Distribution of inhomogeneities in the interstellar plasma in the directions of three distant pulsars from observations with the RadioAstron ground–space interferometer     

M. V. Popov  A. S. Andrianov  N. Bartel  C. Gwinn  B. C. Joshi  D. Jauncey  N. S. Kardashev  A. G. Rudnitskii  T. V. Smirnova  V. A. Soglasnov  E. N. Fadeev  V. I. Shishov 《Astronomy Reports》2016,60(9):792-806

The RadioAstron ground–space interferometer has been used to measure the angular sizes of the scattering disks of the three distant pulsars B1641–45, B1749–28, and B1933+16. The observations were carried out with the participation of the Westerbork Synthesis Radio Telescope; two 32-m telescopes at Torun, Poland and Svetloe, Russia (the latter being one antenna of the KVAZAR network); the Saint Croix VLBA antenna; the Arecibo radio telescope; the Parkes, Narrabri (ATCA), Mopra, Hobart, and Ceduna Australian radio telescopes; and the Hartebeesthoek radio telescope in South Africa. The full widths at half maximum of the scattering disks were 27 mas at 1668 MHz for B1641–45, 0.5 mas at 1668 MHz for B1749–28, and 12.3 at 316 MHz and 0.84 mas at 1668 MHz for B1933+16. The characteristic time scales for scatter-broadening of the pulses on inhomogeneities in the interstellar plasma τ_sc were also measured for these pulsars using various methods. Joint knowledge of the size of the scattering disk and the scatter-broadening time scale enables estimation of the distance to the effective scattering screen d. For B1641–45, d = 3.0 kpc for a distance to the pulsar D = 4.9 kpc, and for B1749–28, d = 0.95 kpc for D = 1.3 kpc. Observations of B1933+16 were carried out simultaneously at 316 and 1668 MHz. The positions of the screen derived using the measurements at the two frequencies agree: d ₁ = 2.6 and d ₂ = 2.7 kpc, for a distance to the pulsar of 3.7 kpc. Two screens were detected for this pulsar from an analysis of parabolic arcs in the secondary dynamic spectrum at 1668 MHz, at 1.3 and 3.1 kpc. The scattering screens for two of the pulsars are identified with real physical objects located along the lines of sight toward the pulsars: G339.1–04 (B1641–45) and G0.55–0.85 (B1749–28).  相似文献   

Scanned superresolution in theory and practice     

E. E. Bloemhof  R. M. Danen  C. R. Gwinn 《Experimental Astronomy》1994,3(1-4):149-150

We discuss some theoretical and practical considerations relating to achieving spatial resolution higher than /D, the nominal diffraction-limited resolution of a telescope of diameter D. Although the Nyquist criterion is often invoked as a justification for choosing pixel scales in array cameras, higher sampling densities are favored for imaging at and somewhat beyond the diffraction limit.  相似文献   

VLBI Imaging of Water Maser Emission from a Nuclear Disk in NGC 1068     

L. J. Greenhill  C. R. Gwinn 《Astrophysics and Space Science》1997,248(1-2):261-267

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Studying Pulsar Emission Regions Using Interstellar Scattering     

Carl R. Gwinn 《Astrophysics and Space Science》2001,278(1-2):65-70

Radio-wave scattering in the interstellar plasma provides the means to circumvent the diffraction limit for earth-based instruments, and to image the emission regions of pulsars. For the past 25 years, observers have sought to exploit this fact to learn how pulsars shine. I review the techniques developed, and summarize measurements of size of emission regions of pulsars to date. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

The role of phosphorus in rhyolitic liquids as determined from the homogeneous iron redox equilibrium   总被引：1，自引：1，他引：1  

Rosa Gwinn  Paul C. Hess 《Contributions to Mineralogy and Petrology》1993,113(3):424-435

The redox ratio of iron is used as an indicator of solution properties of silicate liquids in the system (SiO–Al₂O₃–K₂O–FeO–Fe₂O₃–P₂O₅). Glasses containing 80–85 mol% SiO₂ with 1 mol% Fe₂O₃ and compositions covering a range of K₂O/Al₂O₃ were synthesized at 1400°C in air (fixed fO₂). Variations in the ratio FeO/FeO_1.5 resulting from the addition of P₂O₅ are used to determine the solution behavior of phosphorus and its interactions with other cations in the silicate melt. In 80 mol% SiO₂ peralkaline melts the redox ratio, expressed as FeO/FeO_1.5, is unchanged relative to the reference curve with the addition of 3 mol% P₂O₅. Yet, the iron redox ratio in the 85 mol% SiO₂ potassium aluminosilicate melts is decreased relative to phosphorus-free liquids even for small amounts of P₂O₅ (0.5 mol%). The redox ratio in peraluminous melts is decreased relative to phosphorus- free liquids at P₂O₅ concentrations of 3 mol%. In peraluminous liquids, complexing of both Fe⁺³–O–P⁺⁵ and Al⁺³–O–P⁺⁵ occur. The activity coefficient of Fe⁺³ is decreased because more ferric iron can be accommodated than in phosphorus-free liquids. In peralkaline melts, there is no evidence that P⁺⁵ is removing K⁺ from either Al⁺³ or Fe⁺³ species. In chargebalanced melts with 3 mol% Fe₂O₃ and very high P₂O₅ concentrations, phosphorus removes K⁺ from K–O–Fe⁺³ complexes resulting in a redox increase. P₂O₅ should be accommodated easily in peraluminous rhyolitic liquids and phosphate saturation may be suppressed relative to metaluminous rhyolites. In peralkaline melts, phosphate solubility may increase as a result of phosphorus complexing with alkalis. The complexing stoichiometry may be variable, however, and the relative influence of peralkalinity versus temperature on phosphate solubility in rhyolitic melts deserves greater attention.  相似文献   

Iron and titanium solution properties in peraluminous and peralkaline rhyolitic liquids     

Gwinn  Rosa  Hess  Paul C. 《Contributions to Mineralogy and Petrology》1989,101(3):326-338

The saturation surface of pseudobrookite (Fe₂TiO₅) was determined for melts in the system SiO₂-Al₂O₃-K₂O-FeO-Fe₂O₃-TiO₂ at 1400° C and 1 atm. The variation in concentrations of Fe₂O₃, TiO₂ and Fe₂TiO₅ in liquids can be used to infer relative changes in activity coefficients of these components with changing K₂O/(K₂O+Al₂O₃) of the melts. Saturation concentrations of these components are low and relatively constant in the peraluminous melts and increase with increasing K₂O/(K₂O+Al₂O₃) in peralkaline liquids. The activity coefficients of Fe₂O₃ and TiO₂ and Fe₂TiO₅, therefore, are higher in peraluminous liquids than in peralkaline liquids in this system. In addition, the iron redox ratio was measured as a function of K₂O/(K₂O+Al₂O₃) for liquids just below the saturation surface; was fixed so all variations in redox ratio are entirely due to changes in melt composition. The redox ratio from unsaturated liquids was applied to saturated liquids where redox analysis of the glass is impossible. The homogeneous equilibrium experiments indicate that the activity coefficient of Fe₂O₃ relative to that of FeO is significantly greater in peraluminous melts than peralkaline melts. Both the heterogeneous and homogeneous equilibria suggest that in peralkaline liquids K⁺in excess of that required to charge balance tetrahedral Al⁺³ is used to stabilize both Fe⁺³ and Ti⁺⁴. Calculations show that ferric iron and titanium compete equally effectively for charge-balancing potassium but neither can outcompete aluminum. The observed changes in solution properties of Fe₂O₃ and TiO₂ in the synthetic melts are used to explain variations in Fe-Ti oxide stabilities in natural peraluminous and peralkaline rhyolites and granites. Since the activity coefficients of both ferric iron and titanium are significantly higher in peraluminous liquids than in peralkaline liquids, Fe-Ti oxides should occur earlier in the crystallization sequence in peraluminous rhyolites than in peralkaline rhyolites. In addition, iron will be reduced in peraluminous granites and rhyolites relative to peralkaline ones under comparable P, T, and . Finally, observed crystallization patterns for minerals containing highly charged cations other than ferric iron and titanium are evaluated in the context of this and other experimental studies.  相似文献   

Sub-arcsecond 10 micron imaging of newly-formed stars     

R. M. Danen  C. R. Gwinn  E. E. Bloemhof 《Experimental Astronomy》1994,3(1-4):153-154

We describe initial results of a program to image massive newly-formed stars with sub-arc second spatial resolution. We discuss high-precision diffraction-limited size measurements at =10 m made using the 3 m Lick telescope. The point-spread function has FWHM 0.7; deconvolution yields a spatial resolution of 0.35. We find that the core component of one such object, LkH 101, is unresolved at these scales, and we are able to set a 95%-confidence upper limit of 270 AU for the diameter of the circumstellar dust shell. This places the dust at the same radial scale as a strong ionized stellar wind region seen at radio wavelengths. Our observations, when combined with published spectral observations, rule out an optically thick circumstellar disk but allow a radially thin, anisotropic distribution of dust, or alternatively an isotropic distribution of dust with a narrow range of large grain sizes.  相似文献   

RadioAstron Science Program Five Years after Launch: Main Science Results     

N. S. Kardashev  A. V. Alakoz  A. S. Andrianov  M. I. Artyukhov  W. Baan  V. E. Babyshkin  N. Bartel  O. S. Bayandina  I. E. Val’tts  P. A. Voitsik  A. Z. Vorobyov  C. Gwinn  J. L. Gomez  G. Giovannini  D. Jauncey  M. Johnson  H. Imai  Y. Y. Kovalev  S. E. Kurtz  M. M. Lisakov  A. P. Lobanov  V. A. Molodtsov  B. S. Novikov  A. V. Pogodin  M. V. Popov  A. S. Privesenzev  A. G. Rudnitski  G. M. Rudnitski  T. Savolainen  T. V. Smirnova  A. M. Sobolev  V. A. Soglasnov  K. V. Sokolovsky  E. N. Filippova  V. V. Khartov  M. E. Churikova  A. E. Shirshakov  V. I. Shishov  P. Edwards 《Solar System Research》2017,51(7):535-554

The RadioAstron ground-space interferometer provides the highest angular resolution achieved now in astronomy. The detection of interferometric fringes from quasars with this angular resolution on baselines of 100–200 thousand km suggests the brightness temperatures which exceed the Compton limit by two orders of magnitude. Polarimetric measurements on ground-space baselines have revealed fine structure testifying to recollimation shocks on scales of 100–250 μas and a helical magnetic field near the base of radio emission in BL Lacertae. Substructure within a the scattering disk of pulsar emission on interferometer baselines (from 60000 to 250000 km) was discovered. This substructure is produced by action of the interstellar interferometer with an effective baseline of about 1 AU and the effective angular resolution of better than 1 μas. Diameters of scattering disks were measured for several pulsars, and distances to diffusing screens were evaluated. The ground-space observations of sources of the maser radiation in lines of water and hydroxyl have shown that the maser sources in star-forming regions remain unresolved on baselines, which considerably exceed the Earth diameter. These very compact and bright features with angular sizes of about 20–60 μas correspond to linear sizes of about 5–10 million km (several solar diameters).  相似文献   

Scintillations and Interstellar Lévy Flights     

Stanislav Boldyrev  Carl R. Gwinn 《Astrophysics and Space Science》2004,292(1-4):147-153

We present the new model for interstellar scintillations, which assumes that the electron density fluctuations causing the scintillations obey Lévy statistics rather than Gaussian statistics, as was thought previously. Our model explains the observed anomalous scaling of the signal time-width with respect to the pulsar distance, which remained a puzzle for more than 30 years.  相似文献   

Distribution of interstellar plasma in the direction of PSR B0525+21 from data obtained on a ground–space interferometer     

A. S. Andrianov  T. V. Smirnova  V. I. Shishov  C. Gwinn  M. V. Popov 《Astronomy Reports》2017,61(6):513-520

Observations on the RadioAstron ground–space interferometer with the participation of the Green Bank and Arecibo ground telescopes at 1668 MHz have enabled studies of the characteristics of the interstellar plasma in the direction of the pulsar PSR B0525+21. The maximum projected baseline for the ground–space interferometer was 233 600 km. The scintillations in these observations were strong, and the spectrum of inhomogeneties in the interstellar plasma was a power law with index n = 3.74, corresponding to a Kolmogorov spectrum. A new method for estimating the size of the scattering disk was applied to estimate the scattering angle (scattering disk radius) in the direction toward PSR B0525+21, θ _scat = 0.028 ± 0.002 milliarcsecond. The scattering in this direction occurs in a plasma layer located at a distance of 0.1Z from the pulsar, where Z is the distance from the pulsar to the observer. For the adopted distance Z = 1.6 kpc, the screen is located at a distance of 1.44 kpc from the observer.  相似文献