6,717 research outputs found
Coming From Good Stock: Career Histories and New Venture Formation
We examine how the social structure of existing organizations influences entrepreneurship and suggest that resources accrue to entrepreneurs based on the structural position of their prior employers. We argue that information advantages allow individuals from entrepreneurially prominent prior firms to identify new opportunities. Entrepreneurial prominence also reduces the perceived uncertainty of a new venture. Using a sample of Silicon Valley start-ups, we demonstrate that entrepreneurial prominence is associated with initial strategy and the probability of attracting external financing. New ventures with high prominence are more likely to be innovators; furthermore, innovators with high prominence are more likely to obtain financing
Generation of galactic disc warps due to intergalactic accretion flows onto the disc
A new method is developed to calculate the amplitude of the galactic warps
generated by a torque due to external forces. This takes into account that the
warp is produced as a reorientation of the different rings which constitute the
disc in order to compensate the differential precession generated by the
external force, yielding a uniform asymptotic precession for all rings.
Application of this method to gravitational tidal forces in the Milky Way due
to the Magellanic Clouds leads to a very low amplitude of the warp. If the
force were due to an extragalactic magnetic field, its intensity would have to
be very high, to generate the observed warps. An alternative hypothesis is
explored: the accretion of the intergalactic medium over the disk. A cup-shaped
distortion is expected, due to the transmission of the linear momentum; but,
this effect is small and the predominant effect turns out to be the
transmission of angular momentum, i.e. a torque giving an integral-sign shape
warp. The torque produced by a flow of velocity ~100 km/s and baryon density
\~10^{-25} kg/m^3 is enough to generate the observed warps and this mechanism
offers quite a plausible explanation. First, because this order of accretion
rate is inferred from other processes observed in the Galaxy, notably its
chemical evolution. The inferred rate of infall of matter, ~1 solar-mass/yr, to
the Galactic disc that this theory predicts agrees with the quantitative
predictions of this chemical evolution resolving key issues, notably the
G-dwarf problem. Second, because the required density of the intergalactic
medium is within the range of values compatible with observation. By this
mechanism, we can explain the warp phenomenon in terms of intergalactic
accretion flows onto the disk of the galaxy.Comment: 18 pages, 11 figures, accepted to be published in A&
Type-II surface brightness profiles in edge-on galaxies produced by flares
Previous numerical studies had apparently ruled out the possibility that
flares in galaxy discs could give rise to the apparent breaks in their
luminosity profiles when observed edge-on. However the studies have not, until
now, analyzed this hypothesis systematically using realistic models for the
disc, the flare, and the bulge. We revisit this theme by analyzing a series of
models which sample a wide range of observationally based structural parameters
for these three components. We have considered realistic distributions of bulge
to disc ratios, morphological parameters of bulges and discs, vertical scale
heights of discs and their radial gradients defining the flare for different
morphological types and stellar mass bins, based on observations. The surface
brightness profiles for the face-on and edge-on views of each model were
simulated to find out whether the flared disc produces a Type-II break in the
disc profile when observed edge-on, and if so under what conditions. Contrary
to previous claims, we find that discs with realistic flares can produce
significant breaks in discs when observed edge-on. Specifically a flare with
the parameters of that of the Milky Way would produce a significant break of
the disc at a Rbreak of ~8.6 kpc if observed edge-on. Central bulges have no
significant effects on the results. These simulations show that flared discs
can explain the existence of many Type-II breaks observed in edge-on galaxies,
in a range of galaxies with low-to-intermediate break strength values of
-0.25<S<-0.1.Comment: Published in Astronomy & Astrophysics. 5 pages, 5 figures. Language
corrections by the journal included in this new versio
Photometric scaling relations of antitruncated stellar discs in S0-Scd galaxies
It has been recently found that the characteristic photometric parameters of
antitruncated discs in S0 galaxies follow tight scaling relations. We
investigate if similar scaling relations are satisfied by galaxies of other
morphological types. We have analysed the trends in several photometric planes
relating the characteristic surface brightness and scalelengths of the breaks
and the inner and outer discs of local antitruncated S0-Scd galaxies, using
published data and fits performed to the surface brightness profiles of two
samples of Type-III galaxies in the R and Spitzer 3.6 microns bands. We have
performed linear fits to the correlations followed by different galaxy types in
each plane, as well as several statistical tests to determine their
significance. We have found that: 1) the antitruncated discs of all galaxy
types from Sa to Scd obey tight scaling relations both in R and 3.6 microns, as
observed in S0s; 2) the majority of these correlations are significant
accounting for the numbers of the available data samples; 3) the trends are
clearly linear when the characteristic scalelengths are plotted on a
logarithmic scale; and 4) the correlations relating the characteristic surface
brightnesses of the inner and outer discs and the breaks with the various
characteristic scalelengths significantly improve when the latter are
normalized to the optical radius of the galaxy. The results suggest that the
scaling relations of Type-III discs are independent of the morphological type
and the presence (or absence) of bars within the observational uncertainties of
the available datasets, although larger and deeper samples are required to
confirm this. The tight structural coupling implied by these scaling relations
impose strong constraints on the mechanisms proposed for explaining the
formation of antitruncated stellar discs in the galaxies across the whole
Hubble Sequence (Abridged).Comment: Accepted for publication in Astronomy & Astrophysics, 18 pages, 12
figures, 7 table
An investigation of pre-service teacher and faculty knowledge on RTI
Response to Intervention (RTI) is being used in schools more often and continues to be a more accepted way to help students prior to the need for special education services. RTI is a three-tiered system of support that provides an opportunity for teachers to look at their instruction and curriculum and adjust based upon student progress and needs. The RTI framework, when implemented with fidelity, has been shown to effectively assist students get academic help early and limit the number of special education referrals (VanDerHeyden, Witt, & Gilbertson, 2007).
Although RTI has been shown to promote positive change in student outcomes, there are still several problems with RTI implementation and the framework. One of the biggest issues is the insufficiency of teacher training. Teachers’ are going into the field feeling like they have little to no knowledge of RTI and how to implement it in the classroom (Barrio & Combes, 2014). The purpose of this study is to assess pre-service teachers’ knowledge of the RTI framework and confidence in that knowledge. Additionally, this study will investigate faculty knowledge of RTI in order to better understand student knowledge on this topic. Due to the current lack of research on this topic, the current study will add to the current research on pre-service teacher’s knowledge of RTI
Development and testing of hermetic, laser-ignited pyrotechnic and explosive components
During the last decade there has been increasing interest in the use of lasers in place of electrical systems to ignite various pyrotechnic and explosive materials. The principal driving force for this work was the requirement for safer energetic components which would be insensitive to electrostatic and electromagnetic radiation. In the last few years this research has accelerated since the basic concepts have proven viable. At the present time it is appropriate to shift the research emphasis in laser initiation from the scientific arena--whether it can be done--to the engineering realm--how it can be put into actual practice in the field. Laser initiation research and development at EG&G Mound was in three principal areas: (1) laser/energetic material interactions; (2) development of novel processing techniques for fabricating hermetic (helium leak rate of less than 1 x 10(exp -8) cu cm/s) laser components; and (3) evaluation and testing of laser-ignited components. Research in these three areas has resulted in the development of high quality, hermetic, laser initiated components. Examples are presented which demonstrate the practicality of fabricating hermetic, laser initiated explosive or pyrotechnic components that can be used in the next generation of ignitors, actuators, and detonators
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