We continue to investigate two-dimensional laterally propagating flames in type I X-ray bursts using fully compressible hydrodynamics simulations. In the current study we relax previous approximations where we artificially boosted the flames. We now use more physically realistic reaction rates, thermal conductivities, and rotation rates, exploring the effects of neutron star rotation rate and thermal structure on the flame. We find that at lower rotation rates the flame becomes harder to ignite, whereas at higher rotation rates the nuclear burning is enhanced by increased confinement from the Coriolis force and the flame propagates steadily. At higher crustal temperatures, the flame moves more quickly and accelerates as it propagates through the atmosphere. If the temperature is too high, instead of a flame propagating across the surface the entire atmosphere burns uniformly. Our findings could have implications for the relationship between observed burst rise times and neutron star rotation and accretion rates. All of the software used for these simulations is freely available.

Many astrophysical environments involve convective or explosive flows driven by thermonuclear reactions (Type Ia supernovae, classical novae, X-ray bursts, stellar evolution). Simulation codes need to accurately capture the interactions between reactions and hydrodynamics to produce realistic models of these events. For astrophysical reacting flows, operator splitting is commonly used to couple hydrodynamics and reactions. Each process operates independent of one another, but by staggering the updates in a symmetric fashion (via Strang splitting) second order accuracy in time can be achieved. However, approximations are often made to the reacting system, including the choice of whether or not to integrate temperature with the species. Here we demonstrate through a simple convergence test that integrating an energy equation together with reactions achieves the best convergence when modeling reactive flows with Strang splitting. Additionally, second order convergence cannot be achieved without integrating an energy or temperature equation.

Modelling long-time convective flows in the interiors of stars is extremely challenging using conventional compressible hydrodynamics codes due to the acoustic timestep limitation. Many of these flows are in the low Mach number regime, which allows us to exploit the relationship between acoustic and advective time scales to develop a more computationally efficient approach. MAESTROeX is an open source low Mach number stellar hydrodynamics code that allows much larger timesteps to be taken, therefore enabling systems to be modelled for much longer periods of time. This is particularly important for the problem of convection in the cores of rotating massive stars prior to core collapse. To fully capture the dynamics, it is necessary to model these systems in three dimensions at high resolution over many rotational periods. We present an overview of MAESTROeX’s current capabilities, describe ongoing work to incorporate the effects of rotation and discuss how we are optimising the code to run on GPUs.

We describe recent developments to the Castro astrophysics simulation code, focusing on new features that enable our simulations of X-ray bursts. Two highlights of Castro’s ongoing development are the new integration technique to couple hydrodynamics and reactions to high order and GPU offloading. We discuss how these features will help offset some of the computational expense in X-ray burst models.

Thermonuclear (type Ia) supernovae are bright stellar explosions with the unique property that the light curves can be standardized, allowing them to be used as distance indicators for cosmological studies. Many fundamental questions bout these events remain, however. We provide a critique of our present understanding of these and present results of simulations assuming the single-degenerate progenitor model consisting of a white dwarf that has gained mass from a stellar companion. We present results from full three-dimensional simulations of convection with weak reactions comprising the A=23 Urca process in the progenitor white dwarf.

We discuss the challenges of modeling X-ray bursts in multi-dimensions, review the different calculations done to date, and discuss our new set of ongoing simulations. We also describe algorithmic improvements that may help in the future to offset some of the expense of these simulations, and describe what may be possible with exascale computing.

We present early results from a study addressing the question of how one treats the propagation of incertitude, that is, epistemic uncertainty, in input parameters in astrophysical simulations. As an example, we look at the propagation of incertitude in control parameters for stellar winds in MESA stellar evolution simulations. We apply two methods of incertitude propagation, the Cauchy Deviates method and the Quadratic Response Surface method, to quantify the output uncertainty in the final white dwarf mass given a range of values for wind parameters. The methodology we apply is applicable to the problem of propagating input incertitudes through any simulation code treated as a "black box," i.e. a code for which the algorithmic details are either inaccessible or prohibitively complicated. We have made the tools developed for this study freely available to the community.

We describe the AMReX suite of astrophysics codes and their application to modeling problems in stellar astrophysics. Maestro is tuned to efficiently model subsonic convective flows while Castro models the highly compressible flows associated with stellar explosions. Both are built on the block-structured adaptive mesh refinement library AMReX. Together, these codes enable a thorough investigation of stellar phenomena, including Type Ia supernovae and X-ray bursts. We describe these science applications and the approach we are taking to make these codes performant on current and future many-core and GPU-based architectures.

Thermonuclear (Type Ia) supernovae are bright stellar explosions, the light curves of which can be calibrated to allow for use as "standard candles" for measuring cosmological distances. Contemporary research investigates how the brightness of an event may be influenced by properties of the progenitor system that follow from properties of the host galaxy such as composition and age. The goals are to better understand systematic effects and to assess the intrinsic scatter in the brightness, thereby reducing uncertainties in cosmological studies. We present the results from ensembles of simulations in the single-degenerate paradigm addressing the influence of age and metallicity on the brightness of an event and compare our results to observed variations of brightness that correlate with properties of the host galaxy. We also present results from "hybrid" progenitor models that incorporate recent advances in stellar evolution.

Analog shortwave radio on bands up to 30 MHz has typically been limited in applicability for long-distance broadcast applications requiring high signal fidelity. This is because of the vulnerability of analog modulation schemes to atmospheric dispersion; however, the Digital Radio Mondiale (DRM) standard, introduced in 2001, is a promising solution for shortwave radio. DRM provides various audio and data digital encoding methods that utilize current data compression and error-detection features. These characteristics are attractive for shortwave broadcasters, particularly those desiring to implement economical, reliable studio-to-transmitter links. This paper presents a DRM receiver design which implements these features in addition to possessing firmware scalability for a planned diversity implementation. Significant improvements to the DRM receiver design presented in a previous paper are discussed, and the receiver's increased performance is evaluated, particularly with regards to signal-to-noise ratio. The paper concludes with a discussion of future work.

NOTE: The first page of text has been automatically extracted and included below in lieu of an abstract Diversity Receiver for Digital Radio Mondiale – A Multi-Year Design Project Introduction In nations where the Internet is a rare luxury, radio remains the most efficient way to spread important information to the majority of the populace. Non-profit radio pioneer HCJB, “The Voice of the Andes,” currently works at developing new transmitters and receivers from their technology center in Elkhart, Indiana. HCJB has sponsored the development of many broadcast stations in these less-developed countries, including parts of Africa and Indonesia. Citizens tune in to hear essential public health information, directions for disaster/relief aid, as well as "normal" programming including music, talk shows, and Christian programs. For many listeners this may be their only window to the outside world. Unfortunately, due to the de-centralized and de-urbanized nature of these less-developed countries, many listeners live in isolated communities far from the main cities where the radio studios reside. Transmitting content to these remote listeners and their local radio stations is a major challenge for broadcasters. The preferred method of broadcast is FM, which provides good audio quality but only over short distances. One strategy used by some broadcasters is to "chain" FM repeater links together, stretching the effective transmission range of FM but losing quality in the process. AM radio provides the necessary range to cover entire countries, but its quality suffers in comparison to FM. Furthermore, in many regions, long-range AM transmissions are also hampered by signal degradation brought about by changes in the ionosphere. Digital Radio Mondiale (DRM) is a European digital radio standard designed specifically for shortwave AM broadcasts, including those in the tropical-band frequency range. DRM allows broadcasters to transmit FM-quality audio over AM-grade ranges, and as such, is of particular interest to non-profit broadcasters such as HCJB. DRM Background The Digital Radio Mondiale system was developed by a consortium of over seventy broadcasters and broadcast organizations to satisfy the need for a digital broadcast standard for frequency bands below 30 MHz. 1, 2 It was approved by the European Telecommunication Standards Institute in 2001 and has been extended to frequencies up to 174 MHz. 3,4 A number of broadcasters began digital transmission in 2003, and seven DRM receivers were developed that year. 5 An advantage of DRM over analog radio, in addition to reception quality, is its ability to transmit both audio and data streams. DRM makes use of QAM mapping and Coded Orthogonal Frequency Division Multiplexing (COFDM), which utilizes a convolutional forward error- correcting code with a set of low-bitrate signals at closely spaced frequencies. 6 DRM Receiver as a Senior Design Project In 2007 engineers from HCJB discussed with our faculty the possibility of working with them on a project to develop a receiver which could be used as part of a studio-transmitter rebroadcast link, in which DRM signals from the primary broadcast would be received and demodulated for