Photosensitive Semiconductor Tips in a Scanning Tunneling Microscope

Neutron Star Population Dynamics. I. Millisecond Pulsars

“…The key assumption is that the MSP population includes members with ages ranging uniformly up to the age of the Galaxy, so that the average effect of diffusion will be less than it is for the oldest stars. Using the velocity dispersion data of K and M giants with ages (0.3 − 9) × 10 9 (Wielen 1977), averaging uniformly in time, we infer that the root mean square dispersion is ∼ 50 km s −1 . We suggest that the residual dispersion of 67.5 km s −1 (i.e.…”

“…The model calculations presented above assume a regular background potential. Older stars are well known to have larger velocity dispersions, presumably from interaction with small-scale fluctuations in the gravitational field, but the actual physical source of the irregular field is not well understood (Wielen 1977). The oldest stars, K and M giants of age 9 × 10 9 yrs, reach total dispersions of 77 km s −1 ; for comparison, using interpolated values for the uniform model in Table 5 we estimate that the best fit model for the MSPs (σ V = 53 km s −1 ) implies a total dispersion of 84 km s −1 .…”

Stellar diffusion in barred spiral galaxies

“…First attempts to explain such a heating process in disk galaxies were made by empirically modeling the observed increase of the stellar velocity dispersion with age in the solar neighborhood. Wielen (1977) suggested a diffusion mechanism in velocity space, which gives rise to typical relaxation times for young disk stars of the order of the period of revolution and to a deviation of stellar positions of 1.5 kpc in 200 Myr. The result was obtained without making detailed assumptions on the underlying local acceleration process responsible for the diffusion of stellar orbits.…”

“…The result was obtained without making detailed assumptions on the underlying local acceleration process responsible for the diffusion of stellar orbits. Global acceleration processes, such as the gravitational field of stationary density waves or of central bars with constant pattern speed, were ruled out since their contribution to the velocity dispersion of old stars was found to be negligible and concentrated in particular resonance regions (Wielen 1977;Binney & Tremaine 2008, p. 693). In isolated galaxies, different local accelerating mechanisms have been investigated, such as the gravitational encounters between stars and giant molecular clouds (Spitzer & Schwarschild 1951, 1953Lacey 1984), secular heating produced by transient spiral arms (Barbanis & Woltjer 1967;Carlberg & Sellwood 1985;Fuchs 2001) or the combination of the two processes (Binney & Lacey 1988;Jenkins & Binney 1990).…”

“…The mechanisms driving radial diffusion and heating are still hotly debated, and in many cases the role of the bar is not taken into account. Assuming the whole scatter seen in Edvardsson et al (1993) data was real, Sellwood & Binney (2002) pointed out that the radial excursion predicted by Wielen (1977) was not sufficient to explain the weakness of the AMR in the solar neighborhood. In order to explain both the large scatter in the AMR and the evidence that even old disk stars today have nearly circular orbits, Sellwood & Binney (2002) suggested a new mechanism based on the resonant scattering of stars under the effect of transient spiral waves.…”