6 research outputs found
Observations of Binary and Single Wolf-Rayet Stars with XMM-Newton and Chandra
We present an overview of recent X-ray observations of Wolf-Rayet (WR) stars
with XMM-Newton and Chandra. A new XMM spectrum of the nearby WN8 + OB binary
WR 147 shows hard absorbed X-ray emission, including the Fe K-alpha line
complex, characteristic of colliding wind shock sources. In contrast, sensitive
observations of four of the closest known single WC (carbon-rich) WR stars have
yielded only non-detections. These results tentatively suggest that single WC
stars are X-ray quiet. The presence of a companion may thus be an essential
factor in elevating the X-ray emission of WC + OB stars to detectable levels.Comment: To appear in conf. proceedings: Close Binaries in the 21st Century -
New Opportunities and Challenges, eds. A. Gimenez, E. Guinan, P. Niarchos, S.
Rucinski; Astrophys. and Space Sci. (special issue), 2006. 4 pages, 2 figure
The Physics of Cluster Mergers
Clusters of galaxies generally form by the gravitational merger of smaller
clusters and groups. Major cluster mergers are the most energetic events in the
Universe since the Big Bang. Some of the basic physical properties of mergers
will be discussed, with an emphasis on simple analytic arguments rather than
numerical simulations. Semi-analytic estimates of merger rates are reviewed,
and a simple treatment of the kinematics of binary mergers is given. Mergers
drive shocks into the intracluster medium, and these shocks heat the gas and
should also accelerate nonthermal relativistic particles. X-ray observations of
shocks can be used to determine the geometry and kinematics of the merger. Many
clusters contain cooling flow cores; the hydrodynamical interactions of these
cores with the hotter, less dense gas during mergers are discussed. As a result
of particle acceleration in shocks, clusters of galaxies should contain very
large populations of relativistic electrons and ions. Electrons with Lorentz
factors gamma~300 (energies E = gamma m_e c^2 ~ 150 MeV) are expected to be
particularly common. Observations and models for the radio, extreme
ultraviolet, hard X-ray, and gamma-ray emission from nonthermal particles
accelerated in these mergers are described.Comment: 38 pages with 9 embedded Postscript figures. To appear in Merging
Processes in Clusters of Galaxies, edited by L. Feretti, I. M. Gioia, and G.
Giovannini (Dordrecht: Kluwer), in press (2001
Origin of the Local Bubble
We present a new unbiased search for OB associations in the Solar neighbourhood
which have hosted the progenitor stars of the core collapse supernovae responsible for the
Local Bubble in the interstellar gas. For this purpose we have analyzed a volume complete
set (with a diameter of 400 pc) of B stars drawn from the Hipparcos catalogue and the Arivel
data base, from which candidate members were selected by a kinematical criterion. After
careful dereddening the star colours we have constructed a colour-magnitude diagram and
confirmed that the Upper Scorpius, Upper Centaurus Lupus, and Lower Centaurus Crux
subgroups of the Sco OB2 association are the youngest nearby OB associations. We dated
their ages with theoretical isochrones in the range of 20–30 Myr, in agreement with previous
work. We have traced backwards in time the paths of the stars and found that they entered
the volume of the present bubble at 10 to 15 Myr ago. We argue that the Local Bubble began
to form then and estimate that 14 to 20 supernovae have exploded since. The implied energy
input into the ambient medium can be shown to be sufficient to excavate a bubble of the
presently observed size