165 research outputs found
Bauschinger effect in thin metallic films by fem simulations
Unpassivated free-standing gold and aluminum thin films (thickness ~ 200-400 nm, mean grain size dm,Auâ 70-80nm, dm,Alâ 120-200nm), subjected to tensile tests show Bauschinger effect (BE) during unloading [1, 2]. The focus of this work is to investigate the effect of microstructural heterogeneity such as grain sizes on the BE and the macroscopic deformation behavior in thin metallic films. The finite element code LAGAMINE is used to model the response of films involving sets of grains with different strengths. The numerical results are compared with experimental results from tensile tests on aluminum thin films from the work of Rajagopalan, et al. [2]
Adaptive time stepping approach for Phase-Field modeling of phase separation and precipitates coarsening in additive manufacturing alloys
In the present work, the capacity of phase field method to highlight microstructural changes during the spinodal decomposition of a given binary alloy basing on the Cahn-Hilliard equation is presented. Then, growth and coarsening of precipitates are studied using the KKS (Kim-Kim-Suzuki) model, which includes Cahn-Hilliard and Allen-Cahn equations. The implementation of time stepping algorithms to resolve Phase-Field equations is illustrated. Within Fourier space, using semi-implicit spectral method, it has been demonstrated that it allows faster computing than schemes based on finite difference method. First, spinodal decomposition of a given binary alloy under isothermal loading is implemented and three time stepping approaches are applied: constant time stepping, non- iterative and an iterative method. While the non-iterative method is faster than the constant time stepping scheme, the iterative one, although relatively more CPU consuming, can guarantee the convergence of the computing. These methods are combined in an innovative approach tested on 1D, 2D and 3D grids. The effectiveness of the adopted adaptive time-stepping algorithm allows resolving equations in reasonable CPU time. It predicts different physical phenomena, such as phase separation and growth and coarsening of precipitates induced by important interfacial energies
Confirmation of a recent bipolar ejection in the very young hierarchical multiple system IRAS 16293-2422
We present and analyze two new high-resolution (approx 0.3 arcsec),
high-sensitivity (approx 50 uJy beam-1) Very Large Array 3.6 cm observations of
IRAS 16293-2422 obtained in 2007 August and 2008 December. The components
A2alpha and A2beta recently detected in this system are still present, and have
moved roughly symmetrically away from source A2 at a projected velocity of
30-80 km s-1. This confirms that A2alpha and A2beta were formed as a
consequence of a very recent bipolar ejection from A2. Powerful bipolar
ejections have long been known to occur in low-mass young stars, but this is
-to our knowledge-- the first time that such a dramatic one is observed from
its very beginning. Under the reasonable assumption that the flux detected at
radio wavelengths is optically thin free-free emission, one can estimate the
mass of each ejecta to be of the order of 10^-8 Msun. If the ejecta were
created as a consequence of an episode of enhanced mass loss accompanied by an
increase in accretion onto the protostar, then the total luminosity of IRAS
16293-2422 ought to have increased by 10-60% over the course of at least
several months. Between A2alpha and A2beta, component A2 has reappeared, and
the relative position angle between A2 and A1 is found to have increased
significantly since 2003-2005. This strongly suggests that A1 is a protostar
rather than a shock feature, and that the A1/A2 pair is a tight binary system.
Including component B, IRAS 16293-2422 therefore appears to be a very young
hierarchical multiple system.Comment: Accepted for publication in The Astrophysical Journa
A Surprising Dynamical Mass for V773~Tau~B
We report on new high-resolution imaging and spectroscopy on the multiple T
Tauri star system V773 Tau over the 2003 -- 2009 period. With these data we
derive relative astrometry, photometry between the A and B components, and
radial velocity (RV) of the A-subsystem components. Combining these new data
with previously published astrometry and RVs, we update the relative A-B orbit
model. This updated orbit model, the known system distance, and A subsystem
parameters yields a dynamical mass for the B component for the first time.
Remarkably the derived B dynamical mass is in the range of 1.7 -- 3.0 M_\sun.
This is much higher than previous estimates, and suggests that like A, B is
also a multiple stellar system.
Among these data, spatially-resolved spectroscopy provide new insight into
the nature of the B component. Similar to A, these near-IR spectra indicate
that the dominant source in B is of mid-K spectral type. If B is in fact a
multiple star system as suggested by the dynamical mass estimate, the simplest
assumption is that B is composed of similar 1.2 M_\sun PMS stars in a
close ( 1 AU) binary system. This inference is supported by line-shape
changes in near-IR spectroscopy of B, tentatively interpreted as changing RV
among components in V773 Tau B.
Relative photometry indicate that B is highly variable in the near-IR. The
most likely explanation for this variability is circum-B material resulting in
variable line-of-sight extinction. The distribution of this material must be
significantly affected by both the putative B multiplicity, and the A-B orbit.Comment: 22 pages, 7 figures, accepted for publication ApJ. Companion paper to
R. Torres et al arXiv:1112.0114. Table 2 (RV Data Table) to appear in on-line
versio
A metaheuristic-based method for photovoltaic temperature computation under tropical conditions
Tropical climates have favorable irradiation levels for the development of photovoltaic systems; however, high temperatures have a negative impact on the efficiency of solar cells. Since direct measurement of cell temperature is not common, mathematical models are needed to make predictions. Numerous models have been documented, highlighting the challenge of applying a universal model to different climatic conditions. The main contribution of this study is the proposal of a metaheuristic algorithm to accurately compute the temperature of solar cells. This method is simple and effective in exploring numerous potential states of the reference parameters (i.e., irradiance and ambient temperature). Data collected over a 23-month period in two photovoltaic installations with an output power of 2.2 MW of multicrystalline silicon technology were used to develop the proposed method and validate it. The proposed model was compared with 19 previously reported models in the literature. Compared to the model recommended by the International Electrotechnical Commission (IEC Standard 61215-1), the mean square error, mean absolute error (MAE) and mean absolute percentage error were reduced by 4.9, 4.8, and 2.4 times, respectively. The accuracy of the proposed method is demonstrated by MAE errors ranging from 0.56 °C to 1.88 °C, obtained by considering three different daily profiles of irradiance and ambient temperature. Therefore, the proposed method is recommended to more accurately calculate the temperature of the photovoltaic cell in tropical areas
The Peculiar Debris Disk of HD 111520 as Resolved by the Gemini Planet Imager
Using the Gemini Planet Imager (GPI), we have resolved the circumstellar
debris disk around HD 111520 at a projected range of ~30-100 AU in both total
and polarized -band intensity. The disk is seen edge-on at a position angle
of ~165 along the spine of emission. A slight inclination or
asymmetric warping are covariant and alters the interpretation of the observed
disk emission. We employ 3 point spread function (PSF) subtraction methods to
reduce the stellar glare and instrumental artifacts to confirm that there is a
roughly 2:1 brightness asymmetry between the NW and SE extension. This specific
feature makes HD 111520 the most extreme examples of asymmetric debris disks
observed in scattered light among similar highly inclined systems, such as HD
15115 and HD 106906. We further identify a tentative localized brightness
enhancement and scale height enhancement associated with the disk at ~40 AU
away from the star on the SE extension. We also find that the fractional
polarization rises from 10 to 40% from 0.5" to 0.8" from the star. The
combination of large brightness asymmetry and symmetric polarization fraction
leads us to believe that an azimuthal dust density variation is causing the
observed asymmetry.Comment: 9 pages, 8 Figures, 1 table, Accepted to Ap
Automated data processing architecture for the Gemini Planet Imager Exoplanet Survey
The Gemini Planet Imager Exoplanet Survey (GPIES) is a multi-year direct
imaging survey of 600 stars to discover and characterize young Jovian
exoplanets and their environments. We have developed an automated data
architecture to process and index all data related to the survey uniformly. An
automated and flexible data processing framework, which we term the Data
Cruncher, combines multiple data reduction pipelines together to process all
spectroscopic, polarimetric, and calibration data taken with GPIES. With no
human intervention, fully reduced and calibrated data products are available
less than an hour after the data are taken to expedite follow-up on potential
objects of interest. The Data Cruncher can run on a supercomputer to reprocess
all GPIES data in a single day as improvements are made to our data reduction
pipelines. A backend MySQL database indexes all files, which are synced to the
cloud, and a front-end web server allows for easy browsing of all files
associated with GPIES. To help observers, quicklook displays show reduced data
as they are processed in real-time, and chatbots on Slack post observing
information as well as reduced data products. Together, the GPIES automated
data processing architecture reduces our workload, provides real-time data
reduction, optimizes our observing strategy, and maintains a homogeneously
reduced dataset to study planet occurrence and instrument performance.Comment: 21 pages, 3 figures, accepted in JATI
Improving and Assessing Planet Sensitivity of the GPI Exoplanet Survey with a Forward Model Matched Filter
We present a new matched filter algorithm for direct detection of point
sources in the immediate vicinity of bright stars. The stellar Point Spread
Function (PSF) is first subtracted using a Karhunen-Lo\'eve Image Processing
(KLIP) algorithm with Angular and Spectral Differential Imaging (ADI and SDI).
The KLIP-induced distortion of the astrophysical signal is included in the
matched filter template by computing a forward model of the PSF at every
position in the image. To optimize the performance of the algorithm, we conduct
extensive planet injection and recovery tests and tune the exoplanet spectra
template and KLIP reduction aggressiveness to maximize the Signal-to-Noise
Ratio (SNR) of the recovered planets. We show that only two spectral templates
are necessary to recover any young Jovian exoplanets with minimal SNR loss. We
also developed a complete pipeline for the automated detection of point source
candidates, the calculation of Receiver Operating Characteristics (ROC), false
positives based contrast curves, and completeness contours. We process in a
uniform manner more than 330 datasets from the Gemini Planet Imager Exoplanet
Survey (GPIES) and assess GPI typical sensitivity as a function of the star and
the hypothetical companion spectral type. This work allows for the first time a
comparison of different detection algorithms at a survey scale accounting for
both planet completeness and false positive rate. We show that the new forward
model matched filter allows the detection of fainter objects than a
conventional cross-correlation technique with a Gaussian PSF template for the
same false positive rate.Comment: ApJ accepte
Performance of the Gemini Planet Imager Non-Redundant Mask and spectroscopy of two close-separation binaries HR 2690 and HD 142527
The Gemini Planet Imager (GPI) contains a 10-hole non-redundant mask (NRM),
enabling interferometric resolution in complement to its coronagraphic
capabilities. The NRM operates both in spectroscopic (integral field
spectrograph, henceforth IFS) and polarimetric configurations. NRM observations
were taken between 2013 and 2016 to characterize its performance. Most
observations were taken in spectroscopic mode with the goal of obtaining
precise astrometry and spectroscopy of faint companions to bright stars. We
find a clear correlation between residual wavefront error measured by the AO
system and the contrast sensitivity by comparing phase errors in observations
of the same source, taken on different dates. We find a typical 5-
contrast sensitivity of at . We explore the
accuracy of spectral extraction of secondary components of binary systems by
recovering the signal from a simulated source injected into several datasets.
We outline data reduction procedures unique to GPI's IFS and describe a newly
public data pipeline used for the presented analyses. We demonstrate recovery
of astrometry and spectroscopy of two known companions to HR 2690 and HD
142527. NRM+polarimetry observations achieve differential visibility precision
of in the best case. We discuss its limitations on
Gemini-S/GPI for resolving inner regions of protoplanetary disks and prospects
for future upgrades. We summarize lessons learned in observing with NRM in
spectroscopic and polarimetric modes.Comment: Accepted to AJ, 22 pages, 14 figure
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