2,589 research outputs found
A Note on Frame Dragging
The measurement of spin effects in general relativity has recently taken
centre stage with the successfully launched Gravity Probe B experiment coming
toward an end, coupled with recently reported measurements using laser ranging.
Many accounts of these experiments have been in terms of frame-dragging. We
point out that this terminology has given rise to much confusion and that a
better description is in terms of spin-orbit and spin-spin effects. In
particular, we point out that the de Sitter precession (which has been mesured
to a high accuracy) is also a frame-dragging effect and provides an accurate
benchmark measurement of spin-orbit effects which GPB needs to emulate
Hot Populations in M87 Globular Clusters
We have obtained HST/STIS far- and near-UV photometry of globular clusters in
four fields in the gE galaxy M87. To a limit of m(FUV) = 25 we detect a total
of 66 globular clusters (GCs) in common with the deep HST optical-band study of
Kundu et al. (1999). Despite strong overlap in V- and I-band properties, the
M87 GCs have UV/optical properties that are distinct from clusters in the Milky
Way and in M31. M87 clusters, especially metal-poor ones, produce larger hot HB
populations than do Milky Way analogues. Cluster mass is probably not a factor
in these distinctions. The most metal-rich M87 GCs in our sample are near Z_sun
and overlap the local E galaxy sample in estimated Mg_2 line indices.
Nonetheless, the clusters produce much more UV light at a given Mg_2, being up
to 1 mag bluer than any gE galaxy in (FUV-V) color. The M87 GCs do not appear
to represent a transition between Milky Way-type clusters and E galaxies. The
differences are in the correct sense if the clusters are significantly older
than the E galaxies. Comparisons with Galactic open clusters indicate that the
hot stars lie on the extreme horizontal branch, rather than being blue
stragglers, and that the EHB becomes well populated for ages > 5 Gyr. We find
that 43 of our UV detections have no optical-band counterparts. Most appear to
be UV-bright background galaxies, seen through M87. Eleven NUV variable sources
detected at only one epoch in the central field are probably classical novae.
[Abridged]Comment: 70 pages, 25 figures (including 4 jpgs), 7 tables. To appear in AJ.
Full resolution version available at
http://www.astro.virginia.edu/~rwo/m87/m87-hotpops.pd
Formation of spatially periodic fronts of high-energy electrons in a radio-frequency driven surface microdischarge
The generation of spatially periodic fronts of high-energy electrons (>13.48 eV) has been investigated in a radio-frequency surface microdischarge in atmospheric-pressure argon. Optical emission spectroscopy is used to study the Ar I 2p1−1s2 transition surrounding a filamentary microdischarge, both spatially and with respect to the phase of the applied voltage. The formation of excitation fronts, which remain at a constant propagation distance throughout the RF cycle and for the duration of the pulse, may be explained by a localized increase in the electric field at the tip of surface-charge layers that are deposited during the extension phase
Nanosecond optical imaging spectroscopy of an electrothermal radiofrequency plasma thruster plume
Nanosecond optical imaging spectroscopy is employed to investigate the spatio-temporal dynamics of the plasma plume expanding from a 4.2 mm-diameter, 20 mm-long cylindrical capacitively coupled electrothermal radiofrequency (rf) driven thruster using 10 W of power at 12.50 MHz and an argon pressure of 1.5 Torr. On-axis, the plume exhibits four distinct peaks of optical emission intensity within the rf period. The plume has a spherical shape with a transient radial extension (during half of the rf cycle) at the thruster exit plane due to an rf current to ground when the grounded electrode acts as an anode
Reply to Comment on "Completely positive quantum dissipation"
This is the reply to a Comment by R. F. O'Connell (Phys. Rev. Lett. 87 (2001)
028901) on a paper written by the author (B. Vacchini, ``Completely positive
quantum dissipation'', Phys.Rev.Lett. 84 (2000) 1374, arXiv:quant-ph/0002094).Comment: 2 pages, revtex, no figure
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