Is H3+ cooling ever important in primordial gas?

Studies of the formation of metal-free Population III stars usually focus primarily on the role played by H2 cooling, on account of its large chemical abundance relative to other possible molecular or ionic coolants. However, while H2 is generally the most important coolant at low gas densities, it is not an effective coolant at high gas densities, owing to the low critical density at which it reaches local thermodynamic equilibrium (LTE) and to the large opacities that develop in its emission lines. It is therefore possible that emission from other chemical species may play an important role in cooling high density primordial gas. A particularly interesting candidate is the H3+ molecular ion. This ion has an LTE cooling rate that is roughly a billion times larger than that of H2, and unlike other primordial molecular ions such as H2+ or HeH+, it is not easily removed from the gas by collisions with H or H2. It is already known to be an important coolant in at least one astrophysical context -- the upper atmospheres of gas giants -- but its role in the cooling of primordial gas has received little previous study. In this paper, we investigate the potential importance of H3+ cooling in primordial gas using a newly-developed H3+ cooling function and the most detailed model of primordial chemistry published to date. We show that although H3+ is, in most circumstances, the third most important coolant in dense primordial gas (after H2 and HD), it is nevertheless unimportant, as it contributes no more than a few percent of the total cooling. We also show that in gas irradiated by a sufficiently strong flux of cosmic rays or X-rays, H3+ can become the dominant coolant in the gas, although the size of the flux required renders this scenario unlikely to occur.Comment: 60 pages, 22 figures. Submitted to MNRA

On the photodissociation of H2 by the first stars

The first star formation in the universe is expected to take place within small protogalaxies, in which the gas is cooled by molecular hydrogen. However, if massive stars form within these protogalaxies, they may suppress further star formation by photodissociating the H2. We examine the importance of this effect by estimating the timescale on which significant H2 is destroyed. We show that photodissociation is significant in the least massive protogalaxies, but becomes less so as the protogalactic mass increases. We also examine the effects of photodissociation on dense clumps of gas within the protogalaxy. We find that while collapse will be inhibited in low density clumps, denser ones may survive to form stars.Comment: 13 pages, 10 figures. Minor revisions to match version accepted by MNRA

The Evolution of the CareerAdvance® Program in Tulsa, Oklahoma

CareerAdvance®n collaboration with a multi-disciplinary team of partners, the Ray Marshall Center (RMC) is developing and implementing a sectoral workforce development strategy for low-skilled, low-income parents of children served by early childhood programs in Tulsa, Oklahoma. There is emerging evidence that children whose parents hold stable jobs with progressively rising incomes exhibit better academic and behavioral outcomes. RMC and its partners have undertaken a dual-generation approach to poverty reduction that strengthens the investment in early childhood development by equipping Head Start parents with workforce training and gainful employment opportunities. This approach employs a more holistic model than traditional workforce development programs, as it also includes employee counseling and other support services to help parents complete training and adult basic education, retain their jobs, advance in their careers, and become economically self-sufficient. The goal is to develop a sustainable sectoral strategy that can be replicated beyond Tulsa to other communities across the nation. In the first phase of the project (2008-2009), RMC designed a sectoral job development strategy focused on industries featuring jobs that pay well and provide much-needed employee benefits (e.g., health insurance, annual and sick leave) as well as career advancement opportunities. In April 2009, Community Action Program of Tulsa County launched the pilot, CareerAdvance, at two Head Start sites in Tulsa involving 15 parents. The components of the CareerAdvance are 1) GED and college readiness instruction, as needed; 2) skills training in the healthcare sector progressing from Certified Nursing Aide to Licensed Practical Nurse to Registered Nurse; 3) weekly peer support meetings addressing a flexible set of topics (e.g., life skills, work readiness, family finances); 4) conditional cash incentives (up to $3,000 a year) for participants meeting specified benchmarks to reinforce continued participation and help offset foregone earnings; and 5) workforce intermediation between healthcare employers and training institutions provided through Workforce Tulsa. The report on the project’s first year of operation is available at the link below. In partnership with Harvard University and the University of Oklahoma – Tulsa School of Medicine, a second pilot site was opened in July 2009 at a Tulsa Educare Center. The second pilot, EduCareers, includes all components described above as well as enhanced mental health services for participating households, curriculum enhancements for the children, parent engagement training, and a medical home. The CareerAdvance project has now been expanded to 2015 with support from the U.S. Department of Health and Human Services’ Administration for Children and Families. RMC and partners at Northwestern and Columbia University have been engaged to provide ongoing on data collection, implementation and outcomes analysis of project participants.George Kaiser Family Foundation, W. K. Kellogg Foundation, U.S. Department of Health and Human Services' Administration for Children and FamiliesRay Marshall Center for the Study of Human Resource

Cosmological Implications of the Uncertainty in Astrochemical Rate Coefficients

The cooling of neutral gas of primordial composition, or with very low levels of metal enrichment, depends crucially on the formation of molecular coolants, such as H2 and HD within the gas. Although the chemical reactions involved in the formation and destruction of these molecules are well known, the same cannot be said for the rate coefficients of these reactions, some of which are uncertain by an order of magnitude. Here we discuss two reactions for which large uncertainties exist the formation of H2 by associative detachment of H- with H and the destruction of H- by mutual neutralization with protons. We show that these uncertainties can have a dramatic impact on the effectiveness of cooling during protogalactic collapse

Radiative feedback from an early X-ray background

The first generation of stars (commonly known as population III) are expected to form in low-mass protogalaxies in which molecular hydrogen is the dominant coolant. Radiation from these stars will rapidly build up an extragalactic ultraviolet background capable of photodissociating H2, and it is widely believed that this background will suppress further star formation in low-mass systems. However, star formation will also produce an extragalactic X-ray background. This X-ray background, by increasing the fractional ionization of protogalactic gas, promotes H2 formation and reduces the effectiveness of ultraviolet feedback. In this paper, we examine which of these backgrounds has the dominant effect. Using a simple model for the growth of the UV and X-ray backgrounds, together with a detailed one-dimensional model of protogalactic chemical evolution, we examine the effects of the X-ray backgrounds produced by a number of likely source models. We show that in several cases, the resulting X-ray background is strong enough to offset UV photodissociation in large H2-cooled protogalaxies. On the other hand, small protogalaxies (those with virial temperatures T_vir < 2000K) remain dominated by the UV background in all of the models we examine. We also briefly investigate the effects of the X-ray background upon the thermal and chemical evolution of the diffuse IGM.Comment: 19 pages, 10 figures. Presentation improved, thanks to helpful comments by the referee. Accepted by MNRA

Local structures of free-standing AlₓGa₁ˍₓN thin films studied by extended x-ray absorption fine structure

Local structural information for the first two atomic shells surrounding Ga atoms in free standing AlₓGa₁ˍₓN alloy films has been obtained by extended x-ray absorption fine structure spectroscopy. For an AlN mole fraction ranging from 0 to 0.6, we found that the first shell Ga–N bond length had only a weak composition dependence, roughly one quarter of that predicted by Vegard’s Law. In the second shell, the Ga–Ga bond length was significantly longer than that of Ga–Al (Δ∼0.04–0.065 Å). A bond-type specific composition dependence was observed for the second shell cation–cation distances. While the composition dependence of the Ga–Ga bond length is ∼70% of that predicted by Vegard’s Law, the Ga–Al bond length was essentially composition independent. These results suggested that local strain in AlₓGa₁ˍₓN was also accommodated by lattice distortion in the Al cation sublattice.This work was supported by the Director, Office of Science, Of- fice of Basic Energy Sciences, Materials Science Division of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098. The LLO work was performed at the UC Berkeley Integrated Materials Laboratory which was supported in part by the National Science Foundation. C.J.G. and M.C.R. were supported by the Australian Synchrotron Research Program, funded by the Commonwealth of Australia via the Major National Research Facilities Program. SSRL was supported by the Office of Basic Energy Sciences of the U.S. Department of Energy

Weed Control for Reduced Tillage Systems

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