Modeling active electrolocation in weakly electric fish

In this paper, we provide a mathematical model for the electrolocation in weakly electric fishes. We first investigate the forward complex conductivity problem and derive the approximate boundary conditions on the skin of the fish. Then we provide a dipole approximation for small targets away from the fish. Based on this approximation, we obtain a non-iterative location search algorithm using multi-frequency measurements. We present numerical experiments to illustrate the performance and the stability of the proposed multi-frequency location search algorithm. Finally, in the case of disk- and ellipse-shaped targets, we provide a method to reconstruct separately the conductivity, the permittivity, and the size of the targets from multi-frequency measurements.Comment: 37 pages, 11 figure

Enhanced statistical stability in coherent interferometric imaging

http://iopscience.iop.org/0266-5611/International audienc

Stress response function of a two-dimensional ordered packing of frictional beads

We study the stress profile of an ordered two-dimensional packing of beads in response to the application of a vertical overload localized at its top surface. Disorder is introduced through the Coulombic friction between the grains which gives some indeterminacy and allows the choice of one constrained random number per grain in the calculation of the contact forces. The so-called `multi-agent' technique we use, lets us deal with systems as large as $1000\times1000$ grains. We show that the average response profile has a double peaked structure. At large depth $z$, the position of these peaks grows with $cz$, while their widths scales like $\sqrt{Dz}$. $c$ and $D$ are analogous to `propagation' and `diffusion' coefficients. Their values depend on that of the friction coefficient $\mu$. At small $\mu$, we get $c_0-c \propto \mu$ and $D \propto \mu^\beta$, with $\beta \sim 2.5$, which means that the peaks get closer and wider as the disorder gets larger. This behavior is qualitatively what was predicted in a model where a stochastic relation between the stress components is assumed.Comment: 7 pages, 7 figures, accepted version to Europhys. Let

Entropy maximization in the force network ensemble for granular solids

A long-standing issue in the area of granular media is the tail of the force distribution, in particular whether this is exponential, Gaussian, or even some other form. Here we resolve the issue for the case of the force network ensemble in two dimensions. We demonstrate that conservation of the total area of a reciprocal tiling, a direct consequence of local force balance, is crucial for predicting the local stress distribution. Maximizing entropy while conserving the tiling area and total pressure leads to a distribution of local pressures with a generically Gaussian tail that is in excellent agreement with numerics, both with and without friction and for two different contact networks.Comment: 4 pages, 3 figure

A massive cluster of Red Supergiants at the base of the Scutum-Crux arm

We report on the unprecedented Red Supergiant (RSG) population of a massive young cluster, located at the base of the Scutum-Crux Galactic arm. We identify candidate cluster RSGs based on {\it 2MASS} photometry and medium resolution spectroscopy. With follow-up high-resolution spectroscopy, we use CO-bandhead equivalent width and high-precision radial velocity measurements to identify a core grouping of 26 physically-associated RSGs -- the largest such cluster known to-date. Using the stars' velocity dispersion, and their inferred luminosities in conjuction with evolutionary models, we argue that the cluster has an initial mass of $\sim$40,000\msun, and is therefore among the most massive in the galaxy. Further, the cluster is only a few hundred parsecs away from the cluster of 14 RSGs recently reported by Figer et al (2006). These two RSG clusters represent 20% of all known RSGs in the Galaxy, and now offer the unique opportunity to study the pre-supernova evolution of massive stars, and the Blue- to Red-Supergiant ratio at uniform metallicity. We use GLIMPSE, MIPSGAL and MAGPIS survey data to identify several objects in the field of the larger cluster which seem to be indicative of recent region-wide starburst activity at the point where the Scutum-Crux arm intercepts the Galactic bulge. Future abundance studies of these clusters will therefore permit the study of the chemical evolution and metallicity gradient of the Galaxy in the region where the disk meets the bulge.Comment: 49 pages, 22 figures. Accepted for publication in ApJ. Version with hi-res figures can be found at http://www.cis.rit.edu/~bxdpci/RSGC2.pd

The first observed stellar occultations by the irregular satellite Phoebe (Saturn IX) and improved rotational period

peer reviewedWe report six stellar occultations by Phoebe (Saturn IX), an irregular satellite of Saturn, obtained between mid-2017 and mid-2019. The 2017 July 6 event was the first stellar occultation by an irregular satellite ever observed. The occultation chords were compared to a 3D shape model of the satellite obtained from Cassini observations. The rotation period available in the literature led to a sub-observer point at the moment of the observed occultations where the chords could not fit the 3D model. A procedure was developed to identify the correct sub-observer longitude. It allowed us to obtain the rotation period with improved precision compared to the currently known value from literature. We show that the difference between the observed and the predicted sub-observer longitude suggests two possible solutions for the rotation period. By comparing these values with recently observed rotational light curves and single- chord stellar occultations, we can identify the best solution for Phoebe's rotational period as 9.27365 ± 0.00002 h. From the stellar occultations, we also obtained six geocentric astrometric positions in the ICRS as realized by the Gaia DR2 with uncertainties at the 1-mas level

Yellow Supergiants in the Small Magellanic Cloud (SMC): Putting Current Evolutionary Theory to the Test

The yellow supergiant content of nearby galaxies provides a critical test of massive star evolutionary theory. While these stars are the brightest in a galaxy, they are difficult to identify because a large number of foreground Milky Way stars have similar colors and magnitudes. We previously conducted a census of yellow supergiants within M31 and found that the evolutionary tracks predict a yellow supergiant duration an order of magnitude longer than we observed. Here we turn our attention to the SMC, where the metallicity is 10x lower than that of M31, which is important as metallicity strongly affects massive star evolution. The SMC's large radial velocity (~160 km/s) allows us to separate members from foreground stars. Observations of ~500 candidates yielded 176 near-certain SMC supergiants, 16 possible SMC supergiants, along with 306 foreground stars and provide good relative numbers of yellow supergiants down to 12Mo. Of the 176 near-certain SMC supergiants, the kinematics predicted by the Besancon model of the Milky Way suggest a foreground contamination of >4%. After placing the SMC supergiants on the H-R diagram and comparing our results to the Geneva evolutionary tracks, we find results similar to those of the M31 study: while the locations of the stars on the H-R diagram match the locations of evolutionary tracks well, the models over-predict the yellow supergiant lifetime by a factor of ten. Uncertainties about the mass-loss rates on the main-sequence thus cannot be the primary problem with the models.Comment: Accepted by the Ap

Arcsecond-resolution 12CO mapping of the yellow hypergiants IRC +10420 and AFGL 2343

IRC +10420 and AFGL 2343 are the unique, known yellow hypergiants (YHGs) presenting a heavy circumstellar envelope (CSE). We aim to study the morphology, exceptional kinematics, and excitation conditions of their CSEs, and the implications for mass-loss processes. We have mapped the 12CO J=2-1 and 1-0 emission in these YHGs with the IRAM Plateau de Bure interferometer and the 30m telescope. We developed LVG models in order to analyze their circumstellar characteristics. The maps show that the overall shape of both CSEs is approximately spherical, although they also reveal several aspherical features. The CSE around IRC +10420 shows a rounded extended halo surrounding a bright inner region, with both components presenting aspherical characteristics. It presents a brightness minimum at the center. The envelope around AFGL 2343 is a detached shell, showing spherical symmetry and clumpiness at a level of about 15% of the maximum brightness. The envelopes expand isotropically at about 35 km/s, about two or three times faster than typical CSEs around AGB stars. High temperatures (~ 200 K) are derived for the innermost regions in IRC +10420, while denser and cooler (~ 30 K) gas is found in AFGL 2343. The mass-loss processes in these YHGs have been found to be similar. The deduced mass-loss rates (~ 10E-4 - 10E-3 Msun/yr) are much higher than those obtained in AGB stars, and they present significant variations on time scales of ~ 1000 yr

The Yellow and Red Supergiants of M33

Yellow and red supergiants are evolved massive stars whose numbers and locations on the HR diagram can provide a stringent test for models of massive star evolution. Previous studies have found large discrepancies between the relative number of yellow supergiants observed as a function of mass and those predicted by evolutionary models, while a disagreement between the predicted and observed locations of red supergiants on the HR diagram was only recently resolved. Here we extend these studies by examining the yellow and red supergiant populations of M33. Unfortunately, identifying these stars is difficult as this portion of the color-magnitude diagram is heavily contaminated by foreground dwarfs. We identify the red supergiants through a combination of radial velocities and a two-color surface gravity discriminant and, after re-characterizing the rotation curve of M33 with our newly selected red supergiants, we identify the yellow supergiants through a combination of radial velocities and the strength of the OI $\lambda$7774 triplet. We examine ~1300 spectra in total and identify 121 yellow supergiants (a sample which is unbiased in luminosity above log(L/L\odot) ~ 4.8) and 189 red supergiants. After placing these objects on the HR diagram, we find that the latest generation of Geneva evolutionary tracks show excellent agreement with the observed locations of our red and yellow supergiants, the observed relative number of yellow supergiants with mass and the observed red supergiant upper mass limit. These models therefore represent a drastic improvement over previous generations.Comment: Accepted for publication in the Astrophysical Journa