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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 631 records · Page 35
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DOE OSTI2014

Nova-like cataclysmic variables in the infrared

Nova-like (NL) cataclysmic variables have persistently high mass transfer rates and prominent steady state accretion disks. We present an analysis of infrared observations of 12 NLs obtained from the Two Micron All Sky Survey, the Spitzer Space Telescope, and the Wide-field Infrared Survey Explorer All Sky Survey. The presence of an infrared excess at λ ≳ 3-5 μm over the expectation of a theoretical steady state accretion disk is ubiquitous in our sample. The strength of the infrared excess is not correlated with orbital period, but shows a statistically significant correlation (but shallow trend) with system inclination that might be partially (but not completely) linked to the increasing view of the cooler outer accretion disk and disk rim at higher inclinations. We discuss the possible origin of the infrared excess in terms of emission from bremsstrahlung or circumbinary dust, with either mechanism facilitated by the mass outflows (e.g., disk wind/corona, accretion stream overflow, and so on) present in NLs. Our comparison of the relative advantages and disadvantages of either mechanism for explaining the observations suggests that the situation is rather ambiguous, largely circumstantial, and in need of stricter observational constraints.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Tidal dissipation and obliquity evolution in hot Jupiter systems

Two formation scenarios have been proposed to explain the tight orbits of hot Jupiters. They could be formed in orbits with a small inclination (with respect to the stellar spin) via disk migration, or in more highly inclined orbits via high-eccentricity migration, where gravitational interactions with a companion and tidal dissipation are at play. Here we target hot Jupiter systems where the misalignment λ has been inferred observationally and we investigate whether their properties are consistent with high-eccentricity migration. Specifically, we study whether stellar tides can be responsible for the observed distribution of λ and orbital separations. Improving on previous studies, we use detailed models for each star, thus accounting for how convection (and tidal dissipation) depends on stellar properties. In line with observations suggesting that hotter stars have higher λ, we find that λ increases as the amount of stellar surface convection decreases. This trend supports the hypothesis that tides are the mechanism shaping the observed distribution of λ. Furthermore, we study the past orbital evolution of five representative systems, chosen to cover a variety of temperatures and misalignments. We consider various initial orbital configurations and integrate the equations describing the coupled evolution of the orbital separation, stellar spin, and misalignment. We account for stellar tides and wind mass loss, stellar evolution, and magnetic braking. We show that the current properties of these five representative systems can be explained naturally, given our current understanding of tidal dissipation and with physically motivated assumptions for the effects driving the orbital evolution.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Pulsar wind model for the spin-down behavior of intermittent pulsars

Intermittent pulsars are part-time radio pulsars. They have higher slow down rates in the on state (radio-loud) than in the off state (radio-quiet). This gives evidence that particle wind may play an important role in pulsar spindown. The effect of particle acceleration is included in modeling the rotational energy loss rate of the neutron star. Applying the pulsar wind model to the three intermittent pulsars (PSR B1931+24, PSR J1841–0500, and PSR J1832+0029) allows their magnetic fields and inclination angles to be calculated simultaneously. The theoretical braking indices of intermittent pulsars are also given. In the pulsar wind model, the density of the particle wind can always be the Goldreich-Julian density. This may ensure that different on states of intermittent pulsars are stable. The duty cycle of particle wind can be determined from timing observations. It is consistent with the duty cycle of the on state. Inclination angle and braking index observations of intermittent pulsars may help to test different models of particle acceleration. At present, the inverse Compton scattering induced space charge limited flow with field saturation model can be ruled out.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

NuSTAR and XMM-Newton observations of NGC 1365: Extreme absorption variability and a constant inner accretion disk

We present a spectral analysis of four coordinated NuSTAR+XMM-Newton observations of the Seyfert galaxy NGC 1365. These exhibit an extreme level of spectral variability, which is primarily due to variable line-of-sight absorption, revealing relatively unobscured states in this source for the first time. Despite the diverse range of absorption states, each of the observations displays the same characteristic signatures of relativistic reflection from the inner accretion disk. Through time-resolved spectroscopy, we find that the strength of the relativistic iron line and the Compton reflection hump relative to the intrinsic continuum are well correlated, which is expected if they are two aspects of the same broadband reflection spectrum. We apply self-consistent disk reflection models to these time-resolved spectra in order to constrain the inner disk parameters, allowing for variable, partially covering absorption to account for the vastly different absorption states that were observed. Each of the four observations is treated independently to test the consistency of the results obtained for the black hole spin and the disk inclination, which should not vary on observable timescales. We find both the spin and the inclination determined from the reflection spectrum to be consistent, confirming that NGC 1365 hosts a rapidly rotating black hole; in all cases the dimensionless spin parameter is constrained to be a* > 0.97 (at 90% statistical confidence or better).

79 ASTRONOMY AND ASTROPHYSICS↗
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 OSTI2014

Black hole mass determination in the X-ray binary 4U 1630-47: Scaling of spectral and variability characteristics

We present the results of a comprehensive investigation on the evolution of spectral and timing properties of the Galactic black hole candidate 4U 1630-47 during its spectral transitions. In particular, we show how a scaling of the correlation of the photon index of the Comptonized spectral component Γ with low-frequency quasi-periodic oscillations (QPOs), ν {sub L}, and mass accretion rate, M-dot , can be applied to the black hole mass and the inclination angle estimates. We analyze the transition episodes observed with the Rossi X-Ray Timing Explorer and BeppoSAX satellites. We find that the broadband X-ray energy spectra of 4U 1630-47 during all spectral states can be modeled by a combination of a thermal component, a Comptonized component, and a red-skewed iron-line component. We also establish that Γ monotonically increases during transition from the low-hard state to the high-soft state and then saturates for high mass accretion rates. The index saturation levels vary for different transition episodes. Correlations of Γ versus ν {sub L} also show saturation at Γ ∼ 3. Γ-- M-dot and Γ-ν {sub L} correlations with their index saturation revealed in 4U 1630-47 are similar to those established in a number of other black hole candidates and can be considered as an observational evidence for the presence of a black hole in these sources. The scaling technique, which relies on XTE J1550-564, GRO 1655-40, and H1743-322 as reference sources, allows us to evaluate a black hole mass in 4U 1630-47 yielding M {sub BH} ∼ 10 ± 0.1 solar masses and to constrain the inclination angle of i ≲ 70°.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Architecture of Kepler's multi-transiting systems. II. New investigations with twice as many candidates

We report on the orbital architectures of Kepler systems having multiple-planet candidates identified in the analysis of data from the first six quarters of Kepler data and reported by Batalha et al. (2013). These data show 899 transiting planet candidates in 365 multiple-planet systems and provide a powerful means to study the statistical properties of planetary systems. Using a generic mass-radius relationship, we find that only two pairs of planets in these candidate systems (out of 761 pairs total) appear to be on Hill-unstable orbits, indicating ∼96% of the candidate planetary systems are correctly interpreted as true systems. We find that planet pairs show little statistical preference to be near mean-motion resonances. We identify an asymmetry in the distribution of period ratios near first-order resonances (e.g., 2:1, 3:2), with an excess of planet pairs lying wide of resonance and relatively few lying narrow of resonance. Finally, based upon the transit duration ratios of adjacent planets in each system, we find that the interior planet tends to have a smaller transit impact parameter than the exterior planet does. This finding suggests that the mode of the mutual inclinations of planetary orbital planes is in the range 1.°0-2.°2, for the packed systems of small planets probed by these observations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Chaos in the test particle eccentric Kozai-Lidov mechanism

The Kozai-Lidov mechanism can be applied to a vast variety of astrophysical systems involving hierarchical three-body systems. Here, we study the Kozai-Lidov mechanism systematically in the test particle limit at the octupole level of approximation. We investigate the chaotic and quasi-periodic orbital evolution by studying the surfaces of section and the Lyapunov exponents. We find that the resonances introduced by the octupole level of approximation cause orbits to flip from prograde to retrograde and back as well as cause significant eccentricity excitation, and chaotic behavior occurs when the mutual inclination between the inner and the outer binary is high. We characterize the parameter space that allows large amplitude oscillations in eccentricity and inclination.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Mergers and obliquities in stellar triples

Many close stellar binaries are accompanied by a faraway star. The 'eccentric Kozai-Lidov' (EKL) mechanism can cause dramatic inclination and eccentricity fluctuations, resulting in tidal tightening of inner binaries of triple stars. We run a large set of Monte Carlo simulations, including the secular evolution of the orbits, general relativistic precession, and tides, and we determine the semimajor axis, eccentricity, inclination, and spin-orbit angle distributions of the final configurations. We find that the efficiency of forming tight binaries (≲ 16 days) when taking the EKL mechanism into account is ∼21%, and about 4% of all simulated systems ended up in a merger event. These merger events can lead to the formation of blue stragglers. Furthermore, we find that the spin-orbit angle distribution of the inner binaries carries a signature of the initial setup of the system; thus, observations can be used to disentangle close binaries' birth configuration. The resulting inner and outer final orbits' period distributions and their estimated fraction suggest that secular dynamics may be a significant channel for the formation of close binaries in triples and even blue stragglers.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Scaling of observable properties in rotating stars

The spectral energy distribution as a function of inclination is computed using two-dimensional rotating stellar models and NLTE plane parallel stellar atmospheres. These models cover the range from 1.875 M {sub ☉} to 3.0 M {sub ☉}. The deduced effective temperature is determined by B – V computed from the spectral energy distribution, and the deduced luminosity is computed as the integral of the spectral energy distribution over all frequencies, assuming the distance and reddening are known. These deduced quantities are obtained from the observed spectral energy distribution assuming the objects are spherically symmetric, and thus the results are dependent on the inclination. Previous work has shown that the surface properties between two rotating stellar models with the same surface shape scale, and this is also true for the deduced effective temperature and luminosity over this limited mass range.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Results of the swift monitoring campaign of the X-ray binary 4U 1957+11: constraints on binary parameters

We present new results of uniform spectral analysis of Swift/XRT observations of the X-ray binary system 4U 1957+11. This includes 26 observations of the source made between MJD 54282-55890 (2007 July 01-2011 November 25). All 26 spectra are predominantly thermal and can be modeled well with emission from an accretion disk around a black hole. We analyze all 26 spectra jointly using traditional χ{sup 2} fitting as well as Markov Chain Monte Carlo simulations. The results from both methods agree, and constrain model parameters like inclination, column density, and black hole spin. These results indicate that the X-ray emitting inner accretion disk is inclined to our line of sight by 77.6{sub −2.2}{sup +1.5} deg. Additionally, the other constraints we obtain on parameters such as the column density and black hole spin are consistent with previous X-ray observations. Distances less than 5 kpc are unlikely and not only ruled out based on our analysis but also from other independent observations. Based on model-derived bolometric luminosities, we require the source distance to be >10 kpc if the black hole's mass is >10 M {sub ☉}. If the hole's mass is <10 M {sub ☉}, then the distance could be in the range of 5-10 kpc.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Photon-dominated region modeling of the [C I], [C II], and CO Line Emission From A Boundary In The Taurus molecular cloud

We present [C I] and [C II] observations of a linear edge region in the Taurus molecular cloud, and model this region as a cylindrically symmetric photon-dominated region (PDR) exposed to a low-intensity UV radiation field. The sharp, long profile of the linear edge makes it an ideal case to test PDR models and determine cloud parameters. We compare observations of the [C I], {sup 3} P {sub 1} → {sup 3} P {sub 0} (492 GHz), [C I] {sup 3} P {sub 2} → {sup 3} P {sub 1} (809 GHz), and [C II] {sup 2} P {sub 3/2} → {sup 2} P {sub 1/2} (1900 GHz) transitions, as well as the lowest rotational transitions of {sup 12}CO and {sup 13}CO, with line intensities produced by the RATRAN radiative transfer code from the results of the Meudon PDR code. We constrain the density structure of the cloud by fitting a cylindrical density function to visual extinction data. We study the effects of variation of the FUV field, {sup 12}C/{sup 13}C isotopic abundance ratio, sulfur depletion, cosmic ray ionization rate, and inclination of the filament relative to the sky-plane on the chemical network of the PDR model and resulting line emission. We also consider the role of suprathermal chemistry and density inhomogeneities. We find good agreement between the model and observations, and that the integrated line intensities can be explained by a PDR model with an external FUV field of 0.05 G {sub 0}, a low ratio of {sup 12}C to {sup 13}C ∼43, a highly depleted sulfur abundance (by a factor of at least 50), a cosmic ray ionization rate (3-6) × 10{sup –17} s{sup –1}, and without significant effects from inclination, clumping or suprathermal chemistry.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Optical counterparts of two Fermi millisecond pulsars: PSR J1301+0833 and PSR J1628–3205

Using the 1.3 m and 2.4 m Telescopes of the MDM Observatory, we identified the close companions of two eclipsing millisecond radio pulsars that were discovered by the Green Bank Telescope in searches of Fermi Gamma-ray Space Telescope sources, and measured their light curves. PSR J1301+0833 is a black widow pulsar in a 6.5 hr orbit whose companion star is strongly heated on the side facing the pulsar. It varies from R = 21.8 to R > 24 around the orbit. PSR J1628–3205 is a 'redback', a nearly Roche-lobe-filling system in a 5.0 hr orbit whose optical modulation in the range 19.0 < R < 19.4 is dominated by strong ellipsoidal variations, indicating a large orbital inclination angle. PSR J1628–3205 also shows evidence for a long-term variation of about 0.2 mag, and an asymmetric temperature distribution possibly due to either off-center heating by the pulsar wind, or large starspots. Modeling of its light curve restricts the inclination angle to i > 55°, the mass of the companion to 0.16 < M{sub c} < 0.30 M {sub ☉}, and the effective temperature to 3560 < T {sub eff} < 4670 K. As is the case for several redbacks, the companion of PSR J1628–3205 is less dense and hotter than a main-sequence star of the same mass.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

A substellar-mass protostar and its outflow of IRAS 15398–3359 revealed by subarcsecond-resolution observations of H{sub 2}CO and CCH

Subarcsecond (0.''5) images of H{sub 2}CO and CCH line emission have been obtained in the 0.8 mm band toward the low-mass protostar IRAS 15398–3359 in the Lupus 1 cloud as one of the Cycle 0 projects of the Atacama Large Millimeter/Submillimeter Array. We have detected a compact component concentrated in the vicinity of the protostar and a well-collimated outflow cavity extending along the northeast-southwest axis. The inclination angle of the outflow is found to be about 20°, or almost edge-on, based on the kinematic structure of the outflow cavity. This is in contrast to previous suggestions of a more pole-on geometry. The centrally concentrated component is interpreted by use of a model of the infalling rotating envelope with the estimated inclination angle and the mass of the protostar is estimated to be less than 0.09 M {sub ☉}. Higher spatial resolution data are needed to infer the presence of a rotationally supported disk for this source, hinted at by a weak high-velocity H{sub 2}CO emission associated with the protostar.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

A parallax distance to the microquasar GRS 1915+105 and a revised estimate of its black hole mass

Using the Very Long Baseline Array, we have measured a trigonometric parallax for the microquasar GRS 1915+105, which contains a black hole and a K-giant companion. This yields a direct distance estimate of 8.6{sub −1.6}{sup +2.0} kpc and a revised estimate for the mass of the black hole of 12.4{sub −1.8}{sup +2.0} M {sub ☉}. GRS 1915+105 is at about the same distance as some H II regions and water masers associated with high-mass star formation in the Sagittarius spiral arm of the Galaxy. The absolute proper motion of GRS 1915+105 is –3.19 ± 0.03 mas yr{sup –1} and –6.24 ± 0.05 mas yr{sup –1} toward the east and north, respectively, which corresponds to a modest peculiar speed of 22 ± 24 km s{sup –1} at the parallax distance, suggesting that the binary did not receive a large velocity kick when the black hole formed. On one observational epoch, GRS 1915+105 displayed superluminal motion along the direction of its approaching jet. Considering previous observations of jet motions, the jet in GRS 1915+105 can be modeled with a jet inclination to the line of sight of 60° ± 5° and a variable flow speed between 0.65c and 0.81c, which possibly indicates deceleration of the jet at distances from the black hole ≳ 2000 AU. Finally, using our measurements of distance and estimates of black hole mass and inclination, we provisionally confirm our earlier result that the black hole is spinning very rapidly.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

CONFRONTING THE JET MODEL OF Sgr A* WITH THE FARADAY ROTATION MEASURE OBSERVATIONS

Sgr A* is probably the supermassive black hole being investigated most extensively due to its proximity to Earth. Several theoretical models for its steady state emission have been proposed in the past two decades. Both the radiative-inefficient accretion flow and the jet model have been shown to well explain the observed spectral energy distribution. The Faraday rotation measure (RM) has been unambiguously measured at the submillimeter wavelength, but it has only been tested against the accretion flow model. Here we first calculate the RM based on the jet model and find that the predicted value is two orders of magnitude lower than the measured value. We then include an additional contribution from the accretion flow in front of the jet and show that the measured RM may be reconciled with the model under some tight constraints. The main constraint is that the inclination angle should be greater than ∼73°. However, this requirement is not consistent with an existing observational estimate of the inclination angle.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

ACTIVE-REGION TILT ANGLES: MAGNETIC VERSUS WHITE-LIGHT DETERMINATIONS OF JOY'S LAW

The axes of solar active regions are inclined relative to the east-west direction, with the tilt angle tending to increase with latitude ({sup J}oy's law{sup )}. Observational determinations of Joy's law have been based either on white-light images of sunspot groups or on magnetograms, where the latter have the advantage of measuring directly the physically relevant quantity (the photospheric field), but the disadvantage of having been recorded routinely only since the mid-1960s. White-light studies employing the historical Mount Wilson (MW) database have yielded tilt angles that are smaller and that increase less steeply with latitude than those obtained from magnetic data. We confirm this effect by comparing sunspot-group tilt angles from the Debrecen Photoheliographic Database with measurements made by Li and Ulrich using MW magnetograms taken during cycles 21-23. Whether white-light or magnetic data are employed, the median tilt angles significantly exceed the mean values, and provide a better characterization of the observed distributions. The discrepancy between the white-light and magnetic results is found to have two main sources. First, a substantial fraction of the white-light ''tilt angles'' refer to sunspots of the same polarity. Of greater physical significance is that the magnetograph measurements include the contribution of plage areas, which are invisible in white-light images but tend to have greater axial inclinations than the adjacent sunspots. Given the large uncertainties inherent in both the white-light and the magnetic measurements, it remains unclear whether any systematic relationship exists between tilt angle and cycle amplitude during cycles 16-23.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

ROTATIONAL CORRECTIONS TO NEUTRON-STAR RADIUS MEASUREMENTS FROM THERMAL SPECTRA

We calculate the rotational broadening in the observed thermal spectra of neutron stars spinning at moderate rates in the Hartle-Thorne approximation. These calculations accurately account for the effects of the second-order Doppler boosts as well as for the oblate shapes and the quadrupole moments of the neutron stars. We find that fitting the spectra and inferring the bolometric fluxes under the assumption that a star is not rotating causes an underestimate of the inferred fluxes and, thus, radii. The correction depends on the stellar spin, mass, radius, and the observer's inclination. For a 10 km, 1.4 M {sub ☉} neutron star spinning at 600 Hz, the rotational correction to the flux is ∼1%-4%, while for a 15 km neutron star with the same spin period, the correction ranges from 2% for pole-on sources to 12% for edge-on sources. We calculate the inclination-averaged corrections to inferred radii as a function of the neutron-star radius and mass and provide an empirical formula for the corrections. For realistic neutron-star parameters (1.4 M {sub ☉}, 12 km, 600 Hz), the stellar radius is on the order of 4% larger than the radius inferred under the assumption that the star is not spinning.

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