7,242 research outputs found

    The Link Between Firms? Innovation Decision and the Business Cycle: An Empirical Analysis

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    The sensitivity of innovation activities with respect to the business cycle is often assumed to be small. In this paper the hypothesis on cyclical dependence of innovation activities is tested for firms in the German manufacturing, and additionally for SMEs. To this end firms? innovation decisions are considered. The decision to innovate in one period is modelled via a first-order Markov chain approach. The results suggest that the patterns in innovative behavior are linked to the business cycle. --Innovation,Business Cycle,Panel Model,Markov Chains

    Thermonuclear Bursts with Short Recurrence Times from Neutron Stars Explained by Opacity-Driven Convection

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    Thermonuclear flashes of hydrogen and helium accreted onto neutron stars produce the frequently observed Type I X-ray bursts. It is the current paradigm that almost all material burns in a burst, after which it takes hours to accumulate fresh fuel for the next burst. In rare cases, however, bursts are observed with recurrence times as short as minutes. We present the first one-dimensional multi-zone simulations that reproduce this phenomenon. Bursts that ignite in a relatively hot neutron star envelope leave a substantial fraction of the fuel unburned at shallow depths. In the wake of the burst, convective mixing events driven by opacity bring this fuel down to the ignition depth on the observed timescale of minutes. There, unburned hydrogen mixes with the metal-rich ashes, igniting to produce a subsequent burst. We find burst pairs and triplets, similar to the observed instances. Our simulations reproduce the observed fraction of bursts with short waiting times of ~30%, and demonstrate that short recurrence time bursts are typically less bright and of shorter duration.Comment: 11 pages, 15 figures, accepted for publication in Ap

    The Metallicity Dependence of the Minimum Mass for Core-Collapse Supernovae

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    Understanding the progenitors of core collapse supernovae and their population statistics is a key ingredient for many current studies in astronomy but as yet this remains elusive. Using the MESA stellar evolution code we study the dependence of the lower mass limit for making core collapse supernovae (SNe) as function of initial stellar metallicity. We find that this mass limit is smallest at approximately [Z] = -2 with a value of ~ 8.3 Msun. At [Z] = 0 the limit is ~ 9.5 Msun and continues to rise with higher metallicity. As a consequence, for a fixed initial mass function the supernova rate may be 20% to 25% higher at [Z] = -2. This affects the association of observed SN rates as a probe for the cosmological star formation rate, rate predictions for supernova surveys, and population synthesis studies.Comment: 13 pages, 1 figure, 1 table, submitted to ApJ

    Combined Nucleosynthetic Yields of Multiple First Stars

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    Modern numerical simulations of the formation of the first stars predict that the first stars formed in multiples. In those cases, the chemical yields of multiple supernova explosions may have contributed to the formation of a next generation star. We match the chemical abundances of the oldest observed stars in the universe to a database of theoretical supernova models, to show that it is likely that the first stars formed from the ashes of two or more progenitors.Comment: 3 pages, 2 figures, NIC 2016 Conference Proceeding

    The quest for blue supergiants: binary merger models for the evolution of the progenitor of SN 1987A

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    We present the results of a detailed, systematic stellar evolution study of binary mergers for blue supergiant (BSG) progenitors of Type II supernovae. In particular, these are the first evolutionary models that can simultaneously reproduce nearly all observational aspects of the progenitor of SN 1987A, \text{Sk}-69\,^{\circ}202, such as its position in the HR diagram, the enrichment of helium and nitrogen in the triple-ring nebula, and its lifetime before its explosion. The merger model, based on the one proposed by Podsiadlowski 1992 et al. and Podsiadlowski 2007 et al., consists of a main sequence secondary star that dissolves completely in the common envelope of the primary red supergiant at the end of their merger. We empirically explore a large initial parameter space, such as primary masses (15 MβŠ™15\,\text{M}_{\odot}, 16 MβŠ™16\,\text{M}_{\odot}, and 17 MβŠ™17\,\text{M}_{\odot}), secondary masses (2 MβŠ™2\,\text{M}_{\odot}, 3 MβŠ™3\,\text{M}_{\odot}, ..., 8 MβŠ™8\,\text{M}_{\odot}) and different depths up to which the secondary penetrates the He core of the primary during the merger. The evolution of the merged star is continued until just before iron-core collapse and the surface properties of the 84 pre-supernova models (16 MβŠ™βˆ’23 MβŠ™16\,\text{M}_{\odot}-23\,\mathrm{M}_{\odot}) computed have been made available in this work. Within the parameter space studied, the majority of the pre-supernova models are compact, hot BSGs with effective temperature >12 kK>12\,\text{kK} and radii of 30 RβŠ™βˆ’70 RβŠ™30\,\text{R}_{\odot}-70\,\mathrm{R}_{\odot} of which six match nearly all the observational properties of \text{Sk}-69\,^{\circ}202.Comment: Submitted to MNRAS. 21 pages, 11 figures, 7 table
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