Magnetic hydrogen atmosphere models and the neutron star RX J1856.5–3754     

Wynn C. G. Ho †  David L. Kaplan ‡  Philip Chang  Matthew van Adelsberg  Alexander Y. Potekhin 《Monthly notices of the Royal Astronomical Society》2007,375(3):821-830

RX J1856.5−3754 is one of the brightest nearby isolated neutron stars (INSs), and considerable observational resources have been devoted to it. However, current models are unable to satisfactorily explain the data. We show that our latest models of a thin, magnetic, partially ionized hydrogen atmosphere on top of a condensed surface can fit the entire spectrum, from X-rays to optical, of RX J1856.5−3754, within the uncertainties. In our simplest model, the best-fitting parameters are an interstellar column density   N _H≈ 1 × 10²⁰ cm⁻²  and an emitting area with   R ^∞≈ 17 km  (assuming a distance to RX J1856.5−3754 of 140 pc), temperature   T ^∞≈ 4.3 × 10⁵ K  , gravitational redshift   z _g ∼ 0.22  , atmospheric hydrogen column   y _H≈ 1 g cm⁻²  , and magnetic field   B ≈ (3–4) × 10¹² G  ; the values for the temperature and magnetic field indicate an effective average over the surface. We also calculate a more realistic model, which accounts for magnetic field and temperature variations over the NS surface as well as general relativistic effects, to determine pulsations; we find that there exist viewing geometries that produce pulsations near the currently observed limits. The origin of the thin atmospheres required to fit the data is an important question, and we briefly discuss mechanisms for producing these atmospheres. Our model thus represents the most self-consistent picture to date for explaining all the observations of RX J1856.5−3754.  相似文献   

Quantifying nitrogen cycling beneath a meander of a low gradient,N‐impacted,agricultural stream using tracers and numerical modelling     

Stephen J. Van der Hoven  Nicholas J. Fromm  Eric W. Peterson 《水文研究》2008,22(8):1206-1215

In watersheds impacted by nitrate from agricultural fertilizers, nitrification and denitrification may be decoupled as denitrification in the hyporheic zone is not limited to naturally produced nitrate. While most hyporheic research focuses on the 1–2 m of sediment beneath the stream bed, there are a limited number of studies that quantify nitrogen (N) cycling at larger hyporheic scales (10s of metres to kms). We conducted an investigation to quantify N cycling through a single meander of a low gradient, meandering stream, draining an agricultural watershed. Chemistry (major ions and N species) and hydrologic data were collected from the stream and groundwater beneath the meander. Evidence indicates that nearly all the shallow groundwater flowing beneath the meander originates as stream water on the upgradient side of the meander, and returns to the stream on the downgradient side. We quantified the flux of water beneath the meander using a numerical model. The flux of N into and out of the meander was quantified by multiplying the concentration of the important N species (nitrate, ammonium, dissolved organic nitrogen (DON)) by the modelled water fluxes. The flux of N into the meander is dominated by nitrate, and the flux of N out of the meander is dominated by ammonium and DON. While stream nitrate varied seasonally, ammonium and DON beneath the meander were relatively constant throughout the year. When stream nitrate concentrations are high (>2 mg litre^?1), flow beneath the meander is a net sink for N as more N from nitrate in stream water is consumed than is produced as ammonium and DON. When stream nitrate concentrations are low (<2 mg litre^?1), the flux of N entering is less than exiting the meander. On an annual basis, the meander hyporheic flow serves as a net sink for N. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

Spectral evolution of V838 Monocerotis in the optical and near-infrared in early 2002     

M. T. Rushton  T. R. Geballe  A. V. Filippenko  R. Chornock  W. Li  D. C. Leonard †  R. J. Foley  A. Evans  B. Smalley  J. Th. van Loon  S. P. S. Eyres 《Monthly notices of the Royal Astronomical Society》2005,360(4):1281-1289

We report optical and near-infrared spectroscopy, and optical spectropolarimetry, of the peculiar variable V838 Mon during the multiple outburst phase in early 2002. The spectral evolution is exceptional. Our earliest spectra (2002 January) are noteworthy for their strong absorption lines of barium and strontium in the optical, and bands of CO and circumstellar H₂O in the near-infrared. All but the CO weaken or are absent in later spectra. The behaviour of the CO band during this phase is extraordinary: initially in absorption, it was observed two months later in optically thick emission. The excitation of the CO is probably the result of the propagation of a shock wave at the third maximum. The two spectropolarimetric epochs were taken 6 and 27 d after the second outburst on 2002 February 8. The polarization at both times was measured to be   p_V ≈ 2.7  per cent. Nearly all of the measured polarization is believed to be due to interstellar dust, a conclusion that is consistent with previous studies. At both epochs, however, a weak and variable intrinsic component is thought to be present. Between January and March of 2002 the luminosity of V838 Mon increased by a factor of 15 and the apparent diameter increased fourfold.  相似文献   

Delaunay Tessellation Field Estimator analysis of the PSCz local Universe: density field and cosmic flow     

Emilio Romano-Díaz  Rien van de Weygaert 《Monthly notices of the Royal Astronomical Society》2007,382(1):2-28

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On the prevalence of radio-loud active galactic nuclei in brightest cluster galaxies: implications for AGN heating of cooling flows     

P. N. Best  A. von der Linden  G. Kauffmann  T. M. Heckman  C. R. Kaiser 《Monthly notices of the Royal Astronomical Society》2007,379(3):894-908

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Zum Problem der Kontinentalverschiebung     

W. A. J. M. van Waterschoot van der Gracht 《International Journal of Earth Sciences》1938,30(1-2):8-9

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GRIPES     

Tjeerd van Andel 《地学学报》1995,7(2):300-301

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Energization of interstellar media and cosmic ray production by jets from X-ray binaries     

R. P. Fender  T. J. Maccarone  Z. van Kesteren 《Monthly notices of the Royal Astronomical Society》2005,360(3):1085-1090

Drawing on recent estimates of the power of jets from X-ray binary systems as a function of X-ray luminosity, combined with improved estimates of the relevant  log( N )–log( L _X)  luminosity functions, we calculate the total energy input to the interstellar medium (ISM) from these objects. The input of kinetic energy to the ISM via jets is dominated by those of the black hole systems, in contrast to the radiative input, which is dominated by accreting neutron stars. Summing the energy input from black hole jets L _J in the Milky Way, we find that it is likely to correspond to ≥1 per cent of L _SNe, the time-averaged kinetic luminosity of supernovae, and ≥5 per cent of L _CR, the cosmic ray luminosity. Given uncertainties in jet power estimates, significantly larger contributions are possible. Furthermore, in elliptical galaxies with comparable distributions of low mass X-ray binaries, but far fewer supernovae, the ratio   L _J/ L _SNe  is likely to be larger by a factor of ∼5. We conclude that jets from X-ray binaries may be an important, distributed, source of kinetic energy for the ISM in the form of relativistic shocks, and as a result are likely to be a major source of cosmic rays.  相似文献   

On the assembly history of dark matter haloes     

Yun Li  H. J. Mo  Frank C. van den Bosch  W. P. Lin 《Monthly notices of the Royal Astronomical Society》2007,379(2):689-701

We study the mass assembly history (MAH) of dark matter haloes. We compare MAHs obtained using (i) merger trees constructed with the extended Press–Schechter (EPS) formalism, (ii) numerical simulations and (iii) the Lagrangian perturbation code pinocchio . We show that the pinocchio MAHs are in excellent agreement with those obtained using numerical simulations, while the EPS formalism predicts MAHs that occur too late. pinocchio , which is much less CPU intensive than N -body simulation, can be run on a simple personal computer, and does not require any labour intensive post-simulation analysis, therefore provides a unique and powerful tool to investigate the growth history of dark matter haloes. Using a suite of 55 pinocchio simulations, with 256³ particles each, we study the MAHs of 12 924 cold dark matter (CDM) haloes in a ΛCDM concordance cosmology. This is by far the largest set of haloes used for any such analysis. For each MAH we derive four different formation redshifts, which characterize different epochs during the assembly history of a dark matter halo. We show that haloes less massive than the characteristic non-linear mass scale establish their potential wells much before they acquire most of their mass. The time when a halo reaches its maximum virial velocity roughly divides its mass assembly into two phases, a fast-accretion phase which is dominated by major mergers, and a slow-accretion phase dominated by minor mergers. Each halo experiences about 3 ± 2 major mergers since its main progenitor had a mass equal to 1 per cent of the final halo mass. This major merger statistic is found to be virtually independent of halo mass. However, the average redshift at which these major mergers occur is strongly mass dependent, with more massive haloes experiencing their major mergers later.  相似文献   

Spitzer Space Telescope spectral observations of AGB stars in the Fornax dwarf spheroidal galaxy     

M. Matsuura  A. A. Zijlstra  J. Bernard-Salas  J. W. Menzies  G. C. Sloan  P. A. Whitelock  P. R. Wood  M.-R. L. Cioni  M. W. Feast  E. Lagadec  J. Th. van Loon  M. A. T. Groenewegen  G. J. Harris 《Monthly notices of the Royal Astronomical Society》2007,382(4):1889-1900

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