VADER: A Flexible, Robust, Open-Source Code for Simulating Viscous Thin Accretion Disks

The evolution of thin axisymmetric viscous accretion disks is a classic problem in astrophysics. While models based on this simplified geometry provide only approximations to the true processes of instability-driven mass and angular momentum transport, their simplicity makes them invaluable tools for both semi-analytic modeling and simulations of long-term evolution where two- or three-dimensional calculations are too computationally costly. Despite the utility of these models, the only publicly-available frameworks for simulating them are rather specialized and non-general. Here we describe a highly flexible, general numerical method for simulating viscous thin disks with arbitrary rotation curves, viscosities, boundary conditions, grid spacings, equations of state, and rates of gain or loss of mass (e.g., through winds) and energy (e.g., through radiation). Our method is based on a conservative, finite-volume, second-order accurate discretization of the equations, which we solve using an unconditionally-stable implicit scheme. We implement Anderson acceleration to speed convergence of the scheme, and show that this leads to factor of $\sim 5$ speed gains over non-accelerated methods in realistic problems, though the amount of speedup is highly problem-dependent. We have implemented our method in the new code Viscous Accretion Disk Evolution Resource (VADER), which is freely available for download from https://bitbucket.org/krumholz/vader/ under the terms of the GNU General Public License.Comment: 58 pages, 13 figures, accepted to Astronomy & Computing; this version includes more discussion, but no other changes; code is available for download from https://bitbucket.org/krumholz/vader

Eloquent Wisdom: The Role of Rhetoric and Delight in the Theology of Saint Augustine of Hippo

This study examines Augustine’s conception of the role of delight (delectatio) in the divine acts of creation and redemption. In the first part of the dissertation, I argue that Augustine, who was trained as a rhetor and taught rhetoric before his conversion, came to conceive of theology as the fulfilment of Cicero’s conviction that wisdom and eloquence ought to be united. His approach to Cicero’s rhetorical theory as found in De inventione, De oratore, and Orator shares many similarities with that of Late Antique rhetors (especially Marius Victorinus) in whose works the orator functions less as a statesman than as a physician of the soul. Accordingly, an orator’s role is not to sway the senate or deliberate in law courts (as in Cicero’s thought) but to inform and persuade people towards a fruitful return to the divine. In the second part of the dissertation, I demonstrate how this approach influenced Augustine’s understanding of redemption. He conceives of God as Cicero’s ideal orator, in whom wisdom and eloquence are perfectly united. God engages in a rhetorical contest with the devil whom Augustine portrays in terms of the false orators in Cicero’s De inventione. The devil’s rhetoric comprises an illicit delight in actual sin and an inordinate delight in created goods, both of which exert their power over the human will through suggestion, delight and persuasion and result in a bondage to sin and death. By contrast, God ‘utters’ creation as a delightful song and opposes death’s nihilistic rhetoric by pouring his own delight into the hearts of the faithful; this delight persuades the will to move towards a joyful participation in the divine that is the happy life for which all people long. Ultimately, Augustine identifies this spiritual delight most closely with the Holy Spirit who functions as God’s eternal eloquence

Extending the Globular Cluster System-Halo Mass Relation to the Lowest Galaxy Masses

High mass galaxies, with halo masses $M_{200} \ge 10^{10} M_{\odot}$, reveal a remarkable near-linear relation between their globular cluster (GC) system mass and their host galaxy halo mass. Extending this relation to the mass range of dwarf galaxies has been problematic due to the difficulty in measuring independent halo masses. Here we derive new halo masses based on stellar and HI gas kinematics for a sample of nearby dwarf galaxies with GC systems. We find that the GC system mass--halo mass relation for galaxies populated by GCs holds from halo masses of $M_{200} \sim 10^{14} M_{\odot}$ down to below $M_{200}$ $\sim 10^9 M_{\odot}$, although there is a substantial increase in scatter towards low masses. In particular, three well-studied ultra diffuse galaxies, with dwarf-like stellar masses, reveal a wide range in their GC-to-halo mass ratios. We compare our GC system--halo mass relation to the recent model of El Badry et al., finding that their fiducial model does not reproduce our data in the low mass regime. This may suggest that GC formation needs to be more efficient than assumed in their model, or it may be due to the onset of stochastic GC occupation in low mass halos. Finally, we briefly discuss the stellar mass-halo mass relation for our low mass galaxies with GCs, and we suggest some nearby dwarf galaxies for which searches for GCs may be fruitful.Comment: 16 pages, 5 figures, accepted for publication in MNRA

Age-dependent induction of immunity and subsequent survival costs in males and females of a temperate damselfly

BACKGROUND: To understand variation in resistance to parasites within host populations, researchers have examined conditions under which immunity is induced and/or is costly. Both host sex and age have been found to influence immune expression and subsequently are likely factors influencing the costs of resistance. The purpose of this study was to examine immune expression and associated survival costs for two age groups (newly emerged and sexually mature individuals) of the damselfly, Enallagma boreale Selys. Survival was assessed for experimentally challenged and control damselflies, housed initially at 22°C and then subjected to low temperatures (15°C) associated with reduced foraging activity and food deprivation. Experimental conditions emulated natural local variation in bouts of good weather followed by inclement weather (successions of days with hourly mean temperatures around 15°C and/or rainy weather). RESULTS: At least one of three immune traits was induced to higher levels for both newly emerged and mature E. boreale challenged by Lippopolysaccharide (LPS) relative to saline-injected controls, when housed at 22°C. The immune traits assayed included haemocyte concentration, Phenoloxidase activity and antibacterial activity and their induction varied among ages and between males and females. For matures, those injected with LPS had lowered survivorship compared to saline-injected controls that were housed initially at 22°C and subsequently at 15°C. Newly emerged LPS-injected damselflies did not show reduced survivorship relative to newly-emerged controls, despite showing immune induction. CONCLUSION: Reduced longevity following induction of immunity was observed for reproductively mature damselflies, but not for newly emerged damselflies. Costs of resistance depend only partly on the immune trait induced and more on the age (but not sex) of the host. In four years, we often observed bouts of inclement weather following good days and these bouts occurred primarily during the emergence periods, but also during the flight periods, of E. boreale. The duration of these bouts appear sufficient to compromise survival of mature damselflies that responded immunologically to LPS challenge. We further suggest the environmental conditions likely experienced by different ages of damselflies, following resistance expression, has influenced optimal immune investment by individuals in different age classes and the likelihood of detecting costs of resistance

The Perceptual Northwest

Our goal is to survey cultural perceptions defining the U.S. Northwest region. As geographers, we should concern ourselves with mental constructs of regions, as they can easily impede or facilitate communication. Assumptions of others’ regional boundaries and images may be erroneous. Over the past several decades, a handful of geographers have begun to examine these perceptual (or vernacular) maps and regions. Students at 21 colleges and universities were asked to identify: (1) boundaries of the U.S. Northwest region; (2) Northwest regional characteristics and symbols; and (3) what cities or other places best represent the Northwest. Nationally, student respondents largely followed the “official” area of the region regarding state boundaries, with Washington, Oregon, and Idaho as the core area. Student perception of characteristic cities and places followed this pattern, with referenced population centers spread across the threestate landscape. Regarding descriptive words and symbols, more emphasis is placed on Garreau’s coastal perception of the Northwest through terms such as rainy, trees, and mountainous. Regional differences showed up in perception of area, descriptive words, and symbols of the Northwest. Proximity led to different perceptions. Northwest students showed the smallest perception of the Northwest in geographic area, while those farthest away (Southeast students) mapped the largest Northwest. Difference from the home region also led to different perceptions. Students from less-forested regions emphasized trees more than students from more forested regions, who emphasized open and vast characteristics of the Northwest