The Origin of Primordial Dwarf Stars and Baryonic Dark Matter

I present a scenario for the production of low mass, degenerate dwarfs of mass $>0.1 M_{\odot}$ via the mechanism of Lenzuni, Chernoff & Salpeter (1992). Such objects meet the mass limit requirements for halo dark matter from microlensing surveys while circumventing the chemical evolution constraints on normal white dwarf stars. I describe methods to observationally constrain this scenario and suggest that such objects may originate in small clusters formed from the thermal instability of shocked, heated gas in dark matter haloes, such as suggested by Fall & Rees (1985) for globular clusters.Comment: TeX, 4 pages plus 2 postscript figures. To appear in Astrophysical Journal Letter

The Structure of Isothermal, Self-gravitating Gas Spheres for Softened Gravity

A theory for the structure of isothermal, self-gravitating gas spheres in pressure equilibrium in a softened gravitational field is developed. The one parameter spline softening proposed by Hernquist & Katz (1989) is used. We show that the addition of this extra scale parameter implies that the set of equilibrium solutions constitute a one-parameter family, rather than the one and only one isothermal sphere solution for Newtonian gravity. We demonstrate the perhaps somewhat surprising result that for any finite choice of softening length and temperature, it is possible to deposit an arbitrarily large mass of gas in pressure equilibrium and with a non-singular density distribution inside of r_0 for any r_0 > 0. The theoretical predictions of our models are compared with the properties of the small, massive, quasi-isothermal gas clumps which typically form in numerical Tree-SPH simulations of 'passive' galaxy formation of Milky Way sized galaxies. We find reasonable agreement despite the neglect of rotational support in the models. We comment on whether the hydrodynamical resolution in our numerical simulation of galaxy formation is sufficient, and finally we conclude that one should be cautious, when comparing results of numerical simulations involving gravitational softening and hydrodynamical smoothing, with reality.Comment: 22 pages Latex + 12 figure

Proportion Regulation in Globally Coupled Nonlinear Systems

As a model of proportion regulation in differentiation process of biological system, globally coupled activator-inhibitor systems are studied. Formation and destabilization of one and two cluster state are predicted analytically. Numerical simulations show that the proportion of units of clusters is chosen within a finite range and it is selected depend on the initial condition.Comment: 11 pages (revtex format) and 5 figures (PostScript)

Relationships between Peak Oxygen Uptake and Arterial Function: a Preliminary Study

Please view abstract in the attached PDF file

A Combined Spitzer and Chandra Survey of Young Stellar Objects in the Serpens Cloud Core

We present Spitzer and Chandra observations of the nearby (~260 pc) embedded stellar cluster in the Serpens Cloud Core. We observed, using Spitzer's IRAC and MIPS instruments, in six wavelength bands from 3 to 70 ${\mu}m$, to detect thermal emission from circumstellar disks and protostellar envelopes, and to classify stars using color-color diagrams and spectral energy distributions (SEDs). These data are combined with Chandra observations to examine the effects of circumstellar disks on stellar X-ray properties. Young diskless stars were also identified from their increased X-ray emission. We have identified 138 YSOs in Serpens: 22 class 0/I, 16 flat spectrum, 62 class II, 17 transition disk, and 21 class III stars; 60 of which exhibit X-ray emission. Our primary results are the following: 1.) ten protostars detected previously in the sub-millimeter are detected at lambda < 24 microns, seven at lambda < 8 microns, 2.) the protostars are more closely grouped than more evolved YSOs (median separation : ~0.024 pc, and 3.) the luminosity and temperature of the X-ray emitting plasma around these YSOs does not show any significant dependence on evolutionary class. We combine the infrared derived values of AK and X-ray values of NH for 8 class III objects and find that the column density of hydrogen gas per mag of extinctions is less than half the standard interstellar value, for AK > 1. This may be the result of grain growth through coagulation and/or the accretion of volatiles in the Serpens cloud core.Comment: 69 pages, 16 figures, accepted to ApJ. Higher Resolution Figures at: http://www.cfa.harvard.edu/~ewinston

Comment on piNN Coupling from High Precision np Charge Exchange at 162 MeV

In this updated and expanded version of our delayed Comment we show that the np backward cross section, as presented by the Uppsala group, is seriously flawed (more than 25 sd.). The main reason is the incorrect normalization of the data. We show also that their extrapolation method, used to determine the charged piNN coupling constant, is a factor of about 10 less accurate than claimed by Ericson et al. The large extrapolation error makes the determination of the coupling constant by the Uppsala group totally uninteresting.Comment: 5 pages, latex2e with a4wide.sty. This is an updated and extended version of the Comment published in Phys. Rev. Letters 81, 5253 (1998

Quasi-Elastic Scattering in the Inclusive (3^3He, t) Reaction

The triton energy spectra of the charge-exchange $^{12}$C($^3$He,t) reaction at 2 GeV beam energy are analyzed in the quasi-elastic nucleon knock-out region. Considering that this region is mainly populated by the charge-exchange of a proton in $^3$He with a neutron in the target nucleus and the final proton going in the continuum, the cross-sections are written in the distorted-wave impulse approximation. The t-matrix for the elementary exchange process is constructed in the DWBA, using one pion- plus rho-exchange potential for the spin-isospin nucleon- nucleon potential. This t-matrix reproduces the experimental data on the elementary pn $\rightarrow$np process. The calculated cross-sections for the $^{12}$C($^3$He,t) reaction at $2^o$ to $7^o$ triton emission angle are compared with the corresponding experimental data, and are found in reasonable overall accord.Comment: 19 pages, latex, 11 postscript figures available at [email protected], submitted to Phy.Rev.

Spin gap in the Quasi-One-Dimensional S=1/2 Antiferromagnet: Cu2(1,4-diazacycloheptane)2Cl4

Cu_{2}(1,4-diazacycloheptane)_{2}Cl_{4} contains double chains of spin 1/2 Cu^{2+} ions. We report ac susceptibility, specific heat, and inelastic neutron scattering measurements on this material. The magnetic susceptibility, $\chi(T)$, shows a rounded maximum at T = 8 K indicative of a low dimensional antiferromagnet with no zero field magnetic phase transition. We compare the $\chi(T)$ data to exact diagonalization results for various one dimensional spin Hamiltonians and find excellent agreement for a spin ladder with intra-rung coupling $J_1 = 1.143(3)$ meV and two mutually frustrating inter-rung interactions: $J_2 = 0.21(3)$ meV and $J_3 = 0.09(5)$ meV. The specific heat in zero field is exponentially activated with an activation energy $\Delta = 0.89(1)$ meV. A spin gap is also found through inelastic neutron scattering on powder samples which identify a band of magnetic excitations for $0.8 < \hbar\omega < 1.5$ meV. Using sum-rules we derive an expression for the dynamic spin correlation function associated with non-interacting propagating triplets in a spin ladder. The van-Hove singularities of such a model are not observed in our scattering data indicating that magnetic excitations in Cu_{2}(1,4-diazacycloheptane)_{2}Cl_{4} are more complicated. For magnetic fields above $H_{c1} \simeq 7.2$ T specific heat data versus temperature show anomalies indicating a phase transition to an ordered state below T = 1 K.Comment: 9 pages, 8 postscript figures, LaTeX, Submitted to PRB 8/4/97, e-mail Comments to [email protected]

Study of the magnetic susceptibility in the spin-Peierls system CuGeO3_3

We study numerically, using a one-dimensional Heisenberg model, the spin-Peierls transition in the linear Cu$^{2+}$ spin-1/2 chains in the inorganic compound CuGeO$_3$ which has been recently observed experimentally. We suggest that the magnetic susceptibility, the temperature dependence of the spin gap and the spin-Peierls transition temperature of this material can be reasonably described by including nearest and next nearest neighbor antiferromagnetic interactions along the chain. We estimate that the nearest neighbor exchange parameter J is approximately $160\:\rm K$, and that the next nearest neighbor exchange parameter is approximately $0.36\:\rm J$.Comment: 14 pages, Revtex v2.0, 4 figures available upon reques

Intermediate Temperature Fluids Life Tests - Experiments

There are a number of different applications that could use heat pipes or loop heat pipes (LHPs) in the intermediate temperature range of 450 to 725 K (170 to 450 C), including space nuclear power system radiators, fuel cells, and high temperature electronics cooling. Historically, water has been used in heat pipes at temperatures up to about 425 K (150 C). Recent life tests, updated below, demonstrate that titanium/water and Monel/water heat pipes can be used at temperatures up to 550 K (277 C), due to water's favorable transport properties. At temperatures above roughly 570 K (300 C), water is no longer a suitable fluid, due to high vapor pressure and low surface tension as the critical point is approached. At higher temperatures, another working fluid/envelope combination is required, either an organic or halide working fluid. An electromotive force method was used to predict the compatibility of halide working fluids with envelope materials. This procedure was used to reject aluminum and aluminum alloys as envelope materials, due to their high decomposition potential. Titanium and three corrosion resistant superalloys were chosen as envelope materials. Life tests were conducted with these envelopes and six different working fluids: AlBr3, GaCl3, SnCl4, TiCl4, TiBr4, and eutectic diphenyl/diphenyl oxide (Therminol VP-1/Dowtherm A). All of the life tests except for the GaCl3 are ongoing; the GaCl3 was incompatible. As the temperature approaches 725 K (450 C), cesium is a potential heat pipe working fluid. Life tests results are also presented for cesium/Monel 400 and cesium/70-30 copper/nickel heat pipes operating near 750 K (477 C). These materials are not suitable for long term operation, due to copper transport from the condenser to the evaporator