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Current uncertainty about the most massive observed stars has led to a reexamination of the most massive star that is stable against radial pulsation.The nuclear energy generation equations in the INA linear, nonadiabatic code have been considerably improved, so that it is now appropriate to redo the study to determine the maximtun mass of a ZAMS star that will be stable against pulsation.

The problems of the spatial and velocity distributions of planetary nebulae in our galaxy are still very much with us. In what follows we comment rather hastily on the older work and more especially on the recent work in these fields. We have not been able to carry out an exhaustive literature search, and therefore trust we shall be forgiven by those whose contributions do not appear herein. We first survey the scene in the greater solar neighborhood. Section 2 deals with the local spatial distribution of planetaries in the galactic plane and perpendicular to the plane. Section 3 focuses on the rather meager results that can be obtained on the kinematical properties of the planetaries in our general part of the galactic system. We conclude by moving farther afield and exploring another part of the galaxy and the galaxy as a whole. Section 4 addresses the distribution of planetaries and summarizes recent estimates of the total planetary population of our galaxy. Finally, Section 5 is devoted to what little one can conclude about the overall kinematical properties of the system of planetaries.

Electronic thermal conduction in a magnetoplasma with electron number densities of the order of 10/sup 11/ to 10/sup 12/ cm/sup -3/, where Coulomb interactions predominate, has been studied experimentally in neon discharges at a pressure of the order of 5 mm Hg, and magnetic fields up to 1200 Oe. The temperature distribution resulting from a very localized heating of the electron gas by an electromagnetic wave of frequency 7500 Mc/sec has been measured in a plane perpendicular to the magnetic field and at variable distances from the heated volume. The experimental results are compared with the predictions of Landshoff. (auth)

A special technique for the accurate measurement of thermal conductivity is discussed. The method involves use of the Peltier heat to maintain a temperature gradient along the specimen. Straightforward measurements allow calculation of the absolute value of the thermoelectric power, thermal conductivity, and electrical resistivity. An especially useful feature of the method is that the thermoelectric figure of merit is given in terms of the ratio of two voltages. The theory is presented for the case in which the radiative heat transfer is important. The method has been tested experimentally at 300°K only, but analysis suggests that accurate measurements of thermal conductivity can be made by this technique on low thermal conductivity materials of small dimensions up to 1000°K.

The rapid accumulation of data makes it possible to begin to determine the positions of OH/IR stars on the mass-luminosity plane. With the assumption of fundamental mode pulsation, the accurately determined period-luminosity-color relationships in the Mira region will be extrapolated to the larger OH/IR region, where the Mira variables have evolved. OH/IR star luminosities and periods are needed in order to estimate positions on the mass-luminosity plane.

A punched-card catalogue of planetary nebulae has been prepared, using data extracted from all existing catalogues. A computer program calculates distances and radii using the method of Shklovsky, in which all nebulae are assumed to have the same ionized mass, and allowance for interstellar extinction is made assuming a continuous galactic-dust distribution. The assumption made in Shklovsky’s method, that the nebulae are optically thin, is considered to be satisfied if the calculated radii lie within a certain well-defined interval. The reddening constants obtained are in satisfactory statistical agreement with constants determined by other methods. The local density of planetary nebulae is in agreement with estimates of local white-dwarf densities.

The distribution of current is computed in a cylindrical semiconductor specimen provided with cylindrical metallic end caps of the same diameter as that of the specimen and electrical lead wires of much smaller diameter. Nonuniformity of the longitudinal current density of 1% or less can be obtained in specimens with electrical resistivities at least 200 times greater than that of the end caps if the lengths of the specimens are at least equal to their diameters and if the length of each end cap is at least 0.3 diam.

A survey of available SiO maser observations of the 43.122-GHz, v = 1, J = 1..-->..0 transition has been made in order to further investigate the question of a correlation of maser flux with bolometric flux. This correlation has been shown to exist both for data which have been phase shifted to mean maximum and for those which have not.

We explore several of the consequences of the model of late stellar evolution recently investigated by Wood and Cahn. As stars ascend the asymptotic giant branch in the H-R plane, they evolve from left to right in the (M, log L) -plane, and also sink downward as they suffer a steady stellar-wind mass loss of the Reimers type. Stars in the approximate range of 1--2 M/sub sun/ eventually evolve to the point where they apparently begin first-overtone oscillation; during this phase they are Mira variables for a few hundred thousand years. At the end of their Mira stage, many of these stars evolve directly to white dwarfs; the rest of them eject one or more planetary nebulae before entering white-dwarf territory. Higher-mass stars evolve across the (M, log L) -plane without ever penetrating the Mira region and go on directly to give rise to planetary nebulae and eventually become white dwarfs. Stars of even greater mass presumably end their migration across the (M, logL) -domain by becoming supernovae. We adopt this broad picture and go on first to the Mira variables, seeking to establish an empirical diagram of period of light variation against mean spectral type at maximum light. Curvesmore » of constant period and constant spectral type are superposed on the (M, logL) -plane, along with evolutionary tracks and time scales. Thus a knowlege of the observed mean spectral type at maximum light and the period of a star permits approximate estimates to be made of its mean bolometric luminosity and present mass and gives a bird's-eve view of its past and future evolutionary track.« less

The terminal expansion velocities of circumstellar shells surrounding Mira variables, as deduced by other authors from radio observations of SiO, CO, and OH, are correlated with the mean bolometric luminosities of the parent stars. We find that the greater the photon output of a star the greater is the expansion velocity of its shell. The final gas velocities range from 3 km s/sup -1/ up to 22 km s/sup -1/ over a luminosity factor of 4. We presume that the purely gaseous steady-state outflow in the circumstellar regions close to the parent star is governed by the subsonic stellar wind function. At roughly 2R/sub asterisk/ the temperature becomes cool enough so that grains begin to form out of the heavy-element constituents of the gas. They grow rapidly, and stellar radiation pressure increasingly accelerates the grains relative to the ambient gas. The interaction of grains and gas serves to decelerate the grains and accelerate the gas. At several stellar radii the two components converge on their separate terminal velocities, with the gas moving more slowly than the grains. Together, the observational data and the hydrodynamic equations demand that the most luminous Miras in our sample produce somewhat bigger grains (typical radiimore » approx.6--7 x 10/sup -6/ cm) than do the least luminous Miras (radii approx.3--410/sup -6/ cm).« less

The period distribution of Mira variables has been obtained for a region near the galactic plane, and from this distribution a strip of instability for Mira variables in the mass-luminosity plane has been derived. Mass loss rates that are smaller by factors of 2 to 4 than those obtained by Reimers are required to explain the existence of the high-velocity and halo Miras with periods less than 300 days. Assuming that the high-luminosity edge of the Mira instability strip coincides with ejection of the hydrogen-rich envelope, we derive remnant white dwarf (or planetary nebula nucleus) masses and masses of planetary nebulae as a function of initial stellar mass. Using a mainsequence death rate function, we find that near the plane of the Galaxy the mass distribution of planetary nebula nuclei is highly peaked in the mass range 0.6--0.8 M/sub sun/, while the mass distribution of planetary nebulae is bimodal, with peaks near 0.01 and 0.8 M/sub sun/. The mass distribution of white dwarfs produced near the galactic plane during the lifetime of the Galaxy is also predicted to show a strong peak near 0.75 M/sub sun/. The lower initial mass limit for stars which produce degenerate cores of 1.4 M/submore » sun/ and therefore possibly supernovae lies in the range approx.3.7 to approx.4.7 M/sub sun/.« less

A theory for the propagation of a plasma into an ambient gas is proposed. It is an extension of the usual ambipolar diffusion which takes into account the inertia of the particles and predicts a finite velocity for the propagation of the plasma. The equation ∇2ne=(1/u2)(∂2ne/∂t2)+(1/Da)(∂ne/∂t) is obtained for the electron density in the expanding plasma, where u = (Daνa)½, Da is the ambipolar diffusion coefficient and νa is an ``ambipolar collision frequency.'' u gives the speed of propagation of the plasma interface and is equal to the velocity (kTe/Mi)½, if the electron temperature Te is much higher than the ion temperature. This equation is solved in one‐dimensional geometry and some characteristics of the solution discussed. It is shown that the maximum density of the plasma propagates with speed u in the case of low pressure of the gas but with much lower speed in the case of higher pressure.