Transparent Helium in Stripped Envelope Supernovae

Using simple arguments based on photometric light curves and velocity evolution, we propose that some stripped envelope supernovae (SNe) show signs that a significant fraction of their helium is effectively transparent. The main pieces of evidence are the relatively low velocities with little velocity evolution, as are expected deep inside an exploding star, along with temperatures that are too low to ionize helium. This means that the helium should not contribute to the shaping of the main SN light curve, and thus the total helium mass may be difficult to measure from simple light curve modeling. Conversely, such modeling may be more useful for constraining the mass of the carbon/oxygen core of the SN progenitor. Other stripped envelope SNe show higher velocities and larger velocity gradients, which require an additional opacity source (perhaps the mixing of heavier elements or radioactive nickel) to prevent the helium from being transparent. We discuss ways in which similar analysis can provide insights into the differences and similarities between SNe Ib and Ic, which will lead to a better understanding of their respective formation mechanisms.Comment: Revised version accepted for publication in The Astrophysical Journal Letters, 4 pages, 4 figure

Exploring the Potential Diversity of Early Type Ia Supernova Light Curves

During the first several days after explosion, Type Ia supernova light curves probe the outer layers of the exploding star and therefore provide important clues for identifying their progenitors. We investigate how both the shallow $^{56}$Ni distribution and the presence of circumstellar material shape these early light curves. This is performed using a series of numerical experiments with parameterized properties for systematic exploration. Although not all of the considered models may be realized in nature (and indeed there are arguments why some of them should not occur), the spirit of this work is to provide a broader exploration of the diversity of possibilities. We find that shallower $^{56}$Ni leads to steeper, bluer light curves. Differences in the shape of the rise can introduce errors in estimating the explosion time and thus impact efforts to infer upper limits on the progenitor or companion radius from a lack of observed shock cooling emission. Circumstellar material can lead to significant luminosity during the first few days, but its presence can be difficult to identify depending on the degree of nickel mixing. In some cases, the hot emission of circumstellar material may even lead to a signature similar to interaction with a companion, and thus in the future additional diagnostics should be gathered for properly assessing early light curves.Comment: Revised version with additional figures and discussions. 8 pages, 15 figures, accepted for publication in The Astrophysical Journa

Unifying Type II Supernova Light Curves with Dense Circumstellar Material

A longstanding problem in the study of supernovae (SNe) has been the relationship between the Type IIP and Type IIL subclasses. Whether they come from distinct progenitors or they are from similar stars with some property that smoothly transitions from one class to another has been the subject of much debate. Here we show using one-dimensional radiation-hydrodynamic SN models that the multi-band light curves of SNe IIL are well fit by ordinary red supergiants surrounded by dense circumstellar material (CSM). The inferred extent of this material, coupled with a typical wind velocity of ~10-100 km/s, suggests enhanced activity by these stars during the last ~months to ~years of their lives, which may be connected with advanced stages of nuclear burning. Furthermore, we find that even for more plateau-like SNe that dense CSM provides a better fit to the first ~20 days of their light curves, indicating that the presence of such material may be more widespread than previously appreciated. Here we choose to model the CSM with a wind-like density profile, but it is unclear whether this just generally represents some other mass distribution, such as a recent mass ejection, thick disk, or even inflated envelope material. Better understanding the exact geometry and density distribution of this material will be an important question for future studies.Comment: 9 pages, 7 figures, submitted for publication in The Astrophysical Journa

Modeling the light curve of Type IIn-P SN 2005cl with red supergiant progenitors featuring pre-SN ourbursts

All Type IIn supernovae (SNe IIn) show narrow hydrogen emission lines in their spectra. Apart from this common feature, they demonstrate very broad diversity in brightness, duration, and morphology of their light curves, which indicates that they likely come from a variety of progenitor systems and explosion channels. A particular subset of SNe IIn, the so called SNe IIn-P, exhibit $\sim$100 days plateau phases that are very similar to the ones of the ordinary hydrogen-rich SNe (SNe II). In the past, SNe IIn-P were explained by the models of sub-energetic electron capture explosions surrounded by dense extended winds. In this work, we attempt to explain this class of SNe with standard red supergiant (RSG) progenitors that experience outbursts several month before the final explosion. The outburst energies that show the best agreement between our models and the data ($5\times10^{46}\,{\rm erg}$) fall at the low range of the outburst energies that have been observed for SNe IIn (between few times $10^{46}\,{\rm erg}$ and $10^{49}\,{\rm erg}$). Instead, the inferred explosion energy of SN 2005cl is relatively high ($1-2\times10^{51}\,{\rm erg}$) compared to the explosion energies of the ordinary SNe II. Our models provide alternative explanation of SNe IIn-P to the previously proposed scenarios.Comment: 5 pages, 3 figure

Explaining the subpulse drift velocity of pulsar magnetosphere within the space-charge limited flow model

We try to explain the subpulse drift phenomena adopting the space-charge limited flow (SCLF) model and comparing the plasma drift velocity in the inner region of pulsar magnetospheres with the observed velocity of drifting subpulses. We apply the approach described in a recent paper of van Leeuwen & Timokhin (2012), where it was shown that the standard estimation of the subpulse drift velocity through the total value of the scalar potential drop in the inner gap gives inaccurate results, while the exact expression relating the drift velocity to the gradient of the scalar potential should be used instead. After considering a selected sample of sources taken from the catalog of Weltevrede, Edwards & Stappers (2006) with coherently drifting subpulses and reasonably known observing geometry, we show that their subpulse drift velocities would correspond to the drift of the plasma located very close or above the pair formation front. Moreover, a detailed analysis of PSR B0826-34 and PSR B0818-41 reveals that the variation of the subpulse separation with the pulse longitude can be successfully explained by the dependence of the plasma drift velocity on the angular coordinates.Comment: 14 pages, 6 figures, 2 table

Mechanism of Tau Propagation: Putative Therapeutic Approaches

One of the characteristics of Alzheimer’s disease and associated tauopathies is the accumulation and aggregation of hyperphosphorylated tau protein. The biological activity of tau is to bind to tubulin and promote its assembly into microtubules with subsequent stabilization of the latter. When tau gets hyperphosphorylated it cannot bind to tubulin and carry on its function, instead, it binds to normal tau and sequesters it from microtubules leading to disruption of microtubular assembly and ultimately to the death of neurons. Our lab had previously shown that tau phosphorylation sites 199, 212, 231, and 262, combined with the FTDP-17 mutation R406W (Pathological Human tau or PH-Tau) are critical for inducing a conformational change in the protein similar to the abnormally hyperphosphorylated tau from AD brain. Moreover, it was proposed that hyperphosphorylated tau can transfer between the cells in a prion fashion leading to more microtubular disruption. This research was designed to investigate the role of phosphorylated tau in the mechanism of tau transfer from one cell to another in vitro and in vivo and its effect on memory. I found that tau can be taken up by the cells and neurons when added to the cell media and this uptake is mediated by the M1 and M3 muscarinic receptors. M1 muscarinic receptor inhibitor Pirenzipine similarly to Atropine, a broad muscarinic receptor antagonist is able to block almost 80 percent of tau uptake, while PTX and AF-DX116, M2 antagonist, and M2 and M4 downstream blockers respectively, had no impact on tau PH-PH-Tau uptake, Moreover, once again I confirmed that effect of PH-Tau on primary neuronal cultures is very similar to tau isolated from the brain of the patient with Alzheimer disease and leads to almost complete disruption of neurites and microglial activation. On the other hand, the incubation of primary cultures with tau resulted in enhanced growth of neurites. I also found that all the tau isoforms can be taken up by the neurons and preincubation with atropine significantly reduces their uptake. However, the highest uptake I observed was with the largest tau isoforms, and this uptake was significantly reduced with the isoforms with an absent 2N domain. The intracranial hippocampal injections of PH-Tau lead to cognitive decline in CD1 mice within 6 months after the injection. Furthermore, Direct Current (DC) stimulation is the technique that was shown to upregulate chaperone proteins. These proteins are involved in targeting misfolded proteins and aggregates. Here I investigated the effect of DC stimulation on human cells and primary neuronal cultures that were exposed to pathological tau and the brain of our transgenic mice expressing high amounts of hyperphosphorylated tau. I found that DC stimulation resulted in an increase in HSP70 proteins in vitro and in vivo. I also found the reduction in PH-Tau in vitro and in vivo most likely due to the upregulation of the degradation pathway

Numerical Modeling of the Early Light Curves of Type IIP Supernovae

The early rise of Type IIP supernovae (SN IIP) provides important information for constraining the properties of their progenitors. This can in turn be compared to pre-explosion imaging constraints and stellar models to develop a more complete picture of how massive stars evolve and end their lives. Using the SuperNova Explosion Code (SNEC), we model the first 40 days of SNe IIP to better understand what constraints can be derived from their early light curves. We use two sets of red supergiant progenitor models with zero-age main sequence masses in the range between 9 Msol and 20 Msol. We find that the early properties of the light curve depend most sensitively on the radius of the progenitor, and thus provide a relation between the g-band rise time and the radius at the time of explosion. This relation will be useful for deriving constraints on progenitors from future observations, especially in cases where detailed modeling of the entire rise is not practical. When comparing to observed rise times, the radii we find are a factor of a few larger than previous semi-analytic derivations and generally in better agreement with what is found with current stellar evolution calculations.Comment: 8 pages, 7 figure

Particle acceleration in the polar cap region of an oscillating neutron star

We revisit particle acceleration in the polar cap region of a neutron star by taking into account both general relativistic effects and the presence of toroidal oscillations at the star surface. In particular, we address the question of whether toroidal oscillations at the stellar surface can affect the acceleration properties in the polar cap. We solve numerically the relativistic electrodynamics equations in the stationary regime, focusing on the computation of the Lorentz factor of a space-charge-limited electron flow accelerated in the polar cap region of a rotating and oscillating pulsar. To this extent, we adopt the correct expression of the general relativistic Goldreich-Julian charge density in the presence of toroidal oscillations. Depending on the ratio of the actual charge density of the pulsar magnetosphere to the Goldreich-Julian charge density, we distinguish two different regimes of the Lorentz factor of the particle flow, namely an oscillatory regime produced for sub-GJ current density configurations, which does not produce an efficient acceleration, and a true accelerating regime for super-GJ current density configurations. We find that star oscillations may be responsible for a significant asymmetry in the pulse profile that depends on the orientation of the oscillations with respect to the pulsar magnetic field. In particular, significant enhancements of the Lorentz factor are produced by stellar oscillations in the super-GJ current density regime.Comment: With updated bibliographyc informatio

High-Order, Implicit Time Integration of Discrete, Chaotic Dynamical Systems

A wide range of implicit time integration methods, including multi-step, implicit Runge-Kutta, and Galerkin finite-time element schemes, is evaluated in the context of chaotic dynamical systems. The schemes are applied to solve the Lorenz equations, the equation of motion of a Duffing oscillator, and the Kuramoto-Sivashinsky system, with the goal of finding the most computationally efficient method that results in the least expensive model for a chosen level of accuracy. It is found that the quasi-period of a chaotic system strongly limits the time-step size that can be used in the simulations, and all schemes fail once the time-step size reaches a significant fraction of that period. In these conditions, the computational cost per time-step becomes one of the most important factors determining the efficiency of the schemes. The cheaper, second-order schemes are shown to have an advantage over the higher-order schemes at large time-step sizes, with one possible exception being the fourth-order continuous Galerkin scheme. The higher-order schemes become more efficient than the lower-order schemes as accuracy requirements tighten. If going beyond the second-order is necessary for reasons other than computational efficiency, the fourth-order methods are shown to perform better than the third-order ones at all time-step sizes.Comment: 38 pages, 15 figure