arXiv subjects

H. Sagawa

Publications and source records attributed to H. Sagawa.

At least 127 records · Page 7Linked to original sources

arXiv2012

Wind mapping in Venus' upper mesosphere with the IRAM-Plateau de Bure interferometer

The dynamics of the upper mesosphere of Venus (~85-115 km) have been characterized as a combination of a retrograde superrotating zonal wind (RSZ) with a subsolar-to-antisolar flow (SSAS). Numerous mm-wave single-dish observations have been obtained and could directly measure mesospheric line-of-sight winds by mapping Doppler-shifts on CO rotational lines, but their limited spatial resolution makes their interpretation difficult. By using interferometric facilities, one can obtain better resolution on Doppler-shifts maps, allowing in particular to put firmer constraints on the respective contributions of the SSAS and RSZ circulations to the global mesospheric wind field. We report on interferometric observations of the CO(1-0) line obtained with the IRAM-Plateau de Bure interferometer in November 2007 and June 2009, that could map the upper mesosphere dynamics on the morning hemisphere with a very good spatial resolution (3.5-5.5"). All the obtained measurements show, with a remarkably good temporal stability, that the wind globally flows in the (sky) East-West direction, corresponding in the observed geometry either to an unexpected prograde zonal wind or a SSAS flow. A very localized inversion of the wind direction, that could correspond to a RSZ wind, is also repeatedly detected in the night hemisphere. The presence of significant meridional winds is not evidenced. Using models with different combinations of zonal and SSAS winds, we find that the data is best reproduced by a dominant SSAS flow with a maximal velocity at the terminator of ~200 m/s, displaying large diurnal and latitudinal asymmetries, combined with an equatorial RSZ wind of 70-100 m/s, overall indicating a wind-field structure consistent with but much more complex than the usual representation of the mesospheric dynamics.

astro-ph.EP↗

arXiv2012

Odd-even staggering of reaction cross sections for 22,23,24O isotopes

The interaction cross sections of $^{22,23}$O nuclei at 900 MeV/nucleon have been measured recently by Kanungo {\it et al.}. We show that the odd-even staggering parameter of interaction cross sections deduced from these new data agrees well with the theoretical systematics expected for the neutron separation energy of $S_n$=2.74$\pm$0.120 MeV for $^{23}$O. We also discuss briefly the energy dependence of the staggering parameter.

nucl-th↗

arXiv2011

New air fluorescence detectors employed in the Telescope Array experiment

Since 2007, the Telescope Array (TA) experiment, based in Utah, USA, has been observing ultra high energy cosmic rays to understand their origins. The experiment involves a surface detector (SD) array and three fluorescence detector (FD) stations. FD stations, installed surrounding the SD array, measure the air fluorescence light emitted from extensive air showers (EASs) for precise determination of their energies and species. The detectors employed at one of the three FD stations were relocated from the High Resolution Fly's Eye experiment. At the other two stations, newly designed detectors were constructed for the TA experiment. An FD consists of a primary mirror and a camera equipped with photomultiplier tubes. To obtain the EAS parameters with high accuracies, understanding the FD optical characteristics is important. In this paper, we report the characteristics and installation of new FDs and the performances of the FD components. The results of the monitored mirror reflectance during the observation time are also described in this report.

astro-ph.IM↗

arXiv2011

Pairing correlations and odd-even staggering in reaction cross sections of weakly bound nuclei

We investigate the odd-even staggering (OES) in reaction cross sections of weakly bound nuclei with a Glauber theory, taking into account the pairing correlation with the Hartree-Fock-Bogoliubov (HFB) method. We first discuss the pairing gap in extremely weakly bound nuclei and show that the pairing gap persists in the limit of zero separation energy limit even for single-particle orbits with the orbital angular momenta $l=0$ and $l=1$. We then introduce the OES parameter defined as the second derivative of reaction cross sections with respect to the mass number, and clarify the relation between the magnitude of OES and the neutron separation energy. We find that the OES parameter increases considerably in the zero separation energy limit for $l=0$ and $l=1$ single-particle states, while no increase is found for higher angular momentum orbits with {\it e.g.,} $l=3$. We point out that the increase of OES parameter is also seen in the experimental reaction cross sections for Ne isotopes, which is well accounted for by our calculations.

nucl-th↗

arXiv2011

Global investigation of odd-even mass differences and radii with isospin dependent pairing interactions

The neutron and proton odd-even mass differences are systematically studied with Hartree-Fock+BCS (HFBCS) calculations with Skyrme interactions and an isospin dependent contact pairing interaction. The strength of pairing interactions is determined to reproduce empirical odd-even mass differences in a wide region of mass table. By using the optimal parameter, we perform global HF+BCS calculations of nuclei and compare with experimental data. The importance of isospin dependence of the pairing interaction is singled out for odd-even mass differences in medium and heavy isotopes. The proton and neutron radii are studied systematically by using the same model.

nucl-th↗

arXiv2011

Hyperon effects in covariant density functional theory with recent astrophysical observations

Motivated by recent observational data, the equations of state with the inclusion of strangeness-bearing $Λ$-hyperons and the corresponding properties of neutron stars are studied, based on the covariant density functional (CDF) theory. To this end, we specifically employ the density dependent relativistic Hartree-Fock (DDRHF) theory and the relativistic mean field theory (RMF). The inclusion of $Λ$-hyperons in neutron stars shows substantial effects in softening the equation of state. Because of the extra suppression effect originated from the Fock channel, large reductions on both the star mass and radius are predicted by the DDRHF calculations. It is also found that the mass-radius relations of neutron stars with $Λ$-hyperons determined by DDRHF with the PKA1 parameter set are in fairly good agreement with the observational data where a relatively small neutron stars radius is required. Therefore, it is expected that the exotic degrees of freedom such as the strangeness-bearing structure may appear and play significant roles inside the neutron stars, which is supported further by the systematical investigations on the consistency between the maximum neutron star mass and $Λ$-coupling strength.

nucl-th↗

arXiv2011

Effect of proton-proton Coulomb repulsion on soft dipole excitations of light proton-rich nuclei

We perform three-body model calculations for soft dipole excitations of the proton-rich Borromean nucleus $^{17}$Ne. To this end, we assume that $^{17}$Ne takes the $^{15}$O+p+p structure, in which the two valence protons are excited from the $0^+$ ground state configuration to $1^-$ continuum states. We employ a density-dependent contact force for the nuclear part of the pairing interaction, and discretize the continuum states with the box boundary condition. We show by explicitly including the Coulomb interaction between the valence protons that the Coulomb repulsion does not significantly alter the E1 strength distribution. We point out that the effect of the Coulomb repulsion in fact can be well simulated by renormalizing the nuclear pairing interaction.

nucl-th↗

arXiv2011

Unveiling pairing anti-halo effect in odd-even staggering in reaction cross sections of weakly bound nuclei

We investigate the spatial extension of weakly bound Ne and C isotopes by taking into account the pairing correlation with the Hartree-Fock-Bogoliubov (HFB) method and a 3-body model, respectively. We show that the odd-even staggering in the reaction cross sections of $^{30,31,32}$Ne and $^{14,15,16}$C are successfully reproduced, and thus the staggering can be attributed to the pairing anti-halo effect. A correlation between a one-neutron separation energy and the anti-halo effect is demonstrated for $s$- and p-waves using the HFB wave functions.

nucl-th↗

arXiv2011

Coulomb dissociation of the deformed halo nucleus 31Ne

The recently observed large cross sections for the Coulomb dissociation of $^{31}$Ne nucleus indicate that this nucleus takes a halo structure in the ground state. We analyse these experimental data using the particle-rotor model, that takes into account the rotational excitation of the core nucleus $^{30}$Ne. We show that the experimental data can be well reproduced when the quadrupole deformation parameter of $^{30}$Ne is around $β_2=0.2\sim0.3$, at which the ground state takes the spin and parity of $I^π$=3/2$^-$. This state corresponds to the Nilsson level [330 1/2] in the adiabatic limit of the particle-rotor model. On the other hand, the state corresponding to the Nilsson level [321 3/2] with $β_2\sim 0.55$ can be excluded when the finite excitation energy of the core is taken into account, even though this configuration is a candidate for the ground state of $^{31}$Ne in the Nilsson model analysis. We discuss briefly also a possibility of the $I^π$=1/2$^+$ configuration with $β_2\sim 1$ and $β_2\sim -0.4$.

nucl-th↗

arXiv2010

Two-neutron halo nuclei in one dimension: dineutron correlation and breakup reaction

We propose a simple schematic model for two-neutron halo nuclei. In this model, the two valence neutrons move in a one-dimensional mean field, interacting with each other via a density-dependent contact interaction. We first investigate the ground state properties, and demonstrate that the dineutron correlation can be realized with this simple model due to the admixture of even- and odd-parity single-particle states. We then solve the time-dependent two-particle Schrödinger equation under the influence of a time-dependent one-body external field, in order to discuss the effect of dineutron correlation on nuclear breakup processes. The time evolution of two-particle density shows that the dineutron correlation enhances the total breakup probability, especially for the two-neutron breakup process, in which both the valence neutrons are promoted to continuum scattering states. We find that the interaction between the two particles definitely favours a spatial correlation of the two outgoing particles, which are mainly emitted in the same direction.

nucl-th↗

arXiv2010

Effect of pairing correlations on incompressibility and symmetry energy in nuclear matter and finite nuclei

The role of superfluidity in the incompressibility and in the symmetry energy is studied in nuclear matter and finite nuclei. Several pairing interactions are used: surface, mixed and isovector dependent. Pairing has a small effect on the nuclear matter incompressibility at saturation density, but the effects are significant at lower densities. The pairing effect on the centroid energy of the isoscalar Giant Monopole Resonance (GMR) is also evaluated for Pb and Sn isotopes by using a microscopic constrained-HFB approach, and found to change at most by 10% the nucleus incompressibility $K_A$. It is shown by using the Local Density Approximation (LDA) that most of the pairing effect on the GMR centroid come from the low-density nuclear surface.

nucl-th↗

arXiv2010

First results on Martian carbon monoxide from Herschel/HIFI observations

We report on the initial analysis of Herschel/HIFI carbon monoxide (CO) observations of the Martian atmosphere performed between 11 and 16 April 2010. We selected the (7-6) rotational transitions of the isotopes ^{13}CO at 771 GHz and C^{18}O at 768 GHz in order to retrieve the mean vertical profile of temperature and the mean volume mixing ratio of carbon monoxide. The derived temperature profile agrees within less than 5 K with general circulation model (GCM) predictions up to an altitude of 45 km, however, show about 12-15 K lower values at 60 km. The CO mixing ratio was determined as 980 \pm 150 ppm, in agreement with the 900 ppm derived from Herschel/SPIRE observations in November 2009.

astro-ph.EP↗

arXiv2010

Herschel/HIFI observations of Mars: first detection of O_2 at submillimetre wavelengths and upper limits on HCl and H_2O_2

We report on an initial analysis of Herschel/HIFI observations of hydrogen chloride (HCl), hydrogen peroxide (H_2O_2), and molecular oxygen (O_2) in the Martian atmosphere performed on 13 and 16 April 2010 (L_s ~ 77°). We derived a constant volume mixing ratio of 1400 +/- 120 ppm for O_2 and determined upper limits of 200 ppt for HCl and 2 ppb for H_2O_2. Radiative transfer model calculations indicate that the vertical profile of O_2 may not be constant. Photochemical models determine the lowest values of H_2O_2 to be around L_s ~ 75° but overestimate the volume mixing ratio compared to our measurements.

astro-ph.EP↗

arXiv2010

Effect of the tensor force on the charge-exchange spin-dipole excitations of 208Pb

The charge-exchange spin-dipole (SD) excitations of 208Pb are studied by using a fully self-consistent Skyrme Hartree-Fock plus Random Phase Approximation (HF+RPA) formalism which includes the tensor interaction. It is found, for the first time, that the tensor correlations have a unique, multipole-dependent effect on the SD excitations, that is, they produce softening of 1- states, but hardening of 0- and 2- states. This paves the way to a clear assessment of the strength of the tensor terms. We compare our results with a recent measurement, showing that our choice of tensor terms improves the agreement with experiment. The robustness of our results is supported by the analytic form of the tensor matrix elements.

nucl-th↗

arXiv2010

Diproton correlation in a proton-rich Borromean nucleus 17Ne

We carry out three-body-model calculations for a proton-rich Borromean nucleus $^{17}$Ne by assuming a $^{15}$O + $p$ + $p$ structure. To this end, we use a density-dependent contact interaction between the valence protons, explicitly treating also the Coulomb interaction. We find that the two-particle density distribution for $^{17}$Ne is similar to that for $^{16}$C, which has two valence neutrons outside the $N$ = 8 core. That is, the two protons take a spatially compact configuration, while the Coulomb repulsion plays a minor role. This indicates that there is a strong diproton correlation in the ground state of the $^{17}$Ne nucleus. We also show that the Coulomb interaction reduces the expectation value of the proton-proton interaction by about 14\%.

nucl-th↗

arXiv2010

Preparation for the Solar system observations with Herschel: Simulation of Jupiter observations with PACS

Observations of the water inventory as well as other chemically important species on Jupiter will be performed in the frame of the guaranteed time key project of the Herschel Space Observatory entitled "Water and related chemistry in the Solar system". Among other onboard instruments, PACS (Photodetector Array Camera and Spectrometer) will provide new data of the spectral atlas in a wide region covering the far-infrared and submillimetre domains, with an improved spectral resolution and a higher sensitivity compared to previous observations carried out by Cassini/CIRS (Composite InfraRed Spectrometer) and by ISO (Infrared Space Observatory). In order to optimise the observational plan and to prepare for the data analysis, we have simulated the expected spectra of PACS Jupiter observations. Our simulation shows that PACS will promisingly detect several H2O emission lines. As PACS is capable of spatially resolving the Jovian disk, we will be able to discern the external oxygen sources in the giant planets by exploring the horizontal distribution of water. In addition to H2O lines, some absorption lines due to tropospheric CH4, HD, PH3 and NH3 lines will be observed with PACS. Furthermore, owing to the high sensitivity of the instrument, the current upper limit on the abundance of hydrogen halides such as HCl will be also improved.

astro-ph.EP↗

arXiv2010

First results of Herschel/PACS observations of Neptune

We report on the initial analysis of a Herschel/PACS full range spectrum of Neptune, covering the 51-220 micrometer range with a mean resolving power of ~ 3000, and complemented by a dedicated observation of CH4 at 120 micrometers. Numerous spectral features due to HD (R(0) and R(1)), H2O, CH4, and CO are present, but so far no new species have been found. Our results indicate that (i) Neptune's mean thermal profile is warmer by ~ 3 K than inferred from the Voyager radio-occultation; (ii) the D/H mixing ratio is (4.5+/-1) X 10**-5, confirming the enrichment of Neptune in deuterium over the protosolar value (~ 2.1 X 10**-5); (iii) the CH4 mixing ratio in the mid stratosphere is (1.5+/-0.2) X 10**-3, and CH4 appears to decrease in the lower stratosphere at a rate consistent with local saturation, in agreement with the scenario of CH4 stratospheric injection from Neptune's warm south polar region; (iv) the H2O stratospheric column is (2.1+/-0.5) X 10**14 cm-2 but its vertical distribution is still to be determined, so the H2O external flux remains uncertain by over an order of magnitude; and (v) the CO stratospheric abundance is about twice the tropospheric value, confirming the dual origin of CO suspected from ground-based millimeter/submillimeter observations.

astro-ph.EP↗

arXiv2010

The Herschel-SPIRE submillimetre spectrum of Mars

We have obtained the first continuous disk averaged spectrum of Mars from 450 to 1550 Ghz using the Herschel-SPIRE Fourier Transform Spectrometer. The spectrum was obtained at a constant resolution of 1.4 GHz across the whole band. The flux from the planet is such that the instrument was operated in "bright source" mode to prevent saturation of the detectors. This was the first successful use of this mode and in this work we describe the method used for observing Mars together with a detailed discussion of the data reduction techniques required to calibrate the spectrum. We discuss the calibration accuracy obtained and describe the first comparison with surface and atmospheric models. In addition to a direct photometric measurement of the planet the spectrum contains the characteristic transitions of 12CO from J 5-4 to J 13-12 as well as numerous H2O transitions. Together these allow the comparison to global atmospheric models allowing the mean mixing ratios of water and 12CO to be investigated. We find that it is possible to match the observed depth of the absorption features in the spectrum with a fixed water mixing ratio of 1 x 10-4 and a 12CO mixing ratio of 9 x 10-4

astro-ph.EP↗