Adaptive Optics: Astronomical Results and Perspectives     

P. Léna 《Experimental Astronomy》1997,7(4):281-284

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The variability of Mercury's exosphere by particle and radiation induced surface release processes     

H Lammer  P Wurz  R Killen  S Massetti  A Milillo 《Icarus》2003,166(2):238-247

Mercury's close orbit around the Sun, its weak intrinsic magnetic field and the absence of an atmosphere (P_surface<1×10⁻⁸ Pa) results in a strong direct exposure of the surface to energetic ions, electrons and UV radiation. Thermal processes and particle-surface-collisions dominate the surface interaction processes leading to surface chemistry and physics, including the formation of an exosphere (N?10¹⁴ cm⁻²) in which gravity is the dominant force affecting the trajectories of exospheric atoms. NASA's Mariner 10 spacecraft observed the existence of H, He, and O in Mercury's exosphere. In addition, the volatile components Na, K, and Ca have been observed by ground based instrumentation in the exosphere. We study the efficiency of several particle surface release processes by calculating stopping cross-sections, sputter yields and exospheric source rates. Our study indicates surface sputter yields for Na between values of about 0.27 and 0.35 in an energy range from 500 eV up to 2 keV if Na⁺ ions are the sputter agents, and about 0.037 and 0.082 at an energy range between 500 eV up to 2 keV when H⁺ are the sputter agents and a surface binding energy of about 2 eV to 2.65 eV. The sputter yields for Ca are about 0.032 to 0.06 and for K atoms between 0.054 to 0.1 in the same energy range. We found a sputter yield for O atoms between 0.025 and 0.04 for a particle energy range between 500 eV up to 2 keV protons. By taking the average solar wind proton surface flux at the open magnetic field line area of about 4×10⁸ cm⁻² s⁻¹ calculated by Massetti et al. (2003, Icarus, in press) the resulting average sputtering flux for O is about 0.8-1.0×10⁷ cm⁻² s⁻¹ and for Na approximately 1.3-1.6×10⁵ cm⁻² s⁻¹ depending on the assumed Na binding energies, regolith content, sputtering agents and solar activity. By using lunar regolith values for K we obtain a sputtering flux of about 1.0-1.4×10⁴ cm⁻² s⁻¹. By taking an average open magnetic field line area of about 2.8×10¹⁶ cm² modelled by Massetti et al. (2003, Icarus, in press) we derive an average surface sputter rate for Na of about 4.2×10²¹ s⁻¹ and for O of about 2.5×10²³ s⁻¹. The particle sputter rate for K atoms is about 3.0×10²⁰ s⁻¹ assuming lunar regolith composition for K. The sputter rates depend on the particle content in the regolith and the open magnetic field line area on Mercury's surface. Further, the surface layer could be depleted in alkali. A UV model has been developed to yield the surface UV irradiance at any time and latitude over a Mercury year. Seasonal and diurnal variations are calculated, and Photon Stimulated Desorption (PSD) fluxes along Mercury's orbit are evaluated. A solar UV hotspot is created towards perihelion, with significant average PSD particle release rates and Na fluxes of about 3.0×10⁶ cm⁻² s⁻¹. The average source rates for Na particles released by PSD are about 1×10²⁴ s⁻¹. By using the laboratory obtained data of Madey et al. (1998, J. Geophys. Res. 103, 5873-5887) for the calculation of the PSD flux of K atoms we get fluxes in the order of about 10⁴ cm⁻² s⁻¹ along Mercury's orbit. However, these values may be to high since they are based on idealized smooth surface conditions in the laboratory and do not include the roughness and porosity of Mercury's regolith. Further, the lack of an ionosphere and Mercury's small, temporally and spatially highly variable magnetosphere can result in a large and rapid increase of exospheric particles, especially Na in Mercury's exosphere. Our study suggests that the average total source rates for the exosphere from solar particle and radiation induced surface processes during quiet solar conditions may be of the same order as particles produced by micrometeoroid vaporization. We also discuss the capability of in situ measurements of Mercury's highly variable particle environment by the proposed NPA-SERENA instrument package on board ESA's BepiColombo Mercury Planetary Orbiter (MPO).  相似文献   

The origin of subdwarf B stars – II     

Z. Han  Ph. Podsiadlowski  P. F. L. Maxted  T. R. Marsh 《Monthly notices of the Royal Astronomical Society》2003,341(2):669-691

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Book review     

P.W. Hodge  B.P. Skelton  J. Ashizawa 《Astrophysics and Space Science》2003,286(3-4):529-529

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The relation between Zanstra temperature and morphology in planetary nebulae     

J. P. Phillips 《Monthly notices of the Royal Astronomical Society》2003,344(2):501-520

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Chandra and RXTE spectroscopy of the Galactic microquasar XTE J1550−564 in outburst     

J. M. Miller  H. L. Marshall  R. Wijnands †  T. Di Matteo  D. W. Fox  J. Kommers  D. Pooley  T. Belloni  J. Casares  P. A. Charles  A. C. Fabian  M. van der Klis  W. H. G. Lewin 《Monthly notices of the Royal Astronomical Society》2003,338(1):7-13

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A search for the optical and near-infrared counterpart of the accreting millisecond X-ray pulsar XTE J1751−305     

P. G. Jonker  G. Nelemans  Z. Wang  A. K. H. Kong  D. Chakrabarty  M. Garcia  P. J. Groot  M. van der Klis  T. Kerr  B. Mobasher  M. Sullivan  T. Augusteijn  B. W. Stappers  P. Challis  R. P. Kirshner  J. Hjorth  A. Delsanti 《Monthly notices of the Royal Astronomical Society》2003,344(1):201-206

We have obtained optical and near-infrared images of the field of the accreting millisecond X-ray pulsar XTE J1751−305. There are no stars in the 0.7-arcsec error circle (0.7 arcsec is the overall uncertainty arising from tying the optical and X-ray images and from the intrinsic uncertainty in the Chandra X-ray astrometric solution). We derive limiting magnitudes for the counterpart of   R > 23.1, I > 21.6, Z > 20.6, J > 19.6  and   K > 19.2  . We compare these upper limits with the magnitudes one would expect for simple models for the possible donor stars and the accretion disc subject to the reddening observed in X-rays for XTE J1751−305 and when put at the distance of the Galactic Centre (8.5 kpc). We conclude that our non-detection does not constrain any of the models for the accretion disc or possible donor stars. Deep, near-infrared images obtained during quiescence will, however, constrain possible models for the donor stars in this ultracompact system.  相似文献   

MS 1603.6+2600: an accretion disc corona source?     

P. G. Jonker  M. van der Klis  C. Kouveliotou  M. Méndez  W. H. G. Lewin  T. Belloni 《Monthly notices of the Royal Astronomical Society》2003,346(2):684-688

We have observed the eclipsing low-mass X-ray binary MS 1603.6+2600 with Chandra for 7 ks. The X-ray spectrum is well fit with a single absorbed power law with an index of ∼2. We find a clear sinusoidal modulation in the X-ray light curve with a period of  1.7 ± 0.2 h  , consistent with the period of 1.85 h found before. However, no (partial) eclipses were found. We argue that if the X-ray flare observed in earlier X-ray observations was a type I X-ray burst, then the source can only be an accretion disc corona source at a distance of ∼11–24 kpc (implying a height above the Galactic disc of ∼8–17 kpc). It has also been proposed in the literature that MS 1603.76+2600 is a dipper at ∼75 kpc. We argue that, in this dipper scenario, the observed optical properties of MS 1603.6+2600 are difficult to reconcile with the optical properties one would expect on the basis of comparisons with other high-inclination, low-mass X-ray binaries, unless the X-ray flare was not a type I X-ray burst. In that case, the source can be a nearby soft X-ray transient accreting at a quiescent rate, as was proposed by Hakala et al., or a high-inclination source at ∼15–20 kpc.  相似文献   

Giant Meterwave Radio Telescope observations of an M2.8 flare: Insights into the initiation of a flare–coronal mass ejection event     

Subramanian  Prasad  Ananthakrishnan  S.  Janardhan  P.  Kundu  M.R.  White  S.M.  Garaimov  V.I. 《Solar physics》2003,218(1-2):247-259

We present the first observations of a solar flare with the GMRT. An M2.8 flare observed at 1060 MHz with the GMRT on 17 November 2001 was associated with a prominence eruption observed at 17 GHz by the Nobeyama radioheliograph and the initiation of a fast partial halo CME observed with the LASCO C2 coronagraph. Towards the start of the eruption, we find evidence for reconnection above the prominence. Subsequently, we find evidence for rapid growth of a vertical current sheet below the erupting arcade, which is accompanied by the flare and prominence eruption.  相似文献   

Observations of the neutral gas and dust in the radio galaxy 3C 305     

N. Jackson  R. Beswick  A. Pedlar  G. H. Cole  W. B. Sparks  J. P. Leahy  D. J. Axon  A. J. Holloway 《Monthly notices of the Royal Astronomical Society》2003,338(3):643-654

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