EFFECTS OF RADIONUCLIDES ON THE IONIZATION STATE OF PROTOPLANETARY DISKS AND DENSE CLOUD CORES

Abstract

We comprehensively reinvestigate the ionization rates by radionuclides with the newest data on the abundance of the nuclides for the primitive solar nebula distinguishing the ionization rates of a hydrogen molecule, , from those of a helium atom . The ionization rates by 232Th, 235U, and 238U become an order of magnitude larger than in the previous work of Umebayashi & Nakano by including all the energy released in the decay series, and these nuclides contribute about 20% of the total ionization rate by the long-lived radionuclides, 1.4 × 10–22 s–1 for a hydrogen molecule. The rest (80%) is contributed by 40K. Among the short-lived radionuclides which are extinct in the present solar system, 26Al is the dominant ionization source with the rate (7-10) × 10–19 s–1, overwhelming the long-lived nuclides. In addition, 60Fe and 36Cl are more efficient than the long-lived nuclides though at least 10 times more inefficient than 26Al. The helium abundance in the primitive solar nebula is significantly lower than in the present interstellar medium. We obtain a simple formula which transforms the ionization rates into those for the other values of the helium abundance. Ionization by radionuclides is quite inefficient when the mean dust size is greater than about 1 cm. Using these ionization rates, we investigate the ionization state for some configurations of the clouds. With an improved attenuation law of cosmic rays in geometrically thin disks, we find that the dead zones in protoplanetary disks are significantly larger than those obtained in the previous work.