Radiation Hydrodynamical Instabilities in Cosmological and Galactic Ionization Fronts

Ionization fronts, the sharp radiation fronts behind which H/He ionizing photons from massive stars and galaxies propagate through space, were ubiquitous in the universe from its earliest times. The cosmic dark ages ended with the formation of the first primeval stars and galaxies a few hundred Myr after the Big Bang. Numerical simulations suggest that stars in this era were very massive, 25 - 500 solar masses, with H II regions of up to 30,000 light-years in diameter. We present three-dimensional radiation hydrodynamical calculations that reveal that the I-fronts of the first stars and galaxies were prone to violent instabilities, enhancing the escape of UV photons into the early intergalactic medium (IGM) and forming clumpy media in which supernovae later exploded. The enrichment of such clumps with metals by the first supernovae may have led to the prompt formation of a second generation of low-mass stars, profoundly transforming the nature of the first protogalaxies. Cosmological radiation hydrodynamics is unique because ionizing photons coupled strongly to both gas flows and primordial chemistry at early epochs, introducing a hierarchy of disparate characteristic timescales whose relative magnitudes can vary greatly throughout a given calculation. We describe the adaptive multistep integration scheme we have developed for the self-consistent transport of both cosmological and galactic ionization fronts.Comment: 6 pages, 4 figures, accepted for proceedings of HEDLA2010, Caltech, March 15 - 18, 201

On the Maximum Mass of Accreting Primordial Supermassive Stars

Supermassive primordial stars are suspected to be the progenitors of the most massive quasars at z~6. Previous studies of such stars were either unable to resolve hydrodynamical timescales or considered stars in isolation, not in the extreme accretion flows in which they actually form. Therefore, they could not self-consistently predict their final masses at collapse, or those of the resulting supermassive black hole seeds, but rather invoked comparison to simple polytropic models. Here, we systematically examine the birth, evolution and collapse of accreting non-rotating supermassive stars under accretion rates of 0.01-10 solar masses per year, using the stellar evolution code KEPLER. Our approach includes post-Newtonian corrections to the stellar structure and an adaptive nuclear network, and can transition to following the hydrodynamic evolution of supermassive stars after they encounter the general relativistic instability. We find that this instability triggers the collapse of the star at masses of 150,000-330,000 solar masses for accretion rates of 0.1-10 solar masses per year, and that the final mass of the star scales roughly logarithmically with the rate. The structure of the star, and thus its stability against collapse, is sensitive to the treatment of convection, and the heat content of the outer accreted envelope. Comparison with other codes suggests differences here may lead to small deviations in the evolutionary state of the star as a function of time, that worsen with accretion rate. Since the general relativistic instability leads to the immediate death of these stars, our models place an upper limit on the masses of the first quasars at birth.Comment: 5 pages, 4 figures. Accepted ApJ letter

Patterns of interstate migration in the United States from the survey of income and program participation

The authors describe the Survey of Income and Program Participation (SIPP) as a data source for migration studies. The SIPP is a panel dataset that provides information on income, employment outcomes, and participation in government programs. Survey participants are interviewed for up to four years even if they move to a new household or that household migrates within the United States. This unique longitudinal design gives the survey a strong advantage over traditional data sources. The authors illustrate differences in the propensity for interstate migration among different demographic groups over the 12-year period from 1996 to 2008. They also analyze the relationship between migration choices and life-changing events, such as becoming jobless or dissolution of a marriage. Their findings suggest that future research should consider the migration choices of individuals near retirement age.Demography ; Income ; Emigration and immigration

Forming a Primordial Star in a Relic HII Region

There has been considerable theoretical debate over whether photoionization and supernova feedback from the first Population III stars facilitate or suppress the formation of the next generation of stars. We present results from an Eulerian adaptive mesh refinement simulation demonstrating the formation of a primordial star within a region ionized by an earlier nearby star. Despite the higher temperatures of the ionized gas and its flow out of the dark matter potential wells, this second star formed within 23 million years of its neighbor's death. The enhanced electron fraction within the HII region catalyzes rapid molecular hydrogen formation that leads to faster cooling in the subsequent star forming halos than in the first halos. This "second generation" primordial protostar has a much lower accretion rate because, unlike the first protostar, it forms in a rotationally supported disk of approx. 10-100 solar masses. This is primarily due to the much higher angular momentum of the halo in which the second star forms. In contrast to previously published scenarios, such configurations may allow binaries or multiple systems of lower mass stars to form. These first high resolution calculations offer insight into the impact of feedback upon subsequent populations of stars and clearly demonstrate how primordial chemistry promotes the formation of subsequent generations of stars even in the presence of the entropy injected by the first stars into the IGM.Comment: 4 pages, 2 figures. Some revisions, including enhanced discussion of angular momentum issues. Asrophysical Journal, accepte

Background, current status, and prognosis of the ongoing slush hydrogen technology development program for the NASP

Among the Hydrogen Projects at the NASA Lewis Research Center (NASA LeRC), is the task of implementing and managing the Slush Hydrogen (SLH2) Technology Program for the United States' National AeroSpace Plane Joint Program Office (NASP JPO). The objectives of this NASA LeRC program are to provide verified numerical models of fluid production, storage, transfer, and feed systems and to provide verified design criteria for other engineered aspects of SLH2 systems germane to a NASP. The pursuit of these objectives is multidimensional, covers a range of problem areas, works these to different levels of depth, and takes advantage of the resources available in private industry, academia, and the U.S. Government. A summary of the NASA LeRC overall SLH2 program plan, is presented along with its implementation, the present level of effort in each of the program areas, some of the results already in hand, and the prognosis for the effort in the immediate future

Engaging the Customer: The Impacts of Online Travel Community Engagement on Brand Identification and Behavioral Intentions

Online consumption communities provide opportunities for social engagements targeting conversations about products and services in which individuals’ actions within the community have the potential to influence consumer behavior. According to extant research, these groups effect decision making processes, however they remain an enigma and require further investigation. Understanding the impact of online communities on the hospitality and tourism industry is a crucial area that is still not fully understood. As travel and tourism destinations are highly complex and require travelers to plan thoroughly in order to best enjoy their travels and hopefully minimize travel risks, consumers are turning to alternative sources of information due to availability and preference for online mediums. Using the DIS online community, this study evaluates the impacts of consumer engagement on brand identification and behavioral intentions both towards continued involvement with the community and intentions towards the brand or destination. This research is integral for future understandings of consumer behavior in online communities and the impact of these communities on decision making practices