Hyperon-quark mixed phase in dense matter

高密度物質中でのハイペロン-クォーク混合相

丸山 敏毅  ; 千葉 敏; Schulze, H.-J.*; 巽 敏隆*

Maruyama, Toshiki; Chiba, Satoshi; Schulze, H.-J.*; Tatsumi, Toshitaka*

原子核密度の約23倍でハイペロンが現れると状態方程式が軟化し、その結果重い中性子星が存在できなくなるが、これは中性子星の最大質量が太陽質量の約1.5倍以上という観測事実に矛盾する。これに対して、ある密度でクォーク非閉じ込め相転移が起こると、クォークの固い状態方程式によって重い中性子星が可能になるという主張がされた。しかし非一様構造を考慮しない単純な計算では、ハイペロン物質とクォーク物質の混合相が現れると、状態方程式が再び軟化してしまう。そこで、Brueckner-Hartree-Fock理論に基づく現実的な相互作用模型と、混合相の非一様構造を計算する手法により、ハイペロン物質とクォーク物質の混合相の状態方程式を求めた。その結果、混合相の状態方程式はクーロン遮蔽効果によってあまり軟化しないことがわかった。この状態方程式を用いて中性子星の最大質量を計算したところ、非一様構造を考慮しない単純な計算に比べ、観測されている値により近い値を得ることができた。

Quark matter has been expected inside compact stars. Since the deconfinement transition from hadron to quark phase may occur as a first-order phase transition, the hadron-quark mixed phase should appear, where charge density as well as baryon number density is no more uniform. Owing to the interplay of the Coulomb interaction and the surface tension, the mixed phase can have an exotic shape called pasta structure. In a series of recent papers we have revealed an important role of the charge screening effect in the mixed phase. In this presentation, we incorporate a realistic EOS of hadronic matter which properly includes hyperon degree of freedom and discuss the properties of hadron-quark mixed phase in compact stars. Taking into account the charge screening effects on pasta structures of hyperon-quark mixed phase, we find that the resultant EOS is not so different from that given by the Maxwell construction (MC). Thereby the bulk properties of compact stars such as the mass-radius relation are little affected, compared with the MC case. However, the internal properties of the mixed phase are very different in two cases, especially about hyperons: hyperons always appear in the mixed phase in the MC case, while no hyperon in our case. Non-uniform structure and suppression of hyperons in the mixed phase should have some implications on the neutrino transport and cooling of pulsars as well as the maximum mass problem.