Distributions of Long-Lived Radioactive Nuclei Provided by Star Forming Environments

Radioactive nuclei play an important role in planetary evolution by providing an internal heat source, which affects planetary structure and helps facilitate plate tectonics. A minimum level of nuclear activity is thought to be necessary --- but not sufficient --- for planets to be habitable. Extending previous work that focused on short-lived nuclei, this paper considers the delivery of long-lived radioactive nuclei to circumstellar disks in star forming regions. Although the long-lived nuclear species are always present, their abundances can be enhanced through multiple mechanisms. Most stars form in embedded cluster environments, so that disks can be enriched directly by intercepting ejecta from supernovae within the birth clusters. In addition, molecular clouds often provide multiple episodes of star formation, so that nuclear abundances can accumulate within the cloud; subsequent generations of stars can thus receive elevated levels of radioactive nuclei through this distributed enrichment scenario. This paper calculates the distribution of additional enrichment for $^{40}$K, the most abundant of the long-lived radioactive nuclei. We find that distributed enrichment is more effective than direct enrichment. For the latter mechanism, ideal conditions lead to about 1 in 200 solar systems being directly enriched in $^{40}$K at the level inferred for the early solar nebula (thereby doubling the abundance). For distributed enrichment from adjacent clusters, about 1 in 80 solar systems are enriched at the same level. Distributed enrichment over the entire molecular cloud is more uncertain, but can be even more effective.Comment: 24 pages, 8 figures, accepted for publication in Ap

Raven’s Work in Tlingit Ethno-geography

Chapter in the publication: Holton, Gary and Thomas F.Thornton. (Eds.) Language and Toponymy in Alaska and Beyond: Papers in Honor of James Kari. Language Documentation & Conservation Special Publication no. 17. Honolulu: University of Hawai‘i Press.Ye

The Ultimate Halo Mass in a LCDM Universe

In the far future of an accelerating LCDM cosmology, the cosmic web of large-scale structure consists of a set of increasingly isolated halos in dynamical equilibrium. We examine the approach of collisionless dark matter to hydrostatic equilibrium using a large N-body simulation evolved to scale factor a = 100, well beyond the vacuum--matter equality epoch, a_eq ~ 0.75, and 53/h Gyr into the future for a concordance model universe (Omega_m ~ 0.3, Omega_Lambda ~ 0.7). The radial phase-space structure of halos -- characterized at a < a_eq by a pair of zero-velocity surfaces that bracket a dynamically active accretion region -- simplifies at a > 10 a_eq when these surfaces merge to create a single zero-velocity surface, clearly defining the halo outer boundary, rhalo, and its enclosed mass, mhalo. This boundary approaches a fixed physical size encompassing a mean interior density ~ 5 times the critical density, similar to the turnaround value in a classical Einstein-deSitter model. We relate mhalo to other scales currently used to define halo mass (m200, mvir, m180b) and find that m200 is approximately half of the total asymptotic cluster mass, while m180b follows the evolution of the inner zero velocity surface for a < 2 but becomes much larger than the total bound mass for a > 3. The radial density profile of all bound halo material is well fit by a truncated Hernquist profile. An NFW profile provides a somewhat better fit interior to r200 but is much too shallow in the range r200 < r < rhalo.Comment: 5 pages, 3 figures, submitted to MNRAS letter

Japan's Persistent Trade Surplus: Policies for Adjustment

While part of the recent increase in the Japanese trade surplus can be attributed to the Japanese recession, the surplus has widened despite the appreciation of the yen and enactment of policies to open Japanese markets. We review the trade surplus issue in the light of theories of trade and current account adjustment. We evaluate the potential for exchange appreciation and Japanese fiscal policy to reduce the imbalance, estimating their effects using simulations of the NIRA-LINK model of the US-Japan-world economy. The simulations show that moderate use of macropolicies would not be sufficient to eliminate the trade imbalance.Japanese trade surplus, econometric simulations