An analytic model of the core of a supernova     

Richard C. Adams  Boyd B. Cary  Jeffrey M. Cohen 《Astrophysics and Space Science》1994,213(2):205-232

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World energy supply     

Cohen  R. 《Oceanic Engineering, IEEE Journal of》1984,9(2):121-121

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Geographic Gender Studies: Fresh Approaches,but with Integration     

Saul B. Cohen 《The Professional geographer》1990,42(2):231-232

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Maser Mapping and Dust Properties of Red Supergiant Winds     

Anita M.S Richards  R. J. Cohen  J. A. Yates 《Astrophysics and Space Science》1997,251(1-2):55-62

Maser emission from the circumstellar envelopes of four late-type red supergiants has been mapped with milli-arcsecond resolution using MERLIN1. The wind is driven by radiation pressure on dust and the structure and kinematics of the masing regions reflect the dust properties. The unbeamed radius of water maser blobs, ∼ 10¹² m, has been measured for the first time. The velocity gradient is used to derive the dust absorption coefficient which increases with radius from ≤ 0.1 to ≤ 1.0 m² kg⁻¹. Comparison with laboratory studies suggests that small crystalline grains are formed near the star and are annealed into astronomical silicates at larger distances. This revised version was published online in September 2006 with corrections to the Cover Date.  相似文献   

Virial shocks in galactic haloes?     

Yuval Birnboim  Avishai Dekel 《Monthly notices of the Royal Astronomical Society》2003,345(1):349-364

We investigate the conditions for the existence of an expanding virial shock in the gas falling within a spherical dark matter halo. The shock relies on pressure support by the shock-heated gas behind it. When the radiative cooling is efficient compared with the infall rate, the post-shock gas becomes unstable; it collapses inwards and cannot support the shock. We find for a monatomic gas that the shock is stable when the post-shock pressure and density obey     . When expressed in terms of the pre-shock gas properties at radius r it reads as  ρ r Λ( T )/ u ³ < 0.0126  , where ρ is the gas density, u is the infall velocity and Λ( T ) is the cooling function, with the post-shock temperature   T ∝ u ²  . This result is confirmed by hydrodynamical simulations, using an accurate spheri-symmetric Lagrangian code. When the stability analysis is applied in cosmology, we find that a virial shock does not develop in most haloes that form before   z ∼ 2  , and it never forms in haloes less massive than a few  10¹¹ M_⊙  . In such haloes, the infalling gas is not heated to the virial temperature until it hits the disc, thus avoiding the cooling-dominated quasi-static contraction phase. The direct collapse of the cold gas into the disc should have non-trivial effects on the star formation rate and on outflows. The soft X-ray produced by the shock-heated gas in the disc is expected to ionize the dense disc environment, and the subsequent recombination would result in a high flux of Lα emission. This may explain both the puzzling low flux of soft X-ray background and the Lα emitters observed at high redshift.  相似文献   

Discovery of large-scale methanol and hydroxyl maser filaments in W3(OH)     

L. Harvey-Smith †  R. J. Cohen 《Monthly notices of the Royal Astronomical Society》2006,371(4):1550-1558

Images of the 6.7-GHz methanol maser emission from W3(OH) made at 50- and 100-mas angular resolution with the Multi-Element Radio-Linked Interferometer Network (MERLIN) are presented. The masers lie across the western face of the ultracompact H  ii region in extended filaments which may trace large-scale shocks. There is a complex interrelation between the 6.7-GHz methanol masers and hydroxyl (OH) masers at 1.7 and 4.7 GHz. Together the two species trace an extended filamentary structure that stretches at least 3100 au across the face of the ultracompact H  ii region. The dominant 6.7-GHz methanol emission coincides with the radio continuum peak and is populated by masers with broad spectral lines. The 6.7-GHz methanol emission is elongated at position angle 50° with a strong velocity gradient, and bears many similarities to the methanol maser disc structure reported in NGC 7538. It is surrounded by arcs of ground state OH masers at 1.7 GHz and highly excited OH masers at 13.44 GHz, some of which have the brightest methanol masers at their focus. We suggest that this region hosts the excitation centre for the ultracompact H  ii region.  相似文献   

Wind signatures in the X-ray emission-line profiles of the late-O supergiant ζ Orionis     

David H. Cohen  Maurice A. Leutenegger  Kevin T. Grizzard  Catherine L. Reed  Roban H. Kramer  Stanley P. Owocki 《Monthly notices of the Royal Astronomical Society》2006,368(4):1905-1916

X-ray line-profile analysis has proved to be the most direct diagnostic of the kinematics and spatial distribution of the very hot plasma around O stars. The Doppler-broadened line profiles provide information about the velocity distribution of the hot plasma, while the wavelength-dependent attenuation across a line profile provides information about the absorption to the hot plasma, thus providing a strong constraint on its physical location. In this paper, we apply several analysis techniques to the emission lines in the Chandra High Energy Transmission Grating Spectrometer (HETGS) spectrum of the late-O supergiant ζ Ori (O9.7 Ib), including the fitting of a simple line-profile model. We show that there is distinct evidence for blueshifts and profile asymmetry, as well as broadening in the X-ray emission lines of ζ Ori. These are the observational hallmarks of a wind-shock X-ray source, and the results for ζ Ori are very similar to those for the earlier O star, ζ Pup, which we have previously shown to be well fit by the same wind-shock line-profile model. The more subtle effects on the line-profile morphologies in ζ Ori, as compared to ζ Pup, are consistent with the somewhat lower density wind in this later O supergiant. In both stars, the wind optical depths required to explain the mildly asymmetric X-ray line profiles imply reductions in the effective opacity of nearly an order of magnitude, which may be explained by some combination of mass-loss rate reduction and large-scale clumping, with its associated porosity-based effects on radiation transfer. In the context of the recent reanalysis of the helium-like line intensity ratios in both ζ Ori and ζ Pup, and also in light of recent work questioning the published mass-loss rates in OB stars, these new results indicate that the X-ray emission from ζ Ori can be understood within the framework of the standard wind-shock scenario for hot stars.  相似文献   

Hydroxyl maser disc and outflow in the Orion-BN/KL region     

R. J. Cohen  N. Gasiprong  J. Meaburn  M. F. Graham 《Monthly notices of the Royal Astronomical Society》2006,367(2):541-552

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Old elliptical galaxies at  z ≃ 1.5  and the Kormendy relation     

I. Waddington  R. A. Windhorst  S. H. Cohen  J. S. Dunlop  J. A. Peacock  R. Jimenez  R. J. McLure  A. J. Bunker  H. Spinrad  A. Dey  D. Stern 《Monthly notices of the Royal Astronomical Society》2002,336(4):1342-1350

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ACE Observations of the Bastille Day 2000 Interplanetary Disturbances     

Smith  C.W.  Ness  N.F.  Burlaga  L.F.  Skoug  R.M.  McComas  D.J.  Zurbuchen  T.H.  Gloeckler  G.  Haggerty  D.K.  Gold  R.E.  Desai  M.I.  Mason  G.M.  Mazur  J.E.  Dwyer  J.R.  Popecki  M.A.  Möbius  E.  Cohen  C.M.S.  Leske  R.A. 《Solar physics》2001,204(1-2):227-252

We present ACE observations for the six-day period encompassing the Bastille Day 2000 solar activity. A high level of transient activity at 1 AU, including ICME-driven shocks, magnetic clouds, shock-accelerated energetic particle populations, and solar energetic ions and electrons, are described. We present thermal ion composition signatures for ICMEs and magnetic clouds from which we derive electron temperatures at the source of the disturbances and we describe additional enhancements in some ion species that are clearly related to the transient source. We describe shock acceleration of 0.3–2.0 MeV nucl⁻¹ protons and minor ions and the relative inability of some of the shocks to accelerate significant energetic ion populations near 1 AU. We report the characteristics of < 20 MeV nucl⁻¹ solar energetic ions and < 0.32 MeV electrons and attempt to relate the release of energetic electrons to particular source regions.  相似文献