5,321 research outputs found
Small-Scale Structure of O VI Interstellar Gas in the Direction of the Globular Cluster NGC 6752
In order to study the small-scale structure of the hot interstellar gas, we
obtained Far Ultraviolet Spectroscopic Explorer interstellar O VI interstellar
absorption spectra of 4 four post-extreme horizontal branch stars in the
globular cluster NGC 6752 [(l,b) = (336.50,-25.63), d = 3.9 kpc, z= -1.7 kpc].
The good quality spectra of these stars allow us to measure both lines of the O
VI doublet at 1031.926 \AA and 1037.617 \AA. The close proximity of these stars
permits us to probe the hot interstellar gas over angular scale of only
2\farcm2 - 8\farcm9, corresponding to spatial scales \la 2.5-10.1 pc. On
these scales we detect no variations in the O VI column density and velocity
distribution. The average column density is log = 14.34 \pm 0.02 (log
= 13.98). The measured velocity dispersions of the O VI
absorption are also indistinguishable. Including the earlier results of Howk et
al., this study suggests that interstellar O VI is smooth on scales \Delta
\theta \la 12\arcmin, corresponding to a spatial scale of less than 10 pc, and
quite patchy at larger scales. Although such small scales are only probed in a
few directions, this suggests a characteristic size scale for the regions
producing collisionally-ionized O VI in the Galaxy.Comment: Accepted for publication in the PASP (to appear in the October 2004
issue
A Reservoir of Ionized Gas in the Galactic Halo to Sustain Star Formation in the Milky Way
Without a source of new gas, our Galaxy would exhaust its supply of gas
through the formation of stars. Ionized gas clouds observed at high velocity
may be a reservoir of such gas, but their distances are key for placing them in
the Galactic halo and unraveling their role. We have used the Hubble Space
Telescope to blindly search for ionized high-velocity clouds (iHVCs) in the
foreground of Galactic stars. We show that iHVCs with 90 < |v_LSR| < 170 km/s
are within one Galactic radius of the sun and have enough mass to maintain star
formation, while iHVCs with |v_LSR|>170 km/s are at larger distances. These may
be the next wave of infalling material.Comment: This paper is part of a set of three papers on circumgalactic gas
observed with COS and STIS on HST, to be published in Science, together with
related papers by Tripp et al. and Tumlinson et al., in the November 18, 2011
edition. This version has not undergone final copyediting. Please see Science
online for the final printed versio
Orbital Dynamics of Binary Boson Star Systems
We extend our previous studies of head-on collisions of boson stars by
considering orbiting binary boson stars. We concentrate on equal mass binaries
and study the dynamical behavior of boson/boson and boson/antiboson pairs. We
examine the gravitational wave output of these binaries and compare with other
compact binaries. Such a comparison lets us probe the apparent simplicity
observed in gravitational waves produced by black hole binary systems. In our
system of interest however, there is an additional internal freedom which plays
a significant role in the system's dynamics, namely the phase of each star. Our
evolutions show rather simple behavior at early times, but large differences
occur at late times for the various initial configurations.Comment: 10 pages, 14 figure
Quantum quenches of holographic plasmas
We employ holographic techniques to study quantum quenches at finite
temperature, where the quenches involve varying the coupling of the boundary
theory to a relevant operator with an arbitrary conformal dimension
2\leq\D\leq4. The evolution of the system is studied by evaluating the
expectation value of the quenched operator and the stress tensor throughout the
process. The time dependence of the new coupling is characterized by a fixed
timescale and the response of the observables depends on the ratio of the this
timescale to the initial temperature. The observables exhibit universal scaling
behaviours when the transitions are either fast or slow, i.e. when this ratio
is very small or very large. The scaling exponents are smooth functions of the
operator dimension. We find that in fast quenches, the relaxation time is set
by the thermal timescale regardless of the operator dimension or the precise
quenching rate.Comment: 60 pages, 10 figures, 3 appendice
Numerical Simulations of Hyperfine Transitions of Antihydrogen
One of the ASACUSA (Atomic Spectroscopy And Collisions Using Slow
Antiprotons) collaboration's goals is the measurement of the ground state
hyperfine transition frequency in antihydrogen, the antimatter counterpart of
one of the best known systems in physics. This high precision experiment yields
a sensitive test of the fundamental symmetry of CPT. Numerical simulations of
hyperfine transitions of antihydrogen atoms have been performed providing
information on the required antihydrogen events and the achievable precision
Deuterium toward the WD0621-376 sight line: Results from the Far Ultraviolet Spectroscopic Explorer (FUSE) Mission
Far Ultraviolet Spectroscopic Explorer observations are presented for
WD0621-376, a DA white dwarf star in the local interstellar medium (LISM) at a
distance of about 78 pc. The data have a signal-to-noise ratio of about 20-40
per 20 km/s resolution element and cover the wavelength range 905-1187 \AA.
LISM absorption is detected in the lines of D I, C II, C II*, C III, N I, N II,
N III, O I, Ar I, and Fe II. This sight line is partially ionized, with an
ionized nitrogen fraction of > 0.23. We determine the ratio (2). Assuming a standard interstellar
oxygen abundance, we derive . Using the
value of N(H I) derived from EUVE data gives a similar D/H ratio. The D I/N I
ratio is (2).Comment: accepted for publication in the ApJ
On characteristic initial data for a star orbiting a black hole
We take further steps in the development of the characteristic approach to
enable handling the physical problem of a compact self-gravitating object, such
as a neutron star, in close orbit around a black hole. We examine different
options for setting the initial data for this problem and, in order to shed
light on their physical relevance, we carry out short time evolution of this
data. To this end we express the matter part of the characteristic gravity code
so that the hydrodynamics are in conservation form. The resulting gravity plus
matter relativity code provides a starting point for more refined future
efforts at longer term evolution. In the present work we find that,
independently of the details of the initial gravitational data, the system
quickly flushes out spurious gravitational radiation and relaxes to a
quasi-equilibrium state with an approximate helical symmetry corresponding to
the circular orbit of the star.Comment: 20 pages, 10 figure
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