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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Impact of baryonic physics on intrinsic alignments

We explore the effects of specific assumptions in the subgrid models of star formation and stellar and AGN feedback on intrinsic alignments of galaxies in cosmological simulations of "MassiveBlack-II" family. Using smaller volume simulations, we explored the parameter space of the subgrid star formation and feedback model and found remarkable robustness of the observable statistical measures to the details of subgrid physics. The one observational probe most sensitive to modeling details is the distribution of misalignment angles. We hypothesize that the amount of angular momentum carried away by the galactic wind is the primary physical quantity that controls the orientation of the stellar distribution. Finally, our results are also consistent with a similar study by the EAGLE simulation team.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

The collapse and three-dimensional explosion of three-dimensional massive-star supernova progenitor models

ABSTRACT The explosion outcome and diagnostics of core-collapse supernovae depend sensitively on the nature of the stellar progenitor, but most studies to date have focused exclusively on one-dimensional, spherically symmetric massive star progenitors. We present some of the first core-collapse supernovae simulations of three-dimensional massive star supernovae progenitors, a 12.5- and a 15-M⊙ model, evolved in three dimensions from collapse to bounce through explosion with the radiation-hydrodynamic code fornax. We compare the results using those starting from three-dimensional progenitors to three-dimensional simulations of spherically symmetric, one-dimensional progenitors of the same mass. We find that the models evolved in three dimensions during the final stages of massive star evolution are more prone to explosion. The turbulence arising in these multidimensional initial models serves as seed turbulence that promotes shock revival. Detection of gravitational waves and neutrinos signals could reveal signatures of pre-bounce turbulence.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

SUPERBURST MODELS FOR NEUTRON STARS WITH HYDROGEN- AND HELIUM-RICH ATMOSPHERES

Superbursts are rare day-long type I X-ray bursts due to carbon flashes on accreting neutron stars in low-mass X-ray binaries. They heat the neutron star envelope such that the burning of accreted hydrogen and helium becomes stable, and the common shorter X-ray bursts are quenched. Short bursts reappear only after the envelope cools down. We study multi-zone one-dimensional models of the neutron star envelope, in which we follow carbon burning during the superburst, and we include hydrogen and helium burning in the atmosphere above. We investigate the cases of both a solar-composition and a helium-rich atmosphere. This allows us to study for the first time a wide variety of thermonuclear burning behavior as well as the transitions between the different regimes in a self-consistent manner. For solar composition, burst quenching ends much sooner than previously expected. This is because of the complex interplay between the 3{alpha}, hot CNO, and CNO breakout reactions. Stable burning of hydrogen and helium transitions via marginally stable burning (mHz quasi-periodic oscillations) to less energetic bursts with short recurrence times. We find a short-lived bursting mode where weaker and stronger bursts alternate. Eventually the bursting behavior changes back to that of the pre-superburst bursts. Because of the scarcity of observations, this transition has not been directly detected after a superburst. Using the MINBAR burst catalog we identify the shortest upper limit on the quenching time for 4U 1636-536, and derive further constraints on the timescale on which bursts return.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

ON THE LAUNCHING AND STRUCTURE OF RADIATIVELY DRIVEN WINDS IN WOLF–RAYET STARS

Hydrostatic models of Wolf–Rayet (WR) stars typically contain low-density outer envelopes that inflate the stellar radii by a factor of several and are capped by a denser shell of gas. Inflated envelopes and density inversions are hallmarks of envelopes that become super-Eddington as they cross the iron-group opacity peak, but these features disappear when mass loss is sufficiently rapid. We re-examine the structures of steady, spherically symmetric wind solutions that cross a sonic point at high optical depth, identifying the physical mechanism through which the outflow affects the stellar structure, and provide an improved analytical estimate for the critical mass-loss rate above which extended structures are erased. Weak-flow solutions below this limit resemble hydrostatic stars even in supersonic zones; however, we infer that these fail to successfully launch optically thick winds. WR envelopes will therefore likely correspond to the strong, compact solutions. We also find that wind solutions with negligible gas pressure are stably stratified at and below the sonic point. This implies that convection is not the source of variability in WR stars, as has been suggested; however, acoustic instabilities provide an alternative explanation. Our solutions are limited to high optical depths by our neglect of Doppler enhancements to the opacity, and do not account for acoustic instabilities at high Eddington factors; yet, they do provide useful insights into WR stellar structures.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Thermal Analysis Evaluations Using the Dry Cask Simulator

Different thermal analysis models were developed to simulate the dry cask simulator (DCS). The DCS is an experiment designed to simulate dry storage of a single boiling water reactor fuel assembly under a variety of heat loads and internal pressures. The DCS was set up and tested in both a vertical and horizontal configuration to determine cladding temperatures in vertical and horizontal dry cask storage systems. The models included a detailed STAR-CCM+ model with the fuel assembly geometry explicitly modeled, a porous STAR-CCM+ model with the fuel assembly geometry modeled as porous media region with calculated effective properties, and a COBRA-SFS model. COBRA-SFS is a thermal-hydraulic code developed for steady-state and transient analysis of multi assembly spent-fuel storage and transportation systems. STAR-CCM+ is a commercial computation fluid dynamics (CFD) code. Both a detailed and porous STARCCM+ model were developed to look at the effective thermal conductivity (keff) approach to modeling a fuel assembly. A keff fuel model is typically model in CFD thermal analyses due to its significantly lower computational costs. The models were run for a combination of low and high canister pressures (100 kPa and 800 kPa) and low and high internal heat loads (0.5 kW and 5 kW). Results from all three models were compared against experimental data taken from the DCS for the peak cladding temperature (PCT) and inlet air mass flow.

Suffield, Sarah R.↗
DOE OSTI2016

PHOTO-REVERBERATION MAPPING OF A PROTOPLANETARY ACCRETION DISK AROUND A T TAURI STAR

Theoretical models and spectroscopic observations of newborn stars suggest that protoplantary disks have an inner “wall” at a distance set by the disk interaction with the star. Around T Tauri stars, the size of this disk hole is expected to be on a 0.1 au scale that is unresolved by current adaptive optics imaging, though some model-dependent constraints have been obtained by near-infrared interferometry. Here we report the first measurement of the inner disk wall around a solar-mass young stellar object, YLW 16B in the ρ Ophiuchi star-forming region, by detecting the light-travel time of the variable radiation from the stellar surface to the disk. Consistent time lags were detected on two nights, when the time series in H (1.6 μ m) and K (2.2 μ m) bands were synchronized while the 4.5 μ m emission lagged by 74.5 ± 3.2 s. Considering the nearly edge-on geometry of the disk, the inner rim should be 0.084 au from the protostar on average, with an error of order 0.01 au. This size is likely larger than the range of magnetospheric truncations and consistent with an optically and geometrically thick disk front at the dust sublimation radius at ∼1500 K. The widths of the cross-correlation functions between the data in different wavebands place possible new constraints on the geometry of the disk.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

THE CORRELATION BETWEEN H{alpha} EMISSION AND VISUAL MAGNITUDE DURING LONG-TERM VARIATIONS IN CLASSICAL Be STARS

H{alpha} equivalent widths and UBV magnitudes are calculated for Be star disk models that grow in size and/or density with time. We show that these simple models are consistent with the known Be star classes of positive and inverse correlations between long-term variations in H{alpha} and V magnitude as identified by Harmanec. We support the conclusion of Harmanec that the distinction is controlled by the inclination of the disk to the line of sight. We demonstrate that the strength of these correlations, particularly those of an inverse correlation where the system becomes fainter as the H{alpha} emission strength grows, is strongly influenced by the scale height of the inner Be star disk and the extent of the gravitational darkening of the central B star due to rapid rotation. This dependence may allow coordinated spectroscopic and photometric observations to better constrain these poorly known Be star parameters.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

THE ROLE OF THE MAGNETOROTATIONAL INSTABILITY IN MASSIVE STARS

The magnetorotational instability (MRI) is key to physics in accretion disks and is widely considered to play some role in massive star core collapse. Models of rotating massive stars naturally develop very strong shear at composition boundaries, a necessary condition for MRI instability, and the MRI is subject to triply diffusive destabilizing effects in radiative regions. We have used the MESA stellar evolution code to compute magnetic effects due to the Spruit-Tayler (ST) mechanism and the MRI, separately and together, in a sample of massive star models. We find that the MRI can be active in the later stages of massive star evolution, leading to mixing effects that are not captured in models that neglect the MRI. The MRI and related magnetorotational effects can move models of given zero-age main sequence mass across ''boundaries'' from degenerate CO cores to degenerate O/Ne/Mg cores and from degenerate O/Ne/Mg cores to iron cores, thus affecting the final evolution and the physics of core collapse. The MRI acting alone can slow the rotation of the inner core in general agreement with the observed ''initial'' rotation rates of pulsars. The MRI analysis suggests that localized fields ∼10{sup 12} G may exist at the boundary of the iron core. With both the ST and MRI mechanisms active in the 20 M {sub ☉} model, we find that the helium shell mixes entirely out into the envelope. Enhanced mixing could yield a population of yellow or even blue supergiant supernova progenitors that would not be standard SN IIP.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

The close binary frequency of Wolf-Rayet stars as a function of metallicity in M31 and M33

Massive star evolutionary models generally predict the correct ratio of WC-type and WN-type Wolf-Rayet stars at low metallicities, but underestimate the ratio at higher (solar and above) metallicities. One possible explanation for this failure is perhaps single-star models are not sufficient and Roche-lobe overflow in close binaries is necessary to produce the 'extra' WC stars at higher metallicities. However, this would require the frequency of close massive binaries to be metallicity dependent. Here we test this hypothesis by searching for close Wolf-Rayet binaries in the high metallicity environments of M31 and the center of M33 as well as in the lower metallicity environments of the middle and outer regions of M33. After identifying ∼100 Wolf-Rayet binaries based on radial velocity variations, we conclude that the close binary frequency of Wolf-Rayets is not metallicity dependent and thus other factors must be responsible for the overabundance of WC stars at high metallicities. However, our initial identifications and observations of these close binaries have already been put to good use as we are currently observing additional epochs for eventual orbit and mass determinations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

VOLATILE DELIVERY TO PLANETS FROM WATER-RICH PLANETESIMALS AROUND LOW-MASS STARS

Most models of volatile delivery to accreting terrestrial planets assume that the carriers for water are similar in water content to the carbonaceous chondrites in our solar system. Here we consider how the water content of planetesimals may be higher in many planetary systems, as they could lack the short-lived radionuclides that drove water loss in carbonaceous chondrites in our solar system. Using N-body simulations, we explore how planetary accretion would be different if bodies beyond the water line contained a water-mass fraction consistent with chemical equilibrium calculations, and more similar to comets, as opposed to the more traditional water-depleted values. We apply this model to consider planet formation around stars of different masses and identify trends in the properties of habitable zone planets and planetary system architecture that could be tested by ongoing exoplanet census data collection. Comparison of such data with the model-predicted trends will serve to evaluate how well the N-body simulations and the initial conditions used in studies of planetary accretion can be used to understand this stage of planet formation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Ecological niche and species distribution modelling of sea stars along the Pacific Northwest continental shelf

Abstract Aim Populations of several sea star species were devastated by a disease in 2013–2014 from Alaska to Baja California. However, their exact distributions and ecological requirements are unknown and better knowledge could help identify populations that are at risk. This study aimed to assess patterns of distribution and determine ecological requirements of several sea star species affected by the disease. Location Continental shelf from southern Washington State to northern California, USA . Methods A large occurrence dataset (5385 data points) of 11 taxa was gathered from bottom trawl surveys and underwater video footage, along with data for 13 oceanographic and seafloor environmental parameters. Species distribution modelling ( Maxent ) and ecological niche (outlying mean index) analyses were applied to these biotic and abiotic datasets. Results Each species showed contrasting ecological niches and modelled distributions, the main explanatory factors being depth, temperature, salinity, sediment mean grain size and the presence of rocky outcrops. Despite these species being recorded from Alaska to California, some sea stars showed very narrow distributions, whereas other taxa were widely distributed in the whole area. Main conclusions The modelled ecological niches and distributions of these 11 sea star taxa showed a high degree of differentiation. Because intertidal and shallow subtidal populations were drastically affected by the 2013–2014 disease, recovery will depend on health of subtidal populations and the ability of the species to disperse and recolonize shallower areas. Our study will serve as a useful reference of the species distributions before the disease started. This distributional data and assessment of influential environmental factors will be valuable for future predictions or studies of the effects of bottom‐contact ocean‐use activities on sea star species as well as the effects of climate change. Understanding habitat conditions favourable for these species may be helpful in managing population recovery following disease outbreaks or other catastrophic events.

Hemery, Lenaïg G.↗
DOE OSTI2014

Radiation transfer of models of massive star formation. III. The evolutionary sequence

We present radiation transfer simulations of evolutionary sequences of massive protostars forming from massive dense cores in environments of high mass surface densities, based on the Turbulent Core Model. The protostellar evolution is calculated with a multi-zone numerical model, with the accretion rate regulated by feedback from an evolving disk wind outflow cavity. The disk evolution is calculated assuming a fixed ratio of disk to protostellar mass, while the core envelope evolution assumes an inside-out collapse of the core with a fixed outer radius. In this framework, an evolutionary track is determined by three environmental initial conditions: the core mass M{sub c} , the mass surface density of the ambient clump Σ{sub cl}, and the ratio of the core's initial rotational to gravitational energy β {sub c}. Evolutionary sequences with various M{sub c} , Σ{sub cl}, and β {sub c} are constructed. We find that in a fiducial model with M{sub c} = 60 M {sub ☉}, Σ{sub cl} = 1 g cm{sup –2}, and β {sub c} = 0.02, the final mass of the protostar reaches at least ∼26 M {sub ☉}, making the final star formation efficiency ≳ 0.43. For each of the evolutionary tracks, radiation transfer simulations are performed at selected stages, with temperature profiles, spectral energy distributions (SEDs), and multiwavelength images produced. At a given stage, the envelope temperature depends strongly on Σ{sub cl}, with higher temperatures in a higher Σ{sub cl} core, but only weakly on M{sub c} . The SED and MIR images depend sensitively on the evolving outflow cavity, which gradually widens as the protostar grows. The fluxes at ≲ 100 μm increase dramatically, and the far-IR peaks move to shorter wavelengths. The influence of Σ{sub cl} and β {sub c} (which determines disk size) are discussed. We find that, despite scatter caused by different M{sub c} , Σ{sub cl}, β {sub c}, and inclinations, sources at a given evolutionary stage appear in similar regions of color-color diagrams, especially when using colors with fluxes at ≳ 70 μm, where scatter due to inclination is minimized, implying that such diagrams can be useful diagnostic tools for identifying the evolutionary stages of massive protostars. We discuss how intensity profiles along or perpendicular to the outflow axis are affected by environmental conditions and source evolution and can thus act as additional diagnostics of the massive star formation process.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE SOURCE DENSITY AND OBSERVABILITY OF PAIR-INSTABILITY SUPERNOVAE FROM THE FIRST STARS

Theoretical models predict that some of the first stars ended their lives as extremely energetic pair-instability supernovae (PISNe). With energies approaching 10{sup 53} erg, these supernovae are expected to be within the detection limits of the upcoming James Webb Space Telescope (JWST), allowing observational constraints to be placed on the properties of the first stars. We estimate the source density of PISNe using a semi-analytic halo mass function based approach, accounting for the effects of feedback from star formation on the PISN rate using cosmological simulations. We estimate an upper limit of {approx}0.2 PISNe per JWST field of view at any given time. Feedback can reduce this rate significantly, e.g., lowering it to as little as one PISN per 4000 JWST fields of view for the most pessimistic explosion models. We also find that the main obstacle to observing PISNe from the first stars is their scarcity, not their faintness; exposures longer than a few times 10{sup 4} s will do little to increase the number of PISNe found. Given this, we suggest a mosaic style search strategy for detecting PISNe from the first stars. Even rather high-redshift PISNe are unlikely to be missed by moderate exposures, and a large number of pointings will be required to ensure a detection.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Modelling R Coronae Borealis stars: effects of He-burning shell temperature and metallicity

The R Coronae Borealis (RCB) stars are extremely hydrogen-deficient carbon stars that produce large amounts of dust, causing sudden deep declines in brightness. They are believed to be formed primarily through white dwarf mergers. In this paper, we use mesa to investigate how post-merger objects with a range of initial He-burning shell temperatures from 2.1 to 5.4 × 10 8 K with solar and subsolar metallicities evolve into RCB stars. The most successful model of these has subsolar metallicity and an initial temperature near 3 × 10 8 K. We find a strong dependence on initial He-burning shell temperature for surface abundances of elements involved in the CNO cycle, as well as differences in effective temperature and radius of RCBs. Elements involved in nucleosynthesis present around 1 dex diminished surface abundances in the 10 per cent solar metallicity models, with the exception of carbon and lithium that are discussed in detail. Models with subsolar metallicities also exhibit longer lifetimes than their solar counterparts. In addition, we find that convective mixing of the burned material occurs only in the first few years of post-merger evolution, after which the surface abundances are constant during and after the RCB phase, providing evidence for why these stars show a strong enhancement of partial He-burning products.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

RADIATION TRANSFER OF MODELS OF MASSIVE STAR FORMATION. II. EFFECTS OF THE OUTFLOW

We present radiation transfer simulations of a massive (8 M{sub Sun }) protostar forming from a massive (M{sub c} = 60 M{sub Sun }) protostellar core, extending the model developed by Zhang and Tan. The two principal improvements are (1) developing a model for the density and velocity structure of a disk wind that fills the bipolar outflow cavities, based in part on the disk-wind model of Blandford and Payne; and (2) solving for the radially varying accretion rate in the disk due to a supply of mass and angular momentum from the infall envelope and their loss to the disk wind. One consequence of the launching of the disk wind is a reduction in the amount of accretion power that is radiated by the disk. We also include a non-Keplerian potential appropriate for a growing, massive disk. For the transition from dusty to dust-free conditions where gas opacities dominate, we now implement a gradual change as a more realistic approximation of dust destruction. We study how the above effects, especially the outflow, influence the spectral energy distributions (SEDs) and the synthetic images of the protostar. Dust in the outflow cavity significantly affects the SEDs at most viewing angles. It further attenuates the short-wavelength flux from the protostar, controlling how the accretion disk may be viewed, and contributes a significant part of the near- and mid-IR fluxes. These fluxes warm the disk, boosting the mid- and far-IR emission. We find that for near face-on views, i.e., looking down the outflow cavity (although not too close to the axis), the SED from the near-IR to about 60 {mu}m is very flat, which may be used to identify such systems. We show that the near-facing outflow cavity and its walls are still the most significant features in images up to 70 {mu}m, dominating the mid-IR emission and determining its morphology. The thermal emission from the dusty outflow itself dominates the flux at {approx}20 {mu}m. The detailed distribution of the dust in the outflow affects the morphology, so the model can be constrained by considering detailed intensity profiles along and perpendicular to the outflow axis. For example, even though the outflow cavity is wide, at 10-20 {mu}m, the dust in the disk wind can make the outflow appear narrower than in the near-IR bands.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

DISSECTING MAGNETAR VARIABILITY WITH BAYESIAN HIERARCHICAL MODELS

Neutron stars are a prime laboratory for testing physical processes under conditions of strong gravity, high density, and extreme magnetic fields. Among the zoo of neutron star phenomena, magnetars stand out for their bursting behavior, ranging from extremely bright, rare giant flares to numerous, less energetic recurrent bursts. The exact trigger and emission mechanisms for these bursts are not known; favored models involve either a crust fracture and subsequent energy release into the magnetosphere, or explosive reconnection of magnetic field lines. In the absence of a predictive model, understanding the physical processes responsible for magnetar burst variability is difficult. Here, we develop an empirical model that decomposes magnetar bursts into a superposition of small spike-like features with a simple functional form, where the number of model components is itself part of the inference problem. The cascades of spikes that we model might be formed by avalanches of reconnection, or crust rupture aftershocks. Using Markov Chain Monte Carlo sampling augmented with reversible jumps between models with different numbers of parameters, we characterize the posterior distributions of the model parameters and the number of components per burst. We relate these model parameters to physical quantities in the system, and show for the first time that the variability within a burst does not conform to predictions from ideas of self-organized criticality. We also examine how well the properties of the spikes fit the predictions of simplified cascade models for the different trigger mechanisms.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

FEEDBACK-REGULATED STAR FORMATION: IMPLICATIONS FOR THE KENNICUTT-SCHMIDT LAW

We derive a metallicity-dependent relation between the surface density of the star formation rate ({Sigma}{sub SFR}) and the gas surface density ({Sigma}{sub g}) in a feedback-regulated model of star formation in galactic disks. In this model, star formation occurs in gravitationally bound protocluster clumps embedded in larger giant molecular clouds with the protocluster clump mass function following a power-law function with a slope of -2. Metallicity-dependent feedback is generated by the winds of OB stars (M {approx}> 5 M{sub sun}) that form in the clumps. The quenching of star formation in clumps of decreasing metallicity occurs at later epochs due to weaker wind luminosities, thus resulting in higher final star formation efficiencies (SFE{sub exp}). By combining SFE{sub exp} with the timescales on which gas expulsion occurs, we derive the metallicity-dependent SFR per unit time in this model as a function of {Sigma}{sub g}. This is combined with the molecular gas fraction in order to derive the global dependence of {Sigma}{sub SFR} on {Sigma}{sub g}. The model reproduces very well the observed star formation laws extending from low gas surface densities up to the starburst regime. Furthermore, our results show a dependence of {Sigma}{sub SFR} on metallicity over the entire range of gas surface densities in contrast to other models and can also explain part of the scatter in the observations. We provide a tabulated form of the star formation laws that can easily be incorporated into numerical simulations or semi-analytical models of galaxy formation and evolution.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

A galaxy model from two micron all sky survey star counts in the whole sky, including the plane

We use the star count model of Ortiz and Lépine to perform an unprecedented exploration of the most important Galactic parameters comparing the predicted counts with the Two Micron All Sky Survey observed star counts in the J, H, and K{sub S} bands for a grid of positions covering the whole sky. The comparison is made using a grid of lines of sight given by the HEALPix pixelization scheme. The resulting best-fit values for the parameters are: 2120 ± 200 pc for the radial scale length and 205 ± 40 pc for the scale height of the thin disk, with a central hole of 2070{sub −800}{sup +2000} pc for the same disk, 3050 ± 500 pc for the radial scale length and 640 ± 70 pc for the scale height of the thick disk, 400 ± 100 pc for the central dimension of the spheroid, 0.0082 ± 0.0030 for the spheroid to disk density ratio, and 0.57 ± 0.05 for the oblate spheroid parameter.

79 ASTRONOMY AND ASTROPHYSICS↗