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At least 973 records · Page 54
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DOE OSTI2010

CORRELATIONS BETWEEN PLANETARY MICROLENSING PARAMETERS

Characterization of microlensing planets requires modeling of observed light curves including many parameters. Studying the dependency of the pattern of light curves on the lensing parameters and the correlations between the parameters is important to understand how the uncertainties of the planetary parameters are propagated from other parameters. In this paper, we show that despite the apparent complexity of the pattern of light curves of planetary lensing events, the correlations between the lensing parameters can be understood by studying how the parameters affect the characteristics of lensing light curves such as the height and width, the caustic-crossing time scale, and the location and duration of planetary perturbations. Based on analytic arguments about the dependency of light curve features on the parameters, we obtain the correlations for the two representative cases of planetary events. We also demonstrate the applicability of the correlations to general planetary events by actually obtaining the correlations from modelings of light curves produced by simulations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2010

PERPENDICULAR ION HEATING BY LOW-FREQUENCY ALFVEN-WAVE TURBULENCE IN THE SOLAR WIND

We consider ion heating by turbulent Alfven waves (AWs) and kinetic Alfven waves (KAWs) with wavelengths (measured perpendicular to the magnetic field) that are comparable to the ion gyroradius and frequencies {omega} smaller than the ion cyclotron frequency {Omega}. We focus on plasmas in which {beta} {approx}< 1, where {beta} is the ratio of plasma pressure to magnetic pressure. As in previous studies, we find that when the turbulence amplitude exceeds a certain threshold, an ion's orbit becomes chaotic. The ion then interacts stochastically with the time-varying electrostatic potential, and the ion's energy undergoes a random walk. Using phenomenological arguments, we derive an analytic expression for the rates at which different ion species are heated, which we test by simulating test particles interacting with a spectrum of randomly phased AWs and KAWs. We find that the stochastic heating rate depends sensitively on the quantity {epsilon} = {delta}v {sub {rho}/}v{sub perpendicular}, where v{sub perpendicular} (v {sub ||}) is the component of the ion velocity perpendicular (parallel) to the background magnetic field B {sub 0}, and {delta}v {sub {rho}} ({delta}B {sub {rho}}) is the rms amplitude of the velocity (magnetic-field) fluctuations at the gyroradius scale. In the case of thermal protons, when {epsilon} << {epsilon}{sub crit}, where {epsilon}{sub crit} is a constant, a proton's magnetic moment is nearly conserved and stochastic heating is extremely weak. However, when {epsilon}>{epsilon}{sub crit}, the proton heating rate exceeds half the cascade power that would be present in strong balanced KAW turbulence with the same value of {delta}v {sub {rho}}, and magnetic-moment conservation is violated even when {omega} << {Omega}. For the random-phase waves in our test-particle simulations, {epsilon}{sub crit} = 0.19. For protons in low-{beta} plasmas, {epsilon} {approx_equal} {beta}{sup -1/2{delta}}B{sub {rho}/}B {sub 0}, and {epsilon} can exceed {epsilon}{sub crit} even when {delta}B{sub {rho}/}B {sub 0} << {epsilon}{sub crit}. The heating is anisotropic, increasing v {sup 2}{sub perpendicular} much more than v {sup 2}{sub ||} when {beta} << 1. (In contrast, at {beta} {approx}> 1 Landau damping and transit-time damping of KAWs lead to strong parallel heating of protons.) At comparable temperatures, alpha particles and minor ions have larger values of {epsilon} than protons and are heated more efficiently as a result. We discuss the implications of our results for ion heating in coronal holes and the solar wind.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

HUBBLE/COS OBSERVATIONS OF THE Ly{alpha} FOREST TOWARD THE BL Lac OBJECT 1ES 1553+113

We present new moderate-resolution, far-ultraviolet spectra from the Hubble Space Telescope/Cosmic Origins Spectrograph (HST/COS) of the BL Lac object 1ES 1553+113 covering the wavelength range 1135 A < {lambda} < 1795 A. The data show a smooth continuum with a wealth of narrow (b < 100 km s{sup -1}) absorption features arising in the interstellar medium and intergalactic medium. These features include 41 Ly{alpha} absorbers at 0 < z{sub abs} < 0.43, 14 of which are detected in multiple Lyman lines and 6 of which show absorption in one or more metal lines. We analyze a metal-rich triplet ({Delta}cz {approx} 1000 km s{sup -1}) of Ly{alpha} absorbers at z{sub abs} {approx} 0.188 in which O VI, N V, and C III absorption is detected. Silicon ions (Si III, Si IV) are not detected to fairly strong upper limits and we use the measured Si III/C III upper limit to derive an abundance limit (C/Si) {>=} 4(C/Si){sub sun} for the strongest component of the absorber complex. Galaxy redshift surveys show a number of massive galaxies at approximately the same redshift as this absorption complex, suggesting that it arises in a large-scale galaxy filament. As one of the brightest extragalactic X-ray and {gamma}-ray sources, 1ES 1553+113 is of great interest to the high-energy astrophysics community. With no intrinsic emission or absorption features, 1ES 1553+113 has no direct redshift determination. We use intervening Ly{alpha} absorbers to place a direct limit on the redshift: z{sub em}>0.395 based on a confirmed Ly{alpha}+O VI absorber and z{sub em}>0.433 based on a single-line detection of Ly{alpha}. The current COS data are only sensitive to Ly{alpha} absorbers at z < 0.47, but we present statistical arguments that z{sub em} {approx}< 0.58 (at a 1{sigma} confidence limit) based on the non-detection of any Ly{beta} absorbers at z>0.4.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

TOWARD UNDERSTANDING THE B[e] PHENOMENON. IV. MODELING OF IRAS 00470+6429

FS CMa type stars are a recently described group of objects with the B[e] phenomenon which exhibits strong emission-line spectra and strong IR excesses. In this paper, we report the first attempt for a detailed modeling of IRAS 00470+6429, for which we have the best set of observations. Our modeling is based on two key assumptions: the star has a main-sequence luminosity for its spectral type (B2) and the circumstellar (CS) envelope is bimodal, composed of a slowly outflowing disklike wind and a fast polar wind. Both outflows are assumed to be purely radial. We adopt a novel approach to describe the dust formation site in the wind that employs timescale arguments for grain condensation and a self-consistent solution for the dust destruction surface. With the above assumptions we were able to satisfactorily reproduce many observational properties of IRAS 00470+6429, including the H I line profiles and the overall shape of the spectral energy distribution. Our adopted recipe for dust formation proved successful in reproducing the correct amount of dust formed in the CS envelope. Possible shortcomings of our model, as well as suggestions for future improvements, are discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2010

ECCENTRICITY TRAP: TRAPPING OF RESONANTLY INTERACTING PLANETS NEAR THE DISK INNER EDGE

Using orbital integration and analytical arguments, we have found a new mechanism (an 'eccentricity trap') to halt type I migration of planets near the inner edge of a protoplanetary disk. Because asymmetric eccentricity damping due to disk-planet interaction on the innermost planet at the disk edge plays a crucial role in the trap, this mechanism requires continuous eccentricity excitation and hence works for a resonantly interacting convoy of planets. This trap is so strong that the edge torque exerted on the innermost planet can completely halt type I migrations of many outer planets through mutual resonant perturbations. Consequently, the convoy stays outside the disk edge, as a whole. We have derived a semi-analytical formula for the condition for the eccentricity trap and predict how many planets are likely to be trapped. We found that several planets or more should be trapped by this mechanism in protoplanetary disks that have cavities. It can be responsible for the formation of non-resonant, multiple, close-in super-Earth systems extending beyond 0.1 AU. Such systems are being revealed by radial velocity observations to be quite common around solar-type stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

PROPERTIES OF THE DIFFUSE NEUTRAL HELIUM IN THE INNER HELIOSPHERE

Sensitive SOLARC imaging spectropolarimetric observations from Haleakala reveal a diffuse coronal surface brightness in the He I 1083 nm line. A series of observations suggests that this signal originates from an 'inner source' of neutral helium atoms in the solar corona. Here, we explore the possibility that this cold coronal component originates from helium ions that are neutralized by the near-Sun dust and subsequently excited to the metastable 1s2s {sup 3} S state, which then scatters photons from the solar disk. This picture suggests a deficit of coronal dust inside about 2-4 R{sub sun} in order to account for both the flat radial brightness distribution and the small velocity line width of the observations. We find a strong correlation between the polarized He brightness and coronal white light brightness that supports the argument that electronic collisional excitation of the metastable helium triplet level is responsible for our polarization signal.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

ACCELERATION OF ULTRA-HIGH-ENERGY COSMIC RAYS IN THE COLLIDING SHELLS OF BLAZARS AND GAMMA-RAY BURSTS: CONSTRAINTS FROM THE FERMI GAMMA-RAY SPACE TELESCOPE

Fermi Gamma-ray Space Telescope measurements of spectra, variability timescales, and maximum photon energies give lower limits to the apparent jet powers and, through {gamma}{gamma} opacity arguments, the bulk Lorentz factors of relativistic jets. The maximum cosmic-ray particle energy is limited by these two quantities in Fermi acceleration scenarios. Recent data are used to constrain the maximum energies of cosmic-ray protons and Fe nuclei accelerated in colliding shells of gamma-ray bursts (GRBs) and blazars. The Fermi results indicate that Fe rather than protons are more likely to be accelerated to ultra-high energies in active galactic nuclei (AGNs), whereas powerful GRBs can accelerate both protons and Fe to {approx}>10{sup 20} eV. Emissivity of nonthermal radiation from radio galaxies and blazars is estimated from the First Fermi AGN Catalog, and shown to favor BL Lac objects and FR1 radio galaxies over flat spectrum radio quasars, FR2 radio galaxies, and long-duration GRBs as the sources of ultra-high-energy cosmic rays.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

ENERGY LOSSES BY ANISOTROPIC VISCOUS DISSIPATION IN TRANSIENT MAGNETIC RECONNECTION

Global energy losses associated with transient magnetic reconnection in a viscous resistive plasma are examined. The Braginskii stress tensor is used to model the plasma viscosity for conditions typical of the solar corona. Analytic arguments are used to show that the large-scale advective flows associated with magnetic merging are likely to generate significant viscous losses. It is pointed out that the development of a visco-resistive reconnection scale, predicted for the classical shear viscosity, is not expected in the more realistic case of the Braginskii viscosity. Numerical simulations of planar coalescence merging show that viscous losses should easily dominate resistive losses for physically plausible parameters in flaring regions. Our computations imply that flare-like rates exceeding 10{sup 29} erg s{sup -1} can be achieved under plausible coronal conditions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

A MODEL FOR THERMAL PHASE VARIATIONS OF CIRCULAR AND ECCENTRIC EXOPLANETS

We present a semi-analytic model atmosphere for close-in exoplanets that captures the essential physics of phase curves: orbital and viewing geometry, advection, and re-radiation. We calibrate the model with the well-characterized transiting planet, HD 189733b, then compute light curves for seven of the most eccentric transiting planets: Gl 436b, HAT-P-2b, HAT-P-11b, HD 17156b, HD 80606b, WASP-17b, and XO-3b. We present phase variations for a variety of different radiative times and wind speeds. In the limit of instant re-radiation, the light-curve morphology is entirely dictated by the planet's eccentricity and argument of pericenter: the light curve maximum leads or trails the eclipse depending on whether the planet is receding from or approaching the star at superior conjunction, respectively. For a planet with non-zero radiative timescales, the phase peak occurs early for super-rotating winds, and late for sub-rotating winds. We find that for a circular orbit, the timing of the phase variation maximum with respect to superior conjunction indicates the direction of the dominant winds, but cannot break the degeneracy between wind speed and radiative time. For circular planets the phase minimum occurs half an orbit away from the phase maximum-despite the fact that the coolest longitudes are always near the dawn terminator-and therefore does not convey any additional information. In general, increasing the advective frequency or the radiative time has the effect of reducing the peak-to-trough amplitude of phase variations, but there are interesting exceptions to these trends. Lastly, eccentric planets with orbital periods significantly longer than their radiative time exhibit 'ringing', whereby the hot spot generated at periastron rotates in and out of view. The existence of ringing makes it possible to directly measure the wind speed (the frequency of the ringing) and the radiative time constant (the damping of the ringing).

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

SPECTRAL INDEX AS A FUNCTION OF MASS ACCRETION RATE IN BLACK HOLE SOURCES: MONTE CARLO SIMULATIONS AND AN ANALYTICAL DESCRIPTION

We present herein a theoretical study of correlations between spectral indexes of X-ray emergent spectra and mass accretion rate ( m-dot ) in black hole (BH) sources, which provide a definitive signature for BHs. It has been firmly established, using the Rossi X-ray Timing Explorer (RXTE) in numerous BH observations during hard-soft state spectral evolution, that the photon index of X-ray spectra increases when m-dot increases and, moreover, the index saturates at high values of m-dot . In this paper, we present theoretical arguments that the observationally established index saturation effect versus mass accretion rate is a signature of the bulk (converging) flow onto the BH. Also, we demonstrate that the index saturation value depends on the plasma temperature of converging flow. We self-consistently calculate the Compton cloud (CC) plasma temperature as a function of mass accretion rate using the energy balance between energy dissipation and Compton cooling. We explain the observable phenomenon, index- m-dot correlations using a Monte Carlo simulation of radiative processes in the innermost part (CC) of a BH source and we account for the Comptonization processes in the presence of thermal and bulk motions, as basic types of plasma motion. We show that, when m-dot increases, BH sources evolve to high and very soft states (HSS and VSS, respectively), in which the strong blackbody(BB)-like and steep power-law components are formed in the resulting X-ray spectrum. The simultaneous detections of these two components strongly depends on sensitivity of high-energy instruments, given that the relative contribution of the hard power-law tail in the resulting VSS spectrum can be very low, which is why, to date RXTE observations of the VSS X-ray spectrum have been characterized by the presence of the strong BB-like component only. We also predict specific patterns for high-energy e-fold (cutoff) energy (E{sub fold}) evolution with m-dot for thermal and dynamical (bulk) Comptonization cases. For the former case, E{sub fold} monotonically decreases with m-dot , in the latter case, the E{sub fold} decrease is followed by its increase at high values of m-dot . The observational evolution of E{sub fold} versus m-dot can be another test for the presence of a converging flow effect in the formation of the resulting spectra in the close vicinity of BHs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

X-RAY SIGNATURES OF NON-EQUILIBRIUM IONIZATION EFFECTS IN GALAXY CLUSTER ACCRETION SHOCK REGIONS

The densities in the outer regions of clusters of galaxies are very low, and the collisional timescales are very long. As a result, heavy elements will be under-ionized after they have passed through the accretion shock. We have studied systematically the effects of non-equilibrium ionization for relaxed clusters in the {Lambda}CDM cosmology using one-dimensional hydrodynamic simulations. We found that non-equilibrium ionization effects do not depend on cluster mass, but depend strongly on redshift which can be understood by self-similar scaling arguments. The effects are stronger for clusters at lower redshifts. We present X-ray signatures such as surface brightness profiles and emission lines in detail for a massive cluster at low redshift. In general, soft emission (0.3-1.0 keV) is enhanced significantly by under-ionization, and the enhancement can be nearly an order of magnitude near the shock radius. The most prominent non-equilibrium ionization signature we found is the O VII and O VIII line ratio. The ratios for non-equilibrium ionization and collisional ionization equilibrium models are different by more than an order of magnitude at radii beyond half of the shock radius. These non-equilibrium ionization signatures are equally strong for models with different non-adiabatic shock electron heating efficiencies. We have also calculated the detectability of the O VII and O VIII lines with the future International X-ray Observatory (IXO). Depending on the line ratio measured, we conclude that an exposure of {approx}130-380 ks on a moderate-redshift, massive regular cluster with the X-ray Microcalorimeter Spectrometer (XMS) on the IXO will be sufficient to provide a strong test for the non-equilibrium ionization model.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

THE DYNAMICS OF SPIRAL ARMS IN PURE STELLAR DISKS

It has been believed that spiral arms in pure stellar disks, especially the ones spontaneously formed, decay in several galactic rotations due to the increase of stellar velocity dispersions. Therefore, some cooling mechanism, for example dissipational effects of the interstellar medium, was assumed to be necessary to keep the spiral arms. Here, we show that stellar disks can maintain spiral features for several tens of rotations without the help of cooling, using a series of high-resolution three-dimensional N-body simulations of pure stellar disks. We found that if the number of particles is sufficiently large, e.g., 3 x 10{sup 6}, multi-arm spirals developed in an isolated disk can survive for more than 10 Gyr. We confirmed that there is a self-regulating mechanism that maintains the amplitude of the spiral arms. Spiral arms increase Toomre's Q of the disk, and the heating rate correlates with the squared amplitude of the spirals. Since the amplitude itself is limited by Q, this makes the dynamical heating less effective in the later phase of evolution. A simple analytical argument suggests that the heating is caused by gravitational scattering of stars by spiral arms and that the self-regulating mechanism in pure stellar disks can effectively maintain spiral arms on a cosmological timescale. In the case of a smaller number of particles, e.g., 3 x 10{sup 5}, spiral arms grow faster in the beginning of the simulation (while Q is small) and they cause a rapid increase of Q. As a result, the spiral arms become faint in several Gyr.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

TURBULENT OXYGEN FLAMES IN TYPE Ia SUPERNOVAE

In previous studies, we examined turbulence-flame interactions in carbon-burning thermonuclear flames in Type Ia supernovae. In this study, we consider turbulence-flame interactions in the trailing oxygen flames. The two aims of the paper are to examine the response of the inductive oxygen flame to intense levels of turbulence, and to explore the possibility of transition to detonation in the oxygen flame. Scaling arguments analogous to the carbon flames are presented and then compared against three-dimensional simulations for a range of Damkoehler numbers (Da{sub 16}) at a fixed Karlovitz number. The simulations suggest that turbulence does not significantly affect the oxygen flame when Da{sub 16} < 1, and the flame burns inductively some distance behind the carbon flame. However, for Da{sub 16}>1, turbulence enhances heat transfer and drives the propagation of a flame that is narrower than the corresponding inductive flame would be. Furthermore, burning under these conditions appears to occur as part of a combined carbon-oxygen turbulent flame with complex compound structure. The simulations do not appear to support the possibility of a transition to detonation in the oxygen flame, but do not preclude it either.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

EVOLUTION OF CORONAL MASS EJECTION MORPHOLOGY WITH INCREASING HELIOCENTRIC DISTANCE. I. GEOMETRICAL ANALYSIS

At launch, coronal mass ejections (CMEs) are often approximated as locally cylindrical objects with circular cross sections. However, CMEs have long been known to propagate almost radially away from the Sun along with the bulk solar wind. This has important consequences for the structure of CMEs; an initially circular cross section will be severely flattened by this radial motion. Yet calculations of total flux and helicity transport by CMEs based on in situ observations still use the assumption of a locally cylindrical object. In this paper, we investigate the morphology of an interplanetary CME based upon geometric arguments. By radially propagating an initial cylindrical object that maintains a constant ratio between its expansion speed and bulk flow, A, we show that the flattening, or 'pancaking', of the two-dimensional cross section effectively ceases; the aspect ratios of these CMEs converge to a fixed value as they propagate further into the heliosphere. Thereafter the CME morphology is scale invariant. We predict aspect ratios of 5 {+-} 1 at terrestrial distances. By correlating a planetary shock with an interplanetary shock linked to a CME, these aspect ratios are estimated using in situ measurements in Paper II. These estimates are made at various heliocentric distances.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

AKARI NEAR-INFRARED SPECTRAL OBSERVATIONS OF SHOCKED H{sub 2} GAS OF THE SUPERNOVA REMNANT IC 443

We present near-infrared (2.5-5.0 {mu}m) spectra of shocked H{sub 2} gas in the supernova remnant IC 443, obtained with the satellite AKARI. Three shocked clumps-known as B, C, and G-and one background region were observed, and only H{sub 2} emission lines were detected. Except in clump B, the extinction-corrected level population shows the ortho-to-para ratio of {approx}3.0. From the level populations of clumps C and G-both the one obtained with AKARI and the one extended with previous mid-infrared observations-we found that the {upsilon} = 0 levels are more populated than the {upsilon}>0 levels at a fixed level energy, which cannot be reproduced by any combination of H{sub 2} gas in local thermodynamic equilibrium. The populations are described by the two-density power-law thermal admixture model, revised to include the collisions with H atoms. We attributed the lower (n(H{sub 2})=10{sup 2.8}-10{sup 3.8} cm{sup -3}) and higher (n(H{sub 2})=10{sup 5.4}-10{sup 5.8} cm{sup -3}) density gases to the shocked H{sub 2} gas behind C-type and J-type shocks, respectively, based on several arguments including the obtained high H I abundance n(H I)/n(H{sub 2}) = 0.01. Under the hierarchical picture of molecular clouds, the C-type and J-type shocks likely propagate into 'clumps' and 'clouds' (interclump media), respectively. The power-law index b of 1.6 and 3.5, mainly determined by the lower density gas, is attributed to the shock-velocity diversity, which may be a natural result during shock-cloud interactions. According to our results, H{sub 2} {upsilon} = 1 {yields} 0 S(1) emission is mainly from J shocks propagating into interclump media. The H{sub 2} emission was also detected at the background region, and this diffuse H{sub 2} emission may originate from the collisional process in addition to ultraviolet photon pumping.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

SIMULTANEOUS MONITORING OF THE PHOTOMETRIC AND POLARIMETRIC ACTIVITY OF THE YOUNG STAR PV Cep IN THE OPTICAL/NEAR-INFRARED BANDS

We present the results of a simultaneous monitoring, lasting more than two years, of the optical and near-infrared photometric and polarimetric activity of the variable protostar PV Cep. During the monitoring period, an outburst with declining phase ({Delta}J {approx} 3 mag) lasting about 120 days occurred in all the photometric bands. A time lag of {approx}30 days between optical and infrared light curves has been measured and interpreted in the framework of an accretion event. This latter is directly recognizable in the significant variations of the near-infrared colors, which appear bluer in the outburst phase, when the star dominates the emission, and redder in the declining phase, when the disk emission prevails. All the observational data have been combined to derive a coherent picture of the complex morphology of the whole PV Cep system that is composed of, in addition to the star and the accretion disk, a variable bi-conical nebula. The mutual interaction between all these components is the cause of the high value of the polarization ({approx} 20%) and of its fluctuations. The observational data concur to indicate that PV Cep is not a genuine EXor star, but rather a more complex object; moreover, the case of PV Cep leads to arguments about the classification of other recently discovered young sources in outburst that have been considered, perhaps oversimplifying, as EXor.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

DISCOVERY OF A FAINT X-RAY COUNTERPART AND A PARSEC-LONG X-RAY TAIL FOR THE MIDDLE-AGED, {gamma}-RAY-ONLY PULSAR PSR J0357+3205

The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope opened a new era for pulsar astronomy, detecting {gamma}-ray pulsations from more than 60 pulsars, {approx}40% of which are not seen at radio wavelengths. One of the most interesting sources discovered by LAT is PSR J0357+3205, a radio-quiet, middle-aged ({tau}{sub C} {approx} 0.5 Myr) pulsar standing out for its very low spin-down luminosity (E-dot{sub rot}{approx}6x10{sup 33} erg s{sup -1}), indeed the lowest among non-recycled {gamma}-ray pulsars. A deep X-ray observation with Chandra (0.5-10 keV), coupled with sensitive optical/infrared ground-based images of the field, allowed us to identify PSR J0357+3205 as a faint source with a soft spectrum, consistent with a purely non-thermal emission (photon index {Gamma} = 2.53 {+-} 0.25). The absorbing column (N{sub H} = 8 {+-} 4 x 10{sup 20} cm{sup -2}) is consistent with a distance of a few hundred parsecs. Moreover, the Chandra data unveiled a huge (9 arcmin long) extended feature apparently protruding from the pulsar. Its non-thermal X-ray spectrum points to synchrotron emission from energetic particles from the pulsar wind, possibly similar to other elongated X-ray tails associated with rotation-powered pulsars and explained as bow-shock pulsar wind nebulae (PWNe). However, energetic arguments as well as the peculiar morphology of the diffuse feature associated with PSR J0357+3205 make the bow-shock PWN interpretation rather challenging.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

A BOUND ON THE LIGHT EMITTED DURING THE THERMALLY PULSING ASYMPTOTIC GIANT BRANCH PHASE

The integrated luminosity of the thermally pulsing asymptotic giant branch (TP-AGB) phase is a major uncertainty in stellar population synthesis models. We revise the white dwarf initial-final mass relation (IFMR), incorporating the latest composition and distance measurements for several clusters. Using this IFMR and stellar interiors models, we demonstrate that a significant fraction of the core mass growth for intermediate (1.5 < M{sub sun} < 6) mass stars must take place during the TP-AGB phase. This conclusion holds using models both with and without convective overshoot. We find evidence that the peak fractional core mass contribution for TP-AGB stars is {approx}20% and occurs for stars between 2 M{sub sun} and 3.5 M{sub sun}. Using a simple fuel consumption argument we couple this core mass increase to a lower limit on the TP-AGB phase energy output. We demonstrate that the current TP-AGB models of Pietrinferni et al. and Bertelli et al. systematically grow the core less than we require while the latter predict sufficient integrated light. Our calculated lower bound, coupled with chemical evolution constraints, may provide an upper limit to the integrated luminosity of stars in the TP-AGB phase. Alternatively, a robust measurement of the emitted light in this phase and our constraints could set strong constraints on helium enrichment from TP-AGB stars. We estimate the yields predicted by current models as a function of initial mass. Implications for stellar population studies and prospects for improvements are discussed.

79 ASTRONOMY AND ASTROPHYSICS↗