7,430 research outputs found
The Link Between Firms? Innovation Decision and the Business Cycle: An Empirical Analysis
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
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
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
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
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 (,
, and ), secondary masses
(, , ..., ) 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 () 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 and radii of
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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