Relaxation of the Vela pulsar’s angular velocity following its jumps

The dynamics of the vortex lattice in the inner crust of a neutron star is considered. A general equation of motion is obtained and solved under the assumption that there are regions of pinned and of free vortices. By comparing these solutions with observational data for the Vela pulsar, the relative moments of inertia of regions of relaxation with the corresponding characteristic times are calculated for two model stars with different equations of state. It is shown that the theory can be reconciled with observations of the relaxation of pulsar angular velocity only for model stars with extremely stiff equations of state. Translated from Astrofizika, Vol. 40, No. 1, pp. 67–76, January–March, 1997.     

Modelling Gas And Dust Release From Comet Hale–Bopp

Numerical simulations of the evolving activity of comet Hale-Bopp are presented, assuming a porous, spherical nucleus, 20 km in radius, made of dust and gas-laden amorphous ice. The main effects included are: crystallization of amorphous ice and release of occluded gas, condensation, sublimation and flow of gases through the pores, changing pore sizes, and flow of dust grains. The model parameters, such as initial pore size and porosity, emissivity, dust grain size, are varied in order to match the observed activity. In all cases, a sharp rise in the activity of the nucleus occurs at a large heliocentric distance pre-perihelion, marked by a few orders of magnitude increase in the CO and the CO₂ fluxes and in the rate of dust emission. This is due to the onset of crystallization, advancing down to a few meters below the surface, accompanied by release of the trapped gases. A period of sustained, but variable, activity ensues. The emission of water molecules is found to surpass that of CO at a heliocentric distance of 3 AU. Thereafter the activity is largely determined by the behaviour of the dust. If a dust mantle is allowed to build up, the water production rate does not increase dramatically towards perihelion; if most of the dust is ejected, the surface activity increases rapidly, producing a very bright comet. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Monahans chondrite and halite: Argon‐39/argon‐40 age,solar gases,cosmic‐ray exposure ages,and parent body regolith neutron flux and thickness

Abstract— The Monahans H‐chondrite is a regolith breccia containing light and dark phases and the first reported presence of small grains of halite. We made detailed noble gas analyses of each of these phases. The ³⁹Ar‐⁴⁰Ar age of Monahans light is 4.533 ± 0.006 Ma. Monahans dark and halite samples show greater amounts of diffusive loss of ⁴⁰Ar and the maximum ages are 4.50 and 4.33 Ga, respectively. Monahans dark phase contains significant concentrations of He, Ne and Ar implanted by the solar wind when this material was extant in a parent body regolith. Monahans light contains no solar gases. From the cosmogenic ³He, ²¹Ne, and ³⁸Ar in Monahans light we calculate a probable cosmic‐ray, space exposure age of 6.0 ± 0.5 Ma. Monahans dark contains twice as much cosmogenic ²¹Ne and ³⁸Ar as does the light and indicates early near‐surface exposure of 13–18 Ma in a H‐chondrite regolith. The existence of fragile halite grains in H‐chondrites suggests that this regolith irradiation occurred very early. Large concentrations of ³⁶Ar in the halite were produced during regolith exposure by neutron capture on ³⁵Cl, followed by decay to ³⁶Ar. The thermal neutron fluence seen by the halite was (2–4) × 10¹⁴ n/cm². The thermal neutron flux during regolith exposure was ～0.4‐0.7 n/cm²/s. The Monahans neutron fluence is more than an order of magnitude less than that acquired during space exposure of several large meteorites and of lunar soils, but the neutron flux is lower by a factor of ≤5. Comparison of the ³⁶Ar_n/²¹Ne_cos ratio in Monahans halite and silicate with the theoretically calculated ratio as a function of shielding depth in an H‐chondrite regolith suggests that irradiation of Monahans dark occurred under low shielding in a regolith that may have been relatively shallow. Late addition of halite to the regolith can be ruled out. However, irradiation of halite and silicate for different times at different depths in an extensive regolith cannot be excluded.     

Magnetic field and differential rotation in the radiative zone of the Sun

The problem of the interaction between magnetic fields and differential rotation in the radiative zone of the Sun is investigated. It is demonstrated that effects of magnetic buoyancy can be neglected in the analysis of this interaction. It is shown that hydromagnetic torsional waves propagating from the solar core cannot be responsible for the 22-year solar cycle. A possible geometry of the magnetic field that conforms with stationary differential rotation is considered. A verifying method for hypotheses on the structure of the magnetic field and torsional oscillations in the radiative zone of the Sun is proposed based on helioseismic data.     

Formation of galactic subsystems in light of the magnesium abundance in field stars: The thin disk

Data from our compiled catalog of spectroscopically determined magnesium abundances in stars with accurate parallaxes are used to select thin-disk dwarfs and subgiants according to kinematic criteria. We analyze the relations between the relative magnesium abundances in stars, [Mg/Fe], and their metallicities, Galactic orbital elements, and ages. The [Mg/Fe] ratios in the thin disk at any metallicity in the range ?1.0 dex <[Fe/H] < ?0.4 dex are shown to be smaller than those in the thick disk, implying that the thin-disk stars are, on average, younger than the thick-disk stars. The relative magnesium abundances in such metal-poor thin-disk stars have been found to systematically decrease with increasing stellar orbital radii in such a way that magnesium overabundances ([Mg/Fe] > 0.2 dex) are essentially observed only in the stars whose orbits lie almost entirely within the solar circle. At the same time, the range of metallicities in magnesium-poor stars is displaced from ?0.5 dex < [Fe/H] < +0.3 dex to ?0.7 dex < [Fe/H] < +0.2 dex as their orbital radii increase. This behavior suggests that, first, the star formation rate decreases with increasing Galactocentric distance and, second, there was no star formation for some time outside the solar circle, while this process was continuous within the solar circle. The decrease in the star formation rate with increasing Galactocentric distance is responsible for the existence of a negative radial metallicity gradient (grad _R[Fe/H] = ?0.05 ± 0.01 kpc^?1) in the disk, which shows a tendency to increase with decreasing age. At the same time, the relative magnesium abundance exhibits no radial gradient. We have confirmed the existence of a steep negative vertical metallicity gradient (grad _Z[Fe/H] = ?0.29 ± 0.06 kpc^?1) and detected a significant positive vertical gradient in relative magnesium abundance (grad _Z[Mg/Fe] = 0.13 ± 0.02 kpc^?1); both gradients increase appreciably in absolute value with decreasing age. We have found that there is not only an age-metallicity relation, but also an age-magnesium abundance relation, in the thin disk. We surmise that the thin disk has a multicomponent structure, but the existence of a negative trend in the star formation rate along the Galactocentric radius does not allow the stars of its various components to be identified in the immediate solar neighborhood.     

The Siberian Solar Radio Telescope: the current state of the instrument,observations, and data

The Siberian Solar Radio Telescope (SSRT) is one of the world's largest solar radio heliographs. It commenced operation in 1983, and since then has undergone several upgrades. The operating frequency of the SSRT is 5.7 GHz. Since 1992 the instrument has had the capability to make one-dimensional scans with a high time resolution of 56 ms and an angular resolution of 15 arc sec. Making one of these scans now takes 14 ms. In 1996 the capability was added to make full, two-dimensional images of the solar disk. The SSRT is now capable of obtaining images with an angular resolution of 21 arc sec every 2 min. In this paper we describe the main features and operation of the instrument, particularly emphasizing issues pertaining to the imaging process and factors limiting data quality. Some of the data processing and analysis techniques are discussed. We present examples of full-disk solar images of the quiet Sun, recorded near solar activity minimum, and images of specific structures: plages, coronal bright points, filaments and prominences, and coronal holes. We also present some observations of dynamic phenomena, such as eruptive prominences and solar flares, which illustrate the high-time-resolution observations that can be done with this instrument. We compare SSRT observations at 5.7 GHz, including computed `light curves', both morphologically and quantatively, with observations made in other spectral domains, such as 17 GHz radio images, Hα filtergrams and magnetograms, extreme-ultraviolet and X-ray observations, and dynamic radio spectra.     

HCN fluorescence on Titan

The HCN emission features near 3 μm recently detected by Geballe et al. (2003, Astrophys. J. 583, L39) are analyzed with a model for fluorescence of sunlight in the ν₃ band of HCN. The emission spectrum is consistent with current knowledge of the atmospheric temperature profile and the HCN distribution inferred from millimeter-wave observations. The spectrum is insensitive to the abundance of HCN in the thermosphere and the thousand-fold enhancement relative to photochemical models suggested by Geballe et al. (2003, Astrophys. J. 583, L39) is not required to explain the observations. We find that the spectrum can be matched with temperatures from 130 to 200 K, with slightly better fits at high temperature, contrary to the temperature determination of 130±10 K of Geballe et al. (2003, Astrophys. J. 583, L39). The HCN emission spectrum is sensitive to the collisional de-excitation probability, P₁₀, for the ν₃ state and we determine a value of 10⁻⁵ with an accuracy of about a factor of two. Analysis of absorption lines in the C₂H₂ν₃ band near 3 μm, detected in the same spectrum, indicate a C₂H₂ mole fraction near 0.01 μbar of 10⁻⁵ for P₁₀=10⁻⁴. The derived mole fraction, however, is dependent upon the value adopted for P₁₀ and lower values are required if P₁₀ at Titan temperatures is less than its room temperature value.     

Towards a free–free template for CMB foregrounds

A full-sky template map of the Galactic free–free foreground emission component is increasingly important for high-sensitivity cosmic microwave background (CMB) experiments. We use the recently published Hα data of both the northern and southern skies as the basis for such a template.
The first step is to correct the Hα maps for dust absorption using the 100-μm dust maps of Schlegel, Finkbeiner & Davis. We show that for a range of longitudes, the Galactic latitude distribution of absorption suggests that it is 33 per cent of the full extragalactic absorption. A reliable absorption-corrected Hα map can be produced for ∼95 per cent of the sky; the area for which a template cannot be recovered is the Galactic plane area  | b | < 5°, l = 260°–0°–160°  and some isolated dense dust clouds at intermediate latitudes.
The second step is to convert the dust-corrected Hα data into a predicted radio surface brightness. The free–free emission formula is revised to give an accurate expression (1 per cent) for the radio emission covering the frequency range 100 MHz–100 GHz and the electron temperature range 3000–20 000 K. The main uncertainty when applying this expression is the variation of electron temperature across the sky. The emission formula is verified in several extended H  ii regions using data in the range 408–2326 MHz.
A full-sky free–free template map is presented at 30 GHz; the scaling to other frequencies is given. The Haslam et al. all-sky 408-MHz map of the sky can be corrected for this free–free component, which amounts to a  ≈6  per cent correction at intermediate and high latitudes, to provide a pure synchrotron all-sky template. The implications for CMB experiments are discussed.