Determination of the physical parameters of gas in the neutral atmospheres of comets: The circular aperture method

A new method—the circular aperture method—is proposed for determining the physical parameters of gas in the neutral atmospheres of comets. A check has shown that this method is applicable to calculate the hydrodynamic velocity of gas outflow and the lifetime of molecules in the neutral atmosphere of comet 2009/P1 (Garradd).     

Seismic characterization of vertical fractures described as general linear-slip interfaces

Fluid flow in many hydrocarbon reservoirs is controlled by aligned fractures which make the medium anisotropic on the scale of seismic wavelength. Applying the linear‐slip theory, we investigate seismic signatures of the effective medium produced by a single set of ‘general’ vertical fractures embedded in a purely isotropic host rock. The generality of our fracture model means the allowance for coupling between the normal (to the fracture plane) stress and the tangential jump in displacement (and vice versa). Despite its low (triclinic) symmetry, the medium is described by just nine independent effective parameters and possesses several distinct features which help to identify the physical model and estimate the fracture compliances and background velocities. For example, the polarization vector of the vertically propagating fast shear wave S₁ and the semi‐major axis of the S₁‐wave normal‐moveout (NMO) ellipse from a horizontal reflector always point in the direction of the fracture strike. Moreover, for the S₁‐wave both the vertical velocity and the NMO velocity along the fractures are equal to the shear‐wave velocity in the host rock. Analysis of seismic signatures in the limit of small fracture weaknesses allows us to select the input data needed for unambiguous fracture characterization. The fracture and background parameters can be estimated using the NMO ellipses from horizontal reflectors and vertical velocities of P‐waves and two split S‐waves, combined with a portion of the P‐wave slowness surface reconstructed from multi‐azimuth walkaway vertical seismic profiling (VSP) data. The stability of the parameter‐estimation procedure is verified by performing non‐linear inversion based on the exact equations.     

Midlatitude ocean–atmosphere interaction in an idealized coupled model

Interannual-to-interdecadal ocean-atmosphere interaction in midlatitudes is studied using an idealized coupled model consisting of eddy resolving two-layer quasi-geostrophic oceanic and atmospheric components with a simple diagnostic oceanic mixed layer. The model solutions exhibit structure and variability that resemble qualitatively some aspects of the observed climate variability over the North Atlantic. The atmospheric climatology is characterized by a zonally modulated jet. The single-basin ocean climatology consists of a midlatitude double jet that represents the Gulf Stream and Labrador currents, which are parts of the subtropical and subpolar gyres, respectively. The leading mode of the atmospheric low-frequency variability consists predominantly of meridional displacements of the zonal jet, with a local maximum over the ocean. The first basin-scale mode of sea-surface temperature has a red power spectrum, is largely of one polarity and bears qualitative similarities with the observed interdecadal mode identified by Kushnir. A warm sea-surface temperature anomaly is accompanied by anomalously low atmospheric pressure, an intensified model Gulf Stream and a weakened Labrador current. This mode is found not to be affected significantly by oceanic coupling. In the western part of the basin, this sea-surface temperature pattern is shown to be forced by the slowest components of the surface-wind anomaly through a delayed modulation of the baroclinic time-dependent boundary currents which advect mean SST, with synchronous variations in the two oceanic jets. The response in the east is found to be dominated by local atmospheric forcing. Basin-scale intrinsic oceanic variability consists of a damped oceanic oscillatory mode in the baroclinic flow field that is excited by the atmospheric noise. Its period is around 5.5 years, but it has a negligible influence on the evolution of sea-surface temperature. Important for this mode's excitation is the meridional position of the atmospheric center of action relative to the ocean gyres.     

Spectroscopic studies of southern-hemisphere Cepheids: Six objects in Centaurus (V Cen, V737 Cen) and Sagittarius (BB Sgr, W Sgr, X Sgr, Y Sgr)

This paper is the next one in the series of our works aimed at determining the atmospheric parameters and chemical composition of southern-hemisphere Cepheids. We present the results of our study for six bright Cepheids: V Cen, V737 Cen, BB Sgr, W Sgr, X Sgr, and Y Sgr. We have analyzed 14 high-resolution spectra taken with the 1.9-m telescope at the South African Astronomical Observatory. In addition to determining the chemical composition and atmospheric parameters, we point out and discuss several features in the spectra of individual Cepheids. In particular,we have detected emission in the cores of the Hβ and Hα lines forWSgr near its maximum light, while X Sgr shows the splitting of metal absorption lines into individual components without any change of the hydrogen lines. These peculiarities can be explained by different manifestations of shock waves in the Cepheid atmospheres and by the presence of circumstellar envelopes around X Sgr and W Sgr. The chemical composition has been estimated for V737 Cen, BB Sgr, and X Sgr for the first time. On the whole, our abundance estimates for α-elements, iron-peak elements, and r- and s-process elements are close to the solar ones for all objects, carbon is underabundant, the oxygen abundance is nearly solar, the “odd” elements, Na and Al, are overabundant (except X Sgr), magnesium is underabundant for V Cen and X Sgr and overabundant for the remaining objects. Such a chemical composition is typical of yellow supergiants after the first dredge-up. Keywords: Cepheids, spectra, atmospheric parameters, chemical composition.     

Spectroscopic studies of southern-hemisphere cepheids: WW Car,SX Car,UZ Car,UY Car,GX Car,HW Car,YZ Car

For seven faint southern Cepheids (WW Car, SX Car, UZ Car, UY Car, GX Car, HW Car, YZ Car), we have determined their atmospheric parameters and chemical composition for the first time based on ten high-resolution (R = 50 000) spectra taken with the 1.5-m Hexapod telescope at the Joint Observatory of the Northern Catholic University (Antofagasta, Chile) and the Ruhr University (Bochum, Germany). Six objects from the list demonstrate atmospheric parameters and chemical composition typical of Cepheids that have passed through the first dredge-up phase, while WW Car is probably an anomalous Cepheid. According to our preliminary estimates, it has an overabundance of CNO, a deficit of sodium and aluminum, and a slight deficit of magnesium, with iron and other elements being underabundant relative to the Sun.     

Atlantic Multidecadal Oscillation and Northern Hemisphere’s climate variability

Proxy and instrumental records reflect a quasi-cyclic 50–80-year climate signal across the Northern Hemisphere, with particular presence in the North Atlantic. Modeling studies rationalize this variability in terms of intrinsic dynamics of the Atlantic Meridional Overturning Circulation influencing distribution of sea-surface-temperature anomalies in the Atlantic Ocean; hence the name Atlantic Multidecadal Oscillation (AMO). By analyzing a lagged covariance structure of a network of climate indices, this study details the AMO-signal propagation throughout the Northern Hemisphere via a sequence of atmospheric and lagged oceanic teleconnections, which the authors term the “stadium wave”. Initial changes in the North Atlantic temperature anomaly associated with AMO culminate in an oppositely signed hemispheric signal about 30?years later. Furthermore, shorter-term, interannual-to-interdecadal climate variability alters character according to polarity of the stadium-wave-induced prevailing hemispheric climate regime. Ongoing research suggests mutual interaction between shorter-term variability and the stadium wave, with indication of ensuing modifications of multidecadal variability within the Atlantic sector. Results presented here support the hypothesis that AMO plays a significant role in hemispheric and, by inference, global climate variability, with implications for climate-change attribution and prediction.     

Equilibrium figures with a magnetic field

A set of equations which are magnetohydrodynamic equilibrium conditions in the case of axial symmetry is solved. The possibility of splitting Maclaurin's sequence in the presence of a magnetic field is shown. The effect of a forceless magnetic field on Maclaurin'sP-ellipsoid is investigated.     

Narrowband Electrophotometry of Comet Hale-Bopp (C/1995 O1) Near Perihelion

We present the results of narrowband photoelectric observations of comet Hale-Bopp near perihelion, which were made with the AZT-14telescope at the station Lisniki of Kyiv University Astronomical Observatory. The standard set of IHW cometary filters was used. The total number of cometary observations was more than 500 during the interval March 13–April 29, 1997. The comet's nuclear gas production rates Q of C₂ and C₃ were calculated using the stellar calibration for these filters. The calculations used the Haser model for a neutral cometary atmosphere. The value Afρ, which is characteristic of the dust production rate, was determined too. Mean values of the production rates near perihelion are log Q(C₂) = 28.4; log Q(C₃) = 27.2; (Afρ)_BC = 6.0. This revised version was published online in July 2006 with corrections to the Cover Date.