8,333 research outputs found
Theory and practice of microlensing lightcurves around fold singularities
Among all galactic microlensing events, those involving a passage of the
observed source star over the caustic created by a binary lens are particularly
useful in providing information about stellar atmospheres, the dynamics of
stellar populations in our own and neighbouring galaxies, and the statistical
properties of stellar and sub-stellar binaries. This paper presents a
comprehensive guide for modelling and interpreting the lightcurves obtained in
events involving fold-caustic crossings. A new general, consistent, and optimal
choice of parameters provides a deep understanding of the involved features,
avoids numerical difficulties and minimizes correlations between model
parameters. While the photometric data of a microlensing event around a caustic
crossing itself do not provide constraints on the characteristics of the
underlying binary lens and does not allow predictions of the behaviour of other
regions of the lightcurve, vital constraints can be obtained in an efficient
way if these are combined with a few simple characteristics of data outside the
caustic crossings. A corresponding algorithm containing some improvements over
an earlier approach which takes into account multi-site observations is
presented and discussed in detail together with the arising parameter
constraints paying special attention to the role of source and background
fluxes.Comment: 19 pages with 7 EPS figures embedded, LaTeX2e using mn2e.cls. Final
version, tables clarifying meaning and constraints on parameters added. This
is a preprint of an Article accepted for publication in Monthly Notices of
the Royal Astronomical Society, (C) 2004 The Royal Astronomical Societ
Microlensing of circumstellar envelopes III. Line profiles from stellar winds in homologous expansion
This paper examines line profile evolution due to the linear expansion of circumstellar material obsverved during a microlensing event. This work extends our previous papers on emission line profile evolution from radial and azimuthal flow during point mass lens events and fold caustic crossings. Both "flavours" of microlensing were shown to provide effective diagnostics of bulk motion in circumstellar envelopes. In this work a different genre of flow is studied, namely linear homologous expansion, for both point mass lenses and fold caustic crossings. Linear expansion is of particular relevance to the effects of microlensing on supernovae at cosmological distances. We derive line profiles and equivalent widths for the illustrative cases of pure resonance and pure recombination lines, modelled under the Sobolev approximation. The efficacy of microlensing as a diagnostic probe of the stellar environs is demonstrated and discussed
Giorgio Bellettini: The ISR years (and before)
In honor of Giorgio Bellettini’s 80th birthday, I present stories of his early career, through the Intersecting Storage Ring (ISR) years
Can microlensing fold caustics reveal a second stellar limb-darkening coefficient?
Dense high-precision photometry of microlensed stars during a fold-caustic
passage can be used to reveal their intensity profiles from which the
temperature of the stellar atmosphere as function of fractional radius can be
derived. While the capabilities of current microlensing follow-up campaigns
such as PLANET allow a precise measurement of linear limb-darkening
coefficients, the residual signal of a second coefficient characterizing
square-root limb darkening is ~ 25 times smaller which prevents a proper
determination except for unlikely cases of very high caustic-peak-to-outside
relative magnifications with no adequate event being observed so far or for
source stars passing over a cusp singularity. Although the presence of limb
darkening can be well established from the data, a reliable measurement of the
index of an underlying power-law cannot be obtained.Comment: 4 pages with 4 EPS figures embedded, LaTeX2e using mn2e.cls. Final
version, minor changes. This is a preprint of an Article accepted for
publication in Monthly Notices of the Royal Astronomical Society, (C) 2004
The Royal Astronomical Societ
Quartz Cherenkov Counters for Fast Timing: QUARTIC
We have developed particle detectors based on fused silica (quartz) Cherenkov
radiators read out with micro-channel plate photomultipliers (MCP-PMTs) or
silicon photomultipliers (SiPMs) for high precision timing (Sigma(t) about
10-15 ps). One application is to measure the times of small angle protons from
exclusive reactions, e.g. p + p - p + H + p, at the Large Hadron Collider, LHC.
They may also be used to measure directional particle fluxes close to external
or stored beams. The detectors have small areas (square cm), but need to be
active very close (a few mm) to the intense LHC beam, and so must be radiation
hard and nearly edgeless. We present results of tests of detectors with quartz
bars inclined at the Cherenkov angle, and with bars in the form of an "L" (with
a 90 degree corner). We also describe a possible design for a fast timing
hodoscope with elements of a few square mm.Comment: 24 pages, 14 figure
Central exclusive production in CDF
In the Collider Detector at Fermilab, CDF, we have made the first observations of several “exclusive” processes in p¯p collisions, defined as p + ¯p → p(∗) ⊕ X ⊕ ¯p(∗), where the beam (anti)protons are diffractively scattered, with or
without dissociation, the “⊕” denotes a large rapidity gapΔη > 4.5 with no hadrons, and “X” is a simple state fully measured. The main part of the talk focuses on recent
X = π+π− data, through the double-pomeron exchange mechanism
1318 New Variable Stars in a 0.25 Square Degree Region of the Galactic Plane
We have conducted a deep photometric survey of a 0.5 deg x 0.5 deg area of
the Galactic Plane using the WFI instrument on the 2.2-m ESO telescope on La
Silla, Chile. The dataset comprises a total of 267 R-band images, 204 from a 16
day observation run in 2005, supplemented by 63 images from a six week period
in 2002. Our reduction employed the new numerical kernel difference image
analysis method as implemented in the PYSIS3 code and resulted in more than
500,000 lightcurves of stars down to a magnitude limit of R ~ 24.5. A search
for variable stars resulted in the detection of 1318 variables of different
types. 1011 of these are eclipsing or contact binary stars. A number of the
contact binaries have low mass-ratios and several of the detached binaries
appear to have low-mass components. Three candidate contact binaries have
periods at the known cut off including two with periods lower than any
previously published. Also identified are 3 possible pre-main sequence detached
eclipsing binaries.Comment: 54 pages, 17 figures, 11 tables, accepted by A&A. Photometry will be
available through CD
Associated central exclusive production of charged Higgs bosons
We propose central exclusive production of a charged Higgs boson in
association with a W boson as a possible signature of certain types of extended
Higgs sectors. We calculate the cross section and find that the rate at the LHC
could be large enough to allow observation in some models with two Higgs
doublets, where the charged Higgs and at least one of the neutral scalars can
be light enough. We use the two-Higgs doublet model as a prototype and consider
two distinct regions of parameter space, but we also briefly discuss the
prospects for the next-to-minimal supersymmetric standard model, where the
charged Higgs may very well be quite light.Comment: 9 pages, 5 figures. Minor changes, references added. Version to
appear in PR
Gravitational microlensing as a test of stellar model atmospheres
We present calculations illustrating the potential of gravitational
microlensing to discriminate between classical models of stellar surface
brightness profiles and the recently computed ``Next Generation'' models of
Hauschildt et al. These spherically-symmetric models include a much improved
treatment of molecular lines in the outer atmospheres of cool giants -- stars
which are very typical sources in Galactic bulge microlensing events. We show
that the microlensing signatures of intensively monitored point and fold
caustic crossing events are readily able to distinguish between NextGen and the
classical models, provided a photometric accuracy of 0.01 magnitudes is
reached. This accuracy is now routinely achieved by alert networks, and hence
current observations can discriminate between such model atmospheres, providing
a unique insight on stellar photospheres.Comment: 4 pages, 4 figures, Astronomy & Astrophysics (Letters), vol. 388, L1
(2002
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