We present a measurement of the systemic proper motion of the Small Magellanic Cloud (SMC) made using the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST). We tracked the SMC's ...motion relative to four background QSOs over a baseline of approximately 2 yr. The measured proper motion is k sub(W) = -1.16 c 0.18 mas yr super(-1), k sub(N) = -1.17 c 0.18 mas yr super(-1). This is the best measurement yet of the SMC's proper motion. We combine this new result with our prior estimate of the proper motion of the Large Magellanic Cloud (LMC) from the same observing program to investigate the orbital evolution of both Clouds over the past 9 Gyr. The current relative velocity between the Clouds is 105 c 42 km s super(-1). Our investigations of the past orbital motions of the Clouds in a simple model for the dark halo of the Milky Way imply that the Clouds could be unbound from each other. However, our data are also consistent with orbits in which the Clouds have been bound to each other for approximately a Hubble time. Smaller proper-motion errors and better understanding of the LMC and SMC masses would be required to constrain their past orbital history and their bound versus unbound nature unambiguously. The new proper-motion measurements should be sufficient to allow the construction of improved models for the origin and properties of the Magellanic Stream. In turn, this will provide new constraints on the properties of the Milky Way dark halo.
A study conducted a microlensing experiment to determine whether the dark matter halo in the Milky Way is made up of massive compact halo objects (MACHOs). Evidence from one microlensing event is ...offered.
Strange matter, a form of quark matter that is postulated to be absolute stable, may be the true ground stage of the hadrons. If this hypothesis is correct, neutron stars may convert to 'strange ...stars'. The mass-radius relation for strange stars is very different from that of neutron stars; there is no minimum mass, and for mass of 1 solar mass or less, mass is proportional to the cube of the radius. For masses between 1 solar mass and 2 solar masses, the radii of strange stars are about 10 km, as for neutron stars. Strange stars may have an exposed quark surface, which is capable of radiating at rates greatly exceeding the Eddington limit, but has a low emissivity for X-ray photons. The stars may have a thin crust with the same composition as the preneutron drip outer layer of a conventional neutron star crust. Strange stars cool efficiently via neutrino emission.
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