3,891 research outputs found
Critical bubbles and implications for critical black strings
We demonstrate the existence of gravitational critical phenomena in higher
dimensional electrovac bubble spacetimes. To this end, we study linear
fluctuations about families of static, homogeneous spherically symmetric bubble
spacetimes in Kaluza-Klein theories coupled to a Maxwell field. We prove that
these solutions are linearly unstable and posses a unique unstable mode with a
growth rate that is universal in the sense that it is independent of the family
considered. Furthermore, by a double analytical continuation this mode can be
seen to correspond to marginally stable stationary modes of perturbed black
strings whose periods are integer multiples of the Gregory-Laflamme critical
length. This allow us to rederive recent results about the behavior of the
critical mass for large dimensions and to generalize them to the charged black
string case.Comment: A reference to unpublished work for the case q=2, by J. Hovdebo adde
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
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
Hamiltonian Relaxation
Due to the complexity of the required numerical codes, many of the new
formulations for the evolution of the gravitational fields in numerical
relativity are not tested on binary evolutions. We introduce in this paper a
new testing ground for numerical methods based on the simulation of binary
neutron stars. This numerical setup is used to develop a new technique, the
Hamiltonian relaxation (HR), that is benchmarked against the currently most
stable simulations based on the BSSN method. We show that, while the length of
the HR run is somewhat shorter than the equivalent BSSN simulation, the HR
technique improves the overall quality of the simulation, not only regarding
the satisfaction of the Hamiltonian constraint, but also the behavior of the
total angular momentum of the binary. The latest quantity agrees well with
post-Newtonian estimations for point-mass binaries in circular orbits.Comment: More detailed description of the numerical implementation added and
some typos corrected. Version accepted for publication in Class. and Quantum
Gravit
Low Redshift Intergalactic Absorption Lines in the Spectrum of HE0226-4110
We present an analysis of the FUSE and STIS E140M spectra of HE0226-4110
(z=0.495). We detect 56 Lyman absorbers and 5 O VI absorbers. The number of
intervening O VI systems per unit redshift with W>50 m\AA is dN(O VI)/dz~ 11.
The O VI systems unambiguously trace hot gas only in one case. For the 4 other
O VI systems, photoionization and collisional ionization models are viable
options to explain the observed column densities of the O VI and the other
ions. If the O VI systems are mostly photoionized, only a fraction of the
observed O VI will contribute to the baryonic density of the warm-hot ionized
medium (WHIM) along this line of sight. Combining our results with previous
ones, we show that there is a general increase of N(O VI) with increasing b(O
VI). Cooling flow models can reproduce the N-b distribution but fail to
reproduce the observed ionic ratios. A comparison of the number of O I, O II, O
III, O IV, and O VI systems per unit redshift show that the low-z IGM is more
highly ionized than weakly ionized. We confirm that photoionized O VI systems
show a decreasing ionization parameter with increasing H I column density. O VI
absorbers with collisional ionization/photoionization degeneracy follow this
relation, possibly suggesting that they are principally photoionized. We find
that the photoionized O VI systems in the low redshift IGM have a median
abundance of 0.3 solar. We do not find additional Ne VIII systems other than
the one found by Savage et al., although our sensitivity should have allowed
the detection of Ne VIII in O VI systems at T~(0.6-1.3)x10^6 K (if CIE
applies). Since the bulk of the WHIM is believed to be at temperatures T>10^6
K, the hot part of the WHIM remains to be discovered with FUV--EUV metal-line
transitions.Comment: Accepted for publication in the ApJS. Full resolution figures
available at
http://www.journals.uchicago.edu/ApJ/journal/preprints/ApJS63975.preprint.pd
The DRIFT Project: Searching for WIMPS with a Directional Detector
A low pressure time projection chamber for the detection of WIMPs is
discussed. Discrimination against Compton electron background in such a device
should be very good, and directional information about the recoil atoms would
be obtainable. If a full 3-D reconstruction of the recoil tracks can be
achieved, Monte Carlo studies indicate that a WIMP signal could be identified
with high confidence from as few as 30 detected WIMP-nucleus scattering events.Comment: 5 pages, 3 figures. Presented at Dark 98, Heidelberg, July 1998, and
to appear in conference proceeding
Evolutions of Magnetized and Rotating Neutron Stars
We study the evolution of magnetized and rigidly rotating neutron stars
within a fully general relativistic implementation of ideal
magnetohydrodynamics with no assumed symmetries in three spatial dimensions.
The stars are modeled as rotating, magnetized polytropic stars and we examine
diverse scenarios to study their dynamics and stability properties. In
particular we concentrate on the stability of the stars and possible critical
behavior. In addition to their intrinsic physical significance, we use these
evolutions as further tests of our implementation which incorporates new
developments to handle magnetized systems.Comment: 12 pages, 8 figure
Relativistic MHD and black hole excision: Formulation and initial tests
A new algorithm for solving the general relativistic MHD equations is
described in this paper. We design our scheme to incorporate black hole
excision with smooth boundaries, and to simplify solving the combined Einstein
and MHD equations with AMR. The fluid equations are solved using a finite
difference Convex ENO method. Excision is implemented using overlapping grids.
Elliptic and hyperbolic divergence cleaning techniques allow for maximum
flexibility in choosing coordinate systems, and we compare both methods for a
standard problem. Numerical results of standard test problems are presented in
two-dimensional flat space using excision, overlapping grids, and elliptic and
hyperbolic divergence cleaning.Comment: 22 pages, 8 figure
Two-divisibility of the coefficients of certain weakly holomorphic modular forms
We study a canonical basis for spaces of weakly holomorphic modular forms of
weights 12, 16, 18, 20, 22, and 26 on the full modular group. We prove a
relation between the Fourier coefficients of modular forms in this canonical
basis and a generalized Ramanujan tau-function, and use this to prove that
these Fourier coefficients are often highly divisible by 2.Comment: Corrected typos. To appear in the Ramanujan Journa
Metallicity and Physical Conditions in the Magellanic Bridge
We present a new analysis of the diffuse gas in the Magellanic Bridge (RA>3h)
based on HST/STIS E140M and FUSE spectra of 2 early-type stars lying within the
Bridge and a QSO behind it. We derive the column densities of HI (from
Ly\alpha), NI, OI, ArI, SiII, SII, and FeII of the gas in the Bridge. Using the
atomic species, we determine the first gas-phase metallicity of the Magellanic
Bridge, [Z/H]=-1.02+/-0.07 toward one sightline, and -1.7<[Z/H]<-0.9 toward the
other one, a factor 2 or more smaller than the present-day SMC metallicity.
Using the metallicity and N(HI), we show that the Bridge gas along our three
lines of sight is ~70-90% ionized, despite high HI columns, logN(HI)=19.6-20.1.
Possible sources for the ongoing ionization are certainly the hot stars within
the Bridge, hot gas (revealed by OVI absorption), and leaking photons from the
SMC and LMC. From the analysis of CII*, we deduce that the overall density of
the Bridge must be low (<0.03-0.1 cm^-3). We argue that our findings combined
with other recent observational results should motivate new models of the
evolution of the SMC-LMC-Galaxy system.Comment: Accepted for publication in the Ap
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