59,681 research outputs found
Tidal scattering of stars on supermassive black holes in galactic centers
Some of the mass that feeds the growth of a massive black hole (BH) in a
galactic center is supplied by tidal disruption of stars that approach it on
unbound, low angular momentum orbits. For each star that is disrupted, others
narrowly escape after being subjected to extreme tidal distortion, spin-up,
mixing and mass-loss, which may affect their evolution and appearance. We show
that it is likely that a significant fraction of the stars around massive BHs
in galactic centers have undergone such extreme tidal interactions and survived
subsequent total disruption, either by being deflected off their orbit or by
missing the BH due to its Brownian motion. We discuss possible long-term
observable consequences of this process, which may be relevant for
understanding the nature of stars in galactic centers, and may provide a
signature of the existence of massive BHs there.Comment: 5 pages 4 figures. ApJL in press, minor changes to reflect journal
version including redifinition of unbound tidally disturbed stars and
additional reference
How engaging start-ups in research activities can lead to more effective technology and knowledge transfer from public research organisations
Leveraging on public means for start-ups: a case study on the catalytic role of iMinds' entrepreneurship program
Nonperturbative m_X cut effects in B -> Xs l+ l- observables
Recently, it was shown that in inclusive B -> Xs l+ l- decay, an angular
decomposition provides three independent (q^2 dependent) observables. A
strategy was formulated to extract all measurable Wilson coefficients in B ->
Xs l+ l- from a few simple integrals of these observables in the low q^2
region. The experimental measurements in the low q^2 region require a cut on
the hadronic invariant mass, which introduces a dependence on nonperturbative b
quark distribution functions. The associated hadronic uncertainties could
potentially limit the sensitivity of these decays to new physics. We compute
the nonperturbative corrections to all three observables at leading and
subleading order in the power expansion in \Lambda_QCD/m_b. We find that the
subleading power corrections give sizeable corrections, of order -5% to -10%
depending on the observable and the precise value of the hadronic mass cut.
They cause a shift of order -0.05 GeV^2 to -0.1 GeV^2 in the zero of the
forward-backward asymmetry.Comment: 11 pages, 4 figures, v2: corrected typos and Eq. (25), v3: journal
versio
Color Intensity Projections: A simple way to display changes in astronomical images
To detect changes in repeated astronomical images of the same field of view
(FOV), a common practice is to stroboscopically switch between the images.
Using this method, objects that are changing in location or intensity between
images are easier to see because they are constantly changing. A novel display
method, called arrival time color intensity projections (CIPs), is presented
that combines any number of grayscale images into a single color image on a
pixel by pixel basis. Any values that are unchanged over the grayscale images
look the same in the color image. However, pixels that change over the
grayscale image have a color saturation that increases with the amount of
change and a hue that corresponds to the timing of the changes. Thus objects
moving in the grayscale images change from red to green to blue as they move
across the color image. Consequently, moving objects are easier to detect and
assess on the color image than on the grayscale images. A sequence of images of
a comet plunging into the sun taken by the SOHO satellite (NASA/ESA) and Hubble
Space Telescope images of a trans-Neptunian object (TNO) are used to
demonstrate the method.Comment: 9 pages, 2 figures. Accepted for publication in Publications of the
Astronomical Society of the Pacific. The quality of figure 1 been improved
from the previous posted versio
The orbital statistics of stellar inspiral and relaxation near a massive black hole: characterizing gravitational wave sources
We study the orbital parameters distribution of stars that are scattered into
nearly radial orbits and then spiral into a massive black hole (MBH) due to
dissipation, in particular by emission of gravitational waves (GW). This is
important for GW detection, e.g. by the Laser Interferometer Space Antenna
(LISA). Signal identification requires knowledge of the waveforms, which depend
on the orbital parameters. We use analytical and Monte Carlo methods to analyze
the interplay between GW dissipation and scattering in the presence of a mass
sink during the transition from the initial scattering-dominated phase to the
final dissipation-dominated phase of the inspiral. Our main results are (1)
Stars typically enter the GW-emitting phase with high eccentricities. (2) The
GW event rate per galaxy is a few per Gyr for typical central stellar cusps,
almost independently of the relaxation time or the MBH mass. (3) For
intermediate mass black holes (IBHs) of ~a thousand solar masses such as may
exist in dense stellar clusters, the orbits are very eccentric and the inspiral
is rapid, so the sources are very short-lived.Comment: ApJ Accepte
Shape Function Effects in B -> X_c l \nu_l
Owing to the fact that m_c^2 ~ m_b \Lambda_QCD, the endpoint region of the
charged lepton energy spectrum in the inclusive decay B -> X_c l \nu_l is
affected by the Fermi motion of the initial-state b quark bound in the B meson.
This effect is described in QCD by shape functions. Including the mass of the
final-state quark, we find that a different set of operators as employed in
Ref. hep-ph/0205150 is needed for a consistent matching, when incorporating the
subleading contributions in B -> X_q l \nu_l for both q = u and q = c. In
addition, we modify the usual twist expansion in such a way that it yields a
description of the lepton energy spectrum which is not just valid in the
endpoint region, but over the entire phase space.Comment: 8 Pages, LaTeX, 2 figures; a few typos corrected and some
clarifications added, final journal versio
Resummation and Matching of -quark Mass Effects in Production
We use a systematic effective field theory setup to derive the
production cross section. Our result combines the merits of both fixed 4-flavor
and 5-flavor schemes. It contains the full 4-flavor result, including the exact
dependence on the -quark mass, and improves it with a resummation of
collinear logarithms of . In the massless limit, it corresponds to a
reorganized 5-flavor result. While we focus on production, our
method applies to generic heavy-quark initiated processes at hadron colliders.
Our setup resembles the variable flavor number schemes known from heavy-flavor
production in deep-inelastic scattering, but also differs in some key aspects.
Most importantly, the effective -quark PDF appears as part of the
perturbative expansion of the final result where it effectively counts as an
object. The transition between the fixed-order (4-flavor) and
resummation (5-flavor) regimes is governed by the low matching scale at which
the -quark is integrated out. Varying this scale provides a systematic way
to assess the perturbative uncertainties associated with the resummation and
matching procedure and reduces by going to higher orders. We discuss the
practical implementation and present numerical results for the
production cross section at NLO+NLL. We also provide a comparison to the
corresponding predictions in the fixed 4-flavor and 5-flavor results and the
Santander matching prescription. Compared to the latter, we find a slightly
reduced uncertainty and a larger central value, with its central value lying at
the lower edge of our uncertainty band.Comment: 54 pages, 16 figures. Final version to be published in JHEP (one ref
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