Phase transition in a log-normal Markov functional model

We derive the exact solution of a one-dimensional Markov functional model with log-normally distributed interest rates in discrete time. The model is shown to have two distinct limiting states, corresponding to small and asymptotically large volatilities, respectively. These volatility regimes are separated by a phase transition at some critical value of the volatility. We investigate the conditions under which this phase transition occurs, and show that it is related to the position of the zeros of an appropriately defined generating function in the complex plane, in analogy with the Lee-Yang theory of the phase transitions in condensed matter physics.Comment: 9 pages, 5 figures. v2: Added asymptotic expressions for the convexity-adjusted Libors in the small and large volatility limits. v3: Added one reference. Final version to appear in Journal of Mathematical Physic

Cyclic voltammetry modeling of proton transport effects on redox charge storage in conductive materials: application to a TiO2 mesoporous film

International audienc

A Semi-Empirical Model of the Infra-Red Universe

We present a simple model of the infra-red universe, based as much as possible on local observations. We model the luminosity and number evolution of disk and starburst galaxies, including the effects of dust, gas and spectral evolution. Although simple, our approach is able to reproduce observations of galaxy number counts and the infra-red and sub-millimeter extra-galactic backgrounds. It provides a useful probe of galaxy formation and evolution out to high redshift. The model demonstrates the significant role of the starburst population and predicts high star formation rates at redshifts 3 to 4, consistent with recent extinction-corrected observations of Lyman break galaxies. Starbursting galaxies are predicted to dominate the current SCUBA surveys. Their star formation is driven predominantly by strong tidal interactions and mergers of galaxies. This leads to the creation of spheroidal stellar systems, which may act as the seeds for disk formation as gas infalls. We predict the present-day baryonic mass in bulges and halos is comparable to that in disks. From observations of the extra-galactic background, the model predicts that the vast majority of star formation in the Universe occurs at z<5.Comment: 23 pages including 9 figures. To appear in ApJ. Model results available electronically at http://astro.berkeley.edu/~jt/irmodel.htm

A viscoelastic deadly fluid in carnivorous pitcher plants

Background : The carnivorous plants of the genus Nepenthes, widely distributed in the Asian tropics, rely mostly on nutrients derived from arthropods trapped in their pitcher-shaped leaves and digested by their enzymatic fluid. The genus exhibits a great diversity of prey and pitcher forms and its mechanism of trapping has long intrigued scientists. The slippery inner surfaces of the pitchers, which can be waxy or highly wettable, have so far been considered as the key trapping devices. However, the occurrence of species lacking such epidermal specializations but still effective at trapping insects suggests the possible implication of other mechanisms. Methodology/Principal Findings : Using a combination of insect bioassays, high-speed video and rheological measurements, we show that the digestive fluid of Nepenthes rafflesiana is highly viscoelastic and that this physical property is crucial for the retention of insects in its traps. Trapping efficiency is shown to remain strong even when the fluid is highly diluted by water, as long as the elastic relaxation time of the fluid is higher than the typical time scale of insect movements. Conclusions/Significance : This finding challenges the common classification of Nepenthes pitchers as simple passive traps and is of great adaptive significance for these tropical plants, which are often submitted to high rainfalls and variations in fluid concentration. The viscoelastic trap constitutes a cryptic but potentially widespread adaptation of Nepenthes species and could be a homologous trait shared through common ancestry with the sundew (Drosera) flypaper plants. Such large production of a highly viscoelastic biopolymer fluid in permanent pools is nevertheless unique in the plant kingdom and suggests novel applications for pest control

Leptin and Insulin Act on POMC Neurons to Promote the Browning of White Fat

SummaryThe primary task of white adipose tissue (WAT) is the storage of lipids. However, “beige” adipocytes also exist in WAT. Beige adipocytes burn fat and dissipate the energy as heat, but their abundance is diminished in obesity. Stimulating beige adipocyte development, or WAT browning, increases energy expenditure and holds potential for combating metabolic disease and obesity. Here, we report that insulin and leptin act together on hypothalamic neurons to promote WAT browning and weight loss. Deletion of the phosphatases PTP1B and TCPTP enhanced insulin and leptin signaling in proopiomelanocortin neurons and prevented diet-induced obesity by increasing WAT browning and energy expenditure. The coinfusion of insulin plus leptin into the CNS or the activation of proopiomelanocortin neurons also increased WAT browning and decreased adiposity. Our findings identify a homeostatic mechanism for coordinating the status of energy stores, as relayed by insulin and leptin, with the central control of WAT browning

Constraining Type Ia Supernovae progenitors from three years of SNLS data

While it is generally accepted that Type Ia supernovae are the result of the explosion of a carbon-oxygen White Dwarf accreting mass in a binary system, the details of their genesis still elude us, and the nature of the binary companion is uncertain. Kasen (2010) points out that the presence of a non-degenerate companion in the progenitor system could leave an observable trace: a flux excess in the early rise portion of the lightcurve caused by the ejecta impact with the companion itself. This excess would be observable only under favorable viewing angles, and its intensity depends on the nature of the companion. We searched for the signature of a non-degenerate companion in three years of Supernova Legacy Survey data by generating synthetic lightcurves accounting for the effects of shocking and comparing true and synthetic time series with Kolmogorov-Smirnov tests. Our most constraining result comes from noting that the shocking effect is more prominent in rest-frame B than V band: we rule out a contribution from white dwarf-red giant binary systems to Type Ia supernova explosions greater than 10% at 2 sigma, and than 20% at 3 sigma level.Comment: 14 pages, 15 figures, resubmitted to ApJ, figure 15 modifie

Spectroscopic Observations and Analysis of the Unusual Type Ia SN 1999ac

We present optical spectra of the peculiar Type Ia supernova (SN Ia) 1999ac. The data extend from -15 to +42 days with respect to B-band maximum and reveal an event that is unusual in several respects. Prior to B-band maximum, the spectra resemble those of SN 1999aa, a slowly declining event, but possess stronger SiII and CaII signatures (more characteristic of a spectroscopically normal SN). Spectra after B-band maximum appear more normal. The expansion velocities inferred from the Iron lines appear to be lower than average; whereas, the expansion velocity inferred from Calcium H and K are higher than average. The expansion velocities inferred from SiII are among the slowest ever observed, though SN 1999ac is not particularly dim. The analysis of the parameters v_10, R(SiII), dv(SiII)/dt, and d_m15 further underlines the unique characteristics of SN 1999ac. We find convincing evidence of CII 6580 in the day -15 spectrum with ejection velocity v > 16,000 km/s, but this signature disappears by day -9. This rapid evolution at early times highlights the importance of extremely early-time spectroscopy.Comment: 40 pages, 24 figures, accepted for publication in The Astronomical Journa

NTT and NOT spectroscopy of SDSS-II supernovae

Context. The SDSS-II Supernova Survey, conducted between 2005 and 2007, was designed to detect a large number of Type Ia supernovae (SNe Ia) around z~0.2, the redshift "gap" between low-z and high-z SN searches. The survey has provided multi-band photometric lightcurves for variable targets, and SN candidates were scheduled for spectroscopic observations, primarily to provide SN classification and accurate redshifts. We present SN spectra obtained in 2006 and 2007 using the NTT and the NOT. Aims. We provide an atlas of SN spectra in the range z =0.03-0.32 that complements the well-sampled lightcurves from SDSS-II in the forthcoming three-year SDSS SN cosmology analysis. The sample can, for example, be used for spectral studies of SNe Ia, which are critical for understanding potential systematic effects when SNe are used to determine cosmological distances. Methods. The spectra were reduced in a uniform manner, and special care was taken in estimating the uncertainties for the different processing steps. Host-galaxy light was subtracted when possible and the SN type fitted using the SuperNova IDentification code (SNID). We also present comparisons between spectral and photometric dating using SALT lightcurve fits to the photometry from SDSS-II, as well as the global distribution of our sample in terms of the lightcurve parameters: stretch and colour. Results. We report new spectroscopic data from 141 SNe Ia, mainly between -9 and +15 days from lightcurve maximum, including a few cases of multi-epoch observations. This homogeneous, host-galaxy subtracted, SN Ia spectroscopic sample is among the largest such data sets and unique in its redshift interval. The sample includes two potential SN 1991T-like SNe (SN 2006on and SN 2007ni) and one potential SN 2002cx-like SN (SN 2007ie). In addition, the new compilation includes spectra from 23 confirmed Type II and 8 Type Ib/c SNe.Comment: Accepted for publication in A&

RhoA knockout fibroblasts lose tumor-inhibitory capacity in vitro and promote tumor growth in vivo

Fibroblasts are a main player in the tumor-inhibitory microenvironment. Upon tumor initiation and progression, fibroblasts can lose their tumor-inhibitory capacity and promote tumor growth. The molecular mechanisms that underlie this switch have not been defined completely. Previously, we identified four proteins over-expressed in cancer-associated fibroblasts and linked to Rho GTPase signaling. Here, we show that knocking out the Ras homolog family member A (RhoA) gene in normal fibroblasts decreased their tumor-inhibitory capacity, as judged by neighbor suppression in vitro and accompanied by promotion of tumor growth in vivo. This also induced PC3 cancer cell motility and increased colony size in 2D cultures. RhoA knockout in fibroblasts induced vimentin intermediate filament reorganization, accompanied by reduced contractile force and increased stiffness of cells. There was also loss of wide F-actin stress fibers and large focal adhesions. In addition, we observed a significant loss of a-smooth muscle actin, which indicates a difference between RhoA knockout fibroblasts and classic cancer-associated fibroblasts. In 3D collagen matrix, RhoA knockout reduced fibroblast branching and meshwork formation and resulted in more compactly clustered tumor-cell colonies in coculture with PC3 cells, which might boost tumor stem-like properties. Coculturing RhoA knockout fibroblasts and PC3 cells induced expression of proinflammatory genes in both. Inflammatory mediators may induce tumor cell stemness. Network enrichment analysis of transcriptomic changes, however, revealed that the Rho signaling pathway per se was significantly triggered only after coculturing with tumor cells. Taken together, our findings in vivo and in vitro indicate that Rho signaling governs the inhibitory effects by fibroblasts on tumor-cell growth.Peer reviewe

Hubble Space Telescope studies of low-redshift Type Ia supernovae: Evolution with redshift and ultraviolet spectral trends

We present an analysis of the maximum light, near ultraviolet (NUV; 2900-5500 A) spectra of 32 low redshift (0.001<z<0.08) Type Ia supernovae (SNe Ia), obtained with the Hubble Space Telescope (HST). We combine this spectroscopic sample with high-quality gri light curves obtained with robotic telescopes to measure photometric parameters, such as stretch, optical colour, and brightness. By comparing our data to a comparable sample of SNe Ia at intermediate-z (0.4<z<0.9), we detect modest spectral evolution (3-sigma), in the sense that our mean low-z NUV spectrum has a depressed flux compared to its intermediate-z counterpart. We also see a strongly increased dispersion about the mean with decreasing wavelength, confirming the results of earlier surveys. These trends are consistent with changes in metallicity as predicted by contemporary SN Ia spectral models. We also examine the properties of various NUV spectral diagnostics in the individual spectra. We find a general correlation between stretch and the velocity (or position) of many NUV spectral features. In particular, we observe that higher stretch SNe have larger Ca II H&K velocities, that also correlate with host galaxy stellar mass. This latter trend is probably driven by the well-established correlation between stretch and stellar mass. We find no trends between UV spectral features and optical colour. Mean spectra constructed according to whether the SN has a positive or negative Hubble residual show very little difference at NUV wavelengths, indicating that the NUV evolution and variation we identify do not directly correlate with Hubble residuals. Our work confirms and strengthens earlier conclusions regarding the complex behaviour of SNe Ia in the NUV spectral region, but suggests the correlations we find are more useful in constraining progenitor models than improving the use of SNe Ia as cosmological probes.Comment: 22 pages, 14 figures, accepted in MNRAS with minor changes - Spectra are available on WISeREP, http://www.weizmann.ac.il/astrophysics/wiserep