Amount Of Sweep Research Articles

Low‐Mass Star Formation Triggered by Supernovae in Primordial Clouds

The evolution of a gas shell, swept up by the supernova remnant of a massive first-generation star, is studied with H2 and HD chemistry taken into account and with the use of a semianalytical approximation to the dynamics. When a first-generation star, formed in a parent pregalactic cloud, explodes as a supernova with explosion energy in the range of 1051-1052 ergs at redshifts of z = 10-50, H2 and HD molecules are formed in the swept up gas shell at fractional abundances of ~10-3 and ~10-5, respectively, and effectively cool the gas shell to temperatures of 32-154 K. If the supernova remnant can sweep to gather the ambient gas of mass 6 × 104 to 8 × 105 M☉, the gas shell comes to be dominated by its self-gravity and, hence, is expected to fragment. The amount of swept up gas necessary for fragmentation increases with the explosion energy and decreases with the interstellar gas density (or redshift) of the host cloud, which provides a lower boundary to the mass of the host cloud in which star formation is triggered by the first-generation supernova. Also, the condition for fragmentation is very sensitive to the thermal state of interstellar gas. Our result shows that for a reasonable range of temperatures (200-1000 K) of interstellar gas, the formation of second-generation stars can be triggered by a single supernova or hypernova with explosion energy in the above range in a primordial cloud of total (dark and baryonic) mass as low as a few times 106 M☉. For higher temperatures in the interstellar gas, however, the condition for the fragmentation in the swept up gas shell demands a larger supernova explosion energy. We also follow the subsequent contraction of the fragment pieces assuming their geometry (sphere and cylinder) and demonstrate that the Jeans masses in the fragments decrease to well below 1 M☉ by the time the fragments become optically thick to the H2 and HD lines. The fragments are then expected to break up into dense cores whose masses are comparable to the Jeans masses and collapse to form low-mass stars that can survive to the present. If the material in the gas shell is mixed well with the ejecta of the supernova, the shell and low-mass stars thus formed are likely to have metals of abundance [Fe/H] ≃ -3 on average. This metallicity is consistent with those of the extremely metal-poor stars found in the Galactic halo. Stars with low metallicities of [Fe/H] < -5 such as HE 0107-5240, recently discovered in the Galactic halo, are difficult to form by this mechanism and must be produced in different situations.

Pulsar wind nebulae: theoretical aspects and observational constraints

Of the known pulsar wind nebulae, 8 are good candidates for being in the early stage of evolution where the wind nebula is interacting with the freely expanding supernova ejecta. Several of these have been identified with historical supernovae. Although the identification of SN 1181 with 3C 58 has been thought to be relatively secure, the large size of the nebula, the amount of swept up mass, and the internal energy indicate a larger age. For G11.2-0.3, the nebular size and internal energy are consistent with the identification with the possible supernova of 386. Although the Crab Nebula appears to have approximate energy equipartition between particles and the magnetic field, the nebulae 3C 58 and MSH 15-52 appear to be particle dominated. The low magnetic field is consistent with models in which the nebulae are created by a shocked pulsar wind.

The ray theory of supersonic propeller acoustics

The wavefront surface produced by a supersonic propeller is constructed in the standard way as an envelope of expanding spheres. Focusing of rays is shown to occur, determined by the curvature of this surface and consequent variation of ray tube areas during propagation. Two important factors, the amount of sweep and the length of edge with supersonic normal component of velocity, are investigated analytically by means of a family of “quadratically swept” edges; the length of the supersonic part may be varied from zero to infinity, to give different waveforms. A straight radial edge, which emerges as a special case, is further analyzed. These results are believed to be a helpful preliminary to applying sonic boom methods, such as those based on Whitham's weakly non-linear theory or the transonic equation.

Tone noise of three supersonic helical tip speed propellers in a wind tunnel

A high tip speed turboprop is being considered as a future energy conservative airplane. When the turboprop airplane is at cruise, the combination of the airplane forward speed and the propeller rotational speed results in supersonic helical velocities over the other portions of the propeller blades. These supersonic blade sections could generate noise that is a cabin environment problem. Three 0.622 m (24.5 in.) diam propellers were acoustically tested in the NASA Lewis 8-by 6-ft. wind tunnel. This wind tunnel does not have acoustic damping material on its walls and is therefore not an ideal location for taking noise data. However, it was felt that information obtained about the noise differences among the three propellers would be useable. The propellers were tested at tunnel through flow Mach numbers of 0.6, 0.7, 0.75, 0.8, and 0.85 with different rotational speed and blade setting angles. The three propellers incorporated different plan forms and different amounts of sweep and yielded different noise levels. The acoustically designed propeller had 45° of tip sweep and was significantly quieter than the straight bladed propeller. The intermediate 30° tip sweep propeller exhibited noise that was between the other two blades. Noise trends with varying helical tip Mach number and blade loading were also observed.