Condition for the formation of the compound chondrules in the solar nebula

The conditions for compound chondrule formation described by J. L. GOODING and K. KEIL (Meteoritics, 16,17,1981) are reexamined using the criterion of gravitational instabilities (M. SEKIYA, Prog. Theor. Phys., 69,1116,1983). If a compound chondrule was formed by collisional sticking of a plastic particle with a solid or another plastic particle, the following conclusions are derived : (1) Chondrules were formed after the settling of dust particles had progressed and the number density of dust particles had increased by several orders of magnitude. (2) To reproduce the observed ratio of the compound chondrules to all the chondrules, collisional velocities of pre-chondrule particles must have been larger than about 1.6m s^, and the Weber number for the collision of molten pre-chondrule particles must have been larger than about 40,if we assume that the chondrule formation occurred in the asteroid region in the stage where dust particles were floating in the solar nebula. In this case, collisional sticking would be difficult. If sticking is impossible, then other formation mechanisms for the compound chondrules must be considered (e. g. J. T. WASSON et al., Geochim. Cosmochim. Acta, 59,1847,1995). (3) If chondrules were formed in the inner regions of the solar nebula, the lower limits for the relative velocities of the prechondrule particles would be reduced

Two evolutional paths of an axisymmetric gravitational instability in the dust layer of a protoplanetary disk

Nonlinear numerical simulations are performed to investigate the density evolution in the dust layer of a protoplanetary disk due to the gravitational instability and dust settling toward the midplane. We assume the region where the radial pressure gradient at equilibrium is negligible so that the shear-induced instability is avoided, and also restrict to an axisymmetric perturbation as a first step of nonlinear numerical simulations of the gravitational instability. We find that there are two different evolutional paths of the gravitational instability depending on the nondimensional gas friction time, which is defined as the product of the gas friction time and the Keplerian angular velocity. If the nondimensional gas friction time is equal to 0.01, the gravitational instability grows faster than dust settling. On the other hand, if the nondimensional gas friction time is equal to 0.1, dust aggregates settle sufficiently before the gravitational instability grows. In the latter case, an approximate analytical calculation reveals that dust settling is faster than the growth of the gravitational instability regardless of the dust density at the midplane. Thus, the dust layer becomes extremely thin and may reach a few tenth of the material density of the dust before the gravitational instability grows.Comment: 4 pages, 3 figure

Formation processes of magnetic spherules collected from deep-sea sediments -Observations and numerical simulations of the orbital evolution-

Spherules collected from deep-sea sediments were analyzed for major chemical compositions and examined for textural relationships. Two main types of internal texture are observed in Ni-bearing I-type spherules : spherules with Ni-free Fe-oxide mantle and Fe-Ni metallic cores and Fe-Ni oxide spherules without metallic cores. Cores are often observed off-centered. S-type spherules show porphyritic, barred, and fine-grained types of texture. Relict olivine grains are found in some porphyritic S-type spherules. A cooling rate of 55℃/s was calculated for one of these grains based on chemical zonal patterns. For I-type spherules, numerical simulations were performed to constrain the relationship between temperatures, radii of spherules, and the apparent gravitational acceleration with the spherule as a stationary frame of reference. Results indicate that a) spherules lose most of their original mass by evaporation over 1900℃, and, b) the apparent gravitational acceleration is at a maximum during the last stage of evaporation. Based on the observations and numerical simulations of I-type spherules, we propose the following model for their formation : during ablation, the metallic core is off-centered, a surface of the core is then exposed to the air due to the strong apparent gravitational acceleration and, finally, oxidation reactions can proceed on the exposed core surface

Cosmic-ray exposure age and heliocentric distance of the parent body of H chondrites Yamato-75029 and Tsukuba

Many small pieces of the H chondrites Yamato (Y-) 75029 and Tsukuba were investigated to characterize signatures of light noble gases. These meteorites contain large amounts of solar gases as well as cosmogenic ones. A simple regolith exposure model was developed in order to explain the correlations among solar ^Ne and ^Ar and cosmogenic ^Ne concentrations. Based on the regolith model, the parent body exposure ages, heliocentric distances, and space exposure ages of the two meteorites were calculated. The parent body exposure ages were more than 5.5Ma and 11.8Ma for Y-75029 and Tsukuba, respectively. The heliocentric distances were 2.2±^_ AU and 4.2±^_ AU for Y-75029 and Tsukuba, respectively. The space exposure age of Y-75029 was 5.2-5.8Ma, whereas that of Tsukuba was 8.1±0.6Ma. It has been suggested that the parent bodies of H chondrites are S-type asteroids with orbits that range from 2 to 3.5AU in the present solar system. On the other hand, the obtained heliocentric distances of Y-75029 and Tsukuba indicate locations of the parent bodies in the past when some parts of the meteorites were exposed to the sun. The heliocentric distances for Y-75029 is in a good agreement with current S-type asteroid distribution, while that for Tsukuba is at the upper tail of the distribution

Telephone Cognitive-Behavioral Therapy for Subthreshold Depression and Presenteeism in Workplace: A Randomized Controlled Trial

Subthreshold depression is highly prevalent in the general population and causes great loss to society especially in the form of reduced productivity while at work (presenteeism). We developed a highly-structured manualized eight-session cognitive-behavioral program with a focus on subthreshold depression in the workplace and to be administered via telephone by trained psychotherapists (tCBT).We conducted a parallel-group, non-blinded randomized controlled trial of tCBT in addition to the pre-existing Employee Assistance Program (EAP) versus EAP alone among workers with subthreshold depression at a large manufacturing company in Japan. The primary outcomes were depression severity as measured with Beck Depression Inventory-II (BDI-II) and presenteeism as measured with World Health Organization Health and Work Productivity Questionnaire (HPQ). In the course of the trial the follow-up period was shortened in order to increase acceptability of the study.The planned sample size was 108 per arm but the trial was stopped early due to low accrual. Altogether 118 subjects were randomized to tCBT+EAP (n = 58) and to EAP alone (n = 60). The BDI-II scores fell from the mean of 17.3 at baseline to 11.0 in the intervention group and to 15.7 in the control group after 4 months (p<0.001, Effect size = 0.69, 95%CI: 0.32 to 1.05). However, there was no statistically significant decrease in absolute and relative presenteeism (p = 0.44, ES = 0.15, -0.21 to 0.52, and p = 0.50, ES = 0.02, -0.34 to 0.39, respectively).Remote CBT, including tCBT, may provide easy access to quality-assured effective psychotherapy for people in the work force who present with subthreshold depression. Further studies are needed to evaluate the effectiveness of this approach in longer terms. The study was funded by Sekisui Chemicals Co. Ltd.ClinicalTrials.gov NCT00885014