Oxygen isotopic ratios in first dredge-up red giant stars and nuclear reaction rate uncertainties revisited

We describe a general yet simple method to analyse the propagation of nuclear reaction rate uncertainties in a stellar nucleosynthesis and mixing context. The method combines post-processing nucleosynthesis and mixing calculations with a Monte Carlo scheme. With this approach we reanalyze the dependence of theoretical oxygen isotopic ratio predictions in first dredge-up red giant branch stars in a systematic way. Such predictions are important to the interpretation of pre-solar Al_2 O_3 grains from meteorites. The reaction rates with uncertainties were taken from the NACRE compilation (Angulo etal., 1999). We include seven reaction rates in our systematic analysis of stellar models with initial masses from 1 to 3 M_sun. We find that the uncertainty of reaction rate for reaction O18(p,alpha)N15 typically causes an error in the theoretical O16/O18 ratio of about +20/-5 per cent. The error of the O16/O17 prediction is +-10 to 40 per cent depending on the stellar mass, and is persistently dominated by the comparatively small uncertainty of the O16(p,gamma)F17 reaction. With the new estimates on reaction rate uncertainties by the NACRE compilation, the p-capture reactions O17(p,alpha)N14 and O17(p,gamma)F18 have virtually no impact on theoretical predictions for stellar mass <= 1.5 M_sun. However, this is not the case for masses > 1.5 M_sun, where core mixing and subsequent envelope mixing interact. In these cases where core mixing complicates post-dredge-up surface abundances, uncertainty in other reactions have a secondary but noticeable effect on surface abundances.Comment: 11 pages (with figures and tables at the end), 8 figures (11 .eps files), submitted to MNRA

The Supernova Channel of Super-AGB Stars

We study the late evolution of solar metallicity stars in the transition region between white dwarf formation and core collapse. This includes the super-asymptotic giant branch (super-AGB, SAGB) stars, which have massive enough cores to ignite carbon burning and form an oxygen-neon (ONe) core. The most massive SAGB stars have cores that may grow to the Chandrasekhar mass because of continued shell-burning. Their cores collapse, triggering a so called electron capture supernovae (ECSN). From stellar evolution models we find that the initial mass range for SAGB evolution is 7.5 ... 9.25\msun. We perform calculations with three different stellar evolution codes to investigate the sensitivity of this mass range to some of the uncertainties in current stellar models. The mass range significantly depends on the treatment of semiconvective mixing and convective overshooting. To consider the effect of a large number of thermal pulses, as expected in SAGB stars, we construct synthetic SAGB models that include a semi-analytical treatment of dredge-up, hot-bottom burning, and thermal pulse properties. This synthetic model enables us to compute the evolution of the main properties of SAGB stars from the onset of thermal pulses until the core reaches the Chandrasekhar mass or is uncovered by the stellar wind. Thereby, we determine the stellar initial mass ranges that produce ONe-white dwarfs and electron-capture supernovae. The latter is found to be 9.0 ... 9.25\msun for our fiducial model, implying that electron-capture supernovae would constitute about 4% of all supernovae in the local universe. Our synthetic approach allows us to explore the uncertainty of this number imposed by uncertainties in the third dredge-up efficiency and ABG mass loss rate. We find for ECSNe a upper limit of ~20% of all supernovae (abridged).Comment: 13 pages, 16 figures, submitted to ApJ, uses emulateap

The Zen of Corporate Capital Structure Neutrality

It is well understood that corporate capital structure affects tax collections. Most basically, corporate interest expense is deductible. With each interest accrual, the corporate tax base shrinks. Thus, there is a broad range of circumstances in which corporate managers are encouraged by the Internal Revenue Code (the Code ) to load their corporate capital structures with debt. But there is little support for the proposition that Conpress desires corporations to adopt such debt-laden capital structures. Indeed, much tax legislation suggests congressional displeasure with the achievable degree of corporate self- integration. On the other hand, corporate equity has its charms: shareholders are able to defer their gains essentially forever. Thus, in some circumstances the Code encourages corporate managers to load their corporate capital structures with equity. Based on the numerous provisions in the Code that depress the relative tax cost of equity, it is probably safe to conclude that Congress is more sanguine about equity than it is about debt. But periodically, Congress tempers its enthusiasm. And academicians as a group find the feature of equity deferral - the realization requirement - quite troubling

The Zen of Corporate Capital Structure Neutrality

Given the current tax rate structure - where the marginal tax rate of some persons exceeds the corporate tax rate and the marginal tax rate of others is exceeded by it - corporations are generally well advised to employ both debt and equity in their capital structures. The former will be held by low tax rate taxpayers and will serve to lower the effective aggregate tax rate6 on the corporation\u27s taxable income. The latter will be held by high tax rate taxpayers and will serve to keep low the effective aggregate tax rate on the corporation\u27s unrecognized economic income (such as any increase in the value of corporate assets, including goodwill). From the vantage of the Fisc, this is, of course, the worst of all possible worlds. This Article does not propose to do away with the infirmities of the current corporate tax regime by abolishing double taxation. For while Code § 11 may be the step child of federal income tax theory, there currently appears to be no realistic prospect to repeal it. At least in the case of publicly traded corporations - the most important class of double-taxed entities - Americans tend to view them either as a free good, which can be taxed with economic impunity, or as a proxy for the faceless rich, who are undertaxed in any event. Perhaps this will change in time, as the proliferation of 401(K) plans turns the hoards of middle class taxpayers into capitalists. But a change seems to be yet a good way off. And in any event, as I argue below, integration - at least in its commonly proposed forms - would not necessarily cure all that ails the current corporate tax system. Thus, this Article takes double taxation as a given and as a challenge. It asks how, if at all, a double tax regime can be designed so that economic actors are powerless to use capital structure to influence tax collections. The linchpin to the answer, set forth in Part VI below, is that the Code cannot allow any nontrivial corporate deduction with respect to any returns earned by any corporate capital providers. In particular, and merely as one example, the corporate deduction for interest expense must be abolished

Little Boxes: Can Optimal Commodity Tax Methodology Save the Debt-Equity Distinction?

Optimal commodity tax methodology has been proposed as a way of making difficult line drawing decisions in the income tax. This paper explores some practical difficulties with the approach, and concludes that in one area - the debt-equity divide - the approach is unlikely to prove useful over any significant time horizon

How I Learned to Stop Worrying and Love Double Taxation

This Article is divided into three Parts. The first Part is devotedto an example demonstrating that, while double taxation may be gratuitous in a purely domestic context, it invariably becomes necessary in a multinational context. The second Part formalizes and generalizes the example, and concludes that double taxation is not only necessary in a multinational context, but also in any multi-period domestic context. The third Part contains a few policy prescriptions that, I fervently hope, will guide future administrations

Functional requirements for onboard management of space shuttle consumables, volume 1

A study was conducted to determine the functional requirements for onboard management of space shuttle consumables. A generalized consumable management concept was developed for application to advanced spacecraft. The subsystems and related consumables selected for inclusion in the consumables management system are: (1) propulsion, (2) power generation, and (3) environmental and life support

On the degenerated soft-mode instability

We consider instabilities of a single mode with finite wavenumber in inversion symmetric spatially one dimensional systems, where the character of the bifurcation changes from sub- to supercritical behaviour. Starting from a general equation of motion the full amplitude equation is derived systematically and formulas for the dependence of the coefficients on the system parameters are obtained. We emphasise the importance of nonlinear derivative terms in the amplitude equation for the behaviour in the vicinity of the bifurcation point. Especially the numerical values of the corresponding coefficients determine the region of coexistence between the stable trivial solution and stable spatially periodic patterns. Our approach clearly shows that similar considerations fail for the case of oscillatory instabilities.Comment: 16 pages, uses iop style files, manuscript also available at ftp://athene.fkp.physik.th-darmstadt.de/pub/publications/wolfram/jpa_97/ or at http://athene.fkp.physik.th-darmstadt.de/public/wolfram_publ.html. J. Phys. A in pres

User's manual for the Shuttle Electric Power System analysis computer program (SEPS), volume 2 of program documentation

The Shuttle Electric Power System Analysis SEPS computer program which performs detailed load analysis including predicting energy demands and consumables requirements of the shuttle electric power system along with parameteric and special case studies on the shuttle electric power system is described. The functional flow diagram of the SEPS program is presented along with data base requirements and formats, procedure and activity definitions, and mission timeline input formats. Distribution circuit input and fixed data requirements are included. Run procedures and deck setups are described