Constraints on the Intergalactic Transport of Cosmic Rays

Motivated by recent experimental proposals to search for extragalactic cosmic rays (including anti-matter from distant galaxies), we study particle propagation through the intergalactic medium (IGM). We first use estimates of the magnetic field strength between galaxies to constrain the mean free path for diffusion of particles through the IGM. We then develop a simple analytic model to describe the diffusion of cosmic rays. Given the current age of galaxies, our results indicate that, in reasonable models, a completely negligible number of particles can enter our Galaxy from distances greater than $\sim 100$ Mpc for relatively low energies ($E$ $< 10^6$ GeV/n). We also find that particle destruction in galaxies along the diffusion path produces an exponential suppression of the possible flux of extragalactic cosmic rays. Finally, we use gamma ray constraints to argue that the distance to any hypothetical domains of anti-matter must be roughly comparable to the horizon scale.Comment: 24 pages, AAS LaTex, 1 figure, accepted to Ap

Long Term Evolution of Close Planets Including the Effects of Secular Interactions

This paper studies the long term evolution of planetary systems containing short-period planets, including the effects of tidal circularization, secular excitation of eccentricity by companion planets, and stellar damping. For planetary systems subject to all of these effects, analytic solutions (or approximations) are presented for the time evolution of the semi-major axes and eccentricities. Secular interactions enhance the inward migration and accretion of hot Jupiters, while general relativity tends to act in opposition by reducing the effectiveness of the secular perturbations. The analytic solutions presented herein allow us to understand these effects over a wide range of parameter space and to isolate the effects of general relativity in these planetary systems.Comment: 14 pages, 2 figures, accepted to Ap

Photoevaporation of Circumstellar Disks due to External FUV Radiation in Stellar Aggregates

When stars form in small groups (N = 100 - 500 members), their circumstellar disks are exposed to little EUV radiation but a great deal of FUV radiation from massive stars in the group. This paper calculates mass loss rates for circumstellar disks exposed to external FUV radiation. Previous work treated large disks and/or intense radiation fields in which the disk radius exceeds the critical radius (supercritical disks) where the sound speed in the FUV heated layer exceeds the escape speed. This paper shows that significant mass loss still takes place for subcritical systems. Some of the gas extends beyond the disk edge (above the disk surface) to larger distances where the temperature is higher, the escape speed is lower, and an outflow develops. The evaporation rate is a sensitive function of the stellar mass and disk radius, which determine the escape speed, and the external FUV flux, which determines the temperature structure of the flow. Disks around red dwarfs are readily evaporated and shrink to disk radii of 15 AU on short time scales (10 Myr) when exposed to moderate FUV fields with $G_0$ = 3000. Although disks around solar type stars are more durable, these disks shrink to 15 AU in 10 Myr for intense FUV radiation fields with $G_0$ = 30,000; such fields exist in the central 0.7 pc of a cluster with N = 4000 stars. If our solar system formed in the presence of such strong FUV radiation fields, this mechanism could explain why Neptune and Uranus in our solar system are gas poor, whereas Jupiter and Saturn are gas rich. This mechanism for photoevaporation can also limit the production of Kuiper belt objects and can suppress giant planet formation in sufficiently large clusters, such as the Hyades, especially for disks associated with low mass stars.Comment: 49 pages including 12 figures; accepted to Ap

Superluminality in DGP

We reconsider the issue of superluminal propagation in the DGP model of infrared modified gravity. Superluminality was argued to exist in certain otherwise physical backgrounds by using a particular, physically relevant scaling limit of the theory. In this paper, we exhibit explicit five-dimensional solutions of the full theory that are stable against small fluctuations and that indeed support superluminal excitations. The scaling limit is neither needed nor invoked in deriving the solutions or in the analysis of its small fluctuations. To be certain that the superluminality found here is physical, we analyze the retarded Green's function of the scalar excitations, finding that it is causal and stable, but has support on a widened light-cone. We propose to use absence of superluminal propagation as a method to constrain the parameters of the DGP model. As a first application of the method, we find that whenever the 4D energy density is a pure cosmological constant and a hierarchy of scales exists between the 4D and 5D Planck masses, superluminal propagation unavoidably occurs.Comment: 23 pages. Minor corrections. Version to appear in JHE

Active multi-fidelity Bayesian online changepoint detection

Online algorithms for detecting changepoints, or abrupt shifts in the behavior of a time series, are often deployed with limited resources, e.g., to edge computing settings such as mobile phones or industrial sensors. In these scenarios it may be beneficial to trade the cost of collecting an environmental measurement against the quality or "fidelity" of this measurement and how the measurement affects changepoint estimation. For instance, one might decide between inertial measurements or GPS to determine changepoints for motion. A Bayesian approach to changepoint detection is particularly appealing because we can represent our posterior uncertainty about changepoints and make active, cost-sensitive decisions about data fidelity to reduce this posterior uncertainty. Moreover, the total cost could be dramatically lowered through active fidelity switching, while remaining robust to changes in data distribution. We propose a multi-fidelity approach that makes cost-sensitive decisions about which data fidelity to collect based on maximizing information gain with respect to changepoints. We evaluate this framework on synthetic, video, and audio data and show that this information-based approach results in accurate predictions while reducing total cost.Comment: 37th Conference on Uncertainty in Artificial Intelligenc

Modeling rain-fed maize vulnerability to droughts using the standardized precipitation index from satellite estimated rainfall-Southern Malawi case study

During 1990s, disaster risk reduction emerged as a novel, proactive approach to managing risks from natural hazards. The World Bank, USAlD, and other international donor agencies began making efforts to mainstream disaster risk reduction in countries whose population and economies were heavily dependent on rain-fed agriculture. This approach has more significance in light of the increasing climatic hazard patterns and the climate scenarios projected for different hazard prone countries in the world. The Famine Early Warning System Network (FEWS NET) has been monitoring the food security issues in the sub-Saharan Africa, Asia and in Haiti. FEWS NET monitors the rainfall and moisture availability conditions with the help of NOAA RFE2 data for deriving food security status in Africa. This paper highlights the efforts in using satellite estimated rainfall inputs to develop drought vulnerability models in the drought prone areas in Malawi. The satellite RFE2 based SPI corresponding to the critical tasseling and silking phases (in the months of January, February, and March) were statistically regressed with drought-induced yield losses at the district level. The analysis has shown that the drought conditions in February and early March lead to most damage to maize yields in this region. The district-wise vulnerabilities to drought were up scaled to obtain a regional maize vulnerability model for southern Malawi. The results would help in establishing an early monitoring mechanism for drought impact assessment, give the decision makers additional time to assess seasonal outcomes, and identify potential food-related hazards in Malawi

HIghMass - High HI Mass, HI-Rich Galaxies at z∼0z\sim0: Combined HI and H2_2 Observations

We present resolved HI and CO observations of three galaxies from the HIghMass sample, a sample of HI-massive ($M_{HI} > 10^{10} M_\odot$), gas-rich ($M_{HI}$ in top $5\%$ for their $M_*$) galaxies identified in the ALFALFA survey. Despite their high gas fractions, these are not low surface brightness galaxies, and have typical specific star formation rates (SFR$/M_*$) for their stellar masses. The three galaxies have normal star formation rates for their HI masses, but unusually short star formation efficiency scale lengths, indicating that the star formation bottleneck in these galaxies is in the conversion of HI to H$_2$, not in converting H$_2$ to stars. In addition, their dark matter spin parameters ($\lambda$) are above average, but not exceptionally high, suggesting that their star formation has been suppressed over cosmic time but are now becoming active, in agreement with prior H$\alpha$ observations.Comment: 20 pages, 13 figure

The ALFALFA "Almost Darks" Campaign: Pilot VLA HI Observations of Five High Mass-to-Light Ratio Systems

We present VLA HI spectral line imaging of 5 sources discovered by ALFALFA. These targets are drawn from a larger sample of systems that were not uniquely identified with optical counterparts during ALFALFA processing, and as such have unusually high HI mass to light ratios. These candidate "Almost Dark" objects fall into 4 categories: 1) objects with nearby HI neighbors that are likely of tidal origin; 2) objects that appear to be part of a system of multiple HI sources, but which may not be tidal in origin; 3) objects isolated from nearby ALFALFA HI detections, but located near a gas-poor early-type galaxy; 4) apparently isolated sources, with no object of coincident redshift within ~400 kpc. Roughly 75% of the 200 objects without identified counterparts in the $\alpha$.40 database (Haynes et al. 2011) fall into category 1. This pilot sample contains the first five sources observed as part of a larger effort to characterize HI sources with no readily identifiable optical counterpart at single dish resolution. These objects span a range of HI mass [7.41 < log(M$_{\rm HI}$) < 9.51] and HI mass to B-band luminosity ratios (3 < M$_{\rm HI}$/L$_{\rm B}$ < 9). We compare the HI total intensity and velocity fields to SDSS optical imaging and to archival GALEX UV imaging. Four of the sources with uncertain or no optical counterpart in the ALFALFA data are identified with low surface brightness optical counterparts in SDSS imaging when compared with VLA HI intensity maps, and appear to be galaxies with clear signs of ordered rotation. One source (AGC 208602) is likely tidal in nature. We find no "dark galaxies" in this limited sample. The present observations reveal complex sources with suppressed star formation, highlighting both the observational difficulties and the necessity of synthesis follow-up observations to understand these extreme objects. (abridged)Comment: Astronomical Journal, in pres

The Gravitational Demise of Cold Degenerate Stars

We consider the long term fate and evolution of cold degenerate stars under the action of gravity alone. Although such stars cannot emit radiation through the Hawking mechanism, the wave function of the star will contain a small admixture of black hole states. These black hole states will emit radiation and hence the star can lose its mass energy in the long term. We discuss the allowed range of possible degenerate stellar evolution within this framework.Comment: LaTeX, 18 pages, one figure, accepted to Physical Review