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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 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

IMAGING THE INNER AND OUTER GAPS OF THE PRE-TRANSITIONAL DISK OF HD 169142 AT 7 mm

We present Very Large Array observations at 7 mm that trace the thermal emission of large dust grains in the HD 169142 protoplanetary disk. Our images show a ring of enhanced emission of radius ∼25-30 AU, whose inner region is devoid of detectable 7 mm emission. We interpret this ring as tracing the rim of an inner cavity or gap, possibly created by a planet or a substellar companion. The ring appears asymmetric, with the western part significantly brighter than the eastern one. This azimuthal asymmetry is reminiscent of the lopsided structures that are expected to be produced as a consequence of trapping of large dust grains. Our observations also reveal an outer annular gap at radii from ∼40 to ∼70 AU. Unlike other sources, the radii of the inner cavity, the ring, and the outer gap observed in the 7 mm images, which trace preferentially the distribution of large (millimeter/centimeter sized) dust grains, coincide with those obtained from a previous near-infrared polarimetric image, which traces scattered light from small (micron-sized) dust grains. We model the broadband spectral energy distribution and the 7 mm images to constrain the disk physical structure. From this modeling we infer the presence of a small (radius ∼0.6 AU) residual disk inside the central cavity, indicating that the HD 169142 disk is a pre-transitional disk. The distribution of dust in three annuli with gaps in between them suggests that the disk in HD 169142 is being disrupted by at least two planets or substellar objects.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

SIGNATURES OF LONG-LIVED SPIRAL PATTERNS

Azimuthal age/color gradients across spiral arms are a signature of long-lived spirals. From a sample of 19 normal (or weakly barred) spirals where we have previously found azimuthal age/color gradient candidates, 13 objects were further selected if a two-armed grand-design pattern survived in a surface density stellar mass map. Mass maps were obtained from optical and near-infrared imaging, by comparison with a Monte Carlo library of stellar population synthesis models that allowed us to obtain the mass-to-light ratio in the J band, (M/L){sub J}, as a function of (g - i) versus (i - J) color. The selected spirals were analyzed with Fourier methods in search of other signatures of long-lived modes related to the gradients, such as the gradient divergence toward corotation, and the behavior of the phase angle of the two-armed spiral in different wavebands, as expected from theory. The results show additional signatures of long-lived spirals in at least 50% of the objects.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

MODELING THE NUCLEAR INFRARED SPECTRAL ENERGY DISTRIBUTION OF TYPE II ACTIVE GALACTIC NUCLEI

We present results from model fitting to the spectral energy distribution (SED) of a homogeneous sample of Seyfert II galaxies drawn from the 12 {mu}m Galaxy Sample. Imaging and nuclear flux measurements are presented in an accompanying paper. Here we add Spitzer/IRS observations to further constrain the SEDs after careful subtraction of a starburst component. We use the library of CLUMPY torus models from Nenkova et al. and also test the two-phase models recently produced by Stalevski et al. We find that photometric and spectroscopic observations in the mid-IR ({lambda} {approx}> 5 {mu}m) are crucial to properly constrain the best-fit torus models. About half of our sources show clear near-IR excess of their SEDs above the best-fit models. This problem can be less severe when using the Stalevski et al. models. The nature of this emission is not clear since best-fitted blackbody temperatures are very high ({approx}1700-2500 K) and the Type II classification of our sources would correspond to a small probability to peer directly into the hottest regions of the torus. Crucially, the derived torus parameters are quite robust when using CLUMPY models, independently of whether or not the sources require an additional blackbody component. Our findings suggest that tori are characterized by N{sub 0}{approx}>5, {sigma} {approx}> 40, {tau} {approx}< 25, Angle i {approx}> 40 Degree-Sign , Y {approx}< 50, and A {sup los} {sub v} {approx} 100-300, where N{sub 0} is the number of clouds in the equatorial plane of the torus, {sigma} is the characteristic opening angle of the cloud distribution, {tau} is the opacity of a single cloud, Angle i is the line-of-sight orientation of the torus, Y is the ratio of the inner to the outer radii, and A {sup los} {sub v} is the total opacity along the line of sight. From these, we can determine typical torus sizes and masses of 0.1-5.0 pc and 10{sup 4}-10{sup 6} M {sub Sun }, respectively. We find tentative evidence that those nuclei with detected hidden broad-line regions are characterized by lower levels of extinction than those without one. Finally, we find no correlation between the torus properties and the presence of circumnuclear or more global star formation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Star formation in self-gravitating turbulent fluids

We present a model of star formation in self-gravitating turbulent gas in which the turbulent velocity, v{sub T}, is a dynamical variable and is adiabatically heated by the collapse. The theory predicts the run of density, infall, and turbulent velocity and the rate of star formation in compact massive clouds. The adiabatic heating ensures that the turbulent pressure is dynamically important at all radii. The system evolves toward a coherent spatial structure with a fixed run of density, ρ(r,t)→ρ(r); mass flows through this structure onto the central star or star cluster. We define the sphere of influence of the accreted matter by m{sub ∗}=M{sub g}(r{sub ∗}), where m{sub *} is the stellar plus disk mass in the nascent star cluster and M{sub g}(r) is the gas mass inside radius r. Both v{sub T} and the infall velocity, |u{sub r}|, decrease with decreasing r for r>r{sub ∗}; v{sub T}(r)∼r{sup p}, the size–line-width relation, with p≈0.2−0.3, explaining the observation that Larson’s Law is altered in massive star-forming regions. The infall velocity is generally smaller than the turbulent velocity at r>r{sub ∗}. For r<r{sub ∗}, the infall and turbulent velocities are again similar, and both increase with decreasing r as r{sup −1/2}, with a magnitude about half of the free-fall velocity. The accreted (stellar) mass grows superlinearly with time, M-dot {sub ∗}=ϕM{sub cl}(t/τ{sub ff}){sup 2}, with ϕ a dimensionless number somewhat less than unity, M{sub cl} the clump mass, and τ{sub ff} the free-fall time of the clump. We suggest that small values of p can be used as a tracer of convergent collapsing flows.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1999

Spectral Irradiance Calibration in the Infrared: Comparison of (alpha) Bootis and 1 Ceres with a Laboratory Standard - 11

Infrared spectra of two celestial objects frequently used as flux standards are calibrated against an absolute laboratory flux standard at a spectral resolving power of 100 to 200. The spectrum of the KI.5 III star alpha Boo is measured from 3 to 30 microns, and that of the C-type asteroid 1 Ceres from 5 to 30 microns. While these "standard" spectra do not have the apparent precision of those based on calculated models, they do not require the assumptions involved in theoretical models of stars and asteroids. Specifically, they provide a model-independent means of calibrating celestial flux in the spectral range from 12 to 30 microns, where accurate absolute photometry is not available. The agreement found between the spectral shapes of alpha Boo and Ceres based on laboratory standards and those based on observed ratios to alpha CMa (Sirius) and alpha Lyr (Vega), flux-calibrated by theoretical modeling of these hot stars, strengthens our confidence in the applicability of the stellar models as primary irradiance standards.

Witteborn, Fred C.↗
DOE OSTI2014

THE LOW-SPIN BLACK HOLE IN LMC X-3

Building upon a new dynamical model for the X-ray binary LMC X-3, we measure the spin of its black hole (BH) primary via the continuum-fitting method. We consider over one thousand thermal-state Rossi X-ray Timing Explorer X-ray spectra of LMC X-3. Using a large subset of these spectra, we constrain the spin parameter of the BH to be a{sub ∗}=0.25{sub −0.29}{sup +0.20} (90% confidence). Our estimate of the uncertainty in a {sub *} takes into account a wide range of systematic errors. We discuss evidence for a correlation between a BH's spin and the complexity of its X-ray spectrum.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

TIDAL STREAM MORPHOLOGY AS AN INDICATOR OF DARK MATTER HALO GEOMETRY: THE CASE OF PALOMAR 5

This paper presents an example where the morphology of a single stellar stream can be used to rule out a specific galactic potential form without the need for velocity information. We investigate the globular cluster Palomar 5 (Pal 5), which is tidally disrupting into a cold, thin stream mapped over 22 deg on the sky with a typical width of 0.7 deg. We generate models of this stream by fixing Pal 5's present-day position, distance, and radial velocity via observations, while allowing its proper motion to vary. In a spherical dark matter halo we easily find models that fit the observed morphology. However, no plausible Pal 5 model could be found in the triaxial potential of Law and Majewski, which has been proposed to explain the properties of the Sagittarius stream. In this case, the long, thin, and curved morphology of the Pal 5 stream alone can be used to rule out such a potential configuration. Pal 5-like streams in this potential are either too straight, missing the curvature of the observations, or show an unusual morphology which we dub stream-fanning: a signature sensitive to the triaxiality of a potential. We conclude that the mere existence of other thin tidal streams must provide broad constraints on the orientation and shape of the dark matter halo they inhabit.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1998

Spectral Irradiance Calibration in the Infrared 11: Comparison of (alpha) Boo and 1 Ceres with a Laboratory Standard

Infrared spectra of two celestial objects frequently used as flux standards are calibrated against an absolute laboratory flux standard at a spectral resolving power of 100 to 200. The spectrum of the K1.5III star, alpha Boo, is measured from 3 microns to 30 microns and that of the C-type asteroid, 1 Ceres, from 5 microns to 30 microns. While these 'standard' spectra do not have the apparent precision of those based on calculated models, they do not require the assumptions involved in theoretical models of stars and asteroids. Specifically they provide a model-independent means of calibrating celestial flux in the spectral range from 12 microns to 30 microns where accurate absolute photometry is not available. The agreement found between the spectral shapes of alpha Boo and Ceres based on laboratory standards, and those based on observed ratios to alpha CMa (Sirius) and alpha Lyr (Vega), flux calibrated by theoretical modeling of these hot stars strengthens our confidence in the applicability of the stellar models as primary irradiance standards.

Witteborn, Fred C.↗
DOE OSTI2015

Fully coupled simulation of cosmic reionization. I. numerical methods and tests

Here, we describe an extension of the Enzo code to enable fully coupled radiation hydrodynamical simulation of inhomogeneous reionization in large similar to(100 Mpc)(3) cosmological volumes with thousands to millions of point sources. We solve all dynamical, radiative transfer, thermal, and ionization processes self-consistently on the same mesh, as opposed to a postprocessing approach which coarse-grains the radiative transfer. But, we employ a simple subgrid model for star formation which we calibrate to observations. The numerical method presented is a modification of an earlier method presented in Reynolds et al. differing principally in the operator splitting algorithm we use to advance the system of equations. Radiation transport is done in the gray flux-limited diffusion (FLD) approximation, which is solved by implicit time integration split off from the gas energy and ionization equations, which are solved separately. This results in a faster and more robust scheme for cosmological applications compared to the earlier method. The FLD equation is solved using the hypre optimally scalable geometric multigrid solver from LLNL. By treating the ionizing radiation as a grid field as opposed to rays, our method is scalable with respect to the number of ionizing sources, limited only by the parallel scaling properties of the radiation solver. We test the speed and accuracy of our approach on a number of standard verification and validation tests. We show by direct comparison with Enzo's adaptive ray tracing method Moray that the well-known inability of FLD to cast a shadow behind opaque clouds has a minor effect on the evolution of ionized volume and mass fractions in a reionization simulation validation test. Finally, we illustrate an application of our method to the problem of inhomogeneous reionization in a 80 Mpc comoving box resolved with 3200(3) Eulerian grid cells and dark matter particles.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

The G+M eclipsing binary V530 Orionis: a stringent test of magnetic stellar evolution models for low-mass stars

We report extensive photometric and spectroscopic observations of the 6.1 day period, G+M-type detached double-lined eclipsing binary V530 Ori, an important new benchmark system for testing stellar evolution models for low-mass stars. We determine accurate masses and radii for the components with errors of 0.7% and 1.3%, as follows: M {sub A} = 1.0038 ± 0.0066 M {sub ☉}, M {sub B} = 0.5955 ± 0.0022 M {sub ☉}, R {sub A} = 0.980 ± 0.013 R {sub ☉}, and R {sub B} = 0.5873 ± 0.0067 R {sub ☉}. The effective temperatures are 5890 ± 100 K (G1 V) and 3880 ± 120 K (M1 V), respectively. A detailed chemical analysis probing more than 20 elements in the primary spectrum shows the system to have a slightly subsolar abundance, with [Fe/H] = –0.12 ± 0.08. A comparison with theory reveals that standard models underpredict the radius and overpredict the temperature of the secondary, as has been found previously for other M dwarfs. On the other hand, models from the Dartmouth series incorporating magnetic fields are able to match the observations of the secondary star at the same age as the primary (∼3 Gyr) with a surface field strength of 2.1 ± 0.4 kG when using a rotational dynamo prescription, or 1.3 ± 0.4 kG with a turbulent dynamo approach, not far from our empirical estimate for this star of 0.83 ± 0.65 kG. The observations are most consistent with magnetic fields playing only a small role in changing the global properties of the primary. The V530 Ori system thus provides an important demonstration that recent advances in modeling appear to be on the right track to explain the long-standing problem of radius inflation and temperature suppression in low-mass stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

THE ORIGIN OF COMPLEX ORGANIC MOLECULES IN PRESTELLAR CORES

Complex organic molecules (COMs) have been detected in a variety of environments including cold prestellar cores. Given the low temperatures of these objects, these detections challenge existing models. We report here new observations toward the prestellar core L1544. They are based on an unbiased spectral survey of the 3 mm band at the IRAM 30 m telescope as part of the Large Program ASAI. The observations allow us to provide a full census of the oxygen-bearing COMs in this source. We detected tricarbon monoxide, methanol, acetaldehyde, formic acid, ketene, and propyne with abundances varying from 5 × 10{sup –11} to 6 × 10{sup –9}. The non-LTE analysis of the methanol lines shows that they are likely emitted at the border of the core at a radius of ∼8000 AU, where T ∼ 10 K and n {sub H{sub 2}} ∼2 × 10{sup 4} cm{sup –3}. Previous works have shown that water vapor is enhanced in the same region because of the photodesorption of water ices. We propose that a non-thermal desorption mechanism is also responsible for the observed emission of methanol and COMs from the same layer. The desorbed oxygen and a small amount of desorbed methanol and ethene are enough to reproduce the abundances of tricarbon monoxide, methanol, acetaldehyde, and ketene measured in L1544. These new findings open the possibility that COMs in prestellar cores originate in a similar outer layer rather than in the dense inner cores, as previously assumed, and that their formation is driven by the non-thermally desorbed species.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

REFINED SYSTEM PARAMETERS AND TTV STUDY OF TRANSITING EXOPLANETARY SYSTEM HAT-P-20

We report new photometric observations of the transiting exoplanetary system HAT-P-20, obtained using CCD cameras at Yunnan Observatories and Ho Koon Nature Education cum Astronomical Centre, China, from 2010 to 2013, and Observatori Ca l’Ou, Sant Marti Sesgueioles, Spain, from 2013 to 2015. The observed data are corrected for systematic errors according to the coarse de-correlation and SYSREM algorithms, so as to enhance the signal of the transit events. In order to consistently model the star spots and transits of this exoplanetary system, we develop a highly efficient tool STMT based on the analytic models of Mandel and Agol and Montalto et al. The physical parameters of HAT-P-20 are refined by homogeneously analyzing our new data, the radial velocity data, and the earlier photometric data in the literature with the Markov chain Monte Carlo technique. New radii and masses of both host star and planet are larger than those in the discovery paper due to the discrepancy of the radius among K-dwarfs between predicted values by standard stellar models and empirical calibration from observations. Through the analysis of all available mid-transit times calculated with the normal model and spotted model, we conclude that the periodic transit timing variations in these transit events revealed by employing the normal model are probably induced by spot crossing events. From the analysis of the distribution of occulted spots by HAT-P-20b, we constrain the misaligned architecture between the planetary orbit and the spin of the host star.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

The influence of thermal pressure on equilibrium models of hypermassive neutron star merger remnants

The merger of two neutron stars leaves behind a rapidly spinning hypermassive object whose survival is believed to depend on the maximum mass supported by the nuclear equation of state (EOS), angular momentum redistribution by (magneto-)rotational instabilities, and spindown by gravitational waves. The high temperatures (∼5-40 MeV) prevailing in the merger remnant may provide thermal pressure support that could increase its maximum mass and, thus, its life on a neutrino-cooling timescale. We investigate the role of thermal pressure support in hypermassive merger remnants by computing sequences of spherically symmetric and axisymmetric uniformly and differentially rotating equilibrium solutions to the general-relativistic stellar structure equations. Using a set of finite-temperature nuclear EOS, we find that hot maximum-mass critically spinning configurations generally do not support larger baryonic masses than their cold counterparts. However, subcritically spinning configurations with mean density of less than a few times nuclear saturation density yield a significantly thermally enhanced mass. Even without decreasing the maximum mass, cooling and other forms of energy loss can drive the remnant to an unstable state. We infer secular instability by identifying approximate energy turning points in equilibrium sequences of constant baryonic mass parameterized by maximum density. Energy loss carries the remnant along the direction of decreasing gravitational mass and higher density until instability triggers collapse. Since configurations with more thermal pressure support are less compact and thus begin their evolution at a lower maximum density, they remain stable for longer periods after merger.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

ASTEROSEISMIC DIAGRAMS FROM A SURVEY OF SOLAR-LIKE OSCILLATIONS WITH KEPLER

Photometric observations made by the NASA Kepler Mission have led to a dramatic increase in the number of main-sequence and subgiant stars with detected solar-like oscillations. We present an ensemble asteroseismic analysis of 76 solar-type stars. Using frequencies determined from the Kepler time-series photometry, we have measured three asteroseismic parameters that characterize the oscillations: the large frequency separation ({Delta}{nu}), the small frequency separation between modes of l = 0 and l = 2 ({delta}{nu}{sub 02}), and the dimensionless offset ({epsilon}). These measurements allow us to construct asteroseismic diagrams, namely the so-called Christensen-Dalsgaard diagram of {delta}{nu}{sub 02} versus {Delta}{nu}, and the recently re-introduced {epsilon} diagram. We compare the Kepler results with previously observed solar-type stars and with theoretical models. The positions of stars in these diagrams places constraints on their masses and ages. Additionally, we confirm the observational relationship between {epsilon} and T{sub eff} that allows for the unambiguous determination of radial order and should help resolve the problem of mode identification in F stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Deep crustal heating by neutrinos from the surface of accreting neutron stars

Here, we present a new mechanism for deep crustal heating in accreting neutron stars. Charged pions (π + ) are produced in nuclear collisions on the neutron star surface during active accretion and upon decay they provide a flux of neutrinos into the neutron star crust. For massive and/or compact neutron stars, neutrinos deposit ≈ 1 – 2 MeV of heat per accreted nucleon into the inner crust. The strength of neutrino heating is comparable to the previously known sources of deep crustal heating, such as from pycnonuclear fusion reactions, and is relevant for studies of cooling neutron stars. We model the thermal evolution of a transient neutron star in a low-mass x-ray binary, and in the particular case of the neutron star MXB 1659-29 we show that additional deep crustal heating requires a higher thermal conductivity for the neutron star inner crust. A better knowledge of pion-production cross sections near threshold would improve the accuracy of our predictions.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1989

The compact far infrared emission from the young stellar object IRAS 16293-2422

High resolution far IR observations at 50 and 100 microns were made of the young stellar object (YSO), IRAS 16293-2422. The observations are part of a systematic high resolution study of nearby YSO's. The purpose is to obtain resolution in the far IR comparable to that at other wavelengths. Until recently, the high resolution that has been available in the far IR has been from either IRAS (angular resolution of approx 4 min) or the KAO using standard FIR photometry (approx 35 sec). With scanning techniques, it is possible to obtain 10 sec resolution on bright sources. Such a resolution is necessary to better determine the physical conditions of the YSO, and to compare with model of star formation. In order to better constrain the models for the source, the YSO was observed at both 50 and 100 microns on several flights in 1988 April from the KAO. Estimates are presented of the size both along the major and minor axis of the disk, as well as estimates of the dust temperature and 100 micron opacity for the YSO.

Butner, Harold M.↗
NASA NTRS2017

Finding the Needles in the Haystacks: High-Fidelity Models of the Modern and Archean Solar System for Simulating Exoplanet Observations

We present two state-of-the-art models of the solar system, one corresponding to the present day and one to the Archean Eon 3.5 billion years ago. Each model contains spatial and spectral information for the star, the planets, and the interplanetary dust, extending to 50 au from the Sun and covering the wavelength range 0.3-2.5 micron. In addition, we created a spectral image cube representative of the astronomical backgrounds that will be seen behind deep observations of extrasolar planetary systems, including galaxies and Milky Way stars. These models are intended as inputs to high-fidelity simulations of direct observations of exoplanetary systems using telescopes equipped with high-contrast capability. They will help improve the realism of observation and instrument parameters that are required inputs to statistical observatory yield calculations, as well as guide development of post-processing algorithms for telescopes capable of directly imaging Earth-like planets.

extrasolar planetary systems↗