Inversion doublets of reflection-asymmetric clustering in 28Si and their isoscalar monopole and dipole transitions

[Background] Various cluster states of astrophysical interest are expected to exist in the excited states of $^{28}{\rm Si}$. However, they have not been identified firmly, because of the experimental and theoretical difficulties. [Purpose] To establish the $^{24}$Mg+$\alpha$, $^{16}$O+$^{12}$C and $^{20}$Ne+2$\alpha$ cluster bands, we theoretically search for the negative-parity cluster bands that are paired with the positive-parity bands to constitute the inversion doublets. We also offer the isoscalar monopole and dipole transitions as a promising probe for the clustering. We numerically show that these transition strengths from the ground state to the cluster states are very enhanced. [Method] The antisymmetrized molecular dynamics with Gogny D1S effective interaction is employed to calculate the excited states of $^{28}{\rm Si}$. The isoscalar monopole and dipole transition strengths are directly evaluated from wave functions of the ground and excited states. [Results] Negative-parity bands having $^{24}$Mg+$\alpha$ and $^{16}$O+$^{12}$C cluster configurations are obtained in addition to the newly calculated $^{20}$Ne+2$\alpha$ cluster bands. All of them are paired with the corresponding positive-parity bands to constitute the inversion doublets with various cluster configurations. The calculation show that the band-head of the $^{24}$Mg+$\alpha$ and $^{20}$Ne+2$\alpha$ cluster bands are strongly excited by the isoscalar monopole and dipole transitions. [Conclusions] The present calculation suggests the existence of the inversion doublets with the $^{24}$Mg+$\alpha$, $^{16}$O+$^{12}$C and $^{20}$Ne+2$\alpha$ configurations.Because of the enhanced transition strengths, we offer the isoscalar monopole and dipole transitions as good probe for the $^{24}$Mg+$\alpha$ and $^{20}$Ne+2$\alpha$ cluster bands.Comment: 28 pages, 8 figure

3alpha clustering in the excited states of 16C

The alpha cluster states of 16C are investigated by using the antisymmetrized molecular dynamics. It is shown that two different types of alpha cluster states exist: triangular and linear-chain states. The former has an approximate isosceles triangular configuration of alpha particles surrounded by four valence neutrons occupying sd-shell, while the latter has the linearly aligned alpha particles with two sd-shell neutrons and two pf-shell neutrons. It is found that the structure of the linear-chain state is qualitatively understood in terms of the 3/2 pi- and 1/2 sigma- molecular orbit as predicted by molecular-orbital model, but there exists non-negligible Be+alpha+2n correlation. The band-head energies of the triangular and linear-chain rotational bands are 8.0 and 15.5 MeV, and the latter is close to the He+Be threshold energy. It is also shown that the linear-chain state becomes the yrast sstate at J=10 with excitation energy 27.8 MeV owing to its very large moment-of-inertia comparable with hyperdeformation.Comment: 7 pages, 5 figure

Properties of hadron and quark matter studied with a molecular dynamics

We study the hadron-quark phase transition in a molecular dynamics (MD) of quark degrees of freedom. The hadron state at low density and temperature, and the deconfined quark state at high density and temperature are observed in our model. We investigate the equations of state and draw the phase-diagram at wide baryon density and temperature range. We also discuss the transport property, e.g. viscosity, of $q\bar{q}$ matter. It is found that the ratio of the shear viscosity to the entropy density is less than one for quark matter.Comment: Poster presentation at Quark Matter 200

Three-dimensional orbits of metal-poor halo stars and the formation of the Galaxy

We present the three-dimensional orbital motions of metal-poor stars in conjunction with their metal abundances, for the purpose of getting insight into the formation process of the Galaxy. Our sample stars, which include metal-deficient red giants and RR Lyrae variables observed by the Hipparcos satellite, are least affected by known systematics, stemmed from kinematic bias, metallicity calibration, and secondary metal contamination of stellar surface. We find, for the stars in the metallicity range of [Fe/H]<-1, that there is no evidence for the correlation between [Fe/H] and their orbital eccentricities e. Even for [Fe/H]<-1.6, about 16% of the stars have e less than 0.4. We show that the e distribution of orbits for [Fe/H]<-1.6 is independent of the height |z| away from the Galactic plane, whereas for [Fe/H]>-1.6 the stars at |z|>1 kpc are systematically devoid of low-e orbits with e<0.6. This indicates that low-e stars with [Fe/H]<-1.6 belong to the halo component, whereas the rapidly-rotating thick disk with a scale height about 1 kpc has a metal-weak tail in the range of -1.6<[Fe/H]<-1. The fraction of this metal-weak thick disk appears to be only less than 20%. The significance of these results for the early evolution of the Galaxy is briefly discussed.Comment: 11 pages, 3 figures, AASTeX, to appear in ApJ Letter