This paper describes a newly designed Sun and Aureole Measurement (SAM) aureolegraph and the first results obtained with this instrument. SAM measurements of solar aureoles produced by cirrus and cumulus clouds were taken at the Atmospheric Radiation Measurement Program(ARM) Central Facility in Oklahoma during field experiments conducted in June 2007 and compared with simultaneous measurements from a variety of other ground-based instruments. A theoretical relationship between the slope of the aureole profile and the size distribution of spherical cloud particles is based on approximating scattering as due solely to diffraction, which in turn is approximated using a rectangle function. When the particle size distribution is expressed as a power-law function of radius, the aureole radiance as a function of angle from the center of the solar disk also follows a power law, with the sum of the two powers being -5. This result also holds if diffraction is modeled with an Airy function. The diffraction approximation is applied to SAM measurements with optical depths less than or similar to 2 to derive the effective radii of cloud particles and particle size distributions between similar to 2.5 and similar to 25 mu m. The SAM results yielded information on cloud properties complementary to that obtained with ARM Central Facility instrumentation. A network of automated SAM units [similar to the Aerosol Robotic Network (AERONET) system] would provide a practical means to gain fundamental new information on the global statistical properties of thin (optical depth less than or similar to 10) clouds, thereby providing unique information on the effects of such clouds upon the earth's energy budget.

If \(\gamma\)-ray bursts are produced by hypernovae, a problem that must be confronted is how the core of the hypernova progenitor retains or acquires sufficient angular momentum to produce the requisite axisymmetric collapse. Physical processes during the evolution of an isolated massive star will tend to extract any initial angular momentum from the stellar core, rendering it difficult for such a star to become a hypernova. However, a substantial fraction of massive stars are members of binary systems. Tidal locking, mass transfer, or stellar merger in an evolved massive binary may lead to the transfer of orbital angular momentum to the core of one of the stars (or the merged star), sufficient to produce the progenitor of a hypernova. We have developed a new stellar evolution code that includes rotation and the mixing of nuclear species and angular momentum due to dynamical and secular shear instabilities, convective motions, and gravity waves. Preliminary results indicate that over a wide range of initial conditions, mass transfer or merger during the course of the evolution of a massive binary results in the formation of an evolved massive star with a rapidly rotating core, providing the initial conditions necessary for a hypernova event.

This paper presents the set of plane-parallel model atmosphere equations for a very hot neutron star (X-ray burst source). The model equations assume both hydrostatic and radiative equilibrium, and the equation of state of an ideal gas in local thermodynamic equilibrium (LTE). The equation of radiative transfer includes terms describing Compton scattering of photons on free electrons in fully relativistic thermal motion, for photon energies approaching m_e *c^2. Model equations take into account many bound-free and free-free energy-dependent opacities of hydrogen, helium, and the iron ions, and also a dozen bound-bound opacities for the highest ions of iron. We solve model equations by partial linearisation and the technique of variable Eddington factors. Large grid of H-He-Fe model atmospheres of X-ray burst sources has been computed for 10^7 < T_eff < 3*10^7 K, a wide range of surface gravity, and various iron abundances. We demonstrate that the spectra of X-ray bursters with iron present in the accreting matter differ significantly from pure H-He spectra (published in an earlier paper), and also from blackbody spectra. Comptonized spectra with significant iron abundance are generally closer to blackbody spectra than spectra of H-He atmospheres. The ratio of color to effective temperatures in our grid always remains in the range 1.2 < T_c/T_eff < 1.85. The present grid of model atmospheres and theoretical X-ray spectra will be used to determine the effective temperatures, radii and M/R ratios of bursting neutron stars from observational data.

Compton scattering plays a crucial role in determining the structure of the atmosphere of an X-ray burster and its theoretical spectrum. Our paper presents a description of the plane-parallel model atmosphere of a very hot neutron star and its theoretical flux spectrum of outgoing radiation. Our model equations take into account all bound-free and free-free monochromatic opacities relevant to hydrogen-helium chemical composition and take into account the effects of Compton scattering of radiation in thermal plasma with fully relativistic thermal velocities. We use Compton scattering terms in the equation of transfer, which precisely describe photon-electron energy and momentum exchange for photons with initial energies exceeding the electron rest mass of 511 keV. Model atmosphere equations are solved with the variable Eddington factors technique. The grid of H-He model atmospheres and flux spectra is computed on a dense mesh of 107 K ≤ Teff ≤ 3 × 107 K and a surface gravity of log g. In many cases, the assumed log g approached the critical gravity log gcr, i.e., the Eddington limit. We confirm that H-He spectra of X-ray bursters deviate from blackbody spectra and discuss their shapes. The table of color to effective temperature ratios shows that theoretical values of Tc/Teff do not exceed 1.9 in H-He atmospheres in hydrostatic and radiative equilibrium.

We present model atmospheres and theoretical spectra for X-ray bursters. Our models include the effects of Compton scattering on free electrons. The atmospheres have compositions that are mixtures of hydrogen, helium, and iron. For our models the ratio of color temperature, $T_c$, to effective temperature, $T_{eff}$, is in the range 1.2 - 1.8. This ratio depends on $T_{eff}$, surface gravity, and iron abundance. For fixed $T_{eff}$ and surface gravity, models with non-zero iron abundance exhibit values of $T_c/T_{eff}$ that are slightly lower than pure hydrogen-helium models.

We explore photometric properties of hypothetical iron core white dwarfs and compute their expected colors in UBVRI Johnson broadband system. Atmospheres of iron core WDs in this paper consist of pure iron covered by a pure hydrogen layer of an arbitrary column mass. LTE model atmospheres and theoretical spectra are calculated on the basis of Los Alamos TOPS opacities and the equation of state from the OPAL project, suitable for nonideal Fe and H gases. We have also computed UBVRI colors of the models and determined an area on the B - V vs. U - B and B - V vs. V - I planes, occupied by both pure Fe, and pure H model atmospheres of WD stars. Finally, we search for iron core white dwarf candidates in the available literature.

We present Chandra ACIS and zeroth order HETG/ACIS-S spectra of the Cas A point source extracted from a 50-ksec imaging and a 70-ksec grating observation of the Cas A supernova remnant. There is mounting evidence that the point source is a thermally emitting neutron star. We fit the ACIS spectra with the latest hydrogen/helium models produced by the new ATM model atmosphere code developed by Paul Joss and Jurek Madej. The ATM code calculates the emergent spectrum from a neutron star atmosphere of specified effective temperature and surface gravity. We have previously carried out model atmosphere fits to a spectrum extracted from the archival Chandra 50-ksec observation (ObsID 114). Here we combine this data with a 70-ksec Chandra High Energy Transmission Grating/ACIS-S (HETG) observation of the X-ray point source (XPS) to simultaneously fit the largest number of ACIS spectra photons from the XPS to date. Our superior ATM model atmosphere fits yield surface effective temperatures of similar or equal to 4.1 MK, a surface emission patch size of similar or equal to 1.8 km, and a stellar radius of similar or equal to 6.5 km. We present the results of fitting power law, blackbody and our hydrogen/helium model atmospheres. In our fits, we correct for the presence of absorption features in the spectrum created by silicon and sulfur along the line of sight, as well as general relativistic effects to calculate the temperature, radius of the star, and radius of the emitting region.

We present new results fitting Chandra ACIS and LETG/HRC-S spectra of the Cas A X-ray point source (XPS) and the isolated neutron star RX J1856.5-3754 to H, H–He, Si–ash, and Fe atmosphere models calculated with our compact atmosphere model code. In the case of the Cas A XPS, we find our atmosphere models fit significantly better than blackbody models, although a wide range of radii is formally permitted. The best fit H model is consistent with a neutron star of radius ∼7.6 km, but with a radiating area scale size of only ∼3 km, consistent with a hot spot geometry (discussed by, e.g. [Astrophys. J. Lett. 531 (2000) L53]). We also show the effects of the Cas A supernova remnant material on the XPS spectrum. In the case of RX J1856.5-3754 we find H and Fe spectra provide unacceptable fits to the X-ray and optical data, but we do find physically plausible results with a Si–ash model for a rapidly rotating neutron star.

The strong polarization signatures of many man-made surface targets suggest their use for detection and discrimination in satellite reconnaissance and surveillance. Since thin clouds on average cover more than 40% of the Earth's surface, they are likely to contribute frequently to the obscuration and apparent polarization of such targets. We have carried out theoretical calculations of cloud particle scattering functions in combination with Monte Carlo calculations of radiative transfer in clouds, in order to determine the polarization signatures of thin clouds as a function of the relative spatial orientations of the Sun, target, and detector, the cloud optical depth, the cloud particle phase (water or ice), and the electromagnetic wavelength. Our calculations of the scattering properties of individual cloud particles revealed situations in which the polarization of scattered, initially unpolarized, sunlight is very high. The theoretical cloud signatures were then combined with image polarization data for man-made surface targets and natural backgrounds in order to determine the consequences for the composite image with clouds of various optical depths. For the case investigated here we find that polarization signatures can be a useful discriminant for a surface target even when viewed through a cloud at optical depths as high as two.

In support of the RAMOS ((R) under bar ussian (A) under bar merican (O) under bar bservation (S) under bar atellites) program, an ongoing series of flights is being conducted using FISTA ((F) under bar lying (I) under bar nfrared (S) under bar ignatures (T) under bar echnology (A) under bar ircraft) which, in part, carries instruments to obtain polarization measurements of solar radiation scattered from clouds. Spectral polarization measurements have been made simultaneously across the 2.5-3.5 mum region as a function of solar scattering angle using the Space Dynamics Laboratory's Hyperspectral Imaging Polarimeter (HIP) instrument. The purpose of the experiment is to verify the expected strong sensitivity of polarization as a function of wavelength within this spectral band, and to select the optimum wavelength for making remote diagnostic measurements of cloud compositions. There is also the complementary goal of studying polarization as a potential discriminator between naturally occurring solar-scatter backgrounds and man-made objects.Additional instrumentation onboard during these flights have indicated that the clouds present in these observations were composed primarily of ice rather than water. This paper is therefore focused on our efforts to model polarization due to solar scattering from non-spherical ice crystals. We describe the FISTA experiment, the HIP instrumentation and the data processing methods used to derive the polarization data products. Finally, we present a simulation of the FISTA polarization experiment and compare these modeled results to the KIP data themselves.

We have carried out a systematic study of the storage of nuclear and gravitational energy in the crusts of old neutron stars that are slowly accreting matter from the interstellar medium or distant binary companion stars. We considered neutron-star masses, radii, and accretion rates ṁ in the ranges 0.1-2.8 M☉, 7-150 km, and 1010-1016 g s-1, respectively. We also investigated the effects of variations in ṁ resulting from periodic passages of the neutron star through the Galactic midplane and/or occasional encounters with molecular clouds. We found, consistent with earlier work, that pycnonuclear reactions lead to the development of density inversions in the crust; the strongest inversions (of order 2% in density), which occur at the interface between 16C and heavier nuclei, store ~1041 ergs of gravitational potential energy. If an instability can trigger the overturn of such an inversion, the released energy would ignite a powerful thermonuclear flash, and the resulting starquake might result in magnetospheric effects that lead to the emission of a γ-ray burst or other energetic astrophysical phenomenon. Although the isotropy of the distribution of γ-ray burst sources, as determined by the BATSE experiment, precludes the possibility that all γ-ray bursts are associated with neutron stars in the Galactic disk, it remains possible that a subset of the observed bursts have their origin in events of this type. From our calculations, we determined that the total nuclear energy that can be stored in the crust of a neutron star is limited by the 16C +16C pycnonuclear reaction, which converts the accreted material at a density of ~5 × 1010 g cm-3 into nuclei substantially closer to nuclear equilibrium. As a result, the total (nuclear plus gravitational) energy, Etot, that can be stored is quite generally limited to less than ~1046 ergs, irrespective of neutron-star mass, radius, mean ṁ, or variations in ṁ. For ṁ=1010 g s-1, it takes ~1010 yr for maximum energy storage to be achieved; for higher accretion rates, the required time decreases very nearly as ṁ−1. For ṁ≳3×1015 g s-1, thermonuclear flashes in the helium shell greatly reduce Etot. Irrespective of the γ-ray burst emission mechanism, the upper limit that we have obtained on Etot places a strong constraint on any model that ascribes a subset of γ-ray bursts to the emission of energy stored in the crusts of neutron stars. Our results are also relevant to any other energetic celestial phenomenon that might be associated with a Galactic-disk population of isolated neutron stars.

GRO J1744-28 is the first known X-ray source to display busts, periodic pulsations, and quasiperiodic oscillations. This source may thus provide crucial clues that will lead to an understanding of the differences in the nature of the X-ray variability from various accreting neutron stars. The orbital period is 11.8 days, and the measured mass function of 1.31 x 10(-4) M. is one of the smallest among all known binaries. If we assume that the donor star is a low-mass giant transferring matter through the inner Lagrange point, then we can show that its mass is lower than similar to 0.7 M. and probably closer to 0.25 M.. Higher mass, but unevolved, donor stars are shown to be implausible.We also demonstrate that the current He core mass of the donor star lies in the range of 0.20-0.25 M.. Thus, this system is most likely in the final stages of losing its hydrogen-rich envelope, with only a small amount of mass remaining in the envelope. If this picture is correct, then GRO J1744-28 may well represent the closest observational link that we have between the low-mass X-ray binaries and recycled binary pulsars in wide orbits. We have carried out a series of binary evolution calculations and explored, both systematically and via a novel Monte Carlo approach, the range of initial system parameters and input physics that can lead to the binary parameters of the present-day GRO J1744-28 system. The input parameters include both the initial total mass and the core mass of the donor star, the neutron-star mass, the strength of the magnetic braking, the mass-capture fraction, and the specifics of the core mass/radius relation for giants. Through these evolution calculations, we compute probability distributions for the current binary system parameters (i.e., the total mass, core mass, radius, luminosity, and K-band magnitude of the donor star, the neutron star mass, the orbital inclination angle, and the semimajor axis of the binary). Our calculations yield the following values for the GRO J1744-28 system parameters (with 95% confidence limits in parentheses): donor star mass: 0.24 M. (0.2-0.7 M.); He core mass of the donor star: 0.22 M. (0.20-0.25 M.); neutron-star mass: 1.7 M. (1.39-1.96 M.); orbital inclination angle: 18 degrees (7 degrees-22 degrees); semi-major axis: 64 It-a (60-67 It-s); radius of the donor star: 6.2 R. (6-9 R.); luminosity of donor star: 23 L. (15-49 L.); and long-term mass transfer rate at the current epoch: 5 x 10(-10) M. yr(-1) (2 x 10(-10) to 5 x 10(-9) M. yr(-1)).We deduce that the magnetic field of the underlying neutron star lies in the range of similar to 1.8 x 10(11) G to similar to 7 x 10(11) G, with a most probable value of 2.7 x 10(11) G. This is evidently sufficiently strong to funnel the accretion how onto the magnetic polar caps and suppress the thermonuclear flashes that would otherwise give rise to the type I X-ray bursts observed in most X-ray bursters. We present a simple paradigm for magnetic accreting neutron stars wherein X-ray pulsars, GRO J1744-28, the Rapid Burster, and the type I X-ray bursters may form a continuum of possible behaviors among accreting neutron stars, with the strength of the neutron-star magnetic field serving as the crucial parameter that determines the mode of X-ray variability from a given object.

The presence of a close binary companion can affect the evolution of a massive star through one or more episodes of mass transfer, or by merger in a common-envelope phase. Monte Carlo calculations indicate that ∼20–35% of all massive supernovae are affected by such processes, and that a substantial fraction of these events will be supernovae of type II. The properties of the progenitor star, the distribution of circumstellar material, the peak supernova luminosity, the shape of the supernova light curve, and other observable features of the supernova event can be affected by prior binary membership. Binary interactions may be the cause of much of the variability among type II supernova light curves. In particular, many of the peculiarities of SN 1987A and SN 1993J may well have resulted from the prior duplicity of the progenitors.

The presence of a close binary companion can affect the evolution of a massive star through one or more episodes of mass transfer, or by merger in a common-envelope phase. Monte Carlo calculations indicate that ∼20–35% of all massive supernovae are affected by such processes, and that a substantial fraction of these events will be supernovae of Type II. The properties of the progenitor star, the distribution of circumstellar material, the peak supernova luminosity, the shape of the supernova light curve, and other observable features of the supernova event can be affected by prior binary membership. Binary interactions may be the cause of much of the variability among Type II supernova light curves. In particular, many of the peculiarities of SN 1987A and SN 1993J may well have resulted from the prior duplicity of the progenitors.