Effects of a New Triple-alpha Reaction on X-ray Bursts of a Helium Accreting Neutron Star

arXiv:1105.5484v4 [astro-ph.HE] 9 Jan 2012 1 Prog. Theor. Phys. Vol. ***, No. *, May 2011, Letters Effects of a New Triple-α Reaction on X-ray Bursts of a Helium Accreting Neutron Star Yasuhide Matsuo1, ∗), Hideyuki Tsujimoto1, Tsuneo Noda1, Motoaki Saruwatari1, Masaomi Ono1, Masa-aki Hashimoto1,∗∗) and Masayuki Y. Fujimoto2 1Department of Physics, Kyushu University, Fukuoka 812-8581, Japan 2Department of Physics, Hokkaido University, Sapporo 060-0810, Japan The effects of a new triple-α reaction rate (OKK rate) on the helium flash of a helium accreting neutron star in a binary system have been investigated. Since the ignition points determine the properties of a thermonuclear flash of type I X-ray bursts, we examine the cases of different accretion rates, dM/dt ( ˙M), of helium from 3 × 10−10M⊙yr−1 to 3 × 10−8M⊙yr−1, which could cover the observed accretion rates. We find that for the cases of low accretion rates, nuclear burnings are ignited at the helium layers of rather low densities. As a consequence, helium deflagration would be triggered for all cases of lower accretion rate than ˙M ≃3 × 10−8M⊙yr−1. We find that OKK rate is consistent with the available observations of the X-ray bursts on the helium accreting neutron star. We advocate that the OKK rate is better than the previous rate for the astrophysical phenomena of X-ray burst due to helium accretion. Subject Index: 242, 421 1. Introduction A new challenge has been given for the triple-α (3α) reaction rate, which has been calculated by Ogata et al.4) and found to be very large compared with the previous rate used so far.5),6),7),8) As a consequence, the new rate results in the he- lium (4He) ignition in the lower density/temperature on the stellar evolution of low-, intermediate-, and high-mass stars,9),10) accreting white dwarfs,11),12) and accreting neutron stars.13),14),15) Therefore, it is urgent to clarify quantitatively as possible as how the new rate affects the above astrophysical phenomena, because the rate plays the most fundamental role among the nuclear burning in heavenly bodies and could do some role in the early universe, where any terrestrial experiments for the 3α reaction are very difficult. In the present paper, we investigate the effects of a newly calculated 3α reaction rate (OKK rate)4) on the helium flashes that occur at the bottom layers inside the accreting envelope of a neutron star. We can use the ignition curves to find roughly when the nuclear ignition occurs. We note that X-ray bursts which have been mainly studied so far are limited to the combined burning of hydrogen and helium1) and observational features such as light curves and burst energy have been qualitatively explained well. Fujimoto et al.19) have succeeded in simulating the X-ray bursts by solving the whole structure equations and clarified firstly the importance of the heat flow from the bottom of the accumulated layer. The application has been done for several X-ray burst observations of combined H ∗) E-mail: matsuo@phys.kyushu-u.ac.jp ∗∗) E-mail: hashimoto@phys.kyushu-u.ac.jp typeset using PTPTEX.cls ⟨Ver.0.9⟩ 2 Letters Vol. ***, No. * and He burnings.26),2),3) However, there remained detailed comparison between ob- servations and calculations related to successful bursts, very low accretion rates, and or superbursts (e.g., 1),23)). In the mean time, Fynbo et al.16) (we call the reference as Fynbo) revised the 3α rate of NACRE6) based on new experiments at high temperature of T > 109 K and with an artificial extrapolation to the very low temperature region toward T ∼107 K. However, in the present case of accreting neutron stars we can regard the differences between NACRE and Fynbo as unimportant , because the OKK rate is much larger for the temperature less than 108 K compared to the difference between NACRE and Fynbo. It is noted that although nuclear burning strongly depends on the temperature, the density becomes very important at high densities of ρ ≥106 g cm−3 and low temperatures of T ≤108 K due to the screening effects.11) The new rate has been found to affect significantly the evolution of low-mass and intermediate stars,7),8) where the evolutions from the zero-age main sequence through the core He flash/burning from 1 M⊙to 10 M⊙have been investigated. From the HR diagram obtained with the OKK rate, the results disagrees in detail with the observations. If the OKK rate in the temperature range of 108 K < T < 2 × 108 K is exactly correct, we must invoke some new physical processes such as rotational mixing and/or convection mechanism. On the other hand, Saruwatari and Hashimoto17) have shown the important difference between the ignition points calculated by the two rates for the helium accreting carbon-oxygen white dwarf. They have concluded that for all the accretion rates, nuclear fuel ignites at the accumulated layers of helium and a scenario of Type Ia supernovae changes for low accretion rates of ˙M < 4 × 10−8 M⊙yr−1. Related to this study, since the progenitor o

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