radiation. How long the accretion persists depends on whether the protostellar outflows are present. We set the mass outflow rate to 1% of the stellar sink particle's accretion rate. The outflows open a bipolar cavity extending to the core's outer edge, through which the thermal radiation readily escapes. The radiative flux is funneled into the polar directions while the core's collapse proceeds near the equator. The outflow thus extends the "flashlight effect", or anisotropic radiation field, found in previous studies from the few hundred AU scale of the circumstellar disk up to the 0.1 parsec scale of the core. The core's flashlight effect allows core gas to accrete on the disk for longer, in the same way that the disk's flashlight effect allows disk gas to accrete on the star for longer. Thus although the protostellar outflows remove material near the core's poles, causing slower stellar growth over the first few free-fall times, they also enable accretion to go on longer in our calculations. The outflows ultimately lead to stars of somewhat higher mass.

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arXiv2014

One Hundred First Stars : Protostellar Evolution and the Final Masses

We perform a large set of radiation hydrodynamics simulations of primordial star formation in a fully cosmological context. Our statistical sample of 100 First Stars show that the first generation of stars have a wide mass distribution M_popIII = 10 ~ 1000 M_sun. We first run cosmological simulations to generate a set of primordial star-forming gas clouds. We then follow protostar formation in each gas cloud and the subsequent protostellar evolution until the gas mass accretion onto the protostar is halted by stellar radiative feedback. The accretion rates differ significantly among the primordial gas clouds which largely determine the final stellar masses. For low accretion rates the growth of a protostar is self-regulated by radiative feedback effects and the final mass is limited to several tens of solar masses. At high accretion rates the protostar's outer envelope continues to expand and the effective surface temperature remains low; such protostars do not exert strong radiative feedback and can grow in excess to one hundred solar masses. The obtained wide mass range suggests that the first stars play a variety of roles in the early universe, by triggering both core-collapse supernovae and pair-instability supernovae as well as by leaving stellar mass black holes. We find certain correlations between the final stellar mass and the physical properties of the star-forming cloud. These correlations can be used to estimate the mass of the first star from the properties of the parent cloud or of the host halo, without following the detailed protostellar evolution.

astro-ph.CO↗

arXiv2013

Formation of Primordial Supermassive Stars by Rapid Mass Accretion

Supermassive stars (SMSs) forming via very rapid mass accretion (Mdot >~ 0.1 Msun/yr) could be precursors of supermassive black holes observed beyond redshift of about 6. Extending our previous work, we here study the evolution of primordial stars growing under such rapid mass accretion until the stellar mass reaches 10^{4 - 5} Msun. Our stellar evolution calculations show that a star becomes supermassive while passing through the "supergiant protostar" stage, whereby the star has a very bloated envelope and a contracting inner core. The stellar radius increases monotonically with the stellar mass, until =~ 100 AU for M_* >~ 10^4 Msun, after which the star begins to slowly contract. Because of the large radius the effective temperature is always less than 10^4 K during rapid accretion. The accreting material is thus almost completely transparent to the stellar radiation. Only for M_* >~ 10^5 Msun can stellar UV feedback operate and disturb the mass accretion flow. We also examine the pulsation stability of accreting SMSs, showing that the pulsation-driven mass loss does not prevent stellar mass growth. Observational signatures of bloated SMSs should be detectable with future observational facilities such as the James Webb Space Telescope. Our results predict that an inner core of the accreting SMS should suffer from the general relativistic instability soon after the stellar mass exceeds 10^5 Msun. An extremely massive black hole should form after the collapse of the inner core.

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arXiv2013

On the simultaneous evolution of massive protostars and their host cores

Studies of the evolution of massive protostars and the evolution of their host molecular cloud cores are commonly treated as separate problems. However, interdependencies between the two can be significant. Here, we study the simultaneous evolution of massive protostars and their host molecular cores using a multi-dimensional radiation hydrodynamics code that incorporates the effects of the thermal pressure and radiative acceleration feedback of the centrally forming protostar. The evolution of the massive protostar is computed simultaneously using the stellar evolution code STELLAR, modified to include the effects of variable accretion. The interdependencies are studied in three different collapse scenarios. For comparison, stellar evolutionary tracks at constant accretion rates and the evolution of the host cores using pre-computed stellar evolutionary tracks are computed. The resulting interdependencies of the protostellar evolution and the evolution of the environment are extremely diverse and depend on the order of events, in particular the time of circumstellar accretion disk formation with respect to the onset of the bloating phase of the star. Feedback mechanisms affect the instantaneous accretion rate and the protostar's radius, temperature and luminosity on timescales equal or smaller than 5 kyr, corresponding to the accretion timescale and Kelvin-Helmholtz contraction timescale, respectively. Nevertheless, it is possible to approximate the overall protostellar evolution in many cases by pre-computed stellar evolutionary tracks assuming appropriate constant average accretion rates.

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arXiv2012

On the effects of optically thick gas (disks) around massive stars

Numerical simulations have shown that the often cited radiation pressure barrier to accretion onto massive stars can be circumvented, when the radiation field is highly anisotropic in the presence of a circumstellar accretion disk with high optical depth. Here, these studies of the so-called flashlight effect are expanded by including the opacity of the innermost dust-free but potentially optically thick gas regions around forming massive stars. In addition to frequency-dependent opacities for the dust grains, we use temperature- and density-dependent Planck- and Rosseland mean opacities for the gas. The simulations show that the innermost dust-free parts of the accretion disks are optically thick to the stellar radiation over a substantial fraction of the solid angle above and below the disk's midplane. The temperature in the shielded disk region decreases faster with radius than in a comparison simulation with a lower constant gas opacity, and the dust sublimation front is shifted to smaller radii. The shielding by the dust-free gas in the inner disk thus contributes to an enhanced flashlight effect, which ultimately results in a smaller opening angle of the radiation pressure driven outflow and in a much longer timescale of sustained feeding of the circumstellar disk by the molecular cloud core. We conclude that it is necessary to properly account for the opacity of the inner dust-free disk regions around forming massive stars in order to correctly assess the effectiveness of the flashlight effect, the opening angle of radiation pressure driven outflows, and the lifetime and morphological evolution of the accretion disk.

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arXiv2012

Protostellar Feedback and Final Mass of the Second-Generation Primordial Stars

The first stars in the universe ionized the ambient primordial gas through various feedback processes. "Second-generation" primordial stars potentially form from this disturbed gas after its recombination. In this Letter, we study the late formation stage of such second-generation stars, where a large amount of gas accretes onto the protostar and the final stellar mass is determined when the accretion terminates. We directly compute the complex interplay between the accretion flow and stellar ultraviolet (UV) radiation, performing radiation-hydrodynamic simulations coupled with stellar evolution calculations. Because of more efficient H2 and HD cooling in the pre-stellar stage, the accretion rates onto the star are ten times lower than in the case of the formation of the first stars. The lower accretion rates and envelope density result in the occurrence of an expanding bipolar HII region at a lower protostellar mass M_* \simeq 10Msun, which blows out the circumstellar material, thereby quenching the mass supply from the envelope to the accretion disk. At the same time the disk loses mass due to photoevaporation by the growing star. In our fiducial case the stellar UV feedback terminates mass accretion onto the star at M_* \simeq 17Msun. Although the derived masses of the second-generation primordial stars are systematically lower than those of the first generation, the difference is within a factor of only a few. Our results suggest a new scenario, whereby the majority of the primordial stars are born as massive stars with tens of solar masses, regardless of their generations.

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arXiv2012

Non-Equilibrium Chemistry of Dynamically Evolving Prestellar Cores: II. Ionization and Magnetic Field

We study the effect that non-equilibrium chemistry in dynamical models of collapsing molecular cloud cores has on measurements of the magnetic field in these cores, the degree of ionization, and the mean molecular weight of ions. We find that OH and CN, usually used in Zeeman observations of the line-of-sight magnetic field, have an abundance that decreases toward the center of the core much faster than the density increases. As a result, Zeeman observations tend to sample the outer layers of the core and consistently underestimate the core magnetic field. The degree of ionization follows a complicated dependence on the number density at central densities up to 10^5 cm^{-3} for magnetic models and 10^6 cm^{-3} in non-magnetic models. At higher central densities the scaling approaches a power-law with a slope of -0.6 and a normalization which depends on the cosmic-ray ionization rate ζ and the temperature T as (ζT)^1/2. The mean molecular weight of ions is systematically lower than the usually assumed value of 20 - 30, and, at high densities, approaches a value of 3 due to the asymptotic dominance of the H3+ ion. This significantly lower value implies that ambipolar diffusion operates faster.

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arXiv2012

Non-Equilibrium Chemistry of Dynamically Evolving Prestellar Cores: I. Basic Magnetic and Non-Magnetic Models and Parameter Studies

We combine dynamical and non-equilibrium chemical modeling of evolving prestellar molecular cloud cores, and explore the evolution of molecular abundances in the contracting core. We model both magnetic cores, with varying degrees of initial magnetic support, and non-magnetic cores, with varying collapse delay times. We explore, through a parameter study, the competing effects of various model parameters in the evolving molecular abundances, including the elemental C/O ratio, the temperature, and the cosmic-ray ionization rate. We find that different models show their largest quantitative differences at the center of the core, whereas the outer layers, which evolve slower, have abundances which are severely degenerate among different dynamical models. There is a large range of possible abundance values for different models at a fixed evolutionary stage (central density), which demonstrates the large potential of chemical differentiation in prestellar cores. However, degeneracies among different models, compounded with uncertainties induced by other model parameters, make it difficult to discriminate among dynamical models. To address these difficulties, we identify abundance ratios between particular molecules, the measurement of which would have maximal potential for discrimination among the different models examined here. In particular, we find that the ratios between NH3 and CO; NH2 and CO; NH3 and HCO+ are sensitive to the evolutionary timescale, and that the ratio between HCN and OH is sensitive to the C/O ratio. Finally, we demonstrate that measurements of the central deviation (central depletion or enhancement) of abundances of certain molecules are good indicators of the dynamics of the core.

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arXiv2012

Rapidly Accreting Supergiant Protostars: Embryos of Supermassive Black Holes?

Direct collapse of supermassive stars (SMSs) is a possible pathway for generating supermassive black holes in the early universe. It is expected that an SMS could form via very rapid mass accretion with Mdot ~ 0.1 - 1 Msun/yr during the gravitational collapse of an atomic-cooling primordial gas cloud. In this paper we study how stars would evolve under such extreme rapid mass accretion, focusing on the early evolution until the stellar mass reaches 1000 Msun. To this end we numerically calculate the detailed interior structure of accreting stars with primordial element abundances. Our results show that for accretion rates higher than 0.01 Msun/yr, stellar evolution is qualitatively different from that expected at lower rates. While accreting at these high rates the star always has a radius exceeding 100 Rsun, which increases monotonically with the stellar mass. The mass-radius relation for stellar masses exceeding ~ 100 Msun follows the same track with R_* \propto M_*^0.5 in all cases with accretion rates > 0.01 Msun/yr; at a stellar mass of 1000 Msun the radius is about 7000 Rsun (~= 30 AU). With higher accretion rates the onset of hydrogen burning is shifted towards higher stellar masses. In particular, for accretion rates exceeding Mdot > 0.1 Msun/yr, there is no significant hydrogen burning even after 1000 Msun have accreted onto the protostar. Such "supergiant" protostars have effective temperatures as low as Teff ~= 5000 K throughout their evolution and because they hardly emit ionizing photons, they do not create an HII region or significantly heat their immediate surroundings. Thus, radiative feedback is unable to hinder the growth of rapidly accreting stars to masses in excess of 1000 Msun, as long as material is accreted at rates Mdot > 0.01 Msun/yr.

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arXiv2011

Protostellar Feedback Halts the Growth of the First Stars in the Universe

The first stars fundamentally transformed the early universe by emitting the first light and by producing the first heavy elements. These effects were predetermined by the mass distribution of the first stars, which is thought to have been fixed by a complex interplay of gas accretion and protostellar radiation. We performed radiation-hydrodynamics simulations that followed the growth of a primordial protostar through to the early stages as a star with thermo-nuclear burning. The circumstellar accretion disk was evaporated by ultraviolet radiation from the star when its mass was 43 times that of the Sun. Such massive primordial stars, in contrast to the often postulated extremely massive stars, may help explain the fact that there are no signatures of the pair-instability supernovae in abundance patterns of metal-poor stars in our galaxy.

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