7,355 research outputs found
Expectations on the mass determination using astrometric microlensing by Gaia
Context. Astrometric gravitational microlensing can be used to determine the
mass of a single star (the lens) with an accuracy of a few percent. To do so,
precise measurements of the angular separations between lens and background
star with an accuracy below 1 milli-arcsecond at different epochs are needed.
Hence only the most accurate instruments can be used. However, since the
timescale is in the order of months to years, the astrometric deflection might
be detected by Gaia, even though each star is only observed on a low cadence.
Aims. We want to show how accurately Gaia can determine the mass of the lensing
star. Methods. Using conservative assumptions based on the results of the
second Gaia Data release, we simulated the individual Gaia measurements for 501
predicted astrometric microlensing events during the Gaia era (2014.5 -
2026.5). For this purpose we use the astrometric parameters of Gaia DR2, as
well as an approximative mass based on the absolute G magnitude. By fitting the
motion of lens and source simultaneously we then reconstruct the 11 parameters
of the lensing event. For lenses passing by multiple background sources, we
also fit the motion of all background sources and the lens simultaneously.
Using a Monte-Carlo simulation we determine the achievable precision of the
mass determination. Results. We find that Gaia can detect the astrometric
deflection for 114 events. Further, for 13 events Gaia can determine the mass
of the lens with a precision better than 15% and for 13 + 21 = 34 events with a
precision of 30% or better.Comment: 13 pages; 10 figures; 3 tables; accepted by A&A (April. 28th 2020)
The Python-based code for our simulation is made publicly available
https://github.com/jkluter/ML
(An)isotropy of the Hubble diagram: comparing hemispheres
We test the isotropy of the Hubble diagram. At small redshifts, this is
possible without assumptions on the cosmic inventory and provides a fundamental
test of the cosmological principle. At higher redshift we check for the
self-consistency of the LambdaCDM model. At small redshifts, we use public
supernovae (SNe) Ia data to determine the deceleration parameter q_0 and the SN
calibration on opposite hemispheres. For the complete data sets we fit Omega_M
and the SN calibration on opposite hemispheres. A statistically significant
anisotropy of the Hubble diagram at redshifts z 95%
C.L.). While data from the North Galactic hemisphere favour the accelerated
expansion of the Universe, data from the South Galactic hemisphere are not
conclusive. The hemispheric asymmetry is maximal toward a direction close to
the equatorial poles. The discrepancy between the equatorial North and South
hemispheres shows up in the SN calibration. For the LambdaCDM model fitted to
all available SNe, we find the same asymmetry. The alignment of discrepancies
between hemispheric Hubble diagrams with the equatorial frame seems to point
toward a systematic error in the SN search, observation, analysis or data
reduction. We also find that our model independent test cannot exclude the case
of the deceleration of the expansion at a statistically significant level.Comment: 13 pages, 8 figures; several corrections - conclusions unchanged;
matches published versio
Ongoing Astrometric Microlensing Events of Two Nearby Stars
Context. Astrometric microlensing is an excellent tool to determine the mass
of a stellar object. By measuring the astrometric shift of a background source
star in combination with precise predictions of its unlensed position and of
the lens position, gravitational lensing allows to determine the mass of the
lensing star with a precision of 1 percent, independent of any prior knowledge.
Aims. Making use of the recently published Gaia Data Release 2 (Gaia DR2) we
predict astrometric microlensing events by foreground stars of high proper
motion passing by a background star in the coming years.
Methods. We compile a list of ~148.000 high-proper-motion stars within Gaia
DR2 with > 150 mas/yr. We then search for background stars close to
their paths and calculate the dates and separations of the closest approaches.
Using color and absolute magnitude, we determine approximate masses of the
lenses. Finally, we calculate the expected astrometric shifts and
magnifications of the predicted events.
Results . We detect two ongoing microlensing events by the high proper motion
stars Luyten 143-23 and Ross 322 and predict closest separations of (108.5
1.4) mas in July 2018 and (125.3 3.4) mas in August 2018,
respectively. The respective expected astrometric shifts are (1.74 0.12)
mas and (0.76 0.06) mas. Furthermore, Luyten 143-23 will pass by another
star in March 2021 with a closest separation of (280.1 1.1) mas, which
results in an expected shift of (0.69 0.05) mas.Comment: Submitted to A&A, accepted June 14, 2018. 4 pages, 3 figures, 2
table
Enhanced quantized current driven by surface acoustic waves
We present the experimental realization of different approaches to increase
the amount of quantized current which is driven by surface acoustic waves
through split gate structures in a two dimensional electron gas. Samples with
driving frequencies of up to 4.7 GHz have been fabricated without a
deterioration of the precision of the current steps, and a parallelization of
two channels with correspondingly doubled current values have been achieved. We
discuss theoretical and technological limitations of these approaches for
metrological applications as well as for quantum logics.Comment: 3pages, 4eps-figure
The Star Cluster Population of M51
We present the age and mass distribution of star clusters in M51. The
structural parameters are found by fitting cluster evolution models to the
spectral energy distribution consisting of 8 HST-WFPC2 pass bands. There is
evidence for a burst of cluster formation at the moment of the second encounter
with the companion NGC5195 (50-100 Myr ago) and a hint for an earlier burst
(400-500 Myr ago). The cluster
IMF has a power law slope of -2.1. The disruption time of clusters is
extremely short (< 100 Myr for a 10^4 Msun cluster).Comment: 2 pages, to appear in "The Formation and Evolution of Massive Young
Star Clusters", 17-21 November 2003, Cancun (Mexico
Higgs-Yukawa model on the lattice
We present results from two projects on lattice calculations for the
Higgs-Yukawa model. First we report progress on the search of first-order
thermal phase transitions in the presence of a dimension-six operator, with the
choices of bare couplings that lead to viable phenomenological predictions. In
this project the simulations are performed using overlap fermions to implement
the required chiral symmetry. Secondly, our study for applying finite-size
scaling techniques near the Gaussian fixed point of the Higgs-Yukawa model is
presented. We discuss the analytical formulae for the Higgs Yukawa model and
show results for a first numerical study in the pure scalar sector of
the theory.Comment: 8 pages, 4 figures; Contribution to the proceedings of the 35th
International Symposium on Lattice Field Theory, 18 - 24 June 2017, Granada,
Spai
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