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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 919 records · Page 51
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Dynamical constraints on the core mass of hot Jupiter HAT-P-13B

HAT-P-13b is a Jupiter-mass transiting exoplanet that has settled onto a stable, short-period, and mildly eccentric orbit as a consequence of the action of tidal dissipation and perturbations from a second, highly eccentric, outer companion. Owing to the special orbital configuration of the HAT-P-13 system, the magnitude of HAT-P-13b's eccentricity (e{sub b}) is in part dictated by its Love number (k{sub 2{sub b}}), which is in turn a proxy for the degree of central mass concentration in its interior. Thus, the measurement of e{sub b} constrains k{sub 2{sub b}} and allows us to place otherwise elusive constraints on the mass of HAT-P-13b's core (M{sub core,b}). In this study we derive new constraints on the value of e{sub b} by observing two secondary eclipses of HAT-P-13b with the Infrared Array Camera on board the Spitzer Space Telescope. We fit the measured secondary eclipse times simultaneously with radial velocity measurements and find that e{sub b} = 0.00700 ± 0.00100. We then use octupole-order secular perturbation theory to find the corresponding k{sub 2{sub b}}=0.31{sub −0.05}{sup +0.08}. Applying structural evolution models, we then find, with 68% confidence, that M{sub core,b} is less than 25 Earth masses (M{sub ⊕}). The most likely value is M{sub core,b} = 11 M{sub ⊕}, which is similar to the core mass theoretically required for runaway gas accretion. This is the tightest constraint to date on the core mass of a hot Jupiter. Additionally, we find that the measured secondary eclipse depths, which are in the 3.6 and 4.5 μm bands, best match atmospheric model predictions with a dayside temperature inversion and relatively efficient day–night circulation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

The magnetic furnace: intense core dynamos in B stars

The dynamo action achieved in the convective cores of main-sequence massive stars is explored here through three-dimensional (3D) global simulations of convective core dynamos operating within a young 10 M{sub ⊙} B-type star, using the anelastic spherical harmonic code. These simulations capture the inner 65% of this star by radius, encompassing the convective nuclear-burning core (about 23% by radius) and a portion of the overlying radiative envelope. Eight rotation rates are considered, ranging from 0.05% to 16% of the surface breakup velocity, thereby capturing both convection that barely senses the effects of rotation and other situations in which the Coriolis forces are prominent. The vigorous dynamo action realized within all of these turbulent convective cores builds magnetic fields with peak strengths exceeding a megagauss, with the overall magnetic energy (ME) in the faster rotators reaching super-equipartition levels compared to the convective kinetic energy (KE). The core convection typically involves turbulent columnar velocity structures roughly aligned with the rotation axis, with magnetic fields threading through these rolls and possessing complex linkages throughout the core. The very strong fields are able to coexist with the flows without quenching them through Lorentz forces. The velocity and magnetic fields achieve such a state by being nearly co-aligned, and with peak magnetic islands being somewhat displaced from the fastest flows as the intricate evolution proceeds. As the rotation rate is increased, the primary force balance shifts from nonlinear advection balancing Lorentz forces to a magnetostrophic balance between Coriolis and Lorentz forces.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Structure, dynamics, and deuterium fractionation of massive pre-stellar cores

High levels of deuterium fraction in N{sub 2}H{sup +} are observed in some pre-stellar cores. Single-zone chemical models find that the timescale required to reach observed values (D{sub frac}{sup N{sub 2}H{sup +}}≡N{sub 2}D{sup +}/N{sub 2}H{sup +}≳0.1) is longer than the free-fall time, possibly 10 times longer. Here, we explore the deuteration of turbulent, magnetized cores with 3D magnetohydrodynamics simulations. We use an approximate chemical model to follow the growth in abundances of N{sub 2}H{sup +} and N{sub 2}D{sup +}. We then examine the dynamics of the core using each tracer for comparison to observations. We find that the velocity dispersion of the core as traced by N{sub 2}D{sup +} appears slightly sub-virial compared to predictions of the Turbulent Core Model of McKee and Tan, except at late times just before the onset of protostar formation. By varying the initial mass surface density, the magnetic energy, the chemical age, and the ortho-to-para ratio of H{sub 2}, we also determine the physical and temporal properties required for high deuteration. We find that low initial ortho-to-para ratios (≲0.01) and/or multiple free-fall times (≳3) of prior chemical evolution are necessary to reach the observed values of deuterium fraction in pre-stellar cores.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

The impact of feedback during massive star formation by core accretion

We study feedback during massive star formation using semi-analytic methods, considering the effects of disk winds, radiation pressure, photoevaporation, and stellar winds, while following protostellar evolution in collapsing massive gas cores. We find that disk winds are the dominant feedback mechanism setting star formation efficiencies (SFEs) from initial cores of ∼0.3–0.5. However, radiation pressure is also significant to widen the outflow cavity causing reductions of SFE compared to the disk-wind only case, especially for >100 M{sub ⊙} star formation at clump mass surface densities Σ{sub cl}≲0.3 g cm{sup −2}. Photoevaporation is of relatively minor importance due to dust attenuation of ionizing photons. Stellar winds have even smaller effects during the accretion stage. For core masses M{sub c}≃10–1000 M{sub ⊙} and Σ{sub cl}≃0.1–3 g cm{sup −2}, we find the overall SFE to be ε-bar {sub ∗f}=0.31(R{sub c}/0.1pc){sup −0.39}, potentially a useful sub-grid star formation model in simulations that can resolve pre-stellar core radii, R{sub c}=0.057(M{sub c}/60M{sub ⊙}){sup 1/2}(Σ{sub cl}/gcm{sup −2}){sup −1/2} pc. The decline of SFE with M {sub c} is gradual with no evidence for a maximum stellar-mass set by feedback processes up to stellar masses of m{sub ∗}∼300 M{sub ⊙}. We thus conclude that the observed truncation of the high-mass end of the IMF is shaped mostly by the pre-stellar core mass function or internal stellar processes. To form massive stars with the observed maximum masses of ∼150–300M{sub ⊙}, initial core masses need to be ≳500–1000 M{sub ⊙}. We also apply our feedback model to zero-metallicity primordial star formation, showing that, in the absence of dust, photoevaporation staunches accretion at ∼50 M{sub ⊙}. Our model implies radiative feedback is most significant at metallicities ∼10{sup −2}Z{sub ⊙}, since both radiation pressure and photoevaporation are effective in this regime.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Properties of Dense Cores Embedded in Musca Derived from Extinction Maps and {sup 13}CO, C{sup 18}O, and NH{sub 3} Emission Lines

Using near-infrared data from the Two Micron All Sky Survey catalog and the Near Infrared Color Excess method, we studied the extinction distribution in five dense cores of Musca, which show visual extinction greater than 10 mag and are potential sites of star formation. We analyzed the stability in four of them, fitting their radial extinction profiles with Bonnor–Ebert isothermal spheres, and explored their properties using the J = 1–0 transition of {sup 13}CO and C{sup 18}O and the J = K = 1 transition of NH{sub 3}. One core is not well described by the model. The stability parameter of the fitted cores ranges from 4.5 to 5.7 and suggests that all cores are stable, including Mu13, which harbors one young stellar object (YSO), the IRAS 12322-7023 source. However, the analysis of the physical parameters shows that Mu13 tends to have larger A {sub V}, n {sub c}, and P {sub ext} than the remaining starless cores. The other physical parameters do not show any trend. It is possible that those are the main parameters to explore in active star-forming cores. Mu13 also shows the most intense emission of NH{sub 3}. Its {sup 13}CO and C{sup 18}O lines have double peaks, whose integrated intensity maps suggest that they are due to the superposition of clouds with different radial velocities seen in the line of sight. It is not possible to state whether these clouds are colliding and inducing star formation or are related to a physical process associated with the formation of the YSO.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

ALMA Observations of the Archetypal “Hot Core” That Is Not: Orion-KL

We present sensitive high angular resolution (∼0.″1–0.″3) continuum Atacama Large Millimeter/Submillimeter Array (ALMA) observations of the archetypal hot core located in the Orion Kleinmann-Low (KL) region. The observations were made in five different spectral bands (bands 3, 6, 7, 8, and 9) covering a very broad range of frequencies (149–658 GHz). Apart from the well-known millimeter emitting objects located in this region (Orion Source I and BN), we report the first submillimeter detection of three compact continuum sources (ALMA1–3) in the vicinities of the Orion-KL hot molecular core. These three continuum objects have spectral indices between 1.47 and 1.56, and brightness temperatures between 100 and 200 K at 658 GHz, suggesting that we are seeing moderate, optically thick dust emission with possible grain growth. However, as these objects are not associated with warm molecular gas, and some of them are farther out from the molecular core, we thus conclude that they cannot heat the molecular core. This result favors the hypothesis that the hot molecular core in Orion-KL core is heated externally.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Analytical Core Mass Function (CMF) from Filaments: Under Which Circumstances Can Filament Fragmentation Reproduce the CMF?

Observations suggest that star formation in filamentary molecular clouds occurs in a two-step process, with the formation of filaments preceding that of prestellar cores and stars. Here, we apply the gravoturbulent fragmentation theory of Hennebelle and Chabrier to a filamentary environment, taking into account magnetic support. We discuss the induced geometrical effect on the cores, with a transition from 3D geometry at small scales to 1D at large ones. The model predicts the fragmentation behavior of a filament for a given mass per unit length (MpL) and level of magnetization. This core mass function (CMF) for individual filaments is then convolved with the distribution of filaments to obtain the final system CMF. The model yields two major results. (i) The filamentary geometry naturally induces a hierarchical fragmentation process, first into groups of cores, separated by a length equal to a few filament Jeans lengths, i.e., a few times the filament width. These groups then fragment into individual cores. (ii) Non-magnetized filaments with high MpL are found to fragment excessively, at odds with observations. This is resolved by taking into account the magnetic field (treated simply as additional pressure support). The present theory suggests two complementary modes of star formation: although small (spherical or filamentary) structures will collapse directly into prestellar cores, according to the standard Hennebelle–Chabrier theory, the large (filamentary) ones, the dominant population according to observations, will follow the aforedescribed two-step process.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Distortion of Magnetic Fields in a Starless Core II: 3D Magnetic Field Structure of FeSt 1-457

Three-dimensional (3D) magnetic field information on molecular clouds and cores is important for revealing their kinematical stability (magnetic support) against gravity, which is fundamental for studying the initial conditions of star formation. In the present study, the 3D magnetic field structure of the dense starless core FeSt 1-457 is determined based on the near-infrared polarimetric observations of the dichroic polarization of background stars and simple 3D modeling. With an obtained angle of line-of-sight magnetic inclination axis θ{sub inc} of 45{sup ∘}±10{sup ∘} and previously determined plane-of-sky magnetic field strength B{sub pol} of 23.8 ± 12.1 μG, the total magnetic field strength for FeSt 1-457 is derived to be 33.7 ± 18.0 μG. The critical mass of FeSt 1-457, evaluated using both magnetic and thermal/turbulent support is M{sub cr}=3.70±0.92 M{sub ⊙}, which is identical to the observed core mass, M{sub core}=3.55±0.75 M{sub ⊙}. We thus conclude that the stability of FeSt 1-457 is in a condition close to the critical state. Without infalling gas motion and no associated young stars, the core is regarded to be in the earliest stage of star formation, i.e., the stage just before the onset of dynamical collapse following the attainment of a supercritical condition. These properties could make FeSt 1-457 one of the best starless cores for future studies of the initial conditions of star formation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Features of Accretion-phase Gravitational-wave Emission from Two-dimensional Rotating Core-collapse Supernovae

We explore the influence of progenitor mass and rotation on the gravitational-wave (GW) emission from core-collapse supernovae, during the postbounce, preexplosion, accretion phase. We present the results from 15 two-dimensional (2D) neutrino radiation-hydrodynamic simulations from initial stellar collapse to ~300 ms after core bounce. We examine the features of the GW signals for four zero-age main sequence (ZAMS) progenitor masses ranging from 12 M ⊙ to 60 M ⊙ and four core rotation rates from 0 to 3 rad s -1 . We find that GW strain immediately around core bounce is fairly independent of ZAMS mass and—consistent with previous findings—that it is more heavily dependent on the core angular momentum. At later times, all nonrotating progenitors exhibit loud GW emission, which we attribute to vibrational g-modes of the protoneutron star (PNS) excited by convection in the postshock layer and the standing accretion shock instability (SASI). We find that increasing rotation rates results in muting of the accretion-phase GW signal due to centrifugal effects that inhibit convection in the postshock region, quench the SASI, and slow the rate at which the PNS peak vibrational frequency increases. Finally, we verify the efficacy of our approximate general relativistic (GR) effective potential treatment of gravity by comparing our core bounce GW strains with the recent 2D GR results of other groups.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Atmospheric Temperature Inversions and He ι 5876 Core Profile Structure in White Dwarfs

Here we report distinctive core profiles in the strongest optical helium line, He ι λ5876, from high-resolution high-sensitivity observations of spectral type DB white dwarfs. By analyzing a sample of 40 stars from Keck/HIRES and VLT/UVES, we find the core appearance to be related to the degree of hydrogen and heavy element content in the atmosphere. New Ca K-line measurements or upper limits are reported for about half the sample stars. He ι λ5876 emission cores with a self-reversed central component are present for those stars with relatively low hydrogen abundance, as well as relatively low atmospheric heavy element pollution. This self-reversed structure disappears for stars with higher degrees of pollution and/or hydrogen abundance, giving way to a single absorption core. From our model atmospheres, we show that the self-reversed emission cores can be explained by temperature inversions in the upper atmosphere. We propose that the transition to a single absorption core is due to the additional opacity from hydrogen and heavy elements that inhibits the temperature inversions. Our current models do not exactly match the effective temperature range of the phenomenon or the amplitude of the self-reversed structure, which is possibly a result of missing physics such as 3D treatment, convective overshoot, and/or non-LTE effects. The He ι λ5876 line structure may prove to be a useful new diagnostic for calibrating temperature profiles in DB atmosphere models.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

The Impact of Nuclear Reaction Rate Uncertainties on the Evolution of Core-collapse Supernova Progenitors

We review properties of core-collapse supernova progenitors with respect to the composite uncertainties in the thermonuclear reaction rates by coupling the probability density functions of the reaction rates provided by the STARLIB reaction rate library with MESA stellar models. We evolve 1000 models of 15${M}_{\odot }$ from the pre-main sequence to core O-depletion at solar and subsolar metallicities for a total of 2000 Monte Carlo stellar models. For each stellar model, we independently and simultaneously sample 665 thermonuclear reaction rates and use them in a MESA in situ reaction network that follows 127 isotopes from 1 H to 64 Zn. With this framework we survey the core mass, burning lifetime, composition, and structural properties at five different evolutionary epochs. At each epoch we measure the probability distribution function of the variations of each property and calculate Spearman rank-order correlation coefficients for each sampled reaction rate to identify which reaction rate has the largest impact on the variations on each property. We find that uncertainties in the reaction rates of ${}^{14}{\rm{N}}{({\rm{p}},\gamma )}^{15}{\rm{O}}$, triple-α, ${}^{12}{\rm{C}}{(\alpha ,\gamma )}^{16}{\rm{O}}$, 12 C( 12 C,p) 23 Na, 12 C( 16 O, p) 27 Al, 16 O( 16 O,n) 31 S, 16 O( 16 O, p) 31 P, and 16 O( 16 O,α) 28 Si dominate the variations of the properties surveyed. We find that variations induced by uncertainties in nuclear reaction rates grow with each passing phase of evolution, and at core H-, He-depletion they are of comparable magnitude to the variations induced by choices of mass resolution and network resolution. Yet, at core C-, Ne-, and O-depletion, the reaction rate uncertainties can dominate the variation, causing uncertainty in various properties of the stellar model in the evolution toward iron core-collapse.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1973

A lunar core of Fe-Ni-S.

It has been proposed that lunar samples were magnetized by a field created by a lunar core of molten Fe. Low abundances of siderophile elements in lunar rocks are compatible with formation of a metallic lunar core. A molten Fe core requires that the bulk of the moon was above, or close to, the melting point, a requirement which disagrees with most models of the lunar thermal regime. A core (or perhaps a layer or pockets) of molten Fe-Ni-S, at or close to the eutectic composition would act as a lunar dynamo, and be at a temperature (approximately 1000 C) consistent with some reasonable models of lunar thermal history. The existence of a Fe-Ni-S core would also partly explain the depletion in volatile elements in lunar basalts. Such a core, occupying up to 20% of the moon's radius, requires a bulk S content for the moon of only 0.3 wt %.

Brett, R.↗
NASA NTRS1969

Comments on the feasibility of developing gas core nuclear reactors

Recent developments in the fields of gas core hydrodynamics, heat transfer, and neutronics indicate that gas core nuclear rockets may be feasible from the point of view of basic principles. Based on performance predictions using these results, mission analyses indicate that gas core nuclear rockets may have the potential for reducing the initial weight in orbit of manned interplanetary vehicles by a factor of 5 when compared to the best chemical rocket systems. In addition, there is a potential for reducing total trip times from 450 to 500 days for chemical systems to 250 to 300 days for gas core systems. The possibility of demonstrating the feasibility of gas core nuclear rocket engines by means of a logical series of experiments of increasing difficulty that ends with ground tests of full scale gas core reactors is considered.

Rom, F. E.↗
NASA NTRS1977

Thermal interaction of the core and the mantle and long-term behavior of the geomagnetic field

The effects of temperature changes at the earth's core-mantle boundary on the velocity field of the core are analyzed. It is assumed that the geomagnetic field is maintained by thermal convection in the outer core. A model for the thermal interaction of the core and the mantle is presented which is consistent with current views on the presence of heat sources in the core and the properties of the lower mantle. Significant long-term variations in the frequency of geomagnetic reversals may be the result of fluctuating temperatures at the core-mantle boundary, caused by intermittent convection in the lower mantle. The thermal structure of the lower mantle region D double prime, extending from 2700 to 2900 km in depth, constitutes an important test of this hypothesis and offers a means of deciding whether the geomagnetic dynamo is thermally driven.

Jones, G. M.↗
NASA NTRS1977

Effects of forward motion on jet and core noise

A study was conducted to investigate the effects of forward motion on both jet and core noise. Measured low-frequency noise from static-engine and from flyover tests with a DC-9-30 powered by JT8D-109 turbofan engines and with a DC-10-40 powered by JT9D-59A turbofan engines was separated into jet- and core noise components. Comparisons of the static and the corresponding in-flight jet- and core-noise components are presented. The results indicate that for the DC-9 airplane at low power settings, where core noise is predominant, the effect of convective amplification on core-noise levels is responsible for the higher in-flight low-frequency noise levels in the inlet quadrant. Similarly, it was found that for the DC-10 airplane with engines mounted under the wings and flaps and flap deflection greater than 30 degrees, the contribution from jet-flap-interaction noise is as much as 5 dB in the inlet quadrant and is responsible for higher in-flight low-frequency noise levels during approach conditions. Those results indicate that to properly investigate flight effects, it is important to consider the noise contributions from other low-frequency sources, such as the core and the jet-flap interaction.

Low, J. K. C.↗
NASA NTRS1977

Noble gas evidence for the depositional and irradiational history of 60010-60009 core soils

Isotopic abundances of the noble gases have been determined in grain size separates of eleven soils from different depths in the 60010-60009 double drive tube and in magnetic and plagioclase separates from a few of these soils. Data for the 60010 core are presented here. The entire core was deposited a maximum of approximately 125 m.y. ago as deduced from the Ar-38 cosmic ray exposure age of soil 60009,457. Soils in the topmost 12 cm of the core show loss of cosmogenic He-3 and Ne-21 and gain of trapped solar gases in proportion to the degree of surface reworking by micrometeorites as deduced from FMR data. A variety of compositional and irradiational evidence suggests that soils in the core were formed by mixing of three or more components during or immediately prior to core deposition less than about 125 m.y. ago. Based on cosmogenic noble gases and a variety of other data soils 60009,457 and 60010,3107 are similar (and possibly identical) to two of the end member soils which formed the mixture. More mature soils in the core, however, could not have matured in situ from these two soils because of significant differences in noble gas abundances and chemical composition.

Bogard, D. D.↗
NASA NTRS1975

Noble gas studies on grain size separates of Apollo 15 and 16 deep drill cores

A description is presented of noble gas analyses of grain-size separates of eight Apollo 15 drill-core samples over the depth range 2-80 cm. Noble gas data on grain-size separates of seven Apollo 16 drill-core samples are also reported. On the basis of the obtained results it is suggested that cosmogenic Ne and He in the Apollo 15 core material has been preferentially lost relative to cosmogenic Ar in the Apollo 15 core material in proportion to soil maturity and degree of solar irradiation. A plot of He-4/Ne-20 against Ar-40/Ar-36 demonstrates that material in the lowest 35 cm of the Apollo 16 drill core is distinct from all other analyzed samples in the core, and distinct from nearly all surface soils.

Bogard, D. D.↗
NASA NTRS1979

Core cooling by subsolidus mantle convection

Although vigorous mantle convection early in the thermal history of the earth is shown to be capable of removing several times the latent heat content of the core, a thermal evolution model of the earth in which the core does not solidify can be constructed. The large amount of energy removed from the model earth's core by mantle convection is supplied by the internal energy of the core which is assumed to cool from an initial high temperature given by the silicate melting temperature at the core-mantle boundary. For the smaller terrestrial planets, the iron and silicate melting temperatures at the core-mantle boundaries are more comparable than for the earth; the models incorporate temperature-dependent mantle viscosity and radiogenic heat sources in the mantle. The earth models are constrained by the present surface heat flux and mantle viscosity and internal heat sources produce only about 55% of the earth model's present surface heat flow.

Schubert, G.↗