251 research outputs found
Vintage Capital and the Dynamics of the AK Model
This paper analyzes the equilibrium dynamics of an AK-type endogenous growth model with vintage capital. The inclusion of vintage capital leads to oscillatory dynamics governed by replacement echoes, which additionally influence the intercept of the balanced growth path. These features, which are in sharp contrast to those from the standard AK model, can contribute to explaining the short-run deviations observed between investment and growth rates time series. To characterize the convergence properties and the dynamics of the model we develop analytical and numerical methods that should be of interest for the general resolution of endogenous growth models with vintage capital.
Endogenous growth and wave-like business fluctuations
This paper argues that observed long lags in innovation implementation rationalize Schumpeter's statement that “wave-like fluctuations in business ... are the form economic development takes in the era of capitalism.” Adding implementation delays to an otherwise standard endogenous growth model with expanding product variety, the equilibrium path admits a Hopf bifurcation where consumption, R&D and output permanently fluctuate. This mechanism is quantitatively consistent with the observed medium-term movements of US aggregate output. In this framework, an optimal allocation may be restored at equilibrium by the mean of a procyclical subsidy, needed to generate additional consumption smoothing. Finally, a procyclical R&D subsidy rate designed to half consumption fluctuations will increase the growth rate from 2.4% to 3.4% with a 9.6% (compensation equivalent) increase in welfare
The nucleus of 103P/Hartley 2, target of the EPOXI mission
103P/Hartley 2 was selected as the target comet for the Deep Impact extended
mission, EPOXI, in October 2007. There have been no direct optical observations
of the nucleus of this comet, as it has always been highly active when
previously observed. We aimed to recover the comet near to aphelion, to a)
confirm that it had not broken up and was in the predicted position, b) to
provide astrometry and brightness information for mission planning, and c) to
continue the characterisation of the nucleus. We observed the comet at
heliocentric distances between 5.7 and 5.5 AU, using FORS2 at the VLT, at 4
epochs between May and July 2008. We performed VRI photometry on deep stacked
images to look for activity and measure the absolute magnitude and therefore
estimate the size of the nucleus. We recovered the comet near the expected
position, with a magnitude of m_R = 23.74 \pm 0.06 at the first epoch. The
comet had no visible coma, although comparison of the profile with a stellar
one showed that there was faint activity, or possibly a contribution to the
flux from the dust trail from previous activity. This activity appears to fade
at further epochs, implying that this is a continuation of activity past
aphelion from the previous apparition rather than an early start to activity
before the next perihelion. Our data imply a nucleus radius of \le 1 km for an
assumed 4% albedo; we estimate a ~6% albedo. We measure a colour of (V-R) = 0.
26 \pm 0.09.Comment: 5 pages, 4 figures, accepted for publication in A&
Testing the comet nature of main belt comets. The spectra of 133P/Elst-Pizarro and 176P/LINEAR
We present the visible spectrum of MBCs 133P/Elst-Pizarro and 176P/LINEAR, as
well as three Themis family asteroids: (62) Erato, (379), Huenna and (383)
Janina, obtained in 2007 using three telescopes at "El Roque de los Muchachos"'
Observatory, in La Palma, Spain, and the 8m Kueyen (UT2) VLT telescope at Cerro
Paranal, Chile.
The spectra of 133P and 176P resemble best those of B-type asteroid and are
very similar to those of Themis family members and are significantly different
from the spectrum of comet 162P/Siding-Spring and most of the observed cometary
nuclei. CN gas emission is not detected in the spectrum of 133P. We determine
an upper limit for the CN production rate Q(CN) = mol/s,
three orders of magnitude lower than the Q(CN) of Jupiter family comets
observed at similar heliocentric distances.
The spectra of 133P/Elst-Pizarro and 176P/LINEAR confirm that they are likely
members of the Themis family of asteroids, fragments that probably retained
volatiles, and unlikely have a cometary origin in the trans-neptunian belt or
the Oort cloud.Comment: Paper sumbmited to A&A. 7 pages and 6 figure
Water-ice driven activity on Main-Belt Comet P/2010 A2 (LINEAR) ?
The dust ejecta of Main-Belt Comet P/2010 A2 (LINEAR) have been observed with
several telescopes at the at the Observatorio del Roque de los Muchachos on La
Palma, Spain. Application of an inverse dust tail Monte Carlo method to the
images of the dust ejecta from the object indicates that a sustained, likely
water-ice driven, activity over some eight months is the mechanism responsible
for the formation of the observed tail. The total amount of dust released is
estimated to be 5E7 kg, which represents about 0.3% of the nucleus mass. While
the event could have been triggered by a collision, this cannot be decided from
the currently available data.Comment: Accepted for ApJ Letter
Seasonal and interannual variability of cladoceran communities in two peri-alpine lakes: uncoupled response to the 2003 heat wave
Seasonal and interannual dynamics of cladoceran species were analyzed during the period 1995–2003 in two deep peri-alpine lakes morphologically different but subjected to similar regional climatic forcing. The seasonal succession of cladoceran species was characterized and the impact of extreme climatic events on the annual pattern of species succession was assessed. Using a multivariate method, we show that the cladoceran species display marked seasonality patterns that differed in the two lakes. The differences observed between the lakes were driven by
their trophic state, the plankton species composition and the abundance of predators.
We show that the sensitivity of the annual pattern of species succession to extreme weather changes, illustrated by the 2003 heat wave, differs markedly in these two lakes. In Lake Annecy, the annual pattern of cladoceran succession
observed in 2003 is not different from the one usually observed. In contrast, in Lake Geneva, the annual pattern recorded in 2003 is unusual and characterized by the maintenance of herbivorous cladocera during summer. These findings underline the need to consider the morphology of lakes and trophic state in the assessment of ecological responses to global warming. Our results contribute to the
debate about the predictability of the impacts of climate change on aquatic ecosystems, and their extrapolation from one site to another
P/2010 A2 LINEAR II: dynamical dust modelling
P/2010 A2 is an object on an asteroidal orbit that was observed to have an
extended tail or debris trail in January 2010. In this work, we fit the
outburst of P/2010 A2 with a conical burst model, and verify previous
suspicions that this was a one--time collisional event rather than an sustained
cometary outburst, implying that P/2010 A2 is not a new Main Belt Comet driven
by ice sublimation. We find that the best--fit cone opening angle is about 40
to 50 degrees, in agreement with numerical and laboratory simulations of
cratering events. Mapping debris orbits to sky positions suggests that the
distinctive arc features in the debris correspond to the same debris cone
inferred from the extended dust. From the velocity of the debris, and from the
presence of a velocity maximum at around 15 cm/s, we infer that the surface of
A2 probably has a very low strength (<1 kPa), comparable to lunar regolith.Comment: 14 pages, 25 figures; accepted by Astronomy and Astrophysic
The visible and near-infrared spectra of asteroids in cometary orbits
We study the visible and near-infrared (NIR) spectral properties of different
ACO populations and compare them to the independently determined properties of
comets.
We select our ACOs sample based on published dynamical criteria and present
our own observational results obtained using the 10.4m Gran Telescopio Canarias
(GTC), the 4.2m William Herschel Telescope (WHT), the 3.56m Telescopio
Nazionale Galileo (TNG), and the 2.5m Isaac Newton Telescope (INT), all located
at the El Roque de los Muchachos Observatory (La Palma, Spain), and the 3.0m
NASA Infrared Telescope Facility (IRTF), located at the Mauna Kea Observatory,
in Hawaii. We include in the analysis the spectra of ACOs obtained from the
literature. We derive the spectral class and the visible and NIR spectral
slopes. We also study the presence of hydrated minerals by studying the 0.7
m band and the UV-drop below 0.5 m associated with phyllosilicates.
We present new observations of 17 ACOs, 11 of them observed in the visible, 2
in the NIR and 4 in the visible and NIR. We also discuss the spectra of 12 ACOs
obtained from the literature.
All but two ACOs have a primitive-like class spectrum (X or D-type). Almost
100\% of the ACOs in long-period cometary orbits (Damocloids) are D-types.
Those in Jupiter family comet orbits (JFC-ACOs) are 60\% D-types and
40\% X-types. The mean spectral slope of JFC-ACOs is 9.7 4.6
\%/1000 \AA \ and for the Damocloids this is 12.2 2.0 \%/1000 \AA . No
evidence of hydration on the surface of ACOs is found from their visible
spectra. The slope and spectral class distribution of ACOs is similar to that
of comets.
The spectral classification, the spectral slope distribution of ACOs, and the
lack of spectral features indicative of the presence of hydrated minerals on
their surface, strongly suggest that ACOs are likely dormant or extinct comets.Comment: 11 pages, 10 Figures, published in A&
Technology, Market Structure and the Gains from Trade
We study the gains from trade in a new model with oligopolistic competition, firm heterogeneity, and innovation. Lowering trade costs reduces markups on domestic sales but increases markups on export sales, as firms do not pass the entire reduction in trade costs onto foreign consumers. Trade liberalisation can also reduce the number of firms competing in each market, thereby increasing markups on both domestic and export sales. For the majority of exporters, however, the procompetitive effect prevails and their avera ge markups decline. The incomplete pass-though and the reduction in the number of competitors instead dominate for top-exporters – the top 0:1% of firms which end up increasing their markup. In a quantitative exercise we find that the aggregate effect of trade-induced markup changes is pro-competitive and accounts for the majority of the welfare gains from trade. Trade-induced changes in competition affect survival on domestic and export markets and firms’ decision to innovate. All exporters, and especially the top exporters, increase their market size after liberalisation which, in turn, encourages them to innovate more. Hence, top exporters contribute negatively to welfare gains by increasing their markups but positively by increasing innovation and productivity. Firms’ innovation response accounts for a small but non-negligible share of the welfare gains while the contribution of selection is U-shaped, being negative for small liberalisations and positive otherwise. A more globalised economy is therefore populated by larger, fewer and more innovative firms, each feature representing an important source of the gains from trade
Near-Earth asteroids spectroscopic survey at Isaac Newton Telescope
The population of near-Earth asteroids (NEAs) shows a large variety of
objects in terms of physical and dynamical properties. They are subject to
planetary encounters and to strong solar wind and radiation effects. Their
study is also motivated by practical reasons regarding space exploration and
long-term probability of impact with the Earth. We aim to spectrally
characterize a significant sample of NEAs with sizes in the range of 0.25
- 5.5 km (categorized as large), and search for connections between their
spectral types and the orbital parameters. Optical spectra of NEAs were
obtained using the Isaac Newton Telescope (INT) equipped with the IDS
spectrograph. These observations are analyzed using taxonomic classification
and by comparison with laboratory spectra of meteorites. A total number of 76
NEAs were observed. We classified 44 of them as Q/S-complex, 16 as B/C-complex,
eight as V-types, and another eight belong to the remaining taxonomic classes.
Our sample contains 27 asteroids categorized as potentially hazardous and 31
possible targets for space missions including (459872) 2014 EK24, (436724) 2011
UW158, and (67367) 2000 LY27. The spectral data corresponding to (276049) 2002
CE26 and (385186) 1994 AW1 shows the 0.7 m feature which indicates the
presence of hydrated minerals on their surface. We report that Q-types have the
lowest perihelia (a median value and absolute deviation of AU)
and are systematically larger than the S-type asteroids observed in our sample.
We explain these observational evidences by thermal fatigue fragmentation as
the main process for the rejuvenation of NEA surfaces. In general terms, the
taxonomic distribution of our sample is similar to the previous studies and
matches the broad groups of the inner main belt asteroids. Nevertheless, we
found a wide diversity of spectra compared to the standard taxonomic types.Comment: Accepted in Astronomy & Astrophysics (A&A
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